Viscoelasticity and Lubrication: A Review of Liquid Properties

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Abstract
It is now well established from experimental work on rolling contacts under conditions of elastohydrodynamic lubrication that the lubricant does not behave as a simple highly viscous liquid. Conflicting explanations have been put forward to explain how the observed effects are related to the properties of the lubricant at the high pressures and short contact times encountered under conditions where the elastic deformation of the contact surfaces is significant in comparison with the thickness of the film of lubricant. The majority of these approaches treat the lubricant as a viscoelastic material. Detailed knowledge of the properties which the lubricating fluid exhibits in shear and compression over wide ranges of time scale, pressure, and temperature is therefore essential for a clearer understanding of elastohydrodynamic lubrication. The purpose of this paper is to review the state of existing knowledge concerning the linear viscoelasticproperties of liquids, with particular reference to experiments where the time element is determined by the frequency of applied alternating shear. To a first approximation, the relaxational behavior can be fitted by an empirical equation in which pressure, temperature, and frequency are equivalent variables in determining the value of the triple product ( frequency×viscosity×instantaneous compliance ). In greater detail it is shown that liquids exhibit an equilibrium compliance which depends upon chemical composition. The relaxation region is much broader than that described by a single relaxation time and predictions based upon this latter assumption are erroneous.

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