Enhancement of Journal Load Bearing Capacity by Polymer Containing Oils

Most automotive multigrade oils contain high molecular weight polymeric viscosity index improvers (VII's) and are, to a greater or lesser extent, viscoelastic fluids. For many years the effect of multigrade oil viscoelasticity on journal bearing lubricant load bearing capacity (LBC), (and implicitly minimum oil film thickness) has been a vexed question. The work described in this paper provides experimental evidence that a significant enhancement of journal bearing LBC over that generated by isoviscous Newtonian (single grade) oils can be achieved by the use of multigrade oils under simulated realistic in-senice engine conditions. This effect only occurs under operation at high eccentricity ratios. At lower eccentricity ratios no enhancement in LBC could be found. Examination of the effect of lubricant piezoviscosity on LBC failed to account for the load bearing enhancement phenomenon. Additionally, it is demonstrated that the LBC of transiently loaded journal bearings is a function of both lubricant viscosity and elasticity and that these parameters can be interchanged, for example, to produce a low viscosity (low friction) multigrade oil without compromising journal bearing LBC (and hence durability).