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220 RICHARDSON'S BLUEBOOK OF PROJECTION
loop H by lower sprocket G. The compensation is equal and the action can continue indefinitely.
Inspect Sprocket Teeth
(47) The constant friction of film against metal wears the metal away. Worn intermittent sprocket teeth not only produce an unsteady picture but tear the film sprocket holes. Therefore the intermittent sprockets should be examined after each fifty hours of operation. Use a magnifying glass to discover the flaws that frequently cannot be seen by the naked eye. If there is any visible undercutting in the form of a small notch at the base of the working side of the teeth, the sprocket should be replaced at once. If no notch shows but the teeth are appreciably reduced in width at their base, they are equally useless and harmful.
Lost Motion in Intermittent Sprocket
(48) Adjustment between cam A and star B (Fig. 62 and 63) must be accurate to permit no perceptible movement of the intermittent sprocket. Test it by rocking it back and forth with your finger while the movement is on the lock. At the same time the adjustment must not be too close or it will have a tendency to bind when the projector mechanism flywheel is turned. (49) The adjustment is correct when the flywheel rotates freely and there is no lost rotary motion in the intermittent sprocket when the movement is on the lock — that is, when the locking ring C is engaged with one of the star quarter-circles E.
The intermittent movement may be adjusted either when the mechanism is hot or cold.
Replacement of Intermittent Sprocket
Few projectionists are equipped, either with tools or experience, to perform this task with the necessary accu. racy. Unless an intermittent sprocket is installed on the shaft perfectly, faults will show up on the screen, either as an up-and-down or lateral movement of the whole screen image.