Obtaining the pressure spike and maximum shear stress from optical interferometry data

Abstract

In order to predict and optimise highly loaded contact performance, accurate lubricant data is crucial. The lubricant's high-pressure rheological behaviour is by far the least known parameter. However, this is the key factor to realistic modeling of non-Newtonian Elasto-Hydrodynamic lubrication. In this paper a new approach is described to extract such data from optical interferometric film thickness measurements of EHL contacts. The approach is relatively straightforward and cheap compared to “out of contact” rheological experiments using specialized equipment.A measured high-resolution film thickness distribution was positioned using a computed film thickness distribution as a reference. The reference is computed using the same operating conditions as the measurement. Subsequently, from the computed film thickness difference, a pressure difference file is obtained by deconvolution. Adding this pressure difference to the computed pressure file associated with the computed reference film thickness, provides a corrected pressure distribution, as it has appeared in the experimental contact. In this paper results are presented for the pressure spike region of the contact, in which significant shear stresses occur. The basic approach and its difficulties are described as well as some “tricks”, such as the reduction of (local) noise resulting from the ill-posedness of the deconvolution. It is shown that simple averaging over a circle segment in the pressure spike zone, results in significant noise reduction and a very good ‘measured’ pressure spike.