Non-newtonian effects in rough EHL contacts

Conditions under rough EHL contacts change rapidly with time and a dynamic EHL analysis is necessary to estimate the clearances, pressures and stresses in the conjunction. The results, particularly where there is relative sliding, depend on the assumptions made about the fluid properties, and in particular on whether the relationship between the shear stress and strain rate is Newtonian. There are a number of uncertainties about the actual fluid rheology: for example it is unclear whether the shear rate properties measured in traction tests or in high pressure viscometers apply under the very rapidly changing conditions found in rough contacts. Experimental checks on the accuracy of the predicted results are required and the 'inverse method' provides one way of doing this. In the present paper the method is applied to rough contacts where there is significant sliding and it is shown that, for the particular lubricant used, the theoretical predictions of stress coincide closely with those actually present. Because of the uncertainty about the actual fluid properties it is important to know how sensitive the subsurface stresses are to the shear rate properties. The second part of the paper examines this for two rough surfaces and it is shown that there is a major difference in the stresses predicted for Newtonian and non-Newtonian fluids. However, there is little difference between the stresses for Eyring and limiting shear stress fluids and the actual values of the Eyring stress and the limiting shear stress have only a minor effect on the results. © 2004 Elsevier B.V. All rights reserved.