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Abstract
In many ultrasonic applications frictional effects play an important role (e.g. ultrasonic
machining, ultrasonic motors). For optimising the applications in terms of quality,
efficiency and lifetime it is important to understand the frictional coupling of the
vibrating and the non-vibrating part. This contribution is devoted to give an explanation
for the reduction of friction forces which is often observed when ultrasonic vibrations
are superimposed to macroscopic motions. Usually adopted coefficients of friction
are used for modelling such conditions suggesting special frictional mechanisms for
high frequency oscillations, whereas the present paper shows that Coulomb's friction
law provides a very good description of the observed phenomena if the kinematics of
the system is taken into account. Two systems are investigated. In the first system
the ultrasonic and macroscopic movements are parallel and in the second they are perpendicular
to each other but also within the plane of contact. Both systems were investigated
analytically and experimentally using a specially designed test rig. The measurements
confirmed the analytically derived equations and therefore the validity of Coulomb's
friction law even for ultrasonic conditions.