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504 HANDBOOK OF PROJECTION FOR
slightly, i. e., has from one-sixteenth to one-eighth inch end play, according to size of machine. This tends to prevent the brushes from cutting grooves in the commutator, hence is very important. Unless the machine sets perfectly level the armature will not "float/' hence a level setting is important. (See "Installation" Page 498.) In vertical machines in which the armature is carried on thrust ball bearings, the brushes of opposite polarity overlap, so that the space between the brushes on the one side is covered by a brush on the other. This arrangement causes a very even, uniform wear.
(k) That the copper and mica insulation wear down evenly.
(1) That the generator is not overloaded, and that there are no other faults present which would tend to cause unnecessary sparking, or otherwise injure the commutator.
SPARKING. — Should the brushes of the motor or generator show excessive sparking, it may be attributed to one of the following causes :
(a) If it be a belt driven machine, the belt may be slipping; if the sparking is spasmodic or intermittent the trouble will probably be found in the belt, since belt slip causes sudden variations in speed, and this will, in itself, cause sparking, because it has the effect of producing heavy fluctuations in voltage. The remedy is to tighten the belt or use a belt dressing, and in this connection ordinary black printer's ink is as good an article as we know of to stop belt slipping. Ten cents' worth obtained at any printer's will last a month or more.
(b) In considering the following remember that if the machine is a new one and the rocker arm is set at the position marked by the manufacturer, as before explained, the rocker should under no condition be shifted, since the entire performance of the generator depends, in some cases, on the accurate positioning of the brushes.
Brushes not set correctly, that is to say, the rocker arm too far one way or the other; also the brushes may be too close together, or too far apart. In the first case the remedv is to move the rocker arm until the neutral position is found, whereupon sparking will either cease or be reduced to a negligible quantity. If this fails to remove the trouble, we would see if the brushes themselves are the correct distance from each other. In a two-pole machine they should bear on the commutator at diametrically opposite points. That is to say, the distance from brush-point to brush-point should be