Superfinishing Gears
Superfinishing Gears
Superfinishing Gears
version coating is wiped off the gears one final time to produce the Table 1—Specifications of Scuffing Specimens.
mirrorlike surface. Property Specification
It is important to note that the reactive chemistry producing the Material AMS 6260 (SAE 9310 Air Melt)
conversion coating is only mildly acidic, having a nominal pH of Heat Treatment Carburized
5.5, and the process is normally carried out at ambient tempera- Surface Finish after Grind (RMS) 16 µin. max.
Hardness (HRC) 60–63
ture. Thus there is no possibility of hydrogen embrittlement or
Effective Case Depth (in.) 0.036–0.042
grind burn, as is common with mechanical grinding or honing Core Hardness (HRC) 36–41
operations. Diameter 3.0 in.
When a number of gears are processed simultaneously, all are Crowning radius of disks with
exposed to the same mechanical and chemical environment such transverse radius of curvature 12.0 in.
that every tooth of every gear is processed identically. This elimi-
nates the need for 100% final inspection. Table 2—Testing Parameters of Two-Disk Apparatus.
Depending on the choice and concentration of the active chem- Peripheral Velocity of Fast Shaft (ft./sec.) 65.62
Peripheral Velocity of Slow Shaft (ft./sec.) 16.21
istry, the process can be controlled to remove stock at a rate of
Mean Entraining Velocity (ft./sec.) 42.49
0.00005 to 0.00040 in./hr. Therefore, aerospace gears with an Sliding Velocity (ft./sec.) 52.49
AGMA quality of Q12 or greater and an initial Ra of 12 µin. or
OEM automotive gears with an initial Ra of 60 µin. can be Table 3—Number of Test Specimens Finished in Each Type Of Media.
superfinished to a low surface roughness in approximately 1.0 Media Type No. of Specimen Sets
hour. Non-abrasive ceramic 3
Health, safety and environmental considerations. The chem- Non-abrasive plastic 2
icals used to produce the superfinished gears of alloy steels are
Table 4—Results of Scuffing Tests.
non-toxic and are classified as non-hazardous by 49CFR (Federal
Hazardous Material Transportation Law). Such products have Test Ground Ceramic Plastic
Scuffing Load (lbs.) 522 933* 933* 933* 776 776
been supplied to a wide variety of industries for more than 15
* No scuffing occurred at maximum testing load and 30 minutes hold time.
years without any health or safety incident.
The waste produced by the process is classified as non-haz- Table 5—Specifications of SAE 8620 Carburized Rolling/
ardous according to the Environmental Protection Agency (EPA), Sliding Contact Fatigue Specimens.
but the waste may require standard metals precipitation to meet Property
local and state discharge regulations. Material SAE 8620
Gear Industry Acceptance Hardness (HRC) 60–61
As with any new technology, initially there were some serious Roughness Average (Ra) (µin.) 26
technical questions that needed answering. This was especially
true for gears used in aerospace or military applications where any
failure could be catastrophic.
Metallurgists had apprehensions that gear alloys exposed to an
acidic chemistry would produce hydrogen embrittlement and/or
intergrannular attack. Both in-house and outside testing quickly
dispelled such fears and demonstrated that this process is metal-
lurgically safe. The results have been presented elsewhere, and
these concerns no longer seem to be an issue (Refs. 8–9).
However, other misconceptions about the process have sur-
faced from time to time. It is the purpose of this article to identify Gary J. Sroka
and explain two common misconceptions. is research & development manager for REM
Misconceptions Research Group Inc., located in Brenham,
Texas. He holds a doctorate in physical chem-
Misconception No. 1. Gear teeth having a mirrorlike surface istry. His research interests include develop-
will not have the proper lubrication properties. Residual machine ment of new products and processes for
lines or a dimpled surface are required for oil retention. superfinishing gear and bearing alloys using
chemically accelerated vibratory finishing.
Two basic facts are known about the correlation between sur-
face roughness and tribological properties: Lane Winkelmann
1.) If two mating surfaces are too rough, boundary or mixed lubri- is a senior research associate at REM
cation occurs. The resulting metal-to-metal contact produces a Research Group. He has developed propri-
etary products and processes for the superfin-
higher operating temperature, increased friction and increased ishing of stainless steels, brass, and carbon
wear. steel alloys and has published several gear-
2.) On the other hand, if the two mating surfaces are too smooth, related technical articles.
ic media. Residual machine lines or deep dimples are not essen- FEATURES:
• Composite Gaging
High Speed Automatic Functional Tester
tial, and in fact are detrimental insofar as these can contain dam- • Composite/Center Distance Checks
• Lead/Taper Checks
• SPC Capabilities
aged metal or act as stress raisers leading to contact and/or bend- • Data Acquisition Screens
ing fatigue.
Misconception No. 2. The relationship between surface rough- For additional information on Gear
Burnishing and/or Functional Gear
ness parameters and component functionality is not well under- Inspection, visit our web site at:
www.itwgears.com
stood and requires advanced mathematics and sophisticated soft-
ware. Therefore, there is no simple method of determining what
Precision Checking
surface will give the desired performance benefit. Heads
1205 36th Avenue West
This misconception arises because of the extreme difficulty to Alexandria, MN 56308 U.S.A.
Ph: (320) 762-8782
Fax: (320) 762-5260
characterize a surface. Today there are about 57 different surface E-mail: itwgears@rea-alp.com