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82
R. H. TALBOT
Vol 45, No. 2
the point of perfect mesh has not been clearly established. The following is offered as a possible explanation : In Case I, in which the pitch of the film is greater than that of the sprocket, the film wedges on to the sprocket tooth, the film traveling faster than the circumference of the sprocket. Consequently, all of the driving action is accomplished by the sprocket teeth. In Case II, in which the pitch of the film is less than that of the sprocket, the circumference of the sprocket travels faster than the surface of the film. In this case, a considerable portion of the driving action is accomplished by contact
1.2 1.6 2.0
SHRINKAGE or FILM
FIG. 4. Same as Fig. 3 with the exception that the maximum number of projections does not occur at or near the point of perfect mesh of film and sprocket.
along the circumference of the sprocket as well as by the sprocket teeth, thus causing the number of passages to increase to a certain point as the pitch of the film becomes less than that of the sprocket.
Likewise, the failure of the maximum number of projections to occur at the calculated point of perfect mesh can be explained by the stretching of the film at the point of impact. If the film stretches upon impact of the sprocket tooth, and this appears to be a reasonable assumption, the effective pitch is greater than the calculated or measured pitch. Therefore, to obtain the optimum projections, the pitch of the film should be less than that required to give perfect mesh.
There is another and perhaps more satisfactory manner in which