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June 1947 16 AND 8-MM SPROCKET STANDARDS 493
The sprocket in Fig. 4 (a) is driving the film by the front or lefthand face of tooth A. As the sprocket rotates, tooth A leaves the film, which then slips backward so that the load is transferred to the front face of tooth B . Meanwhile, tooth D enters the perforation without touching the film, as it should if wear is to be kept to a minimum.
Fig. 4(b) shows a properly designed sprocket for the holdback condition. In this case, the film is held back by the rear or left-hand face of tooth A i. As the tooth leaves, the film slips forward, the load is transferred from tooth A\ to tooth BI, and tooth DI freely enters the perforation that is coming into engagement.
Examination shows that in Fig. 4 (a) the circular pitch of the sprocket teeth is greater than the pitch of the film, while in Fig. 4(b) the pitch of the sprocket teeth is less than the pitch of the film. From observation of the action under these two conditions, we can conclude that:
(1) A properly designed drive sprocket should have a circular pitch equal to or greater than the pitch of the film; and
(2) A properly designed holdback sprocket should have a circular pitch equal to or less than the pitch of the film.
These conclusions are not novel but were evident in Jones' paper on Film Sprocket Design.1
Experimental Confirmation of Above Rules. — There is some recent experimental evidence to support the statements made above. Films of three different shrinkages were run in succession on three sprockets having different pitches. Each of the nine combinations of film and sprocket was observed and photographed, under both drive and holdback conditions, with external tensions of 4, 6, 8, and 10 oz applied to the film.
Fig. 5 illustrates qualitatively the results of these tests. It shows, for both drive and holdback conditions, that when the shrinkage of the film and that of the sprocket differ in the theoretically correct direction, the operation is satisfactory. But when the shrinkage difference is in the other direction, there is trouble. In the case of drive sprockets, no actual loss of loop occurred, but it is apparent in the pictures that violation of the theory results in a disturbance caused by contact between the entering tooth and the edge of the perforation. On holdback sprockets, since there was no sprocket clamp to prevent it, the film rose to the tops of the teeth, and the take-up device pulled the film ahead so that the loop at the other side of the sprocket was