HOW THE AUTOMATIC TRANSMISSIONS WORK?

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Planetary Gear sets

Look inside an automatic transmission and you will find a huge assortment of parts in a fairly limited 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

Planetary Gear sets & Gear Ratios

A 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. 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. 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. -2.4:1

	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

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.

This 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. We'll put the two sets of gears together in the next section. The Gears

Automatic transmission uses a set of gears, called a compound planetary gear set, that looks like a single planetary gear set but actually behaves like two planetary gear sets 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. The First Gear

In first gear, the smaller sun gear is driven clockwise by the turbine in the torque converter. The planet carrier tries to spin counterclockwise, but is held still by the one-way clutch (which only allows rotation in the clockwise direction) and the ring gear turns the output. The small gear has 30 teeth and the ring gear has 72, so referring to the chart on this page, the gear ratio is: Ratio = -R/S = - 72/30 = -2.4:1 So the rotation is negative 2.4:1, which means that the output direction would be opposite the input direction. But the output direction is really the same as the input direction -- this is where the trick with the two sets of planets comes in. The first set of planets engages the second set, and the second set turns the ring gear; this combination reverses the direction. You can see that this would also cause the bigger sun gear to spin; but because that clutch is released, the bigger sun gear is free to spin in the opposite direction of the turbine (counterclockwise).

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