Extension of Conventional Optical EHL Technique

Abstract

The term “conventional” optical EHL technique refers to the simple mechanical set up of a steel ball rolling/sliding on a partially Cr-coated glass disc and illuminated using monochromatic light source, as first introduced by Cameron’s group in Imperial College in the 1960s. The technique has been widely used in experimental studies of EHL since its introduction. It gains its popularity not only because of the provision of detailed mapping of the highly pressurized and stressed EHL contact but also its simplicity which, as we recognize, is the beauty of the technique. The film thickness was inferred from the known order of interference fringes, which limits the minimum thickness measurement down to only a quarter of the wavelength of the light source. Recently, the magnitude of both the film thickness measurement and its resolution has been extended to nano-scale, which is achieved by a full optical analysis of the multilayer optical system comprising the lubricated contact, such that the film thickness can be inferred from the fringe intensity instead. The mathematical derivation, at first sight, may be very complicated since it consists of multiplications of complex numbers and lengthy expressions. The aim of this paper is to summarize the derived algebraic equations such that one can easily make use of them. Even though they look lengthy, calculation is quite straightforward and can be readily completed using an ordinary spreadsheet, such as Excel. It is noted that the mechanical set up of the technique and the interferograms obtained are exactly the same as the ones introduced in the 1960s. Thus this current extension of the technique does not obliterate the beauty of its simplicity which was, after all, one of the key considerations in experimental studies as repeatedly stressed by the late Prof. A. Cameron.