In an electrical machine such as a motor or generator stationary brushes radially engage the commutator rings of the rotor to transmit electricity via these rings to the windings of the rotor in a motor or to pick up the electricity generated in the rotor's windings in a generator. The brush is normally a mainly graphite block that is received in a longitudinal passage of a holder. The longitudinal direction of the passage is aligned radially of the rotor or perpendicular to the surface that the brush is to make electrical contact with, and the brush is longitudinally slidable along the passage. A spring is braced between the holder and the brush to move it longitudinally along the passage toward the front end thereof, so that as the front end of the brush is worn off the brush is advanced to compensate.
In an arrangement described in U.S. Pat. No. 3,902,088 the brush holder has a pair of generally parallel side walls having confronting inner faces, a top guide wall extending along and between the inner faces and in turn having an inner face, and a bottom wall in turn having an inner face defining the passage with the other inner faces. In addition this housing has an angled surface adjacent the front end. The brush is snugly received in the passage, in continuous sliding contact with the inner faces of the side and guide walls and has a bottom face spaced from the bottom holder wall. A coil spring has a coiled part engaged between the angled surface and the bottom brush face at the front end of the brush to transversely urge the brush against the inner face of the guide wall and a free end extending longitudinally back along the bottom face and secured near the rear end of the brush thereto. Thus the coiled part pushes the front end of the brush, which is that portion of the brush at the front end of the passage, upward against the guide wall and the free end pulls the whole brush axially forward.
In such an arrangement the concentration of all the laterally effective spring forces at the front end of the brush leads to premature wear in this region. Furthermore the brush and holder must be very accurately made so that the brush fits snugly between the side holder walls while still being able to slide longitudinally in this holder. This tight fit keeps the brush centered and in theory avoids canting or twisting of the brush in the holder. On the other hand such a tight fit makes the system particularly susceptible to jamming when carbon dust clogs between the brush and holder. It is fairly common for the brush to stick or become wedged in place, in particular when subjected to substantial vibration or jarring.