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320 RICHARDSON'S BLUEBOOK OF PROJECTION
round — either of these faults can be tested for easily.
(149) High bars may be detected ordinarily by a slight clicking sound when the machine is run at moderate speed, which can be accomplished without load by manipulation of the switch. There can be also a low bar or bars, in which case the brush would ride inward slightly as the armature is rotated slowly by hand. If the commutator is out of round, that fact may be made evident by rotating the armature slowly by hand, watching the brushes closely for an up and down movement in their holders.
(150) Often a very slight motion of the brushes up and down in the holder means nothing. It is sometimes impossible to get them to ride without some such slight motion. Commutators are turned originally on lathe centers, but in the generator its shaft revolves on bearings. The shaft center may be out of true with the journal a fraction of a thousandth of an inch, or the shaft may be slightly sprung. Perfection here is seldom reached. It will be unwise for anyone except the most experienced machinist to attempt to remedy such faults. In such cases the business of the projectionist is only to detect and report them.
(151) Test for "out-of -round" by resting some blunt pointed tool on a solid support, its face just in contact with the commutator, and rotate the armature slowly by hand, noting whether or not the contact is constant through one complete rotation.
Points that Need Watching
(152) Be sure no bit of copper has dragged across from one commutator bar to another. In that event a ring of light will appear, seemingly entirely around the commutator. Remove the trouble-making copper.
Make sure that the brushes all fit properly in their holders and that the holders are free from all accumulations of dust and dirt.
Be sure, if the set is one in which the armature lies horizontally and is not ball bearing type, that the armature has endwise movement (called "float") of at least