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There are four main types of intermittent movement; the claw, wherein one or more claws engage the perforations in the film; the intermittent sprocket, which has a number of stops to each revolution, and the shoe, wherein a curved bar engages a loop of film and pulls it down by means of its rapid up-and-down movement. The fourth type is known as the "decimittent," but is not in general use in Britain.
The most common in cine equipment is the claw movement, but although four out of every five of the post-war models are fitted with this movement, the intermittent sprocket must be considered of equal merit.
In all projectors the intermittent movement of the film itself must be isolated from the rest of the film running through the machine at an even rate, and this is usually accomplished by maintaining a loop of film at each end of the gate or film channel. This loop must retain the correct size, as a top-loop too large will cause the film to flap and add to the noise of the machine, whilst a lower-loop the wrong size will put the sound out of synchronisation with the picture. In many projectors the manufacturers have included a mark denoting the exact position or size of the loops.
Should the loop be lost during projection the motor must be stopped at once, otherwise the film will be damaged by being pulled between the intermittent and the first take-up sprocket. In some projectors, however, the bottom-loop is formed around a trip switch, which will automatically stop the motor should the loop be lost.
One advantage of the claw mechanism is that it can be more easily serviced than the cross movement, and in this respect you should read your Operators' Instructions for some specific reference to oiling or greasing. But in the absence of such instructions the operator should attend to the servicing of a cross movement at least once in every three months.
If the mechanism of the intermittent-sprocket is sealed within an oil bath, however, it should not be tampered with. Here the operator must ensure that all oil-tight joints are indeed as tight as they should be, and watch, or rather listen, for any signs and indication of wear. An intermittent "beat" in addition to the normal noise made by the mechanism will usually provide ample warning of trouble. And in the same way, a slightly ex
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