Traction Measurements in Mixed Elastohydrodynamic Lubrication with a Controlled Circumferential Roughness

The transition between smooth elastohydrodynamics (EHL) and micro-EHL is investigated. Traction measurements in a disc machine with a controlled circumferential roughness are used to infer film thicknesses under the asperity tops. These are then compared with hydrodynamic theory. At the two extremes where there is little pressure rippling between the asperity tops and the adjoining valleys or where there is little pressure in the valleys the theoretical and experimentally inferred films are in reasonable agreement. In the transition region the experimentally deduced films are not reliable. Traction behaviour is estimated in the transition region by making some simple assumptions. The film thickness under the asperities is taken as that given by theory for no pressure in the valleys, and the valley pressure in the transition region is estimated by assuming a simple variation with speed. Theoretical traction curves are calculated by apportioning the load between the valleys and the asperity tops and agree well with measured curves, suggesting that this is a good picture of the contact. The traction behaviour was found to depend on sliding speed in a way typical of hydrodynamic lubrication even for estimated oil films as small as 8 nm.