Modeling impact of grease mechanical ageing on bleed and permeability in rolling bearings

The evolution of the film thickness in a grease lubricated rolling bearings is given by a continuous process of loss and replenishment of oil in the rolling track. It is believed that in a long run, the oil film is maintained by the oil that is bleeding from grease reservoirs. The bleeding capabilities of a grease thus have an impact on the grease and/or bearing life. The ability of a grease to bleed the oil changes during the operation of the bearing due to mechanical ageing, which affects the fiber structure of the grease. In this study the lubricating grease is considered as a porous medium formed by the grease thickener and the base oil. The oil bleed behavior of a polyurea grease was explored as a function of mechanical degradation of the grease in numerous full bearing tests. The measured degradation in bleed was interpreted as an evolution of the grease matrix permeability based on Darcy's law. A grease ageing master curve was built to describe the change of permeability of the grease due to mechanical ageing.