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Planetary Gear Set

When you take apart and look inside an automatic transmission, you find an amazing assortment of parts in a fairly small space. Among other things you see:

An extremely ingenious planetary gear set
A set of bands that lock parts of a gear set
A set of 3 wet-plate clutches to lock other parts of the gear set
An incredibly odd hydraulic control system that controls the clutches and bands
A large gear pump to move transmission fluid around

The center of attention is the planetary gear set. Everything else in the transmission is there to help the planetary gear set do its thing. An automatic transmission contains two complete planetary gear sets folded together into one component.

Any planetary gear set 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 gear set. Let's take a look at a single planetary gear set.

One of the planetary gear sets from our transmission has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gear set.

Also, locking any two of the three components together will lock up the whole device at a 1:1 gear reduction. Notice that the first gear ratio listed above is a reduction -- the output speed is slower than the input speed. The second is an overdrive -- the output speed is faster than the input speed. The last is a reduction again, but the output direction is reversed. There are several other ratios that can be gotten out of this planetary gear set, but these are the ones that are relevant to our automatic transmission.

Animation of the different gear ratios related to automatic transmissions
Click on the buttons on the left in the table above.

So this one set of gears can produce all of these different gear ratios without having to engage or disengage any other gears. With two of these gear sets in a row, we can get the four forward gears and one reverse gear our transmission needs.