THE FIRST NORMAL STRESS DIFFERENCE IN A SHEAR-THINNING MOTOR OIL AT ELEVATED PRESSURE
The recent revolution in quantitative elastohydrodynamics resulted from the use of realistic rheological properties, particularly real shear-thinning, in analysis of film behavior. Similar advances in the understanding of the hydrodynamic lubrication may result from applications of realistic rheology. Currently, the only aspect of non-Newtonian behavior being modeled is the shear dependence of viscosity. There is recently renewed interest in the shear-dependent viscosity of multigrade engine oils. Seemingly lost in the present discussion is the fact that real shear-thinning is accompanied by a large difference between the normal stress in the flow direction and the normal stress in the cross-film direction. In this work, the rheology of an off-the-shelf 10W-40 commercial motor oil was examined. Three different high-pressure instruments were employed; a falling body viscometer, a thin-film Couette viscometer and a parallel plate rheogoniometer.