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A reduction drive is a device used in many types of engines to shift the speed of rotating components. The original drives featured ball bearings that were mounted around a central bushing, though the most common type of reduction drive is the planetary gear, which is used in most automobiles. The planetary gear features a large, central, rotating gear with several other smaller gears mounted around it. The speed from the center gear will be transferred to the smaller gears that will rotate around the central gear. The system is known as a planetary system because the smaller gears revolve around the center gear, much like planets revolve around the sun.
The smaller gears will then transfer the speed of the center gear to an outer bushing, plate, or gear that will turn a component such as a wheel axle. The purpose of this reduction drive is to effectively transfer power to components without increasing torque that can damage axles or other components. Many automobiles feature planetary reduction drive systems in the gearbox; some large trucks will feature a similar system in each axle, since the drive shafts of larger trucks tend to produce so much torque that the axles can snap. To counteract this, planetary gears are mounted at each axle end to help transfer power more efficiently. The reduction drive can also be used to increase the amount of torque produced per each revolution of the shaft.
Another advantage of the reduction drive system is the efficiency with which torque can be applied. There is far less slop between a rotating axle and any components that rely on that axle's rotation. "Slop" essentially means play or excess movement between the components that can lead to inefficiency or difficulty in fine tuning. Radio knobs, for example, will use a reduction drive system to transfer the movement of the knob to the radio's tuning capacitor. Without the reduction system, it would be very difficult to fine tune the radio knob to get a clear signal on the radio.
Sometimes rubber belts are used in a reduction drive system to help transfer torque. The belt will usually be notched in order to work in conjunction with gear teeth that rotate off a central shaft. The size of the gears will help provide the transfer of torque, and the notched belt will help facilitate that transfer as it is wrapped around both larger and smaller gears.