Dynamic effects in EHL contacts

Abstract

Virtually all EHL contacts operate under conditions where the load, surface velocities and contact radii change with time. In gears the load, slip and effective contact radius change over the tooth meshing cycle; in cams the surface velocities, load and contact radii change, often very rapidly, during cam rotation while in rolling element bearings the load changes continuously as the balls or rollers rotate around the track but may change more rapidly if the external load alters. Predictions of film thickness (and pressure distribution) are relatively straightforward when conditions are constant but under dynamic conditions a separate, lengthy EHL analysis is required for each situation. This paper explores dynamic effects in EHL contacts and attempts to define criteria that can be used to decide where steady state results will give reasonable estimates of the contact's behaviour and where a more detailed analysis is required. It examines the way in which clearances and pressures change when the contact conditions alter. Detailed results are presented showing the effect of changing the entrainment velocity and load. It is shown, for example, that sudden increases in load produce large, localised increases in film thickness and significant increases in the maximum hydrodynamic pressure and stress. It is also shown that high frequency load variations can produce very large changes in clearance, maximum pressure and contact stress. Use of a quasi steady state analysis in these types of situation could overestimate the minimum film thickness and underestimate the maximum stress.