ABSTRACT
In line with the preceding evidence, theoretically as well as experimentally, metal fiber brushes typically have much lower electrical resistances and can perform to much higher speeds and current densities than monolithic brushes. This superiority derives from their much smaller and more numerous contact spots, in combination with the fact that flash temperatures, which are the truly limiting factor for brush performance, are proportional to the power density at the contact spots, q, multiplied with the contact spot radius, r (Eq. (20.35)). The benefit of fiber brushes is thus twofold: Firstly, q is reduced both on account of much lower resistance and thus applied voltage as well as on account of local reduction of mechanical pressure below H. Secondly, and decisively, r is inversely proportional to the root of the number of contact spots, ranging in the many thousands, and thus very
small. By contrast, the contact spot number of monolithic brushes is typically n ti 10 and
the applied pressures higher, thus yielding much larger contact spot diameters. Correspondingly, not only is the constriction resistance essentially eliminated in metal fiber brushes but, relative to comparable monolithic brushes, flash temperatures are reduced by factors of tens, and achievable current densities at tolerable wear rates are increased by one or two orders of magnitude.
