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310 A. S. HOWEU, AND J. A. DUBRAY [J. S. M. P. E.
Base diameter of the sprocket 1 . 1842 in.
Circular pitch of the sprocket 0. 1870 in.
Tooth thickness for a maximum shrinkage of approximately 2.5 per cent 0 . 0500 in.
The increase in the number of sprocket teeth and consequent reduction in perforation pitch, as compared with the condition illustrated at A, offers a greater number of film bearing surfaces and is, therefore, more closely corresponding to the ideal condition previously expressed, while the film portion between perforations is of sufficient width to withstand the stress imposed upon the film and therefore offers greater protection against breakage.
The above expressed considerations and dimensions apply to an intermittent, as well as to a film feeding sprocket, both of which function as film driving elements.
A different condition is met when a take-up or hold-back sprocket is considered. The function of these sprockets is inverse. They hold back the film instead of driving it and are therefore driven instead of acting as drivers. Since only one of the teeth comprised in the arc of contact (except for the exceptional case of perfect mesh) can drive or be driven (as the case may be) in engagement with the film perforation, and since the leaving tooth is in both cases the only one in contact with the perforation, it results that the side paying off film is under tension, while the side paying in is loose in contrast to an intermittent sprocket for which the opposite condition exists.
The slightest interference of free engagement of the film perforation with the entering tooth, which may be caused by the looseness of the film at this end, will cause the film to take a longer path and be immediately crowded out of mesh.
It is evident that the remedy for this condition is that the holdback sprocket teeth shall have a circular pitch slightly less than the pitch of the perforation of the maximum shrunk film.
The reversal of the function of the driven sprocket, as compared with that of the driver, calls for a reversal of its ability of accommodation. This can be secured by so reducing the base diameter of the sprocket that it will bring in perfect mesh the film when in its condition of maximum shrinkage. In other words, for a driven sprocket, the circular pitch of its teeth must be identical with the pitch of the shrunken film.
This condition is illustrated at A1 and J51, Fig. 1, where it is again shown that for the predetermined perforation pitch of 70 mm.