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How Automatic Transmissions Work

Introduction to How an Automatic Transmission Works
Planetary Gearsets & Gears
How it Works
Benefits and Shortcomings of the Torque Converter

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

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

Any planetary gearset has three main components:

• the sun gear
• the planet gears and the planet gears' carrier
• the ring gear.

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.

	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:

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.

Gears
This automatic transmission uses a set of gears, called a compound planetary gearset, that looks like a single planetary gearset but actually behaves like two planetary gearsets 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.

Let's look at some of the parts.

Figure 3 shows how the gears in the transmission are put together.

Figure 3. From left to right -- the ring gear, planet carrier, and two sun gears.

Figure 4 shows the planets in the planet carrier. Notice how the planet on the right sits lower than the planet on the left. The planet on the right does not engage the ring gear -- it engages the other planet. Only the planet on the left engages the ring gear.

Figure 4. Planet carrier -- note the two sets of planets.

Figure 5 shows the inside of the planet carrier. The shorter gears are engaged only by the smaller sun gear. The longer planets are engaged by the bigger sun gear and by the smaller planets.

Figure 5. Inside of the planet carrier -- note the two sets of planets.

Figure 6 shows all of the parts are hooked up in a transmission.

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 the previous page, the gear ratio is:

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).

Second Gear
This transmission does something really neat in order to get the ratio needed for second gear. It acts like two planetary gearsets connected to each other with a common planet carrier.

The first stage of the planet carrier actually uses the larger sun gear as the ring gear. So the first stage consists of the sun (the smaller sun gear), the planet carrier, and the ring (the larger sun gear).

The input is the small sun gear; the ring gear (large sun gear) is held stationary by the band, and the output is the planet carrier. For this stage, with the sun as input, planet carrier as output, and the ring gear fixed, the formula is:

The planet carrier turns 2.2 times for each rotation of the sun gear. At the second stage, the planet carrier acts as the input for the second planetary gear set, the larger sun gear (which is held stationary) acts as the sun, and the ring gear acts as the output, so the gear ratio is:

To get the overall reduction for second gear, we multiply the first stage by the second, 2.2 x 0.67, to get a 1.47:1 reduction. This may sound wacky, but it works.

Third Gear
Most automatic transmissions have a 1:1 ratio in third gear. You'll remember from the previous section that all we have to do to get a 1:1 output is lock together any two of the three parts of the planetary gear. With the arrangement in this gearset it is even easier -- all we have to do is engage the clutches that lock each of the sun gears to the turbine.

If both sun gears turn in the same direction, the planet gears lockup because they can only spin in opposite directions. This locks the ring gear to the planets and causes everything to spin as a unit, producing a 1:1 ratio.

Overdrive
By definition, an overdrive has a faster output speed than input speed. It's a speed increase -- the opposite of a reduction. In this transmission, engaging the overdrive accomplishes two things at once. If you read How Torque Converters Work, you learned about lockup torque converters. In order to improve efficiency, some cars have a mechanism that locks up the torque converter so that the output of the engine goes straight to the transmission.

In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. Let's go back to our chart again, this time with the planet carrier for input, the sun gear fixed and the ring gear for output.

So the output spins once for every two-thirds of a rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow.

Reverse
Reverse is very similar to first gear, except that instead of the small sun gear being driven by the torque converter turbine, the bigger sun gear is driven, and the small one freewheels in the opposite direction. The planet carrier is held by the reverse band to the housing. So, according to our equations from the last page, we have:

So the ratio in reverse is a little less than first gear in this transmission.

Gear Ratios
This transmission has four forward gears and one reverse gear. Let's summarize the gear ratios, inputs and outputs:

After reading this section, you are probably wondering how the different inputs get connected and disconnected. This is done by a series of clutches and bands inside the transmission. In the next section, we'll see how these work.

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Table of Contents:

Introduction to How an Automatic Transmission Works
Planetary Gearsets & Gears
The Clutches and Bands
The Hydraulic System and How It Shifts

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