A clutch is a coupling that can be engaged and disengaged as required and used to transmit torque (rotational force) from a driving shaft to a driven shaft. Most clutches are designed to drive an initially stationary shaft from a continuously rotating shaft. Thus, the matching members of the clutch are designed to allow slippage until the driven member is brought up to speed. However, in positive clutches, where the matching members are interlocking jaws or teeth, no slippage is possible. The most common positive clutch is the square-jaw clutch, which can be engaged only when both members are stationary or moving slowly. Spiral-jaw clutches can be engaged at low to moderate speeds, but not without a high shock load being transmitted to the driven member.
The most common type of clutch is the friction clutch, which is generally placed between an engine and its load. In automobiles, a friction clutch is located to connect the engine to a transmission and the drive shaft.
The plate, or disk, friction clutch, the type used in automobiles, consists of two sets of disks, or plates, faced with high-friction materials, that rotate around the same axis. One set is attached to a shaft (usually the driven shaft). The driven and driving disks are mounted alternately. When the clutch is engaged, a spring compresses the disks together so that they are brought into contact with the rotating plates. As the driven member is brought up to speed, heat is generated by the slippage between parts. In most automobile clutches, which are run dry, the heat escapes through the clutch housing to the atmosphere. In heavier machinery, the parts of a friction clutch are immersed in circulating oil, which carries off the heat.
Other widely used friction clutches are the cone clutch and the rim clutch. The members of a cone clutch are matching sections of a cone faced with high-friction materials. As one member is inserted into the other, contact is made and the driven member turned. Cone clutches are used to drive machinery such as hoists from a constantly rotating shaft.
In rim clutches, one member is the rim of a wheel, and the other member consists of shoes that grip either the interior or the exterior of the rim. The shoes are brought into contact with the rim by springs, hydraulic pressure, or rotational forces.
Materials for friction clutches should have a high and uniform coefficient of friction, an ability to withstand the high temperatures and to dissipate the heat generated by slippage, a low response to environmental changes, and a resistance to abrasion. Typical pairs of surfaces are woven asbestos or bronze with chromium-plated steel, molded asbestos with cast-iron or steel, hard steel with hard steel, or powder metal with cast iron.
Hydraulic clutches, or fluid couplings, provide a smoothness of operation not possible with friction clutches or positive clutches. The matching members of a hydraulic clutch are hollow halves of a doughnut shape lined with vanes. The space between them is filled with a viscous fluid that transmits torque from the vanes of the driving member to the vanes of the driven member. A third set of vanes, with adjustable pitch, is placed between the members, and it acts to disrupt the flow of fluid when the clutch is to be disengaged. About 3% to 5% of the power input is lost in the form of heat, which must be dissipated to prevent clutch failure.
Electromagnetic forces are utilized in a number of clutches. Disk clutches that are engaged by electromagnetic attraction between disks and plates are sometimes called "electromagnetic clutches." Magnetic powder clutches are simple disk clutches in which the space between disk and plate is filled with a powder that coalesces into a friction-bearing mass when a magnetic field is applied. A magnetic fluid clutch is similar, except that the powder is in an oil suspension. In eddy current and hysteresis clutches, the mating members are kept together by electromagnetic effects and are not in physical contact.