A Comparison of the High-Pressure Viscometry and Optical Inference Methods Used to Determine the Pressure-Viscosity Coefficient

Abstract

Pressure-viscosity coefficients (PVC) are used in the predictions of elastohydrodynamic lubricated (EHL) componentry. These coefficients are obtained by either viscometry or optical EHL inference. The literature indicates that each method differs in its conclusion. Those who favor viscometry believe optical methods yield a misleading coefficient. Those who favor optical methods suggest low shear viscometric results over-predict the high shear-influenced film thickness. This work compares each method relative to di-(ethylhexyl) sebacate (DEHS), and five MIL and DOD spec lubricants. PVC results from viscometry and two optical methods are presented. Comparisons are made relative to other published measurements. Conclusions show PVCs inferred from optical film thickness measurements, differ from those obtained by viscometry. Viscometry methods are demonstrated as being consistent. Optically inferred results have uncertainty and require ample data to align with classical dimensionless speed exponents. While the optical measurements are truly EHL, the test conditions fall outside the fitted window of classical algebraic film equations, like that of Hamrock and Dowson. The PVC discrepancies, between optical inference and viscometry and for the studied fluids, cannot be explained by the proper account of the refractive index, shear thinning models, and/or film thickness correction models.