ELASTOHYDRODYNAMIC FILM THICKNESS FOR SHEAR-THINNING LUBRICANTS

Mineral oils and synthetic lubricants that are thickened by polymers o f large molecular weight are being promoted for automobiles as well as aircraft gas turbines. These multiweight lubricants are found to have a complicated Newtonian and non-Newtonian viscosity depending upon sh ear rate in the bearing. Ln general, polymer-thickened mineral oil lub ricants show a first Newtonian behaviour at a low shear rate, shear-th inning non-Newtonian behaviour at a higher shear rate and a second New tonian behaviour at a very high shear rate, with a second Newtonian vi scosity approximately equal to the base oil viscosity. Because of high shear thinning in the inlet region of rolling element bearings, predi cting the film thickness using the low shear rate first Newtonian visc osity can be in error, in particular examples, by a factor of 1/2 for mineral oil plus 4% methacrylate thickener and 1/7 for mineral oil plu s 20% polybutene thickener. The case of naturally shear-thinning silic one fluids is analysed and it is shown that the elastohydrodynamic (EH D) film thickness is nearly the same for silicones with widely varying first Newtonian viscosity. A general EHD analysis for shear-thinning lubricants in pure rolling is presented and shown to agree with known special cases. A closed-form EHD equation for power law shear-thinning lubricants is derived, which gives very accurate results for a bearin g where the inlet state of the rolling element falls in the region whe re the non-Newtonian viscosity is expected. A comparison with some pub lished experimental results by Bair and Khonsari is presented.