MIXED LUBRICATION MODEL FOR MICROBEARINGS CONTAINING MICROASPERITIES OF DEFINED GEOMETRY

For applications involving small-size bearings, the hydrodynamic lubri cation conditions are not fulfilled and a mixed lubrication model, inv olving geometrical characteristics of the microasperities which are re sponsible for abrasion, is presented. In micromechanical systems, the mixed lubrication condition is assumed to be a steady-state regime. Ab rasion is considered as the dominant phenomenon and its dependence on the geometry of microasperities is studied. It is assumed that the lub ricant properties do not significantly influence the lubrication condi tions in this model. The degree of lubrication is averaged by a modifi er. Conical, pyramidal and spherical shapes of microasperities are ass umed. It is found that for conical and pyramidal microasperity shapes, the friction coefficient depends only on the tip, angle of the protru sions. For a spherical shape, the friction coefficient depends also on the depth of the wear plough. Experiments done with macroscopic sampl es for dry and mixed lubrication show no detectable correlation betwee n the macroshape of the test sample and the friction coefficient. Assu ming in turn, for the same samples, uniform and regular shapes of the microasperities, geometric characteristics of these regular shapes can be determined. Reciprocal, for a certain configuration of the microas perities, the friction coefficient can be anticipated. This model prov ides a quantitative guide to the proper design of surface topography i n order to get a minimal friction coefficient during the mixed lubrica tion regime.