Lubricant Behavior in Concentrated Contact—Some Rheological Problems

Abstract

Several problems are reviewed concerning the flow behavior of lubricants under the high pressures and shear stresses which occur in the contact zones of ball bearings and gears. A description is given of experiments on this topic with a machine involving contact between cylindrical and spherical rollers. An ester-based lubricant under a mean Hertz pressure of 102,900 lb/in² (7.1 × 10⁹ dyn/cm²) is shown to behave under shear stresses below 10⁸ dyn/cm² as a liquid of great viscosity giving a film thickness proportional to rolling speed. At high rates of shear the ester behaves as a plastic solid. A silicone fluid shows no such viscous behavior and is presumed to lose viscosity or to undergo plastic shear under much lower shear stresses than the ester. Such rheological differences affect the load-carrying capacity of lubricants by determining both their film thickness and perhaps also their response to extreme-pressure additives.