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How Automatic Transmissions Work

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

The Clutches and Bands
In the last section, we discussed how each of the gear ratios is created by the transmission. For instance, when we discussed overdrive, we said:

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.

To get the transmission into overdrive, lots of things have to be connected and disconnected by clutches and bands. The planet carrier gets connected to the torque converter housing by a clutch. The small sun gets disconnected from the turbine by a clutch so that it can freewheel. The big sun gear is held to the housing by a band so that it could not rotate. Each gear shift triggers a series of events like these, with different clutches and bands engaging and disengaging. Let's take a look at a band.

Bands
In this transmission there are two bands. The bands in a transmission are, literally, steel bands that wrap around sections of the gear train and connect to the housing. They are actuated by hydraulic cylinders inside the case of the transmission.

Figure 7. One of the bands.

In Figure 7 you can see one of the bands in the housing of the transmission. The geartrain is removed. The metal rod is connected to the piston, which actuates the band.

Figure 8. The pistons that actuate the bands are visible above.

In Figure 8 you can see the two pistons that actuate the bands. Hydraulic pressure, routed into the cylinder by a set of valves, causes the pistons to push on the bands, locking that part of the gear train to the housing.

Clutches
The clutches in the transmission are a little more complex. In this transmission there are four clutches. Each clutch is actuated by pressurized hydraulic fluid that enters a piston inside the clutch. Springs make sure that the clutch releases when the pressure is reduced.

Figure 9 shows the piston and the clutch drum. Notice the rubber seal on the piston -- this is one of the components that is replaced when your transmission gets rebuilt.

Figure 9. One of the clutches in a transmission.

Figure 10 shows the alternating layers of clutch friction material and steel plates. The friction material is splined on the inside, where it locks to one of the gears. The steel plate is splined on the outside, where it locks to the clutch housing. These clutch plates are also replaced when the transmission is rebuilt.

Figure 10. The clutch plates.

The pressure for the clutches is fed through passageways in the shafts. The hydraulic system controls which clutches and bands are energized at any given moment.

When You Put the Car in Park
It may seem like a simple thing to lock the transmission and keep it from spinning; but there are actually some complex requirements for this mechanism:

• You have to be able to disengage it when the car is on a hill (the weight of the car is resting on the mechanism).
• You have to be able to engage the mechanism even if the lever does not line up with the gear.
• Once engaged, something has to prevent the lever from popping up and disengaging.

The mechanism that does all this is pretty neat. Let's look at some of the parts first.

Figure 11. The output of the transmission - the square notches are engaged by the parking brake mechanism to hold the car still.

Figure 11 shows the output section of the transmission. The parking brake mechanism engages the teeth on the output to hold the car still. This is the section of the transmission that hooks up to the driveshaft -- so if this part can't spin, the car can't move.

Figure 12. The empty housing of the transmission with the parking brake mechanism poking through, as it does when the car is in park.

Figure 12 shows the park mechanism protruding into the housing where the gears are located. Notice that it has tapered sides. This helps to disengage the parking brake when you are parked on a hill -- the force from the weight of the car helps to push the parking mechanism out of place because of the angle of the taper.

Figure 13. This rod actuates the park mechanism.

Figure 13 shows the rod that actuates the park mechanism. This rod is connected to a cable that is operated by the shift lever in your car.

Figure 14. The park mechanism.

Figure 14 shows the park mechanism from the top. When the shift lever is placed in park, the rod pushes the spring against the small tapered bushing. If the park mechanism is lined up so that it can drop into one of the notches in the output gear section, the tapered bushing will push the mechanism down. If the mechanism is lined up on one of the high spots on the output, then the spring will push on the tapered bushing, but the lever will not lock into place until the car rolls a little and the teeth line up properly. This is why sometimes your car moves a little bit after you put it in park and release the brake pedal -- it has to roll a little for the teeth to line up to where the parking mechanism can drop into place.

Once the car is safely in park, the bushing holds down the lever so that the car will not pop out of park if it is on a hill.

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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

• Automatic Transmissions, how do they work?
• Torque Converters... What? What are those?
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