WEAR AND FRICTION OF CARBON BRUSHES AS A FUNCTION OF AIR HUMIDITY

Abstract

That atmospheric humidity plays an important role in helping to maintain low friction films and long brush life has been well established. Data are presented which indicate that the critical air humidity level, below which carbon brush friction and wear increase to disastrous proportions, is determined, at least partially, by the particular combination of brush type and the slip ring or commutator material on which it rides. For example, the performance of electrographitic brushes operating on copper slip rings sharply declines as the ventilating air humidity drops below 0.16 gr water per cubic foot. Graphite brushes operating on steel slip rings show a greater sensitivity toward humidity since brush wear rates and friction rapidly increase to abnormally high values below a critical humidity level of 0.8 gr water per cubic foot of air. Since air humidities well below these critical values are known to exist at certain locations on the earth's surface and particularly at high altitudes above the earth's surface, special precautions must be taken to assure satisfactory performance in these cases. The humidity requirements for special high altitude brushes are lowered by the addition of special chemicals within the brush body. Even with these “treated” brushes, however, the quality of performance is shown to be affected by variations in humidity which are known to exist in the extremely dry, high altitude atmosphere. Through the careful selection of brush materials and treatments, tolerable brush wear rates are obtained even under the lowest humidity conditions existing at altitudes up to 60,000 ft.