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304
RICHARDSON'S BLUEBOOK OF PROJECTION
is unsteady. (55) Too much tension, on the other hand, imposes a great strain upon many parts of the intermittent mechanism, upon the delicate edges of the sprocket holes of the film, upon the motor that drives the projector, upon the projector gear train and consequently upon the shaft bearings which give the gears their support, and generally upon all working surfaces of the intermittent movement including the edges of the sprocket teeth and the tension shoes and aperture plate film tracks.
Even under the best of conditions the intermittent mechanism represents a sensitive adjustment of precisely accurate parts working at top speed under strain. Some sections of the intermittent assemblage must start, reach high speed and stop twentyfour times each second for long and continuous periods of time each day. This truly "nerve-wracking" action takes place against the braking effect of the aperture tension.
(56) If the tension applied at the gate is just enough to permit the film to stop perfectly still for a brief interval over the aperture (without unsteadiness or overshooting of the screen image), then the intermittent is accomplishing its work smoothly. But if the aperture tension is excessive, the actuating pin D in Fig. 134 will press against the side of the star slot with sufficient force to overcome the abnormal tension. It is obvious that the excessive tension at the aperture will, in time, destroy the delicate adjustment of the whole intermittent mechanism, which moves 1 ,440 times per minute.
Checking Aperture Tension
Figure 137
(57) One method of securing proper aperture ten