Power Transmission

a mechanism for the transmission of mechanical power, usually with a transformation of the forces, torques, speeds, and, in some cases, the nature of the motion.

In machine drives, power transmission makes it possible to match the operating conditions of the motor with those of the machine’s working elements. It also allows several mechanisms to be driven from one motor, reverses the direction of motion, alters torques and rotational speeds while maintaining a constant torque and speed for the motor, and converts rotational motion into reciprocating, helical, or other type of motion. The most common power transmissions in machine building have mechanical linkages, although hydraulic (seeHYDRAULIC MACHINE DRIVES), pneumatic, and other types are occasionally used. In order to drive different mechanisms in one machine, different types of power transmissions are sometimes used, or combinations of transmissions, as in hydromechanical power transmissions. The economic advantage of using high-speed motors in machines (because of their smaller dimensions, weight, and cost) accounts for the preference for power transmissions that reduce the speed of the driven shaft in comparison to that of the driving shaft.

Very high power can be transmitted with gear transmissions; for instance, there are reduction gears for marine turbines that handle powers of over 50 megawatts. The power-handling capacity of worm-gear transmissions is limited (generally to 200 kilowatts) by the poor efficiency of such designs and by the production of heat. Chain-drive power transmissions can transmit as much as 4 megawatts, friction transmissions up to 300 kilowatts, and belt drives up to 1.5 megawatts.

Mechanical power transmissions are compact, suitable for machine layouts, and very reliable. They make it possible to achieve in a relatively simple fashion the required conversion of motion and practically any transmission characteristics. With proper manufacturing, the majority of power transmissions have a high efficiency.