HOW THE AUTOMATIC TRANSMISSIONS WORK?

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Gear Ratios

This transmission has four forward gears and one reverse gear. Let's summarize the gear ratios, inputs and outputs: Gear Input Output Fixed Gear Ratio 1st 30-tooth sun 72-tooth ring Planet carrier 2.4:1 2nd 30-tooth sun Planet carrier 36-tooth ring 2.2:1 Planet carrier 72-tooth ring 36-tooth sun 0.67:1 Total 2nd 1.47:1 3rd 30- and 36-tooth suns 72-tooth ring 1.0:1 OD Planet carrier 72-tooth ring 36-tooth sun 0.67:1 Reverse 36-tooth sun 72-tooth ring Planet carrier -2.0:1 After reading these sections, 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.

Clutches and Bands

In the last section, 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. In the figure above, you can see one of the bands in the housing of the transmission. The gear train is removed. The metal rod is connected to the piston, which actuates the band. Above 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. The 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. Below you can see 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. The next figure 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.

When You Shift the Gear into 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. 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 drive shaft -- so if this part can't spin, the car can't move. Above you see the parking 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.

This rod is connected to a cable that is operated by the shift lever in your car. 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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