Experience with a Disc Rig Micropitting Test

The experimental work carried out was aimed at developing a test method that was able to consistently produce micropitting damage and could discriminate between a good oil (i.e., one that rarely produces micropitting in service) and a poor oil (i.e., one that does produce micropitting in service). The small-scale 3-Disc test rig that was used for this work employs 3 discs to apply the test load to a 12mm–diameter test roller. This test geometry allows a large number of stress cycles (typically 600,000 to 800,000 cycles/h) to be generated at the contact track on the roller.

The disc rig control system allows test parameters such as entrainment velocity, contact stress and slide/roll ratio at the disc/roller contacts to be accurately and independently controlled. This enables the effect of key parameters to be studied in isolation, which is something that cannot be easily achieved using conventional gear test rigs.

The early work carried out using the disc rig was aimed at producing micropitting damage by operating the rig at contact conditions similar to those used in the FZG micropitting gear test method. These early tests confirmed that the damage produced to the roller track exhibits characteristics that are typical of micropitting damage, and showed that the severity of the micropitting produced was affected by the amount of running–in carried out on the roller prior to applying the full test load. A test procedure has been developed which provides a good level of repeatability and which allows discrimination between oils which produce micropitting in service and those which do not. In addition, a study of the effect of slide/roll ratio (SRR) has shown that the severity of micropitting damage increases as SRR increased, whereas at 0% SRR no micropitting occurred and, at negative SRRs, microcracking occurred but not micropitting. This is the way that SRR seems to affect micropitting in gears.