Free-Surface Grease Flow: Influence of Surface Roughness and Temperature

Grease flow in grease-lubricated systems can often be qualified as free-surface flow. It occurs, for example, in rolling bearings after the churning phase or on open gears. Here only a fraction of the bearing or gearbox volume is filled with grease. Part of the grease is flowing in relatively thin layers induced by centrifugal forces caused by rotation of the various components. In this paper, a model problem is investigated in the form of a free-surface flow of grease on a rotating disc. Experiments have been performed where the onset of flow and remaining grease have been studied varying the surface roughness, temperature and the centrifugal forces. The experiments have been coupled to analytical models describing the flow and temperature distribution in the grease. It was found that the impact of surface roughness could be neglected. The flow is determined by the centrifugal forces and rheology of the grease. Temperature effects the rheology but also the oil separation creating low shear strength/low viscosity layers at the surface.