Derivation of basic rheological parameters from experimental elastohydrodynamic lubrication traction curves of low-viscosity lubricants

The rheological parameters characterizing the non-linear viscosity of lubri cants having a low viscosity at a Hertzian contact were determined on an as sumption of a parallel film, a Hertzian pressure distribution and the Eyrin g model expressing the relationship between the shear stress and the shear rate in an elliptical contact. Making a reasonable approximation and taking the variation in viscosity with pressure in the Hertzian contact into acco unt, an approximate formula expressing the mean shear stress with shear rat e was derived as a function of a viscosity-pressure coefficient and the rep resentative stress determining the onset of non-Newtonian behaviour. Tracti on characteristics of two kinds of paraffinic mineral oil were determined w ith an elastohydrodynamic lubrication tester. Then, a regression analysis u sing a least-squares method with the proposed formula was applied to the tr action measurements, from which the available rheological parameters and th e effective pressure were obtained. The viscosity-pressure coefficients der ived from traction measurements fall on the extrapolated line from the rela tion between the viscosity-pressure coefficient and the pressure in the pre ssure range from atmospheric pressure to 0.3 GPa obtained with a high-press ure viscometer.