A clutch is any coupling that allows shafts or other rotating parts to be connected or disconnected at will i.e., without the removal or refitting of any components. Clutches are useful in devices that have two rotating shafts. In these devices, one shaft is typically driven by a motor or pulley, and the other shaft drives another device. In a drill, for instance, one shaft is driven by a motor, and the other drives a drill chuck. In the claw clutch (Fig.1) one half of the clutch can slide on its shaft, so that the claws can be engaged or disengaged. This type of clutch can be engaged only when the shafts are stationary or rotating at low speed.

The geared clutch (Fig.2) is widely used in machine tools and motor vehicles. The two clutch bosses are each supported with external teeth which can mesh with a sleeve that has corresponding internal teeth and can be slid over both bosses so as to establish a positive connection between the two shafts. To permit engagement of the clutch while the shafts are rotating, the sleeve and the shaft end to be coupled are respectively provided with friction surfaces which are brought into contact with each other and thus equalize the speed of the rotating parts before the teeth on the shaft and inside the sleeve are brought into mesh. Friction clothes transmit power through contact friction surfaces on the two halves to be connected.

Different types of friction clutch are illustrated in Fig.3. In the disc clutch (or plate clutch), the cone clutch, the boss of the movable part slides in longitudinal grooves in the shaft on which it is mounted. The movements for engaging and disengaging the clutch are performed by the action of a lever whose forked ends fit into a circumferential recess in the boss of the clutch plate or cone. The internal-expanding shoe-type clutch comprises an outer shell attached to one shaft and two semicircular shoes which are mounted on arms attached to a sliding sleeve on the other shaft and which can be brought internally into contact with the shell. The forks of the clutch-operating lever engage with a recess on the sliding sleeve.

A type of friction clutch commonly used in machine tools and other motor vehicles is the multiple-disc clutch (Fig.4). It is based on the principle that a series of discs or plates alternately connected to the driving and the driven shaft will increase the power-transmitting capacity in proportion to the number of pairs of contact surfaces. In the form of clutch illustrated in Fig.4 the boss mounted on the driving shaft is provided with external teeth with which the internal teeth on a series of thin steel plates engage.

The outer shell of the clutch is mounted on the driven shaft and has internal teeth with which the external teeth of a second series of plates (alternating with those of the first series) likewise engage. When the clutch lever shifts a collar to the left, the plates are pressed together and thus transmit power by friction. Multiple-disc clutches in machine tools usually operate immersed in oil; those in motor vehicles are usually of the dry type. A magnetic clutch is a friction-disc clutch that is engaged by the energizing of a magnet coil, which attracts a set of steel friction discs and thus establishes the connection.

A double-acting clutch based on this principle is illustrated in Fig.5. When the coil a is energized, the discs b is compressed together by magnetic attraction, thereby connecting the gear c to the shaft d. When the coil e is energized, the discs f is pressed together, so that now power is transmitted from the shaft “d” to the gear g.

An automatic clutch is often installed between the driving shaft of a motor and the machinery it drives. It does not allow the shaft to reach a predetermined speed before engagement is effected and is especially useful in a case where the driven machinery requires a high starting torque.

For such purposes a centrifugal clutch (Fig.1) may suitably be utilized. It comprises two or more shoes which, when the driving shaft on which they are mounted has reached a certain speed, overcome the pressure of restraining springs by the action of centrifugal force and move outwards to press against the inner surface of the rim mounted on the driven shaft. In this way the transmission of power to the driven shaft is gradually and automatically increased, so that smooth engagement is effected.

The speed at which engagement takes place can be increased by fitting the clutch with more power-restraining springs. When the shafts are not rotating, the shoes are retracted and not in contact with the rim. Various other types of automatic clutch are likewise based on the centrifugal principle. Freewheeling clutches drive in one direction only and permit free movement when the speed of the driven shaft exceeds that of the driving shaft. In the grip-roller type of freewheeling clutch (Fig.2) each roller is gripped, i.e., jammed, in the wedge-shaped space as soon as the movement of the outer race in relation to the inner race causes the roller to move into the shallower part of this space.

The friction produced at these faces will depend on the contact pressure exerted by the springs. If the pressure is low, the friction will also be low, so that slip in the clutch will occur at a low value of the torque. By means of a screw it is possible to increase the spring pressure and therefore the friction, so that the clutch will be able to transmit a greater torque without slipping. The torque can thus be adjusted to a predetermined value, and the clutch can serve as a safety device against overloading of the driven machinery. A simpler safety device for this purpose is the shear-bolt coupling (Fig.4). It comprises two flanges connected by bolts that are designed to fail in shear (i.e., to break off) when the torque exceeds a predetermined value.