Doctor of Philosophy (PhD)


Mechanical Engineering

Document Type



The present dissertation investigates the effect of surface roughness in the elastohydrodynamic lubrication (EHL). Since many essential components of the machinery such as gears, rolling element bearings, cam-followers, and heavily loaded journal bearings operate under the mixed EHL condition, and given the fact that nearly all engineering surfaces are rough to some extent, there is a need for an extensive research which can realistically quantify the effect of the surface roughness in such applications. We thus seek to develop an applied engineering approach for the treatment of the mixed EHL for prediction of the performance of machinery. Both line-contact and point-contact EHL are studied in this dissertation with a major focus on the line contact. In the present work, numerical solutions are developed to treat the problem of elastohydrodynamic lubrication of rough surfaces (referred to as the mixed EHL). The common EHL equations are solved in conjunction with the statistically-based elasto-plastic deformation of the surface asperities. Formulas are developed that can be readily applied to tribological contact involving both line and point contacts to predict the thickness of the lubricant’s film in such applications. For the line contact, a thermo-elastohydrodynamic approach is presented to predict the traction coefficient, where this model is also used to evaluate the wear rate. In addition, an engineering approach to estimate the traction coefficient and the wear rate with reasonable accuracy is presented. Since a rough surface does not necessarily have an isotropic orientation, a study on the effect of different surface pattern is also included to show how this factor influences the results. Also studied is the effect starvation where it is shown how inadequate lubricant supply at the inlet affects the film thickness, the traction, and the wear rate. To show the utility of the approach, the applications of the developed models in evaluating the performance of spur gear under the mixed EHL regime are presented.



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Committee Chair

Khonsari, Michael M.