The effect of brush spring pressure on the wear behaviour of copper-graphite brushes with electrical current

Electrical brushes are used to conduct current between stationary part and moving part of a motor or a generator. To ensure proper current transfer and continuous contact, brushes must be loaded against the sliding contact surface with a sufficient force. High loads increase frictional losses and wear of the brushes and/or sliding surface. While relatively low contact pressure causes arcing and higher voltage drop. In this study, a novel pin-on-slip ring-type friction and wear test machine was designed and manufactured for the purpose of brush testing. Copper-graphite-based electrical brush containing 90 wt% copper and 10 wt% graphite was manufactured by powder metallurgy and the tribological behaviour and voltage drop were investigated at different brush spring pressures at 10-200 kPa with current. It was found that the specific wear curve showed three distinct wear rate regimes, such as low, mild, and severe. Severe wear was observed below 30 kPa and above 120 kPa brush spring pressures (BSP) (3 and 12 N loads, respectively). Arc erosion was the main wear mechanism below 30 kPa brush spring pressure while abrasion was dominant above 120 kPa BSP. Low and mild regimes were observed between 30-50 and 50-120 kPa BSP, respectively. SEM observations showed that a continuous surface layer was formed at the sliding surfaces of the wear samples in low and mild wear regimes. The wear debris was examined by SEM and X-ray diffractometer. (c) 2007 Elsevier Ltd. All rights reserved.