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406 Transactions of S.M.P.E., August 1927
The movement of the intermittent gear shall be expressed in degrees of rotation during which the pin of the driver is in contact with the slot of the driven gear. For example, a gear in which the pin is engaged with the slot for one-quarter of a revolution of the driver shall be called a 90-degree movement; that in which the pin is engaged with the slot for one-sixth revolution shall be called a 60-degree movement, etc.
This is evidently Nomenclature and we ask that the rule of double approval of all Standards and Nomenclature be made unanimous by second approval of this definition and that it be listed under Nomenclature.
{Motion carried to adopt above recommendation.)
Sprocket Dimensions In the No. 27 Transactions is printed a report by Mr. J. G. Jones on the dimensions of sprocket wheels for projectors. The method of arriving at this standardization of sprocket dimensions, proposed by Mr. Jones, has I believe the approval of the Society. There has been raised an objection to the dimensions which Mr. Jones proposed for the take-up sprocket which is a hold-back sprocket. In the design of this sprocket it was assumed that if it is of such size that it corresponds to a film having a shrinkage of 2.92 per cent that no injury will be done to new film having zero shrinkage provided, of course, other considerations such as tooth thickness treated in this recommendation be complied with. It has been pointed out, however, that when perfectly new film is used, the last tooth of the sprocket wheel holds back the film until it lets go and, when it does so, there is a sudden jump of the film to the tooth just preceding it. Thus the sprocket of the dimensions recommended would give the greatest smoothness of action to film shrunk to 2.92 per cent but allows slipping from tooth tooth in the case of new film. This slipping action for new film was not assumed to be in the least injurious. It has been pointed out, however, that the standard tension of the wind-up which is 16 ounces on the periphery of a 10 in. reel is perhaps five times that amount or five pounds when a new reel is started and the film is wound near the hub. This jumping action on the new unshrunken film when used on a hold-back sprocket adjusted for the maximum 2.92 per cent shrinkage is highly injurious and moreover does the greatest damage to new film which is presumably both more valuable and is tenderer than old film.