Effect of Surface Roughness on Rolling Friction

The minimum tilt at which bearing balls roll down an inclined plate of stainless steel is greater for rough than for smooth surfaces. The roughest surfaces (finish No. 1, average height of elevations 3×10−4 cm, and finish No. 2D, average height 0.9×10−4 cm) gave an almost quantitative agreement between this height of elevations and the height of hills calculated from a theory attributing rolling friction to surface roughness. No similar agreement was observed for fine finishes, presumably because the balls used (radii between 0.159 and 0.635 cm) were too big for the irregularities on these surfaces. The elastic deformation of the surfaces in contact was irrelevant for coarse finishes. Capillary attraction presumably did not interfere as superficial drying of the system had no effect. Surface roughness which is the cause of Coulomb's sliding friction thus seems to cause also rolling friction at low pressures.