Roughness-induced dynamic loading at dry and boundary-lubricated sliding contacts

Measurements of instantaneous normal and frictional forces have been made at smoothly sliding hemispherical steel contacts under nominally dry and boundarylubricated conditions. Normalized contact force fluctuations generated by surface irregularities are presented. Instantaneous frictional relations are characterized by performing spectral and transfer function computations on the measured signals. The qualitative dynamic frictional behavior is shown to be similar at dry and boundary-lubricated contacts. However, the magnitude of the dynamic loading is considerably reduced in the presence of lubrication. In both cases, an apparent time-varying frictional behavior is found to exist.