Interactions Between Dynamic Normal and Frictional Forces During Unlubricated Sliding

The results of measurements showing large normal and frictional force oscillations during unlubricated smooth sliding between steel surfaces are presented. The measurements were made on a pin-on-disk type apparatus instrumented with piezoelectric force and acceleration transducers. Spectral analysis of the contact forces (including inertia forces) up to frequencies of 2 kHz indicate that the fluctuations have their major components in this frequency range. The force oscillations are primarily associated with normal and tangential contact vibrations which are excited by surface irregularities being swept through the contact region during sliding. Transfer function analyses between the normal and frictional forces show 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) friction coefficient is indicative of fluctuations in the instantaneous coefficient of friction which are shown to occur in synchronism with the contact force oscillations.