Abstract
An optical elastohydrodynamic rig was constructed to measure the inwards radial force of a ball rolling on a plate. The axis of rotation of the ball could be set to give known amounts of side slip and spin. Hertz pressures up to 2 GPa could be achieved. Four fluids were used, a tractant, 5-phenyl 4 ether, a paraffinic cylinder stock and a highly refined synthetic hydro­carbon. It is shown that the film changes from a viscous to an elastic material the shear modulus and ultimate shear strength of which rise mostly linearly with pressure. Their ratio approaches 1/(2π) (as predicted by Frenkel’s theory (1926).

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