Society of Motion Picture Engineers : incorporation and by-laws (1923)

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Starting again from perfect mesh, let us now assume the film to be shrunk or shortened, then the only tooth in engagement is the leaving tooth, "G", Figure 8. This by numerous wear tests, running endless or belted film, indicates the best wearing condition. There is here a backward slip of the film equal to the difference between the pitch of the film and the pitch of the sprocket, "H"3 Figure 8, which MOTION PICTURE FILM IN MESH WITH FILM SPROCKET. Good condition. Pitch of film less than that of sprocket, but not sufficiently so as to cause interference. Fig. 8. amounts to deceleration and tends to ease up the impact of the teeth against the edge of the perforations, especially on the intermittent sprocket. Also the friction due to wrap of the film on the base diameter helps to pull the film along and eases the stress on the edge of the perforations. This is a better running condition than when the film is being crowded forward as mentioned in the case of film having longer pitch than the pitch of the sprocket. This favorable running condition holds until the film is shrunken enough to interfere at the back of the entering tooth, "I", Figure 9. Any further shrinkage will result in excessive interference so that destruction of film will result, as at "K", Figure 10. MOTION PICTURE FILM IN MESH WITH FILM SPROCKET. Interference due to too much shrinkage. Pitch of film less than that of sprocket. Fig. 9 59