Brushes should be checked for wear periodically. When new they are approximately 1/2" long. Replace them when they have worn to 1/4". Be sure to replace both motor brushes even if one of them is less worn than the other.

 

More Details About Motor Brushes:
All Foredom electric motors are the 2-pole type, and use one set of brushes, one on each side of the motor case, opposite the shaft end. The brushes are installed in brush holders, which are copper or brass tubes encased in plastic, with shallow plastic screw caps to retain, and provide access to, the brushes. The leads from the motor power cord are connected to the metal brush tubes, from which current flows into the brushes. A brush consists principally of a length of compressed graphite, with carefully controlled characteristics to provide the lowest possible resistance, low friction, and long life. A flexible copper pigtail is imbedded in the brush, terminated in a “U” or disc terminal. The pigtail is surrounded by a coil spring. The free end of the brush is curved, to match the curvature of the commutator. When the brush is inserted in the brush holder and the screw cap is in place, the spring maintains proper brush pressure on the commutator as the brush wears, and the pigtail terminal contacts the inner metal tube to provide a current path to the brush.