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June 1947 PROJECTION LlFE OF 16-MM FlLM 525
experiments is somewhat lower than that which should be obtained with the film feeding normally into the drive-sprocket assembly.
These data indicate that any elements which cause added resistance to movement of the film by the drive sprocket should be avoided. In general, it is better to provide rollers instead of stationary surfaces where the film enters the sprocket assembly. Snubbing could be eliminated by arranging the film path from the supply roll to the free loop so that no guiding elements are required. Although these methods would eliminate the undesirable snubbing action, other factors causing increased damage to the film might be introduced. Experiments to determine the effect of these changes are indicated.
At the holdback sprocket, the snubbing action of the stripper post absorbs part of the applied tension. This friction decreases the strain on the perforations and results in longer film life.
A comparison of the corrected curves, H2 and D2, shows that, even when the influence of the "snubbers" is eliminated, the life of film running on a drive sprocket is significantly less than the life of film running on a holdback sprocket. These data indicate that the action of a drive sprocket is more damaging than that of a holdback sprocket. The relationship of the face of the tooth to the edge of the perforation during their disengagement may account for this difference. This relationship is not the same for the two sprockets, as is shown in a later section of this paper.
Since a drive sprocket is more damaging to film than a holdback sprocket, a special effort must be made to keep the tension on a drive sprocket as low as is consistent with good projector performance. This is particularly .true since on every projector there are two, and sometimes three, drive mechanisms that wear the same edge of each perforation :
(1) A supply sprocket, which pulls the film from the supply reel into the free loop. Tension is present here because there must be a restraining torque on the supply spindle in order to prevent excessive overrunning of the supply reel when the projector is stopped.
(2) An intermittent device, which pulls the film through the gate. Tension here arises from the friction that is necessary for steadiness of the projected image, and from the acceleration of the film during the pull-down cycle.
(5) A sound sprocket, which governs the movement of the film through the sound-reproducing mechanism. This sprocket acts