Fig. 474.
Fig. 475.
As the keys in through keyways often require to be driven in very tight, and as the parts keyed together often remain a long time without being taken apart and in some situations become rusted together, it is often a difficult matter to get them apart. First, it is difficult to drive it out because the blows swell the end of the key so that it cannot pass through the keyway, and secondly, driving the socket off the plug of the two parts keyed together often damages the socket and may bend the rod to which it is keyed. Furthermore, as the diameter of the socket is usually not more than half as much again as the diameter of the plug, misdirected blows are apt to fall upon the rod instead of upon the socket end and damage it. Hence, a piece of copper, of lead, or a block of wood should always be placed against the socket end to receive the hammer blows. To force a plug out of a socket, we may use reverse keys. These are pieces formed as shown in [Fig. 474]. a, a and b, b are edge and face views respectively of two pieces of metal, formed as shown, which are inserted in the keyway as shown in [Fig. 475], in which a is the plug or taper end of a rod and b the socket, c is one and d the other of the reverse keys, while e is a taper key inserted between them, b driving e through the keyway, a and b are forced apart. The action of the reverse keys is simply to reverse the direction of the draft in the keyway so that the pressure due to driving e through the keyway is brought to bear upon the rod end in the part that was previously the draft side of the keyway, and in like manner upon the keyway in the socket on the side that previously served as draft.
Reverse keys are especially serviceable to take off cross heads, piston heads, keyed crank-pins, and parts that are keyed very firmly together.
