A Review on Grease Lubrication in Rolling Bearings

Grease lubrication is widely applied to rolling bearings. The consistency of grease prevents it from leaking out of the bearing, makes it easy to use, and will give it good sealing properties. The same consistency prevents an optimal lubrication performance. Most of the grease is pushed out of the bearing during the initial phase of bearing operation and no longer actively participates in the lubrication process, leaving only a limited quantity available, which is stored inside the bearing geometry and on the bearing shoulders (covers or seals). This stored volume strongly determines the remaining lubrication process in the bearing. The distribution of this volume is determined by the grease flow, which is very complex to understand due to the strong nonlinear rheology. There is no consensus on the next phase in the lubrication process. The grease may bleed and provide oil to the raceway; it may be severely sheared in the raceway releasing oil; or small fresh quantities of grease may be sheared off from the volume stored on the shoulder. In addition, the lubrication process may be dynamic. Grease has self-healing properties where fresh grease is supplied in case of film breakdown and self-induced heat development. This article describes the state-of-the-art knowledge on grease lubrication, including grease flow, film formation, film reduction, dynamic behavior, and grease life.