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80
R. H. TALBOT
Yol 45, No. 2
film is less than that of the sprocket. Here, the tooth that is about to leave the film does the driving, and the entering tooth engages the film without touching the edge of the perforation. The film is well seated at the base of the tooth when it makes contact with the driving face. Theoretically, this condition should cause less wear on the film than the previous case, in which the pitches of the film and the sprocket are reversed. Actually, experiments show that lower wear may occur on either side of perfect mesh, but, in general, so far as film wear is concerned, it is better to have the pitch of the film less
than that of the sprocket rather than greater than that of the sprocket, as is the case today.
This fact is illustrated in Fig. 3, which shows graphically the increased number of projections, plotted along the ordinate, as the shrinkage* of the film, plotted along the abscissa, causes the pitch of the film to become equal to, or slightly less than, the pitch of the intermittent sprocket. The shrinkage of film necessary to give perfect mesh with a 0.935-in. sprocket is illustrated by the dotted line. All points to the left of this line represent Case I, in which the pitch of the film is greater than that of the sprocket, and all points to the right of the dotted line represent Case II, in which the pitch of the film is less than that of the sprocket.
The actual number of projections required to produce complete breakdown of the film at the various film shrinkages will vary widely with machine conditions and the manner of operation. The maximum may occur under some conditions at several hundred to one
* The values along the abscissa are expressed as "per cent shrinkage" of the film, for convenience. Since it would be impossible to obtain shrinkages of such great range with the present motion picture films, many of the samples were perforated less than standard pitch on specially designed equipment in order to simulate films having these different shrinkages.
FIG. 2. Action of film on a sprocket when the pitch of the film perforations is less than the pitch of the sprocket teeth.