A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as an assembly of parts, including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a live axle.
Often transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device. In British English, transmission refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive), differential, and final drive shafts.
In American English, however, the term refers more specifically to the gearbox alone, and the usage details differ. The most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such machines must operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are also used on pedal bicycle fixed machines, where different rotational speeds and torques are adapted.
Often, a transmission has multiple gear ratios (or simply “gears”), with the ability to switch between them as speed varies. This switching may be done manually (by the operator) or automatically. Directional (forward and reverse) control may also be provided. Single-ratio transmissions also exist, which simply change motor output’s speed and torque (and sometimes direction).
In motor vehicles, the transmission generally is connected to the engine crankshaft via a flywheel and clutch and fluid coupling, partly because internal combustion engines cannot run below a particular speed. The transmission output is transmitted via driveshaft to one or more differentials, which drive the wheels.
While a differential may also provide gear reduction, its primary purpose is to permit the wheels at either end of an axle to rotate at different speeds (essential to avoid wheel slippage on turns) as it changes the direction of rotation. Conventional gear/belt transmissions are not the only speed/torque adaptation mechanism. Alternative mechanisms include torque converters and power transformation (diesel-electric transmission and hydraulic drive system). Hybrid configurations also exist.