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310 RICHARDSON'S BLUEBOOK OF PROJECTION
clearance between armature and pole pieces. Naturally any wear in the bearings will reduce the clearance at the bottom and increase it at the top, the shorter gap below becoming a danger point. Projectionists should therefore check up on bearing wear at frequent periods — at least every thirty days. This does not mean that a properly lubricated bearing will wear appreciably in thirty days; it is simply a precaution. If the bearing should fail because of lack of oil distribution trouble may come very quickly.
(112) Ball bearings usually proclaim that they are worn by becoming exceedingly noisy. Sleeve bearings give no warning except through heat.
There is no one best way to check up on bearing wear. A good mechanic will do it almost intuitively. A strip of hard cardboard, about an inch wide and a foot long may be slipped between a pole and the armature. Then, grasping both ends, it can be worked gradually around until it has passed between all poles and the armature surface, thus making sure that no friction exists between the two elements. Because of the dirt that settles on the pole face it is sometimes impossible to see through the air gap; also the gap below the armature may seem smaller than the gap above because more dirt is gathered on the faces of the lower poles. The cardboard removes some of the accumulated dust.
(113) A set of gauges or "feelers" may be purchased from automobile supply dealers. This is a series of thin steel strips varying usually from .002-in. to .010 or .015inc. in thickness. They are held together in a frame like the blades of a jack-knife and can be laid together in any desired combination so that by building up to any desired thickness and inserting it into the gap, the width of the gap can be tested accurately.
(114) An equally efficient check is to pry with a bar inserted under some exposed part of the armature shaft. Pressure will make the armature rise in the bearing if the bearing is worn. The "looseness" of the bearing indicates the wear.
(115) Lubrication of sleeve bearings usually is accomplished by means of oil wells. Under each shaft