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June 1947 PROJECTION LlFE OF 16-MM FlLM 539
possibly others, that makes the drive sprocket damage film more than does the holdback sprocket.
In order to observe the engagement and disengagement of the sprocket teeth with the film and the influence of certain variables on this action, high-speed motion pictures were made of film passing over drive and holdback sprockets. The pictures were taken with an Eastman High-Speed Camera, Type III, operating at a speed of approximately 2000 frames per sec. On the test projector, the portion of the shoe covering the teeth was removed so that the action could be photographed.
With films and drive sprockets combined to obtain film shorter than the sprocket by 0.7 and 1.7 per cent, pictures were taken with film tensions of 4, 6, and 9 oz. The following observations were made from these pictures :
(1) After the leaving tooth disengages from the film, the film moves back to the next tooth so rapidly that the perforation humps up or distorts considerably. This effect was noticed only on the shortest film.
(2) As this tooth starts to disengage, the front of the tooth "saws" on the edge of the perforation, and the film is distorted downward.
(3) The impact on the tooth that assumes the load is more pronounced at greater tensions and at greater deviations from sprocket pitch.
(4) For the film which was measured to be only 0.7 per cent shorter than the sprocket, there was some evidence of wedging-on at the entering tooth when the film tension was increased — indicating that the film was actually longer than the sprocket. This effect can be explained only by the assumption that the pitch of the film increased because of stretching or distortion under the applied tension.
At low tensions, these effects are not readily seen. It is only when the tension is as high as 6 or 9 oz that they become pronounced. It is believed that they are present at lower tensions, but to a lesser extent. Pictures of the holdback sprocket with films longer than the sprocket by 0.2 and 1.3 per cent at tensions of 6 and 10 oz show the following :
(1) The film slides very easily and smoothly off the leaving tooth, with no evidence of resistance to stripping.