The process of churning in grease lubricated bearings: Identification and Thermo-mechanical characterization

Most rolling bearings are lubricated by grease. The main disadvantage of using grease is that it has a finite life, which is usually shorter than the life of the bearing. It is therefore important to select grease for which grease life is expected to be long and to be able to predict the end of grease life. Grease life is governed by grease degradation which is a thermodynamic process. The degradation process happens during the two phases a grease-lubricated bearing goes through during its life: the ‘churning phase’ and the ‘bleeding phase’. This thesis describes the evolution of some important properties of grease during the ‘churning phase’. One could also see this as a ‘running-in phase’, where much of the mechanical degradation will take place. This thesis will describe bearing experiments in combination with studies on the evolution of the microstructure of lubricating grease using Atomic Force Microscopy, the flow properties (rheology), and the
grease distribution in the bearing. First, it will be shown that greases can be characterized as ‘good churning’ and ‘poor churning’ greases. The research in this thesis is aimed at understanding the grease degradation process during churning and towards the development of a theory/model to predict the impact of this degradation on grease life. The thesis is concluded with the formulation of new grease material properties and methods to measure them, laying the foundation for
the development of even better models in the future, for improved grease selection methods, and the formulation of novel lubricants. This will lead to further extension of grease life, improvement in efficiency, predictability, and reduction of bearing friction and energy losses.