Planetary gears borrowed their layout from ancient models of the heavens
Inside many gearboxes, small planet gears circle a central sun gear while meshing with a ring around the outside. The arrangement is named after epicycles, the circles on circles that Greek astronomers used to explain wandering planets, and a version of it was already turning inside the Antikythera mechanism around 80 BC.
An epicyclic, or planetary, gear set has three main members. A sun gear sits at the centre, one or more planet gears mesh with it, and a carrier links their centres and swings the planets around. Usually an outer ring gear, also called an annulus, meshes with the planets too. Lock any one of the three members, carrier, ring or sun, and the other two turn at a fixed relationship, so a single compact unit can deliver three different gear ratios.
The set can also blend two rotating inputs into one output. A linear analogy makes this intuitive: picture a wheel squeezed between two racks. Hold the top rack still and push the wheel's axle forward one inch, and the bottom rack slides two, because the wheel both travels and spins. In gear terms, the speeds of any two members determine the third; if only one is driven and nothing is held, the motion is undefined, so a brake, load or one-way clutch is needed. A car differential works on a related principle, with its carrier turning at the average speed of the two wheels.
Greek thinkers developed epicycles around 500 BC, and Ptolemy's Almagest used them in the second century AD to predict the paths of the Sun, Moon and five visible planets. The Antikythera mechanism used a pin-and-slot pair of gears on slightly offset centres to speed up and slow down its Moon display, mimicking the uneven orbit.
Later builders found other uses. Richard of Wallingford, abbot of St Albans, wrote about using it in a 14th-century clock that tracked the heavens. In 1588 Agostino Ramelli designed a rotating bookstand whose two-level planetary gears kept every volume upright, and Desargues built a mill with epicycloidal teeth around 1650. Compound sets with several planet stages now give larger reductions and more torque for their weight, and engineers have proposed modified versions for wave-energy machines.
Source: Epicyclic gearing
