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June 1947 PROJECTION LlFE OF 16-MM FlLM 535
sprocket and film, it seems clear that sprockets .should be designed to give optimum conditions for forward projection. This is essential if film life in excess of a few hundred runs is desired. The increased damage caused by such sprockets running in reverse should be more than compensated for by the decreased damage during the far more frequent forward projections.
To determine whether or not reverse operation is feasible on. sprockets in the proper pitch range, films of 0 and 1 per cent shrinkage were run in reverse on projectors with experimental eight-tooth drive and holdback sprockets of approximately the correct pitch. (Drive sprockets of 0.761-in. root diameter and holdback sprockets of 0.746in. root diameter were used.) The films projected quite satisfactorily in reverse, with no loss of loop or other trouble. After the rolls had been run forward and in reverse ten times, inspection of the films showed no appreciable damage to the perforations.
These results indicate that sprockets of optimum pitch for forward projection do not prohibit reverse projection. In these experiments, the sprockets were provided with shoes which prevented the film from climbing off the sprockets. Also, the tooth shape was an involute with a low-pressure angle, so that wedging-on did not result in as severe strain on the perforation as would result from wedging on a tooth of more rounded profile. It should be emphasized that elimination or modification of the sprocket shoe, or a change in the shape of the tooth, may result in prohibitive damage to the film perforations when a projector provided with sprockets designed for optimum conditions during forward projection is run in reverse.
Comparison of the Sprocket-Film Interaction at Drive and Holdback Sprockets. — It will be recalled that the experimental data in Figs. 2, 7, and 8 show two essential points of difference in the life of film at the drive and holdback sprockets:
(1) With the film engaging the sprocket under optimum pitch conditions, the drive sprocket damages film more severely than does the holdback sprocket at the same tension.
(2) On a drive sprocket, the life of the film is at a maximum at an optimum pitch difference between the film and sprocket and decreases when the film is longer or shorter than this optimum. On a holdback sprocket, there is no such maximum in the life of the film within the same range of pitch differences. Rather, the projection life of the film increases as the pitch of the film becomes progressively longer than that of the sprocket.