Interactions between dynamic normal and frictional forces during unlubricated sliding

The results of measurements showing large normal and frictional force oscillationsduring unlubricated smooth sliding between steel surfaces are presented. Themeasurements were made on a pin-on-disk type apparatus instrumented withpiezoelectric force and acceleration transducers. Spectral analysis of the contactforces (including inertia forces) up to frequencies of 2 kHz indicate that thefluctuations have their major components in this frequency range. The forceoscillations are primarily associated with normal and tangential contact vibrationswhich are excited by surface irregularities being swept through the contact regionduring sliding. Transfer function analyses between the normal and frictional forcesshow the frictional force fluctuations to be in phase with the normal force fluctuations, but related in magnitude by a ratio larger than the concurrently-measured(average) kinetic friction coefficient. This larger oscillatory (or AC) frictioncoefficient is indicative of fluctuations in the instantaneous coefficient of frictionwhich are shown to occur in synchronism with the contact force oscillations.