Surface Characterization of Contact Fatigue Tested Ground Spur Gears

Abstract

A gearbox with gears of different sizes is a part of the vehicle transmission system that plays an important role in transmitting the engine power. In service, gears fail in many ways but mostly due to surface contact fatigue. Gear failures can cost thousands of dollars. Failure due to contact surface fatigue represents an important fraction on all gear failures (about 20% according to ASM, 1975) and other failure types can have origin from surface contact fatigue. Micropitting is a kind of surface contact fatigue failure which causes pits on the surface of the gear. It looks like dark gray patches to the naked eye. It can be observed in all types of hardened gears and it can occur at a relatively low number of cycles say less than 106 cycles. Surface roughness has the greatest influence on micro pitting. Compressive residual stresses on the surface of the gears can be beneficial in improving the fatigue life of the gears. The aim of this project is to closely study the evolution of residual stresses and surface topography of ground gears that are contact fatigue tested in FZG test rig. To understand the residual stress state and surface roughness change pattern closely test cycles with relatively small intervals are performed. Residual stress and surface roughness were measured on all the tested gears. This report discusses the evolution of residual stresses and the change of the surface roughness on contact fatigue tested carburized ground spur gears. It is found that micropitting starts to initiate at 200 cycles. Compressive residual stresses on the surface tend to increase after 200 cycles and follow a pattern in decreasing over further testing. But, these stress changes are confined only to 10 microns in depth from the gear surface. It is also found from this study that magnitude of residual stress in the axial direction is only one-third of that in the profile direction. The surface roughness keeps reducing gradually until 63k Cycles.