Abstract

Background

When seeking to eliminate gear failure of case carburized gears the most practical approach is to eliminate the primary form of failure: micropitting (gray staining). The distressed micropitted surface will, as two gears move in mesh together, increase the wear of the opposite tooth flank. As expected the longer a micropitted surface is run against another, the greater the resulting wear. This manifestation of wear causes damage to the flanks of gear teeth by enlarging microcracks and other microscopic disruptions of the surface. This damage decreases the life of case hardened gears and limits the load- bearing capacity by serving as initiation sites for catastrophic gear failure.

Various methods have been used to reduce micropitting in the past, but often sacrifice much. For instance, lubricant additive packages are able to reduce micropitting, but may also increase the wear rate or degrade under elevated temperatures or pressures. Alternatively, applying a coating to the gear flanks can be used to alleviate micropitting[1]. During testing cycles coatings may perform well, but under long term working conditions, often wear or flake off. This increased debris increases the contact area wear rate, and/or travels to and destroys bearings.

The FZG Brief Test of Gray Staining (BTGS) is a supplement to the FZG micropitting test[2] and is economical in terms of time and costs. The BTGS is a method of determining how variables such as lubricants, lubricant temperature, coatings and surface finishes influence micropitting.

Specimen

Three sets of FZG gears were tested; all gears were the sliding-speed-balanced tooth configuration “FZG”, type C-GF. Each set was comprised of a gear and a pinion. Geometric data of the gears, tooth quality, heat treatment and material data are consistent across all of the gear sets. Refer to Table 1 for geometric data of the gears and Table 2 for tooth quality heat treatment and material data.

Table 1: Geometric data of the test gear and pinion teeth for the BTGS Material 16 MnCr 5 ( DIN 1721 0)

Table 2: Test gear specifications for C-GFU specimens for all gears prior to superfinishing

All three sets were specified to have an initial roughness average (Ra) of 0.5 ± 0.1 μm. One set was to be used for baseline comparison. The two remaining sets were superfinished by vibratory finishing as well discussed elsewhere. [3,4] As a result of the superfinishing, the surface Ra was under 0.15 μm. Both sides of the gear teeth were tested.

Test conditions

In accordance with DIN51354 the BTGS is performed on a standard FZG warping test bench with splash lubrication. The test gear is installed on the motor shaft while the test pinion is the driving gear. The standard test conditions for the DGMK BTGS were used, and are compiled in Table 3, while Table 4 contains torque and Hertzian stresses for each power level.

Table 4: Power levels of the BTGS performed.

The lubricant used was FVA 2 +2% LZ 677A. This oil and additive is known to have relatively low micropitting resistance, and was selected for this test based on this criteria. Since operational temperature is a separate factor of a lubricant’s micropitting resistance, the sump temperature in all three tests was held constant. A thermostat held the oil sump at 90 oC on all six test runs to ensure that the lubricant’s additive package would perform equally across all tests.  

The test consisted of a ~1.0 hour run-in cycle at power level 3 followed by ~16 hour duration loaded cycles at power level 7 and 9 respectively. The gears were measured and observed following each loaded cycle. Each measurement was of three teeth spaced equally around the circumference of the gear. The measurement consisted of a profile form deviation, and the final measurement included a picture of the final condition of the three tooth flanks that were measured.

The test for micropitting on the FZG gears is correlated to the maximum profile deviation, or wear. This correlation is due to the deformation caused by micropitting. As such, the tests conducted for this study use this profile deviation (wear) to determine the effects of superfinishing on micropitting reduction.

Data

Concluding all test runs, each gear set was classified into BTGS low, medium, or high resistance to micropitting. The BTGS classifications are defined relative to when the wear exceeds the 7.5 μm failure limit. As shown in Table 5, BTGS low fails after the power level 7 loaded cycle, BTGS medium fails after the power level 9 loaded cycle, and BTGS high does not fail by the conclusion of testing.

Table 5: Categorization of micropitting resistance.

In two runs, the gears specified to have a Ra of 0.5 ± 0.1 μm (or non- superfinished, baseline gears) had average micropitting resistance, according to the researcher’s experience. The two runs exhibited the same results with minimum dispersion. The first run, A-1, had an arithmetic roughness average (Ra test) of 0.48 μm (where Ra test = ((Ra gear + Ra pinion) / 2)) prior to testing and ended with 9.7μm wear. The second run, A-2, had a Ra test of 0.55 μm initially and finished with an wear of 10.3 μm. Both tests resulted in a failure after power level 9, and confirm that the lubricant selected exhibits average micropitting (BTGS-medium) for the investigated operational temperature range of 90 °C. The progression of the wear of these trials is present in Graph 1, and images of the final condition of the three equally spaced tooth flanks along the circumference are given in Figures 1 and 2.