Study on the lubrication state and pitting damage of spur gear using a 3D mixed EHL model with fractal surface roughness

For high-speed, heavy-load gear units, the meshing tooth surfaces are generally under the mixed lubrication state, which is crucial for gear pitting or micro pitting damage. To clarify the effects of lubrication state on tooth pitting damage, carrying out both numerical and experimental studies on the contact severity of gears under different lubrication states is necessary. In this paper, a 3D line-contact elasto hydrodynamic lubrication model considering fractal surface roughness was developed and used to investigate the lubrication characteristics of involute gears. Both the distributions of pressure and film thickness fluctuated under the rolling contact of rough surfaces. The fluctuations became more substantial with the increase of load and roughness. Furthermore, from the calculated film thickness ratio lambda, the contact of gear tooth surfaces with a fractal roughness of Ra = 0.8 mu m was always under full lubrication state (lambda > 1). When the roughness was increased to Ra = 3.2 mu m, the contact was first transmitted into the mixed lubrication state (0.2 < lambda < 1) under the load of 1000 N*m and finally deteriorated to boundary lubrication state (lambda < 0.2) under the load of 2000 N*m. The boundary lubrication state that occurred under the contact of highly rough surfaces could induce the formation of gear pitting damage. The contact fatigue test showed the gear tooth surface roughness increased from 0.7 mu m to around 2.7 mu m after 8 million running cycles and then suffered pitting failure after another 2 million cycles, which was consistent with the simulation analysis prediction.