A planetary gearbox is a gearbox with the input shaft and the output shaft aligned. A planetary gearbox is used to transfer the biggest torque in the most compact form (known as torque density).
The bicycle’s acceleration hub is an excellent exemplory case of a planet-wheel mechanism: Perhaps you have ever wondered how you can get so much power and capabilities in such a little hub? For a three-quickness hub, a one-stage planetary gear system is used, for a five-acceleration hub a 2-stage. Each planet gear program has a reduction state, a primary coupling and an acceleration mode.
In mathematical 
terms, the smallest reduction ratio is 3: 1, the largest is 10: 1. At a ratio of less than 3, the sun gear becomes too big against the planet gears. At a ratio higher than 10 sunlight wheel becomes too small and the torque will drop. The ratios are often absolute i.electronic. an integer number.
Whoever invented the planetary gearbox is not known, but was functionally described simply by Leonardo da Vinci in 1490 and has been used for centuries.