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June 1947 PROJECTION LlFE OF 16-MM FlLM 529
or a similar mechanism. This wedging of the film on the sprocket tooth produces a severe strain on the perforation and decreases the life of the film.
On a drive sprocket, these conditions are reversed. The film is in contact with the forward faces of the sprocket teeth; and when the pitch of the film is longer than that of the sprocket, the film wedges on the entering tooth. This is shown in Fig. 5.
On the other hand, Fig. 6 shows that when the pitch of the film is less than that of the sprocket (Case 2), the film is driven by the leaving tooth, and the entering tooth engages the film without touching the edge of the perforation. These conditions are similar to those of
FIG. 4. Action of film on a holdback sprocket when the pitch of the film perforations is less than the pitch of the sprocket teeth.
Case 1 on the holdback sprocket, and should result in less damage than when the film wedges on the sprocket teeth.
Theoretically, the best wear life should be obtained when there is perfect mesh between the sprocket teeth and the film perforations. Under these conditions, the load is carried by more than one sprocket tooth, and each tooth engages perfectly with each film perforation as it enters. However, because film shrinks as it ages, and changes dimensionally with moisture content, the pitch of the film is not constant.
Since perfect mesh is not always attainable, these considerations indicate that the pitch of the film should be longer than that of the holdback sprocket and shorter than that of the drive sprocket.
Experimental verification of these principles is shown in Figs. 7 and 8, where the number of projections required to produce complete breakdown of the film at a constant tension is plotted against the