Relating profile instrument measurements to the functional performance of rough surfaces.

An easily programmed method is proposed for translating the rms height (Rq) and rms slope (DELTAq) determined using a profile measuring instrument, into more readily interpreted measures of functional severity such as the density of plastic contacts or the mean real contact pressure. The method involves estimation from the ratio Rq/DELTAq of the exponent k of an assumed power function relation between the profile spectrum and the spatial frequency. Having estimated k, the mean square curvature is computed analytically and used together with Rq and DELTA to determine the three input variables needed for the Greenwood-Williamson microconcontact model. This model is then used to compute, as a function of the separation of two rough surfaces, the contact density, the plastic contact density, the mean load per unit area and the mean load per unit of real contact area. The mean square curvature estimated in this manner is compared to the directly measured mean square curvature for 12 distinct surface types including a bearing ball and the ground inner ring rolling path of a cylindrical roller bearing.