On the evolution of oil-separation properties of lubricating greases under shear degradation

The bleed rate of a grease may change over time due to degradation of the thickener microstructure and a decrease in oil content, as oil is released from the grease. In this work, the bleed properties-quantified by the permeability-of five types of lubricating greases are studied. Controlled shear ageing is performed using a grease worker. It is shown that the bleed rate does not change significantly in a grease worker at room temperature, as opposed to changes in rheology and microstructure. This is explained using a permeability model. It is shown, via a comparison with rolling bearings results, that ageing grease in a grease worker is too mild to yield significant changes in the permeability of lubricating greases.