The Epicyclic gearbox was devised in 1781 (as a sun and planet gear) by James Watt for use in conjunction with his steam engines. The concept was adopted in 1902 by Sturmey and Archer for a multi-speed rear bicycle hub. The initial design was for two speeds, but was rapidly developed to give a three speed gearbox. Presently, hubs with up to five gears are manufactured.

The diagram shows the essential features of an epicyclic gearbox. At the center is a 'Sun' gear which meshes with the three 'Planet' gears that run on spindles on the planet carrier. These gears in turn mesh with teeth of the outer 'Ring.' If all of the gear carriers are locked together, the input and output shafts of the box rotate at the same rate. The overall gear ratio would then be expressed as 1:1. Fixing the planet gear carrier and allowing independent rotation of the sun and ring gears gives them a relative rotation rate determined by their tooth counts, N_S and N_R. This overall gear ratio is (N_R / N_S).

From: Dudley, "Gear Handbook,"
McGraw Hill (1962)