A twin disc test rig for contact fatigue characterization of gear materials

Pitting on gear tooth flanks is one of the major causes of failure in power transmissions. Cracks originate at the surface and propagate at a small depth causing the detachment of material debris, which results in craters. Pitting is detrimental as it leads to vibration, noise, loss of efficiency and eventually to the gear un-serviceability. The contact fatigue characterization of gear materials requires a great number of endurance tests on reference gears and is rarely affordable for industries. For this reason, the ISO standard 6336 suggests that tests on rolling pair of disks may be performed in order to compare the pitting durability of either different materials or manufacturing processes. However, the standard does not provide guidance about the geometry of the specimens and the correlation between the results of disc tests and actual gears durability. In this paper a twin-disc test rig is presented, that was conceived to reproduce the contact pressure and the sliding velocity of gears at one particular point along the tooth profile. A criterion for specimens design is also described. The discs were sized to resemble the working conditions experienced by sun gears mounted in the final drive of an axle for medium power Off-Highway vehicles. In particular, the Lower Point of Single Tooth Contact (LPSTC) was considered to design the tests, being the most favourable location for pitting occurrence because of the high contact pressures and unfavourable kinematic conditions.