The First Normal Stress Difference in a Shear-Thinning Motor Oil at Elevated Pressure

Currently, the only aspect of non-Newtonian behavior being modeled in lubrication is the shear dependence of viscosity. However, shear thinning is accompanied by a large difference between the normal stress in the flow direction and the cross-film direction. This stress difference can increase the load capability of a lubricant film without increased frictional penalty. A commercial 10W-40 motor oil was characterized at elevated pressures. Three different high-pressure instruments were employed: a falling-body viscometer, a thin-film Couette viscometer, and a parallel-plate rheogoniometer. Ordinary shear thinning with a second Newtonian inflection was observed. A first normal stress difference of 0.6 MPa was measured under what may be mild conditions for a crankshaft journal bearing. Elevated pressures are essential to the measurement of rheological properties that govern hydrodynamic film thickness and friction in automotive components. Time-temperature-pressure superposition was validated for the first normal stress difference. The first normal stress difference in the terminal regime may be estimated from the upper-convected Maxwell model, where the shear modulus is assumed to be equal to the Newtonian limit shear stress obtained from a measurement of shear thinning. The first normal stress difference in the shear-thinning regime may be estimated from an extant empirical rule. These results will be of substantial importance when analytical techniques are developed for hydrodynamic lubrication with real non-Newtonian shear response. The results are immediately useful for calculating the shear stress for cavitation in ambient pressure high-shear viscometers.