There are three main components in a planetary gearbox: the sun gear, the planet gears and the ring gear. The gears are attached to the planet carrier that has teeth on the inside, which is the ring gear.

Planetary gearboxes are able to produce many different reduction ratios because of the different planet gears that revolve around the sun gear. Planetary gearboxes are constructed of heavy duty metal such as steel and are able to handle large shock loads well. However, different planetary gearboxes are designed for specific speed, load and torque capacities. The predominant use for planetary gearboxes is in motor vehicles with automatic transmissions. Unlike manual transmission, in which the operator is responsible for switching gears, automobiles that have automatic transmissions use clutches, brake bands and planetary gearboxes to change the inputs and outputs, thereby adjusting the speeds accordingly.

An automatic transmission contains two complete planetary gearsets placed together into one component. Planetary gearboxes are also used in electric screwdrivers, sprinklers and applications that require large or multiple reductions from a compact mechanism. Planetary gearboxes are one of many variations of speed reducers and it is important that the right mechanism is used. Gearboxes may even be combined to produce the desired results and the most common kinds are helical gear reducers, worm gear reducers and inline gear reducers.

Planetary gearboxes gain their advantages through their design. The sun gear’s central position allows the planet gears to rotate in the same direction and for the ring gear (the edge of the planet carrier) to turn the same way as the sun gear. In some arrangements the sun gear can simultaneously turn all the planets as they also engage the ring gear. Any of the three components may be the input, the output or held stationary, which results in many different reduction ratio possibilities.

In many planetary gearboxes one component is held stationary with another component serving as the input and the other as output. The reduction ratios for planetary gearboxes are dependent upon the number of teeth in the gears and what components are engaged. Generally, the load ability and torque increases with the number of planets in the system because the load is distributed among the gears and there is low energy waste; planetary gearboxes are highly efficient, averaging between 96 and 98%. The design is complex, however, and difficult to access for repairs or maintenance.