Debris and Roughness in Machine Element Contacts: Some Current and Future Engineering Implications

Abstract

The influence of surface roughness and third-body particulates on tribological contacts will be reviewed in general, but in particular their influence on rolling contacts and contacts where rolling and sliding occur simultaneously will be considered in some detail. Such non-conforming contacts are often referred to as Hertzian, and when lubricated are often considered to be protected by an elastohydrodynamic (EHD) lubricant film. Transferring these concepts to the loaded interfaces of real engineering components such as gears, rolling bearings, constant-velocity joints and cams, etc., can have serious shortcomings and lead engineers to many misconceptions. Roughness often appears to dominate over the theoretical size of lubricant films and can create a contact geometry and local stress distribution far removed from the smooth surface case. Even in ball-bearings where surfaces are some of the smoothest of all mass-produced engineering components, lubricant-borne debris particles are usually many times the theoretical film thickness and the damage they can cause can quickly destroy the production engineer's good work in creating smoothness, as well as having a significant influence on component performance. The paper reviews such aspects in terms of recent research and explains some of the many practical implications of roughness and debris that are faced at present, and also speculates on the implications for the future, when roughness geometry and surface properties might be engineered to suit a particular lubricant and its potential contaminant population.