A planetary gearbox is a gearbox with the insight shaft and the output shaft aligned. A planetary gearbox can be used to transfer the largest torque in the most compact form (known as torque density).
The bicycle’s acceleration hub is a great exemplory case of a planet-wheel mechanism: Have you ever wondered how you can get so much power and features in such a little hub? For a three-swiftness hub, a one-stage planetary gear system is used, for a five-rate hub a 2-stage. Each planet gear system includes a reduction state, a direct coupling and an acceleration mode.
In mathematical terms, the tiniest reduction ratio is 3: 1, the biggest is 10: 1. At a ratio of less than 3, the sun gear becomes too large against the earth gears. At a ratio higher than 10 the sun wheel becomes too small and the torque will drop. The ratios are usually absolute i.e. 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.