A Quantitative Method to Measure Oil-Separation Properties of Lubricating Greases

The oil-separation properties of lubricating greases are responsible for transporting base oil to the bearing contacts. Therefore, a good understanding of these properties is necessary to predict grease life based on physical grease properties. Currently, oil separation for small, aged grease samples collected from bearings, is studied using so-called maintenance tools. These tools give qualitative insight into the grease properties, e.g., increases or decreases in oil separation after ageing of the grease. In this work, a quantitative, lab-scale method to study oil separation is presented. Using this method, the amount of base oil transferred from a grease sample to a piece of blotting paper is measured based on the difference in light transmission through the oil stain and the dry paper. Translation of transmitted light intensity to oil concentration is accomplished using a modified Lambert-Beer’s law. This enables the determination of the instantaneous bleed rate and oil content. In combination with a physical model, this method can help to improve the understanding of the driving forces behind oil separation, e.g., the affinity pressure and permeability.