The proportions of sunk keys are thus given by the Manchester (England) rule. The key is square in cross section and its width or depth is obtained by subtracting 1⁄2 from the diameter of the shaft and dividing the sum thus obtained by 8, and then adding to the subtrahend 1⁄4.
Example.—A shaft is 6 inches in diameter, what should be the cross section dimensions of its key diameter of shaft?
6 - 1⁄2 = 51⁄2, 51⁄2 ÷ 8 = .687, and .687 + .25 = 937⁄1000 inch.
In general practice, however, the width of a key is made slightly greater than its depth, and one-half its depth should be sunk in the shaft.
Fig. 466.
Fig. 467.