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Nov. 1947
INTERMITTENT MECHANISMS
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about ten times the average speed of the film through the projector. The lower curve shows that the film is accelerated for the first half of the pull-down. The maximum acceleration or deceleration is approximately 60,000 inches per second per second.
The curves of Fig. 2B show characteristics which are related particularly to tension on the film when driven by a Geneva movement. The upper curve shows the pull on the film caused by its acceleration which is determined by the movement of the intermittent mechanism.
A B
FIG. 2. A, Travel, velocity, and acceleration of the film. B, Pull on the film caused by acceleration and film-trap drag.
This pull is determined by multiplying the film-acceleration curve by the mass of the film, the determination of which was described earlier. In addition to the accelerating pull on the film imposed upon it by the intermittent mechanism there is an additional stress due to the friction in the film gate. At the mid-point of the pull-down period the film is traveling at 180 inches per second and if no means were provided to decelerate it, it would tend to continue this speed and would try to overshoot the intermittent sprocket. In order that the film will never tend to overshoot and will come to rest precisely under the control of the intermittent mechanism the film-trap drag must be at least equal to the peak inertia force on the film at its peak point of deceleration, which is approximately 0.5 pound. The middle curve,