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538 C. F. VlLBRANDT Vol 48, No. 6.
As is shown in Fig. 10, the divergence of the path of the perforation and the corresponding face of the tooth on a holdback sprocket allows a relatively large movement of the film on the sprocket before the tooth disengages. Thus, for any reasonable pitch difference, impact loading of the perforations cannot occur. The absence of this shock action can account for the fact that the life of the film on the holdback sprocket does not decrease as the pitch difference between the sprocket and the film increases. However, some other explanation, not apparent at the present time, is necessary to account for the observed increase in life under these conditions.
A comparison of the data of Figs. 7 and 8 shows that, even under optimum conditions on the drive sprocket, the life of the film is considerably less than that obtained for films at comparable engagement conditions with the holdback sprocket. This same difference in life level is shown by comparison between curves H^ and D2 in Fig. 2. These data indicate that some action at the drive sprocket is more damaging than that at the holdback sprocket. A possible explanation for this difference is suggested by a comparison of Figs. 9 and 10. As the film slides out along the back face of a holdback sprocket tooth, the angle between the face and the film changes rapidly, so that sliding becomes easier. On the other hand, as the drive sprocket turns and the perforation slides out along the front face of the tooth, the angle between the face and the film decreases only slightly. Thus, it is much more difficult for the film to slide up and off the leaving tooth of the drive sprocket. The friction between the face of the tooth and the edge of the perforation tends to pull the film past the ideal point of disengagement for the sprocket. On the drive sprocket, this tendency for the perforation to cling to the sprocket tooth is enhanced by the fact that the only force stripping the film from the tooth is the spring-like action of the film in the free loop. Thus, the film and the tooth tend to remain in engagement so long that when the perforation finally is stripped from the tooth, the tooth effectively drives down into the film and produces a strain on the perforation.
Moreover, to disengage from the drive sprocket, the perforation must slide under tension on the tooth face for practically the full length of the face, whereas on the holdback sprocket, the film must slide only a very short distance up the face of the tooth before the film touches the next tooth. This difference results in a greater sawing action of the tooth on the perforation in the case of a drive sprocket. It is probably a combination of all these effects, and