TUANGALIE PLANETARY GEAR SET

The Planetary Gear set

     From left to right: the ring gear, planet carrier, and two sun gears

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

·         An ingenious planetary gear set

·         A set of bands to lock parts of a gear set

·         A set of three wet-plate clutches to lock other parts of the gear set

·         An incredibly odd hydraulic 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. About the size of a cantaloupe, this one part creates all of the different gear ratios that the transmission can produce. 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. See How gear ratio works for an introduction to planetary gear sets.

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.

Planetary Gear set Ratios

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.

	Input	Output	Stationary	Calculation	Gear Ratio
A	Sun (S)	Planet Carrier (C)	Ring (R)	1 + R/S	3.4:1
B	Planet Carrier (C)	Ring (R)	Sun (S)	1 / (1 + S/R)	0.71:1
C	Sun (S)	Ring (R)	Planet Carrier (C)	-R/S	-2.4:1

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.

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.