System in an engine that transmits power generated by the engine to the point where it is to be used. Most mechanical transmissions function as rotary speed changers; the ratio of the output speed to the input speed may be either constant (as in a gearbox) or variable. On variable-speed transmissions, the speeds may be variable in individual steps (as on an automobile or some machine-tool drives) or continuously variable within a range. Step-variable transmissions, with some slip, usually use either gears or chains and provide fixed speed ratios with no slip; stepless transmissions use belts, chains, or rolling-contact bodies.
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