A planetary gearbox is a gearbox with the insight 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 example of a planet-wheel mechanism: Have you ever wondered ways to get so much power and capabilities in such a small hub? For a three-velocity hub, a one-stage planetary gear system can be used, for a five-swiftness 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 biggest is 10: 1. At a ratio of less than 3, the sun gear becomes too big against the planet gears. At a ratio greater than 10 the sun wheel becomes too little and the torque will drop. The ratios are often absolute i.e. an integer number.
Whoever invented the planetary gearbox isn't known, but was functionally described simply by Leonardo da Vinci in 1490 and has been used for years and years.