This paper describes an experimental study of the influence of surface roughness on elastohydrodynamic film thickness in a steel ball on glass flat contact under nominally pure rolling conditions. Steel balls with a range of isotropic roughnesses have been employed and measurements have focused on the mixed lubrication regime where the film thickness is comparable to or below the composite surface roughness. It has been found that full film lubrication is only realised when the nominal lambda ratio (X) has a value of 2 or higher. Below this value, the film thickness decreases, first gradually and then rapidly, as the nominal smooth surface lambda ratio decreases. The mixed regime, where both roughness and entrainment velocity influence film separation, appears to span a large range in terms of the lambda ratio, from λ = 2 to λ = 0.1.
The mean separation between rough surfaces in static contacts has been measured over a range of loads and the problem of the precise definition of lubricant film thickness in rough surface conditions is discussed.