Fig. 472.
Through keys and keyways are employed to lock two pieces, and sometimes to enable the taking up of the wear of the parts. [Fig. 471] represents an example in which the key is used to lock a taper shaft end into a socket by means of a key passing through both of them. When the keyway is completely filled by the key as in the figure it is termed a solid key and keyway, indicating that there is no draft to the keyway. [Fig. 472] represents a key and keyway having draft. One edge, a c, of the key binds against the socket edges only, and the other edge e binds against the edge b of the enveloped piece or plug, so that by driving in the key with a hammer the two parts are forced together. The space or distance between the edge d and the key, and between edges e and f, is termed the draft. The amount of this draft is made equal to the taper of the key, hence, when the key is driven in so that its head comes level with the socket or work surface, the draft will be all taken up and the key will fill the keyway.
Fig. 473.
Draft is given to ensure all the strain of the key forcing the parts together, to enable the key to be driven in to take up any wear and to adjust movable parts, as straps, journal boxes or brasses, &c. When the bore of the socket and the end of the rod are parallel, the end of the rod f, [Fig. 473], should key firmly against the end e of the socket, while the end d of the socket should be clear of the shoulder on the rod; otherwise instead of the key merely compressing the metal at f it will exert a force tending to burst the end f from g of the rod, furthermore, the area of contact at the shoulder d being small the metal would be apt to compress and the key would soon come loose.
In some cases two keys are employed passing through a sleeve, the arrangement being termed a coupling, or a butt coupling.
The usual proportions for this class of key, when the rod ends and socket boxes are parallel, is width of key equals diameter of socket bore, thickness of key equals one-fourth its width, with a taper edgeways of about 1⁄4 inch in 10 inches of length.