On the Behavior of Friction in Lubricated Point Contact With Provision for Surface Roughness

This paper presents a simple approach to predict the behavior of friction coefficient in the sliding lubricated point contact. Based on the load-sharing concept, the total applied load is supported by the combination of hydrodynamic film and asperity contact. The asperity contact load is determined in terms of maximum Hertzian pressure in the point contact while the fluid hydrodynamic pressure is calculated through adapting the available numerical solutions of elastohydrodynamic lubrication (EHL) film thickness formula for smooth surfaces. The simulations presented cover the entire lubrication regime including full-film EHL, mixed-lubrication, and boundary-lubrication. The results of friction, when plotted as a function of the sum velocity, result in the familiar Stribeck-type curve. The simulations are verified by comparing the results with published experimental data. A parametric study is conducted to investigate the influence of operating condition on the behavior of friction coefficient. A series of simulations is performed under various operating conditions to explore the behavior of lift-off speed. An equation is proposed to predict the lift-off speed in sliding lubricated point contact, which takes into account the surface roughness. DOI: 10.1115/1.4000306 Research in elastohydrodynamic lubrication EHL study added a new dimension when researchers turned their attention from rectangular to elliptical conjunctions in the early 1970s. This began in 1970 when Cheng 1 developed a Grubin type inlet analysis applicable to elliptical Hertzian contact. Later, Hamrock and Dowson 2 conducted a comprehensive study on EHL analysis of ball bearings where they showed that the theoretical features are in overall agreement with experiments. More recently, computational schemes that utilize multilevel solvers are extended for prediction of film thickness in lubricated elliptic contacts. The pressure and film thickness on the center-line of the contact are predicted accurately from an equivalent point contact analysis 3. EHL research has progressively continued. Yet most of the published works are limited to “smooth” interacting surfaces where roughness and surface features are absent in the formulation. In fact, the classical analysis of elastic contact between two spheres as formulated and solved by Hertz pertains to smooth surfaces while engineering surfaces are rough. Therefore, when two mating surfaces are pressed together, most of the real contact occurs at the tips of the surface asperities. The asperities of mating surfaces behave like a spring as they interact under an applied load, so that contact is extended over a larger area than the contact between two smooth surfaces. Under a high applied load, the contact region in the central area is surrounded by a fringe in which only higher asperities are involved in contact. On the other hand, a reduction in the real contact area may occur due to light applied load where the asperities are not flattened; this may result in an increase in contact pressure due to asperity peak interaction of a rough surface. It is clear that the classical Hertzian analysis cannot hold for describing the contact of rough surfaces. The elastic contact of rough spheres has been studied by several investigators. Greenwood and Tripp 4 extended the Hertzian theory of elastic contact to a number of discrete microcontacts in an attempt to find the real pressure distribution over the contact area. Yip and Venart 5 formulated the contact between curved rough bodies based on the assumptions that the roughness height distribution is Gaussian and that the deformation of the surface roughness is elastic while that due to waviness is fully plastic. Zhu and Ai 6 presented a numerical solution for the EHL analysis of point contacts where they investigated the surface roughness effects on the average film thickness and the pressure peak. Hu and Zhu 7 proposed a new, simple, and robust numerical approach for the mixed EHL analysis of point contact. The system of equations and the numerical procedure are unified for a full coverage of all the lubrication regions including the full-film-, mixed-, and boundary-lubrications. A comprehensive study con