Regimes of Traction in Elastohydrodynamic Lubrication

The rheological behaviour of the lubricant in an elastohydrodynamic contact depends upon the properties of the fluid and the imposed conditions of load, speed and temperature. For a lubricant of known rheological properties, it is shown how a ‘map’ can be constructed in which different areas of the map correspond to different regimes of behaviour. The coordinates chosen for the map are non-dimensional pressure and a parameter which is closely related to the film thickness. Given the load, speed and temperature, the operating conditions can be located as a point in the map. Maps have been constructed for three fluids: a mineral oil, polyphenyl ether 5P4E and a traction fluid. In general the maps display four regimes: (a) Newtonian, (b) Eyring, (c) viscoelastic and (d) elastic-plastic. Appropriate constitutive equations are proposed for each regime from which the traction forces can be predicted. The maximum traction coefficients calculated in this way are well supported by experiment.