Doctor of Philosophy (PhD)


Mechanical Engineering

Document Type



A comprehensive literature review on physical and chemical degradation monitoring and life estimation models for lubricating greases is presented in chapter one. Degradation mechanisms for lubricating grease are categorized and described, and an extensive survey of the available empirical and analytical grease life estimation models including degradation monitoring standards and methods are presented. In chapter two, irreversible thermodynamic theory is employed to study the mechanical degradation of lubricating grease. A correlation between the mechanical degradation and entropy generation is established and the results are verified experimentally using a rheometer, a journal bearing test rig, and a modified grease worker machine. It is shown that the degradation rate is linearly related to the entropy generation, and that it can be used for estimation of the mechanically degraded grease life. In chapter three, a model is presented that uses the principles of irreversible thermodynamics to predict the life of a lubricating grease undergoing mechanical shearing action. Here we restrict our attention to operating temperatures far below the initial activation energy needed to initiate chemical degradation or base oil evaporation. Thus, mechanical degradation is the dominant degradation process. The predictions of the model are validated using the experimental results obtained by testing three greases subjected to different shear rates and temperatures. In chapter four, mechanical life of grease in an elastohydrodynamic (EHL) line contact between two steel rollers is studied. Grease traction curves are measured and reported in different conditions. Three successive lubricating phases of “Fully grease covered rollers”, “Slippage and grease separation” and “Formation of liquid lubricant reservoir” are observed and their behaviors are examined. The traction of the grease is monitored during a long term mechanical degradation process. Our mechanical life prediction model is applied to the lubricating grease at the contact. In chapter five, chemical degradation is studied from an energy point of view. A theory is introduced based on acquired experimental results, and is verified using a roller tester rig. The theory is used to estimate the chemical life of a grease at different temperatures. Summary and conclusions are given in Chapter six along with recommendations for future studies.



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Committee Chair

Khonsari, Michael M.