The influence of the conditions of sliding on the wear of electrographitic brushes

Abstract

The rates of wear of an electrographitic brush sliding on copper have been determined over a wide range of loads, speeds, apparent areas of contact and currents. The magnitude of the rate of wear depends primarily on the way in which these variables influence the type of surface film generated on the copper. At high speeds and heavy loads a continuous layer of transferred graphite is formed and the rate of wear per unit load is relatively high and unaffected by the magnitude of the current. At low speeds and light loads, however, a surface film of cuprous oxide develops which prevents transfer of graphite and reduces the rate of wear. The amount of oxidation depends on the time available between repeated contacts on the copper, and hence upon the area of the brush and the diameter and speed of the slip-ring. Current breaks down the oxide film, leading to transfer of graphite, and the rate of wear increases with current. At high currents the rate of wear reaches a limiting value when the transferred layer of graphite on the copper becomes continuous. It is concluded that the wear of the brush is a `fatigue' process resulting from a succession of repeated elastic stresses over the localized regions of true contact.