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GENERATORS AND MOTORS 155
indicated by the mark, which should be present on both the generator frame and yoke. Any error in the yoke set in a normally loaded machine will cause brush sparking and a decrease in efficiency.
Be sure that there is some protection over the commutator to keep things from dropping upon it. This does not mean simply a screen such as the manufacturer furnishes as a* guard against accidents. Cases have occured where the commutator was ruined and the winding burned out as a result of pieces of plaster dropping from a wall overhanging the machine. There should be a solid baffle of metal, or its equivalent, not too far above the commutator surface.
Sticking Brushes
. (87) Brushes often stick in their holder when they fit too snugly or bits of dirt and grease work their way in between the brush and the holder or the brush itself is not true on its surfaces. The last is an unlikely fault, but is sometimes found. To correct it tack No. 0 sandpaper on a perfectly flat board and rub the brush on it gently until its surfaces are true. Kerosene is excellent to remove the sticky grease and dirt mixture that some times accumulates on the brush rigging. Carbon tetrachloride is still better.
Commutator Too Thin
(88) Commutators gradually wear clown through friction. After being trued up repeatedly in a lathe the bars may become too thin to serve adequately. As the bars get thinner from wear the ends no longer have a firm mechanical support, since the clamp or lock is near the center of the length of the bar. This condition will cause chattering, sparking and heating, because the individual bars are disturbed when the brushes pass over them. Install new bars.
Uneven Wear
(89) It is possible for some commutator bars to be