The Effect of Superfinishing on Gear Micropitting II

The Effect of Superfinishing on Gear Micropitting, Part II

The Effect of Superfinishing on Gear Micropitting Part II

Lane Winkelmann, Omer ElSaeed and Matt Bell, REM Chemicals, Inc.

Introduction

It should be noted from the outset that the data presented in Part I and Part II of this paper was generated by independent laboratories. Superfinishing of the gears was the authors’ sole contribution to these studies. The authors provided no input on the selection of the testing facilities, procedures or parameters. The conclusions listed at the end of this paper were solely those of the testing laboratory.

Part I of this paper discussed the FZG Brief Test of Gray Staining (BTGS), which was designed to quickly induce micropitting. It is an economical test in terms of cost and time to determine how lubricants, lubricant temperature, coatings and surface finishes influence micropitting. The BTGS, showed that superfinishing significantly reduces micropitting in comparison to baseline gears.[4] This finding stresses the importance of surface finish for resisting the formation of micropitting.

Part II of this paper discusses the results of a more intensive micropitting testing performed according to FVAInformationsheet 54/IIV. The mineral oil used for lubrication was an ISO viscosity class 200 which contains a special additive (the nature of the additive is unknown to the authors) to reduce the micropitting carrying capacity. Baseline tests with a nonmodified standardFZGCgear were carried out to demonstrate the micropitting properties of the oil. The test gears were standardFZGCgears which had the surface modified by superfinishing to a low Roughness Average (Ra).  The pitch line velocity during all testing was set to 8.3 m/s and the lubricant was injected at 60°C.

A brief summary of the test procedure taken from FVAInformationsheet 54/IIV is given below:

The micropitting test may be used to determine quantitatively the influence of lubricants (especially additives), the lubricant temperature and other influential factors on micropitting. The micropitting test differentiates between oils and thus facilitates the choice of a lubricant with sufficient micropitting load capacity.

The operating conditions (circumferential speed and lubricant temperature) may be suitably adapted for testing lubricants for a large variety of applications in the micropitting test. To differentiate between the various test options, which are carried out according to the same test sequence, but with different test conditions, they are designated similarly to the FZGscuffing test by test gear type/ circumferential speed/and lubricant (inlet tempera- ture in accordance with the selected test conditions (e.g., standard test: GTCl8.3190; GT = micropitting test).

The micropitting test consists of two parts. It comprises a load stage test followed by an endurance test. In the load stage test, the ability of the gearlubricant tribological system to resist micropitting is determined under specified operating conditions (lubricant temperature. circumferential speed) in the form of a failure load stage. The endurance test provides information on the progress of the damage after higher numbers of load cycles.(FVA 54.1IV Test procedure for the investigation of the micropitting capacity of gear lubricants).[5]

Experimental

Gear design

The gears used were the standard FZGC type gears for micropitting testing. Table 1 gives the general data for these gears.

Baseline Gears

Baseline gears were unmodified from the specifications given in the FVA Information Sheet 54.

Superfinished gears

A set of gears conforming to the specifications given in FVA  Information sheet  54 were finished  using

chemically accelerated vibratory finishing as described in detail elsewhere.[6], [7], This process utilizes   high   density   nonabrasive   media   to

enhance the performance of components that are subjected  to  metaltometal  contact  or bending

fatigue. The Isotropic Superfinish (ISF®) process generates a unique surface when compared to even

the finest honing and lapping in that it has no directionality with a final surface roughness of 0.25 mm Ra or less. This ISF® surface will be referred to as superfinished throughout this paper.

 

Table 1. Specifications are given for FZG C- type gears for use in micropitting testing ac- cording to FVA Information Sheet 54 [5]

 

Figure 1 shows scanning electron microscope (SEM) images at 1000X of a typical ground surface with an Ra of approximately 0.25 mm (top image) and a superfinished surface with an Ra < 0.05 mm (bottom image). Only slight scratches and small dents are visible amongst smooth, plateaus of the superfinished surface.

The Ra of the baseline and superfinished gears were measured in the involute direction of the gear. The values are tabulated in Table 2.

Test rig

The test rig was the same as used in scuffing testing according to DIN 51 354 Part 1, but of reinforced construction and with spray lubrication.

Test runs description

Both the baseline gears and the superfinished gears underwent the following tests:

S Test run 1 was the load stage test in which the loading was increased every 16 hours starting with load stage 5 and ending after load stage 10.

S Test run 2 consisted of a completed load stage test followed by an endurance test. The endurance test starts with an 80hour cycle at load stage 8, followed by five 80hour cycles at load stage 10 (see Table 3).

Figure 1. SEM images of a ground surface (top) and a superfinished surface (bottom)

 

 

Table 2. Listing of the Ra values for the baseline and superfinished FZG gears

 

Baseline gears Superfinished gears
Test run 1 (mm) Test run 2 (mm) Test run 1 (mm) Test run 2 (mm)
Ra1 of Pinion 0.52 0.51 0.13 0.12
Ra2 of Gear 0.44 0.42 0.07 0.07
Ra = (Ra1 + Ra2)/2 0.48 0.47 0.10 0.095

 

Table 3. Contact stresses, duration, and failure limits for load stage test and endurance test

 

Load stage Contact stress, N/mm2 Cycle duration,

hours

Failure criteria
 

 

Load stage test

5 795.1 16  

 

ffm >7.5 mm

6 945.1 16
7 1093.9 16
8 1244.9 16
9 1395.4 16
10 1547.3 16
 

 

Endurance test

8 1244.9 80  

 

ffm >20 mm

10 1547.3 80
10 1547.3 80
10 1547.3 80
10 1547.3 80
10 1547.3 80

 

 

After each 16hour stage of the load stage test and every 80hour cycle of the endurance test, the following inspection and measurements were made on the pinion:

  1. ffm, average profile form deviation, in mm,
  2. GF, micropitting area of gear flank, in %, and
  3. W, weight loss of gear, in mg.

Failure was defined by the average profile deviation (ffm). For the load stage test failure occurred when ffm exceeded 7.5 mm. For the endurance test failure occurred when ffm exceeded 20 mm (see Table 3).

Experimental data

Test run 1

The results of ffm, GF and W are given in Figures 2a,

2b and 2c, for the baseline and superfinished gears, respectively. For the baseline gears, failure occurred at load stage 8, since ffm was approximately 8.5 mm. By the end of load stage 8, approximately 30% of the gear tooth flank was covered with micropitting, which increased to 60% by the completion of test run 1 (Load Stage 10), with W at 54 mg.

 

The superfinished gears however, showed no measurable variation for ffm, or GF at the end of load stage 10. Meanwhile, there was only approximately 8 mg of weight loss on the pinion.

 

Figures 3 and 4 show the presence of micropitting for the baseline pinion and their absence on the superfinished pinion.

 

 

 

Figure 2. Measurements of test run 1 (load stage test) (a) ffm, (b) GF, (c) W on the baseline and superfinished pinions

 

 

Test run 2

Test run 2 consisted of a load stage test followed by an endurance test.

The results of ffm, GF, and W are given in Figures 5a, 5b and 5c, for the baseline and superfinished gears, respectively. For the baseline gears, failure again occurred at load stage 8, since ffm was approximately 8.5 mm. By the end of load stage 8,

approximately 28% of the gear tooth flank was covered with micropitting, which increased to 60% by the end of the load stage test with W at 57 mg.

 

In the endurance test the base line pinion exceeded the 20mm failure limit during the third 80hour cycle at load stage 10 with a ffm  of approximately 20.2 mm. By the conclusion of testing, ffm, GF and W reached 28 mm, 80%, and 128 mg, respectively.

 

 

 

 

 

Figure 3. Pictures of three teeth on the baseline pinion after the completion of load stage 10 of test run 1 show micropitting on approximately 60% of the tooth flank

Figure 4. Images showing the lack of micropitting on the superfinished pinion following the completion of load stage 10 of test run 1 showing no micropitting on the tooth flanks

 

 

 

 

Figure 5. Measurements of test run 1 (load stage test) (a) ffm, (b) GF, (c) W on the baseline and superfinished pinions

 

 

The superfinished gears, showed no measurable change for ffm or GF at the end of the load stage test. There was only approximately 6 mg of weight loss on the pinion.

In sharp contrast to the baseline gears, the superfinished gears showed a negligible ffm of 0.5 mm, no mi-

cropitting (GF of 0%), and a mere 13 mg W by the conclusion of Test Run 2.

 

Figure 6 and Figure 7 show the presence of micro- pitting for the baseline pinion and their absence on the superfinished pinion.

 

 

 

 

Figure 6. Three teeth on the baseline pinion after test run 2 showing 79% of the tooth flank covered in micropitting, with the band of the densest micropitting specified

 

The thin (0.5mm) gray mark on the superfinished pinion was attributed to the lack of tip relief on the mating gear and was not a manifestation of micro- pitting. A better view of the gray mark is shown in Figure 8 where it was investigated under a micro- scope.

Conclusions

  1. The baseline gears had a lower resistance to micropitting.
    1. Profile form deviation was 28 mm by the end of the endurance
    2. Micropitting coverage at the end of the load stage test was 60% and 79% by the end of the endurance
    3. Weight loss was 38 mg after the load stage test, and 129 mg by the end of the endurance test.

Figure 7. Picture of 3 teeth on the superfinished pinion after test run 2 (load stage test and endurance test) showing a thin gray mark that was attributed to the gears not having any tip relief. The gears show no micropitting

 

  1. The superfinished gears never showed micropit- ting nor reached any of the specified failure criteria.
    1. Profile form deviation was 0 mm at the end the load stage test and only 0.5 mm at the completion of the endurance
    2. Micropitting coverage at the end of both the load stage test and endurance test was nonexistent (0 %).
    3. Weight loss was 6 mg after the load stage test, and 13 mg by the end of the endurance test.
  2. The superfinishing results are remarkable de- spite the use of unfavorable oil which showed damage at load stage 8 on the baseline

 

 

Figure 8. Microscope image of the thin gray mark of the dedendum circled on two flanks of the superfinished pinion. An investigation determined that this mark was not micropitting

 

Acknowledgements

The authors would like to thank Dipl.Ing. G Lützig, and Prof. Dr.Ing. W. Predki with the University of Bochum for performing the testing along with Win- ergy for their part in making this information avail- able.

 

References

  1. Snidle, R.W., Evans, P., Alanou, M.P., Holmes, M.J.A., Understanding Scuffing and Micropitting of Gears, RTOAVT Specialists Meeting. Willimsburg, USA, 79 June 2003. RTOMPAVT 109.
  2. Astrene, T., 20 Minutes with Walt Musial, Tribology & Lubrication Technology. May 2004, p. 2634.
  3. Barnett, D., Elderkin, P., and Bennett, W. An Analytical Approach to the Prediction of Micro- pitting on Case Carburized Gears, Gear Indus- try Journal. 4th Quarter 2006. p.3246.
  4. Winkelmann, L., Bell, M. and King, B., The Effect of Superfinishing on Gear Micropitting, Part I, 63rd Annual STLE Meeting, Cleveland, Ohio, USA, May 21,
  5. Forschungsvereinigung Antriebstechnik, V, FVA Information Sheet, Research Projects Nr. 54/IIV, 1993.
  6. Arvin, J., Manesh, A., Michaud, M., Sroka, G., and Winkelmann, L., The Effect of Chemically Accelerated Vibratory Finishing on Gear Metrology, 02FTM01, AGMA Fall Technical Meeting, St. Louis, MO, October
  7. Winkelmann, L., Bell, M., and El Saeed, O., The Capacity of Superfinished Vehicle Compo- nents to Increase Fuel Economy, ASME 2007 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference, September 47, 2007, Las Vegas, Nevada,  USA,  DETC2007

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