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XB U n d e r Royalty A g r e e m e n t
i AGMA911-A94

A M E R IC A N G E A R M A N U F A C T U R E R S A S S O C IA T IO N

D e sig n G u idelines fo r A e r o s p a c e G e a rin g

c.

AGMA INFO R M A T IO N SHEET

(This In fo r m a tio n S h e e tis N O T a n A G M A S ta n d a r d )
AGMA 911-A94

AGMA 91%A94, Design Guidelines for Aerospace Gearing

CAUTION NOTICE: AGMA standards are subject to constant improvement, revision, or withdrawal as
dictated by experience. Any person who refers to any AGMA Technical Publication should be sure that the
publication is the latest available from the Association on the subject matter.
[Tables or other self-supporting sections may be quoted or extracted in their entirety. Credit lines should read:
Extracted from AGMA 911-A94, information Sheet - Design Guidelines for Aerospace Gearing, with the
permission of the publisher, the American Gear Manufacturers Association, 1500 King Street, Suite 201,
Alexandria, Virginia 223141.

ABSTRACT:
This Information Sheet covers current gear design practices as they are applied to air vehicles and spacecraft.
The material included goes beyond the design of gear meshes and presents the broad spectrum of factors
which combine to produce a working gear system, whether it be a power transmission or special purpose
mechanism. Although a variety of gear types, such as wormgears, face gears and various proprietary tooth
forms are used in aerospace applications, this document covers only spur, helical, and bevel gears.

Copyright 0 1994 by American Gear Manufacturers Association

Published by

American Gear Manufacturers Association

1500 King Street, Suite 201, Alexandria, Virginia, 22314

ISBN: l-55589-8294

ii
 AGMA 911-A94

Contents Page

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..*............... vi

1 Scope ....................................................................... 1
1.1 Application ................................................................... 1
1.2 References ................................................................... 1
2 Application ................................................................... 1

3 Definitions and symbols ........................................................ 2

3.1 Definitions ................................................................... 2
3.2 Symbols ..................................................................... 2
4 Designapproach .............................................................. 5
4.1 Design requirements and goals ... ;. ............................................ 5
4.2 Identify design criieria ......................................................... 6
4.3 Preliminary design .................................... . ....................... 8
4.4 Detail design ................................................................ 12

5 Lubrication .................................................................. 15
5.1 Cooling vs. lubrication requirements ............................................ 15
5.2 Lubricants...................................................................l 5
5.3 Distribution systems .......................................................... 18
5.4 Lubrication system design considerations ....................................... 19
5.5 Filtration .................................................................... 21
5.6 Oiipumps ................................................................... 21
5.7 Lube system condition monitoring .............................................. 23
6 Environmental issues ......................................................... 24
6.1 Ambient temperature effects ................................................... 24
6.2 Ambient pressure effects ...................................................... 25
6.3 Attitude effects .............................................................. 25
6.4 Contaminant effects (water, corrosives, dirt, dust, and sand) ....................... 26
6.5 Vibration/Shock effects ....................................................... 26
6.6 Fire resistance requirements .................................................. 29
6.7 Electromagnetic effects ....................................................... 29
6.8 Nuclear, biological, and chemical (NBC) effects ......................... :. ....... 29
7 Vibration and noise ........................................................... 30
7.1 Causes of gear vibration ...................................................... 30
7.2 Consequences of vibration .................................................... 31
7.3 Design ...................................................................... 32
7.4 Analyzing vibration problems .................................................. 35
7.5 VibratiorYNoise reduction techniques ........................................... 37

8 Load Capacity ............................................................... 39

8.1 Introduction ............................................................... ..3 9
8.2 Spur, helical, and bevel gear tooth breakage and surface durability ................. 41
8.3 Spur, helical, and bevel gear scuffing (scoring) -flash temperature index ............ 45
9 Gear materials and heat treatment ............................................. 47
9.1 Class and grade definitions .................................................... 47
9.2 Mechanical properties ........................................................ 47

...
III
AGMA 911-A94

Contents, continued
9.3 Cleanliness ............................................................... ..4 8
9.4 Heat treatment ............................ ................................... 48
9.5 Microstructure ............................................................... 48
9.6 Hardenability ................................................................ 48
9.7 Dimensional stability .......................................................... 48
9.8 Pm-machining stock removal .................................................. 48
9.9 Ferrousgearing .............................................................. 48
9.10 Non-ferrous gearing .......................................................... 49
9.11 Material grades and heat treatment ............................................. 49
9.12 Gear surface hardening ....................................................... 49
9.13 Gear through hardening ....................................................... 53
10 Surface treatment ............................................................ 54
10.1 Introduction ................................................................. 54
10.2 Shot peening ................................................................ 55
10.3 Surfacecoatings.............................................................6 0
10.4 Ion implantation of gears ...................................................... 61
11 Manufacturing considerations .................................................. 63
11.1 Introduction ................................................................. 63
11.2 Spur and helical gears ........................................................ 63
11.3 Bevel gears ................................................................. 64
11.4 Stress relief treatment ........................................................ 67
12 Gear inspection .............................................................. 68
12.1 General ..................................................................... 68
12.2 Spur and helical involute gears ................................................ 68
12.3 Bevelgears ................................................................. 69
13 Rocket & space gearing .... .................................................. 70
13.1 Introduction ................................................................. 70
13.2 Lubrication .................................................................. 71
13.3 Gear materials for space application ............................................ 73

1 Symbols used in equations ..................................................... 2

Aerospace lu&icant viscosities ................................................ 16
Aerospace lubricant densities .................................................. 16
Aerospace lubricant pressure/viscosity coefficients ............................... 16
Aerospace lubricant specific heat values ........................................ 17
Aerospacegreases ........................................................... 17
Aircraft dry lubricants ......................................................... 17
Advantages & disadvantages of a common engine & transmission lubrication system . 19
9 Particle size distribution by weight .............................................. 26
10 Suggested functional test levels for propeller aircraft and turbine engine equipment ... 27
11 Suggested functional test peak levels for equipment installed on helicopters ......... 28
12 Potential influence of design features on noise and vibration ....................... 35
13 Vibration testing ............................................................. 37
14 Sound and vibration reduction techniques ....................................... 39

iv
 AGMA Qll-AQ4

Tables, continued
15 Typical aerospace gear materials .............................................. 49

16 Surface coatings used in aerospacegear units ..................................... 62

17 Candidate solid-film lubricants for space application .............................. 73

18 Candidate fluid lubricants for space application .................................. 74
19 Working fluid lubricants ....................................................... 74

Figures
1 Retative life as function of lambda ............................................... 7
2 The general parallel-axis epicyclic gear train ..................................... 9
3 Goodman diagram for combined load ........................................... 11

Typical aerospace lubrication system schematic ................................. 22

Spur gear pump ............................................................. 23
Vane pump .................................................................. 23
Gerotor Pump ............................................................... 23

8 Typical gearbox attitude limits .................................................. 25

9 Suggested vibration spectra for propeller aircraft and turbine engine equipment ...... 27
10 Suggested vibration spectrum for equipment installed on helicopters ................ 27
11 Terminal-peak sawtooth shock pulse configuration and its tolerance limits ........... 28

12 Sound and vibration paths ..................................................... 30

13 Typical damping ring ......................................................... 38

14 Diierent methods for determining tooth root stress ............................... 40

15 Directions of crack propagation in gear teeth .................................... 40
16 Reliabilii versus number of standard deviations ................................. 46

17 Schematic of material ground from a gear tooth .................................. 53

18 Schematic of material ground from a distorted gear tooth .......................... 53

19 Fatigue strength in ground AISI 4349 (50 HRC) .................................. 54

20 Example of residual stress profile created by shot peening ........................ 55
21 Peening 1045 steel at 48 HRC with 330 shot .................................... 56
22 Peening 1045 steel at 62 HRC with 330 shot .................................... 56
23 Stress profile of carburized gear tooth root, ground and then shot peened ........... 57
24 Increase in fatigue resistance of spiral bevel gear ................................ 57
25 Fatigue tests on rear axle shafts ............................................... 57
26 Fatigue tests on notched shafts ................................................ 57
27 Fatigue life comparison ....................................................... 58
28 Correlation of Almen intensities as indicated by arc heights of peened strips ......... 59

29 Heat treat coupon ............................................................ 68

Annexes
A Spur gear geometry factor including internal meshes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
B Gearbox test and mission requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
C References and bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

V
AGMA 911-A94

[The foreword, footnotes, and annexes are provided for informational purposes only and should not be
construed as a part of AGMA 911-A94, information Sheet - Design Guidelines for Aerospace Gearing,]
This Information Sheet supersedes AGMA Standard 411.02, Design Procedure forAirm&Engine and Power
Take-OlT Spur and Helical Gears. Its purpose is to provide guidance to the practicing aerospace gear
engineer in the design, manufacture, inspection, and assembly of aerospace gearing. In addition, it
addresses the lubrication, environmental, and application conditions which impact the gearbox as a working
system of components.
Material in the Information Sheet is supplemental to current AGMA Standards, but does not constitute a
Standard itself. By definition, Standards reflect established industry practice. In contrast, some of the
practices discussed here have not seen enough usage to be considered standard, but they do provide insight
to design techniques used in stat-f-the-art aerospace equipment. It is expected that the user of this
Information Sheet will have some general experience in gear and machine design, and some knowledge of
current shop and inspection practices.
Suggestions for the improvement of this information sheet will be welcome. They should be sent to the
American Gear Manufacturers Association, 1500 King Street, Suite 201, Alexandria, Virginia, 22314.

vi
 AGMA 911-A94

PERSONNEL of the AGMA Committee for Aerospace Gearing

Chairman: A. Meyer . . . . . . . . . . . . . . . Textron - Lycoming
Vice Chairman: K. Buyukataman . . . . Pratt & Whitney

ACTIVE MEMBERS
J. Abrahamian . . . Pratt & Whitney R. Drago . . . . . . . . Boeing Helicopters
N. Anderson . . . . . GM Technical Center B. Dreher . . . . . . . Kaiser Aerospace
I. Armitage . . . . . . Spar Aerospace R. C. Ferguson . . Taiga Group
E. J. Bodensieck . Bodensieck Engineering W. D. Glasow . . . . Sikorsky
M. Brogiie . . . . . . . Dudley Technical Group T. Heiliger . . . . . . . Sikorsky
R.C. Bryant . . . . . . General Electric M. Howes . . . . . . . IIT Research
Ft. Burdick . . . . . . . Aero Gear J. G. Kish . . . . . . . Sikorsky
J. Daly . . . . . . . . . . Metal Improvement Co. E. A. Lake . . . . . . Wright Patterson A.F.B.
R. Dayton . . . . . . . Wright Patterson A. F. 9. W. Michaels . . . . . Sundstrand
ASSOCIATE MEMBERS
G. Belling . . . . . . . American Pfauter W. Marquadt * . . . Norwood Precision/Textron
J. D. Black . . . . . . General Motors D. Merritt . . . . . . . Lion Precision Gear
E. R. Braun . . . . . Eaton R. Miller . . . . . . . . Pratt &Whitney
C. E. Breneman . Advance Gear J. Mogul* . . . . . . . Metal Improvement Co.
A. T. Brunet . . . . . Allied Signal Aerospace J. O’Donnell . . . . . Naval Air Warfare Center
J. Cadisch . . . . . . Reishauer A. E. Phillips . . . . Emerson Power Transmission
H. S. Cheng . . . . . Academic Member T. L. Porter . . . . . . ITW/Spiroid
L. Cloutier . . . . . . Academic Member A. K. Rakhit* . . . . Solar Turbines
9. Cluff . . . . . . . . . American Pfauter J. R. Reed . . . . . . Klingelnberg Soehne
F. W. Cumbow . . . M&M Precision T. Riley . . . . . . . . . NWL Control System
R. J. Cunningham Boeing E. Ropac . . . . . . . Bachan Aerospace
P. A. Deckowitz . . ITWlllitron S.S. Sachdev . . . Spar Aerospace
J. W. Dern . . . . . . SPECO Corporation B. Schneider . . . . NASA, Johnson Space Center
K. R. Dirks . . . . . . Allied-Signal, Garrett Eng. Div. D. J. Schreiner . . General Motors
R. DiRusso” . . . . . Kaman A. Seireg . . . . . . . Academic Member
D. W. Dudley* . . . Honorary Member S. V. Shebelski . . Lion Precision Gear
R. Durwin . . . . . . . Sikorsky E. E. Shipley . . . . Mechanical Technology
W. C. Emmerling Naval Air Propulsion Center G. Skirtich . . . . . . Lion Precision Gear
R. L. Errichello . . Academic Member L. J. Smith . . . . . . Invincible Gear
J. A. Ferrett . . . . . National Broach N. Sonti . . . . . . . . Academic Member
D. J. Fessett . . . . Lucas Western IncJATD D. A. Sylvester . . Power-Tech
H. K. Frint . . . . . . Sikorsky K. Tower . . . . . . . . Metal Improvement Company
R. Gefron . . . . . . . Superior Gear D. P. Townsend* . NASA, Lewis
N. L. Grace . . . . . Gleason Works F. Uherek . . . . . . . Flender
M. J. Gustafson . Kaman M. Valori . . . . . . . . Naval Air Propulsion Center
D. R. Houser . . . . Academic Member L. Vesey . . . . . . . . iTW/Spiroid
C. lsabelle . . . . . . Sikorsky D. A. Wagner . . . . General MotorsIAGT
D. E. Kosal . . . . . National Broach H. Wagner . . . . . . Advance Gear
C. Layer . . . . . . . . Mmg R. D. Wagner . . . National Broach
A. J. Lemanski Academic Member 9. R. Walter . . . . . Liebherr Machine
A. A. Lewis . . . : : Pratt & Whitney, Canada R. F. Wasilewski . Arrow Gear
M. Lonergan . . . . National Broach S. R. Winters . . . . General Motors
P. Mangione . . . . Naval Air Warfare Center T. J. Witheford . . . Teledyne Vasco
W. Mark . . . . . . . . Academic Member G. I. Wyss . . . . . . Reishauer

* Contributed technical material to the document.

vii
 This page is intentionally blank.

. ..
VIII
 AGMA 911-A94

AGMA 91%A94

AGMA 390.03a, - 1980, Gear Handbook - Gear

Design Guidelines for Classification, Materials and Measuring Methods
Aerospace Gearing for Bevel, Hypoid, Fine Pitch Wormgearing and
Racks Only as Unassembled Gears.
1 Scope
ANSVAGMA 110.04- 1989, Nomenclature of Gear
This Information Sheet covers current gear design Tooth Failure Modes.
practices as they are applied to air vehicles and ANSVAGMA 2000-A88, Gear Classification and
spacecraft. The material included goes beyond the inspection Handbook - Tolerances and Measuring
design of gear meshes per se, and presents, for the Methods for Unassembled Spur and Helical Gears
consideration of the designer, the broad spectrum of (Including Metric Equivalents).
factors which combine to produce a working gear
ANSVAGMA 2001-988, Fundamental Rating
system, whether it be a power transmission or spe-
Factors and Calculation Methods for Involute Spur
cial purpose mechanism. Although avariety of gear
and Helica/ Gear Teeth.
types, such as wormgears, face gears and various
proprietary tooth forms are used in aerospaceappli- ANSVAGMA 2003-A86, Rating the Pitting
cations, this document covers only conventional Resistance and Bending Strength of Generated
spur, helical, and bevel gears. straight Bevel, ZEROLB Bevel, and Spiral Bevel
Gear Teeth.
1 .l Application
ANSVAGMA 20044389, Gear Materials and Heat
The working environment of the aerospace gear Treatment Manual
has become so diverse that a single set of guide-
ANSVAGMA 6023-A88, Design Manual for
lines will no longer suffice. The operating conditions
Enclosed Epicyclic Gear Drives.
imposed on a high speed, high powered, transmis-
sion or actuator are quite different than those expe- ANSVAGMA 6123-A88, Design Manual for
rienced by the spacecraft mechanism which must Enclosed Epicyclic Metric Module Gear Drives.
function in a hard vacuum for long periods of time 2 Application
without maintenance. This Information Sheet ad- A listing of aerospace geared applications by type of
dresses these differences and provides guidance to service or function performed is useful in segregat-
the designer for these demanding applications. ing the diverse gearing tasks into mechanism fami-
1.2 References lies which experience similar load and environ-
mental spectra.
The following standards contain provisions which,
Applications can be identified by general grouping
through reference in thii text, constitute provisions
as follows:
of this American Gear Manufacturers Information
Sheet. At the time of publication, the editions - Main propulsion systems;
indicated were valid. All standards are subject to - Propeller gearboxes reduce engine
revision, and parties to agreements based on this speed to propeller speed;
American Gear Manufacturers lnfomration Sheet - Fan gearboxes allow the use of optimum
are encouraged to investigate the possibility of turbine and fan speeds for maximum effi-
applying the most recent editions of the standards ciency;
indicated below. - Helicopter transmissions. A system of
gearboxes and shafting to drive the helicopter
AGMA 230.01 - 1974, Surface Temper Inspection
rotors from the engine(s);
Process.
- Mechanical interconnection between
AGMA 246.02A - 1983, Practice for Carburked engines allow for independent engine opera-
Aerospace Gearing. tion on multiingine systems;

1
AGMA 911-A94

- Accessory drive gearboxes driie accessory The terms used, wherever applicable, conform to
devices, such as generators, fuel pumps, hy- the following standards:
draulic pumps, oil pressure and scavenge ANSI Y10.3-1968, Letter Symbok for Quantities
pumps, blowers, alternators, etc; Used in Mechanics of Solids
- Auxiliary/secondary power units (APU/ AGMA 1012-FQO, Gear Nomenclature, Definitions
of Terms with Symbols
SPU) consist of an auxiliary turbine engine inte-
grated with a gearbox to provide powerfor main AGMA 904-689, Metric Usage
engine starting, electrical services, emergency 3.2 Symbols.
hydraulic power, cabin air conditioning, etc.; The symbols used in this information sheet are
shown in table 1.
- Actuators. A general class of geared devices
used to cause a position change of an object. The NOTE - The symbols and definitions used in this
information sheet may differ from other AGMA
objects may include aerodynamic control sur-
publications. The user should not assume that fa-
faces, winch cables, doors, landing gear, or miliarsymbols can be used without a careful study
telerobotic arms. Actuators are distinguished of these definitions.
from most aerospace gearing in that they only SI (metric) units of measure are shown in parenthe-
move on command; ses in table 1 and in the text. Where equations re-
quire a different format or constant for use with SI
- Space systems. A specialized grouping of
units, a second expression is shown after the first,
power (as in rocket turbo-pump drives), and ac- indented, in smaller type, and with “M” included in
tuatortype devices which have been designed to the equation number.
be compatible with the unique rigors of outer-
Example
space environments. These include the high
power, short life rocket applications as well as the wt & pd Ks % KB
S . ..(n)
long life satelliie or space platform systems. *=K,K, J

3 Definitions and symbols w,Ka 1 4 KmKB

St=-- . ..(llM)
K~K~ mF J
3.1 Definitions. The second expression uses SI units.
Table 1 - Symbols used in equations

Symbol Name Units First Reference

equation paragraph

C Center distance in (mm) 9 8.2.2

G Application factor for pitting resistance ---- 12 8.2.2

Cf Surface condiiion factor for pitting resistance --a-

12 8.2.2

CH Hardness ratio factor for pitting resistance -w--

18 8.2.8

CL Life factor for pitting resistance 18 8.2.8

Gl Load distribution factor for pitting resistance 12 8.2.2

cp Elastic coefficient 12 8.2.2

CR Reliability factor for pitting resistance 18 8.2.8

G Size factor for pitting resistance 12 8.2.2

2
 AGMA 91%A94

Table 1 (coMwo”

Symbol Name Units First Reference

equation paragraph
CT Temperature factor for pitting resistance ---- 18 8.2.8
G Dynamic factor for pitting resistance ---- 12 8.2.2
ch Lubricant specific heat BTlJ/lbm “F 8 5.1.2
&J&l K)
dp Pinion operating pitch diameter in (mm) 10 8.1.2
E, Reduced modulus of elasticity IWir?(N/mm2) 4 4.2.4
F Face width in (mm) 9 8.1.2
Fe Effective or net face width in (mm) 12 8.2.2
H Oil film thickness in (mm) 1 4.2.4
HG Heat generated at design point BTU/min 8 5.1.2
(kJ/min)
&ill Film thickness, minimum ---- 4 4.2.4
I Geometry factor for pitting resistance ---- 12 8.2.2
J Geometry factor for bending strength ---- 11 8.2.1
K Contact load factor for pitting resistance lb/in* (MPa) 9 8.1.2
Ka External application factor for bending strength . - - - - 11 8.2.1
KB Rim thickness factor ---- 11 8.2.1
KL Liie factor for bending strength ---- 13 8.2.7
Km Load distribution factor for bending strength - - - - 11 8.2.1
KR Reliability factor for bending strength ---- 13 8.2.7
% Size factor for bending strength ---- 11 8.2.1
KT Temperature factor for bending strength ---- 13 8.2.7
KY Dynamic factor for bending strength ---- 11 8.2.1
KX Lengthwise curvature factor for bevel gear - - - - 11 8.2.1
bending strength
M Lubricant flow rate Ibmlmin(kg/min) 8 5.1.2
m Module ( = 25.4/pd ) (mm) 11 M 8.2.1
m Gear ratio (never less than 1 .O) ---- 9 8.1.2
n Number of standard deviations ---- 14 8.2.7
np Pinion speed rpm 9 8.1.2
P Transmitted power hp NW) 9 8.1.2
AGMA 91 l-A94

Table 1 (concluded)
Symbol Name Unite First Reference
iquatior paragraph
pd Diametral pitch ( = 25.4/m ) in-l 11 8.2.1
% Reliabilii constant ---- I4 8.2.7
sac Allowable contact stress number lb/in* (MPa) 18 8.2.8
sat Allowable bending stress number lb/in* (MPa) 13 8.2.7
SC Contact stress number lb/in* (MPa) I2 8.2.2
St Bending stress number lb/in* (MPa) II 8.2.1
%vc Working contact stress number lb/in* (MPa) I8 8.2.8
swt Working bending stress number lb/in* (MPa) I3 8.2.7
Till Inlet oil temperature OF(“C) 8 5.1.2
Tout Outlet oil temperature OF(“C) 8 5.1.2
82 Contact temperature “F (“C) I9 8.3.1
tfl Flash temperature OF(“C) I9 8.3.1
tM Bulk temperature OF(“C) I9 8.3.1
u Speed parameter ---- 6 4.2.4
u’ Average rolling speed inlsec (mm&c) 1 4.2.4
Ve Entraining velocity in/s (m/s) 6 4.2.4
W Load parameter ---- 7 4.2.4
W’ Unit tangential load lb/in (N/mm) I 4.2.4
WVr Normal unit load lb/in (N/mm) 7 4.2.4
wt Tangential tooth load lb 0’4 I1 8.2.1
Xl- Load sharing factor ---- 7 4.2.4
a Pressure viscosity coefficient in*/lb (l/MPa) 5 4.2.4
h Specific film thickness inlin (mm/mm) 2 4.2.4
CL Viscosity reyns (kPa s) 1 4.2.4
PO Absolute viscosity reyns (kPa s) 6 4.2.4
V Coefficient of variation or standard deviation ---- I4 8.2.7
Pn Normal relative radius of curvature in (mm) I 4.2.4
(Ja Composite surface roughness tin @ml 2 4.2.4
Ul,(x Surface roughness of pinion, gear crin tw) 3 42.4

4
 AGMA 911-A94

4 Design approach 4.1J Maintainability

Guidelines for field service work, space require-
4.1 Design requirements and goals
ments, and tool limitations must be specified early in
The design procedure begins with a definition of the the project.
application, requirements, and goalsforthe project. 4.1.6 Cost
It is sometimes diiicuft to clearly define all aspects
of the project at the start, but a complete tabulation Aerospace gearing is generally more costly than
of the following parameters should be made to commercial gearing because of the necessary
provide a working definition of the project. performance, qualii and traceability requirements.
Life cycle cost is often established at the start of the
4.1 .I Power/speed and torque/position project as a goal or as a requirement. Life cycle cost
The complete range of power and speed or torque is defined as the total cost of ownership of a system
and position (actuators) must be tabulated including over its operating fife.
a definition of growth capabili. A duty cycle 4.1.9 Efficiency
definition is required for calculation of life. Within In most aerospace applications, gearbox efficiency
these parameters a design point must be selected is an important design consideration because it
for sizing purposes. influences system weight and power requirements.
4.1.2 Gear ratlo and direction of rotation Efficiency requirements and goals will provide the
designer a clear indication of the project objectives
Gear ratio must be specified with an indication of and may affect key decisions in the design process.
allowable deviation. Input and output directions of
rotation are required and are important in selection 4.1.I0 Altitude/attitude requirements
of the hand of helix or hand of spiral for thrust Altitude and attitude specifications are required for
direction and lubrication considerations. lubrication system design, since oil pump and oil
passage design are dependent on these parame-
4.1.3 Life
ters. In lieu of any specific application data
A clear definition of required gear and bearing MIL-E-3!593C provides general requirements for
system life must be provided. Life is defined at a aerospace applications.
specified survival level. 4.1.I1 Externally generated gearbox loads
4.1.4 Weight External loads can be generated by rotor loads,
System weight is criiical in aerospace applications. flight maneuvers, gravity and gyroscopic effects,
A value for gear system weight should be specified hard or crash landing requirements, or vibration, as
as dry gearbox weight or gearbox plus lubrication applicable. All must be considered in the design of
system weight. the gearbox housing, mounts and their effects on
misalignment of bearings and gears within the
4.1.5 Size limita%ons gearbox. Typical loads are given in MIL-E-3593C.
In most applications, gearbox location and maxi- 4.1.I2 Mount locations
mum envelope will be defined. These details must Mount locations must be specified to allow design
be made available to the designer. and analysis of the housing and internal structure
4.1.6 Reliability under external loading conditions. Mount location
requirements may also affect maintainability con-
Reliability requirements are typically specified in siderations.
terms of mean time between failure (MTBF). A
historical data base of typical component reliability 4.1 .I3 Loss of lubricant
will permit calculation of system reliability. New All military and some commercial aircraft have
products are more difficult to characterize. Tech- requirements for operation with loss of lubricant ,
nique& quantify reliability levels must be specified typically specifying a time and power level of
for a new gearbox system. operation after loss of lubricant. These require-

5
 AGMA 911-A94

ments must be known to allow the design of a as aircraft flap drive systems, winches, and space-
suitable lubrication system. craft robotic manipulator arms. These loads are the
4.1 .I4 Test requirements highest loads specified for the gears, and are often
two to three times higher than the maximum
Test requirements are sometimes different than continuous operating loads. This is particularly true
those used to design the gearbox. If an unusual test for low speed actuator gearing where there are no
is required it can affect the gearbox design. significant “dynamic” loads. To properly accommo-
4.1.15 Noise requirements date these conditions, the designer must evaluate
the gear design for maximum compressivestresses
The recent trend in air vehicle specification has
been to require meeting specified internal noise at the maximum holding loads.
levels in cabin and cockpit. Holding loads are usually specified as limit loads,
where there may be no permanent deformation or
4.2 identify design criteria
yielding allowed, and ultimate loads, where de-
It is sometimes difficult to clearly define design formation is allowed but the gears may not fracture.
objectives or goals of a gearbox or gearset. Proper
A value of 3.1 times the shear yield strength may be
identification of design criteria requires application
used as the allowable contact stress for most steels.
of many disciplines such as elastohydrodynamics,
involutometry, geometry, stress analysis, system High strength, through hardened stainless gears,
dynamics, materials, kinematics, vibration, heat may also be utilized where environmental condi-
transfer, processes, manufacturing, economics, tions warrant. The surface durability of these gears
etc. Each of the above disciplines requires that may be improved, if required, by nitriding.
design limits be imposed such as: 4.2.2 Allowable bending stress
- Stress limits; The allowable tooth root bending stress is a function
- Scuffing (scoring); of the hardness and residual stress near the surface
- Minimum oil film thickness; of the root fillet and at the core.
- Type of mounts, deflections and locations; 4.2.2.1 Power transmission
- Weight and Cost: Power transmission gears are usually case hard-
- Vibration; ened by either nitriding or carburizing to obtain
- Noise. adequate high cycle bending and contact fatigue
The design criteria which have the largest influence life.
on the final configuration are as follows. A method for calculation of bending stress, along
4.2.1 Allowable contact stress with allowable limits, is given in ANSVAGMA
2001-B88.
The tooth contact (Hertz) stress limit depends on
the type of application, required service life, proper- 4.2.2.2 Actuator gearing
ties of materials used, and the shape of the tooth Gears which are manufactured from high strength
surfaces near the point of contact before the load through hardening steels (260 ksi and above), and
transfer begins. heat treated to through hardness in the Rockwell C
4.2.1 .I Power transmission 50+ range, have shown higher bending fatigue
strength in the lower fatigue cycle range (i.e. less
In high pitch linevelocity gearsets, thedistribution of than lo6 tooth bending cycles) than conventional
dynamic load is required for accurate determination case hardened gears. Thus, a designer seeking
of tooth contact stress. A method for calculation of optimum minimum weight gearing should consider
contact stresses, along with allowable limits, is the actual cycle life imposed prior to making a
given in ANSI/AGMA 2001-888. selection of either case hardened or high strength
4.2.1.2 Actuator gearing through hardened gears for a particular application.
Actuator gears are subject to “holding”loads which Allowable bending fatigue limits are given in ANSI/
are static loads. These loads occur in systems such AGMA 2001-B88.

6
 AGMA 911-A94

4.2.3 Surface temperature d is unit tangential load, lb/in (N/mm).

The mechanism of surface failure due to a sudden The currently used lubricant film thickness analysis
temperature rise is one of the major considerations is the extension of a bearing film thickness study by
in aircraft gearing. Osborne Reynolds. Ertel, Gruben, Hamrock, Dow-
son, and Higginson contributed to the equation in its
Each oil has a characteristic criiical temperature in- current form.
dependent of gear design and operating conditions.
The most influential parameter in the calculation of
Appendix A of ANSVAGMA 2001-888 defines film thickness is the speed parameter U, which
scuffing as related to the instantaneous tempera- represents the average rolling speed and the
ture rise on tooth surfaces caused by frictional heat. surface condition of the point at which the EHD film
The equations which define the surface tempera- thickness is calculated.
ture rise have begun to adapt dynamic conditions Surface geometry and finish are important to the
and have become more representative of what EHD lubrication process. EHD theory is based on
happens at the gear mesh, including: constrained the assumption of perfectly smooth surfaces, that is,
heat source on the tooth profile, sliding velocity no interaction of surface asperities. In reality, this is
variations, tooth surface conditions, load sharing, oil not true for boundary lubrication. Therefore, the
jet cooling, oil jet impingement depth and air/oil mist relative life chart was introduced.
cooling.
h=+- . ..(2)
Experiments have verified that minimum values of u
surface temperature occur at operating pitch
diameters. A method of calculating surface
temperature is presented in Appendix A of (31 and 02 are the roughnesses of the two surfaces
ANSVAGMA 2001-B88. Maximum values in contact and his the ratio of EHD film thickness to
generally occur at or near the highest point of single composite surface roughness. A plot of h vs.
tooth contact. Although the above procedure is relative life is shown in figure 1. This figure assumes
currently in use, the method is only applicable under sufficient loading and otherwise satisfactory opera-
boundary lubrication conditions. Allowable scuffing tion of the gears.
temperature values should be based on the NOTE-& issupplantingrms as a way of describ-
lubricant temperature at which lubricant breakdown ing roughness. Both terms are still in use but are
occurs, the material tempering temperature, or the not equivalent.
user’s experience whenever possible.
4.2.4 Lubricant film thickness
Lubricant film thickness has received ever-increas-
ing attention since the time it was introduced by 2.2 Aerospace gears
Martin in London Engineering in 1914. //
-i
g
a, , A Bkaririgs
5
H= .
4896 ’ “h
/
H = ,,,,g:” ” pn
W’
where 4 -6 1 2 4
H is oil film thickness, in (mm); ecific film thickness, h
p is viscosity, reyns (kPa s); h < 0.4 Danger of scuffing for carburized gears
U’ is average rolling speed, inls (mm/s); li 5 0.4 Acceptable, assuming boundary layer
lubrication
h is normal relative radius of curvature, in
(mm): Figure 1 - Relative life as a functio~~~&n@&
AGMA 911-A94

Further studies by NASA simplified the general accessory gears. If equation 4 is used for power
equation to the form presented in Appendix A of gearing without the previously noted enhance-
ANSVAGMA 2001-B88 for dimensionless mini- ments, the definition of when boundary lubrication
mum film thickness: occurs may be as low as R = 0.2 to E.= 0.4.
GO54 uo.70 42.5 Structural integrity
H mh = 2.65 .*p-(4)
WO.13 Structural integrity is achieved by the proper
where the following are dimensionless parameters: definition of gear, bearing and gearbox mounts;
materials parameter, G; gear configuration and materials: selection of
G= a,?$ . ..(5) bearings; type of bearings and bearing location;
seals and type of sealing surfaces.
speed parameter, V;
4.3 Preliminary design
u= - PO Ve

2w+a The areas of concern during the preliminary phase

load parameter, W, of aerospace gearbox design consist primarily of
performance, cost, configuration and packaging.
e(7)
45.1 Configuration study
where In the preliminary design stage, it is generally
a is pressure-viscosity coefficient, in2/lb necessary to lay out various gearbox configurations
(mm*/N); which meet the basic speed, power, and ratio
requirements. These configurations can be com-
p. is absolute viscosity, reyns (kPa s); pared against design requirements and rated
Ve is entraining velocity, in/s (m/s); against each other in terms of reliability, efficiency,
E, is reduced modulus of elasticity, lb/in2 maintainability, cost, size, weight, and similarity to
(N/mm2); past experience. From this process the most
suitable configuration for the particular application
p,, normal relative radiusof curvature, in (mm); is selected.
Xr load sharing factor; 4.3.1.1 Gearing
wr normal unit load, lb/in (N/mm). A large number of gearbox configurations are
The following enhancements may be added to the possible to achieve the desired design goal, some
calculation as follows: of which are described below. The gearbox
- Transient squeeze film effects from change in envelope is generally set by the space available
entrainment velocity, surface geometry and dy- plus the speed, power and ratio requirements.
namic load; However, the configuration may be further compli-
-Actual dynamic&ad profile in place of average cated by pitch change mechanisms, accessories,
tangential load; overrunning clutches, engine air intake, etc.
Possible configurations include:
- Equilibrium surface temperature and oil inlet
temperature which defines the temperature of - Offset. This refers to a gearbox in which the
the oil film; input and output shafts have a parallel offset;
- Use of optimal, experimental heat transfer co- - Inline. This refers to a gearbox axis in which
efficients when oil jet cooling is used for minimi- the input shaft and output shafts are concentric;
zation of surface temperature; -Angular. This refers to a gearbox in which the
- Effects of oil entrapment on long face width input and output shaft are at an angle to each
gears may be included and equations may be other.
separated from short face width gears. 4.3.1.2 Epicyclic
The relative film thickness, as calculated using In the same sense that some gearformsare specific
equation 4 for ZYZ~ has been derived and used cases of a more general configuration (example: A
successfully using narrow face width gears such as spur gear is the special case of a helical gear with a

8
 AGMA 911-A94

zero helix angle), a gear system can be general or tiple, equally spaced planets to assure a bal-
specific. In the context discussed here, we will anced system, and most importantly, provide
consider the parallel axis epicyclic rather than the multiple load paths for reduced weight.
more general bevel epicyclic. Refer to ANSVAGMA
Standard 6028-A88 or 6128-A88 Metric.
Kinematically, the general case for the parallel axis
epicyclic is an arrangement of six gears in two
planes as shown in figure 2. By definition a sun gear
is a gear element whose axis is coincident with the
system axis. Thus, the system shown contains four
sun gears; i.e., two external suns and two internal
suns. Internal sun gears are sometimes called ring
gears. The sun gears of each plane are meshed 11
with an idler. If the two idlers are assumed to be
mounted on a common shaft which is, in turn,
supported by bearings to a rotatable structure we
have the general parallel axis epicyclic system.
By controlling the location of the instant center of
rotation in the above system of gears, the designer
can produce 88 epicyclic variations, each with its
T
Figure 2 - The general parallel-axis epicyclic
own unique properties.
gear train
Some of the more important variations have been
given names and appear in countless transmission If the input to the simple planetary is to the external
systems. For example: sun gear, the resulting gear box will be a speed
reducer, and conversely if the input is to the carrier,
- The simple epicyclic: If each of the
the resulting gearbox will be a speed increaser. In
corresponding gears in the general system are
assigned identical tooth counts, then the gearing application the practical usable reduction ratio will
in one of the planes becomes redundant and may lie between 2.5 and 7 and the input and output
be eliminated, leaving a single external sun, a shafts will have the same direction of rotation.
single internal sun, each meshed with a common - The star gearbox: If we constrain the carrier
idler which is finally supported by the rotatable against rotation, the system instant center of
structure usually called a “carrier’ rotation is coincident with the axis of the idler and
In the general simple epicyclic, everything in theory the rotating components become the central
external sun, the idler, and the internal sun.
can rotate. However, by controlling the location of
Since the idler no longer orbiis about the system
the instant center of rotation, we can produce some
axis it is usually called a “star”. Again, for reasons
very interesting and important gear systems. These
of equilibrium and load division it is common
include: practice to fit the stationary carrier with multiple,
- The simple planetary: If we constrain the in- equally spaced stars.
ternal sun against rotation its pitch circle has zero If the input to the star gearbox is to the central
angular velocity and the remaining three compo- external sun, the resulting unit will be a speed
nents, the external sun, the idler, and the carrier reducer, and conversely if the input is to the internal
are free to rotate. As the idler rolls in mesh with
sun, the resulting unit will be a speed increaser. In
the fixed internal sun it orbits about the system
application the practical usable reduction ratio lies
axis as it rotates about its own axis, thus the idler
between 1.5 and 6 and the input and output shafts
in a simple planetary has come to be called a
“planer. The use of a single planet would place will have opposite directions of rotation.
serious balance constraints on the gear system, The star gear system has found extensive use in the
so it is common practice to fit the carrier with mul- first reduction of high speed systems because it is

9
AGMA 911-A94

free from high centrifugal bearing loading caused by cautioned however, that some compound planetary
orbiting planets. variations exhibit very poor transmission efficiency
- The solar gearbox: If we constrain the due to high effective pitch line velocity in the high
external sun against rotation the system instant torque meshes. A thorough analysis of each
center of rotation is coincident with the pitch application is recommended before committing the
circle of the external sun, and the rotating design to detailing.
components become the internal sun gear, the 4.3.1.3 The parallel axis differential
planet, and the carrier. Since, in this system, all This special case of the parallel axis epicyclic will be
components orbit about the central fiied mem-
mentioned here because of its extensive use in
ber the name “solar” is quite descriptive.
spacecraft and other systems that require a redun-
Of the simple epicyclics described so far, the solar dant drive source. In such a system, use is made of
system is the least popular since for a given two suns, and two planet pairs. Each planet pair is in
reduction ratio it has higher mesh velocities and a mesh, and the first planet of each pair is in mesh
lower transmission efficiency. Usable ratios lie in a with one of the sun gears while the second planet of
narrow band between 1.14 and 1.5 with driving and each pair is in mesh with the other sun gear. The
driven shafts rotating in the same direction. carrier is free to rotate and is usually assigned to be
the output member. A motor/brake combination is
- The compound epicyclic: Referring once
again to figure 2, if the tooth counts of the gear fitted to each of the input suns. In service, either of
elements on each end of the idler shaft are not the motors can be the system input, and the
the same, then all elements in the system can be opposite brake can serve as the system reaction
relevant to the creation of useful gear arrange- member. The reduction ratio of the differential is 2.
ments. A few of the possible arrangements are 4.3.1.4 Accessory drive system
noteworthy and will be discussed further: The accessory drive system is a drive train dedi-
- The compound planetary. If either of the cated to drive accessories (i.e., lube and scavenge
internal suns is constrained against rotation its pumps, alternators, generators, etc.) which are
pitch circle has zero angular velocity and the requirements of the application. The size and
remaining four components are free to rotate; location of the gearbox are dependent on the
i.e., the two external suns, the compound planet, accessory requirement, positioning of these acces-
and the other internal ring gear. In theory, the sories and the position of the gearbox input drive.
designer could produce a transmission with When positioning the accessory gearbox, consid-
three output shafts, but it would be a rare system
eration needs to be given to the overall configura-
where such a configuration would be useful.
tion to ensure that a compact package is obtained.
There are numerous examples of flight systems
with counter rotating propellers which use the Definition of an accessory drive system depends on
concept of a compound planetary withtwooutput the spaces and the location available to driie the
shafts. As with the simple epicyclics, it is usual accessories. One concern is the selection of gear
practice to configure the gearbox with multiple and bearing diameters to fill the distance between
equally spaced planets to assure a balanced the power input and available accessory mount
drive, and multiple load paths. locations. Another concern is to ensure that system
In space robotic systems, extensive use is made of life is compatible with the general requirements.
Both concerns are equally essential for a successful
the compound planetary using a single driving
external sun, one fixed internal sun and one output drive train.
internal sun. In this latter case, the carrier and the Refer to ANSVAGMA 6123-A88 for specific
second external sun of the general arrangement are arrangements.
not utilized, and are therefore discarded. 45.2 Gear sizes
Usable ratios available from the compound plane There usually are two modes of operation which
tary cover a very wide range and can be found as size gears as follows:
low as 5 to values well over 1000. The user is - Start up conditions;

10
 AGMA 911-A94

- Spectrum of speed and torque or torque and apply to a given material and number of cycles of
position operating conditions. operation, as shown in the note.
Gear tooth geometry definition plays an important Erduy limit Case carburized
role in optimization of design. In general, proper AMS6265 Material
selection of tooth proportion, pressure angle, and
spiral angle or helix angle are important to increase
the overall contact ratio and to provide better
balance between operational stresses. Most recent
experience shows increased use of high transverse
contact ratio spur and helical gears.
4.3.3 Loads
4.3.3.1 Dynamic loads Steady stress
High power density gearing depends on designing Figure 3 - Goodman diagram for combined
gears to maximum load capacity. For high speed loads
gears, a major requirement becomes the ability to
accurately calculate dynamic loads so the essential 4.3.4 Rotating components
performance and design characteristics can be Preliminary design is not complete unless all
accurately predicted. rotating components such as splines, shafts, bear-
Dynamic load consists of three parts. The first part ings and seals are defined along with the gears.
is defined by the component or system resonance. 4.3.4.1 Splines
The second part is defined by the gear tooth mesh Splines are used to transmit torque between two
transmission error, and the third part consists of parts with a common axis. In a gearbox, splines
inputtorquefluctuation. Resonancecondiiions may transmit torque between a shaft and a gear or a
be controlled by changing gear and web configura- shaft and another shaft. In practice there are
tion, by damping, or by changing gear ratio. The straight sided, serrated, and involute splines but in
gear tooth mesh generated conditions can be aerospace transmissions, involute splines are nor-
controlled by changing the tooth form, contact ratio, mally used. Involute splines transmit torque through
and damping. Additional mass elastic analysis can contact between external and internal spline mem-
also be performed to ascertain torsional dynamics. bers independent of the fii clearance. This high
4.3.3.2 Centrifugal loads degree of contact reduces the wear and the length
of spline required. The mating internal and external
Centrifugal Loads in gearboxes result from compo- involutes provide a centering effect and the distribu-
nent rotation. These loads impose steady stresses, tion of force from top to bottom is also very good.
for a given speed, on components such as gears, Requirements for involute splines are usually speci-
bearings, and gearbox mounts. The stresses are fied in one of the following standards:
accounted for under combined loading.
-ANSI B92.1, Involute Splines and Inspection;
4.3.3.3 Combined loads (Goodman diagram) -ANSI B92.2M, Metric Module /nvo/ute Splines
For high speed gears the steady stress (centrifugal) and Inspection;
should be combined with the alternating stress at - IS0 4156, Straight Cyl~ndricallnvolute Splines.
the gear tooth root, as shown on a Goodman Involute splines can be of the side bearing, major
diagram in figure 3. Combined operating stresses diameter, or minor diameter fii type. In the side
such as point A fall within the area bounded by the bearing fit types of spline, which is the most widely
endurance limit and yield strength and are accept- used type, the mating members contact only on the
able. The same gear operating at a higher speed, driving sides of the teeth with clearance between
point B, might fail since the combined stress the major and minor diameters. In the major
exceeds the allowable limit. Goodman diagrams diameter fit type of spline, the mating members

11
AGMA 911-A94

contact and are piloted by the major diameters with accessory drives are sometimes designed with
clearance on the minor diameters. Minor diameter non-metallic muff inserts between spline members.
fit splines are only used in situations where the These serve as an inexpensive compliant part
diameter is too small for the cutter of the internal which mitigates metallic spline wear.
member. 4.3.4.2 Bearings
The splines can be designed to act as fiied, Bearings used in aerospace applications generally
non-working types or flexible, working types. In the are one of the following types:
fixed spline, the members are piloted on one or both
- Deep groove ball bearings;
ends, so that the pilots, rather than the spline teeth,
carry any radial load. The fiied type of splined joint - Cylindrical roller bearings;
is often clamped in the axial direction. The objective - Needle bearings;
in the fiied spline design is to force the spline to - Angular contact ball bearings;
carry only torque while other elements carry radial - Angular contact ball bearings with split
and axial load. Fixed splines must have clearance inner race;
because of non-concentricity between the spline - Tapered roller bearings;
pitch diameter and the mounting diameters. -Journal bearings;
Without clearance, the internal and external mem- - Thrust bearings:
bers could bind, leading to increased operating
- Duplex bearings.
stresses.
As the bearing size increases, it is generally more
A flexible spline is not held radially by a diametral fit. difficult to obtain calculated life due to changes in
This permits both radial and angular misalignments preload caused by mounting, thermal and centrifu-
of the mating members. There is generally no axial gal load variations and deflections.
clamping in a flexible spline since this would tend to
restrain angular or radial motion. The spline should 4.3.4.3. Seals
have enough clearance to allow it to move in a The gearbox design is required to minimize the
misaligned condition without binding. Splines which number of static oil or grease seals to prevent
must accommodate excessive misalignment lubricant loss. Experience has shown that the use of
should be crowned along the flank to prevent end flat gaskets as static seals has been so poor that
loading and keep the load toward the center of the they should be used only if absolutely necessary.
tooth. Outside diameter crowning is also used to O-ring seals are generally used.
ensure adequate root clearance under misaligned The dynamic seals can either be spring or magneti-
conditions. cally loaded face seals, bore rubbing seals, laby-
A spline subject to angular misalignment carries an rinth seals, or lip seals. Efforts should be made to
induced bending moment across mating members positively drain, and to provide pressure balance
because friction at the spline teeth does not permit and damping for any dynamic seal system.
free angular motion. The magnitude of the induced Consideration should be given to the surface finish
moment is a function of torque, coefficient of friction, and lay of shafts and journals which have contact
angular misalignment, and component bending with seals. Either too fine or too coarse a surface
stiffnesses. finish could be detrimental.
Lubrication is beneficial to fixed splines and is 4.3.5 Lube system requirements
recommended for flexible splines, especially at high Details of the lube systems are discussed in clause
speeds where the teeth tend to have more sliding 5. Consideration should be given to cool, lubricate
and wear. Filtered oil supplied to the spline joint and scavenge all rotating power transmission
provides cooling and also washes away abrasive components.
particles. Grease packed splines are also used.
However, they tend to trap the abrasive particles, 4.4 Detail design
which can accelerate wear and thus will require Detail design of a geared system requires accurate
periodic maintenance. Flexible splines used as evaluation of dynamic gear tooth loads caused by

12
 AGMA 9ll-A94

load transfer from one mesh to another and 4.4.1 Finite element modeling considerations
momentary overloads caused by system reso-
Single flank element models can be used to
nance. In detail design, structural gear analysis
determine tooth stress. To develop a finite element
requires an assessment of tooth load capacity, to
methodology and a design tool to analyze the load
select or calculate derating factors. The design
sharing behavior from simple spur gear systems to
process may be based on conventional AGMA or
more complex helical and spiral bevel gears on
FE analysis.
combined systems, an attempt should be made to
Manufacturing tolerances, tooth errors, profile address the factors influencing load sharing dis-
modifications and system misalignment will signifi- cussed earlier.
cantly influence gear tooth load along the contact
path, thus affecting load sharing. 4.42 Tooth bending and contact stress con-
siderations
Accurate evaluation of gear tooth load sharing
behavior under dynamic conditions is not only Once the load distribution along the contact path is
important in minimizing the weight of the entire obtained, the calculated load can be transferred to
system but also is valuable to enhance over- all gear tooth pair mesh locations to obtain stresses at
system reliability. the root or along the contact surfaces. The calcula-
tions and limits are discussed in clause 8.
Detail design of aircraft gears can also involve
modifications of analysis methods, using nonlinear Gear stresses are a valuable design tool in deter-
multibody dynamic analysis including equilibrium mining thesize of the gears, and thus minimizing the
analysis, kinematic analysis, vibratory analysis with gear system weight. It is particularly important in
open loop systems, closed loop systems and elastic sizing (where possible) to base the selection of
(flexible) and/or rigid body systems. derating factors of a new design on old designs
All of the above can be used to perform an which are similar and have been successful in the
assessment of the load distribution along the past.
contact line. ANSVAGMA 20014388 defines load The tendency of gear teeth to pit has traditionally
distribution for gears of general use. been thought of as a surface fatigue problem in
In addition to materials and design configurations, which the prime variables are the compressive
the following items greatly influence the rate of load stress at the surface, the number of repetitions of
transfer, or a system’s response to input torque: the load, and the endurance strength of the gear
- Geometry of Pinion and Gear Teeth; material. In steel gears the surface endurance
strength is quite closely related to hardness, so
- Thermal Distortions;
stress, cycles, and hardness become the key items.
- Gear Rim Centrifugal Forces; Gear work in the 1970’s led to two very important
- Profile Modifications and Crowning; conclusions.
- Manufacturing and Alignment Errors; - Pitting isvery much affected by lubrication con-
- Instantaneous Angular Position of Gears; ditions;
- Rotational Delay of Driven to Driving Gear - There is no pitting endurance limit. (S-N
(Angular Acceleration); diagram does not become asymptotic.) The
- Total Tooth Deflections (Rim, Web, etc.); allowable stress used for design purposes con-
- Shaft Deflections (Bearing, Housing, etc.). siders such items as the number of cycles and
the types of material and oil used.
Load distribution is influenced by the above factors
and is non-uniform along the contact lines of Work on the theory of EHD showed that gears and
meshing gears. To determine tooth load distribu- rolling-element bearings often developed a very
tion, tooth and rim deflections are required. These thin oil film that tended to separate the two
deflections vary with the load position and affect the contacting surfaces so that there was little or no
dynamics and tooth root stress as the tooth rotates metal-to-metal contact. When this favorable
through the entire mesh. situation was obtained, the gear or the bearing

13
AGMA Qll-A94

could either carry more load without pitting or run for finish, and designed to controlled surface finish and
a longer time without pitting at a given load. waviness.
Gears in service frequently run for several thousand Aircraft bearings are typically AFBMA grade 5 to 7
hours before pitting starts, or becomes serious. A or better, selectively designed to meet performance
gear can often run for up to a billion (1Og)cycles with requirements.
little or no pitting, but after 2 or 3 billion (2 or 3 x 1Og) High speed aircraft seals are in general carbon face
cycles, pitting, and the wear resulting from pitting, and rotating. Their designs are selected to be flat
can make the gears unfit for further service. within two Helium light bands, where each band
4.4.3 Regimes of lubrication step measures 11.6 pin (294 pmm). In lower speed
To handle the problem of EHD lubrication effects, applications, lip seals are often used.
three regimes of lubrication should be considered 4.4.5 Lube system considerations
(see figure 2.12 in [19]*). These are: Details of lube systems are discussed in clause 5.
- Regime I: No appreciable EHD oil film Aircraft or aerospace gearbox components rely on
(boundary); direct and pressurized lubrication for the formation
- Regime II: Partial EHD oil film (mixed); of EHD films and cooling.
- Regime Ill: Full EHD oil film (full film). Lube system design includes internal coring or
Regime I is encountered in aircraft gears when external piping, jets, spray bars, and into mesh or
speeds are jaw, such as in the final stages of out of mesh lubrication. Lube pumps,deaeration,
gearing in a helicopter gearbox. and filtering requirements are also considered an
Regime II is characterized by partial metal to metal integral part of the lube and cooling systems.
contact. The asperities of the tooth surfaces hit 4.4.6 Tradeoff considerations
each other, but substantial areas are separated by a Completion of final design can also include a
thin film. Regime II is typical of medium speed comparative study for advanced materials vs.
gears, highly loaded, running with a relatively thick conventional materials. This study includes all
oil and fairly good surface finish. Most helicopter or rotating components and housings. Life, weight,
final stage turboprop gears are in regime II. cost and maintainability can be compared.
In Regime Ill the EHD oil film is thick enough to 4.4.7 Test considerations
essentially avoid metal-to-metal contact. Even the
asperities generally miss each other. The high Completion of any aircraft or aerospace gear
speed gear is generally in Regime Ill. In the system design also includes modification of test
aerospace gearing field, turboprop drives are high tools and test setups to run the following:
speed and in Regime Ill. Helicopter gears are in the - Manufacturing Tests;
high speed gear region at the input sections of the - Component Tests;
gearbox. - Loss of Oil Tests;
Definition of endurance limits and regime of lubrica- - Power Plant Tests;
tion are outlined in clauses 5 and 8. - Overload Tests:
4.4.4 Considerations for quality levels - Ground Tests;
Quality levels of aircraft and aerospace gears, - Flight Tests.
bearings and seals are usually as high as system These tests are conducted at specified environ-
cost limitations permit or as good as can be mental conditions outlined in clause 6.
obtained by using today’s manufacturing methods. Vibrations, fire resistance, weapons effects, emis-
Aircraft engine gears are generally ground to obtain sions, and attitude are also integral parts of the
quality 12 or better, honed to obtain good surface above defined tests.

* Numbers in brackets[ ] refer to references listed in Annex C.

14
 AGMA 91%A94

5 Lubrication Typically, convection and radiation are ignored such

that the entire heat load is to be transferred to the
5.1 Cooling vs. lubrication requirements cooling oil by conduction and then removed from the
Proper lubrication of gears consists of: system with a separate oil cooler. When using
grease lubrication, solid lubrication and low flow
a) selecting the correct lubricant; splash lubrication, heat must be removed entirely by
conduction through the housing walls or through
b) ensuring that the lubricant gets into the gear shafting. Cften cooling is a major limitation of these
mesh; systems. Knowing the heat load, the lubricant
c) providing adequate lubricant flow so that heat characteristics and the allowable temperature rise,
generated in the mesh is removed. the required oil flow rate can be calculated:

There are a number of other considerations in the HG = M ch (T,,,,,- Td . ..(8)

design of an aerospace gearbox lubrication system where
but all are related to these three basic requirements.
Failure modes that can occur due to inadequate lu- HG is heat generated at design point, Btu/min
brication include: scuffing, micropitting and spalling. (kJ/min);

5.1 .l Elastohydrodynamic (EHD) lubrication M is lubricant flow rate, Ib/min (kg/min);

and lambda ratio is lubricant specific heat at ( Tout+ Th) /2,
ch
The thickness of the protective EHD oil film can be Btu/lbm”F (kJ/kg”K);
calculated using the techniques described in ap- Tau is average oil out temperature, “F (“C);
pendix A of ANSVAGMA ZOOl-B88. The ratio of
film thickness to composite surface roughness is l-ill is average oil in temperature, “F (“C).
called the lambda ratio. At a lambda ratio of one,
5.2 Lubricants
there is theoretically no metal to metal contact. As
the lambda ratio decreases, more and more contact 52.1 Liquid lubricants
occurs. However, carburiied aerospace gears
operate successfully at lambda ratios as low as 0.4 Liquid lubrication predominates in the aerospace in-
without incurring suface damage. Aerospace gears dustry today. Many gear systems must be designed
can operate successfully at lambda ratios below 0.4 to utilize lubricants that were originally formulated
if adequate boundary lubrication is available. for high temperature turbine engine applications
Boundary lubrication utilizes the chemistry of the (MIL-L-23699 and MIL-L-7808). In some cases
tooth surfaces, the lubricant and its additives to the engine and gearbox use a common lubrication
provide a protective film. Since this type of lubrica- system and thus must utilize engine oil. In other
tion is not well understood today, the designer must cases a common lubricant has been required to pre-
match the application to past successful1 designs vent mixing of two diierent types of oil. These lubri-
operating under similar condiiions. cants were formulated to meet criteria such as cold
flow/cold start requirements, high temperature li-
5.1.2 Cooling the gear mesh mitations, material compatability requirements and
cost. These properties are derived from fluid base
In oil lubricated systems, the amount of lubricant
stocks that are not necessarily ideal for lubricated
supplied to the gear mesh depends on the heat
contacts in a gear drive system. Recently a new
generation rate. The amount of oil required in the
version of these engine lubricants has been put in
formation of an oil film is miniscule compared to that
service for helicopter applications (DOD-L-85734).
required for cooling. Most aerospace lubrication
This lubricant isvery similarto MIL-L-23699 butad-
systems are designed to handle the highest heat
diiives beneficial to the transmission are included.
load and have excess capacity at all other operating
conditions. Heat generation in gears and bearings Tables 2 through 5 list pertinent properties of the
can be estimated by various techniques [l] thru [7]. most commonly used aircraft lubricants today.

15
AGMA 91%A94

Table 2 - Aerospace lubricant viscosities

Tern1 lrature ViSCOS I,csf
OF “c MI L-L-23699’ MIL-L-7808’ DOD-L-85734* Dexron II3

400 204 1.25 1.00 - -

350 177 1.63 1.25 - 2.23
320 160 2.00 1.47 - 2.8
212 100 5.00 3.00min” - 7
210 98.9 5.ooto5.50* - 5.00 to 5.50 -
104 40 25.00 12.00 - 42
100 37.8 25.00 min* - 25.00 min -
-40 -40 13000max* 2000 c9500 20000
-65 - 13OOOmax - -
dotes-
Reference - AFAPL-TR-71-35;

1
! QOD-L-85734(AS) specification
) General Motors Dexron II Specification
’from MIL-L-23699D or MIL-L-7808J specifications

Table 3 - Aerospace lubricant densities

Teml Densi , s/ml
OF MIL-L-23699’ MlL-L-7808’ DOD-L-85734* Dexron II3

392 200 0.86 0.81

320 160 0.89 0.84
302 150 0.90 0.85
212 100 0.94 0.89
104 40 0.98 0.93 -
60 16 - - 0.87

Jotes -
Reference - AFAPL-TR-71-35
1Exxon Datasheet - ET0 25
I General Motors Dexron II Specification

Table 4 - Aerospace lubricant pressure-viscosity coefficients

Temperature Pressure-viscosity coefficient,
(in2/lb)x10000[(mm2/N)x10 000]
“F “C MIL-L-23699* MIL-L-7808G*
400 204 0.498 (72.2) 0.428 (62.1)
350 177 0.532 (77.2) 0.462 (67.0)
320 160 0.556(80.6) 0.486 (70.5)
212 100 0.681 (98.8) 0.613(88.9)
104 40 0.966(140.1) 0.918 (133.2)
l Reference AFAPL-TR-75-26

16
 AGMA 911-A94

5.2.2 Greases Table 5 - Aerospace lubricant

Greases are commonly used to lubricate actuator specific heat values
gearing and gearbox components such as bearings
and splines. Several grease lubricated helicopter Temperature Specific heat,
transmissions are in production but are not com- btu’lb OF[kJ/(kg OK)]
mon. The most common greasesand their uses are
listed in table 6. MIL-L-23699 or
“F “C MIL-L-7808G*
5.2.3 Dry lubricants
400 204 0.562 (2.35)
Dry lubricants are widely used in spacecraft sys- 350 177 0.551 (2.31)
tems where liquids or greases cannot be used due 300 149 0.538 (2.25)
to out-gassing problems (see clause 13) and also in 250 121 0.524 (2.19)
aircraft systems where liquids or greases cannot be 200 93 0.508 (2.13)
contained. These lubricants do not provide the 0.486 (2.03)
150 66
same level of protection as liquids or greases.
100 38 0.464 (1.94)
Thus, the applied loads and sliding velocities must t
be significantly lower in these systems. Table 7 lists ’Reference AFAPLR-T&75-26
common dry lubricants in aircraft use today.

Table 6 - Aerospace greases*

ML Specification Description Application

MIL-G-6032 Oil resistant grease Tapered plug valves, gaskets
MIL-G-21164 Molybdenum disulfide Splines, sliding steel surfaces
MIL-G-23827 Gear and actuator grease Bearings, gears, etc.
MIL-G-25013 Aircraft bearing grease Ball and roller bearings to 200 000 DN
MlL-C-38220 High speed bearing grease Ball and roller bearings to 400 000 DN
MIL-L-27617 Oil resistant grease Tapered plug valves and gaskets
MlL-G-46006 Aircraft grease Driveshaft couplings
MIL-G-81322 General purpose grease Bearings and gearboxes
MIL-L-81827 High load capacity grease Splines and bearings
MIL-G-83261 Extreme pressure grease Gearboxes, actuators
MIL-G-83363 Helicopter transmission grease Tail rotor and intermediate gearboxes
l Military Handbook, Guide for Selection of Lubricant and Compounds for Use in Flight Vehicles and
Components, MIL-HDBK-2754, May, 1969
NOTE -The above greases are not to be used in vacuum applications(see clause 13).

Table 7 - Aircraft dry lubricants*

MIL Specification Description Application

m-G-659 Graphite Dry lubricant or mix with oil
MIL-M-7866 Molybdenum disulfide Threads, gears
MIL-L-8937 Corrosion inhibiting Gears, flap hinges
MIL-L-23398 Air drying solid film Steel, titanium, aluminum
l Military Handbook, Guide for Selection of Lubricant and Con-pounds for Use in Fli&ht Vehicles and
Components, MIL-HDBK-275A, May, 1969

17
AGMA 911-A94

5.3 Distribution systems cooling is maximized. In some cases only a small

in this subclause, guidelines are described for amount of oil is required to provide adequate
selection of various gearbox lubrication schemes. cooling. into mesh lubrication can increase
The development of any new gearbox must, churning losses due to the presence of excess oil in
however, include testing to determine the success the mesh.
of the lubrication system design. There are many In high power applications it may be necessary to
factors that can unexpectedly affect system use multiple oil jets to provide the required oil flow
performance. A successful lubrication system
and still maintain jet velocity. Oil jets can be placed
design prevents gear tooth scuffing, meets oil
on both the out of mesh and into mesh sides to ob-
temperature rise expectations and limits power
consumption due to oil distribution and removal. tain the required oil flow rates. In this case the into-
mesh jet should supply a lesser flow.
53.1 Pressure-fed
5.3.2 Splash
A pressure fed system, for this discussion, consists
of an oil supply line directed at a gear mesh which is Splash lubrication is used when heat generation
fitted with a metering orifice. rates are low compared to cooling available either
through conduction through the gear or through
5.3.1 .l Oil pressure requirements convection to a nearby lubricant. A first order heat
Oil pressure is required to propel lubricant into a transfer analysis can be performed to determine the
rapidly moving ,gear mesh. An oriiice is used to cooling available from the gear and the lubricant.
convert oil pressure into kinetic energy creating the
Splash lubrication can be provided by dipping the
oil velocity required to penetrate the mesh. By
properly selecting both the oil pressure and the gear through a pool of oil or by placing the gear in
orifice (jet) diameter, the flow rate and oil the path of a stream of oil exiting from another mesh.
penetration depth can be controlled. Development testing is typically performed to deter-
mine adequacy of the lubrication scheme.
53.12 Analysis for pitch line velocities to ap-
proximately 25 000 feet per minute (127 mkec) Consideration should be given to pre-lubing gear-
boxes of this type to ensure that oil is available dur-
For gear meshes operating at 25 000 ft/min or less ing startup.
the technique described in [8] can be used. Oil
penetration depth into the mesh is simply a time of 5.3.3 Mist
flight calculation. At these speeds the jet still acts as Mist lubrication is similar to splash lubrication ex-
a solid stream and is not significantly affected by air cept that the cooling provided by the oil is very low.
turbulence. Ati long as the oil flow is not interrupted
The oil available in the mist is sufficient only to allow
by the passage of another tooth, oil will penetrate to
the depth as determined by this method. generation of an oil film. In some cases an air/oil
mist is created with the intent that the air will provide
5.3.1.3 Analysis for pitch line velocities greater cooling of the mesh.
than 25 000 feet per minute (127 m&c)
5.3.4 Grease
Standard analytical methods have not been con-
Grease is used for gearbox lubrication when a
firmed above 25 000 feet per minute. Windage of ro-
sealed area can be provided to prevent contamina-
tating parts must be considered to ensure proper
tion and lubricant loss. The advantages include low
targeting of the oil jet. Development testing of the
cost and simplicity particularly when it would be
unit is required to determine the adequacy of the lu-
diicult to introduce a liquid lubricant system. Heat
brication scheme employed.
generation in these components must be low
5.3.1.4 Oil jet orientation compared to the heat sink available through
Oil isfrequently supplied toward thedisengagement conduction to the housing walls and the shafting to
side of the mesh on the driving side of the tooth. prevent high operating temperatures.
Power losses are minimized, cooling oil will reach Sealed, grease lubricated gearboxes have been
the tooth flank at its highest temperature thus used in helicopter tail rotor and intermediate gear-
promoting heat transfer and the length of time for box applications. In addition to the advantages

18
 AGMA 911-A94

stated above for bearings and spiines they are also transmission and also has less tendency to chum
less vulnerable in combat situations. the oil. Wet sumps may not be practical for some
applications due to the increased frontal area
Flap actuator gear drives are another common
required or other envelope limitations.
aerospace application for grease lubrication.
5.3.5 Powder lubrication 5.4.3 Oil deaeration

Powder lubrication is being investigated for very Oil foaming is a major concern in aircraft gear
high temperature applications where only dry lubri- systems. Air is easily trapped in oil due to the mixing
cants can survive. in these systems dry lubricants action that occurs during high speed rotation of
are blown into bearings and gears as an aerosol to gears and bearings. Foaming can be controlled by
lubricate and cool the system. allowing air to escape naturally in an oil tank,
providing deaeration trays in the oil tankor by using
5.4 Lubrication system design considerations air/oil separators. Oil circulation rates must be
5.4.1 Common vs. separate lubrication systems selected to provide time for air to escape in an oil
tank or tray. Since this usually requires a larger oil
In some cases the designer may have the choice tank it is sometimes necessary to supplement the
between a self contained lubrication system or an natural deaeration with an air/oil separator.
external system. Frequently, however, the gearbox
lubrication system must becommon with the engine 5.4.4 Oii scavenging systems and oil baffles
or other equipment and the resulting complexities In order to prevent heat generation due to churning
must be considered. Table 8 lists some of the ad- of excess oil near rotating bearings, gears or seals,
vantages and disadvantages of each type of sys- it is frequently necessary to use oil scavenge pumps
tem. to return oil to the sump or tank. If this excess oil is
5.4.2 Dry sump vs. wet sump not removed several problems can arise: excessive
power consumption due to oil churning, high oil
in a dry sump system, oil is stored in a separate oil temperatures, oil foaming, oil leaks through seals,
tank, not part of the transmission housing. In opera- and larger oil tank capacity requirement due to
tion, oil supplied to transmission components drains change in level on start-up.
to scavenge ports where the air/oil mixture is
pumped out to the tank. In the design of the scavenge system the designer
must consider attitude and altitude requirements of
The wet sump system integrates the oil tank into the the system. The scavenging action must continue
transmission housing, typically at the bottom. through all aircraft maneuvers and thus through a
Scavenge pumps are required only for areas that
range of system attitude alignments. This require-
are diiicult to drain. Otherwise, gravity is used to ment frequently results in multiple oil scavenge
return oil to the tank area. Oil pump inlets are
pumps with oil pick-up passages placed in strategic
designed to remain covered with oil during all locations. It is generally not feasible to use one
attitudes of operaion. pump with multiple scavenge pick-up points since
The wet sump system offers the advantages of no only air will be removed if any one passage
external plumbing for connecting the oil tank to the becomes exposed to air.

Table 8 - Advantages and disadvantages of a common engine and transmission

lubrication system

Common system Separate systems

Lower weight - common parts Increased weight - redundant parts
Engine oil must be used Transmission oil can be used
Transmission failure can affect engine Transmission failure is self contained
External plumbing required Self containedplumbing

19
AGMA 911-A94

Scavenge oil pumps are typically rated at two to When taking an oil sample it is good practice to
three (and sometimes more) times the supply pump clean the tap priorto taking the sample and to let the
flow rate. This ensures that oil will not build up in oil flow for several minutes before taking the actual
critical areas under adverse operating conditions. sample.
One of these conditions occurs at altitudes where 5.4.7 Fill/drain considerations
air is less dense and is easily dissolved into the oil.
Removal of this foamy m’tiure requires additional It is often possible to introduce more contaminants
flow capacity since the volume of this mixture is intoagearsystem during top-off than during normal
greater than solid oil. system operation. This is particularly true for
systems with fine filtration. A fine filtration system
Design and placement of oil baffles can also affect cleanses the entire system over a period of time to
oil scavenging and gearbox performance. The very low contaminant levels. Opening the system
purpose of an oil baffle is to prevent oil from during top-off may allow debris to fall into the
becoming entrained in rotating gears and bearings system. Control of the top-off function can partially
and to remove cooling oil that has done its job. be aided by design and placement of the fill
Many of the same problems discussed above will mechanism but is primarily controlled by training
occur if oil is allowed to flow into rotating parts. maintenance personnel.
An oil baffle diverts oil toward the sump or scavenge Oil changes are common in systems with coarse
port to prevent a build-up of oil within the housing. A filtration. Change intervals are determined from
baffle can also be used to control air windage and its past experience with similar systems. Oil changes
effect on oil foaming. A baffle can be cast into the in systems with fine filtration are less frequent. The
housing or fabricated separately and bolted into cleanliness levels found in fine filtration systems are
position. The baffle should be tested to determine maintained continuously. The lack of circulating
its natural frequency to ensure that it will not be debris tends to reduce the amount of new debris
subject to high cycle fatigue in the high vibration that is formed. In systems in which the engine and
environment inside the gearbox. gearbox are common, oil consumption by the
5.4.5 Pressure drop in oil passages engine requires a routine tovff of new oil. This
replenishes any additives that may have been lost.
Consideration should be given to pressure drop in
oil passages, particularly if the length of the 5.4.8 Vulnerability/safety
passage is long, if there are a number of sharp turns Some gearbox specifications include the require-
or if the surface roughness of the passage is poor as ment for an emergency lubrication system
in cast passages. If this is not done, the calculated independent of the primary system. The
flow rate could be significantly less in operation. A emergency system allows safe operation of the
rule of thumb used in the past for sizing passages aircraft for a limited time in the event of the loss of
has been to design for a velocity not to exceed 15 the primary system. The degree of sophistication in
ft/sec (4.6 rn/sec) on the pressure side and 5 ft/sec these systems can range from a complete
(1.5 m/se@ on the suction side. secondary pumping system to a very simple gravity,
5.4.6 Oil sampling oil drip system. In addition, some military
transmissionsare required to operate for a specified
If oil sampling capability is required it must be period of time after loss of the lubrication system.
designed into the system at an early stage.
Frequently this feature is added to an existing 5.4.9 Low temperature operation
system and good results are not possible. In order Most aircraft gearboxes must be designed for
to obtain a representative oil sample, the oil tap operation at a very low temperature, typically4 OF
should be located at the end of a line just before a (-54 “C). Since lubricant viscosities are quite high
turn is made. When the sample is taken, oil from the at this temperature, it is diiicult to circulate oil in the
main stream is obtained. In many cases an oil tap is lubrication system. The requirement led to the
placed in a region where debris is allowed to build development of the MIL-L-7808 lubricant with its
up and the sample is artificially contaminated. lower cold temperature viscosity. Even with this

20
 AGMA 911-A94

lubricant, oil heaters may be required to aid starting. points such as a main distribution line leading to
It is difficult to pump lubricant during start-up at low gear mesh oil jets. Screens are also used to protect
temperature. scavenge pumps.

5.4.10 Oil coolers 5.5.3 Filter rating nomenclature

Some type of oil cooling system is required for most A typical filter rating might be: 10 micron absolute,
aerospace gear systems. Convective heat loss Beta=1 00. This terminology means:
through the housing may not remove enough heat
under all flight conditions to maintain acceptable oil The fitter will reduce the number of particles
temperatures. Typical oil cooling systems include: greater than or equal to 10 microns in size
entering the filter by a factor of 100. This equates
- ram air/oil heat exchangers;
to an efficiency of 99 percent. Or, 1 percent of the
- air/oil heat exchanger with blowers; particles greater than 10 microns will pass
- fueVoil heat exchangers for lower heat load through the filter.
systems.
Nominal filter ratings are also used today but are
Oil temperature is sometimes controlled in these
less stringent. Nominal efficiencies are typically 90
systems by varying the amount of air or fuel
to 98 percent.
supplied to the coolers. A typical aerospace lube
system schematic is shown in figure 4. Some 5.5.4 Fine filtration
gearboxes that have low power consumption can be
operated without oil coolers if the heat loss is Pine fittration is becoming more common today for
sufficient to keep operating temperatures within aircraft engine and gearbox systems for several
acceptable ranges. reasons: longer bearing and gear life due to lower
5.5 Filtration levels of debris, fewer or no oil changes and poten-
tially longer filter life. Fine filtration requires a 3 to 5
5.5.1 Filter types micron absolute rating with a Beta factor of at least
100. Fitter life has been found to increase after the
There are three types of aircraft filters in use today initial system clean-up phase. Once the system is
characterized by the media used to trap debris: clean, it tends to stay clean.
steel mesh, paper and fiber mesh. Steel mesh fil-
ters are being utilized less due to: 5.5.5 Filter location

-the maintenance required to clean them; Filters can be located on the pressure side of the
system, the scavenge side or both. Selection of the
-the loss in effectiveness after cleaning; site depends on the designer’s philosophy, e.g. a
pressure side filter will always protect the compo-
- cost.
nent, whereas a scavenge filter will protect the heat
Paper media filters are generally used for coarse exchanger, sump or tank, and will not proliferate the
filtration levels. Throw-away fibrous media filters debris. The overall system design must be consid-
are becoming more common as fine filtration is ered to reach the best decision on filter location.
becoming more popular.
5.6 Oil pumps
5.5.2 Screens
The three most common aerospace gearbox oil
Screens or last-chance-filters are used to protect pumps are the gear pump, the vane pump and the
system components from large pieces of debris, gerotor pump . All are positive displacement pumps
The mesh is usually coarse and is intended to stop since output flow is a function of fifed internal
debris from a failure that occurs upstream of the geometry. Flow increases linearly with shaft speed.
screen but downstream of the main filter. Screens However, the vane pump can be designed to
are typically placed ahead of critical lubrication provide a variable flow rate.

21
 AGMA 91%A94

Oil tank From nacelle

Prjp gearbox lube and scavenge pu$ps

To dacelle
+

Gear mesh

I
Gear mesh

I
Bearings
c/l kf Gear mesh
i
Bearings )-I Bearings

I b H Spline
Spline
1
I

Prop gearbox sump Accessory gearbox sump

LEGEND
Screen mesh 0, Regulating valve

0 Filter * By-pass valve

Figure 4 - Typical aerospace lubrication system schematic

56.1 Gear pumps gear versions of this type of pump can increase the
flow with little increase in pump size.
A gear pump moves a fixed volume of oil from the
inlet side to the outlet as shown in figure 5. Plow 5.62 Vane pumps
volume is a function of tooth depth and width. As Vane pumps consist of a rotor mounted off center in
resistance to flow increases at the outlet, the power a circular cavity (see figure 6). Vanes in the rotor are
required to rotate the input shaft increases and forced out due to centrifugal force under rotation
discharge pressure increases accordingly. Three and contact the cavity walls. The amount of

22
 AGMA 916A94

eccentricity determines the volume of flow per

revolution. Variable flow rates are achieved by
allowing adjustment of the eccentricity during op-
eration. Variable displacement pumps are more
efficient since excess flow (and thus excess power
r Housing body/cam ring
r Rotor

loss) is eliminated.

Figure 5 - Spur gear pump

5.6.3 Gerotor pumps Figure 6 - Vane pump
Gerotor pump action is shown in figure 7. Oil is
5.7 Lube system condition monitoring
drawn into an area equivalent to one tooth space at
position 1 and is rotated and then forced out due to 5.7.1 Chip detectors
compression as it moves toward the exit at position Magnetic chip detectors are used to collect debris
4. Both rotors rotate but the inner rotor driies the
circulating in the lubrication system. In some
w-w systems the pilot receives an indication when the
Spring type pressure regulators are often used to number of particles collected is sufficient to trigger a
maintain a constant oil pressure under varying con- signal in the detection circuit. In other systems the
ditions of operation, e.g. speed and temperature. chip detector is visually checked as a maintenance
These regulators are commonly built into the pump function. Chip detector operation is very sensitive to
body. Oil flow rates are then controlled by selecting systemconfiguration and sensor location. Testing of
orifice (jet) sizes to supply the correct oil flow at the chip migration in the housing is recommended for
regulated oil pressure. placement of chip detectors.

Figure 7 - Gerotor Pump

23
AGMA 91%A94

There are many types of chip detectors but their MlL-STD-462, Measurement of Electromagnetic
function is similar to that just described. The burn- Interference Characteristics.
off chip detector was developed to reduce the inci- Another source of environmental information is the
dence of nuisance indications during flight. This de- NASA Technical Memorandum 82473, Terrestrial
vice automatically passes a strong current through
Environment (Climatic) Criteria Guidelines for Use
the collected chips, melts them, and may provide an
in Aerospace Vehicle Development.
indication to the pilot. This will clear the indicator. If
a true failure has occurred chips will collect again 6.1 Ambient temperature effects
causing another indication shortly thereafter. The operating ambient temperature range for the
Another type of chip detection system under devel- gearbox will be determined by the aerospace sys-
opment has the ability to count particles of several tem application operating envelope in conjunction
sizes and track the build-up of debris. If the rate of with data presented in the International Civil Aero-
build-up is rapid, there is a good chance that a fail- nautical Organization (ICAO) Standard Atmos-
phere. [Note that the ICAO Standard Atmosphere is
ure has occurred.
identical to the US Standard Atmosphere for alti-
5.7.2 Temperature/Pressure sensors tudes below 65 617 feet (20 000 meters) .]
Lubricant temperature and pressure are the primary The gearbox should be capable of operation at
parameters used to track system condition. These continuous rated power or less at any ambient
sensors must be placed in the system correctly to temperature within the specified operating enve-
obtain good results. As an example, a temperature lope. It is important to note that at temperature
sensor that is to detect the temperature of the oil ex- extremes material characteristics and properties
iting the gearbox must be placed in an area where can be significantly affected. For example, materi-
good lubricant flow is taking place. Otherwise, it als with dissimilar coefficients of thermal expansion
may just indicate air temperature. (e.g., aluminum versus steel) will significantly
influence operating fiis and clearances within the
5.7.3 Spectrometric oil programs gearbox and need to be adequately considered in
Spectrometric oil analysis programs have been the design. Also, at very low temperatures lubricant
used for many years to detect wear of components viscosity will be orders of magnitude higher than at
in aircraft systems. The procedure consists of tak- typical operating temperature. As a result, lubricant
ing oil samples at regular intervals and analyzing flow through heat exchangers may not occur even
them for metal content in a laboratory. These pro- after the remainder of the gearbox is at normal
grams have shown varying degrees of success. operating temperatures. The net consequence is
Due to the size of the particles analyzed, this that power loss through the gearbox will be signifi-
method may not work in fine-filtration systems. cantly increased.
In addition, increased brittleness and notch
6 Environmental issues sensitivii can become an issue at very low
temperatures. For example, the fracture toughness
This clause illuminates the environmental issues of gear materials can be reduced by more than
likely to be encountered by an aerospace gearbox twenty fiie percent when going from room
during its lifetime, including storage, transport, and temperature to -65°F (-54%). Other potential low
operation. temperature problems are loss of O-ring resilience,
Data on the occurrence of environmental extremes resulting in leakage of lubricant from the gearbox
may be found in several publications. MIL- and contraction of the gearbox at a greater rate than
STD-210, Climatic information to Determine De- the gears and bearings, causing abnormal loading
sign and Test Requirements for Military Systems ofthesecomponents. Refertoclause13forvacuum
and Equipment provides data on free air (i.e. ambi- environment applications
ent) conditions. During the development phase of a At the other extreme, lubricant viscosity will be very
gearbox, MIL-STD-810, Environmental Test Meth- low leading to the possibilii of very thin oil films in
ods and Engineering Guidelines, can be used to the gear meshes and in the internal bearing con-
gain additionalinsights. Electromagnetic
interfer- tacts. If the oil film thicknessis too small,allowable
ence test and analysis requirements are covered in compressive stress limits will be reduced for the

24
 AGMA 911-A94

gears and bearings and the probabilii of gear tooth 6.3 Attitude effects
scuffing (scoring) or bearing surface distress will be The gearbox operating attitude limits will vary
significantly increased. The judicious selection of greatly depending upon the particular aerospace
oils with appropriate properties at both extremes application. For reference, figure 8 shows typical
(see clause 5 for detailed information) can help limits for military aircraft. The aircraft/gearbox
avoid these potential problems as can the proper should be capable of continuous operation in the
design and sizing of oil coolers to maintain a desired unshaded area and capable of some period of
oil operating temperature. transient operation in the shaded area. Sometimes
Another consideration associated with very high diierent duration requirements are specified de-
ambient temperature operation is the retention of pending upon whether the system is at zero g
adequate surface hardness for both gears and (acceleration due to gravity) or negative g operating
bearings. Sufficient loss of hardness (e.g., two conditions. At any attitude, the gearbox should be
points below design minimum values) can radically capable of operation at continuous rated power or
degrade compressive fatigue life. Therefore, less. In addition to operating requirements, the
retention of adequate hardness is a very important gearbox should be capable of being stowed or
consideration in the proper selection of gear and transported across the specified attitude spectrum.
bearing material.
* Above
Special mention is madeforaerospace applications horizontal
required to operate at cryogenic temperature (i.e.,
at liquified gaseous conditions). At these extremely
low temperatures, very specialized design prac-
tices, materials, and lubricants are needed to en-
sure proper gearbox operation.
6.2 Ambient pressure effects
The operating ambient pressure range for the gear-
box will be determined by the operating envelope of
the particular aerospace application. Typical fac-
tors affecting pressure include operating altitude of
4, ,Roll &-rgle
P
the aircraft, rates of climb/dive, and the effects of nu-
clear or conventional weapons. At extremes, pres-
sure changes, at a rate of 1000 psilsec (6.895 x 1O6
Pa/se@ with magnitudes of 2.5 psi (1.724 x 1O4Pa)
and 70 psilsec (4.826 x lo5 Pa/set) with magni-
tudes of 5 psi (3.447 x 1O4Pa) can be anticipated.
The gearbox should be capable of operation at con-
tinuous rated power or less at any ambient pressure
A *Below A
horizontal
or pressure change within the specified operating * Reference to ground
envelope. Design of the gearbox should include n - Test points
consideration of the possible pressure changes on Notes -
performance. Pressure differentials across gearbox 1. The gearbox shall be capable of operating at all pos-
seals can fluctuate and even reverse in flight if not sible conditions; however, for the purpose of defining
properly designed for and cause seal contamination the direction of acceleration vector from the engine CG,
the figure assumes no acceleration other than grav.w.
and oil leakage. Likewise, venting of the lube sys- 2. Gearbox centerline perpendicular to plane of paper.
tem is often accomplished within the gearbox 3. Continuous operation in clear area.
through breather pressurizing valves which need to 4. Thirty second operation in shaded area.
be designed for all possible pressure variations. Figure 8 - Typical gearbox attitude limits
Adverse consequences include loss of oil, oil en-
trapment, and oil foaming, all of which can lead to The requirements for variable attitude limits derive
early failure of the gearbox components. from the fact that the aircraft application is required

25
AGMA 911-A94

to climband bank, and formilitaryaircraft, tosustain conducted to validate the adequacy of the gearbox
air combat maneuvers and terrain following opera- or other component design to operate for some
tion and possibly undergo inverted flight. If the oil extended period of time. For the required duration
system for the gearbox is not properly designed to the gearbox should be capable of operation without
operate within the specified attitude limits, serious any degradation in performance. Obviously, the
consequences for the gearbox can result. For ex- best wayto handle the contaminant is to preclude its
ample, extended operation in zero or negative g introduction into the gearbox by prudent design. For
conditions can result in oil system malfunctions the larger contaminants, like sand, this may be
which can effect the gearbox such as oil starvation, possible. However, for water or dirt this is probably
gearbox flooding, oil foaming problems, and seal not possible and other solutions have to be found to
leaks. Internal pressurization techniques are often accommodate them such as filters to remove the
used as solutions to these problems. Special provi- dirt after it has entered or protective coatings to
sions, such as an auxiliary lube system, may be resist the deleterious effects of water.
necessary in the gearbox lubrication system for ex- 6.5 Vibration/Shock effects
tended inverted flight operation. Placement of suc-
tion/feed points to the pumps can alter the attitude This subclause covers vibration and shock effects
envelope. See 5.4.4. on the gearbox ensuing primarily from external
sources. Those initiated internally within the gear-
6.4 Contaminant effects (water, corrosives, dirt, box are covered extensively in clause 7. The ran-
dust, and sand) dom vibration environment which an aerospace
Due to worldwide all weather operation require- system gearbox might be subject to can originate
ments for many aerospace systems, gearbox op- from the following sources:
eration in the presence of contamination has be- - Turbulent aerodynamic airflow along external
come a necessity. For example, aircraft operating surfaces of the airframe structure;
out of airports near arid parts of the world often take- - Engine noise impinging on the airframe struc-
off and land in conditions where the air contains ture;
sand, dust, and dirt. Water gets ingested during - Rotor/propeller blade induced effects;
rainstorms. The fact that the lubrication system - Airframe structural motions causing
which services the gearbox has to breathe means sympathetic gearbox response;
that atmospheric water and vapor is ingested into - General aircraft motions caused by such
the gearbox. This moisture may or may not contain factors as runway roughness, landing, and
salt which presents a whole host of corrosion re- gusts;
lated problems. The incidence of contamination is - General motions encountered during
so prevalent that specifications have been devel- transport of the gearbox, such as by rail, ship,
oped for them. The table below shows the data for or truck;
US Specification Sand. The specified sand should - For military systems, the gun blast pressure
include 90 percent silicon dioxide (SiO;z)of angular impinging on the aircraft structure from high
structure with total particle size distribution by speed repetitive firing of installed guns and vi-
weight as shown in table 9. bration from antiaircraft fire (flak).
Table 9 - Particle size distribution by weight Typical vibration spectra tested for in aircraft are
Size, fin Percent shown below in table IO and figure 9 for propeller
Size, CPM
by weight aircraft and turbine engine equipment, and in figure
10 and table 11 for helicopters.
o-2 953 (o-75) 5+2
2 953-4 921 (75-l 25) 15+2 The shock environment is often a consequence of
4 921-7 874 (125-200) 28f2 the following effects: aircraft launch/catapult and
7 874-15 748 (200-400) 36+2 landing; aircraft crash; handling, such as dropping
15748-35433 (400-900) 3.5 lk 0.5 of the gearbox; and transport of the gearbox.
35 433-39 370 (900-l 000) 1.5kO.5 Typical shock pulse levels and durations tested for
are shown in figure 11. Generally, specific require-
Using the specification contaminant at some prede- ments are specified by the customer. These may
termined concentration level, qualification tests are diier from those in the figure.

26
 AGMA 911-A94

Table 10 - Suggested functional test levels for propeller aircraft and turbine eng,ine equipment
Equipment location*- 3 ! Vibration level (Ll) at F1 4$5, g*/Hz
In fuselage or wing forward of propeller 0.1
In fuselage or wing aft of propeller 0.3
In engine compartment or pylons 0.6
Equipment mounted directly on aircraft engines 1.0
1 Fl= fundamental excitation frequency; Fi= source frequency (i = 1 -4), FZ = 2 Fl, F3= 3 FI, F4= 4 F1
2 When panels and racks are not available for equipment installed on vibration isolated panels or racks, or when
the equipment is tested with isolators removed, use “fuselage or wing forward of propeller” category with levels
reduced 4 dB.
3 Increase test levels 6 dB for equipment mounted on fuselage or wing skin within one propeller blade radius of
the plane of the propeller disc. For all other skin mounted equipment, increase levels by 3 dB.
4 Bandwidth vibration around each Fiwill equal f 5% F fpr constant-speed excitation. When excitation is not
constant-speed, bandwidth will encompass operating speeds for cruise and high power operation.
5 Fl= 68 Hz for most C-l 30 aircraft.

_---_-_ __
3
m
Li
4. -
-6 dB/octave
octave = 2 to 1
frequency range
Li
8 I
I II I
s
2 l . I
45
--
l .I

!
I

E .
I

N4
5 .
. 1

1
.03

Q
I

6 -01 -
n I h-l

I I I I I I I
15 F, F2 F3 4 2000 i5 F, 200;
Frequency (Hz) Frequency (Hz)
A. Propeller aircraft spectrum B. Turbine engine equipment spectrum
Figure 9 - Suggested vibration spectra for propeller aircraft and turbine engine equipment

AA rl
:I’ Note: See table 11 for
&I57 i 22 L’t , Lf2, L13and L/4 g-levels.
g-g L’3 L’4

ES 1
I

*; $02 Equipment
w, , g*A-lz Ft , Hz
i5gj
5m
location
General 0.002 500
8a
at instrument panel 0.002 500
% External stores 0.002 500
% 8 t 1 8
On/near drive 0.02 2000
I 8
I
t
t
8 train elements
I I I I t

10 F, F2 F3 F4 300 5
Frequency (Hz)

Figure 10 - Suggested vibration spectrum for equipment installed on helicopters

27
AGMA 91%A94

Table 11 - Suggested functional test peak levels for equipment installed on helicopters
Equipment location Source frequency (5) range, Hz Peak vibration level at F,, L’,, g’s
General’ 5-25 0.1 F,
25-40 2.5
40-50 6.5 - 0.1 F,
50-500 1.5
Instrument panel’ 5-25 0.07 F,
25-40 1.75
40-50 4.55 - 0.07 F,
m-500 1.05
External stores’ 5-25 0.15 Fx
25-40 3.75
40-50 9.75 - 0.15 F,
50400 2.25
On/near drive train elements* 5-50 0.1 F,
50-2000 5 + 0.01 F,
1 F,= Source frequency of interest = Fr, Fe, FJ, or F.4
Fj = Fundamental source frequency
Fp = 2F1 ; F3=3Fj ; F4 = 4F1
Upon determining values of Fl, F2, F3, or F4 (figure lo), select the appropriate source frequency range for each
when determining peak vibration levels. The source frequency ranges are not presented in order of FI = F4.
2 Ft, F2, F3 and F4 must be determined from drive train areas for the particular helicopter. Note (1) is then
applicable.

[[
Note - The oscillogram shall include a time
Ideal sawtooth pulse about 30 long with a pulse located approxi-
mately in the center. The peak acceleration
Tolerance limits magnitude of the sawtooth pulse is P and its
duration is D. The measured acceleration
ulse shall be contained between the broken
line boundaries and the measured velocity
change (which may be obtained by integra-
tion of the acceleration pulse) shall be within
the limits of K fO.l I$ , where X is the veloc-
ity change associated with the ideal pulse
which equals 0.5DR The integration to deter-
mine velocity change shall extend from 0.4 D
before the pulse to 0.1 D after the pulse.

Test Minimum peak value, (P) g’s Nominal duration, (0) ms

Flight Ground Flight vehicle Ground Flight vehicle
vehicle equipment equipment’ equipment equipment’
Operation test 402 20 11 11
Crash safety 75 40 6 11
1 Shock parameters recommended for equipment not shock-mounted and weighing less
than 300 pounds (136 kg).
2 Equipment mounted only in trucks and semitrailers may use a 20 g peak value.

Figure 11 -Terminal-peak sawtooth shock pulse configuration and its tolerance limits
(for use when shock response spectrum analysis capability is not available)

28
 AGMA Qil-AQ4

Some potential adverse effects of shock and resistance requirements. These tests are con-
vibration on a gearbox include cracking of gearbox ducted while conveying the oil within the gearbox at
housings and covers, breaking of gearbox mounts, the lowest oil flow rate, highest ambient system
cracking of oil supply and scavenge lines, misal- pressure, and the highest oil temperature expected
ignment of internal components, and brinelling/ over the complete gearbox operating range.
false brinelling of the gearbox bearings and gears. 6.7 Electromagnetic effects
Several means are typically employed to minimize
these adverse effects. For example, vibration Electromagnetic compatibility between any gear-
damping coatings can be applied to gearbox box electrical/electronic component and those of
housings, elastomeric dampers are used for gear- the rest of the aerospace system is essential for
box mounting, pneumatic or hydraulic mounts are proper and safe operation. For example,
employed during transport, and oil lines are amply electromagnetic interference could conceivably
supported to preclude the presence of potentially ’ cause a malfunction of the gearbox condition
harmful resonant vibration frequencies. monitoring system such as that used to monitor
gearbox vibration levels or to detect the presence of
Qualification tests should be conducted on the foreign or wear-generated contamination. Addi-
gearbox at the appropriate vibration and shock tionally, other components including gears and
levels and durations (again depending on the bearings may become magnetized due to electro-
particular aircraft application) to validate the gear- magnetic effects and attract harmful ferrous-based
box design. Because of potential airframe effects, contamination that can lead to wear and premature
testing of the total aerospace system is necessary failure. To mitigate electromagnetic effects,
for final evaluation. shielding can be used with electrical leads and/or
6.6 Fire resistance requirements with the electronic component itself. During periodic
gearbox inspections and teardowns, procedures
The capability of the aircraft and its many often include passing bearings and gears through
components including the gearbox to have demagnetizing coils to preclude rebuilding the
adequate fire resistance is critical to overall gearbox with magnetized components. Electro-
aerospace system operational safety. It is essential magnetic interference testing methods, such as
that any system component containing flammable those prescribed in MlL-STD-462 for military
fluids, such as the gearbox with its oil lubrication aircraft, will ensure compatibility between all aircraft
system and fuel-oil heat exchangers, will not electrical and electronic components.
contribute to an aircraft fire and will allow sufficient 6.8 Nuclear, biological, and chemical (NBC)
time to isolate the fire zone. Gearbox seals, covers, effects
and ftiings are primary areas of concern for leakage
and potential fire initiation sources. Some aerospace system vehicles may have to
operate in a strategic or tactical nuclear warfare
In the design of a gearbox, materials and fire environment. Therefore, all system equipment
resistant coatings are selected that offer adequate including the gearbox must be optimized to with-
resistance to fire and which can also maintain stand that environment. Potential nuclear weapon
appropriate mechanical strength properties. As effects include the following: blast/gust, thermal,
examples, aluminum and magnesium, which are transient radiation, and electromagnetic pulse (see
often used for gearbox housings, have repeatedly 6.7). In addition, chemical and biological weapons
demonstrated more than adequate fire resistance are becoming more widely available and have been
and sufficient resistance to thermally degraded used in recent warfare. Thus, the gearbox must be
mechanical properties. capable of sustaining operation in a total NBC
environment. NBC contamination can have direct
Reduction in mechanical properties can lead to effects upon the critical properties (e.g., physical,
permanent deformation of the gearbox resulting in chemical, mechanical, thermal, or electrical) of the
degraded gearbox performance or causing oil materials employed. In addition, materials or
leakage paths. Qualification tests of sufficient procedures used to decontaminate equipment
duration should be conducted on the complete items may also degrade the properties and reduce
gearbox assembly, includingappropriatefittings, to gearbox/drive system capabilities. To overcome
validate its compliance with specification fire NBC effects, selection of materials known to be

29
AGMA 9ll-A94

resistant to the expected NBC condition or specific 7.1 .I Transmission error

design protection, such as nuclear hardening, must
be provided. Material resistance can be established One of the most significant sources of gear vibration
either through testing or by analysis of effects on is the result of the nonuniform transfer of torque be-
similar materials or designs. Use DOD-BTU-21 69 tween mating gears due to variations in the mating
for reference and additional information, especially parts, the elastic deformation (bending and contact)
for military applications. of the gear teeth under load, and the deformation of
the tooth backup rim and web. The variations can
include pitch line runout, tooth spacing variation
(both tooth-to-tooth and accumulated pitch errors),
7 Vibration and noise and profile variation. Each type of variation will pro-
7.1 Causes of gear vibration duce a characteristic vibration pattern. For exam-
ple, pitch line runout will produce a strong once per
Gearbox vibration is the result of a complex interac- revolution excitation while spacing variation will pro-
tion among the gears, bearings, shafts, and hous- duce excitation at mesh frequency (rpm times num-
ings which make up a gearbox assembly. In order to ber of teeth) and harmonics of mesh frequency. The
reduce this vibration, the precise mechanisms by resulting nonuniform transfer of torque produces a
which it is generated must be understood. Several dynamic force which in turn excites torsional/lateral/
mechanisms have been identified which are pri- axial vibratory modes of the gear shafts. The shaft
mary contributors. There are also some secondary response produces displacements at the shaft sup-
contributors which may amplify or reduce vibration port bearing locations. These excite the housing
levels. Figure’12 is a schematic representation of and cause it to vibrate at all mesh frequencies. The
the way vibration and noise passes from the source dynamic characteristics of the internal components
at the teeth to the foundation and the air. Note that may magnify this excitation. Furthermore, the dy-
the path includes the gear bodies, shafts, bearings namic characteristics of the housing may also am-
and housing. plify its displacements and resulting noise.

r noise

1 e-’
Airborne structure noise

m n Structure-borne noise
Figure 12 - noise and vibration paths

30
 AGMA 911-A94

Due to deterioration after many hours of service, is the case where a turboprop gearbox must avoid
many gearboxes will exhibit increases in noise or natural frequencies at or near the propeller blade
vibration levels relative to the levels when they were passing frequency; i.e., number of blades times
new. This effect is usually the result of changes propeller rpm.
occurring in tooth profile accuracy and even in tooth 7.1.5 Entrainment
spacing accuracy. These changes are the result of
Another source of vibration which is generally of sig
surface deterioration due to either wear or localized
nificanceonly on very high speed units or those with
pitting, and are most common with through hard-
very wide face widths and moderate to high speeds
ened gears, but can occur even with case
is that created when the mixture of air and oil which
carburized teeth. The condition is aggravated if the
occupies the space between adjacent teeth is
operating oil film is marginal, either due to low
forced out by the entrance of a tooth on the mating
viscosity or low oil film thickness.
gear. In a high speed mesh, the speed of the air-oil
7.1.2 Unbalance mixture as it exits the mesh area is many times the
All rotating machinery is subject to vibratory excita- pitchline velocity of the gear set and may, in some
tion due to the dynamic unbalance of the rotating cases, reach the speed of sound. At the speed of
components. Any unbalance will produce a rotating sound, this effect dominates the overall noise level
force with a frequency equal to the rotational of the box.
frequency of the component. This force will be 7.2 Consequences of vibration
proportional to the square of the rotational speed 72.1 Structural issues
and will cause a response from all the parts of the
gearbox which depends on their dynamic charac- 7.2.1 .l Fatigue
teristics, i.e., natural frequencies and damping. One of the most severe consequences of excessive
7.1.3 Misalignment with connected equipment vibration in a geared system is the possibility of
fatigue failure of internal components due to
Many gearboxes have flexible couplings to provide vibration induced stresses. When a critical
both input and output connections. Misalignment of component has a resonant frequency within the
the connected equipment to these couplings is a operating range there is a risk that high stressescan
possible source of vibratory excitation. In general, if occur and some method of reducing the response
flexible couplings are used which are not of a con- should be undertaken. This can include redesigning
stant velocity type, misalignment will produce a tor- the gear to change its natural frequencies or adding
sional excitation with a frequency of two times the some kind of damping.
rotational frequency. The gearbox can respond to
7.2.1.2 Fretting
this excitation just as to any other vibratory source.
Fretting occurs when parts are in intimate contact
7.1.4 Resonance
and are subjected to microscopic motion. Although
Although resonance is not a vibratory source, since fretting can occur even if vibration levels are low, the
it represents the coincidence of an excitation risk of serious fretting problems is greatly increased
frequency with a system natural frequency, it is when the parts are also subjected to high vibration.
probably the most common cause of vibration
problems. Once the gearbox is designed, the 7.2.1.3 Fasteners and lockwire
frequency characteristics of vibratory sources in the A high level of vibration can be a significant
system are well defined. If any of the gearbox contributor to locknut loosening, breakage of lock-
components has a natural frequency which is close wire or broken tab washers in gearboxes.
to one or more of these excitation frequencies or
7.2.1.4 Brackets
harmonics of them, vibration or noise can occur.
One interesting example of this is the so called If external brackets are used to provide support for
“beat” phenomenon where a gearbox will exhibit accessories or for gearbox mounting, they can be
noise at a frequency equal to the difference particularly susceptible to vibration. Since brackets
between a system characteristic frequency and a are often designed with simple structural shapes
nearly equal exciting frequency. Another example connected together, they can have high stress con-

31
 AGMA 91%A94

centrations at the intersections of these shapes. b) Natural frequency analysis of the gear blanks
When subjected to vibratory loads, these intersec- to determine the mode shapes and frequencies
tion points can become highly stressed areas. of these components;
7.2.1.5 Lube system components c) Determination of the dynamic gear loads ap-
plied to the components;
Internal lube system components such as nozzles
and windage baffles often have low natural d) A detailed finite element model of the static
frequencies and as such can be susceptible to gearbox structure;
vibration problems. Particular attention should be e) An analysis of the modes of the entire system.
paid to attachment points and any bolt flanges. 7.39 Strain energy considerations
7.2.2 Noise In recent years, a trend to dynamic optimization by
strain energy techniques has evolved. This has
There is a very close relationship between mechani-
mainly been for the alteration of an undesirable
cal vibration and audible noise. If all mechanical
natural frequency. To understand the technique,
vibration were eliminated, noise also would be consider that in general each natural mode of the
eliminated. However, it is not necessarily true that structure contributes to vibration in proportion to its
reducing the vibration of any single component will amplification factor. Consequently, each mode
reduce noise. On the other hand, selectively whose frequency is in the vicinity of the forcing
increasing the vibration of certain components, if frequency would be a major contributor to the
coupled with node relocation, can produce a noise overall dynamic response. In the modal method,
reduction. which operates principally on the amplification
factor, the natural frequency immediately above the
The noise which is ultimately radiated is composed
exciting frequency is usually increased. One could
of two components: airborne and structure borne
also reduce the natural frequency immediately
noise (see figure 12). Airborne noise is transmitted
below the exciting frequency if it is possible
directly from the source. Structurebome noise is structurally.
due to the transmission of the vibratory energy
through the internal structure of the gearbox and A finite element analysis is first employed in the
into the external supporting structure. The external modal method to yield a dynamic solution. The
structure provides a path for noise since it may be mode shapes are obtained, then the modal strain
excited by the gearbox at its attachment points and energy distribution throughout the structure is found
for any given mode shape whose natural frequency
produce or amplify noise.
is to be modified. The strain energies for all
7.3 Design structural elements are obtained and then tabulated
from the highest to lowest. The structural elements
7.3.1 Finite element analysis with the highest strain would be the best candidates
The ideal time for minimizing vibration is in the for modification of the natural frequency. For
preliminary design stage. The ability to analyze a example, in the case of increasing the lowest mode,
given gearbox and modify its design, based solely the elements with the highest strain density, when
on this analysis, in order to minimize its operating deformed in this mode, would be the best
vibration level requires the use of several finite candidates for modification to obtain a maximum
element modeling techniques. These analyses frequency shii for a minimum material addition
define the excitation due to the gears, the response (weight) penalty.
of the shaft support system to these excitations, the 7.3.3 Design parameters
manner in which these shaft responses are trans-
Inordertoprovidesomefeelforthemannerinwhich
ferred to the housing through their bearings, and the
various transmission design parameters affect vi-
response to these various stimuli. In general, the
bration and noise level, some of the more significant
approach involves the following analyses:
ones will be addressed in the next subclauses.
a) Modeling the gearteeth for local dynamicflexi- When utilizingthis information,the designer should
bilii and kinematic loading; beaware that they represent trends only. Due to the

32
 AGMA 91%A94

complex interactions which exist in a gearbox, the noise is seldom the single driving force in the design
results of similar investigations utilizing significantly of a transmission system, other considerations will
different test boxes may vary substantially. dictate the basic type of gearing to be used. Given
7.3.3.1 Tooth combinations that the box is to be of the parallel shaft type, the
designer may choose from simple spur gears
The selection of tooth combinations which avoid (contact ratio 1.25 - 1.70), single or double helical
potential resonances is sometimes possible. By gears (total contact ratio* 1.8 - 5.0, or greater in
varying the selected tooth numbers, the meshing some cases), high profile contact ratio (HCR) spur
frequency can be raised or lowered. Obviously, this gears (contact ratio greater than 2), or any of a
must be done within the limitations imposed by variety of special purpose geartypes. The choice of
stress and scuffing (scoring) requirements. which specific type of gearing to be used depends
7.3.3.2 Contact ratio on many considerations, only one of which is its
vibratory characteristic.
The properties inherent in certain types of gears can
have a large effect on gearbox vibration levels. Ac- 7.3.3.3 Tooth shape modification
curacy aside, contact ratio (simply stated, contact
In many cases, it is desirable to modify the gear
ratio is the average number of teeth in contact dur-
tooth profile shapes so that they are no longer true
ing each mesh cycle) is one of the most important
involute curves, especially for high load, high speed
parameters which affects gear tooth excitation and
drives. This is accomplished by relieving the tip
thus noise and vibration level. Virtually all other
andor the flank of the profile. This is done to avoid
gear tooth parameters which affect vibration level
tooth interference on engagement and disengage-
do so largely by virtue of their effect on contact ratio.
ment during rotation. This interference is due to
For example, decreasing a spur gear’s pressure deflections, both within the gear teeth themselves
angle generally decreases noise level, however the and in the housing and shafts. If these interferences
same effect can be achieved by extending the are not relieved, the load capacity of the gears can
addenda of the higher pressure angle gears slightly be seriously impaired. In addition, since these
to achieve the same contact ratio as the lower conditions generate high dynamic loads, vibration
pressure angle gear. Unfortunately, due to tooth excitation and noise is also increased.
geometry limitations, this approach can be used
In addition to modifying the tooth profile shape, it is
only for relatively small changes in pressure angle.
also often desirable to modii the tooth shape in the
As noted earlier, the primary exciting force within the axial direction. This can take the form of crowning or
gear mesh is due to the non-uniform transfer of load lead correction. Lead or helii corrections compen-
between successive pairs of meshing teeth. sate for deflections of the teeth and shafts and also
Increasing the number of tooth pairs in contact for the housingsand supports. If pinions have a high
reduces the total load carried by any single pair of length to diameter ratio (above l/i), and are sub-
teeth, thus the dynamic forces generated at en- jected to heavy loads, they will often have enough
gagement and disengagement are reduced. In “windup” to cause heavy end loading if compensa-
addition, as the number of tooth pairs in contact tion is not provided. This heavy concentration of
increases, individual tooth errors tend to average load at the ends will contribute to vibration and
out, again reducing the dynamic loads generated. noise. Crowning may also be used to relieve end
One way to improve contact ratio and thus reduce load.
noise is to use helical rather than spur gears or spi- It must be emphasized that any tooth shape modifi-
ral bevel rather than straight or Zero1bevel gears. cation is optimum for only a single load level. Vibra-
Generally, when considering vibration and noise tion levels will increase both above and below this
levels, the designer should strive to achieve the load level. If the modification is not very severe, the
highest contact ratio possible within the constraints increase at lower loads may not even be perceptible
of the system being designed. Since vibration or but heavily modified gears will be noisy at offpeak
.-
* Total contact ratio is equal to the transverse contact ratio plus the face contact ratio.

33
AGMA Qll-A94

loads. For this reason, the actual modification ap- reduce the propagated levels. Often, these ribs can
plied will often be a compromise. be spaced so that they also contribute to improved
7.3.3.4 Gear accuracy cooling.
The second approach to housing design is more
It is usually true that a more accurate gear also has
quantitative. Utilizing the results of the finite ele-
less vibration, assuming appropriate tooth modifi-
ment analysis, the strain energy density of the entire
cations. If the gear mesh is exciting a resonance
housing can be evaluated. With this information in
somewhere in the system, nothing short of a
hand, thoseareas whichare highest in strain energy
uperfect” gear set will substantially reduce the
density, and thus the most likely candidate areas for
vibration and noise. In most other cases however,
vibration reduction, can be identified and modified.
improving accuracy will reduce the vibratory levels.
This is especially true when relatively low quality 7.3.3.7 Bearings
gears are being used but much less so for higher Vibration or noise generated by the bearings is gen-
quality gears. Among the most important character- erally overshadowed by that due to the gears and
istics that affect noise and vibrations are involute their interaction with the housing.
hollowness, lead variation, and control of profile Vibration created by rolling element bearings can
modification. sometimes be reduced by control of manufacture,
7.3.3.5 Design of shafts and location of through tight tolerance and surface finish. This can
bearings lead to an increase in bearing cost. Another
approach to reducing vibration may be to select the
The design of the shafts which support the gears
quietest bearings from a production lot. Many
and the location of the bearings which support the
bearing manufacturers utilize a sound checkas one
shafts are a critical concern for two quite different
of their quality control devices. The bearing to be
reasons. The first is the need to provide the gears
checked is mounted in a standard fixture and run at
with a support system which maintains their relative
a standard speed. its noise level is checked and
alignment. A misaligned gear system is subject to
those which exceed a given value are rejected.
higher unit tooth loads which not only degrade load
capacity but also increase the overall noise and 7.3.3.8 Tooth stiffness
vibration level of the box. Several factors affect tooth stiffness, such as pres-
In many cases, especially for high speed, light- sureangle, andtheratiooftooththicknessto height.
weight gears, the support shafts are frequently In general, the more flexibility the tooth possesses
sized based on deflection and not solely strength the lower the dynamic loading will be, and thus vi-
restrictions. The location of the bearing supports, bratory excitation will be minimized. Changing
as noted earlier, should be chosen so that they these parameters in the directions required to im-
coincide as nearly as possible to node points in the prove flexibility may compromise tooth strength.
shaft mode shape. This will ensure minimum 7.3.3.9 Backlash
vibration transmission to the housing and thus Sufficient backlash should be provided to avoid tight
minimum noise. mesh (drive and coast side contact simultaneously)
7.3.3.6 Design of housing under all load and temperature conditions. This
backlash should be obtained by tooth thinning and
There are two major approaches to housing design.
not by spreading the center distance since the for-
First, the housing design can be evaluated qualita- mer method does not affect contact ratio while the
tively. The design should be reviewed to ensure that latter does. Under conditions of reversing loads or
there is a rigid load path between each bearing during periods of zero load operation, excessive
location and the housing mounting points to reduce backlash will result in gearbox noise. The loss in
deflections. Large flat or gently curved areas on the tooth bending fatigue strength due to tooth thinning
housing surface should be avoided since these must also be considered; however, for most gear
areas tend to vibrate freely (like a drum) when systems the actual reduction in strength is very
excited. Ribs can be used to stiffen these areas to small.

34
 AGMA 911-A94

For very high speed gears or wide face gears with 7.3.3.12 Gear material
moderate to high speed, an increase in backlash, For steel gearing, the choice of a specific material
above standard values, may be required to reduce has little effect on thevibration level. If a non-metal-
the velocity of the air-oil mixture which is forced lic material can be utilized, however, it can have a
from the tooth space during meshing. Small in- significant effect. This is due to two factors. First,
creases in backlash can result in measurable im- the increased compliance of these materials de-
provements when the overall noise orvibration level creases the magnitude of the dynamic loads and
is dominated by this phenomena. Spur gears are second, the materials themselves are usually good
more susceptible to this phenomena because of the dissipative dampers. These factors combine to
large trapped volume which exists during meshing. make non-metallic materials a good choice when
the operating environment (temperature, load,
7.3.3.10 Root clearance
speed, lubricant compatibility, etc.) permits. A
The effect of root clearance is similar to that of secondary benefti of this approach is that such
backlash in that a sufficient amount must be gears when run in combination with a steel mate
provided to avoid any tip interference with the may be run with minimal or no lube.
mating gear. Additional root clearance (increased 7.3.3.13 Surface finish
tooth whole depth) above this value will have the The surface finish on the flanks of gear teeth has
following effects: only a small effect on vibration and noise level.
- Tooth bending stress will be increased since 7.3.3.14 Relative influence of factors
the beam length will be increased; Table 12 is a summary of the design approaches
discussed above showing the relative influence
- The smaller fillet radius in the root will also
each characteristic has on vibration and noise.
contribute to increased bending stress;
7.4 Analyzing vibration problems
-There will be a reduction in noise caused by the 7.4.1 The excitation mechanism
exit velocity of the air/oil mixture. This is due to
the reduction in the exit velocity provided by the The initial step in analyzing vibration problems is
increased clearance. In most cases, exit veloci- usually to define the excitation at each gear mesh.
ties are not high enough to be significant; The factors described in table 12 can all contribute
to this excitation.
-There will be an increase in tooth flexibility and Table 12 - Potential influence of design
thus an improvement in load sharing and error features on noise and vibration
tolerance.
Design feature Influence
7.3.3.11 Gear mesh phasing
Accuracy High
Sound transmission is accomplished through the Contact ratio High
propagation of waves which can be considered Load intensity High
sinusoidal. The addition of two sine waves with Phasing High
identical frequencies which are separated in phase Tooth profile modification High
would produce a third wave of the same frequency Alignment Medium
but with a third amplitude. While gear noise may not Bearing location Medium
be exactly sinusoidal it can be phased. Studies of Bearing type Medium
such phasing have proved that a significant reduc- Housing design Medium
tion in noise level can thus be achieved in applica- Tooth numbers Medium
tions having multiple load paths. Examples of Tooth stiffness Medium
methods to use phasing to reduce noise and Backlash Low
vibration are: tooth selection in multiple path Material Low
systems, unequal spacing of planet pinions, and Root clearance Low
change of double helical gear intersection point. Surface finish Low
AGMA 91%A94

A gear system transmitting power is also suscepti- loading can be applied. This method offers the
ble to torsional vibration. The inertia may be con- advantage that the parts are subjected to the
centrated as in the body of a gear or distributed as in operating dynamic conditions, so that results will
the shafting. Similarly, the elasticity may be concen- closely resemble field experience.
trated as in a coupling, or it may be distributed with 7.4.3.1.2 Siren
the inertia in the shaft sections. The excitation may
This is usually a non-running test in which the part
come from externally applied pulsating torques or
to be tested is excited by variable frequency air
from a fluctuating resistance to the steady rotation.
pulses. A stream of pressurized air is passed
However, in a geared system there is also an excita-
through a rotating serrated disk whose speed can
tion due to displacement which comes from the im-
be varied over the frequency range of interest. The
perfect transfer of motion (as described in 7.1 .l) be-
air is directed at a location which would excite the
tween the meshing gears. Having defined the gear
mode shapes of interest. Since several diierent
tooth excitations, a modified Holzer analysis can be
disks can be used with differing hole patterns,
used to calculate the dynamic tooth forces [9].
frequencies up to about 20 kHz can be investigated.
7.4.2 Response characteristics The response can be determined either by using a
7.4.2.1 Shaft response camera if there is sufficient motion or with strategi-
cally located strain gages. Since this method
Once the dynamic forces at the gear mesh have applies a low dynamic force to only one specific
been calculated, they can be applied to an analytical component, it is not usually practical to simulate the
model of the shaft system. The results are dynamic response of a complete gearbox by this technique.
forces at the shaft support (bearing) locations and
7.4.3.1.3 Impact
vibration mode shapes of the shafts. The objective
of the shaft analysis is to reduce the shaft response This is also a non-rotating test in which the
in general and to change the shaft geometry such component to be tested is supported in a manner
that the bearings are located close to node points for which will not affect its frequency response, and
the critical modes. then is struck with an impact device, typically a
hammer with a dynamic force gage, selected for the
7.4.2.2 Gear and housing response frequency range of interest. The response is
The final step in the analysis is the evaluation of the measured with accelerometers or strain gages
response of the gears and housings to the excita- placed at critical locations. By impacting the part in
tions transmitted to them. This task is usually various locations and monitoring the responses, it is
accomplished through the use of a finite element possible to determine natural frequencies and
program. The responses are obtained by applying mode shapes. However, since it is a static test, any
the appropriate forcing functions at the tooth contact change in frequencies or damping characteristics
points in the case of the gears or at the bearing due to the effect of rotation is not included.
locations within the housing. In the development of 7.4.3.1.4 Shaker
the model, the parts are supported at their normal
This type of test is similar to the impact test except
mount points by appropriate constraints [lo].
that an electrodynamic shaker head is mounted so
7.4.3 Test methods as to impart a force to the component being tested.
The force can be varied in frequency so that the
Many different methods of testing are used to deter-
response can be determined at critical frequencies,
mine vibratory characteristics. Each method is de-
either with accelerometers or other transducers.
signed to answer certain kinds of questions, but no
Since it is a non-rotating test, the same cautions
one test will resolve all vibration or noise issues.
apply as with impact testing.
7.4.3.1 Excitation methods 7.4.3.2 Response measurement
7.4.3.1 .I Running tests 74.3.2.1 Strain gages
If the parts to be tested can be operated under When one of the parameters to be studied is the
realistic conditions, simulated or actual service stress in the part during vibratory response, strain

36
 AGMA 911-A94

gages can be applied to locations which are addressed, corrective measures must center
expected to show high stresses. The gages are around modification to the existing hardware. Sev-
used to determine both the frequency response and eral of the more effective methods will be discussed.
the operating stresses. For the results to be useful, Table 13 - Vibration testing
the excitation method must simulate operating
conditions. Test objectives
7.4.3.2.2 Accelerometers
Excitation:
Since the application of strain gages can be diiicult Measurement g
and time consuming, accelerometers are often
used as the transducers. Similar results with
respect to frequency response and mode shape can Running:
be obtained except with no information relative to Strain gage yes X X X
stress. The availabilii of small, lightweight acceler- Accelerometer x x x x
ometers has made this technique very popular. Siren:
7.4.3.2.3 Powder pattern Strain gage no X X
Holography no X X X
This technique for determining modes and frequen-
Impact:
cies involves coating the part with a special powder
Accelerometer no X X
and exciting it with a variable frequency shaker. By
Shaker:
observing the patterns developed in the powder at X
Holography no X X
the part’s natural frequencies, the modes for those
Powder Yes x x
frequencies can be determined. It is also a nonrotat-
Strain gage no X X X
ing test, so effects of rotation are not included. The
part is normally excited during the observation by a 7.5.1 Isolation
shaker, since the air siren would interfere with the
patterns. Isolation means that excitation forces are prevented
from being transmitted from one part of the system
7.4.3.2.4 Holography to another. This interruption yields a reduction in the
This is a laser based optical technique of observing transmitted forces. It is most common to isolate
and photographing mode shapes. The components either the transmission from its supporting structure
are excited, usually by a shaker, at the selected or the gear rim from its blank and hub. The former
frequency. The holographic camera records a approach is generally easier to accomplish and is
hologram of the part which is vibrating during the therefore the most common.
exposure. The resulting image is covered with light 7.5.1 .l Gearbox mounting isolation
and dark fringes which are contour lines related to
Vibration isolators are often used to control the
the vibration amplitude. From this fringe pattern, the
transmission of high frequency energy across the
mode shapes can be determined and, if desired, the
mounting points to the supporting structure. This
pattern can be quantitatively evaluated to deter-
method involves the use of elastomeric mounts be-
mine vibration ampliiude at any point.
tween the transmission mounting pointsand its sup-
The approach is most useful for non-rotational porting structure. Reductions as high as 10 dB can
determination of modes, since obtaining a fringe be obtained through the use of rubber mounts hav-
pattern on rotating parts requires an elaborate ing a static spring rate equal to that of the supporting
setup. Table 13 lists some common combinations of structure at the gear box attachment point.
these techniques and shows the characteristics of
7.5.1.2 Gear rim isolation
each.
A second method of reducing vibration and noise is
7.5 Vibration/Noise reduction techniques
to ‘s olate the gear rim from its hub with an elas-
In cases where hardware is in existence before a tomeric insert. The elastomer will reduce blank ex-
noise or vibration problem has been recognized and citation and, consequently, shaft and housing exci-

37
 AGMA 911-A94

tation are also reduced. One of the difficulties with frictional damping to occur at the interface of the
this approach is the limitation on torque transmis- rings and grooves. Care must be taken to avoid
sion imposed by the strength of the elastomer. The having the rings become a wearing part and thereby
effect of the lubricant on the elastomer and the ac- introducing an additional mode of failure.
curacy of the position of the gear with respect to its
mate may also limit its use to non-critical, low load
situations. In those systems where load capacity is
secondary and vibration or noise is primary, this
method has been shown to work quite well.
7.5.2 Dissipation
7.5.2.1 Externally applied damping material
Application of damping material to the surface of a
structure by spraying, gluing, plasmaflamecoating,
Figure 13 - Typical damping ring
etc., may be effective. Since most of the straining
action (and thus energy conversion into heat) will be
confined to the layers closest to the structure, thin 7.5.3 Screens or barriers
layers of damping material will be more cost and A well constructed screen can shield a high fre-
weight effective than thick layers. quency source and result in a significant noise level
reduction. However, if the gear drive is operating at
Objections to the use of damping materials applied
avery low speed, it will produce IowfrequWcy noise
to the exterior surface of transmission housings
and the gap around the screen can act as a new
. include greater cost, added weight, concealment of
source and radiate sound into the space with little
cracks, and heat retention. Each of these must be
attenuation. Some noise will still enter the working
traded off against the potential benefits.
area via reflection from the adjacent walls as well as
There are many methods by which dissipative by transmission through the-screen itself. In this
damping may be achieved. The simplest of these is case, the use of absorbent materials, in conjunction
to fill a hollow gear shaft with an elastomeric with a screen, can prove quite effective.
material, or to provide constrained layer damping.
7.5.4 Absorbent materials
Dissipative damping is effective when constrained Sound absorbing materials serve the purpose of
layer damping is applied to gears or housings with minimizing reflection of sound waves. They are not
large flat surface areas which tend to “ring” when very effective at blocking the path of sound trans-
excited. mission. For this reason, they are usually used to
7.5.2.2 Damping
_ - rings
- alterthe sound characteristics of an area, and in this
manner, reduce the sound pressure level within that
Rotating parts such as bevel or spur gears have
area. This type of material is not normally used
many natural modes of vibration which can be
alone as a barrier between a source and a receiver,
excited within the operating range. It is often
since the sound pressure level would be reduced
impractical to redesign the part to avoid all of these
very little at the receiver. Some factors affecting the
frequencies, so some means of reducing the
absorption characteristics of a material are mass,
response is the next best solution. The use of
surface condition, pore size and structure, flow
damping rings is quite common in this situation.
resistance, thickness and the frequency of imping-
These can take the form of special snap rings
ing sound. Typical materials used are glass fiber
installed in grooves in the gear rim, or more complex
and certain types of polyurethane foam.
rings designed specifically for the purpose. Figure
13 shows an example of a typical damping ring for a 7.5.5 Enclosures
spiral bevel gear rim. The location of the grooves Reduction of airborne noise by the use of enclo-
and the shape of the ring are determined by the surescan be effective, but the noiseattenuation that
mode shapes of concern and by the need for the can be achieved is dependent upon the complete-

38
 AGMA 911-A94

ness of the enclosure. The noise reduction limita- from the stress index prediction. The reasons for
tion in the speech frequency range with typical disagreement between actual stresses and stress
acoustical enclosures and seals is about 25 dB, with index numbers can be caused by any number of
up to 35 dB obtainable through use of improved seal factors including the following:
configurations. Further reductions in noise level, up
- Size effects;
to 50 to 60 dB, can be achieved with fume-tight
- Fine pitch vs. coarse;
enclosures, such as those employed in some com-
- Blank configuration;
mercial helicopters and in some commercial trans-
port aircraft engine installations operating today. - Rim;
-Web;
Practical enclosures are limited in noise attenuation - support;
by unavoidable sound leaks in seams and access
doors. Not only do these enclosures impose - Dynamic effectslresonance;
considerable weight and maintainability penalties, - Temperature effects;
but they do not reduce the harmful effect of the - Loading conditions/prelubing;
accompanying vibrations which contribute to - Deflections of teeth, shaft, housings.
material fatigue and fretting at joints.
Thus the designer faces the dilemma of whether or
A summary chart showing the relative effectiveness not to use the stress index method or to conduct a
of the various methods discussed in this clause is more complex analysis, such as finite element
given in Table 14. analysis. This is not to say that the classical method
is inaccurate for it can be shown that the final
Table 14 - Noise and vibration application factor by the stress index method and
reduction techniques FEA are in agreement if the proper values for the
factors have been chosen.
I Relative effectiveness I
Airborne Local The conventional AGMA method permits calcula-
noise vibration tion of the maximum tooth tensile stress. The
minimum stress is assumed to be zero. Thus the
Mounting isolation Medium MedIHigh vibratory and steady stress is taken to be l/2 of the
Rim isolation Medium MedlHigh maximum tensile stress. The true minimum bend-
External damping Medium High ing stress in the gear is usually negative and occurs
Internal damping rings Low High prior to the initial point of contact for a driving gear
Screens Low Low and after the final point of contact for a driven gear
Absorbent material Low Low (see figure 14). Afinite elementanalysiscan predict
Enclosure MedIHigh Low the maximum and minimum stress by conducting a
series of static solutions of stress as the pinion and
8 Load capacity
gear are incrementally “rolled” through mesh. The
8.1 Introduction stress allowables for the finite element method are
8.1.1 Analytical considerations for fatigue generally taken to be the material allowables found
bending from material testing or from actual gear teeth test
results.
The conventional AGMA method of bending stress
calculation is in reality a stress index method of A design approach that has been employed is to
design. Production designs based on this method conduct the design using the classical AGMA stress
have countless hours of successful field operation. index method and then to conduct an optional finite
The allowables are based on this successful field element analysis to refine the design. A digital com-
experience and successful/unsuccessful bench puter analysis using AGMA equations can quickly
test experience. When the stresses are determined and efficiently compare many preliminary designs
experimentally by strain gaging gear teeth, it is whereas the finite element method is very tedious
found that the actual stresses may be quite different and time consuming. On the other hand, the FEA

39
39 39
39
AGMA Qll-A94

Load A A

Time
Maximum stress, thick rim (AGMA stress index)
Load

Load

Time

Minimum and maximum stresses, thin rim (FEA method)

Figure 14 - Different methods for determining tooth root stress
can more readily account for variables such as and web can be calculated by FEA. The user should
suppottstiSfnessand temperature effects. Account- satisfy himself that the gear blank construction is
ing for these factors permits the load distribution to representative of the basic theory embodied. Fig-
be determined. ures 14 and 15 illustrate the above discussion.

In aerospace gearing where the gear rims are

designed to minimum thickness, the gear rim and
web can have a profound effect on the bending
stress. Thin rims make the calculated stress
number too low, giving an optimistic estimate of life.
As the thickness of the gear rim is reduced, there is
a point where the stress in the gear rim itself is
higher than the tooth root bending stress. This is
generally not a good design condition because propagation Maximum tooth
cracks may propagate through the gear rim instead through tooth rim stress
of through the tooth. It is of course not desirable to 1 9
have any cracks which propagate, but assuming a Direction of propagation through rim
crack were to be present, it is desirable to control the Figure 15 - Directions of crack propagation
direction of propagation. in gear teeth
Cracks through the gear rim should be avoided be- 8.12 Design criteria
cause they can cause sudden and catastrophic loss Prior to beginning the gear analysis several design
of the mesh without warning since chips are usually parameters must be established. In aerospace
not generated by this type of fracture. Hence the gearing, the design is usually based on three
stress in the root of the tooth should be lower than considerations: bending fatigue, surface
the tooth bending stress which is usually maximum compression (Hertz stress), and scuffing (scoring)
at a position near the TIF. Stresses in the gear rim resistance. Each of these conditions must be

40
 AGMA Wl-A94

checked independently for adequacy of design. For w is gear ratio;

aerospace designs which incorporate condition
np is pinion speed, rpm.
monitoring, bending tooth breakage probably has
the most severe consequence of the three failure NOTE:K= 500400 istypicalforcontinuousoperation
modes whereas pitting and scuffing are durability of carburizedaerospacegearing.
type failures.
The teeth and diametral pitch can be selected for
Durability type failures can cause gearbox removals bending. The gearset scuffing resistance is then
but are not catastrophic unless the gearbox is per- analyzed.
mitted to operate in that condition for a sufficient 8.1.3 Design life considerations
time period in which the surface breakdown degen-
erates to the point where the mesh is lost. Pitting Of primary importance in the preliminary design are
and scuffing are a phenomenon of the mesh while reliability requirements. The life can be calculated
using Miner’s cumulative damage theory (see
bending is individual to the pinion and gear, there-
fore both members must be analyzed for bending. Appendix B of ANSVAGMA 2001-B88 for explana-
Optimization of scuffing and a bending stress bal- tion of Miner’s Rule). The gear designer must have
ance or life balance between pinion and gear can be a spectrum of estimated operating power, rpm, and
achieved by varying addendums and dedendums percent time. This usage spectrum should be
established by analysis of the operating environ-
and/or tooth thicknesses of both members.
ment and intended use of the gearbox being
In aerospace designs, it is desirable to achieve a designed. Even in the case of relatively slow speed
balanced tooth design. That is to say that the life in drives, the cycle build up is very rapid. Unless
bending, contact, and scuffing resistance should be operation at the highest powers is for very limited
optimized. This is not always possible for a number durations, finite life design becomes impractical.
of reasons, but the gear designer generally has On the other hand, if the high power condition is truly
control of basic parameters such as the number of transient in nature and the rpm is relatively low, finite
teeth, diametral pitch, and center distance. In life design can achieve reduced size and weight. To
general, the gearset is designed for Hertz stress (for conduct a finite life design, material stress vs. cycle
single external mesh) as follows: curves must also be known (see ANWAGMA
2001-B88 for allowable stress and life factors). If it
315ooP (V&+ 1)3
C2F = ...(9) is desired to have unlimited life, the gearing can be
A- nP “G designed for limit load or the maximum load
6.923 x lo8 P (mG+ 1)3 expected in service.
62F = .*..(9M)
8.2 Spur, helical and bevel gear tooth breakage
’ nP mG
and surface durability
From equation 3.1 of ANSVAGMA 1012-F90, The bending strength of gear teeth is a fatigue
phenomenon related to cracking at the tooth root
C= . ..(lO) fillet. See ANSVAGMA 110.04-1980 for a more
complete discussion.
and if F is set equal to d, and substituted in 9, the The basic theory assumes that the tooth is a
cubic equation can be solved ford, (and F), where cantilever beam fixed at its base (the gear rim) with
c is center distance, in (mm); a reduction in tensile stress caused by the
compression of the radial component of the
dp is operating pitch diameter of the pinion,
transmitted load. A stress concentration factor is
in (mm);
applied because the tooth is subjected to alternate
F is face width, in (mm); cycles of maximum to zero stress. Load sharing
K is contact load factor for pitting between adjacent teeth in contact is calculated.
resistance, lb/in* (N/mn?); The pitting of gear teeth is a fatigue phenomenon
P is transmitted power, hp (kVV); caused by repeated application of surface com-

41
AGMA 91%A94

pressive stresses as the teeth roll through mesh.

Initial pitting and destructive pitting are illustrated w* G cs cnl Cf
and discussed in ANSVAGMA 110.04. SC = s J C, dTe I
. ..(12)

where
8.2.1 Fundamental bending stress formula
S_ is contact stress number, lb/in* (N/mm2>;
(Refer to ANSVAGMA 2001-888 for spur or helical dp is elastic coefficient, W (m );
gears or ANSVAGMA 2008-A86 for bevel gears)
ca is application factor for pitting resistance;
cv is dynamic factor for pitting resistance;
5 --v wt Ka pd Ks Km KB
...(n)
‘-K,,K F J cs is size factor for pitting resistance;
dP is operating pitch diameter of pinion, in
s _ Wt Kca 1 5 Km KB . ..(ll M)
(mm);
t ?TlF
KVKX J Fe is effective or net face width of narrowest
where member, in (mm);
% is load distribution factor for pitting
is calculated tensile bending stress number
resistance;
at the root of the tooth, lb/in* (NImrr?);
Cf is surface condition factor for pitting
is tangential tooth load, lb (N);
resistance;
is external application factor for bending
strength. This factor takes into account the I is geometry factor for pitting resistance.
effect of any externally applied load in ex- 8.2.3 Dynamic factor
cess of the nominally applied torque load; A dynamic factor is applied to the tooth bending and
KB is the rim thickness factor for spur and heli- contact stress to account for increased dynamic
cal gears (use 1 .Ofor bevel gears); tooth load. lnertias and spring rates of the
KV is dynamic factor for bending strength. This transmission system influence the dynamic tooth
factor takes into account the effect of gear load as does operating speed compared to reso-
tooth quality, as related to speed and load; nance speed. Gear tooth tolerances have a large
is size factor for bending strength. This effect on dynamic load. As the gear tooth rolls
factor takes into account nonuniformity of through mesh, teeth are engaging and disengaging
material properties; as a function of the tooth geometry. For example in
is load distribution factor for bending a conventional mesh, there are alternate cycles
strength. This factor modifies the rating where the load is carried by varying numbers of
equation to reflect the non-uniform dis- teeth. In the worst case, since the spring rate of the
tribution of load across the face of the tooth; mesh is considerably different when one tooth is
meshing and when two teeth are in mesh, the
K, is tooth lengthwise curvature factor for
tangential deflections are different at different times
bevel gear bending strength (use 1 .O for
during the mesh cycle.
spur or helical gears);
J is tooth geometry factor for bending This change of mesh stiffness with time causes the
strength; mesh point tangential deflection to vary with time at
the tooth mesh frequency. The resultant change in
pd is diametral pitch, in-l;
tangential deflection caused by a change in velocity
m is module, (mm);
from the theoretical constant velocity is sometimes
F is face width of gear for which bending referred to as transmission error or TE. Transmis-
stress is desired, in (mm). sion error induces a vibratory forcing function at the
mesh point and is the root cause of dynamic tooth
8.2.2 Fundamental contact stress formula
load and noise.
Refer to ANSVAGMA 2001-888 for spur or helical Profile modifications on gear teeth are required to
gears or ANSVAGMA 2008-A86 for bevel gears. accommodate tooth mesh deflections and other

42
 AGMA 911-A94

variables. Modifications can also have an effect on Dimensional allowances that affect load distribution
tooth spring rate. The modification specified is are as follows:
optimum for only one load condition, usually some - accumulation of tolerances;
high load, high speed operating condition. When - alignment of the axes of rotation;
operated at other load conditions, the gearset may
- bearing clearances;
produce higher dynamic loads. Excessive profile
modification can actually reduce mesh contact ratio - profile accuracy;
and introduce error in action. - lead;
- crowning.
A complete discussion and calculation of transmis-
sion error and resultant dynamic load is beyond the For a particular application, each of the above
scope of this guide. A simplified dynamic factor can influences should be evaluated as to its effect on
be obtained by referring to ANSVAGMA 2001-S88 load distribution. Modem finite element methods
for spur or helical gears or ANSVAGMA 2003-R% can be used if the pinion and gear are modeled as
for bevel gears. separate parts and the tooth load induced by torque
applied to the shafts through the use of gap
8.2.4 Siie factor elements or with three dimensional contact model-
Size effect factor has been established to account ing. The finite element technique is suitable for
for detrimental effects as the volume of material in calculating load distribution because all of the
the gear set increases. This can be thought of as a factors which influence deflection and manufactur-
chain analogy. The more links in the chain (units of ing deviations can be evaluated.
stressed volume), the higher the probability of a 89.8 Geometry factor, J
defect in one of the links. For aerospace spur and
The geometry factor is used to calculate gear tooth
helical gears the gear size has not been established
bending stress. The geometry factor takes into
to have a detrimental effect and a factor of 1.O is
account bending of the tooth as a cantilever beam
used. For bevel gears a size factor is used and can
as well as the compression caused by the radial
be determined by referring to ANSVAGMA
component of the normal tooth load. The maximum
2006-A86.
stress occurs where the load is at the highest point
8.2.5 Load distribution factor of single tooth contact for a conventional low contact
The load distribution factor accounts for ratio gear tooth. For high contact ratio gears a load
non-uniformity of load across the lines of contact on sharing factor is applied to account for distribution of
the teeth. The deviation from the theoretical uniform the transmitted load among the teeth.
load is caused by operating deflections and Internal gears are often neglected in texts but are
dimensional allowances. important in aerospace design because of heavy
reliance on planetaries which have an internal ring
Deflections along the teeth are caused by the fol-
gear. Annex A shows a procedure for calculation of
lowing:
the geometry factor for an internal gear mesh. The
- elastic deflections of the gear elements; procedure is suitable for calculation of either
- shaft deflections; internal or external gears or mates. The equations
- bearing deflections; are generalized by the use of “signed integers”
which are +l for external gears and -1 for internal
- housing deflections;
gears.
- foundations which support the gear
elements; Helical gear geometry factors are based on a mean
normal tooth section and also account for load
-thermal expansion from non-uniformity of
sharing. After the mean normal section is estab-
temperatures;
lished, the geometry factor iscalculated in the same
- differential thermal expansion from manner as for a spur gear. See Annex A for spur
different materials; gear geometry factors. Further data on the
- centrifugal effects. calculation of spur and helical gear geometry

43
 AGMA 911-A94

factors is shown in AGMA 908-B89. For bevel gear 16. For highly reliable aerospace design, a
geometry factors, see ANSVAGMA 2003-A86. reliability of 3 standard deviations has been used in
8.2.7 Allowable bending stress the past (or 3o). This results in a reliability of
0.99875 and a reliability constant of 0.7. Thus the
The allowable bending stress is calculated as an working 3o allowable is found by multiplying the
allowable stress index number and represents the mean stress by 0.7.
allowable stress when the stresses are calculated
by the procedure outlined above. The allowable From this discussion it is seen that there can be
stress number is a function of the material and heat different allowable design stresses for the same
treatment used as well as the desired life, design and that each stress will have a
temperature of operation, and reliability desired. corresponding reliability associated with it. It is up to
The relationship is given by: the designer to establish reliability goals before the
design begins so that allowable stress can be
Swt Ct KL determined. The allowable stress numbers shown
I- ...(13)
KTKR in ANSVAGMA 2001-B88 are based on a reliability
where of 1 failure in 100 at 1Oscycles. This corresponds to
n = 2.326 (number of standard deviations
sWt is working bending stress number, lb/in*
corresponding to 99% reliability). Thus for a spur or
(N/mm*);
helical gear with v = 0.1, the reliability constant for
S is allowable bending stress number, lb/in* 99% reliability is 0.7674 and for a bevel gear with
at (N/mm*); v = 0.156, the reliability constant is 0.6371. To
determine the reliability factor for multiplication of
KL is life factor for bending strength;
the stress index:
K, is temperature factor for bending strength;
=- R99 . ..(15)
KR
is reliability factor for bending strength. KR
Rdesired
The reliability factor accounts for the normal statisti-
cal variations found when materials fracture. From where
the analysis of probability, for a normal distribution,
R99 is reliability constant for 99% (0.7674
the reliability constant is given as:
for spur or helical gears and 0.6371 for
...(i 4) a bevel gear);
-Rel = 1-nv
where Rdesired = 1-nv = reliability constant for desired
reliability.
Rel
is reliability constant;
The allowable bending stress numbers and factors
n is number of standard deviations; are shown and discussed in ANSVAGMA
V is coefficient of variation = standard 2001-888 for spur and helical gears and in ANSI/
deviation/mean. AGMA 2003-A86 for bevel gears.
It has been found experimentally that for steel, the
Reliability (or probability) is related to the number of
coefficient of variation is approximately 10% (v =
standard deviations by the term:
0.1). This number can be used for spur and helical
n
gears but the variation in spiral bevel gears has
been found to be higher because of problems & (in (...16)
Rel =
encountered in shimming or other dimensional s
variables which influence gear tooth patterns. A
standard deviation of 0.156 has been calculated for Unfortunately, there is no closed form solution to
bevel gears using data from a large number of test this integral. However, it may be solved by a
and field bevel gearfractures. A plot of reliabilityvs. numerical approach. For a numerical approach to
number of standard deviations is depicted in figure calculate reliability, knowing the numberof standard

44
 AGMA 91 l-A94

deviations, n: bending stresses. The allowable contact stress

numbers and factors are shown and discussed in
Rer= 1 - ANSVAGMA 2001-B88 for spur and helical gears
and in ANSVAGMA 200%A86 for bevel gears.
(...17)
8.3 Spur, helical, and bevel gear scuffing
where
(scoring) - flash temperature index
1
t 8.3.1 Fundamental flash temperature index
= l+ 0.2316419n
formula
Cl = 0.319381530 The fundamental scuffing (scoring) formula is
c:! = -0.356563782 derived by the method proposed by Blok which
calculates a temperature at the gear mesh point as
c3 = 1.781477937 the gear bulktemperature added to the temperature
rise in the mesh. The temperature rise in the mesh
c4 =- 1.821255978 is referred to as the flash temperature index.
c5 = 1.330274429 Appendix A of ANSUAGMA 200%B88 gives a
detailed discussion of the flash temperature index
method for spur or helical gears, but there is no
n = number of standard deviations
(horizontal axis of figure 16) generally accepted AGMA method for bevel gears.
For bevel gears, the flash temperature index
Rel= reliability (vertical axis of figure 16) method outlined by Gleason has been widely used.
Both the Gleason and AGMA methods are based on
By the above method, Relwill be accurate to 4 deci- Blokand Kelley and the general form of the equation
mals. is as follows:
8.2.8 Allowable contact stress
tc = tM + tfl . ..(19)
The allowable contact stress is a function of tc = t&f + t, c 17.78 . ..(19M)
parameters similar to those used for allowable
bending stress number and in addition includes a where
hardness ratio factor. The relationship is given by:
tc is contact temperature in “F (“C);
?M is bulk temperature in OF(“C);
SWC sac cL cH ..I418)
I tn is flash temperature in “F (“C).
‘T cir
where
8.33 Film thickness method
%C is working contact stress number in pounds
per square inch (megapascals); The film thickness method is based on the work of
sac is allowable contact stress number in Dowson and Higginson. The minimum oil film thick-
pounds per square inch (megapascals); ness is calculated under load and is divided by the
composite surface roughness which is an average
is life factor for pitting resistance;
surface roughness of the pinion and gear. When the
is hardness ratio factor for pitting resis- ratio of film thickness to surface roughness, h, is 1.O
tance; or greater, the surface asperities cannot touch and
cT is temperature factor for pitting resistance: there is no scuffing (scoring) or wear. When the
is reliability factor for pitting resistance. pitch line velocity of the mesh is low [PLV < 1500 ft/
cR min (7.6 m/s)], acceptable values of surface film
To calculate CR , use the equations forKR shown in thickness and h are achieved through the use of
8.2.7. For bevel gears, a lower coefficient of varia- high viscosity oils. The calculations are summa-
tion has been observed for contact stresses than for rized in ANSVAGMA 2001-B88 in Appendix A.

45
AGMA 91%A94

*a 6o
r=
%
yg 407
cc
s
20

10

5-

2
1
0.5

5 4 3 2 1 0 -1 -2 -3
n
Figure 16 - Reliability versus number of standard deviations

46
 AGMA 911-A94

9 Gear materials and heat treatment the material must endure. Mechanical properties of
the material determine the allowable stress levels for
Aerospace gears are manufactured from appropri-
the application.
ate materials to perform under the imposed operat-
ing conditions for the life of the gear application. 9.2.1 Hardness
Gear material specifications are determined by the
The strength properties are closely related to
requirements of the application for mechanical
material hardness for ferrous materials. Hardness
properties, material quality, dimensional stability,
indicates the strength of the material and resistance
hardenabilii, and manufacturing characteristics.
to tooth bending failure. Surface hardness provides
Heat treatment is specified to achieve the required
resistance to gear wear, pitting and scuffing.
mechanical properties of the gear materials.
Ferrous and non-ferrous materials are used for 9.2.2 Fatigue strength
aerospace gearing. Steel alloy materials are used in Contact and bending fatigue strengths predict the
most primary gear applications for strength and number of cycles that a gear can endure at a given
durability. Stainless steels are used in special stress level before surface pitting or tooth fracture
applications for corrosion resistance. Bronze alloys occurs. Contact and bending fatigue strengths are
are occasionally used in worm gearing for wear influenced by hardness, microstructure, material
resistance and reduced friction coefficient. cleanliness, surface conditions and residual stress.

9.1 Class and grade definitions 9.2.3 Tensile strength

Aerospace gearing is divided into three classes de- Tensile strength is the maximum possible tensile
pending on the nature of the intended application. stress before fracture occurs. Tensile strength is
considered as a mechanical property used for rela-
Gear materials are specified by grade of metallurgi-
tive comparison of materials. Hardness is recom-
cal control factors and allowable stress numbers ac-
mended in lieu of tensile strength for specifications
cording to ANSVAGMA 2006B88 and ANSVAGMA
in gear manufacture.
2003-A88 The three grade numbers are: Grade l-
moderate quality, Grade 2 - superior quality, and 9.2.4 Yield strength
Grade 3 - Premium quality.
Yield strength is the maximum possible stress
Gear application classes are described with the ma- before permanent deformation occurs.
terial grade numbers that are used as follows:
9.2.5 Toughness
a) Typically Grade 3 material is used in main
drive system components, where failure of a Toughness is a measure of ability to absorb impact
gear could result in the loss of the vehicle or energy and is important for high impact or low tem-
endanger operating personnel or passengers; perature applications. Brittle fracture may occur in
b) Typically Grade 2 or 3 material is used where high strength materials as a sudden failure or rapid
failure of a gear may render the primary system crack propagation below the tensile strength due to
inoperative, but where a secondary system can low fracture toughness properties. Toughness of
be engaged to perform the same function; steel gearing is adversely affected by the following
c) Typically Grade 1 or 2 material is used where factors:
failure of a gear may affect mission capability or - Low temperature;
an auxiliary system but will not result in the loss - Improper heat treatment causing defective
of a vehicle nor endanger operating personnel microstructure:
or passengers.
- High sulphur content;
9.2 Mechanical properties - High phosphorus and embriiling grain bound-
The particular design configuration and duty cycle ary precipitates;
loads of the application determine stress levels that - Non-metallic inclusions ;

47
 AGMA 91%A94

- Large grain size; The surface is often required to have high hardness
- Absence of nickel alloying element ; and high strength, while the core is required to be
tough for impact resistance. A correctly hardened
- Stress concentrating notches, fillets, tool steel gear will consist primarily of tempered
marks or rough surface finish.
martensite. Furthermore, a gas carburized gear will
9.3 Cleanliness contain a high carbon tempered martensite case
with a low carbon tempered martensite core.
Cleanliness determines the extent of homogeneous Excessive case carbon content can result in
material properties. Alloy steel manufactured with retained austenite if not transformed and can also
electric furnace practice is commonly vacuum lead to the formation of undesirable carbide
degassed, inert atmosphere (argon) shielded and networks at grain boundaries. Improper hardening,
bottom poured to improve cleanliness and reduce such as quenching, can result in undesirable bainite,
gas content. Reduced non-metallic inclusion con- free ferrite or pearlite, in the from of banding.
tent improves transverse ductility and impact
strength. Further refinement by vacuum arc remelt- 9.6 Hardenability
ing o/AR) or electroslag remelting (ESR) reduces Hardenability of steel is the property that determines
gas and inclusion content more for improved fatigue the hardness gradient produced by quenching from
strength. Cleanliness requirements of alloy steels the austenitizing temperature. The asquenched
are controlled according to AMS 2300 (premium surface hardness is dependent primarily on the
aircraft quality steel) and AMS 2301 (aircraft quality carbon content of the steel and cooling rate. The
steel). depth to which a particular hardness is achieved with
a given quenching condition is a function of the
9.4 Heat treatment
hardenabilii due to carbon and alloy content of the
Most wrought ferrous materials used in aerospace steel.
gearing are heat treated to meet. hardness and
mechanical property requirements. Gear blanks are 9.7 Dimensional stability
generally annealed and normalized to produce Improper heat treatment processes cause dimen-
homogeneous microstructure for uniform machin- sional distortion and possible cracking. Distortion is
abitii and improved response to subsequent heat caused by mechanical and thermal stresses with
treatment. Quenching and tempering increase phase transformation during quench and tempering
material hardness and strength properties. Case due to variations in: 1) section thickness, and 2) the
hardening increases surface hardness and strength duration at transformation temperature.
while maintaining a softer core for toughness.
9.6 Pre-machining stock removal
9.5 Microstructure A specified minimum thickness. of surface stock
The microstructure is the material structure should be removed from ferrous gear forgings to
observed at 100X or higher magnification and eliminate decarburization, seams, and other surface
reveals the constituents of the material. The imperfections.
constituents include, but are not limited to,
martensite, ferrite, pearlite, and bainite. The 9.9 Ferrous gearing
microstructure also reveals grain size, carbides, Ferrous materials for aerospace applications are
carbide networks, and retained austenite. These primarily wrought alloy steels. Wrought steels are
constituents are a result of the heat treat process mechanically worked to form round stock, flat stock,
and can help determine if a heat treat process was or forgings. Anisotropic mechanical properties
done correctly or incorrectly. lt should be noted that (tensile ductilii, fatigue strength, and impact
on a cross section, the microstructure near the strength) vary according to the direction of hot
surface may be dierent from the microstructureof working or inclusion and grain flow. Improved steel
the core. cleanliness improves the transverse and tangential

46
 AGMA 911-A94

properties of forged steel that approach, but do not 9.11 Material grades and heat treatment
equal, longitudinal properties.
Common gear materials and heat treatment used in
Other ferrous materials may include H-series tool
aerospace applications are shown in table 15.
steels or austenitic, marten&c, and precipitation
hardening stainless steels for special requirements.
Powdered metal materials may be used in certain 9.12 Gear surface hardening
non-critical applications.
Most aerospace power gears are produced with
9.10 Non-ferrous gearing heat treated alloy steels and surface hardened to
Copper base (bronze) gears are used in worm gear provide tooth bending strength and resistance to pit-
applications with steel worms to improve wear ting and wear. Carburizing is the primary method of
resistance and reduce the coefficient of sliding surface hardening gear teeth. Nitriding is an alter-
friction. Manganese bronze and aluminum bronze nate surface hardening process specified where
have higher strength than phosphor or tin bronze carburizing and quenching would cause excessive
materials. distortion.

Table 15 - Typical aerospace gear materials

Typical hardness
AMS Heat Typical
Material treatment* Case, Core, applications
SPec HRC** HRC
Alloy St-L .
AISI 9310 626516260 58-62 3442 Main drive,
Accessory, actuators
4330M 6427 C-H 58-62 4248 Actuators
VASCO X2M”’ N/A 58-62 3844 Main drive,
High temperature
HP 9-4-30 6526 58-60 48-52 Actuators
PYROWEAR 53*** 6308 C-H 59-64 3642 Main driie,
High temperature
M50NiL 6278 C-H 58-62 35-45 High temperature
CBS600 6255 GH 58-62 3442 High temperature
Nitralloy 135M 6471 TH-N 60-64 3442 Accessory drive
Niiralloy N 6475 TH-N 60-64 38-44 Accessory drive
‘AISI 4340 6414 TH-N 48-53 27-35 Accessory drive
AISI 4340 6414 TH Accessory drive
300M 6419 TH 52-55 Actuators
Stainless steel:
PH13-8 MO 5629 PH Accessory drive
Custom 455 5617 ST Actuators
Bronze: Worm gear
C63000 4640 ST (1OOHRB) Actuators
NOTES- * Rockwell hardness scales (HRC and HRB) are shown for
C-H = Carburize and harden direct comparison only. In general, those scales are not
TH-N = Through harden and nitride specifically recommended for measurement where other,
TH 5: Through harden more accurate hardness scales are commonly used.
= Precipitation harden
E = Solutionheat treat -* Proprietary material designation.

49
AGMA 911-A94

However, nitriding does not increase tooth bending After carburizing the gear is cooled slowly in the
strength as much as carburizing. Other methods of carburizing medium or protective atmosphere to
selective direct hardening may be used for less criti- control decarburiiation. Next the gear may be
cal gear applications with medium carbon alloy reheated to 1200°F (649OC)for subcriiical anneal to
steels. reduce sudden changes in carbon content across
grain boundaries in the microstructure. After cooling
The minimum depth of surface hardened case in
again, the gear is reheated to 1500°F (816°C) to
gear teeth is required to resist: (1) sub-surface
austenitize the carburized steel. When the gear is
shear stress caused by tooth contact pressure and
quenched in oil, much of the austenite transforms
(2) root fillet tensile stress caused by tooth bending.
into hard and brittle martensite. A deep freeze cycle
Minimum required case depth is typically specified
may be used to complete the transformation and
as the greater value of: (1) two times (2x) the depth
minimize retained austenite. Finally the gear is
of maximum sub-surface shear stress at the pitch
tempered at 300-360°F (149-182OC) to reduce
line, or (2) 10 percent of the tooth thickness to
brittleness and microcracking.
accommodate sub-surface fillet tensile stress.
Refer to ANSVAGMA 2001-B88 for carburiied or The carburizing and hardening cycles produce a
nitrided case depth recommendations. very hard, mattensitic layer on the surface with a
less hard, tough core. In addition, the carburiiing
The case must not be so great as to result in brittle
and quenching processes cause high residual
teeth tips and edges, or high residual tensile
compressive stresses in the surface for increased
stresses in the core. Maximum case depth at the
material strength and resistance to fatigue failure.
tooth tip should be limited to 56 percent of the tooth
top land thickness when possible.
9.12.1.1 Carburizing process control
The effective case depth for carburized and
hardened gears is defined as the depth below the Gas carburizing atmospheres can be in equilibrium
surface at which the hardness is 50 HRC equivalent with a wide variety of carbon content. Proper surface
by conversion from microhardness. carbon content must be maintained to achieve the
required properties after heat treatment. Surface
The effective case depth for induction hardened
carbon content should be maintained between 0.6%
gears is defined as the depth below the surface at
carbon and 1.0% carbon. Less than 0.6% surface
which the hardness is 10 HRC points below the
carbon will lead to low surface hardness which may
specified minimum surface hardness.
not meet final requirements. Surface carbon con-
For nitrided gears, case hardness is specified as to- tents over 1 .O% may lead to excessive retained aus-
tal case depth, and is defined as the depth below the tenite and/or grain boundary carbide networks. With
surface at which the hardness has dropped to 110 higher surface carbon contents it may be necessary
percent of the core hardness. The practical limit for to sub-zero cool to transform retained austenite to
maximum nitrided case depth is 0.040 inch (1 mm). martensite.
The gas carburizing atmosphere is controlled to
9.12.1 Carburizing
obtain the desired carbon content in the surface of
In a typical process for alloy steel, gas carburiiing the gear. It is diicult to measure carbon potential
diffuses carbon into steel from a hydrocarbon gas at- directly. Cften one or more components of the gas
mosphere while the gear is heated to 1660°F mixture, such as CO,, CH4, H20, or 02 are
(899OC)or higher in a furnace. Carbon diision into measured and the other component (usually CO) is
the steel is affected by (1) timeof exposure, (2) tem- assumed to be in proportion. However, the un-
perature in the carburizing atmosphere, and (3) per- measured component may vary due to variable gas
centage of carbon in the atmosphere. The higher source, air leak or carbon build up. If CO, hydrogen
concentration of carbon near the surface provides or other componentsvary,the reactionswill shii and
maximum local hardness. make the carbon potential relationship inaccurate.

50
 AGMA Qll-A94

Shim stock (a thin, flat material sample) can be used mm). Some critical applications require removal of
to accurately assess the initial carbon potential of the white layer.
the carbutizing gas. However, conditions may be Even with the two-stage process, nitriding is slow,
different later when the gears are exposed in the taking about ten times as long as carburizing to
furnace. produce a specified case. The nitride process
produces a very hard case with minimum distortion.
9.12.1.2 Other carburizing methods Volume increases during nitriding and causes favor-
Vacuum carburizing and plasma carburizing pro- able compressive stress to build up in the case.
duce results similar to gas carburiiing. Vacuum However, nitrides tend to accumulate more at tooth
carburizing is often faster because vacuum carbu- edges.
iiing is carried out at higher temperatures [I 80&
9.12.2.1 ion nitriding
1950°F (982-l 066 “C)], rather than the
1650-l 800°F (899-982 “C) typically used ip gas Ion nitriding or plasma nitriding is similar to plasma
carburizing. Plasma carburiiing occurs in a DC carbutizing in that a plasma is formed around the
electrically charged furnace with the gears acting as work during treatment.
the cathode. A plasma is formed around the gears 9.12.3 Selective direct hardening
which enhances the absorption of carbon at the
Selective direct hardening produces a hard case by
carburizing surface of the gear.
heating the surface layer above the austenitiiing
9.12.2 Nitriding temperature and rapidly quenching, while leaving
the core in the original condition. A medium carbon
Nitriding is an alternate case hardening process steel is used with the required carbon already in the
often specified for gears when distortion would be steel and heat treated to proper core hardness.
difficult to control if the gears were case carburized Since a large proportion of the part remains cool,
and quenched. In nitriding, nitrogen is introduced thus stabilizing the material, distortion is much less
into the surface of the steel at relatively low than it would be if the entire part were heated. The
temperature [925-1050°F (496-566 “C)] from a higher carbon containing material may make
nitrogen containing atmosphere such as ammonia. machining more diicult than a carbuiiing grade of
A hard case is produced by the formation of hard -
steel.
nitride compounds in the surface making quenching
Induction hardening is the one method of selective
unnecessary.
directive hardening that may be suitable for some
Special steels are needed that contain elements production aerospace gears. However, the induc-
such as aluminum or chromium to form hard nitrides tion hardening process should be developed to
during treatment. The steel is nitrided in the hard- control residual stresses and annealing between the
ened and tempered condition. The process is case and core. Two other surface hardening
controlled by adjusting the dissociation of the processes, laser heat treatment and electron beam
ammonia. More often the Floe process is used which heat treatment, are also being developed.
is a double-stage process analogous to the boost-
diffuse cycle in carburizing. In the first stage, 9.12.3.1 Induction hardening
dissociation level is controlled at 1530% by using a Induction hardening is achieved by using an
temperature range of 925- 975”F( 496-524 “C) alternating current in a work coil that surrounds the
producing a white nitride layer which is diised in part to be heated. An alternating magnetic field is
the secpnd stage by increasing the dissociation to established that induces a potential in the part
80-85%. The high dissociation can be achieved by causing a current to flow in the closed circuit.
increasing the temperature to 1025-l 050°F Heating is produced by the resistance to the induced
(552-566 “C) and using an external dissociation. current. The rate of heating depends on the strength
The remaining white layer should be limited to a of the magnetic field. The depth of the field varies
maximum allowable depth of 0.0005 inch (0.013 inversely with the frequency of alternation. The

51
AGMA 91%A94

higher the frequency, the more shallow the heating stresses help counteract tensile stresses produced
effect. during tooth loading and thus increase the expected
If a circular coil is used to heat a gear then the tips of life. A part that has been carburized is heated above
the gear are coupled closer to the coil and thus they the austenitizing temperature and then quenched.
heat more, resulting in a deeper case depth at the The surface cools faster than the center of the
tooth tips. section because heat is abstracted from the surface
by the quenching media. The net result is that
After the heating is complete, the current is turned off transformation of austenite to martensite starts at
and the part is quenched by synchronized jets of a the case/core interface with an expansion as
quenching fluid, usually water-based. martensite is formed. The case is the last material to
transform and expand to martensite, causing
9.12.3.2 Laser heat treatment
compressive stresses because the core has already
Laser heat treatment is a surface-hardening proc- transformed and restrains the case.
ess in which laser energy heats the surface above The conditions are different in selective hardening,
the austenitiiing temperature. When the source of but the results are similar. Energy is transmitted
energy is removed, the part self-quenches by quickly into the surface resulting in a surface layer
diffusion of the heat into the mass of the part. The heated above the austenitizing temperature. This
laser causes a steep temperature gradient due to layer will later become the hardened case. When the
the extremely rapid heating rate. As the rate of heat energy is turned off, rapid cooling progresses and
input increases, the depth of hardening is reduced, again the case is the last to transform and the
since the temperature gradient becomes steeper restraint induces residual compressive stresses as
and the surface temperature must be limited to avoid the surface expands during transformation from
melting. austenite to martensite.
To spread the laser energy over required coverage
9.12.5 Dimensional problems caused by heat
area, the beam is usually defocused. Alternatively
treatment
oscillating optics or integration optics with a faceted
mirror may be used. Heat treatment tends to cause more quality prob-
lems than any other manufacturing step. Heat
Laser beam penetration is controlled by power level treatment causes dimensional changes due to
and rate of beam traverse. Penetration increases volume change resulting from phase transforma-
with increased power and decreases with increased tion. Distortion occurs from a combination of aeo-
beam traverse rate. metric factors and uncontrolled stress relief. These
two factors acting together often cause unpredict-
9.12.3.3 Electron beam heat treating
able results. Variables that contribute towards the
This method is similar in principle to laser heat treat- dimensional changes include:
ing, except that heating is achieved by an acceler-
- Variations in material composition;
ated stream of electrons instead of a light or infrared
beam. When the electron beam is turned off, the part - Residual stress differences;
self-quenches. The electron beam heat treat pro- - Size of part (within tolerance range) before
cess occurs in a vacuum environment. This require- heat treatment;
ment introduces some complications into the f&k- - Surface condition;
ing. The electron beam is manipulated by magnetic - Cart&zing heating cycle;
coils. - Carburizing atmosphere control;
9.12.4 Residual stress patterns - Depth of case;
- Quenching parameters;
One advantage of case carburized parts is that when
the treatment is properly carried out it produces - Quenching die dimensions;
compressive stress at the surface. Compressive - Post heat treatment.

52
 AGMA 9ll-A94

Gear manufacturers try to bring the component size First, there is lackof uniformity in case depth leading
under control in the finish grinding stage. If excess to uneven residual stress distribution. Second (and
material is left on the part prior to heat treatment, worse) is that the gear appears satisfactory in a
there will be enough stock to enable the size to be nondestructive inspection, even though the perfor-
brought under control. However, if too much is taken mance of the gear will be less than optimum. Third, a
off, the most effective portions of the carburiied (or considerable thickness of material has to be
nitrided) case are removed. Figure 17 shows removed during grinding, increasing the probability
uniform material being removed from a tooth after of grinding bums. Some problems that are blamed
heat treatment. In the example shown in figure 18, on grinding can in reality be traced back to heat
the tooth has distorted to the right. To correct the treatment. Thus the effects of heat treatment have to
profile, excess stock has to be ground from the right be considered before and after the process in both
side of the tooth. This has several serious the soft machining and hard finishing stages.
consequences. Selective direct hardening processes minimize
distortion and associated rework problems.

Size before 9.13 Gear through hardening

Heat Treatment
Some aerospace gears that are not critically loaded
may be heat treated by through hardening
processes to obtain the required mechanical
properties for the gear application. Typical material
hardnesses range from 32 HRC to 54 HRC with
allowable gear tooth bending and contact stresses
that are lower than those for surface hardened
gears.
The process includes annealing, normalizing,
quenching and tempering. For annealing, ferrous
’ Case alloys are heated to 1475-l 650°F (802-899%) and
Figure 17 - Schematic of material ground from furnace cooled below 600°F (316%). In normaliz-
a gear tooth ing, ferrous alloys are heated to 1600-l 800°F
(871-982°C) and cooled in air. Annealing and
normalizing are used as homogenizing pm-heat
m-h Tooth
Distortion treatment processes to reduce metallurgical non-
uniformities.
For the quench and temper process, ferrous alloys
Heat Treatment
are heated to 1475-16OOOF(802-871 “C) to form
austenite, followed by rapid quenching which
causes the gear to become harder and stronger by
formation of martensite. The gear is then tempered
to a specific temperature below 1275OF (691 “C) to
achieve the desired hardness and strength with
improved ductility and toughness.

Figure 18 - Schematic of material ground from

a distorted gear tooth

53
 AGMA 91%A94

IO Surface treatment
This information covers a variety of currently utilized
surface treatments, generally applied after harden-
ing, that are used to enhance the durability of aero-
space gears with respect to their resistance to metal
fatigue, wear, and environmental corrosion. ‘B
Y
10.1 Introduction L1oo
53
Post hardening surface treatments are usually ts
employed on gearing to accomplish one or more of :
the following: .g 80
- Raise the bending fatigue strength at the tooth
8
root fillet radii, as well as in the rim, web, hub and
integral shafts;

- Increase the contact fatigue properties along

the tooth flanks near the pitch line;
40
- Improve the resistance of gears, bearings, and -80 0 80 160
other mating surfaces to adhesive wear compression tension
Peak residual stress, ksi
phenomenasuch asfretting, scuffing and galling;
Figure 19 - Fatigue strength in ground AISI
- Improve the resistance to abrasive wear, such
as scuffing (scoring) from ingested abrasive and 4340 (50 HRC) [ll]
wear debris;
Tensile residual stresses may be present at the
- Improve lubricity, both for break-in and contin- surface of gears after heat treatment, particularly in
uous operation; high strength, through hardened steels tempered at
temperatures which provide only moderate relief of
- Increase resistance to general and to localized quenching stresses. Other processes which may
corrosion (pitting, stress corrosion, etc.) during also introduce residual tensile stresses in gears
storage, from aggressive service environments, include grinding, abusive machining, hard plating
or due to contaminated lubricants. (such as nickel and chromium) and hard thermal
Tooth bending fatigue is of primary importance spray coatings. Some of the non-traditional
since fatigue fractures of gear teeth can result in machining processes such as electro-discharge
catastrophic destruction of a gear train. Contact machining (EDM) and electrochemical machining
fatigue pitting, wear, and corrosion, however, can (ECM) may also damage surfaces through the
also lead to tooth fatigue fractures from a reduction formulation of a brittle recast layer by EDM or the
in load carrying area and/or through the formulation introduction of pitting and/or shallow intergranular
of local stress raisers. Since most geartooth fatigue attack by ECM.
cracking initiates at or very near the surface, fatigue On the other hand, surface hardening heat treat-
properties are highly sensitive to surface character- ments such as carburizing and nitriding introduce
istics such as roughness, and to residual surface beneficial residual compressive stresses in gears.
stresses introduced during the manufacturing pro- Some of their benefits may be offset, however, by
cess. As an example shown in figure 19, surface the presence of shallow, partial decarburization and
residual tensile stresses can lower fatigue strength, intergranular oxidation in unground roots of some
while fatigue strength is raised by the presence of carburized gears, and the presence of a brittle white
surface residual compressive stresses. nitride layer in some nitrided gears. Honing, polish-

54
 AGMA 911-A94

ing, burnishing and other “superfinishing” tech- forces between surface and core are balanced.
niques are often eff e&e in removing fatigue lower- Since the offsetting tensile stresses act over a sig-
ing surface layers and defects, as well as in reduc- nificantly greater cross-section than the compres-
ing surface roughness. sive stresses, the tensile stresses are generally of
low magnitude. See figure 20. In shot ieening thin
Another effective process for improving fatigue
sections, the depth of peening is controlled so as to
properties by inducing high surface residual
keep the core tensile stresses at moderate values
compressive stresses in gears is “controlled shot
and also to prevent distortion of the workpiece.
peening.” The process is utilized for both
carburized case hardened and through hardened Tension Compression
aerospace gears. Controlled shot peening should (+) % Ultimate tensile strength (-)
not be confused with “shot blasting,” “grit blasting,” +1 00 40 0 -50 -100
or “abras.ive blasting.” The latter processes are I I 0
t
employed for cleaning or abrading surfaces and do - ss-l
not produce consistent residual surface compres- -28
I- E
save stress profiles or predictable increases in gear 4 wnax -4:
fatigue properties. 6
- 6$
10.2 Shot peening L -6nax
-8%
Controlled shot peening is a surface cold working
25
process in which hard, spherical shaped media 28
(steel, ceramic or glass) are propelled at relatively
high velocity and at a nearly normal incidence angle - 12
against a workpiece. Its purpose is to promote sur-
face strain hardening and to induce predictable sur- SS is surface stress
CS is compressive stress
face and near surface residual compressive TS is tensile stress
stresses.
Figure 20 - Example of residual stress profile
Each particle of round shot striking the surface acts
like a tiny peening hammer, producing a small created by shot peening [12]
indentation or dimple. The surface fibers are The maximum residual compressive stresses from
stretched (yielded in tension) by the dimple forma- shot peening are at least as great as half the yield
tion, which also forms a sub-surface hemisphere of strength of the workpiece material, providing that
strained metal below the dimple. Overlapping the media used for peening has a minimum
dimples develop an even layer of plastically de- hardness at least as hard as the workpiece. For
formed surface fibers and a sut+sutface zone of example, the use of regular hardness steel shot
strained material. If unrestrained by the core, the (HRC 45-52) will not induce the same magnitude of
surface of the workpiece would elongate under compressive stress in a carburized gear as special
these deformation induced strains. The greater hardness shot (HRC 55-62). This is illustrated in
mass of unaffected metal in the core, however, figures 21 and 22.
restricts this expansion, producing high magnitude
The stress profiles in these and subsequent figures
residual compressive stresses in the surface and
were determined by standard X-ray diffraction
near surface layers.
techniques for measuring residual stress (SAE
The maximum compressive stress is generally lo- J784a). Subsurface measurements are made after
cated just below the surface and decreases with in- elecropolishing away surface layers and are
creasing depth. The depth at which the residual corrected for subsurface stress gradients [20].
compressive stress becomes zero is usually re- Since the use of the harder shot increases surface
ferred to as the effective depth of peening. Beyond roughness, it may be necessary to final finish after
this point, the compressive stresses are offset by peening when surface finish is critical. Lapping and
sub-surface residual tensile stresses so that the honing may be used if the operation does not

55
55
 AGMA 911-A94

remove more than 10% of the compressive stress life is primarily dependent on the propagation or
depth. non-propagation of these cracks, which in turn
Regular hardness shot without post peen finishing depends on the stress conditions at the crack tip.
may also be used; however, the maximum com- The theory of crack arrest due to the residual com-
pressive stresses will be lower than that obtainable pressive stresses induced by shot peening is based
with the harder shot. on the following:
- A crack will not propagate unless a tensile
0 .
/- G stress forces it open near the crack tip;
‘Y5
5 - The crack tip will not open as long as a
- -500 compressive force acts upon it.
Some important points to be considered with
respect to shot peening are:
- -1000
-Gears should always be cleaned and inspected
I I thoroughly for cracks before shot peening. If
0 0.004 0.008 0.012 cracksarefound, the cause should be thoroughly
Depth in inches investigated and corrected. If allowed, the parts
should then be repaired by blending or re-ma-
Figure 21 - Peening 1045 steel at 46 HRC with
chining. Shot peening tends to obscure cracks
330 shot [13] and should not be used as a method for repairing

-Shot peening is effective as long as the residual

compressive stresses do not fade out or relax
due to exposure to high temperatures (generally
500°F max. for most steels) or to over-stressing
(applied stresses in excess of the yield strength);
- Shot peening effectiveness is greatest when
to an area of a part where there is an
applied stress gradient, such as in bending or
0.004 0.008 torsion, or in the presence of geometric notches,
Depth in inches such as fillet radii;
-Shot peening is more effective in the high cycle
Figure 22 - Peening 1046 steel at 62 HRC with fatigue (HCF) domain; however, it is also benefi-
330 shot [13] cial in the low cycle fatigue (LCF) region up to the
point where the applied stresses correspond to
10.2.1 Crack arrest due to shot peening the net section yield strength.
Metal fatigue cracking takes place in two distinct 10.22 Effect of shot peening on tooth bending
phases. The initiation phase encompasses the fatigue
development and early growth of a small crack, The tooth root fillet radii in aerospace gears are
almost always at a “free surface.” The propagation generally subject to relatively high cyclic bending
phase is that portion of the total life during which the stresses. Carburized case hardened gears, as well
crack grows to the point of failure. It is often difficult as through hardened gears, are frequently shot
to define the transition from initiation to propagation. peened in the root and root fillet area to introduce a
In assessing the effect of residual compressive high magnitude compressive stress, as illustrated in
stresses in increasing fatigue strength and life, it is figure 23. Significant increases in bending fatigue
usually simpler to consider fatigue from the stand- strength can be realized when shot peening
point of fracture mechanics theory. This assumes carburiied and hardened gears with unground
that minute cracks or crack-like flaws are always roots, as well as in gears with ground rcotswhen the
present in engineering structures and that fatigue peening is performedafter grinding. See figure 24.

56
 AGMA 911-A94

Non-cased areas in carburized gears, such as the 6I-

fillet radii at rims, webs, hubsand shafts, also exhibit
improved fatigue properties after shot peening. See
r”
figures 25 and 26. ‘T 5
t
5
NOTE - As bending stresses approach the yield t
strength of the material, the effectiveness of shot E4
peening is reduced due to plastic deformation which I
causes fading of the residual stresses. 8
P
a3 I I

d Spiral bevel gear, 3.625 DP

0 Material, 8620H bar stock, heat treated:
z2 Carburizsd & hardened to 61 HRC

.I
0 Load bansmitt& 135 HP @ 5000 RPM
Y Shot peeningspecifications:
z! Shot size 170 H
n1 Intensity 010 -014A
Coverage 200%

0
104 165 lb6 107
Cycles to failure

Figure 24 - Increase in fatigue resistance of

spiral bevel gear 1151

Figure 23 - Stress profile of carburized gear

tooth root, ground and then shot peened with
special hardness shot [14]

60 l
420

350
‘G
y-40, 4 zz 280
i! Shot peened - 43 ksi (300 N/mm2)
nl I \ . II I I
I I I I I

“::;f:;
1% 5x105 106 5x106 107
Revolutions
5x107
YFymnl(o(0124”) 1 1

Cycles tti failure

Figure 25 - Fatigue tests on Figure 26 - Fatigue tests on

rear axle shafts [16] notched shafts [17]

57
AGMA 911-A94

109.3 Effect of shot peening on tooth contact investigator has also indicated that this lower friction
fatigue can also reduce gear noise.
Contact fatigue pitting and spalling in gear teeth The modem “delamination theory” of wear suggests
generally initiates at the dedendum surface of the that all adhesive-wear processes are related to
tooth flank, just below the pitch line. Surface initia- fatigue and that the residual compressive surface
tion results from the combined effect of subsurface stresses from shot peening, as a result, may also
shear stresses from rolling and tensile frictional increase resistance to wear. This has not as yet
traction stresses from negative sliding (rolling and been confirmed in any rigorous test program.
sliding in opposite directions). Testing of peened
and unpeened carburized and hardened gears has 10.2.5 Effect of shot peening on corrosion
indicated that shot peening of the tooth flanks in- In gear steels, shot peening has no effect on either
creases contact fatigue life by a factor of about 1.6. general corrosion or pitting corrosion. Shot
See figure 27. peening, however, is effective in retarding the
10.2.4 Effect of shot peening on wear initiation of fatigue cracking from corrosion pits until
the depth of the pits exceeds the effective
High hardness gear steels exhibit only low to compressive depth of the peening.
moderate strain hardening when shot peened. The
increase in surface hardness due to shot peening, Several of the ultra-high strength, through hard-
therefore, has little effect on increasing resistance ened gear steels are susceptible to stress corrosion
to adhesive or abrasive wear. The tiny indentations cracking (SCC) when exposed to a moist chloride
on the gear flanks produced by peening, however, atmosphere. The residual surface compressive
act as very small oil reservoirs which can help to stresses from shot peening, often in combination
promote better lubrication, thereby reducing fretting with a sacrificial coating (such as cadmium, alumi-
and scuffing (scoring), as well as lowering operating num and zinc) may prevent or retard SCC in these
temperatures by reducing friction. At least one steels as well as fatigue cracks initiated by SCC.
99 1 I I I I I I 1 -8
z
95 s
a0 E
z
60 $
t
&
3
ii
E
10 E
0.
6
4

I I I I I
20 40 100 20 40 100 200
Number of cycles xl O6 Number of cycles xl O6
Standard gears Shot-peened gears

Comparison of surface (pitting) fatigue lives of Speed - 10 000 RPM

standard ground and shot-peened carburized Lubricant - synthetic paraffinic oil
and hardened CVM AISI 9310 steel spur gears: Gear temperature - 77°C (170°F)
Figure 27 - Fatigue life comparison 1181

58
 AGMA 91%A94

10.2.6 Shot peening process controls of the media size, material, hardness, velocity and
There is presently no non-destructive production impingement angle. In order to specify, measure
method to determine the proper shot peening of a and calibrate intensity, a method developed by J. 0.
gear. Strict control of the shot peening process is, Almen utilizing SAE 1070 spring steel specimens
therefore, essential to ensure repeatability and called “Almen strips” is still in use. The unpeened
uniformity on a part-to-part and lot-to-lot basic, strip is fastened to a steel block and exposed to a
while conforming to applicable specifications. The steam of peening shot for a given period of time.
shot material, size, shape and hardness, as well as Upon removal from the block, the residual compres-
velocity and impact angle, must be rigidly controlled sive stress and surface plastic deformation pro-
to provide consistency in peening results. In order duced by the peening impacts cause the Almen strip
to effectively control the shot peening process, the to curve, convex on the peened surface.
following parameters must be addressed: The height of this curvature when measured in a
- Media control standard Almen gauge is called arc height. There
- Intensity control; are three standard Almen strips currently in use: the
- Coverage control; “A” strip, 0.051 in (1.3 mm) thick for intermediate
- Equipment control. intensities; the “C” strip, 0.094 in (2.4 mm) thick for
Details can be found in shot peening specifications high intensities; and the “N”strip, 0.031 in (0.79 mm)
such as MIL-S-1316X, AMS2430, AMS2431 and thick for low intensities. Three strip thicknesses are
AMS2432, as well as the SAE Manual of Shot Peen- required since the useable range of curvature on the
ing, J808a. The following, therefore, is meant as Almen strips is 0.004 to 0.024 inches (0.1-0.6 mm).
only a brief summary. A comparison of the intensities of the A strip for the
C strip and the N strip, as indicated by arc heights, is
10.2.6.1 Media control
also shown. See figure 28.
Media typically used for shot peening are small
spheres of cast steel, conditioned cut wire (both Intensity designations must include both the arc
carbon and stainless steel), ceramic and glass ma- height and type of strip used. Substitution between
terials. Peening media must be uniform in size and strips is not permitted. An Almen arc height is not
essentially spherical in shape with no sharp edges properly termed intensity unless “saturation” is
or broken particles. Broken or sharp edge particles achieved. This is done by developing an intensity
can be potentially damaging to the part surface. saturation curve. Saturation is defined as the
earliest point on the curve where doubling the
10.2.6.2 intensity control exposure time produces no more than a 10%
Calibration of the impact energy orpeening intensity increase in arc height. Most important to the user is
of the shot stream is essential to controlled shot the fact that the depth of the compressive layer is
peening. The energy of the shot stream is a function proportional to the Almen intensity.
0.001 inch 0.001 inch
2 4 6 8 4 8 12 16 20
130 0.30

0.20

- 10 0.10
r
.-E
E
0 B
0.05 0.10 0.15 0.20 6
Almen intensity of C strip, mm
Figure 28 - Correlation of Aimen intensities as indicated by arc heights of A, C, and N strips
peened under identical blast and exposure conditions.

59
 AGMA 91%A94

10.2.6.3 Coverage control When shot peening is selected for man-flight ve-
Coverage is defined as the extent, in percent, of hicle geared components, for geared components
uniform and complete dimpling or obliteration of the where it is used as part of the design strength, or for
original workpiece surface. Inspection of percent geared components which are considered critical to
coverage can be accomplished by using a ten system success, the following are suggested:
power magnifying lens or through the use of an - Shot peening by computer monitored/con-
approved fluorescent liquid tracer system. It is trolled equipment per an applicable, approved
extremely difficult to visually determine coverage in specification such as AMS2432, including the
hardened gears with a ten power visual examina- use of liquid tracer systems to verify coverage;
tion. For determining coverage on gears, the liquid - Prior approval of a strength and life analysis
fluorescent tracer system is widely used. The which justifies the approach. This includes life
coating, which is applied prior to peening, iS certification/qualification testing on hardware
removed during peening at a rate proportional to the conforming to the production configuration;
percent of shot peening coverage. Examination
-The establishment and validation of an inspec-
under black light (UV) provides a practical and
tion interval that takes into account the potential
superior method of verifying the 100% minimum
for in service degradation of peening benefits;
coverage required when shot peening gears for
fatigue resistance. - Any repair/rework peening required at pre-
scribed inspection intervals to attain the required
102.6.4 Equipment control fatigue life.
The machines used for shot peening provide means 10.2.8 Guidelines for media and intensity selec-
for propelling shot by air pressure or centrifugal tion
force against the workpiece, as well as mechanical The proper selection of media type, size, hardness
systems for moving the work through the shot and intensities for peening of aerospace gearing is
stream or moving the shot stream through the work dependent on numerous variable including:
by translation and/or rotation. The equipment also
continuously removes broken or defective shot so - Minimum fillet radii in gear;
that it is not used for peening. Many modem shot - Thickness of peened section of gear and/or
peening machines are computer controlled. Typical effective depth of carburized case, if any;
peening parameters which are monitored, con- - Hardness of gear in areas to be peened;
trolled and documented are:
- Surface finish requirements;
- Air pressure of each nozzle or wheel speed of - Requirements for peening internal surfaces
each wheel; and/or intersecting holes.
- Shot flow of each nozzle or wheel; The tables of guidelines found in numerous specifi-
- Part rotation or translation rates; cations, such as MIL-S-13165C, are valuable statt-
ing points for selection.
- Nozzle reciprocation rates, distances and run
times; NOTE - Magnetic particle, penetrant, ultrasonic or
otherflaw andcrack detectionmethodsshouldbe per-
- Total cycle time. formedprior to shot peening.
102.7 Shot peening design considerations 10.3 Surface coatings
When the gear user does not wish to take advan- Table 16 lists of some of the most widely used coat-
tage of the fatigue strength increases in the design ings for aerospace gears. The table is divided into
calculations, but wishes to use the process to over- three broad categories of coating application: anti-
come many of the residual surface tensile stresses fretting barriers, corrosion resistance, and build-up
or similar problems from previous manufacturing repair.
processes, shot peening controlled per AMS2430 Under the anti-fretting barrier list are coatings used
or the equivalent is generally suitable. to reduce the effects of fretting at joints. Aluminum

60
 AGMA Qll-A94

bronze econal has been successfully used between fields. The ion implantation process is primarily
mounting surfaces of gears which are bolted to used for two reasons:
flanges. Silver and copper plate have also been - Improving surface hardness, wear resistance
used on gear bolted connections, but are more against adhesive, abrasive and scuffing condi-
normally used as coatings for splines. Tungsten tions;
carbide has had application to cone seat connec-
- For improving corrosion resistance of
tions between helicopter rotor heads and main rotor
materials.
shafts under high loads and stresses.
In the ion implantation process, energetic ions
Steel parts exposed to the atmosphere have used impinge on the surface of interest. The ions
baked resin (or other forms of paint) in non-working penetrate into a substrate material, modifying the
areas for corrosion resistance. Cadmium plate has surface by changing crystallinity and chemical
been used on steel parts exposed to the structure of the material.
atmosphere with brush cadmium being used on
Ion implantation has proven to be an effective and
high strength steel (over 150 ksi) and conventional
technically attractive approach for changing the
cadmium on lower strength components. Thin
surface properties of high value added and high
dense chrome has been used on grease lubricated
precision components of various criiical systems.
bearings exposed to the atmosphere. There is
disagreement over the benefits vs. cost of black Ion implantation of gears was introduced and has
oxide or phosphate coatings on steel parts. been used in the last ten years for a variety of
reasons, primarily for improving the mechanical
The third broad category of coatings used in performance in a wide range of applications and
aerospace applications is in the area of repair of improving corrosion resistance in helicopter
surfaces that are under the minimum material transmissions and aircraft engines.
condition. Sulfamate nickel plate has been used as
10.42 Improved mechanical performance of the
a repair for bearing journals that are up to 0.010 inch
undersized, in carburized and uncarburized mr
conditions. Since there may be a reduction in Ion implantation has been used on a variety of gears
fatigue strength from sulfamate nickel plating, for improving surface hardness, providing lubrica-
repairs should be made in areas of low stress only. tion and minimizing scuffing wear of gears. The
Electroless nickel and brush nickel plate have been most common material studied in this field is AISI
used for very thin coatings where precision is 9310 steel, a commonly used material for gear
required. The parts need not be ground after plating fabrication.
as the thickness is uniform and the plating takes the Ion implantation is also used extensively for
form and finish of the base material. treatment of titanium alloy gears. In aerospace
applications, for weight saving purposes, titanium
For any coating that is applied in areas that are
gears are occasionally used. Ion implantation of ion
clamped or otherwise in contact, consideration
species, such as nitrogen and/or carbon, into
must be given to the consequence of loss of coating
titanium alloys will induce hard phase precipitates of
thickness, loss of preload on bolted joints, loss of fits
titanium nitride and/or titanium carbide. The
for press fitted parts, etc.
increased surface hardness of the gears and lower
10.4 Ion implantation of gears coefficient of friction induced by ion implantation
has proven very effective in reducing scuffing wear
10.4.1 Introduction and lowering wear of titanium components.
Ion implantation isan effective processforchanging In hydraulic fuel applications, ion implantation has
the surface properties of materials without been used to modify the surface properties of the
adversely affecting the bulk properties. Ion face of the gear. In this case, the main concern is
implantation technology is used in a variety of excessively high friction, adhesive wear and particle
applications in aerospace, automotive, cutting tool, debris generation. ion implantation of titanium and
biomaterials, metal stamping and metal piercing carbon into commonly used steels used in fuel

61
AGMA 911-A94

pumps has proven very effective in minimizing testing, as well as field appiications. Implantation of
friction and improving resistance of the surface ion species such as chromium and molybdenum
against wear. have significantly improved pitting corrosion and
10.4.3 Ion implantation for corrosion resistance aqueous corrosion in salt bath applications.

ion implantation has been used extensively to 10.4.4 Conclusion

modify corrosion resistance of steel gears. The
most common recipes include ion species such as High precision gears appear to be ideally suited for
chromium, molybdenum, tantalum and titanium the ion implantation process. The available data
followed by carbon ion implantation. Depending on supports the use of the technology on a broader
the nature of corrosive attack, one or the other of range of applications requiring superior corrosion
these ion species have been studied in laboratory and wear resistance.
Table 16 - Surface coatings used in aerospace gear units

Coating Application Specification

Ant--fretting
Aluminum bronze econal Fretting barrier (plasma spray) METCO 604NS
Copper Fretting barrier MIL-C-14550
Silver plate Fretting barrier (SplinesIJoints) QQ-S-365
Sulfamate nickel plate Limited fretting barrier QQ-N-290
Tungsten carbide Hard, wear resistant, fretting barrier None
Detonation thermal spray
Tungsten carbide plasma Hard, wear resistant, fretting barrier METCO 072VF
spray
Corrosion resistance
Baked resin Corrosion resistance MIL-R-3043
Black oxide Corrosion resistance (on shelf) MIL-C-13924
Brush cadmium plate Corrosion resistance. MIL-STD-665
Used on high strength parts
Cadmium plate Corrosion resistance for QQ-P-416
parts outside gearbox
Gun blue Touch up for Black Oxide MIL-C-13924
Ion vapor deposition Corrosion resistance MlL-C-6637
Phosphate coat Corrosion resistance & MIL-P-16232
break in surface for gears
Thin dense chrome Corrosion resistance, hard wear surface QQ-C-320
Build-uo reoair
Brush nickel plate Thin build up repair (0.0002 inches thick). MlL-STD-665
Precision plate with no grinding.
Electroless nickel plate Thin build up repair (0.0002 inches thick) MIL-C-26074
No machining after plating.
Uniform plating thickness even in grooves.
Loss of fatigue strength.
Sulfamate nickel plate Build up repair (0.005 inches thick) QQ-N-290

62
 AGMA 911-A94

11 Manufacturing considerations Special shaping machines are available to shape,

crown, and/or taper the spur or helical tooth. The
11.I Introduction involute profjle is a function of the involute shaping
The performance of an aerospace transmission is cutter and the generating machine motion.
directly related to the manufacturing process used 11.2.2 Hobbing gears
to fabricate the gears. Due to the unique character-
istics of the sliding mesh, close control of the gear Hobbing is recommended for semi-finishing
tooth geometry is paramount to yield a successful external spur and helical gears where the blank
application. The metallurgical and geometric qual- configuration permits the clearance for the cutter
ity must be controlled to meet the drawing require- path. Hobbing is generally the most cost effective
ment. In addition, surface finish, including surface means of generating spur and helical gears.
roughness and waviness, is important to reduce Hobbing machines can generate crown and/or
surface distress. The Engineering drawing require- taper. Helical gear teeth are generated through a
ments should dictate the manufacturing source and differential gear box or by computer numerical
process used to fabricate the gears. The selection control (CNC)/electronic control.
process for the manufacturer should include the 112.3 Semkfinish tooth geometry
approval of Engineering, Manufacturing Engineer- The sem’tiinish tooth geometry and accuracy
ing, and Quality Assurance. The manufacturer should be controlled by measuring the first part and
should provide tangible evidence of their capability last part on a qualified measuring machine. Addi-
to meet the drawing requirements. tional parts should be measured when a cutter is
11.2 Spur and helical gears changed or the machine setup is changed. Gener-
ally, gears are semi-finished by cutting on a hob or
Manufacturing spur and helical gears require
shaping machine but may be ground “from solid”
unique machines to cut (generate) the gear teeth
(directly from a blank), or milled, forged, rolled, etc.
prior to the carburizing and hardening process.
Generally, the parallel axis gear teeth are semi-fin- 112.4 Heat treat considerations
ished, using a shaping or hobbing machine. The Consideration should be given to heat treat
shaping or hobbing cutters should be designed to distortion. It may be required to adjust the
accommodate the appropriate stock on the gear measurement over or between pins to accommo-
teeth to meet the needs to clean up the gear teeth date dimensional change in the carburizing and
after hardening. Care should be taken not to leave hardening cycle. A lead adjustment may also be
too much stockon the teeth, which would reduce the required to counteract lead variations introduced by
effective case depth and hardness. The tooth form heat treat. Generally, a pilot lot should be
(involute and lead) and tooth accuracy (pitch and processed through the heat treat cycle to confirm
index variation) should be controlled within reason- the effects of hardening. The larger the gear
able tolerances to assure proper stock allowance diameter and the longer the face width, the more
for final grinding of the teeth. Sometimes, 200 per- critical this process may be. For consideration of
cent of the drawing tolerance for the above require- microstructure, see clause 9 of this document.
ments can be used to control the as-cut dimen- Nitriding and induction hardening of some aircraft
sions. The actual amount needs to be carefully gears are also used. These processes reduce
chosen based on the material and configuration. much of the distortion of the gear teeth and the gear
11.2.1 Shaping gears blank experienced during the carburizing and
The shaping machine process can be used to gen- hardening process.
erate internal and external gears, especially gears The nitriding process produces a thin, very hard,
which have a shoulder or other physical obstruction brittle layer in some materials that may cause dam-
limiting the path of thecutter. Shaping machinesare age in certain applications. Some manufacturers al-
unique in that they can generate most or all parallel low this material to wear off during break-in. Others
axis gearswhen properly tooled and fuctured. Shap remove it by either light grinding or by electro-
ing helical gears will require a special helical guide. chemical machining (ECM). This non-abrasive

63
AGMA 91%A94

process removes the white layer with chemicals that - The selection of the grinding wheel and the
slowly dissolve the surface. A non-destructive method of dressing the wheel are factors in con-
chemical check can be performed to determine if trolling the involute form and the surface finish.
the white layer has been sufficiently removed. The wheel abrasive, grii size, hardness, bond,
coolant oil, wheel surface feed, and machine
112.5 Gear blank control feed rates are all factors in the performance.
Gear blank dimensional control before, during, and Plated cubic boron nitride (CBN) grinding wheels
after heat treat is important. The operation sheets can be used successfully to grind precision gears
are used to control the manufacturing process, with good involute and surface finish.
including the dimensions and tolerances for the 11.2.7 Measurement and control
surfaces used to support the work holding fixtures All precision gears should be measured and con-
and the surfaces used to contact the quench die trolled based on engineer requirements. Special
where applicable. requirements will include involute, lead, pitch, and
The gear tooth accuracy is directly related to the index variation and should be measured on a preci-
control of the tooling points of the gear blank. In sion certified measuring machine. As the precision
addition, the contact points related to the quench die and accuracy requirements of the gear increase, so
will dictate the consistency of the gear teeth through does the accuracy requirements for the measuring
the hardening,process. system. A calibration method should be established
to certify the accuracy of the measuring machine.
112.6 Grinding gear teeth. Finish grinding gear 11.2.8 Other measurements
teeth is important to provide accurate and consis-
tent gears. Selecting the grinding machine may be In addition, the major and minor diameter (whole
the most important decision to assure the gear teeth depth), root fillet radius, and measurement over or
will meet the engineering drawing requirements for between pins and/or balls (tooth thickness) should
geometry and accuracy. Every gear is unique and be measured and controlled. Surface finish, rough-
one machine may be able to perform better than ness, and waviness are important elements to be
others. The following are some suggestions in se- measured and controlled. Catburized, hardened,
lecting a gear grinding machine: and ground gears should be evaluated for abusive
grinding (burning) by surface temper inspection.
- A shoukier or physical obstruction adjacent to 112.9 Deburring
the gearteeth restrictstheselection of a machine
type. Form grinding permits the use of small Deburring and radiusing the edges of gear teeth
wheels in this case; shoukl be accomplished using care not to damage
the working surface of the gear teeth and use
- Gears which require the root fillet radius to be caution not to abuse (overheat) the surface, which
ground restrict the use of some gear grinding ma- may cause surface temper on the edges.
chines; 11.3 Bevel gears
- Most aerospace gears have geometry and ac- This document will address spiral bevel, straight
curacy tolerances requiring gear grinding ma- bevel, and Zero1 bevel gears. It should be
chines to be maintained to excellent condition. lt understood that the spiral bevel gear tooth form is a
is not unusual for gear designs to require AGMA three dimensional curve, which is not traceable to
Class No. 13 and 14. In order to meet AGMA any known curve. The geometry of spiral bevel,
Class No. 14 successfully, consistently, and eco- straight bevel, and Zero1bevel gears is traceable to
nomically, it is recommended to use electronic the machine that is used to manufacture the tooth
and CNC grinders, or other machines that have form. Most precision aerospace bevel gears are
demonstrated they can meet this requirement. ground using modified roll. Wiih the advent of the
Temperature control of the grinding machine and CNC controlled spiral bevel gear grinding ma-
the ambient temperature is important to obtain chines, which have the abilii to grind other tooth
the specified accuracy; fans, the aerospace industry will continue to use

64
 AGMA 911-A94

the modified roll system for most spiral bevel gears instant, and the Engineer has a more scientific
for some time. Straight bevel gears are usually not approach. The Engineer has the opportunity to
ground because there are limited machines change any one or all of the eighteen machine
available to grind straight bevel gear teeth. settings, which control(s) the gear tooth geome-
Note: The majority of aerospace bevel gears are de- try of the gear, and the eighteen machine settings
signed and fabricated using the Gleason system. which control the pinion. Conventionally, the
gear is cut and ground spread blade (grinding or
11.3.1 The source for data
cutting both sides of the tooth at the same time),
The origin of the machine settings used on the but the Engineer has the option to produce the
cutting and grinding machines are developed by gear single side in the same manner as the
using the many computer programs available to the pinion. The tooth contact analysis will display the
industry. Initially, the Engineer will design the gear lines of contact and the path of contact at the toe,
mesh based on the requirements of the application. heel, and mean of thetooth. In addition, the tooth
The power, velocity, tooth load, shaft angle, ratio, contact analysis displays the motion curves
temperature, the mechanical application, and many (transmission error) at the toe, heel, and mean of
other characteristics are considered. The following the tooth. During the development of the tooth
is some of the basic data used by the manufacturing contact pattern required by the development of
engineer who will develop the machine settings the tooth contact analysis, the Manufacturing
necessary to meet the engineering design: Engineer is evaluating the following:
- Number of Teeth; i. Lengthwise curvature;
- Pressure Angle; ii. Tooth bias;
- Diametral Piich;
- Spiral Angle; iii. Tooth profile curvature;
- Face Width; iv. The effect of the blade angle (longer at
- Shaft Angle; the dedendum and shorter at the adden-
- Whole Depth; dum, etc.);
- Working Depth; v. Length of the toe versus the length of the
- Addendum;
heel;
- Root Angle;
- Root Fillet Radius; vi. The path of contact;
- Addendum Angle. vii. The profile width at the toe versus the
11.3.2 Develop flank form geometry profile width at the heel.
Once the basic gear requirement is established by 11.3.3 Engineering skills
Design Engineering, the Gear Manufacturing The development of the tooth contact analysis
Engineer develops the toothflankform geometry by requires either second or third order changes in
using a computer system. This is accomplished by order to correct for an undesirable pattern (load
conducting the following computer analysis: distribution). The Manufacturing Engineer’s level of
a) First, a dimension sheet which includes the experience and understanding of the gear mesh
basic dimensions of the bevel gear and pinion application is directly related to the level of success
and also includes some of the strength and in this work Generally, further development work is
durability factors for the design must be estab- conducted at the time the gears are generated (cut
lished based on the Engineering requirements; and ground).

b) Second, a tooth contact analysis is developed. 11.3.4 Loaded tooth contact analysis
The computer has a working model of the The Manufacturing Engineer has an option to run
generating motion, including the kinematics of the loaded tooth contact analysis program. This
the meshing of the gear and pinion teeth. In program is similar to the standard tooth contact
principle, this program develops the tooth flank analysis but can display the line and path of contact
form geometry on the computer in the same as well as motion curves based on seven diierent
mannerasthe machine operatordoes except it is loads. This program may further define required

65
AGMA 911-A94

changes to the flank form geometry to improve the computer system. It will also provide the machine
load distribution. This would require changes to the changes required to correct the errant tooth form.
input data for the tooth contact analysis. 11.3.9 Manufacturing bevel gears
11.3.5 Summary of machine settings Manufacturing bevel gears requires a good manu-
facturing plan. The Manufacturing Engineer should
When the tooth contact analysis has been devel-
have a good technical background in bevel gear
oped to the satisfaction of the gear Manufacturing
manufacturing. The manufacturing plan should use
Engineer, the summary should be produced. This
good logic for processing the gear, taking into con-
data provides the machine settings for cutting and
sideration the following:
grinding the gear and pinion. In addition to machine
settings, basic gear data is given as well as settings a) The dimensional tolerances on the Engineer-
for the Blank Checker, which measures the face and ing drawing;
back angle, root depth, and root angle. Proportional b) The gear tooth geometry and accuracy;
changes are given for making second order c) The heat treat distortion and processing;
changes to the pinion.
d) The finish processing, including deburring,
11.3.6 Grinding sequence plating, and final inspection;
Running the Grinding Sequence Program will pro- e) Each operation should be described clearly
vide information for sizing the grinding wheel, type and concisely using picture sheets when
of grinding wheel, feeds and speeds, and other se- appropriate.
quence data necessary to grind the gear and pinion. 11.3.10 Generating
11.3.7 Fillet details Generating the gear teeth prior to carburizing and
hardening is important. The primary considerations
The gear Manufacturing Engineer has the option of are as follows:
running “Tooth Profile and Fillet Details” program.
This program will define the generated root fillet ra- a) Controlling the flank form geometry to permit
dius of the gear and pinion based on the generator minimum stock removal for finish hard grinding
and grinding machine settings. the gear teeth;
b) Control the root depth and root fillet radius to
11.3.8 Current technology permit proper stock for finish grinding the gear
Todays current technology permits defining the teeth;
working flanks of spiral bevel gears scientifically c) The tooth accuracy, pitch, and index variation
using a three-dimensional, coordinatesystem. The should be controlled to provide the requirements
computer system will define the theoretical tooth for hard grinding the gear teeth;
form established in the special analysis file located d) The surface finish should be controlled to
in the mainframe computer, while developing the assure that the cutter flats and scratches do not
tooth contact analysis and summary of machine endanger cleaning up the tooth surface while
settings. The theoretical coordinate data estab- finish grinding gear teeth.
lished on the computer can be downloaded to the
coordinate measuring machine for use in measur- 115.11 Heat treat distortion
ing theflankform of the spiral bevel gear teeth. With Consideration of heat treat distortion during all
the software system, it is possible to get first and phases of gear blank fabrication and generating the
second order corrections to the machine settings bevel gear teeth must be maintained. Carburizing
used to produce the gear teeth. That is, a gear and hardening gears, which causes the gear blank
produced on a cutting or grinding machine can be to distort, has always proven to be difficult,
measured on a coordinate measuring machine especially on gears with large diameters and thin
using the computer program. It will define the cross sections. The spiral angle of spiral bevel gear
variance of the machined surface as compared to teeth tends to unwind during heat treat. To control
the theoretical or measured data stored in the the stock removal of the carburized gear teeth, the

66
 AGMA 911-A94

face and back cone and crown diameter must be accurate system. An alternate method is using the
maintained within close limits. Quench dies are rolling method on a test machine for taking tooth
generally used on large gears by placing contact patterns. The tooth pattern record should include
pressure on the outside diameter, inside diameter, vertical (V) and horizontal (H) measurements to the
and top face angles during the quench cycle to toe and heel to control bias, and (V) only
reduce some of the distortion. Gears must be flat measurements to the toe and heel to control the
and round after hardening. pattern length (see AGMA 390.03a). Additional
Nitriding and induction hardening are also used for V&H measurements should be taken to control the
tooth profile width.
some bevel gears. These processes reduce much
of the distortion of the gear teeth and the gear blank. 11.3.14.2 Other measurements
11.3.12 Control after hardening The root depth and root fillet radius must be
measured and controlled to meet the Engineering
Controlling the gear blanks after hardening is drawing requirement. The surface finish, pitch and
accomplished by using a good manufacturing index variation, and face and back angle of the gear
process and tooling. The pitch line of the gear teeth should also be measured and controlled
must be concentric to the mounting diameter and
mounting surface. This part of the manufacturing 11.3.47 Deburring and radiusing
process is critical in order to maintain stock removal Deburring and radiusing gear teeth must be
within limits of the case depth and surface hardness accomplished, being cautious not to temper the
on the gear teeth. The accuracy of the gear teeth tooth edges after the gear teeth are ground.
are traceable to the accuracy of the gear blank. Abusive grinding during the debut-ringand radiusing
11.3.13 Grinding bevel gear teeth can be detrimental to the strength of the gear tooth.
Surface temper inspection should be performed
Grinding the spiral bevel gear teeth is generally the after final machining, including deburring to assure
last machining operation and is among the most that the gear is free of temper. Automated deburring
important. Using the grinding machine that can and brushing are the preferred methods.
meet all the requirements of the engineering
NOTE - One way to avoid surface temper is to ade-
drawing is the first and most critical step. Other
quately chamfer the gear teeth prior to hardening.
considerations are as follows:
11.3.18 Final processing
a) Using the established summary and final ma-
chine settings for the gear grinder; The final processing of the gears, which includes
surface temper inspection, plating, shotpeening,
b) Controlling and recording the machine set- magnetic particle inspection, and coating must be
tings used to grind the gears; conducted according to approved operating
c) Stock dividing the gear to the grinding wheel to procedures.
remove equal stock from the flanks and control 11A Stress relief treatment
stock from thwoot of the gear tooth while main-
Stress relief should be performed on all gears that
taining the root depth requirement;
have been carburized and hardened as soon as
d) Use the grinding wheel, which maintains form possible after grinding and should be performed
and produces the required surface finish without prior to any subsequent processing. 9310 steel
tempering the gear tooth. parts should be stress-relieved at 275 to 300 “F
11.3.14 Measurement of bevel gears (135 to 150 “C) for at least one hour (depending on
the cross section of the gear) followed by still air or
11.3.14.1 Tooth flank form oven cooling to room temperature. Stress relief
Measuring and controlling ground bevel gear teeth treatment will minimize the possibility of residual
is among the important steps in fabricating bevel stress cracking. Since the possibility of such
gears. The recommended method for measuring cracking increases with time between grinding and
bevel gear teeth flank form is using the coordinate stress relief, it is recommended that stress relief be
system. This method is the most scientific and performed within eight hours of grinding.

67
AGMA 91-l-A94

12 Gear inspection retire other gears from service, it is recommended

that all gears be permanently identified with a lot
12.1 General
number or with an individual serial number. The lot
This clause describes the recommended methods, number or serial number should be traceable to a
practices and controls used for the assurance of the raw material heat and melt number and be in
desired quality level of aerospace gears. The evidence throughout manufacture and be docu-
clause is divided into two parts and discusses spur mented at significant manufacturing, inspection or
and helical involute gears and bevel gears. The processing operations. Traceability with serial
dimensional and gear tooth element inspection of numbers can significantly assist in the investigation
aerospace gears is the same as for gears used in of any problem or failure.
other applications. However, the controls which
maintain the integrity, reliability and life are more 12.29 Heat treat verification
stringent and more supportive dccumentation and
The verification of final heat treat results is of the
records are required.
utmost importance, particularly if the gears are
12.2 Spur and helical involute gears processed to produce a high hardness layer such as
The inspection methods and practices for the by carburizing or nitriding. A metallurgical labora-
inspection of spur and helical involute gears are tory analysis of an actual part, part section, or
comprehensively discussed in ANSVAGMA coupon (see figure 29 for coupon example) repre-
2000-A88, Gear Classification and inspection senting the gear teeth and processed with the lot of
Handbook, Tolerances and Measuring Methods for gears is the recommended method of assuring the
Unassembled Spur and Helical Gears (including results of the heat treating process. This analysis
Metric Equivalents). The following covers addi- would be traceable to the lot either by permanently
tional considerations for the inspection and control ident’@ing the part with the heat treat lot number or
of aerospace spur and helical involute gears. by documentation of part serial numbers. The
laboratory analysis would also be used to calculate
122.1 Identification/traceability
the minimum and maximum amount of stock that
Because the integrity and lie of aerospace gears is can be removed to meet the specified finish depth of
so important, and in the event of a failure having to the hardened layer.

Surface A, TYP, 2 PL

Drill thru 306 (0.187) diameter

Figure 29 - Heat treat coupon

68
 AGMA 911-A94

122.3 Hardened layer finished depth manufacturing, plating or coating operation has
induced defects that would be detrimental to the use
When required to assure that the finished depth of
of the gear.
the hardened layer meets specification, the recom-
mended method is to monitor through process 12.2.6 Inspection of root fillet radii
control at the grinder by stockdividing and recording Inspection of the root fillet radii should be accom-
the machine in-feed setting at the first contact with plished by the use of a magnifying tracer instrument
the gear and the final machine setting. The stock which charts the radius form, or by the use of a cast
removal can then be easily calculated. This method and optical comparator. To assure maximum tooth
proves the entire process of the gear from the strength, the fillet radii should meet specifications
calculated root and flank stock through cutting, heat for size and smoothness.
treat, quench and final gear grind.
12.2.7 Verification of dimensions after shot
129.4 Surface temper inspection peening
Inspection of high hardness ferrous gears, Precision gear configurations that are shot peened
particularly those with carburized surfaces, for should have close tolerance features such as
abusive grinding should be accomplished by diameters, lead and runout m-inspected after shot
in-process surface temper inspections during the peen to assure that no undesirable dimensional
grind operations. It is also recommended that all change has taken place. Refer to clause 10 for
carburized gears be resurface temper etched after other shot peen requirements.
all machining and deburring operations are 12.2.8 Conformity inspections
complete. A discussion of surface temper may be
Assurance that a first run gear has been
found in AGMA 230.01, Surface Temperlnspection
manufactured correctly to the design requirements
Process. The requirements for this process should
may be accomplished through a comprehensive
be mutually agreed upon by the manufacturer and
destructive test of a completed part by experienced
the purchaser because failure to detect abusive
laboratory personnel. This activity can also be
grinding can have serious consequences.
conducted on a periodic basis through the
122.5 Inspection for surface or subsurface manufacturer/purchaser agreement to assure
flaws process control.
Essential to assurance of the reliability, lie and 12.3 Bevel gears
control of the cost of gears is the need to verify the The inspection methods and practices for the
integrity of their material condition at various stages inspection of bevel gears are discussed in AGMA
of manufacture. If there is a concern that the raw 390.03a, Gear Handbook - Gear Classification,
material, i.e. forgings or bar, may contain Materials and Measuring Methods for Bevel,
undesirable characteristics such as laps, seams or Hypoid, Fine Pitch Wormgearing and Racks only as
inclusions they should be non-destructively tested Unassembled Gears. The following covers addi-
prior to or immediately after final heat treatment. tional considerations for the inspection and control
The most commonly used tests are magnetic of aerospace bevel gears.
particle, which detects surface or slightly
subsurface defects, or ultrasonic which will detect 12.3.1 Master gears
all defects through the material. To successfully maintain proper pattern position
and interchangeability during the production of
If gears are shot peened, a non-destructive test of
bevel gears, it is necessary to establish master
the shot peened areas prior to peening will assure
gears during preproduction development of the
that no defects open to the surface will be peened
gear sets. When the proper working pattern is
closed.
achieved at least three sets of mating gears should
An appropriate final non-destructive test should be be selected by evaluating the tooth contact patterns
performed after all manufacturing, plating and on bevel gear test machines and designating them
coating operations are completed to assure that no as master gears at the following levels:

69
AGMA 91%A94

a) The top level master set is selected as the best Wih the proper equipment and software, pitch,
set to duplicate successfully tested gears; index, spacing and the finished tooth surface tooth
form, as well as other characteristics, may be very
b) The second level master set is usually
precisely measured. Measurement of the tooth
designated as the reference master set and is
flank form consists of a digitized map of the working
periodically tested with the top level master mate
surface of either flank that is compared to a
to assure that the correct contact pattern is
computer stored digitized master tooth.
maintained for transfer to the third level master;
This method of tooth flank form measuring is being
c) The third level master sets are used to test the
used successfully in the gear industry to maintain
production gears for contact pattern position and process control during manufacture. Also, the
backlash and are normally called working mas- technology is available to allow automatic calcula-
ters or working control masters. tions from the digitized data to corrected machine
To preserve the patterns from the master gears for settings for optimization of tooth form.
wear comparisons at a later date, tape transfers of 12.3.6 Surface temper inspection
the pattern positions are made using a colored
compound on the gear teeth during testing and Refer to 12.2.4.
lifting the contact pattern from the tooth with a 12.3.7 Inspection for surface or subsurface
transparent tape to a white hardboard card. flaws
The top level masters should be well protected, Refer to 12.25.
preserved and used only to test second level 12.3.8 Inspection of root fillet radii
masters.
Refer to 12.2.6.
Second level masters may require replacement 12.3.9 Verification of dimensions after shot
occasionally, dependent upon production volume peening
and the frequency of testing with third level masters.
Refer to 12.2.6.
Third level masters must be controlled closely
because they are used frequently to test the working 12.3.10 Conformity inspections
production gearing. Refer to 12.2.8.
12.3.2 Identificatio~nkaceability 13 Rocket and space getiring
Raw Material Control and Traceability - Refer to 13.1 introduction
12.2.1.
Gears are in common use in space vehicles of all
12.3.3 Heat treat verification types. As experience is accumulated in space, it
Veriication of Heat Treat Results - Refer to 12.2.2. becomes increasingly clear that space imposes its
own set of rules for survival. As the distance of
123.4 Hardened layer finished depth operation from the earth increases, the environ-
Verification of Stock Removal - Refer to 12.2.3. ment becomes a dominant factor in the design
However, the calculation of the stock removal may equation. The behavior of many materials changes
be somewhat more complex due to the mechanics in the vacuum of space, and these changes must be
of the bevel gear grinder. considered in the design of space mechanisms.
12.3.5 Coordinate measuring machine Vehicles designed for operation in space fall into
inspections three general categories:

In addition to the measuring methods and practices a) Rockets - contain single use, short life span,
for bevel gears that are described in AGMA high power gearboxes or mechanisms. These
390.03a, multi-axis coordinate measuring units are expended by a single flight;
machines may be used to inspect many features of b) Reusable spacecraft and space planes -
bevel gears. gearing and mechanisms must be suitable for

70
 AGMA 911-A94

repeated reuse over a series of flights and mission is extended and can be in excess of 10
launches. Operation in a harsh environment years. As a consequence, the opportunity for
usually involves a short mission duration typically inspection and refurbishment does not exist, and
measured in days or weeks; the mechanisms will be subject to degradation of
c) Space Station and Satellites - geared lubricant from evaporation and erosion, and ex-
systems for these applications must survive their tended wear on working surfaces.
operating environment for extended periods of 13.1.4 Gear forms for space use
time (to more than ten years). Functional Many gear forms have found a place in space
reliability is paramount and there is no opportu- mechanisms. These include extensive use of spur
nity for preventive maintenance. gears, some bevel, helical and worm gearing, and
13.1 .I Rocket gearboxes an occasional proprietary tooth form. Since light
weight structure is of paramount importance, gear
One of the first applications of gears for space was
forms requiring high precision mountings are more
in the liquid fueled rocket. The turbopump gearing
diiicult to apply successfully in the space mecha-
on these vehicles transmit the power to pump the
nism. Similarly, forms that are relatively heavy for
thousands of gallons of fuel consumed during the
the torque transmitted, or forms that exhibit a high
first few minutes of flight.
level of sliding at their contacting surfaces, are less
All of the considerations for light weight power suitable for the space mechanism.
gearing outlined in clause 4of this information sheet
Motors which power space mechanisms are them-
apply to rocket gearing. The power density for
selves small and usually have only modest torque
rocket gearing is extremely high since the mission
outputs. As a consequence, the efficiency of the
time is short and minimum weight is critical. In
drive is of prime importance. In some drives, such
qualification tests, these gears must function suc-
as those used on robotic arms, provision for
cessfully for approximately 5000 seconds at full
backdriiing in case of a collision or momentary
power.
overload is a requirement. To achieve back-
13.1.2 Spacecraft and space planes driieablilty in a gear train whose transmission ratio
These craft embody the use of all the usual may be in excess of 18OO:l points to the use of high
mechanisms found on aircraft, such as control precision spur or helical gears.
surface actuators, bay door actuators, etc. In Althoughoperating ina weightlessenvironment, the
addition, there are mission specific actuators used payload mass which must be manipulated is often
in the deployment of satellites and other payloads large, and output torques of a few thousand foot
from the cargo bay. When the mission involves pounds are not uncommon. The contact stress
manipulation of large bodies, the vehicle, such as a capacity of space qualified lubricants imposes
space shuttle, is fitted with a robotic arm. serious constraints on usable tooth loads. Hence in
Mechanisms aboard space shuttles and planes are high reduction dries, the designer must consider
typically designed for repeated usage, and there- dividing the tooth load over a number of gear
fore must withstand the vibration and shock of meshes. The planetary gearbox is often utilized to
repeated lift-offs and landings in addition to the meet these demands.
function they perform on a space mission. Since a 13.2 Lubrication
shuttle operates at an attitude less than 150 miles The approach to lubricant and material selection for
above the earth’s surface, the environment includes spacecraft gears must be tempered by considera-
exposure to highly reactive atomic oxygen and tion of the environment in which they will be
micro-meteorites. expected to operate.
13.1.3 Space station and satellites Conventional oil and grease lubricants are not
Although geared systems in this last category of suitable for space application since in the presence
space vehicle may not be especially different than of a high vacuum, normal lubricants will outgas and
the ones found on a shuttle, the duration of the their usual lubricating properties will be destroyed.

71
AGMA 911-A94

Some dry compounds, such as graphite, which 40 000 lb/in2 (240-280 N/mrr?) is prudent. Each
exhibit lubricity at sea level become extremely application should be verified by test in vacuum
abrasive in a vacuum. Other lubricants, although conditions before being approved for flight.
technically usable in a vacuum, will migrate and can Some typical solid-film lubricants with space history
contaminate sensitive optical and electronic equip- are listed in table 17.
ment. Clean, non-lubricated metal, under the
influence of contact pressure and sliding in a 13.2.3 Wet lubricants
vacuum, can cold weld. Subsequent separation of Both gears and bearings have been successfully
welded surfaces will tear the welded spots apart lubricated in space with oil and/or grease for short
leaving severe craters and pits. periods of time. However, the behavior .of these
Fortunately, certain grease and dry compounds lubricants in the space environment imposes some
have been identified, which offer a solution to the constraints on the design which must receive
lubrication problem and have been used with careful attention.
reasonable success. These lubricants fall into two 13.2.3.1 Vapor pressure
categories: dry and wet. Each has its own
The vapor pressure of fluid lubricants for space use
limitations.
is extremely important. Space lubricants typically
13.2.1 Application have a vapor pressure of I Od to 10-l” torr (1.35 x
To be effective, a lubricant must remain in the load 1o-4to 1.35 x 10” Pa) at 1OO°F(38%). Evaporation
area in order to prevent metal to metal contact on and migration of fluid lubricants necessitate the use
working surfaces. In ball bearings, the churning of an extensive sealing system which can be
action created by the rolling balls tends to complex and occupy limited weight and space. The
redistribute lubricant on a continuous basis. In more viscous oils typically have lower evaporation
gears, the tooth action tends to wipe or squeeze the rates.
contact area free of lubricant, especially in slow,
high torque meshes. A planetary gear train will 13.2.3.2 Surface migration (creep)
minimize such lubricant loss since the internal gear Oils vary in their migration characteristics. Mineral
will serve to contain the lubricant, and the circulating oils like Apiezon C have the lowest migration rates,
planets will redistribute some lubricant among while silicone and fluropolymer oils like Versilube
working meshes. A successful, long life, wet
F-50 and Ktytox have the highest. Lubricant creep
lubricated spur gear mesh in space remains to be
may be minimized by the use of flurochemical
demonstrated.
barrier films. However, these chemicals must not
13.2.2 Dry lubricants enter the load contact area.
One dry lubricant which has been in extensive use is 13.2.3.3 Greases
a molybdenum disulphide compound combined
Greases are oils that have been thickened by the
with a carrier. The material is applied to the surface
to be lubricated, and may be subsequently baked, addition of either soaps (e.g. stearates) or fillers
and finally burnished. The thickness of material (e.g. finely divided silica). In most cases, the
after burnishing is typically in the range of 0.0002 thickening agent is not a good lubricant itself, and
inch (0.005 mm). Since metallic geartooth surfaces most of the important properties of the grease are
must be separated by a thin residual film of dry derived from the oil in it. Under vacuum conditions,
lubricant, the Hertzian contact stress which the the oil will eventually outgas, leaving no lubricant
lubricant can carry establishes the load limits for the and sometimes an abrasive thickening agent. A
gear train. Once the lubricant film is destroyed, gear partial listing of space qualified wet lubricants is
failure from scuffing, pitting, and welding can be shown in table 18.
expected. As a rule of thumb, gear contact stress
13.2.3.4 Rocket lubricants
up to 100 000 lb/in2 (690 N/mrr?) has given good
service for low speed, robotic gearing. On meshes Rocket gear trains have been successfully
where the pitch line speed may approach 1000 lubricatedwith either oil or engine fuel, as listed in
ft/min (5 mls), limiting the contact stress to 35 OOO- table 19.

72
 AGMA 91%A94

Table 17 - Candidate solid-film lubricants for space application

Lubricant* Method of Constituents Remarks

application
Lubeco 905 Coated MoS,, PbS & graphite, Good load carrying capacity.
Sprayed + inorganic binder. Low coef. of friction.
Oil contaminates deteriorate lube.
Viirolube Coated MoS,, Ag & graphite, Ideal for hi temp applications.
1220 Sprayed + ceramic binder. Good load capacity. Approx 1000
“F cure temp. limits substrate
hardness. Ceramic binder.
Microseal Impinged on MoS,, + ceramic binder. Moderate load capacity.
200-I Surface Insensitive to oil contamination.
Electrofilm Sprayed/ MoS,, & graphite with Used on moderately stressed
2396 Coated sodium silicate binder. components. Sensitive to oil
contamination.
Electrofilm Sprayed/ MoS, & graphite, phenolic Good load carrying capacity.
4396 Coated binder.
MoS2 Sputtered MoS, ion bombardment Good. solid-film coating. Fair to
in vacuum. good load capacity.
Molykote - - Sodium Silicate binder, Gear and bearings, deployment
x-1 5 MoS, and graphite. mechanisms.
Duroid 5813 - - Teflon reinforced with Self-lubricating material used as
glass MoS, thrust washers, bearing element
separator, etc.
Vespel -- Polymide Used as thrust washers, bearing
element separator, etc.
Lead Ion plated Lead Good load capacity.
* This information is given for the convenience of users of this AGMA information sheet and does not constitute an
endorsement by AGMA of the products named. Equivalent products may be used if they can be shown to lead to
the same results.

13.3 Gear materials for space application gearing are hygroscopic and may therefore
promote rusting of the substrate material. To avoid
In selecting a gear material, the intended duty,
lubricating media, service life, and environment are rust contamination, corrosion resistant steels have
found wide application in steel spacecraft gearing
all factors to be considered.
and structures. For high loads the material can be
13.3.1 Rocket gearing gas plasma or ion nitrided. On some designs
The high speed, high power gearing found in precipitation hardening or maraging steels heat
turbopump drives requires more premium grade treated to the 44 - 48 HRC hardness range are
material than is used in helicopter and turboprop used. In general, tooth wear rate is found to be
units and the suggestions of clause 4 apply. proportional to the square of the Hertzian stress.
One such material which has found extensive use
13.3.2 Spacecraft gearing for space gears is AMS 5617 in the precipitation
Some of the dry film lubricants applied to spacecraft hardened condition.

73
AGMA 911-A94

Table 18 - Candidate fluid lubricants for space application

Lubricant*
T Viscosity
CSt

1OO°F 210°F
T Other
properties
Space history
and remarks
138%) (100°C)

Viscous VI, 100 10 Vapor pressure 6 x 1O-” Used on OS0 II-

Vat-Kote torr (8 x 10 -‘Pa) at INTELSAT III, IV for bearings,
:Hydrocarbon base oil) 77°F (25 “C). slip ring assemblies,
DC motor brushes, etc.
Versilube F-50 52 16 Pour point -100°F (-75 Used on Gemini, Manner
Vlethylchloropheny “C). Useful operating 3 & 4, OGO, Nimbus,
ISilicone oil temp. in air is -1 OOOFto Ranger, Mercury & others.
+450°F (-75 to +230°C). Used on ball bearings & gears.
Aeroshell7 Soap -- 3.1 Pour point -100°F (-75. Used on Ranger 6 - 9.
thickened diester “C). Useful operating
temp. -100 to +3OO”F
(-75% to +15O”C).
Apiezon “c’ 100 100 10 Pour point +I 5°F (-9°C) OS0
Molecularly distilled Vapor pressure 1O*
hydrocarbon oil. ton (1.3 x 1OGPa) at
68OF (20 “C).
Braycote 601 140 45 Dropping point 1500°F Perfluorinated polyether
Grease (820 “C). Vapor pres- with tetrafluoroethylene
sure 1 0-gtorr (1.3 x 10v7 telomere
Pa) at 38.8”F (3.8 “C).
l This information is given for the convenience of users of this AGMA information sheet and does not constitute an
endorsement by AGMA of the products named. Equivalent products may be used if they can be shown to lead to
the same results.

Table 19 - Working fluid lubricants

For lightly loaded applications, plastic gears have Lubricant 1 ~Space history
been applied to space gearing. These are used in MIL-L-7808 Aerojet
combination with stainless steel gears. Plastic ~ Titan
gears giving the lowest wear rate have been MoS,
MIL-L-23699 Rocketdyne
filled polyimide. Turbopump
Mark 3
Hard anodized aluminum gears have been used on
Fuel additive
a few satelliies, however the coating is subject to
~ Kerosene & 3% Zinc diacoele ~ Delta engine
cracking under high loads. Such gears have severe MIPHI Phosphate RS-27
limitations when run against the same material.
i Gaseous hydrogen RL-10
Improved performance will result if the hard
Pratt & Whitney
anodized aluminum gear is run against a plastic
gear in the presence of MoS,. Liquid oxygen I Aerojet

74
 AGMA 911-A94

Annex A
(Informative)
Spur gear geometry factor including internal meshes
[The foreword, footnotes, and annexes are provided for informational purposes only, and should not be
construed as a part of AGMA 911-A94, Design Guidelines forAerospace Gearing.]

A.1 Introduction b is dedendum of gear, in (mm);

Many texts dealing with the capacity of spur gears %ate is addendum of mate, in (mm);
neglect a discussion of internal gear meshes. is circular tooth thickness of gear,
Internal gears are especially important in aero- *P
in (mm);
space applications since planetary reduction
stages are widely used. The definitions of the NOTE - tp may be any value so long as:
involute portion of the internal gear are the same as BLm~ + tp + *pnme =n;
the imaginary external gear comprised of the tooth is fillet radius of gear, in (mm);
spaces of the internal gear. The equations ‘f
*pmate is circular tooth thickness of mate, in
presented herein are written in a form to be
compatible with a computer program. (mm);
%otd is total backlash, in (mm).
To determine the geometry factor, J, for bending
strength of an internal mesh, two general cases A.2 Geometry factor calculation
must be considered as follows: R NG /2 = pitch radius of gear,
- External gear with internal mate: in (mm);
- Internal gear with external mate. Rr R-(k) b = root radius of gear,
The equations presented below are written in a in (mm);
general form and include “signed” integers, & and Rb R cos Cp= base circle radius of
AM, for the gear and mate respectively. The signed gear, in (mm);
integers used in the J factor equation permit their N naate/2= pitch radius of mate,
Rmate
use for both internal and external gears. & and AM in (mm);
are defined as follows:
RGmate R mate + (AM) amate = outside
& = +I for external gear; radius of mate, in (mm);
4 = -1 for internal gear;
Rbmate Rmate cos q5= base circle radius
AM = +1 for external mate; of mate, in (mm);
AM = -1 for internal mate. 7ccos q5= base pitch, teeth/in
pb
The “gear”is defined, as the member for which the J (teeth/mm);
factor is desired, whereas the “mate” is defined as
c (AM)R + (4) Rmate = center
the mating gear. All calculations are based on a
distance, in (mm);
diametral pitch of 1 .OO and a standard center
distance. The equations apply to a true radius
ground fillet (not generated). tan+* =
--
(&#$, @d(Ad~R~

Rb &

The following basic data is required prior to starting

the calculation. + h%f~
c SWJ . ..(A.l)
&
NO?E - all angles are in radians.
where
9 is pressure angle:
tan @h is tangent of pressure angle at
NG is number of teeth in gear;
highest point of single tooth
Nmate is number of teeth in mate; contact on involute.

75
AGMA 911-A94

or
*d +(AG)wq Al= R;+ [d[R, +&I lj-12-Rb+k) p12

% =fan& -(AG) - *i . ..(A.7)

mb
for internal or external gears with tangency point
Rx =* ...(A.4) above the base circle, and
n
where cos lY,=$ I A, for all internal gears and for
tt, is tooth thickness on base circle (may be external gears where the tangency
negative for internal gears whereAo =-l), in point is above the base circle diameter
inches (millimeters); where
rjn is angle which the normal force makes with a r, is the pressureangle at the intersection of the ,
line perpendicular to the tooth centerline at fillet and involute;
highest point of single tooth contact, in A, is the radius to tangency point of fillet and
radians; gear tooth profile, in inches (mm).
Rx is radius on tooth centerline to point of appli- To determine coordinates for the center of the fillet
cation of worst load, in inches (millimeters). radius:
An undesirable condition is a design where the fillet l+bk)
radius for an external gear is tangent to a radial line
below the base circle. This can happen for small
1 2 1 ‘b -6%) *b’
aI= invI;+
fillet radii, small pressure angles, and small num- 2Rb . ..(A.8)
bers of teeth. Since the tangency point of the fillet is
x’=A,$n a’
below the base circle, there can be no involute from
the end of the fillet to the base circle. The equations
are valid for this condition if a “radial line”connector Y’=A,cos a’
is assumed between the base circle and end of fillet. A =a’+I’,
For internal gears, it is obvious that this condition
can not occur since the fillet must always be at a aa= x’ -64~) 7 cos A . ..(A.9)
larger radius than the base circle. For an external
gear, & = +l and the check for this condition is as
follows: bb=y’+&) rf sinA . ..(A.8)
R; -R2
where
(for external gears only)
rf ’ 2R rr aa isxcoordinate of fillet radius center, in inches
...(A.!$
If the above expression is true for an external gear, (millimeters);
then the tangency point of the fillet is belowthe base bb is y coordinate of fillet radius center, in inches
circle diameter and a radial line connector is as- (millimeters).
sumed. To calculateA, for this case, equation A.6 is
used for external gears only. For all internal gears, Figure A.1 illustrates some of the nomenclature
or for external gears whose fillet is above the base used for external spur gears while figures A.2 and
circle, the above check is false (for external gears) A.3 are for internal gears. Figure A.4 demonstrates
and equation A.7 is used to calculate A,. the special case where the fillet center is below the
base circle diameter and the fillet and involute are
A, = y/m @=&%;I connected by a radial line portion.
...(A.6) Afterthe coordinatesof the center of the fillet radius
have been calculated, the next step is to determine
for tangency point below base circle diameter, and h and t, the height and thickness respectivety of the
cos r, = 1.0000 where the tangency point is tooth at the critical section. Figure A.5 illustrates h
below the base circle diameter and tfor external and internal gears.

76
 AGMA 911-A94

& space

bb

I-
B
B-
--Jc
Nl%’
-L

--I r- ‘. -

Figure A.1 - External spur gear nomenclature

77
 AGMA 911-A94

&tooth

A!---
t-
I
1II T
i
A

Figure A2 -Angles locating fillet radius center for internal gear

78
 AGMA 911-A94

k space ctooth &space

bb ++‘i jFx*Y 1 1 T&D

\\II
I
I
4\
\
1RX I
I I Base circle I
I\
1 1II-
L
ri cp.I

I J I

I
I
1I
Figure A.3 - Internal gear nomenclature

79
AGMA 911-A94

I
-1 I I
a ---+jiji fillet

Internal gear

. SPACE

Figure A.4 - External spur gear with fillet

tangency point below base circle

The equations for the calculation of h and t are

different in the zone of the involute and in the zone of
the fillet and are given below for the generalized
radius, A.

In the zone of the involute:

A is radius to the point on involute, in inches
(millimeters); External gear

A lies between A1 and Rx for zone of involute.

[(AG>O)AND(A>AI)AND(A>R~)I
OR [(AG < 0) AND (A <Al)] Figure A.5 - Definition of built-in
section of tooth
cosr= -Rb ...(A.ll)
A A is radius to point on fillet, in inches (millime-
ters).
1 + (AG)
[ 2 1 ‘b -(AG) fb’
[(&>O)~tA<Adl
a =inv r+
2Rb ...(A.12) OR [(AG< 0) AND (A >A1 11
In the zone of the fillet: A lies between Al and R, for zone of fillet.

80
 AGMA 911-A94

[ ~;!+?3;$]
u=arctan~+(&)arccos
ences other than pure bending which are functions
of h and t:
- The axial component of the normal tooth load
...(A.l3) which produces a compressive stress that
NOTE-if (A1sAsRb )thena=a’ (radiallineportion) subtracts from the bending;
- The stress concentration factor which is a
h=(&)R,-(&)Acos[( ‘+;AG’>” NG- wo;]
function of h and t.
...(A.l4) Therefore, the point at which hand tact is defined as
the point which produces the highest total stress
t=2Asin l+@G z;n (&) a] ...(A.%)
I[( 2 9 and not the highest bending stress. Since all the
influencing factors are contained in the geometry
Similar equations may be developed forthe case of factor, J, the object is to determine h and t where J is
a generated fillet. minimum.
After defining the general equations to calculate h J is defined by:
and t at any point, it remains to determine the 1
location where the stress will be the highest. The J=
cos 4n 1.5 tan 4%
classical method which dates from the Lewis
equation was to assume that the tooth had an
Kt---- cost [ x t 1 . ..(A.l6)

inscribed parabola with the apex at the load point t2

and which was target to the tooth as shown in X=4h
figure A.6.
Kt =H + [+]” [$]M . ..(A.l8)
,
where H, M, and L are given in table A.1 .

Table A.1 - H, L, and M for use in Kt

(from linear extrapolation of
Dolan and Broghamer method)
Pressure
angle $ H L M
14.5* 0.22 0.20 0.40
20* 0.18 0.15 0.45
22.5 0.1618 0.1272 0.4728
25 0.1437 0.1045 0.4955
30 0.1073 0.0590 0.5409

Figure A.6 - Constant stress parabola * Original Dolan and Broghamer values

Since a parabola has the property of constant Assuming that H, M, and L are straight line functions
bending stress, section B-B, which is at a point of of the pressure angle, they may be expressed as
tangency between the parabola and tooth profile will follows:
have the maximum bending stress. Note at any H = 0.3255- 0.4167Cp
other section such as A-A, the stress would be the L = 0.3318 - 0.52094
same as at section B-B only if the tooth were of
M = 0.2682+ 0.5209$I
thicknesst~. Howeverintheactualtoothatsection
A-A has more metal outside of tM and therefore where $ is in radians.
has lower stresses. This classical assumption is To find the point where J is a minimum, an iterative
based on constant bending stress. However,in the procedure is used utilizing the equations for h and t
modern definition of tooth stress, there are influ- which have been previously defined as a function of

81
the general radius, A. A simple method is to divide Figures A.7 through A.9 are for external gears with
the section between R, and R, into fBed number internal mates for gears of varying pressure angles,
of intervals, n , and to start at some point such as while figures A.10 through A.12 are for internal
R, . J is then successively calculated using a new gears with external mates. These curves may be
value of A each time defined by: used for preliminary design purposes prior to the
detailed Jfactor calculations using the actual tooth
A=A+(&)A . ..(A.19) dimensions.
J will continue to diminish until a minimum is found, Figures A.7 through A.12 have been developed
at which point the next value of J will increase. The using the equations in thisannex, with the input data
accuracy of the answer will depend on the magni- shown in tables A2 through A.7. Note that all values
tude of the interval A. With the advent of high speed shown are for diametral pitch = 1 .OO. Maximum
computers, the calculation of Jby the above method value a = 1 .O was used for the addendum and
is easily accomplished. Figures A.7 thru A.12 are Br,=O.O4 for the the backlash; minimum value
plots of the geometry factors for internal/external d = 1.24 was used for the dedendum and t = 1.5508
gear meshes for gears with standard proportions. for the tooth thickness.

82
 AGMA 911-A94

.60

56

‘-15 20 30 40 60 80 100 150

Number of teeth in external gear

Figure A.7 - Geometry factor for standard proportions, external gears with internal mates,
addendum = 1 .OO,dedendum = 1.24, tooth thickness = 1.5508, backlash = 0.04, (I = 20”
Table A.2 - Fillet radius used to generate J factor curves in figure A.7
Fillet radius, inches
Number of internal teeth
Number of external teeth
50 70 90 110 150 200 300
17 0.613 0.613 0.616 0.617 0.617 0.616 0.614
20 -- 0.596 0.596 0.596 0.596 0.596 0.596
25 -- 0.561 0.565 0.562 0.560 0.560 0.560
35 -- -- 0.502 0.512 0.526 0.532 0.531
50 -- -- -- 0.448 0.467 0.479 0.492
75 -- -- -- -- 0.412 0.426 0.441
100 -- -- -- -- -- 0.396 0.413
AGMA Qil-A94

-60

56
Number of teeth in

.52

.36 ’
15 20 30 40 60 80 100 150
Number of teeth in external gear

Figure A.8 - Geometry factor for standard proportions, external gears with internal mates,
addendum = 1 .OO,dedendum = 1.24, tooth thickness = 1S508, backlash = 0.04, $ = 22.5”
Table A.3 - Fillet radius used to generate J factor curves in figure A.8
Fillet radius, inches
Number of internal teeth Number of internal teeth
50 70 90 110 150 200 300
17 0.546 0.552 0.554 0.552 0.550 0.546 0.544
20 0.513 0.513 0.532 0.532 0.530 0.525 0.522
25 -- 0.494 0.500 0.498 0.499 0.498 0.493
35 -- -- 0.462 0.463 0.470 0.463 0.466
50 -- -- -- 0.431 0.442 0.441 0.444
75 -- mm -- -- 0.415 0.419 0.429
100 -- -- -- -- -- 0.411 0.417

84
 AGMA 911-A94

.60

.56

‘T
% t$ -52
al5
-cd
-U-
Es

G&8

30 40 60 80 100 150
Number of teeth in external gear

Figure A.9 - Geometry factor for standard proportions, external gears with internal mates,
addendum = 1 JO, dedendum = 1.24, tooth thickness = 1.5508, backlash = 0.04, + = 25O
Table A.4 - Fillet radius used to generate J factor curves in figure A.9

Fillet radius, inches

Number of internal teeth
Number of external teeth
50 70 90 110 150 200 300
17 0.474 0.473 0.474 0.477 0.474 0.471 0.467
20 0.450 0.452 0.454 0.452 0.447 0.441 0.439
25 -- 0.420 0.423 0.419 0.421 0.417 0.412
35 -- i- 0.386 0.393 0.394 0.389 0.384
50 -- -- -- 0.365 0.370 0.366 0.359
75 -- -- -- -- 0.341 0.346 0.337
100 -- -- -- -- -a 0.343 0.330

85
AGMA 911-A94

.62

.60

58

8L 5 .56
--
is?
SE
=g.5454

50
.50

.48 '
40 60 80 100 150 200 300 400
Number of teeth in internal gear
Figure A.10 - Geometry factor for standard proportions, internal gears with external mates,
addendum = 1 .OO,dedendum q 1.24, tooth thickness = 1.5508, backlash = 0.04,$ = 20”
Table A.5 - Fillet radius used to generate J factor curves in figure A.10

Fillet radius, inches

Number of external teeth
Number of internal teeth
17 20 25 35 50 75 100
50 0.412 _- _- -- -- -_ _-
70 0.422 0.425 0.424 -- -- -- _-
90 0.431 0.433 0.434 0.437 -- -- --
110 0.437 0.438 0.439 0.439 0.438 -- --
150 0.444 0.444 0.446 0.448 0.448 0.448 --
200 0.451 0.451 0.450 0.450 0.452 0.453 --
0.455 0.455 0.454 0.453 0.454 0.455 --

86
 AGMA 91%A94

I I
I I

Number of teeth in 75 -
matinn external near 50 .
.62 - ’
35
\
17 20 $
.60 \ \ \ \
\ F
\
h \

.58

.56

.52 ~~

.50
40 60 80 100 150 200 300 400
Number of teeth in internal gear
Figure A.11 - Geometry factor for standard proportions, internal gears with external mates,
addendum = 1 .OO,dedendum = 1.24, tooth thickness = 1 S506, backlash = 0.04, $ = 22.5”
Table A.6 - Fillet radius used to generate J factor curves in figure A.11

I Fillet radius. inches

of external teeth
Number of internal teeth
l--TnsY 75 100
50 0.333 0.360 -- yF
70 0.351 0.345 0.344 -- --
i
90 0.365 0.363 0.357 0.357 --
110 0.368 0.362 0.366 0.364 0.366 -- --
150 0.372 0.372 0.372 0.374 0.370 0.370 --
200 0.375 0.372 0.378 0.378 0.383 0.375 0.385
300 0.374 0.383 0.374 0.378 0.383 0.385 0.382

87
AGMA 911-A94

3 5 .62 -
aao
-s
E6
i2g
= g.60

58
I I II I I I

80 100 150 200 300 400

Number of teeth in internal gear
Figure A.12 - Geometry factor for standard proportions, internal gears with external mates,
addendum = 1.99, dedendum = 1.24, tooth thickness = 1.5599, backlash = 0.94, $ = 25”
Table A.7 - Fillet radius used to generate J factor curves in figure A.12
I Fillet radius, inches
Number of external teeth
Number of internal teeth

I
17 20 25 35 50 75
0241 0241 -- -- -- --
0260 0262 0.267 -- -- --
0.270 0.271 0.274 0.280 -- --
0278 0.279 0.276 0.282 0.282 -- I --
0288 0288 0.288 0.291 0.288 0.288 --
0295 0294 0.294 0.305 0.303 0.290
0295 0.295 0.294 0.306 0.298 0.280

88
 AGMA 911-A94

Annex B
(Informative)
Gearbox Test and Mission Requirements

[The foreword, footnotes, and annexes are provided for informational purposes only, and should not be
construed as a part of AGMA 91 l-A94, Design Guidelines for Aerospace Gearing.]
B.l Introduction For gearbox design and test purposes values of
power and speed are needed for each phase of
Mission profiles are established to determine load,
flight. Figures B.2 and B.3 show typical power and
speed, operating time, and environmental require-
speed values versus time for both a fixed wing and a
ments for gearbox operation during service. The
rotary wing application.
designer uses these parameters to size the gearbox
while testing is performed to assess the integrity of 10 J_Aad-l
----
the design and to uncover any unmanifested F-r --!
problems. Gearbox testing is performed in test rigs
where possible, and finally in the actual application
for certification.
B.2 Mission Requirements
_---m-m------- -

Missionsvary depending on the customer, military or 5

commercial, and on the type of aircraft/ engine such
as propeller driven, helicopter and turbojet engines. 20 _ - - - - -4’o- - - - - - . . - - -

-i

The basic phases of flight are shown, for a 4 lib --

I I I I I i
commercial aircraft, in figure B.l which displays
OO 20 40 60 80 100
altitude versus time. The definition for each % lime
numbered phase is listed as follows:
Figure B.2 - Commercial application,
1 - Takeoff 4 - Altitude change 7 - Landing turbofan aircraft
2 - Climb 5 - Descent 8 - Ground
3 - Cruise 6 - Approach Operations

Other maneuvers include Holding Patterns while

above airports and Go-Arounds during landing
attempts. For some military aircraft Loitering is a
phase that may consume 25% or more of the
mission time.

0 I I I I
0 20 40 60 80 100
% Time
Figure B.3 - Military application, helicopter
For actuator gearboxes, the highest loads are
usually experienced at the lowest speeds and in
many cases these loads occur at zero speeds (zero
Time “power”transmitted). These loads are referred to as
Figure B.l - Commercial application, “holding loads”, “stall ioads”, or “limit loads”. They
phases of flight are def&f by the customer for a specific applica-

89
AGMA 911-A94

tion, and hence, they become a major criteria engine flight certification. The endurance and
governing the design. environmental testing outlined in the following
Commercial applications are often more stringent sections is extracted from the Army Aviation Specifi-
than military in terms of gearbox loading and hours cation AV-E-9593D, 1934 for turboprop and
of use. Military aircraft, however, operate in a more turboshaft engines. Numerical values are listed to
severe environment which may shorten gearbox fife show the stringency of the test requirements which,
for reasons other than load. when completed successfully, will lead to safer
operation of the aircraft. Limit and ultimate load
Figure B.4 represents a 150 hour certification test for testing are included to cover actuator testing.
a helicopter engine that containsa Power Reduction
Gearbox that must be certified also. The test is
designed to be more severe in terms of load than an
equivalent number of hours of field operation. For
example, if power levels below 90 % do not have a
significant effect on the accumulated fatigue dam-
g60
age of the gearing, then only 2-l/2 % of the mission t
time shown in figure B.3 would be applicable to 2
reducing fatigue life, a matter of l-1/2 minutes. By $40
I
comparison, 50 % of the time, 4 500 minutes, is
spent at or above 90 % power in the 150 hour
qualification test. This illustrates that sufficient
operational hours can be accumulated during a
certification test to reveal long term field related
operational problems.
Figure 8.5 shows power usage spectrumsfortypical
2::
0 20 40
% Time
60 80

Figure 8.5 - Milita~~~l,icopters, large and

100

small and large military helicopters. In general, as

the size of the helicopter increases, the percent time 8.3.1 Types of Testing
near the top power levels increase.
8.3.1 .l Development Testing
B.3 Testing Development testing is required to assess the
Testing is performed to assess the integrity of the capability of a transmission design prior to qualifica-
gearbox design and is a requirement for aircraft and tion or flight. Certain tests, as outlined below, are

Repeat the 5-hour cycle 25 times

Figure B.4 - 150 Hour engine certification test

90
 AGMA 911-A94

required to establish and develop basic operational B.3.1.2 Endurance Testing

capabilities such as lubrication system tests and
gear pattern development tests. Other development Endurance tests are performed to assess the
tests are conducted to prove out structural integrity, integrity of the gearbox design and are usually
search out “weak links” and gain confidence in the performed with maximum parameter conditions
design. The ultimate goal is to verify that the design acting simultaneously.
requirements for reliability and maintainability have
- Accessory Gearbox Drive Test - 300 hour
been achieved by the time the transmissions are put
endurance/ 600 starts. Simultaneous operation
into service.
of all drives with each drive subjected to the
- Lubrication Systems Tests - Testing at no load maximum permissible torque or power rating
and varying speeds up to overspeed conditions. including the maximum overhung moment and
Bearing temperatures are monitored for misalignment angle of each accessory. A vibra-
over-temperature conditions. Oil quantity, pump tory and resonant search from engine idle up to
flow rate and pressure are established. Then 125 % of maximum speed under varying loads.
power is applied for development of cooling Tests to be conducted at maximum oil inlet
system requirements; temperature and minimum oil flow. The same
gearbox is to be used for the Static Test;
- Gear Pattern Development Tests - Gears are
tested to determine load distribution at various - Accessory Gearbox Static Test - 150%
power conditions. Visual techniques as well as maximum static torque on all accessory pads
strain gaging are used to determine load diitribu- simultaneously and with the starter loaded to 250
tion as a function of applied load. Machine % of max. starting torque for five seconds;
grinding corrections are used to assure full - Engine Output Shaft Drive Spline - Demon-
contact at the desired power level and to correct strate ability to absorb thrust In either direction
for deflections. After testing, final machine equal to 20% of the circumferential force acting
corrections are added to the production manufac- on the output drive shaft spline at the maximum
turing procedure; continuous torque rating. Perform endurance
- Over-torque Development Tests - Tests are testing at maximum allowable misalignment and
conducted at over-torque conditions using accel- at a speed of 1.15 times the maximum continuous
erated mission profiles to substantiate a higher rating. Demonstrate a continuous operating
equivalent operating life and to demonstrate capability of 1.2 times the maximum continuous
non-catastrophic and fail-safe modes through torque rating;
the use of different inspection and detection - Oil Interruption Test - Operate engine for 30
techniques. Over-torque testing often acceler- minutes at intermediate power (a rating some-
ates failure modes. This is beneficial in a what higher than maximum continuous power)
development test program so that design or with air only at the oil pump inlet. Then with oil
manufacturing”changes can be made in a timely supplied, operate the engine for 30 more minutes.
manner; There shall be no damage to any components as
- Bench Tests - Used to substantiate systems a result of the test;
which have loads induced from external sources, - Windmilling Test (backdriving) - 8 hours
i.e., a shaft and housing which support a propeller continuous operation without component dam-
or a rotor; age or excessive loss of lubricant:
- Vibration Surveys - Vibration surveys are - Low Cycle Fatigue - A test cycle from engine
conducted to determine resonances in the oper- idle to maximum power and back to idle lasting
ating range. Mode shapes for the resonant approximately 5 minutes with a shutdown every
frequencies are found by holographic inter- 15 minutes. Repeat cycle for a total of 3 750
ferometry, rap testing with accelerometers, or cycles. The minimum LCF life of the components
other methods. shall be 15 000 cycles.

91
AGMA 911-A94

B.3.1.3 Environmental Testing induced dynamics. Testing duration is usually from

A number of environmental requirements exist for 50 to 200 hours. Drive shafting, cooling systems, tail
the safe operation of airborne gearboxes. Some of and intermediate gearboxes, and accessory drives
the Military requirements and reason for testing, as are also tested.
outlined in MlL-STD-31 OC, are listed as follows: After ground testing has shown that no major
NOTE - Numericalvalues are not listed here as they development issues exist, the transmission is tested
are extensivelycoveredin the MIL-STD. in flight. Flight and ground testing continue simulta-
neously until qualification is complete. The objective ’
- Low Pressure (altitude) - Gearbox venting, oil
of the flight test program is generally fourfold:
leakage and effect on heat convection and
conduction characteristics; 1) Structural substantiation;
- High Temperature - High temperature storage 2) Demonstration of handling qualities and the
or service conditions; automatic flight control system;
- Low Temperature - Loss of resiliency of gasket 3) Verification of the propulsion/drive system;
materials, congealing of lubricants and tight 4) Confirmation of the mission equipment.
meshing of gears; Structural substantiation testing establishes and
- Solar Radiation - Damaging effects on natural expands flight envelopes including evaluation of all
rubber and plastics; attitudes and conditions of flight. This includes
- Attitude -. Insure proper oil scavenging for all extreme maneuvers and maximum and minimum
flight maneuvers and for leakage during trans- loads on all major components. Handling qualities
portation; confirms basic data for the controls and control
- Rain, Humidity and Fungus - Component system characteristics and evaluates control
damage resulting from wet and warm atmo- reliability and fidelity. Drive system testing confirms
sphere; structural integrity and measures flight loads
required to establish component life. Mission
- Salt and Sand - Component corrosionlerosion;
equipment testing includes instrumentation, naviga-
- Flame - Fires during storage or operation; tion and communications systems quality, human
-Vibration and Noise- Performance degradation factors criteria, and related systems capabilities and
or component breakage and airport operating performance.
requirements;
B.3.1.5 Actuator qualification testing
- Shockor Impact-Operation and transportation
requirements; Testing requirements are determined largely by the
part reliability requirements. Test programs often
- Combined Effects - Where multiple conditions
include, but are not limited to the following:
may exist simultaneously i.e. Altiiude - combin-
ing low pressure with high temperature. Space - - Baseline performance testing;
combining low pressure and temperature with - Thermal vacuum testing;
radiation. - Vibration/shock testing;
NOTE - Referto clause 6 for a more detaileddiscus- - Humidity testing;
sion. - Lie testing;
8.3.1.4 Flight Testing - Structural load testing.
Prior to conducting flight testing, development Baseline performance runs normally are conducted
testing has usually been completed or is well along before and after each test. Structural load tests are
in schedule. Some qualification agencies require tie performed to validate the stress model and to verify
down testing of a helicopter with the complete the load capacity. Thermal vacuum tests are often
dynamic system installed prior to flight test. The expensive, but they are a reliable method of
helicopter is tied to the ground and the rotors determining the mechanism characteristics under
operated to full capabilii levels to simulate flight. environmental extremes. Life testsoften exceed the
The transmission receives full torque as well as rotor operational requirements by a factor from 1.5 to 4.0.

92
 AGMA 911-A94

This is done to ensure that all gearboxes meet the - Visual, dimensional, and non-destructive in-
requirement. spection record of all gearbox components;
Large systems such as a robotic manipulator often - Spectrometric analysis of the oil and chip de-
can only be qualified by testing individual compo- tectors;
nents due to the large size of the assembled unit. - Evaluation of all test data in preparation for a
final report.
Guidance for testing can be found in Environmental
Test Methods and Engineering Guidelines, MIL- 8.4 Gearbox Test Rigs
S-1 OE.
Accessory and actuator gearboxes may be loaded
B.3.2 Test cell and installation requirements by the actual accessories or by using waterbrakes or
The condition of test cell apparatus is paramount to other load absorbing devices on the mounting pads.
the successful completion of a program. Data Electric motors coupled to step up gearboxes are a
acquisition should be complete and should be common method for providing power. Variable
carried out in a timely manner. The following attitude rigs are used to evaluate the effects of oil
checklist may serve as a guideline for complete and churning and scavenging during a simulated mis-
orderly accrual of data: sion. In this rig the whole gearbox is mounted in a
a) Pm-test: stand capable of rotation around two axes, pitch and
- Calibration of test apparatus and instrumen- roll, during operation.
tation; Main reduction gear driie systems would require
- Means of recording test cell temperature, hu- such large loading as well as power absorption
midi and atmospheric pressure; devices as to make this method of testing quite
- Inspection records of all gearbox compo- expensive. The four square system eliminates the
nents;
need for a load dissipating device and reduces the
- Test log for proper certification of test data;
size of the loading device considerably. The system
- Test parameters and sequence to be corn-
is closed loop in that input and output shafts are
mensurate with expected mission require-
coupled together.
ments;
- List of all test equipment and facilities; Figure B.6 illustrates a four square system that uses
- Adherence to all Safety requirements: two gearboxes arranged back-to-back. One end of
- Review normal and emergency shutdown the two piece output (low speed) shaft is held fiied
procedures; while the other end is rotated to take up backlash
- Clean room conditions when required. and then rotated until the desired torque is reached.
2) Startup: The two halves are then locked together and
- Clean chip detectors connected to the prime mover through a clutch. Slip
- Heat or cool test item and oil; rings are used to measure torque, the clutch assures
- Operate scavenge pumps; smooth acceleration of the rig tospeed, and the
- Operate oil pump; power required by the prime mover need only be
- Apply load with load cell; enough to overcome the system friction which
- Check temperature, pressure, speed, vibra- amounts to approximately 2% of the gearbox design
tion, flow indicators and any special test equip- point power. On one side of the rig, the gears will be
ment. loaded as in normal operation (test). On the other
3) During Testing: side (slave), the gears will be loaded on the coast
- Record all data for the established test se- side. Thrust load direction, for helical gears, and oil
quence; jet location may require special consideration, for
- Proper documentation of all problems and the slave gearbox, when designing this type of
shutdowns. system. The advantage of this system is the minimal
4) Post-test: amount of new hardware required, i.e. input and
-Verification of test apparatus and instrumen- output shafts. The disadvantage is that the load is
tation calibration; not variable during testing.

9393
AGMA 911-A94

inet3lkatinn
,IIY.UII~.I”II ‘J ,

T’
c

Test side Slave side Magnetic Prime mover

Output (low speed) shaft

Figure B.6 - Test rig arrangement, back-to-back gearboxes

The top view of another type of four square rig is test for larger complex gearboxes/transmissions.
shown as in figure 6.7. In this arrangement a single The complex testing may be performed in test rigs as
planetary reduction gearbox is coupled to a low described in B.4, or on the next higher assembly
speed gearbox on the left side and a high speed dynamic test as in the case of turbine engine
gearbox on the right. Slip ring assemblies measure accessory or speed reduction gearboxes.
both input and output torque. The gears numbered B.5.1 Rig acceptance tests
Na and Nb have the same values in both the
reduction gearbox and the rig. The low speed shaft These tests should be described in detail by
can be reduced further in speed by choosing 3Na or acceptance test plans (ATPs). The ATP should, as a
4Na etc. The Input Torque Drive is located on the minimum, cover:
low speed shaft and incorporates splines that are a) Pretest procedures;
hydraulically loaded to provide the desired torque.
b) Startup procedures;
The advantages of this configuration are a single c) Load spectrums;
test gearbox is required with all gear teeth loaded
d) Data to be recorded and documented;
and rotating as in normal operation. Also, loading
may be varied during running. The disadvantage is e) Inspections to be performed during test;
the extra gears and attendant bearings that must be 9 Shut down procedures;
designed for each side of the rig and the increase in
size of the prime mover to overcome system friction. g) Post test inspections to be performed.
8.5.2 Next assembty subsystem testing
B.5 Production dynamic testing
Written procedures should be used to define the test
After each air vehicle gearbox/transmission under- parameters as listed in B.5.1 for the gearbox/trans-
goes final assembly, it is prudent to substantiate mission being tested. This type of testing is cost
design performance by a dynamic test. The test effective, but does not have the risk of high
performed can be simple or complex, i.e. spin test for disassemblycosts when infant mortality failures are
small accessory gearboxes or a mini qualification experienced.

94
 AGMA 911-A94

Test gearbox

,- Slip ring Slip ring

/

\-/ Input
1 motor

I
x
I\ I
Low speed gearbox ‘7 Higk&Z$ubox

Hydra& fluid in
Figure 8.7 - Test rig arrangement, variable torque

95
AGMA 911-A94

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96
 AGMA 911-A94

Annex C
(Informative)
References and Bibliography

[The foreword, footnotes, and annexes are provided for informational purposes only, and should not be
construed as a part of AGMA 911-A94, Design Guidelines for Aerospace Gearing.]
1. Anderson, N.E., Loewenthal, S.H.: “Comparison 9. Drago, R. J., Then Anal’icalExperimentalEvalu-
of Spur Gear Efficiency Prediction Methods”, Ad- ation of Resonant Response in High Speed, Light-
vanced Power Transmission Technology, NASA CP weight, High/y Loaded Gearing, ASME Paper
2210, AVRADCOM TR 82-C-l 6, June, 1981. 8O-C2/DET-22, August 1980.

2. Anderson, N.E., Loewenthai, S.H., Black, J.D.: 10. Lisp, T. C. and Zakrapak, J. J., ModalAnalysis of
“An Analytical Method to Predict Efficiency of Air- Gear Housing and Mounts, Seventh International
craft Gearboxes”, ASME Journal of Mechanical De- Modal Analysis Conference, Jan. 30,1989.
sign, v. 108, Sept., 1986. Il. Koster, W-P., Surface /ntegrity of Machined Ma-
terials, Technical Report, AFML-TR-74-60, April
3. Anderson, N.E., Loewenthal, S.H.: Efficiency of 1974.
Nonstandard and High Contact Ratio Involute Spur
Gears, ASME paper 84-DIET-172, presented at the 12. Fuchs, H.O., Shot Peening Stress Profi/es.
Fourth ASME International Power Transmissioin 13. Lauchner, E., Westech Presentation, March
and Gearing Conference, Cambridge, Mass., Oct. 1974, Northrop Corporation, Hawthorne, California.
1984.
14. Ahmad, Aquil, Eaton Corporation.
4. Harris, T.A.: Rolling Bearing Analysis, Wiley, New 15. Daly, J., A Concept for Using Controlled Shot
York, 1966, pp446-450. Peening in Original Gear Design, American Gear
Manufacturers Association Technical Paper
5. Kleckner, R.J., and Pirvics, J.: High-Speed Cy/in-
87FrMl3.
drical Roller Beating Ana&sis (CYBEAN) User’s
Manual, SKF Report No. AL78P023, SKF Indus- 16. Horger, 0-J. and Lipson, C., Automotive Rear
tries, Inc., (NASA Contract No. NAS3-22807), July, Ax/es and Means of improving Their Fatigue Resis-
1978. tance, American Society for Testing and Materials,
Technical Publication No. 72, 1947.
6. Kleckner, R.J., Dyba, G.J., and Ragen, M.A.:
Spherical Roller Bearing Anal&is (SPHERBEAN) 17. Lowenthal, S.H.; Design of Power Transmission
User’s Manual, SKF Report No. AT81D007, SKF In- Shafting, NASA Report, RP-1123.
dustries, Inc., (NASA contract NAS3-22807) Feb., 18. Townsend, D-P., and Zaretsky, E.V., Effect of
1982. Shot Peening on Su/face Fatigue Life of Canburized
and HardenedAlSl9310 Spur Gears, NASA Techni-
7. Hadden, G.B., Kleckner, R.J., Ragen, M.A., and cal Paper 2047,1982.
Sheynin, L.: System lnciuding Ball, Cylindrical, and
Tapered Roller Bearings (SHABERTH) User’s Man- 19. Dudley, D-W., Handbook of Practical Gear De-
ual, SKF Report No. AT81D040, SKF Industries, sign, McGraw-Hill, Inc., 1984.
Inc., (NASA Contract No. NAS#-22690) May, 1981. 20. Prevey, P.S., X-Ray Diffraction Residual Stress
Techniques, Metals Handbook, Ninth Edition,
8. Akin, L.S., Townsend, D.P.: Lubricant Jet Flow Vol.1 0, ASM International, Ohio, 1986.
Phenomena in Spurand Helical Gears with Modified
Addendums-for Radially Directed individual Jets, 21. A. Gerve, B. Kehrwald and L. Wiesner, T. W.
NASA TM 101460, AVSCOM TR 88-C-034, pre- Conlon and G. Dearnaley, Materialscience and En-
sented at the Ffih International Power Transmission gineering 69 (1966), pp. 221-225.
and Gearing Conference, Chicago, Illinois, April, 22. G. Hubler, I.L. Singer and C. R. Clayton, Materi-
1989. a/s Science and Engineering 69 (1985), p. 203.

97
 PUBLISHED BY
AMERICAN GEAR MANUFACTURERS ASSOCIATION
1500 KING STREET, ALEXANDRIA, VIRGINIA 22s14