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Planetary Gearing
A planetary gear train consists of a stationary "sun" gear, surrounded by several (usually 3 or 4) identical "planet" gears which mesh with it. The planet gears, in turn, mesh with a hollow "gear ring" which has teeth on the inner surface. The gear ring rotates faster than the planet gears.
If the drive sprocket is connected to the gear ring, and the cage that holds the planet gears is used to turn the wheel, the wheel will turn slower than the sprocket, thus providing a lower gear, compared with a simple hub driven by the same-sized sprockets.
If the drive sprocket is connected to the planet cage, and the gear ring drives the wheel, the result is a higher gear. Most three-speed internally-geared hubs use these two configurations, along with the direct drive. to provide the three speeds.
Internally-geared hubs with more than three speeds use multiple sun pinions and sometimes multiple planet gears in the same manner.
You could make a case that "planetary gearing" is the earliest use of space technology in vehicle design...
See also my articles on English 3-speeds and internal-gear hubs.
