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 torque per revolution of a shaft: the gearbox of any car is a ubiquitous example of a reduction drive. Common household uses are washing machines, food blenders and window-winders.
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 smooth movements of the knob. Planetary drives are used in this situation to avoid "backlash", which makes tuning easier. If the capacitor drive has backlash, when you attempt to tune-in a station, the tuning knob will feel sloppy and it will be hard to perform small adjustments. Gear-drives CAN be made to have no backlash by utilising split gears and spring tension but the shaft bearings have to be very precise.
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