How Automatic Transmissions Work

Compound Planetary Gearset

This automatic transmission uses a set of gears, called a compound planetary gearset, that looks like a single planetary gearset but actually behaves like two planetary gearsets combined. It has one ring gear that is always the output of the transmission, but it has two sun gears and two sets of planets.

Let's look at some of the parts:

automatic transmission gear housing
How the gears in the transmission are put together
Left to right: the ring gear, planet carrier, and two sun gears

The figure below shows the planets in the planet carrier. Notice how the planet on the right sits lower than the planet on the left. The planet on the right does not engage the ring gear -- it engages the other planet. Only the planet on the left engages the ring gear.

automatic transmission planet carrier
Planet carrier: Note the two sets of planets.

Next you can see the inside of the planet carrier. The shorter gears are engaged only by the smaller sun gear. The longer planets are engaged by the bigger sun gear and by the smaller planets.

automatic transmission planet carrier
Inside the planet carrier: Note the two sets of planets.

The animation below shows how all of the parts are hooked up in a transmission.

Move the shift lever to see how power is transmitted through the transmission.

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