The influence of system dynamics on the frictional resistance: insights from a discrete model

In order to examine the influence of system dynamics on sliding friction, we introduce the so-called micro-walking machine. This model consists of a rigid body with a number of elastic contact spots that is pulled by a constantly moving base. The system slides with dry friction on a rigid substrate. The kinematic coupling of the rotation and the translation of the rigid body results in varying normal and tangential forces at the contact spots. For certain parameter ranges this leads to self-excited oscillations in the vertical direction. A particular dynamic mode occurs which is characterized by a strong correlation between low or even zero normal forces and a fast forward motion. This effect is referred to as micro-walking. In addition to an experimental rig we use numerical integration and an extensive parameter study for the analysis. In theory, the reduction of the frictional resistance reaches up to 98%. These results are confirmed by the experiments where the maximal reduction was 73%. Our model shows that micro-vibrations play an important role for the dynamic influences on the frictional resistance of systems that exhibit apparently smooth sliding. The identification of the critical parameter range enables the systematic control of frictional resistance through the adjustment of attributes such as geometry and stiffness. In addition, it is possible to deduce guidelines for how tribological test rigs should be designed in order to get reliable results.