Modeling Oil-Separation Properties of Lubricating Greases

Base oil separation from the thickener structure of a lubricating grease, also known as “bleed,” is an essential process for grease lubrication of bearings. To get more insight in this process, a model is presented to describe the bleed behavior in terms of grease parameters such as base oil content, base oil-thickener affinity, thickener permeability and elasticity, and viscosity of the base oil. With this model the bleed rate can be calculated for a given external pressure gradient. Blotting paper is used to create this external pressure gradient as well as to measure the amount of bled oil. The model calculations are compared with experimental results of the bleed rate of grease samples with varying initial oil content and describe the overall bleeding process quite well.