Prediction of Traction and Microgeometry Effects on Rolling Contact Fatigue Life

Abstract

A refined mathematical model for the prediction of rolling contact fatigue is presented. It analyzes the effect of frictional traction in the contact surface, and of surface asperity slope, on the failure hazard functions applicable to surface and subsurface originated spalls. Major effects of traction on life arise from three sources: (a) increased surface distress micropitting; (b) increased microscopic shear stresses beneath surface furrows; (c) greatly increased macroscopic shear stresses in the zone relatively free from shear-stress which exists, in the absence of traction, between the asperity stress region and the Hertzian shear stress region. The major effect of steeper asperity slopes is to increase surface distress micropitting. A strong effect of traction on the angular orientation of the Hertz stress field is used to correlate experimentally observed changes in the Martin angle of orientation of deformation bands. The correlation permits calculation of the variation in the effective traction coefficient as a function of film thickness/roughness ratio. The traction coefficients obtained are then used as input to numerical life prediction. Satisfactory agreement is obtained between theory and experiment in predicting the life of seven groups of fatigue tested ball bearings with different surface roughness, run at different film thickness/roughness ratios.