A planetary gearbox is a gearbox with the insight shaft and the result shaft aligned. A planetary gearbox can be used to transfer the largest torque in the many compact form (referred to as torque density).
The bicycle’s acceleration hub is a great 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-acceleration hub, a one-stage planetary gear system can be used, for a five-speed 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, sunlight gear becomes too large 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.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 centuries.