A Study of Metal Fiber Brush Operation on Slip Rings and Commutators

The results of an investigation to determine the wear mechanism of metal fiber brushes on commutators and slip rings are presented. Metal fiber brushes were operated, while conducting direct current, on gold-plated copper rotors with and without unfilled gaps to simulate slip rings and commutators. Wear rates on unfilled-groove commutators were found to be only modestly higher than on slip ring rotors. Three possible causes for enhanced metal fiber brush wear on commutators were considered: i) accelerated "adhesive" wear controlled by contact spots, ii) fatigue induced wear and iii) "fiber chopping". All observations on wear debris and brush fibers are consistent with "adhesive" fiber brush wear on commutators as well as on slip rings; due to interlocking contact spots. Little to no evidence was found to support the other two hypotheses of fatigue induced wear and fiber chopping. The same conclusion also follows from comparing commutator wear rates with that on slip rings. A geometrical factor to account for the local elevation of brush pressure due to commutator grooves not supporting brush fiber tips was determined. This factor, of approximately 1.2, brought the wear data for commutators and slip rings into coincidence.