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530 C. F. VlLBRANDT Vol 48, No. 6
per cent difference in pitch between the film and the sprocket. As was recommended by Talbot3, these data are plotted so that zero on the abscissa represents a perfect fit of the film on the sprocket. All points to the left of the vertical line through this point represent Case 1, in which the pitch of the film is greater than that of the sprocket. All points to the right of this line represent Case 2, in which the pitch of the film is less than that of the sprocket. These data were obtained from tests of film loops run on the apparatus shown in Fig. 1. The range of differences was made possible by combinations of films perforated to different pitches and sprockets of three different pitch diameters.
FIG. 5. Action of film on a drive sprocket when the pitch of the film perforations is greater than the pitch of the sprocket teeth.
As is predicted by sprocket theory, Fig. 7 shows that better wear is obtained at the holdback sprocket when the pitch of the film is longer than that of the sprocket (Case 1). However, there is no evidence of a maximum in the curve at or near perfect mesh of the sprocket with the film. Instead, throughout the range of differences studied, the life of the film increases as the pitch of the film becomes progressively longer than that of the sprocket.*
* It is surprising that longer life is not obtained when the tension is divided between two perforations instead of having the entire tension carried by one perforation. A possible explanation for this effect is as follows :
In Case 2, where the pitch of the film is less than that of the sprocket, the film wedges on the sprocket tooth. The face of the tooth shears against the entering perforation when the film is forced down on the sprocket by the shoe. As the film becomes longer, the initial point of contact on the face of the entering tooth