Measurements of Elasticity in Multigrade Motor Oil at Elevated Pressure

Abstract

It has been recognized for about 50 years that the addition of polymer to motor oil will result in shear-dependent viscosity at relatively low shear stress. This shear-thinning can be shown to reduce the elastohydrodynamic film thickness and there has been an expectation of a reduced film in hydrodynamic lubrication as well. The expected associated increase in the journal bearing wear, however, has not been observed. The load-supporting benefit of the elastic properties contributed by the polymer has been used as an explanation for the success of the polymer blended oils in this application. The available measurements of elasticity in the form of normal stress differences have been inconclusive regarding their contribution to load support. These measurements have all been at atmospheric pressure, however, and at a very high shear rate. The oil in the load-carrying region of a bearing is at elevated pressure and the elevated pressure may enhance the elasticity. A new experimental technique for the measurement of elasticity in oils at elevated pressure was developed. A Weissenberg rheogoniometer of the parallel plate, torsional flow type has been miniaturized and placed in a 350-MPa pressure vessel. This is essentially a parallel face thrust washer that can support a load unrelated to the usual viscous action. The load capacity due to the liquid elasticity vanishes for low-molecular-weight liquids of the same viscosity. The device has been validated with an NIST non-Newtonian standard reference material and the preliminary measurements on commercial multigrade oils clearly show the effect of elevated pressure. The possibility exists to provide additional load support without a viscous frictional penalty.