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MANAGERS AND PROJECTIONISTS 507
There should, however, be only sufficient tension on the brush to insure its making good contact with the commutator. Be careful, therefore, and don't get your springs too strong. If you do there will be unnecessary wear both on the brush and the commutator, which will to some extent add the element of mechanical heat generated by undue friction.
Reasons for the brush sticking in the holder are: (1) Dirt in the holder or on the brush; (2) brush not true; (3) hammer that rests on the brush (where that type of tension is used) not working true on the slot-end of the brush. The brush should slip freely in its holder, though not freely enough to allow of any considerable amount of play, and the hammer should be so adjusted that it lies true in the slot at the end of the brush. A brush which is not true may be evened up by tacking No. 1 sandpaper on a perfectly flat surface and rubbing the brush thereon.
(e) Commutator worn too thin. If the commutator wears down too much, although it may wear evenly and appear to be in good condition, the brushes will spark in spite of everything you may do, particularly when the machine is working at capacity. The reason may lie in the fact that since the segments are wedge shape, as they wear down they become narrower, thus allowing the brush to span more of the circumference of the commutator than was intended, or there may be a slight error in the setting of the brush holder, and this error becomes greater as the distance between the brush holder and the commutator increases. The only remedy is a new commutator, but the sparking may possibly be lessened somewhat by moving the brush holder closer to the commutator. This trouble appears at its worst in a series type machine.
(f) A high or low commutator segment. This fault may usually be detected by the clicking sound made by the brush in passing over the defective segment when the machine is run at moderate speed. When the segment is low the brush rides in toward the shaft each time the bad bar passes under it. If it is high the brush will jump. The remedy will depend somewhat upon the cause. It may be that the segment has become loose, in which case it may be driven back into place by tapping lightly with a wooden mallet, or by using a wooden block and hammering gently, but the armature will probably have to be taken out and sent to the repair shop unless you yourself can tighten the clamp ring — a rather delicate operation.