Influence of the geometry on the transient and steady flow of lubricating greases

The objective of this paper is to investigate the influences that geometry, roughness of the sensor system and gap size exert on the rheological measurements of a commercial lubricating grease. With this aim, creep and stress growth experiments, stepped strain rate ramps and dynamic measurements were performed using different geometries and roughness. A significant influence of gap size was found in transient and steady-state experiments in a certain range of strain rate. Roughening the surface of the geometry increases considerably the values of the stress in comparison to the measurements performed with smooth surfaces. The experimental results are discussed taking into account the wall slip phenomenon. In addition to this, different grooves made in the surfaces of the tools seem to eliminate or considerably decrease these wall slip effects. However the influence of geometry or roughness was not detected in oscillatory tests performed inside the linear viscoelastic range.