Thickness of very thin films in elastohydrodynamic lubrication

Abstract

Films of lubricant were formed between two loaded rolling steel discs, and the thicknesses of the films were estimated from measurements of the electrical capacitance between the discs. Two mineral oils and one synthetic diester were used as lubricants. The minimum film thicknesses ranged down to approximately 10 nm (100 A), and agreed approximately with the predictions of an isothermal Newtonian theory in which the viscosity of the lubricant, measured in bulk, was used. Of the four sets of results examined, only one, that for the di-ester, gave measured film thicknesses greater than the theoretical ones. The discrepancy of 13 % is probably within the systematic errors of the experiment and of the theory. With this exception, there was therefore no evidence that the viscosity of the lubricant was affected by the close proximity of the metal surfaces. The maximum shear rate in these experiments was of the order of 107 s–1, i.e., several orders of magnitude greater than those obtaining in other experiments reported in the literature, in which the viscosity of a fluid was increased by the presence of the surfaces.