Lubricant Film Thickness and Wear in Rolling Point Contact

Abstract

Lubricant film thickness in the partial elastohydrodynamic range was measured as a function of rolling speed in a rolling 4-ball configuration, for four mineral oils, two esters and a polyphenylether covering a viscosity range of 8–360 cs. Film thickness is shown to vary with speed as a power function. At the point where a full elastohydrodynamic film is formed, the product of viscosity and speed is constant for most lubricants. Wear rate w after run-in follows a simple wear law w = Ka where a is the total area of asperity contact and K is a constant characteristic of the lubricant, of the order of 1 μg/inch³ for compounded ester type lubricants rating these as best, and 4 μg/inch³ for ester base stock, 6 μg/inch³ for polyphenylether and 13 μg/inch³ for mineral oil base stock, rating these lubricants progressively poorer in the order given, regarding their wear-preventing ability. A hypothesis regarding size and shape of wear particles is derived, suggesting platelet shaped particles. Limited data appear to confirm an exponential distribution of wear particle diameters with a mean of the order of 10⁻⁴ inch.