A novel numerical analysis method for predicting non-Newtonian lubricant (grease) distribution in ball bearings

Abstract

Grease is a visco-elastic fluid and the most common lubricant for rolling bearings. During the churning phase (initial phase) of bearing operation, most of the grease is collected at various locations in the bearing, e.g. by the seal, at the bottom cage bar, in the cage pockets, which are then called grease reservoirs. These are formed based on bearing operating conditions, grease type, bearing geometry, bearing type. The grease reservoirs supply lubricant to the bearing contacts via bleeding: during operation the oil bleeds out of the reservoirs and lubricates the contacts. Since the distribution of grease is crucial to the bearing lubrication, the design of bearing components e.g., the cage, shall aim at an optimal grease reservoir creation during the churning phase. It is difficult to observe the effect of component design on the grease distribution through real life experiments, so CFD simulations with MPS (Moving Particle Semi-implicit) method are performed here. Flow Simulations with MPS method can help determine the factors influencing the grease distribution inside the bearing after churning, with the final aim of enhancing lubrication. For the current simulations, a CAE software based on MPS, namely Particleworks, was used.

Grease consists of a base oil, a thickener, and additives. Since it is a non-Newtonian fluid, it has a nonlinear relation shear stress vs shear rate. It is a visco-elastic material, so it does not flow in the absence of a threshold force. For current simulations, the grease rheology has been modelled using the Herschel-Bulkley model. The model was fitted on in-house rheometer test data and was reduced to a single cage-ball segment. All critical kinematic properties were preserved, that is the speeds of both the inner ring and ball surfaces and the oscillation of the ball position within the pocket as it travels through the loaded and unloaded zones of the bearing. The location and the duration of grease injections resulted in different grease distributions in the model. For demonstration purposes, this document only focuses on the results obtained injecting a grease layer on the inner ring surface in front of the contact. This realistically predicted grease reservoirs to be located at the cage bottom, on the top of cage and in the cage pockets. Sensitivity study on particle size, time step, surface tension and the parametric variation of the bearing speed were also performed. This is an important milestone, as we are able for the first time to virtually simulate the onset of grease reservoirs at locations corresponding to evidence from real bearing operations. The distribution of grease observed at the cage pockets is very important for oil film replenishment. This outcome gives us confidence that the MPS method can be used to support the design of bearing components contributing to enhanced lubrication.