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PROJECTOR MECHANISM 465
be great enough to hold the film flat in the aperture when it is stationary, yet small enough to permit the film to be moved readily when the claw propels the film through the gate. The surfaces of the body plate and of the pressure plate are relieved or undercut so that only the safety areas of the film bear upon the cooperating plates. A good projector has all support parts {e.g., sprockets, rollers, transport parts) relieved so that the film is supported only at the safety areas. A loop of film will go through a well-designed projector as much as 5000 times when a good machine is in good order, but less than 100 times in the case of a reputable machine that is poor by comparison. After 1000 times, such a loop should show little or no scratch or abrasion marks at those parts of the picture that are projected and at those parts of the sound track that are scanned by the sound light beam. To reduce wear to a minimum on the working parts of the surfaces of the sprockets, gates, and rollers the parts are made either of materials with excellent resistance to abrasion, such as stainless steel, or of softer materials such as brass or steel and are chrome-plated. In either event the parts are highly polished to reduce wear and abrasion of the projected film to a minimum. All rollers should turn very freely, since a roller that binds causes more film damage than no roller at all.
The sound translation system, although usually relatively simple, deserves special discussion and will be treated later.
The takeup reel is rotated either by the projector motor through a belt drive or by a separate motor. In either case there is some form of friction clutch or variable-speed drive between the takeup spindle that holds the takeup reel and the member from which the spindle is driven. There is no fully satisfactory takeup drive on any regular commercial machine that takes up very smoothly and with neither excessive tension nor too slack tension for all portions of all commercial reels from the inside of a 200-ft. American War Standard reel of 2-in. inside diameter to the outside of a 2000-ft. American "War Standard reel of 15-in. outside diameter. It is entirely too much to expect of a simple friction clutch that is to be reliable and cheap to manufacture, to operate over a torque radius range from 2 to 15 in. With almost any commercial projector that uses 400-ft. reel — even the American \Yar Standard 400ft. reel — cinching will always be present on the first 25 to 50 ft. because of excessive takeup tension. The practical solution to takeup difficulties on commercial projectors is to use only reels with an inside diameter not less than about 4-7 8 in.: this dimension is maintained on all