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178
W. G. HILL AND C. L. SCHAEFER
[J. S. M. P. E.
Since the perforating standards for film are well established and there is reason to believe that no changes will be made in the principal dimensions, we have expressed the base diameter for the sprocket in terms of film pitch. It Will be recognized, then, that as the shrinkage characteristics of motion picture film change, and they no doubt will through improvements in manufacturing, the method of sprocket design is not altered. Another point that should be mentioned is that for any motion picture film, for instance 16-mm, classified in group /, and a sprocket of a definite number of teeth and with anywhere from 1 to 10 teeth in mesh, the same sprocket is used. This means that the same cutter can be used in forming the teeth of such sprock
py-r.
FIG. 1. Hold-back sprockets B, base sprocket dia. (Dl ~ G}N
F
3.1416
DI, film perforation pitch = D — DS D, nominal pitch of freshly perforated film S, shrinkage
G, clearance of second tooth in mesh N, number of teeth on sprocket F, film thickness
F, number of teeth in mesh minus one P, sprocket-tooth pitch
ets. American Sprocket Standard Z22.6-1941 for 16-mm film calls for different tooth thicknesses for each different number of teeth in mesh, which means that different cutters must be used.
Fig. 1 represents a hold-back or take-up sprocket; the sprocket is shown rotating counter-clockwise, the film feeding from a loop onto the sprocket at the right, and under tension as it leaves engagement at the left. It will be noticed that the clearance G has been so chosen that the tooth pitch is less than the film perforation pitch. This is because for best operation it is deemed advisable to relate the film and sprocket pitch in such a manner that the film is always free to pass onto the sprocket teeth and have the disengaging tooth carry the load.