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532
C. F. VlLBRANDT
Vol 48, No. 6
those used for Fig. 8. It was noted throughout the investigation that tests involving the use of one projector and one batch of film showed very good reproducibility. But another series, run with a different lot of film on another projector, might show a different level of projection life under comparable conditions, even though the shape of the curves remained unchanged. These differences are due to variations in alignment of film on the sprocket, history of the film, condition of tooth surfaces, etc. Although there are not enough experimental points to define the shape of the curves with any great accuracy, it
WEDGING ON OCCURS
o I NO TENSION ON DRIVE SPROCKET
X 13 OZ TENSION ON HOLD-BACK SPROCKET
SERIES: B-4-5e.7
PROJECTOR A
+ 1.5 +1.0 1-0.5 0 -0.5 -I.O I.S
PERCENT DEVIATION OF PITCH OF FILM FROM PITCH OF SPROCKET
FIG. 7. Relationship between the per cent deviation of the film pitch from the holdback sprocket pitch and number of projections that produce film [breakdown.
appears that longest film life is obtained when the pitch of the film is somewhat shorter than that of the drive sprocket. Accordingly, the use of a drive sprocket with a pitch longer than that of the film is indicated. These findings are in agreement with the theoretical conditions discussed earlier and with the results on 35-mm intermittent sprockets (which are drive sprockets) presented by Talbot3 at the Hollywood meeting in 1945.
When the pitch of the film is greater than that of the drive sprocket (Case 1), the film wedges on the entering tooth, severely straining the perforations. As the difference between the pitch of the film and the sprocket becomes less and less, this "wedging-on" decreases and the life of the film increases. This same effect was observed for film