Film thickness anomalies in very thin elastohydrodynamic oil films

Abstract

Experimental studies of very thin elastohydrodynamic (EHD) oil films have shown, or suggested, that several aspects of lubricant behaviour do not fully conform to the model presumed by classical EHD theory. This study examines three forms of behaviour which appear to be anomalous. The first concerns the pressure-viscosity response of the lubricant in the EHD inlet zone. Although this appears to be weaker than expected, the disparity appears more likely to arise from a modest systematic error in the method by which film thickness is determined than from the behaviour of the lubricant. The second phenomenon investigated is a mechanism by which additive systems improve the durability of rolling surfaces. A transition is detected at a film thickness of about 5 nm, below which the effective viscosity of an additive-treated lubricant approximately doubles. Finally, the effect of polymeric viscosity index improvers on film thickness is examined for both simple base oil solutions and a simplified range of fully formulated engine oils. Both film thickness enhancements and a severe film thinning phenomenon are observed at film thicknesses in the region of 10 nm.