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PROJECTOR TENSION SHOES
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tension device could easily be turned farther than the screw on the other projector in the booth. It is possible, therefore, that there could be a variance in the projected picture from two machines in the same booth, even though the print being run were in identical condition.
In order to determine the proper degree of shoe tension that should be applied to film in various conditions to produce maximum screen results and minimum wear on film, Motiograph conducted a long series of tests. Brand new film was run at varying degrees of shoe pressure to locate the lowest possible degree of pressure at which a fine quality picture resulted. After running a long series of tests, it was decided that a pressure sufficient to cause a film pull tension of eight ounces would produce best results. Similar tests were made with film that had passed the first run stage and it was found that a tension of twelve ounces was most desirable. Badly buckled, often spliced film required the much greater tension of twenty-four ounces to produce anywhere near satisfactory picture results.
Cam Operating Unit
To give any of the three degrees of pressure as required, Motiograph provides in its projectors a cam-operated tension device that permits the projectionist to flip a small
lever to bring about the desired degree of tension. Careful checks of installations have shown the wisdom of having but three definite degrees of tension instead of a large number of different degrees.
While it is true that wear on film is increased as shoe tension is increased, Motiograph tests showed that the extent of wear at the greatest degree of tension employed in its projector was not of great importance, considering that the heavier tension is needed only on films which are near the end of their useful life. These tests did clearly prove, however, that extreme shoe pressure could create excess film wear by forcing the film out of alignment if the film were not properly edge-guided through the film gate. This is particularly true with shrunken film.
The tests also proved that the best method of guiding the film through the film gate was the provision of two film guide rollers at top and bottom of the film gate which would automatically adjust themselves to the width of the film being run. For this reason, Motiograph discarded the conventional studio guides employed in many projectors in favor of two self-adjusting film guide rollers. Studio guides work only on film of the exact dimensions for which they are set, and are either useless or cause buckling of film if other degrees of shrinkage