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540 C. F. VILBRANDT Vol 48, No. 6
(2) As the leaving tooth disengages, the film slides on the sprocket circumference and the load is assumed by the next tooth with no evidence of humping of the film on the tooth.
(3) The film only 0.2 per cent longer than the sprocket touches the entering tooth at these high tensions. This is somewhat surprising, since, with film longer than the holdback sprocket, the tooth should enter freely. This effect can be explained only by the assumption that the distance between the leaving and entering perforations has been decreased by distortion of the perforation by the leaving tooth under the applied tension.
These observations substantiate the explanation advanced earlier for the difference in the damage to the film caused by drive and holdback sprockets.
These considerations suggest the desirability of altering the profile of the tooth of the drive sprocket to facilitate backward sliding of the film as the perforation disengages from the tooth and to ensure the gradual engagement of the film with the next tooth. If the sprocket tooth is cut away more sharply, there will be more damage to the film when the diameter of the sprocket is such that the entering tooth strikes the edge of the perforation and the film is wedged on the sprocket by the shoe. This factor makes it imperative to use drive sprockets with a large circular pitch, in relation to the pitch of the film, to make certain that wedging-on never occurs. It is believed that a section at the base of the tooth should be left as nearly vertical as is practicable, for a height approximately equal to the thickness of the film, to act as a driving face. The paper by Chandler, Lyman, and Martin4 proposes a method for specifying the shape of the tooth to comply with these considerations.
Some improvement should result from the placement of a stripper post or roller at the drive sprocket to prevent the film from being carried down past the proper point of disengagement.
There is some evidence to indicate that the stretch of film under tension on a sprocket is considerable. It is possible that this stretching may have to be considered in the design of sprockets. The extent and mechanism of this stretching are under study and the results will be reported at a later date.
Conclusions. — It has been shown that the drive sprocket damages film more severely than does the holdback sprocket at the same tension. This indicates that film tensions must be kept as low as is