Optical Elastohydrodynamics at Extreme Pressures

Abstract

THE theoretical predictions of Dowson and Higginson¹ concerning the size of the lubricant film generated in rolling-bearing line contacts have been adequately confirmed over a fairly large range of pressures and for a wide selection of temperatures, speeds and fluids (see, for example, ref. 2). The theory has been extended to include point and elliptical contact conditions^3–5 and various aspects of non-Newtonian fluid behaviour (see, for example, ref. 6). Lately, however, there has been a need to test the predictions at much higher pressures as the loading in modern bearings constantly increases. Since optical elastohydrodynamic techniques^7,8 have been limited to a pressure of about 7×10⁸ N m⁻² (100,000 pound inch⁻²) by the use of glass as one lubricated element, this investigation has largely fallen to the X-ray method⁹. This technique has been doubted by some workers although it has generally reaffirmed the data obtained with earlier methods of film thickness measurement. Unfortunately, recent results¹⁰ with this method differ greatly from theory. Film thicknesses have been found an order of magnitude below predicted values at pressures of 2×10⁹ N m⁻², and the dependence on load has been generally too great. It is therefore uncertain whether this is a failure of the Dowson and Higginson type of theory or of the X-ray method itself.