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.
 One of the bands |
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.
 The pistons that actuate the bands are visible here. |
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.
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.
 One of the clutches in a transmission |
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.
 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.
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