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236 RICHARDSON'S BLUEBOOK OF PROJECTION
the gear train the master blade of the shutter is made to cut off all light from the screen and to turn it on again at precisely the right instant of time.
(99) There is lost motion even in a new gear train — about 3/16 of an inch free movement of the intermittent flywheel while the shutter is held stationary. Projectionists should watch this carefully. New or repaired mechanisms that have a greater ratio of lost motion should be rejected.
(100) As parts of the mechanism wear, lost motion will increase. It should never be permitted to be more than 5/16 of an inch. If the ratio is higher it is evidence that the mechanism needs a general overhauling.
The gear train may not be altogether responsible for lost motion. Often it is due to worn bearings and other factors in the intermittent movement.
(101) To measure lost motion affix a prick-punch mark to some part of the mechanism frame as close as possible to the rim of the flywheel of the intermittent movement and a similar one near to the inner edge of the flywheel rim. By fixing one point of a calliper, or an ordinary carpenter's compass, in the mark on the frame with the other point at the mark on the flywheel rim, the distance the flywheel can be rotated may be accurately measured. Be very sure to hold the shutter perfectly still while testing the flywheel rotation.
The foregoing test was developed by projection manufacturers especially for this edition of the bluebook and has become the official, standard method of checking the gear train for lost motion.
Gear Lubrication
Projectionists differ as to what constitutes the best form of gear lubrication. (102) Manufacturers recommend a fairly ample lubricant of a good grade of medium weight oil. A good quality lubricant keeps up a slow but constant washing of the gear surfaces and works out much more rapidly than heavy oil or grease, which collects dust and grit from the air.