Elastohydrodynamic lubrication at high pressures

Film thickness and traction have been measured in a two-disk machine over a range of rolling and sliding speeds, by using two mineral oils which have previously been studied at lower pressures. The results show: (1) That the lubricant film thickness is correctly given by ‘classical’ elastohydrodynamic theory, even when the behaviour of the lubricant in the high pressure regions is quite different from the Newtonian viscous behaviour postulated in the theory. (2) That over the range of pressure 0.7-2.5GPa both oils behave as elastic solids with a well defined shear modulus. (3) That the elastic compliance of the disks may be comparable with or exceed that in the oil film and must be taken into account in the calculation of the shear modulus of the oils. (4) That at these high pressures the elastic properties, not the viscous properties, of the oil determine the traction when the shear is small. (5) That there is an elastic limit at a critical shear stress above which the shear increases more rapidly than the stress. The magnitude of this critical stress increases with the pressure. (6) That the transition from viscous to elastic behaviour takes place over a relatively narrow range of pressure above which the elastic behaviour becomes insensitive to the rolling speed at which the tests are performed.