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sion to deposit upon the tension shoes, and to increase the tendency of such deposit to do serious damage to the film.
In connection with the foregoing we must call attention to the fact that by proper construction it is possible to so support the film in its path between the upper end of the film gate and the intermittent sprocket that the film itself will in some measure act to reduce the amount of tension necessary ; also that closeness of intermittent sprocket to aperture, and the holding of the film in firm contact with the intermittent sprocket has some bearing on the amount of aperture tension necessary for a given speed.
Before closing this phase of the subject let me cite, in proof of the strain under which the intermittent mechanism of a projector and the edges of film sprocket holes work, the following facts.
So great is the pull of the film upon the intermittent sprocket teeth that even though the teeth be of the finest grade of steel and hardened as much as possible, still the film cuts or wears away the metal in a comparatively short time. In considering this fact as applied to the total amount of pull exerted upon the film, as against its almost negligible inertia plus the braking power exerted by the tension springs, let it be remembered that there are eight of the intermittent sprocket teeth contacting with sprocket hole edges at all times. I think you will agree with me that when the pull is so great that glass-hard steel is cut away from eight teeth at the same time, and the cutting is done by a celluloid sprocket hole edge six one-thousandths of an inch in thickness, and that those sprocket hole edges are contained in a film having considerable money value, the reducing of the pull on the intermittent sprocketteeth to its lowest possible value is an important item, while increasing it needlessly is a very serious matter indeed; that overspeeding does increase it, always presuming to be reduced to its necessary value for normal speed, cannot, I think, be seriously questioned, though, as before remarked, there may be a less percentage of increased tension pressure for a given increase in projection speed in a mechanism in which the film is well supported in its passage through the film gate, and where the distance from intermittent sprocket to aperture is short. Such claim is made, and it seems reasonable, though the author has made no actual test of the matter.
Summed down, we find that with normal projection speed as against over-speeded projection, there is decidedly less wear of the projection mechanism, intermittent sprocket teeth, film sprocket hole edges, aperture plate tracks, and tension shoes, as well as less tendency for deposit of emulsion on the tension shoes, and less liability of such deposit to do serious damage to the film. We find, also that wear of intermittent movement, gears, bearings and intermittent sprocket holes and sprocket hole edges increases very rapidly with increase in projection speed, as also does tendency to over-oil and the throwing off of oil from moving parts.
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