Paper 1: The Hydrodynamics of Flat Contacts: Friction Studies in the Pin and Disc Machine

Abstract

The friction of run-in flat steel specimens lubricated with plain mineral oils has been measured in a pin and disc machine over a wide range of operating conditions. The hydrodynamic region was identified with the aid of electrical contact measurements and the corresponding friction data were considered in terms of the various theories which have been proposed to explain hydrodynamic action in nominally flat sliding contacts. It was concluded that the Lewicki inflow, the surface roughness, and the viscosity-density wedge mechanisms could not explain the observed friction; but that it could be explained if the surfaces formed a wedge whose angle was constant over the whole range of operating conditions. It is suggested that the wedge may be formed during the running-in process by mechanical effects such as flexural distortion. Particularly striking is the strength of the hydrodynamics which can occur in such ‘flat’ contacts: in terms of the wedge analysis the contact was operating close to the maximum theoretical load-carrying capacity. The implications with respect to the use of the pin and disc machine as a test rig are discussed and it is suggested that there might be a practical application in the design of ‘self-adjusting‘ slider bearings.