Collector ring marking or brush footprinting

Collector ring marking, also known as brush footprinting or photographing, has been a persistent problem for many years on carbon-brush collectors of synchronous machines from different manufacturers. The theory that the footprints are formed by simultaneous loss of electrical contact of all brushes on the ring is proposed. The trigger for the loss of contact is mechanical, but adverse ring film conditions, such as oil contamination, can greatly accelerate the appearance of the marks. The test results which led to the loss-of-contact theory are explained, and corrective steps, as well as their efficacy, are described. In particular, modifications consisting of increased spring pressure, more brushes, greater peripheral brush spacing, and stiffer brushholder supports where needed were devised to eliminate or minimize simultaneous loss of brush contact