Copper drag on commutators

The paper includes a survey of the very sparse literature on copper drag and gives a general outline of the metallurgy of commutator bars and copper drag from various sources. Laboratory experiments are described which simulate the production of copper drag, and typical specimens obtained are shown to be very similar to practical examples. The basic cause of copper drag appears to be a vibration and hammering action of the brushes predominantly occurring under low-load conditions. This produces a flowed surface layer on the commutator of heavily cold-worked material. The effects of temperature, time, bar material and sparking or arcing are discussed. Several recommendations are given for reducing or preventing the occurrence of copper drag on bothnew and existing machines.