EASA Bearing Failure Root Cause PDF
EASA Bearing Failure Root Cause PDF
EASA Bearing Failure Root Cause PDF
ICA R
TR
Tech Note No. 27
AT
Cause and Analysis of Bearing and Shaft Failures in Electric Motors
C
ELE
US
TECH NOTE NO. 27 Reliable Solutions Today! EA SA
SER
N
IO
IC
V
T
E A IA
SSOC
them exceeds the bearing capacity, then the life may be dras-
tically diminished and a catastrophic failure could occur.
10
BEARING FUNDAMENTALS L50 is the LIFE where 1/2 the
The prediction of rating fatigue life, commonly referred to bearings have failed.
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-1
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
OVER-HUNG
For anti-friction bearings, normally used in electric mo-
DRIVE END BEARING
LOAD (OHL) tors, the bearing operating temperature should not normally
AXIAL REACTION exceed 100° C, assuming that the bearing is properly ap-
FOR DRIVE END BEARING: DRIVE END BEARING
plied and lubricated. Exceeding this limit can result in a
RADIAL REACTION = OHL (A + B) + SP & W(A) RADIAL REACTION permanent change in the bearing size due to metallurgical
A 2
AXIAL REACTION = THRUST
changes of the steel and thermal expansion. Both of these
TOTAL
LOAD conditions can cause loss or reduction of the radial internal
P = TOTAL LOAD = X(RADIAL) + Y(AXIAL)
clearances, which can generate excessive temperatures and
L10 LIFE (HRS.) = 1 x 10 6 C a a = 3 FOR BALL BEARING reduced life. At excessive temperatures, hastened by rapid
60n P n = SPEED (RPM)
C = BEARING DYNAMIC LOAD RATING oxidation of the lubricant, softening of the steel will lower the
Y = AXIAL TO RADIAL CONVERSION FACTOR bearing fatigue limit and shorten the bearing life.
X = RADIAL CONVERSION FACTOR
Figure 2
BEARING TEMPERATURE
VERTICAL BEARING LOADING PRINCIPLES Factors that influence the bearing temperature include:
(ANTI-FRICTION) 1. Winding temperature
2. Lubricant temperature
3. Motor thermal circuit (cooling path and method)
4. Oil and grease viscosity
OPPOSITE END
BEARING 5. Bearing seals, shields and lubricant type
REACTION
6. The amount of grease in the bearing and cavity
SIDE PULL 7. Radial internal clearance
8. Ambient conditions, including contamination
A
WEIGHT 9. Bearing load and speed
10. Bearing type and size
(Update - 4/99)
TN27-2 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Tech Note No. 27 Cause and Analysis of Bearing and Shaft Failures in Electric Motors
ADJUSTMENT FACTOR
STANDARD CLEANLINESS
ditives. Soaps such as sodium, calcium, aluminum, lithium
and barium are most commonly used. Polyurea is a synthetic
1 EP ADDITIVE REQ'D.
organic thickener that has been widely used for electric mo-
tor bearings due to its elevated temperature properties.
Polyurea is usually suitable for operating at temperatures in UNFAVORABLE CONDITIONS
UNSUITABLE LUBRICANT
excess of 120° C, assuming the bearing material and clear- CONTAMINANTS
ance are sized properly. Rust and oxidation inhibitors and
tackiness additives are included to enhance performance. 0.1
Present research is making it possible to predict bearing 0.1 1 10
VISCOSITY RATIO
life more accurately. The use of Elasto Hydrodynamic Lubri-
cation theory (EHL), introduced in the 1960s for calculating Figure 4
film thickness and pressure profiles, has been the key to many
investigations and the base for understanding failure modes.
LIFE ADJUSTMENT FACTOR VS. CONTAMINATION
Since the early 1970s, lubrication and film thickness have
LOAD FACTOR BASED ON A SKF FACTOR
been recognized as a significant factor in the life equations.
100
The AFBMA Standard 9/ANSI B3.15 and IS0 281 standards
were modified in 1972 and 1977, respectively, to include this
effect by the addition of the a2(material) and a3(operating con- K = 4
ditions) life adjustment factors [7]. Typical factors that have ADJUSTMENT FACTOR
10
been used are shown in Figure 4. The latest efforts have 1
UNLIMITED OXYGEN
with similar actions taken by other manufacturers commonly
400
lead to a more precise determination of bearing life.
1 2 3 4 5
In addition to new life prediction theories, new lubricants 300
MINERAL GREASE
UNLIMITED OXYGEN
and lubrication methods are being devised which will extend
200
the operating life. Synthetic greases are capable of extend- 6206/1500N/6000 RPM/L50
1. MINERAL LITHIUM
ing grease life significantly as indicated by the oxidation 100 2. MINERAL POLYUREA
3. MIN/PAO POLYUREA
characteristics depicted in Figure 6. Although grease life is a 4. PAO POLYUREA
function of more than just oxidation life, it is a good indicator 0
5. ESTER POLYUREA
of the type of gain that can be made using synthetic grease. 1.00E + 00 1.00E + 01 1.00E + 02 1.00E + 03 1.00E + 04 1.00E + 05 1.00E + 06
Synthetic greases can be formulated with a lower sensitivity LIFE - HOURS
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-3
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
The question frequently asked about greases deals with 2. Inadequate or excessive lubrication
the compatibility of them if mixed during the relubrication pro- 3. Contamination
cess. Table 1 is a guideline to assist in this process. If in doubt, 4. Excessive loading (axial/radial combined)
do not mix without checking with the lubricant manufacturer.
5. Vibration
TABLE 1. RESULTS OF GREASE 6. Misalignment
INCOMPATIBILITY STUDY 7. Improper shaft and housing fits
8. Machinery defects
9. Shaft-to-ground currents
10. Incorrect handling or mounting
11. Load life and fatigue factors
12. Improper application
13. Damaged during transportation or storage
The challenge is to learn how to identify each type of fail-
ure with a high level of certainty and repeatability.
METHODOLOGY OF ANALYSIS
Five key areas that should be considered and related to
one another in order to accurately diagnose the root cause
of bearing failures are:
1. Failure mode
2. Failure pattern
3. Appearance
4. Application
5. Maintenance history
Brief discussions of each of these areas follow.
FAILURE MODE
B = Borderline incompatibility C = Compatible
I = Incompatible The failure modes can be grouped into the following 12
categories which are usually the result of the combined
stresses acting on the bearing to the point of damage or fail-
MOTOR BEARING LUBRICATION PRECAUTIONS ure. This is arbitrarily referred to as the failure mode.
1. All motor housings, shafts, seals and relubrication paths 1. Fatigue spalling, flaking
must be kept thoroughly clean throughout the motor’s life. 2. Fretting
2. Avoid any dirt, moisture, chips or foreign matter contami- 3. Smearing
nating the grease. 4. Skidding
3. Identify the temperature range for the application and se- 5. Scoring
lect a grease that will perform satisfactorily. 6. Abrasive, abnormal wear
4. Over greasing may cause elevated bearing and/or wind- 7. Corrosion
ing temperatures, which can lead to premature failures.
8. Lubrication failure
Be sure to properly purge excess grease.
9. True or false brinelling
5. When regreasing, be sure that the new grease is compat-
ible with the existing grease and that it has the desired 10. Electric pitting or fluting
performance characteristics. 11. Cracks
6. Be aware that synthetic grease may not be as suitable as 12. Seizures
petroleum greases in high-speed applications. Some ap- These modes do not represent the cause of the bearing
plications may require an extreme pressure (EP) grease. problem; instead, they are the result or way that the problem
7. Be aware that some common greases are not desirable is manifested. This is arbitrarily referred to as the failure mode.
for motor applications. If they are too soft, whipping can
occur. If too stiff, noise and poor bleeding characteristics BEARING FAILURE PATTERNS
can occur. Each bearing failure has a certain pattern that is closely
associated with, yet different from, the failure mode. These
CAUSE OF FAILURE
patterns can be grouped into some combination of these
The most common causes of bearing failures are: categories:
1. Thermal overloads 1. Temperature levels (discoloration)
(Update - 4/99)
TN27-4 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Tech Note No. 27 Cause and Analysis of Bearing and Shaft Failures in Electric Motors
2. Noise levels 10. Is the belting or method of connection to the load cor-
3. Vibration levels rect for the application?
4. Lubrication quality MAINTENANCE HISTORY
5. Condition of mounting fits
An understanding of the past performance of the motor
6. Internal clearances
can give a good indication as to the cause of the problem.
7. Contamination Again, a checklist may be helpful.
8. Mechanical or electrical damage 1. How long has the motor been in service?
9. Load paths and patterns (alignment) 2. Have any other motor failures been recorded, and what
was the nature of the failures?
APPEARANCE OF MOTOR AND BEARING
3. What failures of the driven equipment have occurred?
When coupled with the mode and pattern of failure, the Was any welding done?
motor bearing and load appearance usually give a clue as to 4. When was the last time any service or maintenance was
the possible cause of failure. The following checklist will be performed?
useful in the evaluation:
5. What operating levels (temperature, vibration, noise, etc.)
1. Are there signs of contamination in the area of the bear- were observed prior to the failure? What tripped the motor
ings? Any recent welding? off the line?
2. Are there signs of excessive temperature anywhere in 6. What comments were received from the equipment op-
the motor or driven equipment? erator regarding the failure or past failures?
3. What is the quality of the bearing lubricant? 7. How long was the unit in storage or sitting idle prior to
4. Are there signs of moisture or rust? starting?
5. What is the condition of the coupling device used to con- 8. What were the storage conditions?
nect the motor and the load? 9. How often is the unit started? Were there shutdowns?
6. What levels of noise or vibration were present prior to 10. Were correct lubrication procedures utilized?
failure?
11. Have there been any changes made to surrounding
7. Are there any missing parts on the rotating member? equipment?
8. What is the condition of the bearing bore, shaft journal, 12. What procedures were used in adjusting belt tensions?
seals, shaft extension and bearing cap?
13. Are the pulleys positioned on the shaft correctly and as
9. What was the direction of rotation, overhung load and close to the motor bearing as possible?
axial thrust? Are they supported by the bearing wear
patterns? SUMMARY AND CONCLUSIONS
10. Does the outer or inner face show signs of fretting?
There are numerous ways to go about failure analysis.
11. Is the motor mounted, aligned and coupled correctly? The procedure proposed is a simple one that can be easily
Do not destroy the failed bearing until it has been properly taught and communicated to employees with a wide range of
inspected. It is also important to save a sample of the bear- skills and backgrounds. This type of analysis will usually lead
ing lubricant. to the quick elimination of those factors that are not contrib-
uting to the failure. When the problem is reduced to the one
APPLICATION DATA or two most likely culprits, thoughtful analysis will usually lead
It usually is difficult to reconstruct the actual operating con- to the correct conclusion. It is not one’s brilliance that leads
ditions at the time of failure. However, knowledge of the to the truth; instead, it is the ability to sort that which is impor-
general operating conditions will be helpful. The following tant from among all of the unrelated data available.
items should be considered:
REFERENCES
1. What are the load characteristics of the driven equip-
ment and the loading at time of failure? [1] A.H. Bonnett, “The Cause of Winding Failures in Three
Phase Squirrel Cage Induction Motors,” PCI-76-7.
2. Does the load cycle or pulsate?
[2] A.H. Bonnett, “Analysis of Winding Failures in Three
3. How many other units are successfully operating?
Phase Squirrel Cage Induction Motors,” PCI-77-4.
4. How often is the unit started?
[3] A.H. Bonnett, “Prevention of Winding Failures in Three
5. What type of bearing protection is provided? Phase Squirrel Cage Induction Motors,” PCI-78-7.
6. Where is the unit located and what are the normal envi- [4] A.H. Bonnett and G.C. Soukup, “Rotor Failures in Squir-
ronmental conditions? rel Cage Induction Motors,” PCI C85-24, published IAS
7. Is the motor enclosure adequate for the application? IA Vol. 2, September/October, 1986.
8. What were the environmental conditions at time of [5] A.H. Bonnett and G.C. Soukup, “Analysis of Rotor Fail-
failure? ures in Squirrel Cage Induction Motors, “ PCI 87, CH
9. Is the mounting base correct for proper support to the 2495-0/87.
motor?
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-5
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
LIMIT 0.08
)
1g
25
(p)
shown in Table 2.
0
"
25
LIMIT 0.02
TABLE 2
00 0.0
0.
5
"
LIMIT 0.01
2" 063
g
LUBRICANTS
0.
0.2
5g
6" 003
STANDARD SYNTHETIC
0
0.
(Update - 4/99)
TN27-6 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Tech Note No. 27 Cause and Analysis of Bearing and Shaft Failures in Electric Motors
damage to the balls and raceways, such as spalling and pit- the way to spalling. Appendix II provides a more detailed sum-
ting. They are usually employed as part of a predictive mary of the various methods used to eliminate bearing
maintenance program. Some of the keys to success with these currents.
methods are proper selection of the time intervals between
testing, location and measurement methods. Properly done,
these methods can detect a flaw before any detectable change APPENDIX II: METHODS OF REDUCING BEARING
in vibration or temperature occurs. CURRENTS IN MOTORS OPERATED ON PWM DRIVES
There are several other methods available for bearing fault An estimated 25 percent of all bearing failures on PWM
detection, such as acoustic emission, stress wave energy, applications are dv/dt- and carrier-frequency related, accord-
fiber optics, outer race deflection, spectrum analysis, and lube ing to Dr. Tom Lipo of the University of Wisconsin. The following
oil analysis. Some of them can, if done properly, detect faults briefly summarizes the various methods that are being used
at very early stages. For more detailed information, see cita- or investigated to reduce or eliminate damaging bearing cur-
tion 10 in the Reference section. rents on AC industrial motors operated on low-voltage PWM
drivers:
UNUSUAL FAILURES 1. Insulate shaft bearing journals
One or both ends; very effective on larger motors
SHAFT CURRENTS above 200 HP.
During the past few years, a significant increase in prob- 2. Insulate bracket bore to bearing O.D. fit
lems associated with shaft voltages and currents has been One or both ends; use on vertical and horizontal motors;
observed. In many cases, these currents have caused bear- not as durable as shaft insulation at journal.
ing failures, which are identified as fluting or pitting type 3. Use bearing with insulated l.D. or O.D
failures. There are at least three known causes for the Long lead time; not always available.
phenomena: 4. Insulated balls or lubricants
1. Electromagnetic dissymmetry, which is usually inherent in In experimental stage only.
the design and manufacture of the motor. 5. Grounding brush between shaft and ground
2. Electrostatic charges (associated with friction) accumu- Subject to contamination problems and expensive, but it
lated on the rotor assembly. Also, shaft couplings and air does work in many cases.
passage are known causes.
6. Clean up the VFD output waveform
3. Electrostatic coupling caused by extreme power supplies Best overall solution and could include single output re-
such as PWM inverters. actors, limit filters, or motor terminators; can be
Other abnormalities in sinewave power supply associated expensive.
with grounding, unbalances, harmonic content and high com- 7. Reduce drive switching frequency to less than 5kHz
mon mode voltages may also result in induced shaft voltages.
Will cause some noise and loss of efficiency.
In the case of motors used in conjunction with PWM in- 8. Reduce or eliminate common-mode voltage
verters, it is theorized that the terminal motor voltage supplied
Under investigation; can be done by use of filters and
by the drive is not balanced or symmetrical in some aspect.
reactors.
Another possible source of this problem is electrostatic
9. Improve grounding techniques
coupling, which induces a voltage into the shaft large enough
a. Proper cable selection.
to cause currents that damage the bearings. The high dv/dt’s
associated with the GTO and IGBT transistors are the major b. Optimize grounding location; eliminate floating
source of this problem. The amount of load, rotor speed, grounds.
method of coupling and type of bearing lubricant can each c. Proper cable termination—use of current
aggravate the situation. In some cases, insulated bearings connectors.
may not solve this type of problem. 10. Other options
Regardless of the cause of the induced shaft voltage, if its a. Resonant link, zero switching in development stage;
value exceeds .3 to .5 VRMS sinewave, it may produce cur- high cost.
rents large enough to permanently damage the bearings. This b. Switched reluctance driver with slower waveforms;
problem has heretofore been limited to larger motors, usually not yet ready.
500 frame and up (where the stator outside lamination diam-
eters exceeds 20”). In most cases, the current passes through c. Mechanical driver or gear; a step backward!
both bearings. This condition can be corrected by insulating d. Faraday shields in prototype stage.
the outboard bearing on horizontal motors or top bearing on The motor manufacturers have done a good job of ad-
vertical motors. dressing the increased insulation stress on the winding and
This approach is usually not practical on smaller size mo- also offer insulated bearings on larger motors. However, there
tors, where it is now starting to appear when a PWM inverter is no single, clear-cut solution that is economically feasible
with IBGTs is used. for 1-200 HP motors. When asked for recommended solu-
tions on their smaller motors, the first choice is usually output
Figure 22 shows a typical bearing when fluting has oc-
filters at the drive; this seems to give the best results to date.
curred. Depending upon the amount of running time on the
bearing, the raceways may show signs of straight frosting all
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-7
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
DEFECTIVE FITS/SEATS
FATIGUE
(Update - 4/99)
TN27-8 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Tech Note No. 27 Cause and Analysis of Bearing and Shaft Failures in Electric Motors
MECHANICAL—Continued CONTAMINATION—Continued
Figure 14 Figure 18
TEMPERATURE
Excessive thrust on a spherical roller bearing. Color variation due to excess temperature.
Figure 16 Figure 20
Figure 17 Figure 21
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-9
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
SHAFT CURRENTS—Continued
THE CAUSE AND ANALYSIS OF SHAFT STRESS by bending can be treated as a combination of tension and
compression when the convex side is in tension and the con-
The majority of all shaft failures are caused by a combina-
cave side is in compression.
tion of various stresses that act upon the rotor assembly. As
long as they are kept within the intended design and applica-
tion limits, shaft failures should not occur during the expected
life of the motor. These stresses can grouped as follows:
• Mechanical • Environmental
- Overhung load and - Corrosion
bending - Moisture
ELASTIC-STRESS ELASTIC-STRESS ELASTIC-STRESS
- Torsional load - Erosion DISTRIBUTION DISTRIBUTION DISTRIBUTION
- Cyclic • Electromagnetic
- Shock - Side loading Figure 1. Free-body diagrams showing orientation of nor-
- Out of phase reclosing mal stresses and shear stresses in a shaft under simple
• Residual
tension, torsion and compression loading.
- Manufacturing
processes
- Repair processes THE TOOLS OF SHAFT FAILURE ANALYSIS [1] [3] [4]
• Thermal
- Temperature gradients The ability to properly characterize the microstructure and
- Rotor bowing the surface topology of a failed shaft are critical steps in ana-
lyzing failures. The most common tools available to do this
STRESS SYSTEMS ACTING ON SHAFTS can be categorized as follows:
Before the causes of shaft failures can accurately be de- • Visual
termined, it is necessary to clearly understand the loading • Optical microscope
and stress acting on the shaft. These stresses can best be • Scanning electron microscope
illustrated by the use of simple free-body diagrams. • Transmission electron microscope
Figure 1 is taken from the Metals Handbook, Volume 10 • Metallurgical analysis
[1], and illustrates how tension, compression and torsion act
The material presented in this paper assumes that it may
on the shaft for both ductile and brittle materials. In the case
be necessary to employ the services of a skilled metallurgi-
of motor shafts, the most common materials can be classi-
cal laboratory to obtain some of the required information.
fied as ductile. It should be pointed out that failures caused
However, experience shows that a significant number of fail-
* Numbering of figures, references, and tables in Part 4 begin ures can be diagnosed with a fundamental knowledge of
again with Number 1. motor shaft failure causes and visual inspection. This may
(Update - 4/99)
TN27-10 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Tech Note No. 27 Cause and Analysis of Bearing and Shaft Failures in Electric Motors
then lead to seeking confirmation through a metallurgical labo- TYPICAL MOTOR SHAFT LOADING [11]
ratory. Regardless of who does the analysis, the material
presented here will help lead to an accurate assessment of HORIZONTAL BEARING LOADING PRINCIPLES
the root-cause and failure. (ANTI-FRICTION)
OPPOSITE END
FAILURE ANALYSIS SEQUENCE BEARING REACTION
There is no specific sequence for determining the cause
of failure. The order of steps may depend on the type of fail- SIDE PULL THRUST
ure. However, the following may be a useful guideline [4]: & WEIGHT
CAUSES OF FAILURE
Studies have been conducted to try to quantify the causes
of shaft failures. One industry study [2] provided the results DRIVE END BEARING
for rotating machines shown in Table 1. RADIAL REACTION
TABLE 1 TOTAL
LOAD DRIVE END
CAUSE OF SHAFT FAILURES PERCENT BEARING
AXIAL
Corrosion 29% OVER-HUNG REACTION
Fatigue 25% LOAD (OHL)
Other informal studies [6] [8] suggest that the majority of OVERHUNG LOAD
all shaft failures are fatigue related (in the 80 - 90% range).
For motor applications, it is at least the majority of all shaft
failures. The number climbs into the 90% range when the WEIGHT
result of corrosion and new stress raisers are added. Hence,
the main focus of this paper is on failure associated with CRITCAL
fatigue. AREA
Figures 2 and 3 show typical free-body diagrams for typi- Failure Mode: Bending Fatigue & Shaft Rub
cal motor shaft loading.
Figure 4
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-11
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
TORSIONAL LOAD
CRITCAL
AREA
Keyways are used commonly to secure fans, rotor cores The appendix provides a more complete breakdown of
and couplings to the shaft. All of these cause stress raisers. failure modes [7].
However, the keyway on the take-off end or driven end of the As stated previously, shaft fatigue failures can be classi-
shaft is the one of most concern because it is located in the fied as bending fatigue, torsional fatigue and axial fatigue. In
area where the highest shaft loading occurs. When this load- the case of axial fatigue for motors, the bearing carrying the
(Update - 4/99)
TN27-12 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Tech Note No. 27 Cause and Analysis of Bearing and Shaft Failures in Electric Motors
load will fatigue (contact fatigue) before the shaft does. due to rotational, bending fatigue. The surface of a fatigue
Spalling of the bearing raceways usually evidences this. In fracture will usually display two distinct regions as shown in
the bending mode, almost all failures are considered “rota- Figure 14. Region A includes the point of origin of the failure
tional,” with the stress fluctuating or alternating between and evolves at a relatively slow rate (seconds through years),
tension and compression. This is a cycling condition that is a depending on the running and starting cycle and, of course,
function of the shaft speed. Torsional fatigue is associated the load. Region B is the instantaneous or rapid growth area
with the amount of shaft torque present and transmitted load. (cycles through seconds) and exhibits very little plastic de-
Since most shaft failures are related to fatigue, which is formation. If the conchoidal marks were eccentric that would
failure under repeated cyclic load, it is important to under- indicate an unbalanced load.
stand fatigue strength and endurance limits. One way to
establish the strength and limits is to develop an S-N dia-
gram as shown in Figure 10 for a typical 1040 steel.
70 INSTANTANEOUS
60 ZONE WITH
MINIMAL PLASTIC
50 DEFORMATION.
40
REGION A OF
FRACTURE ZONE:
30 SLOW GROWTH
EVIDENCED BY
CONCHOIDAL
20 MARKS.
103 104 105 106 107
LOG CYCLES, N
Figure 10
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-13
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
INSTANTANEOUS
REGION
FRACTURE FRACTURE
SURFACE SURFACE
SLOW
GROWTH SURFACE FINISH EFFECTS
REGION
In most applications, the maximum shaft stress occurs on
the surface. Hence, the surface finish can have a significant
impact on fatigue life. During the manufacturing process and
INITIATION
SITES future handling and repairs, it is important not to perform op-
erations that would result in a coarser shaft. The impact of
surface finish and fatigue life in cycles can be seen in Table 4.
TABLE 4
Figure 14
CORROSION FAILURES
In corrosion failures, the stress is the environment and
the reaction it has on the shaft material. At the core of this
FAILURE PATTERNS
problem is an electrochemical reaction that weakens the shaft.
Failure patterns can be associated with how the shaft Pitting is one of the most common types of corrosion, which
“looks” at the time of failure. Depending upon the type of is usually confined to a number of small cavities on the shaft
material, shaft fractures can be identified by classifying them surface. Only a small amount of material loss can cause per-
as ductile or brittle. foration, with a resulting failure without warning in a relatively
short period of time. On occasion, the pitting has caused
Plastic deformation is always associated with ductile frac-
stress raisers that result in fatigue cracks.
tures, since only part of the energy is absorbed as the shaft
is deformed. In brittle fractures, most of the energy goes into RESIDUAL STRESS FAILURES
the fracture, and most of the broken pieces fit together quiet
well. Ductile failures have rough surfaces, and brittle failures These stresses are independent of external loading on
have smooth surfaces, as shown in Figures 15A, 15B and the shaft. A wide variety of manufacturing or repair opera-
15C [1] [2] [8]. These are an expansion of Figure 2, where tions can affect the amount of residual stress. They include
the stresses are shown. [1]:
• Drawing • Straightening
• Bending • Machining
(Update - 4/99)
TN27-14 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Tech Note No. 27 Cause and Analysis of Bearing and Shaft Failures in Electric Motors
• Grinding • Short blasting or peening The Metals Handbook, Volume 10 [1], Pages 395-396, pro-
• Surface rolling • Polishing vides additional information on this subject.
All of these operations can produce residual stresses by MISCELLANEOUS NON-FRACTURE
plastic deformation. In addition to the above mechanical pro- TYPES OF SHAFT FAILURES
cesses, thermal processes that introduce residual stress
include: There is a broad category of shaft failures or motor fail-
• Hot rolling • Torch cutting ures that do not result in the shaft breaking. The following is
a list of the more common causes. Fatigue failures that are
• Welding • Heat treating caught in the early stages would also fit in the non-fracture
All residual stress may not be detrimental. If the stress is category:
parallel to the load stress and in an opposite direction, it may • Bending or deflection, causing interference with station-
be beneficial. Proper heat treatment can reduce these ary parts.
stresses if they are of excessive levels.
• Incorrect shaft size causing, interference, run out or in-
SHAFT FRETTING [4] correct fits.
• Residual stress, causing a change in shaft geometry.
Shaft fretting can cause serious damage to the shaft and
the mating part. Typical locations are points on the shaft where • Material problems.
a “press” or “slip” fit exists. Keyed hubs, bearings, couplings, • Excessive corrosion and wear.
shaft sleeves and splines are examples. Taper fits seem to • Excessive vibration caused by electrical or mechanical
be an exception to this rule and experience little or no fret- imbalance.
ting. The presence of ferric oxide (rust) between the mating In many cases, bearing failures that are catastrophic will
surfaces, which is reddish-brown in color, is strong confirma- cause serious shaft damage but usually will not result in a
tion that fretting did occur. The cause of this condition is some fracture.
amount of movement between the two mating parts and oxy-
gen. Once fretting occurs, the shaft is very sensitive to fatigue CHECKLIST
cracking; this eventually leads to a fatigue mode failure. Shaft
vibration can worsen this situation if it is not corrected. The following section provides a checklist for use in gath-
ering critical information pertaining to the appearance,
CAVITATION [4] [9] application and maintenance history of the motor and other
related equipment. Some of these questions overlap.
In pumping applications where a liquid rapidly passes over
the shaft, a phenomena known as cavitation can occur. Cavi-
APPEARANCE OF MOTOR AND SYSTEM
ties, bubbles or voids are created in the fluid for short
durations. As they collapse, they produce shock waves that When coupled with the class and pattern of failure, the
ultimately cause craters on the shaft surface. The shaft can general appearance of the motor usually gives a clue as to
be weakened and fail in a relatively short period of time. A the possible cause of failure. The following checklist will be
common approach to minimizing this condition is to use a useful in evaluating assembly conditions:
stainless steel shaft, which has a much enhanced abrasion • Does the motor exhibit any foreign material?
resistance and wear quality. There are also some elastomeric
coatings that often increase resistance to erosion. • Are there any signs of blocked ventilation passages?
• Are there signs of overheating exhibited by insulation, lami-
SURFACE COATING nation, bars, bearings, lubricant, painted surfaces, etc.
Metallic coatings to protect or restore a shaft can cause • Has the rotor lamination or shaft rubbed? Record all loca-
harmful residual stresses, which can reduce the fatigue tions of rotor and stator contact.
strength of the base metal. In most cases, there are enough • Are the topsticks, coils, or coil bracing loose?
safety factors to handle this additional stress. However, if the
• Are the motor cooling passages free and clear of clogging
shaft is being stressed to its design limits, then such pro-
debris?
cesses as electroplating, metal spray or catalytic deposition
could be a source of fatigue failures. • What is the physical location of the winding failure? Is it
on the connection end or end opposite the connection? If
During some plating processes, it is possible to introduce the motor is mounted horizontally, where is the failure with
hydrogen into the base metal. If it is not removed by the ap- respect to the clock? Which phase or phases failed? Which
propriate heat treatment process, severe hydrogen group of coils failed? Was the failure in the first turn or first
embrittlement may occur, which can greatly reduce the ten- coil?
sile strength of the shaft.
• Are the bearings free to rotate and operating as intended?
Shafts repaired by welding are beyond the scope of this
paper. However, caution must be used in this process. The • Are there any signs of moisture on the stator or rotating
selection of the proper weld material, method of application, assembly, contamination of the bearing lubricant, or cor-
stress relieving, surface finish, and diameter transition are rosion on the shaft?
all critical to a successful repair. Not all shaft materials are • Are there any signs of movement between rotor or and
good candidates for repair by welding. shaft or bar and lamination?
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-15
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
• Is the lubrication system as intended or has there been • How would you describe the driven load method of cou-
lubricant leakage or deterioration? pling and mounting and exchange of cooling air?
• Are there any signs of stalled or locked rotor?
MAINTENANCE HISTORY
• Was the rotor turning during the failure?
• What was the direction of rotation and does it agree with An understanding of the past performance of the motor
fan arrangement? can give a good indication as to the cause of the problem.
Again, a checklist may be helpful:
• Are any mechanical parts missing (such as balance
weights, bolts, rotor teeth, fan blades, etc.) or has any • How long has the motor been in service?
contact occurred? • Have any other motor failures been recorded, and what
• What is the condition of the coupling device, driven equip- was the nature of the failures?
ment, mounting base and other related equipment? • What failures of the driven equipment have occurred? Was
• What is the condition of the bearing bore, shaft journal, any welding done?
seals, shaft extension, keyways and bearing caps. • When was the last time any service or maintenance was
• Is the motor mounted, aligned and coupled correctly? performed?
• What is the shaft loading, axial and radial? • What operating levels (temperature, vibration, noise, in-
• Is the ambient usual or unusual? sulation, resistance, etc.) were observed prior to the
failure?
• Do the stress raisers show signs of weakness or crack-
ing? (The driven end shaft keyway is a weak link.) • What comments were received from the equipment op-
erator regarding the failure or past failures?
When analyzing motor failures, it is helpful to draw a sketch
of the motor and indicate the point where the failure occurred, • How long was the unit in storage or sitting idle prior to
as well as the relationship of the failures to both the rotating starting?
and stationary parts (such as shaft keyway, etc.). • What were the storage conditions?
• How often is the unit started? Were there shutdowns?
APPLICATION CONSIDERATIONS • Were correct lubrication procedures utilized?
• Have there been any changes made to surrounding
It usually is difficult to reconstruct conditions at time of
equipment?
failure. However, knowledge of the general operating condi-
tions will be helpful. Consider the following items: • What procedures were used in adjusting belt tensions?
• What are the load characteristics of the driven equipment • Are the pulleys positioned on the shaft correctly and as
and the loading at time of failure? close to the motor bearing as possible?
• What is the operating sequence during starting? PREVENTION
• Does the load cycle or pulsate?
In general terms, a number of practices can be used to
• What is the voltage during starting and operation; is there minimize the probability that a premature shaft failure might
a potential for transients? Was the voltage balanced be- occur. The following are some of the more critical steps.
tween phases?
1. Be sure that the application and the possible loading on
• How long does it take for the unit to accelerate to speed? the motor are well understood and communicated. It is
• Have any other motors or equipment failed on this imperative to know if there is an overhung load. The envi-
application? ronmental conditions are also critical.
• How many other units are successfully running? 2. The motor manufacturer must be sure that proper materi-
• How long has the unit been in service? als are selected. For the most part, steel with the properties
• Did the unit fail on starting or while operating? of hot rolled 1045 steel is adequate.
• How often is the unit starting and is this a manual or auto- 3. The manufacturing processes are critical. During the pro-
matic operation? Part winding, wye/delta, or ASD or across cessing of the shaft, care must be taken not to introduce
the line? stress raisers and to achieve the required shaft finish.
• What type of protection is provided? 4. The installation phase and operation phases are also criti-
• What removed or tripped unit from the line? cal. Care must be taken not to damage the shaft when
coupling it to the driven equipment. For belt driven loads,
• Where is the unit located and what are the normal envi- remember the MOMENT principle (force x distance) in
ronmental conditions? placement of the pulley.
• What was the ambient temperature around the motor at
time of failure? Any recirculation of air? ACKNOWLEDGEMENTS
• What were the environmental conditions at time of failure? The author wishes to express appreciation to the follow-
• Is the mounting base correct for proper support to the ing companies for their contribution of material and pictures
motor? for this project: Weyerhaeuser, Inc.; Goulds Pumps, Inc.; Buck-
• Was power supplied by a variable-frequency drive? How eye Pumps, Inc.; Longo Industries; Brithinee Electric; and
far away is it? Darby Electric.
(Update - 4/99)
TN27-16 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Tech Note No. 27 Cause and Analysis of Bearing and Shaft Failures in Electric Motors
SUMMARY AND CONCLUSIONS [7] C.Y.P. Qiao and C.S. Wang, “A Taxonomic Study of
Fractograph Assisted Engineering Materials Failure
All too often when a motor fails, the major (and some-
Analysis,” Maintenance and Reliability Conference, May
times only) focus is the repair or replacement and getting it
20-22, 1997, Knoxville, TN, P. 501.
“up and running again.” Without down playing the importance
of this goal, time should be spent collecting valuable infor- [8] N. Sachs, “Failure Analysis of Mechanical Components,”
mation that will assist in a root-cause analysis that can be Maintenance Technology, September 1993, Pgs. 28-33.
conducted after the fact. This paper, along with the previous [9] Jeff Hawks of Buckeye Pump, Inc, “Cavitation in a Nut-
ones [10] [11], provides the methodology to analyze and prop- shell,” Pumps and Systems Magazine, December 1997,
erly identify failures, so that, hopefully, the necessary steps Pgs. 22-26.
can be taken to eliminate them. [10] A.H. Bonnett and G.C. Soukup, “The Causes and Analy-
One of the best methods to assist in the analysis of shaft sis of Stator and Rotor Failures in AC Machines,”
failures is to develop a reference library of pictures of known Maintenance and Reliability Conference, May 20, 1997,
causes of shaft failures. The following is a sample of some of Knoxville, TN, P. 29.01.
the more typical types of failures. [11] A.H. Bonnett, “Cause and Analysis of Anti-Friction Bear-
ing Failures in AC Induction Motors,” IEEE Industrial
TABLE 4 Application Society Newsletter, September/October
1. Loading 1993.
• Impact loading [12] J.H. Holdrege, W. Sobier, and W. Frasier, “AC Induction
Motor Torsional Vibration Considerations,” IEEE, PCB-
• Rotational bending
81-2, P. 23.
• Torsional loading
[13] J.E. Shigley, Mechanical Engineering Design, McGraw-
2. Environment Hill, 1963, P. 160.
• Wear
• Pitting
APPENDIX
• Cavitation
The following pictures show some of the more common
• Fretting
shaft failures.
• Temperature
Figures 16 and 17 are of a 1045 series carbon steel mo-
3. Manufacture tor shaft that failed due to rotational bending fatigue over time.
• Excessive stress raisers The point of failure was at the shoulder of the customer take-
• Residual stress off end.
• Surface coatings
• Surface finish
4. Design
• Improper material selection
• Lack of application knowledge
• Design strength
5. Repair
• Welding
• Machining
REFERENCES
[1] Metals Handbook, Volume 10, “Failure Analysis and Pre-
vention,” 8th edition.
[2] C.R. Brooks and A. Choudhury, Metallurgical Failure
Analysis, McGraw-Hill, 1993.
[3] V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical
Failures, John Wiley & Sons, 1974.
[4] A. Das, Metallurgy of Failure Analysis, McGraw-Hill,
1996. Figure 16
[5] J.E. Shigley, Mechanical Engineering Design, McGraw-
Hill, 1963.
[6] N. Sachs, “Failure Analysis of Mechanical Components,”
Maintenance Technology, September 1993.
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-17
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
Figure 20
Figure 17
Figure 21
Figure 19
Figure 22
(Update - 4/99)
TN27-18 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Tech Note No. 27 Cause and Analysis of Bearing and Shaft Failures in Electric Motors
APPENDIX—Continued
Figure 26
Figure 27
Figure 24
Figure 28
Figure 25
(Update - 4/99)
Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.” TN27-19
Cause and Analysis of Bearing and Shaft Failures in Electric Motors Tech Note No. 27
L APPA
ICA R
TR
Electrical Apparatus Service Association, Inc.
AT
C
ELE
US
EA SA 1331 Baur Boulevard • St. Louis, MO 63132 U.S.A. • (314) 993-2220 • Fax (314) 993-1269 • www.easa.com
SER
N
IO
IC
V
T
E A
SSOC
IA Reliable Solutions Today! Copyright © 1999
(Update - 4/99)
TN27-20 Place in Section 9 of your EASA Technical Manual; note its location in Section 15, “Future Tech Notes.”
Version 499DP5M-499