Keys are wedges, generally rectangular in section, but sometimes circular; they are made of wrought iron or steel, and are used for securing wheels, pulleys, cranks, &c., to shafts.

Various sections of keys are shown in fig. 20. At (a) is the hollow or saddle key. With this form of key it is not necessary to cut the shaft in any way, but its holding power is small, and it is therefore only used for light work. At (b) is the key on a flat, sometimes called a flat key. The holding power of this key is much greater than that of the saddle key. At (c) is the sunk key, a very secure and very common form.

The part of the shaft upon which a key rests is called the key bed or key way, and the recess in the boss of the wheel or pulley into which the key fits is called the key way; both are also called key seats. With saddle, flat, and sunk keys the key bed is parallel to the axis of the shaft; but the key way is deeper at one end than the other to accommodate the taper of the key. The sides of the key are parallel.

The round key or taper pin shown at (d) is in general only used for wheels or cranks which have been previously shrunk on to their shafts or forced on by great pressure. After the wheel or crank has been shrunk on, a hole is drilled, half into the shaft and half into the wheel or crank, to receive the pin.

When the point of a key is inaccessible the other end is provided with a gib head as shown at (e), to enable the key to be withdrawn.

A sliding or feather key secures a piece to a shaft so far as to prevent the one from rotating without the other, but allows of relative motion in the direction of the axis of the shaft. This form of key has no taper, and it is secured to the piece carried by the shaft, but is made a sliding fit in the key way of the shaft. In one form of feather key the part within the piece carried by the shaft is dovetailed as shown at (f). In another form the key has a round projecting pin forged upon it, which enters a corresponding hole as shown at (g). The feather key may also be secured to the piece carried by the shaft by means of one or more screws as shown at (h). The key way in the shaft is made long enough to permit of the necessary sliding motion.

Cone Keys.—These are sometimes fitted to pulleys, and are shown in fig. 32, page 38. In this case the eye of the pulley is tapered and is larger than the shaft. The space between the shaft and the boss of the pulley is filled with three saddle or cone keys. These keys are made of cast iron and are all cast together, and before being divided the casting is bored to fit the shaft and turned to fit the eye of the pulley. By this arrangement of keys the same pulley may be fixed on shafts of different diameters by using keys of different thicknesses; also the pulley may be bored out large enough to pass over any boss which may be forged on the shaft.

Proportions of Keys.—The following rules are taken from Unwin's 'Machine Design,' pp. 142-43.

The following table gives dimensions agreeing with average practice.

Dimensions of Keys.