Optical Measurement of Oil Film Thickness under Elasto-hydrodynamic Lubrication

Abstract

THE use of Newton rings for the measurement of oil film thickness in point or line contact is rendered difficult by the similarity of the refractive index of normal glass and oil. For this reason it is usually applied when the lubricant is not oil (Skinner¹ used glycerine) or when one of the surfaces is not glass. Recently, Kirk² has used interferometry with ‘Perspex’ ; this limits the pressures obtainable, and the peculiar characteristic of elasto-hydrodynamic lubrication, namely, the enormous increase of viscosity with pressure, does not operate. However, by the use of special high refractive index glass (Pilkington, μ = 1.93) we have obtained Newton's fringes with a rotating 1-in. steel ball loaded against a glass plate. Fig. 1 shows typical results; the constriction at the end of the contact zone is clearly visible. The load on the ball was 0.815 kg and its surface speed 12 cm/sec with castor oil as lubricant. The diameter of the static Hertzian contact zone was 2.24 × 10⁻² cm, which is shown in Fig. 1 to give the scale. By increasing the speed and noting when the light bands changed from dark to light the oil film thickness was recorded with the surface speeds up to 40 cm/sec. The results fell midway between the two expressions (equations 15 and 16) published by Archard and Kirk³.