The animation
below shows how all of the parts are hooked up in a
transmission.
Move the shift lever to see how power is transmitted
through the 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.
Second Gear
This transmission does something really
neat in order to get the ratio needed for second gear. It
acts like two planetary gear sets 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.
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 gear set 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. 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.
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 the ratio in
reverse is a little less than first gear in this
transmission.
We give our thanks to
HOW STUFF WORKS for allowing us to use their info and
graphics!
For more information on how engines
or anything else in this world works, visit
www.howstuffworks.com.
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