The influence of surface material and topography on the wall effect of grease

The phenomenon of grease slip in the boundary layer of a lubricating pipe or a journal bearing is discussed here. A correlation between this phenomenon and the grease structure has been found. Based upon earlier work, it has been shown that a generalised Casson rheological model as well as a proposed yield stress distribution in the lubricant wall layer are useful for an analytical description of the phenomenon. A Couette rheometer was used for testing. As a result of the investigations it is shown that the wall material significantly affects the yield stress distribution in the boundary layer. Essential differences in the stress values are found, especially for plastics and metals.