Evaluation of Lubricants Using Optical Elastohydrodynamics

A system is described whereby a clear map is obtained of the contact between a metal (steel) ball rolling against a plate glass disk. This is achieved by mounting the disk in an air bearing and coating its surface with a thin layer of chromium that partially reflects the light, allowing good interference patterns to be formed. The effect of load (giving stresses to 100,000 psi) and speed and viscosity is demonstrated for a wide range of fluids. A complete run can be carried out with at most 5 ml of sample. It is shown that load hardly alters the film thickness in the middle of the contact but does affect the minimum film thickness, which is in the side lobes. By using a calibrated oil the pressure-viscosity coefficients of the fluids can be determined. The rate of shear is very high, between 0.1 and 40 million sec−1 and the flow orientation of polymer thickened oils is clearly marked as well as the molecular orientation postulated by Bondi. For most fluids, except the polymer thickened oils and silicones, the fluid structure influences the film thickness primarily through its effect on viscosity and pressure viscosity.