The Planetary Gearset in Automatic Transmissions

While automatic transmissions are relatively small, they consist of a wide assortment of different parts. One of the most important features of an automatic transmission system is the planetary gearset. Also known as the epicycloidal gearset, the planetary gearset serves as the focal point of any automatic transmission system. Essentially, all other components in the system are created to facilitate the operation of the planetary gearset. Use this guide to better understand the role of the planetary gearset in automatic transmissions.

What is a planetary gearset?

A planetary gearset is a component of an automatic transmission system that consists of two gears that are combined into a single component. The gears are mounted in a way that allows the center of one of the gears to revolve around the center of the other.

Generally, the center of the two gears are connected by a moveable arm or carrier that rotates and carries the planet gear around the sun gear or sun wheel. Because the gears revolve around the sun wheel similar to the way planets revolve around the sun, the gear aptly received its name: the “planetary gearset.”

The role of a planetary gearset in an automatic transmission

A planetary gearset creates all the gear ratios that a transmission can produce. It gives vehicles with automatic transmission systems the ability to change gears smoothly as a vehicle reaches different speeds without having to move the gear shift like in a manual system. In other words, it facilitates the chief function of an automatic transmission system by allowing the system to change gear ratios without the need to engage or disengage individual gears.

Components of a planetary gearset

Planetary gearsets consist of three primary components:

  • The sun gear
  • The planet gear
  • The ring gear

The gear ratio for the gearset is determined by which of these component plays which role—either serving as the input (the gear that is generating the power), output (the gear that receives power), or as a stationary element. Locking different parts will produce different gear ratios.

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