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639 HANDBOOK OF PROJECTION FOR
The projectionist should carefully examine his aperture plate tracks, with reasonable frequency, and should renew them as soon as they show any appreciable unevenness of the surface.
GENERAL INSTRUCTION NO. 12— SPROCKET IDLERS.
— It is essential to proper performance of sprocket idlers that the idler set equidistant from the face of the sprocket at both ends of the sprocket, and that the distance of the idler from the face of the sprocket be a trifle more than the thickness of a film. If the sprocket idler be not carefully and accurately set as per the foregoing, there is likely to be more or less trouble, particularly at the lower sprocket. The losing of the lower loop, while not of course necessarily chargeable to improper setting of the lower sprocket idler, nevertheless is very frequently due to that cause, particularly if there is excessive take-up tension. If the lower sprocket idler be out of line with the sprocket, or too far away from the sprocket, or close enough to the sprocket to pinch the film, there will probably be trouble through losing of the lower loop.
Never allow your sprocket idler to "ride the film" — that is to say, to bear on it with any pressure. This is particularly bad if the pressure be greater on one side of the sprocket than on the other. Under that condition, the film is likely to climb the sprocket at the first bad splice.
Examine your sprocket idlers frequently, and make sure that they are turning freely. If they do not they will soon develop a flat spot, which sooner or later means trouble. All professional projectors have an adjustment by means of which the projectionist may determine the distance of the idlers from the face of the sprocket. A fairly good plan is to place two thicknessess of film on the sprocket, and then adjust the idlers so they rest on the film; with this adjustment and only one thickness of film, the idler should be about the right distance from the sprocket.
GENERAL INSTRUCTION NO. 13 — LINING CAM SHAFT. — When using a projector in which the adjustment between the star and cam is accomplished by turning an intermittent sprocket shaft bearing at either end, great care should be taken that the intermittent sprocket and the cam shaft are kept in perfect alignment with each other. The position of the cam shaft is not adjustable, but the sprocket shaft is raised or lowered by turning the bearing bushings, and it is possible, by turning one bushing more than the other,