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