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374 MOTION PICTURE HANDBOOK
ing of the commutator, and if a soft brush is used no lubrication should be given. This caution is necessary with undercut insulation by reason of the fact that the lubricating medium will have a tendency to combine with carbon dust and fill up the space between the commutator bars, thus in time possibly short circuiting the bars. Also where soft brushes are used the brushes themselves as a rule contain sufficient paraffine to provide all necessary lubrication.
(d) See to it that sufficient oil, or combined oil and carbon dust, has not collected at any point or spot, either on the commutator or face of any brush, to form a semi-insulation:
(e) That there are no high or low bars and that the commutator is perfectly round.
(f) That a fragment of copper does not drag across the insulation between two adjacent bars, or that oil and carbon dust does not form such a bridge. This fault will be evidenced by a thin, sparkling ring of light around the commutator.
(g) That the brush springs do not carry sufficient current to heat them.
(h) That the brushes fit properly in their holders, and are kept free from accumulation of dirt, dust, etc. They should be taken out and cleaned once in every 60 hours run.
(i) That the brushes are neither too hard nor too soft.
(j) That the armature "floats" slightly, i.e., has from onesixteenth to one-eighth inch end play, according to size of machine. This tends to prevent the brushes from cutting grooves in the commutator; is very important. Unless the machine sets perfectly level the armature will not "float," hence a level setting is important.
(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.
Should the brushes of the motor or generator shpw excessive sparking, it might be attributed to one of the following causes;
(a) If 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