Use of the inverse method to check the stress predictions in rough elastohydrodynamic lubrication contacts lubricated with a non-Newtonian fluid for a range of asperity geometries

Abstract

The inverse method enables a check to be made on the pressures and stresses calculated in rough elastohydrodynamic lubrication (EHL) contacts. Essentially, a rough, soft surface is run against a smooth, hard counterface in a twin disc machine for a limited time. This tends to deform the asperities on the soft disc and, at the end of the run, the profile is measured and used as input to an EHL solver to determine the hydrodynamic pressures. From these the stresses are calculated. If the material has deformed and deformation has ceased, the maximum stress should be equal to the yield strength of the soft disc. This comparison provides a quantitative check on the accuracy of the EHL analysis and on the assumptions made about the fluid rheology. Allowance has, of course, to be made for the build-up of residual stress in the disc material. The method is applied here to surfaces in which defects with a range of sizes have been manufactured. A non-Newtonian fluid with reasonably well-established characteristics was used under conditions of moderate slip. Good agreement is found between the predicted stresses and the material strength for all defect geometries, suggesting that the fluid properties are sufficiently well defined for accurate predictions to be made of the pressures and stresses in EHL conjunctions.