A planetary gearbox is a gearbox with the insight shaft and the result shaft aligned. A planetary gearbox is utilized to transfer the biggest torque 
in the most compact form (known as torque density).
The bicycle’s acceleration hub is an excellent example of a planet-wheel mechanism: Have you ever wondered how you can get so much power and features in such a small hub? For a three-swiftness hub, a one-stage planetary equipment system is used, for a five-velocity hub a 2-stage. Each planet gear system includes a reduction state, a primary coupling and an acceleration setting.
In mathematical terms, the tiniest reduction ratio is 3: 1, the biggest is 10: 1. At a ratio of significantly less than 3, sunlight gear becomes too large against the planet gears. At a ratio higher than 10 the sun 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.