Like a compound gear train, planetary trains are used when a large change in speed or power is needed across a small distance. There are four different ways that a planetary train can be hooked up.

A planetary gear train is a little more complex than other types of gear trains. In a planetary train at least one of the gears must revolve around another gear in the gear train. A planetary gear train is very much like our own solar system, and that’s how it gets its name. In the solar system the planets revolve around the sun. Gravity holds them all together. In a planetary gear train the sun gear is at the center. A planet gear revolves around the sun gear. The system is held together by the planet carrier. In some planetary trains, more than one planet gear rotates around the sun gear. The system is then held together by an arm connecting the planet gears in combination with a ring gear.

The planetary gear set is the device that produces different gear ratios through the same set of gears. Any planetary gearset has three main components:

· The sun gear

· The planet gears  and the planet gears’ carrier

· The ring gear

Each of these three components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset.

These four combinations and the resulting speed and power outputs are listed in Table

• They have higher gear ratios.

• They are popular for automatic transmissions in automobiles.

• They are also used in bicycles for controlling power of pedalling automatically or manually.

• They are also used for power train between internal combustion engine and an electric motor.