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Aug., 1945
THE PROJECTION LIFE OF FILM
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The foregoing remarks relate to the difference in the nature of the wear on the sprocket teeth. The increased extent of wear on the 0.935-in. intermittent sprocket as compared to that on the oversized sprocket is explainable. In the case of the former, the additional force necessary to thrust the film ahead of the rotational speed of the sprocket manifests itself in increased wear of both film and sprocket teeth. When the pitches of film and sprocket are equal, the driving force is spread over 2, or even 3 teeth simultaneously — resulting in decreased wear on both film and sprocket. This condition should hold true even when the 2 pitches are nearly equal, as a result of the local stretching of the film upon impact.
CASE I
CASE I
U
FIG. 9. Impact marks of an intermittent-sprocket tooth on a film perforation. Case I — Correct Alignment; Case II — Incorrect Alignment.
It is therefore gratifying that by the use of increased diameter intermittent sprockets substantially increased film life can be achieved with no increase in sprocket wear.
Relationship Between the Alignment of the Film on the Intermittent Sprocket and Film Wear. — Fig. 9 is a drawing of the marks made on the standard positive-type perforation by the intermittent sprocket on one passage through a projector. It is impossible to align the film on the intermittent sprocket so that the latter will strike .the film in the center of each row of perforations. This is because the transverse pitch of the sprocket is less than that of the film. If the film is centered on the intermittent sprocket on one