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THE PROJECTION LIFE OF FILM
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of the film perforations is changing more or less continually, owing to the loss of volatile materials and also to the dimensional changes which accompany variations in moisture content of the film. What then should be the pitch of the intermittent sprocket teeth?
The projector manufacturer has several choices in the design of the intermittent sprocket. He may make the pitch of the teeth such that there will be almost perfect mesh between film and sprocket when the film is freshly processed and less and less close mesh as the film shrinks, or he may make the pitch of the teeth such that there will be closer and closer mesh as the film ages. He may even choose a compromise pitch so that the film fits the sprocket best after the film has shrunk a portion of the total amount.
In order to decide which course is the best to follow from the standpoint of film wear, it might be of interest to review the behavior or movement of film on a sprocket in each of the 2 cases, i. e., Case I— The pitch of the film perforations is greater than that of the sprocket teeth; Case II— The pitch of the film perforations is less than that of the FlG L Action of mm on a
Sprocket teeth. sprocket when the pitch of the film
T?« 11, j.1, u t, perforations is greater than the pitch
Fig. 1 shows the behavior or Sf the sprocket teeth.
movement of film on a sprocket
when the pitch of the film is greater than that of the sprocket. This was described before the Society in 1923 by J. G. Jones, of the Eastman Kodak Company.1 Note tkat the entering tooth strikes the edge of the perforation and shifts the film forward, the film traveling faster than the surface of the sprocket. This wedging-on of the film on the sprocket teeth produces a severe strain on and eventually a tearing of the perforations, and decreases the life of the film. This is the condition which exists today with the 0.935-in. diameter intermittent sprocket which was designed for films of much higher shrinkage than those in use at the present time.
Fig. 2 shows the action of film on a sprocket when the pitch of the