The measurement of dynamic normal and frictional contact forces during sliding.

Dynamic normal and frictional forces, including inertia forces, have been measured at frequencies between 0 and 2khz during the nominally smooth sliding of unlubricated steel surfaces using a pin-on-disc type apparatus. It is found that large audio-frequency normal and frictional force fluctuations are present, long before, significant loss of contact or other obvious changes in frictional force behaviour begin to occur. These fluctuations, which cannot be measured using conventional friction testing devices are shown to be largely caused by normal vibratory excitation associated with the interactions of surface irregularities on the sliding surfaces and are therefore inherent in the sliding process itself. Transfer function analysis is used to explore phase and amplitude relationships between normal and frictional forces in the frequency domain. An oscillatory (or AC) friction coefficient, interpreted in the frequency domain, is found to be largely independent of frequency but greater than the concurrent average kinetic (or DC) friction coefficient. Detailed measurements of slider motions and fluctuating force components are presented. (A)