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How Automatic Transmissions Work

Planetary Gearset Ratios

One of the planetary gearsets 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 gearset.

Gear Ratio
Sun (S)
Planet Carrier (C)
Ring (R)
1 + R/S
Planet Carrier (C)
Ring (R)
Sun (S)
1 / (1 + S/R)
Sun (S)
Ring (R)
Planet Carrier (C)

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. You can try these out in the animation below:

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 gearsets in a row, we can get the four forward gears and one reverse gear our transmission needs. We'll put the two sets of gears together in the next section.


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