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The Radial-Tooth, Variable-Pitch Sprocket
By J. G. STREIFFERT
A unique sprocket tooth whose driving face is a plane lying on a radius of the sprocket is used to improve longitudinal registration of the film over that obtained with conventionally shaped, curved-profile teeth. By supporting the film or films by means of an appropriately decentered drum while the films are in engagement with this sprocket, shrinkage accommodation is effected by virtue of the varying effective pitch of the sprocket. The variablepitch effect also makes it possible to strip the film off the sprocket.
Calculated and measured flutter in sound prints and measured steadiness in picture prints made on a sprocket of this type in a 16-mm continuous contact printer are found to be substantially independent of film shrinkage and to be markedly better than in prints made on conventional printers.
I
T is WELL KNOWN that ordinary sprockets can impart uniform continuous motion to film only if the film pitch happens to be identical with the sprocket pitch and the tooth profile is one which clears the path of the perforation as the film engages and disengages the sprocket. If the film pitch does not match the sprocket pitch exactly, only one tooth will be driving at any one instant, and, at the time of transfer of load from one tooth to the next, the film motion will not be uniform.
Various expedients have been employed to reduce this type of noncommunication No. 1449 from the Kodak Research Laboratories, a paper presented on October 17, 1951, at the Society's Convention at Hollywood, Calif., by J. G. Streiffert, Eastman Kodak Co., Kodak Park Works, Rochester 4, N.Y.
uniformity. In some cases, brute force is used to stretch the film so that it matches the sprocket or matches another film on which it is to be printed. This usually requires the application of inordinately high tensions and/or pressure on the film so that excessive wear of the film perforations and driving members is likely to occur.
In other cases, various forms of what may be called "shrinkage-accommodating sprockets" have been proposed. Among the more promising of these sprockets have been those of Elmer,1 Mechau,2 and Chandler.3
The Elmer disclosure described a sprocket whose base diameter was determined by the pitch of the longest film likely to be encountered, whereas the tooth profile was a curve intended to allow film of maximum shrinkage to
December 1951 Journal of the SMPTE Vol.57
529