Planetary gear train

A planetary set has a sun gear in the middle, two or more planet gears orbiting it on a carrier, and a ring gear with internal teeth surrounding everything. Any one of sun, ring or carrier can be held still, driven, or used as the output — which is why a single planetary set can replace several ordinary gear pairs, and why it sits inside almost every automatic transmission and electric bike hub.

1. 1With the ring gear held fixed, drive the sun and watch the carrier turn slowly in the same direction — this is the classic reduction mode.
2. 2Hold the sun fixed instead and drive the ring: the carrier now turns the other way round, at a different ratio.
3. 3Lock any two members together and the whole unit spins as one solid piece — a 1:1 direct-drive lockup, exactly like a car gearbox's top gear.