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.
Bookwheel, from Agostino Ramelli's Le diverse et artifiose machine, 1588
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
Gear train
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A gear train or gear set is a machine element of a mechanical system formed by mounting two or more gears on a frame such that the teeth of the gears engage.
Transmission of motion and force by gear wheels, compound train.
Illustration by Georgius Agricola (1580) showing a toothed wheel that engages a slotted cylinder to form a gear train that transmits power from a human-powered treadmill to mining pump.
2 gears and an idler gear on a piece of farm equipment, with a ratio of 42/13 = 3.23:1
Bicycle with toothed belt drive to transmit torque from crank to rear sprocket