Elastic Stresses Below Asperities in Lubricated Contacts

The presence of contacting asperities in lubricated rolling bearings modifies the subsurface stress field strongly in the neighborhood of the surface and, to a lesser extent, at larger depths where the maxima of the shear or von Mises stress of a smooth Hertzian contact normally exist. The near surface stresses are of importance because they may result in micropitting, a mode of surface distress which leads to the eventual fatigue failure of the contacting surfaces. A mathematical method is presented in this paper which allows the statistical calculation of important parameters (maximum von Mises stress or maximum shear stress amplitude) of the stress fields generated under elastically deforming asperities during their passage through a lubricated contact. The asperities themselves are modelled using estimates of the surface spectral moments obtained from single-profile trace measurements. The method is applicable to both isotropic and anisotropic rough surfaces. Moreover, the important effect of the shear surface tractions, including tractions over the asperities, is contained in the analysis. Computed examples are presented for different surface textures and film thicknesses in the case of a deep groove ball bearing. Finally, a qualitative attempt is made to correlate features of these stress fields with the presence of surface pitting, and the limitations of the analysis are discussed.