In contrast with bearings, friction is a useful and essential agent in belts, clutches, and brakes. Frictional forces are commonly developed on flat or cylindrical surfaces in contact with shorter pads or linkages or longer bands or belts. A number of these combinations are employed for brakes and clutches, and the band (chain) and wheel pair is used in belt (chain) drives as well. Hence, only a few different analyses are required, with surface forms affecting the equations more than the functions of the elements. Also common operating problems relate to pressure distribution and wear, temperature rise and heat dissipation, and so on. The foregoing devices are thus effectively analyzed and studied together.

A belt or chain drive provides a convenient means for transferring motion from one shaft to another by means of a belt or chain connecting pulleys on the shafts. Part A of this chapter is devoted to the discussion of the flexible elements: belts and chains. In many cases, their use reduces vibration and shock transmission, simplifies the design of a machine substantially, and reduces the cost. Power is to be transferred between parallel or nonparallel shafts separated by a considerable distance. Thus, the designer is provided considerable flexibility in location of driver and driven machinery [1-16]. The websites www.machinedesign.com and www.powertransmission.com on mechanical systems include information on belts and chains as well as on clutches and brakes.

Brakes and clutches are essentially the same devices. Each is usually associated with rotation. The brake absorbs the kinetic energy of moving bodies and thus controls the speed. The function of the brake is to turn mechanical energy into heat. The clutch transmits power between two shafts or elements that must be often connected and disconnected. A break acts likewise, with exception that one element is fixed. Clutches and brakes, treated in Part B of the chapter, are all of the friction type that relies on sliding between solid surfaces. Other kinds provide a magnetic, hydraulic, or mechanical connection between the two parts. The clutch is in common use to maintain constant torque on a shaft and serve an emergency disconnection device to decouple the shaft from the motor in the event of a machine jam. In such cases, a brake also is fitted to bring the shaft (and machine) to a rapid stop in urgency. Brakes and clutches are used extensively in production machines of all types as well as in vehicle applications. They are classified as follows: disk or axial types, cone types, drum types with external shoes, drum types with internal shoes, and miscellaneous types [17-23].