A transmission is a mechanical device which uses a gear set—two or more gears working together—to change the speed or direction of rotation in a machine. Many transmissions have multiple gear ratios, but there are also transmissions that use a single fixed-gear ratio.
Hydraulic automatic transmission (cutaway view)
Transmission for a Bristol Sycamore helicopter
Mazda M5OD manual transmission (viewed from the engine side)
Transmission internals for a 2009 Volkswagen Golf
Epicyclic gearing
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An epicyclic gear train is a gear reduction assembly consisting of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates, to carry the planet gear(s) around the sun gear. The planet and sun gears mesh so that their pitch circles roll without slip. If the sun gear is held fixed, then a point on the pitch circle of the planet gear traces an epicycloid curve.
Compound planets of a Sturmey-Archer AM bicycle hub (ring gear removed)
A spur gear differential constructed by engaging the planet gears of two co-axial epicyclic gear trains. The casing is the carrier for this planetary gear train.
Stepped planet series of the Rohloff Speedhub internally geared bicycle hub with the smaller planet series meshing with the sun wheel and the larger planet series meshing with the ring gear