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What is the planetary gear set in an automatic transmission?

The components and function of the planetary gears.

The planetary gears are a set of sun gears, planet gears and ring gears inside the automatic transmission which produce every gear ratio your transmission delivers when your car is in motion and is the biggest function of the overall transmission in which everything else depends. It is not very large either. The whole device is about 7 to 10 inches big but it is the heart of your transmission.

Think of the whole set of gears as planets rotating with the largest gear on the outside as the ring gear with the core of this centered in the middle as the sun gear and small planetary pinions or gears between those two gears rotating around the sun like a solar system as seen in the illustration here. These gears never disengage and they are all locked together inside one device.

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The sun gear is in the center of them all and the ring gear connects to the planet gears as seen in this illustration. Each gear ratio depends on the state of the planet gears are being locked in or unlocked as they move around the center sun gear.

Each of the planetary pinion gears can do different things individually delivering what the transmission needs at any given time such as deliver torque or speed. The torque convertors are connected to clutch system which is connected to the planetary gear set and feeds pressurized oil into the gear set. The propelling movement sends in motion the necessary thrust to the output shaft that turns your automobile's wheels.