Oil-Bleeding Model for Lubricating Grease Based on Viscous Flow Through a Porous Microstructure

Abstract One of the criteria in selecting lubricating grease for rolling-element bearing applications is its ability to bleed oil, sometimes called “grease bleeding.” Oil bleeding is assumed to be the dominating mechanism supplying new oil to the rolling track for lubrication. In this study, a physical model has been developed to understand the relation between parameters that control oil bleeding. In the model, lubricating grease is described as a porous network, formed by the thickener fibers, that contains the base oil. This type of structure is confirmed by SEM and AFM images of a lithium complex grease showing a matrix of rigid fibers with random orientation. A relatively simple flow model based on Darcy’s law for viscous flow in porous media and an anisotropic microstructure deformation model was developed. The model relates the pressure gradient, oil viscosity, thickener structure deformations, and permeability to the volumetric oil flow out of the thickener network. The permeability depends strongly on the thickener microstructure. The model was verified with experiments at a wide variety of temperatures and rotational speeds. KEY WORDS: Oil BleedMicrostructure PermeabilityGrease LubricationRolling-element Bearing ACKNOWLEDGMENTS The work described in this article was conducted at SKF Engineering & Research Centre, and the authors thank Prof. E. Ioannides, Director Product R&D, for his kind permission to publish this article. Review led by Jane Wang