In search of dynamic effects in dry sliding friction

There are many instances in mechanical systems when sliding contacts are subject to unsteady loads. Of interest for system modeling is the behavior of friction processes under such conditions. If quasi-static models can be applied one can focus on other, possibly more important aspects of the problem. In this paper, we suggest that a distinction between a constant (scalar) and non-constant friction coefficient be made. When unsteady friction and normal force measurements are made with care, we find that the average (or DC) friction coefficient can still be found to apply during contact vibrations at thousands of Hz. One only needs to invoke simple concepts (e.g., the Greenwood-Williamson theory of rough surface contact) to explain the relation between instantaneous normal and friction forces. When additional materials, e.g., wear particles or a solid lubricant are introduced at the interface, part of the dynamic normal contact load will be carried via damping mechanisms which do not, in general, contribute to tangential friction. It is unclear whether this effect should be considered a dynamic friction behavior or just a reflection of system and contact dynamics. The results of a series of concurrent average and time-varying friction measurements are used to illustrate these ideas.