We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
Aug., 1941 J
DESIGNING FILM SPROCKETS
181
stretched 1.51 per cent or shrunk 0.54 per cent. These shrinkage factors, of course, refer to freshly perforated stock.
If we examine the curves (Fig. 3) for sprockets designed to take film that has shrunk 0.70 per cent, we find the curves the same as the previous set but displaced. The stretch curve SB now crosses the zero line and becomes positive between 8 and 9 teeth in mesh. This may be interpreted, for example, for 10 teeth in mesh, simply that the film must shrink at least 0.17 per cent from freshly perforated film dimensions if interference is not to be encountered but may shrink as much as 1.23 per cent. To show the amount of film slip on the sprocket for each tooth leaving engagement, let it be as
FIG. 4. Feed sprockets.
F
B, base sprocket dia. = — Ti^ia
DI, film perforation pitch = D — DS
D, nominal pitch of freshly perforated film
S, shrinkage
G, clearance of second tooth in mesh
N, number of teeth on sprocket
F, film thickness
Y, number of teeth in mesh minus one
P, sprocket-tooth pitch
sumed that film which has shrunk 1.0 per cent is operating on the sprocket. Then the vertical distance from this point (1.0 per cent) to the Sc curve represents the slip which in this case is 0.23 per cent. For films with less shrinkage the slip of course will be greater.
Fig. 4, for feed sprockets, shows the sprocket rotating counterclockwise, receiving film from the right under tension and feeding into a loose loop on the left. In order that the film may pass freely onto the teeth, the sprocket-tooth pitch is greater than the perforation pitch. The sprocket base diameter then, as shown by the equation, equals (Dl + G)N/r-F. In this case G, for 10 or less teeth in mesh, is taken equal to 0.0015 inch as against 0.0010 inch for holdback sprockets.