The effect of variable high-altitude humidity on the wear of nondusting brushes

THE important effect which water vapor has on the performance of carbon brushes has been recognized for many years. Prior to 1935,1 cases of excessively short brush life were found to be due to the low humidity of the cold air of midwestern United States winters. Early in World War II, when bombers began flying at altitudes above 20,000 feet, this low-humidity problem was more spectacularly demonstrated when it was found that the brushes on the dc generators were wearing out completely during flights as short as one hour. Tests in simulated high-altitude atmospheres showed, conclusively, that the extremely rapid wear, or “dusting,” of carbon brushes was caused by the inability of the brushes to maintain lubricating films on commutators in the extremely low atmospheric humidities which were found to exist at high altitudes.