Reduction drive
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A reduction drive is a mechanical device to shift rotational speed. A planetary reduction drive is a small-scale version using ball bearings in an epicyclic arrangement instead of toothed gears.
Reduction drives are used in engines of all kinds, to increase the amount of power per revolution of a shaft: the gearbox of any car is a ubiquitous example of a reduction drive.
Planetary reduction drives are typically attached between the shaft of the variable capacitor and the tuning knob of any radio, to allow fine adjustments of the tuning capacitor with large movements of the knob.