Fourth Paper: Shear Behaviour of Elastohydrodynamic Oil Films at High Rolling Contact Pressures

Abstract

The variation with sliding speed of the traction transmitted by E.H.L. oil films has been measured in a rolling contact disc machine, at contact pressures up to 260,000 lbf/in2 and at various rolling speeds and disc temperatures. From the traction measurements at very low sliding speeds, under isothermal conditions, the variation in apparent viscosity of the oil with pressure and temperature has been found. The variation with temperature at high pressure was exponential with an index similar to that at low pressure. The variation with pressure showed a striking reduction in the rate of increase with pressure in the high pressure region (> 105 lbf/in2). At high sliding speeds the traction coefficient was found to approach a common ‘ceiling’, which was largely independent of contact pressure, rolling speed and disc temperature. The ceiling appears to be a function of sliding speed only; it falls from 0.07 at 5 in/s to 0.04 at 60 in/s. Using the measurements of apparent viscosity obtained at low rolling speed, it has been shown conclusively that the limitation (or ceiling) in traction at intermediate sliding speeds cannot be explained by Crook's theory of a reduction in Newtonian viscosity by frictional heating. Alternative explanations based upon plastic shear of the lubricant when a critical shear stress is reached are much more consistent with the observations. But further work is necessary to elucidate fully the behaviour of E.H.L. films in sliding.

History, Origins and Prediction of Elastohydrodynamic Friction

“…Throughout the 1960s and 1970s a series of increasingly accurate friction measurements were obtained using twin disc machines, in particular by the research groups of Johnson [18][19][20][21][22][23][24][25] and Hirst [26][27][28][29][30][31]. Traction curves were obtained over a range of entrainment speeds, contact pressures, temperatures, lubricant viscosities and lubricant types.…”

“…In 1967 Johnson and co-workers measured EHD friction in a power-circulating twin disc machine that enabled very accurate control of sliding speed over a wide range of conditions [18] [19]. They deduced that it was not possible to explain the observed friction behaviour at high sliding speeds solely in terms of either shear heating, as suggested by Crook [14] or a shear rate-independent limiting shear stress, as proposed by other researchers [13][32] [33].…”

Flow of a Fluid Through a Porous Solid Due to High Pressure Gradients

“…Bridgman [6] in his book cites a personal communication from Andrade that provides a relationship of the viscosity to the temperature, density and pressure 1 (see also the discussion in [10]). There has been a great deal of experimental data since Bridgman's book that document the variation of viscosity with pressure (see [11,12,13,14,15,16,17,18,19,20,21,22]). In most liquids, while the variation in the viscosity can be of the order of 10 8 (see Bair and Kottke [21]), the variation of the density due to variation in the pressure is of the order of a few percent (see Dowson and Higginson [23], Rajagopal [24]), and thus it would be reasonable to assume that the liquid is incompressible while the visocity varies with the pressure.…”

Influence of Elastohydrodynamic Lubrication on the Life and Operation of Turbine Engine Ball Bearings. Bearing Design Manual

“…In the ball-race and ball-cage inlet regions the heat generated due to the shearing for:e FS and sliding force FR is calculated as, = 2 FRV + FsU (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) where V = fluid entrainment velocity at the contact center u = sliding velocity at the contact center…”

“…The heat generated by fluid shearing between the cage and land is calculated as the product of the cage friction moment and rotational speed, i.e., qc Mc "1• -WcI (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) where Mc is given by Eq. (3-12) and 1w -wcl is the absolute value of the difference between the cage speed and the speed of the ring that guides the cage.…”