Vibration-induced friction modulation for a general frequency of excitation

Applying an oscillatory load is one of the most efficient ways to alter friction forces. Several theoretical and experimental studies on the influence of oscillatory loads on friction have been conducted, investigating the effect of both in-plane and out-of-plane oscillations for different tribological pairings. However, in the literature, the effect of an oscillatory load on the friction force has been studied with an emphasis on dynamic loads characterized by a high-frequency content, while a clear statement as to what is considered “high-frequency” is missing. Moreover, the effect of a combination of load directions on the friction reduction is not accounted for. Therefore, this study aims to determine the vibration-induced effect on friction regardless of the frequency range and direction of harmonic force for a single and multi-degree-of-freedom system. Analytical methods are used to obtain the friction modulation due to harmonic loads, considering a classical mass–spring–dashpot system on a moving belt and the Amontons–Coulomb law. It is found that, in the case of continuous slip, a general relation for the vibration-induced friction modulation is obtained utilizing the velocity response function of the investigated system. The latter is used to highlight a threshold from which the high-frequency regime starts and to determine the stick–slip boundaries. Moreover, through the velocity response function, the influence of different external harmonic forces is investigated and discussed. This includes considerations of phase, excitation frequency, system characteristics, and the choice of the normal contact force expression.