Ques. What should be done after adjusting the brushes to their correct positions upon the commutator?
Ans. Their tips or rubbing ends should be examined while in position to see that they bed accurately on the surface of the commutator.
In many instances it will be found that this is not the case, the brushes sometimes bearing upon the point or toe, and sometimes upon the heel, so that they do not make contact with the commutator throughout their entire thickness and width. The angle of the rubbing ends will therefore need to be altered by filing to make them lie flat.
Ques. How is the proper brush contact secured?
Ans. When the brushes do not bed properly they should be refitted to secure proper contact.
Ques. How is the pressure adjustment made?
Ans. This is effected by regulating the tension of the springs provided for the purpose upon the brush holders.
Ques. With what pressure should the brushes bear against the commutator?
Ans. The tension of the springs should be just sufficient to cause the brushes to make a light yet reliable contact with the commutator.
The contact must not be too light, otherwise the brushes will vibrate, and thus cause sparking; nor must it be too heavy, or they will press too hard upon the commutator, grinding, scoring and wearing away the latter and themselves to an undesirable extent, and moreover, giving rise to heating and sparking.
The correct pressure is attained when the brushes collect the full current without sparking, while their pressure upon the commutator is just sufficient to overcome ordinary vibration due to the rotation of the commutator.