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320 RICHARDSON'S BLUEBOOK OF PROJECTION
Sprocket Idlers
(104) The function of holding the film close to the face of the various sprockets is performed by sprocket idlers, consisting either of one or two rollers per sprocket, or by a thin steel shoe.
Whether it is a shoe, a roller or rollers (some projectors have two rollers to each sprocket and some only one), the rule is (105) that the distance from the face of the roller or shoe to the face of the sprocket must be approximately one and one-half times the thickness of the film. (106) To arrive at this lay two thicknesses of film on the sprocket and adjust the roller bracket or the shoe firmly against the film with the adjustment screw in firm contact with the stop it rests upon.
(107) If the idler has been set too close to the sprocket face it will pinch the film. It will then start a tendency to side-weave, causing the sprocket teeth to climb out of the sprocket holes. If the shoe or roller is too far away the same condition may be brought about, though for a different reason, resulting in the loss of one of the loops. Proper adjustment of sprocket idlers is important.
(108) It is also important that each end of the sprocket idler be exactly the same distance from the sprocket face, since any variation will cause the film to pull sidewise away from and off the sprocket teeth. (109) If the idler is a roller it must be kept well lubricated with a thin oil, else it will stick and quickly wear flat on one side. In that case it must be replaced immediately. Do not use heavy oil. The much advertised household oils are excellent for this purpose; or the oil you use for the projector, mixed half and half with kerosene, will do very well. All idler rollers, including those of the fire trap, should be inspected each day before the first run to make sure they are rotating freely. Sprocket idler rollers should be given one drop of oil each day, wiping all surplus oil carefully away.
Upper Magazine Tension
Every first class projector is equipped with some sort