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150 RICHARDSON'S BLUEBOOK OF PROJECTION
the best evidence that the commutator is in the finest possible condition. If the condition is good there will probably be a slight squeak from the brushes when the armature is rotated slowly, though its absence does not necessarily indicate that anything is wrong.
Yoke Mark
(69) On all modern types of motor-generator sets the correct setting of the brush yoke is indicated by a mark on the yoke and another on the frame. These should be exactly opposite each other. This setting does not contemplate a continuous overload or underload. If that is the case the brushes may need advancing or retarding.
To Test Brush Pressure
(70) An experienced man can test brush pressure with sufficient accuracy by raising the brush finger or spring. If the commutator has little or no sparking and the proper glaze, it is,best not to touch it at all. For the brushes usually found on a commutator used for dynamos of this type a pressure of from one and one-quarter to one and one-half pounds per square inch of brush surface is approximately correct. A small spring balance will measure such a pull quite accurately. This may be hooked to the brush holder finger or spring and the latter raised just out of contact.
(71) If at this point — when the finger is worked up and down by raising and lowering the scale — it is found that there is a wide difference in the scale of. reading, it shows that the brush finger is too tight and the chances are that a good cleaning is in order. The purchase price of the scale is fully justified by the discovery of an undesirable condition even in a single brush holder.
If the brush is, for example one and one-half inch wide by three-eighths of an inch thick its contact area is roughly nine-sixteenths of a square inch, so that the balance reading should be fourteen ounces.
The width times thickness will serve for calculating