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Almost all automatic transmissions use a pair of gear groups called
epicyclic, or planetary gears. Each group consists of; an outside
"ring" gear, a shared "sun" gear in the center, and a set of "planet
gears", which mesh in between the sun and the ring gear. Planet gears
are so named because each one turns on its own axis as they orbit the
sun gear, like planets do. Each group of planet gears is held in a
"planet gear carrier". By clamping the ring gears, the sun gear, and
the carriers together in various combinations, and by locking some of
them in stationary positions, it is possible to achieve three forward
gear ratios, and reverse as well.
Single epicyclic gear sets work like this:
To increase torque:
When the ring gear is stopped, and the power is applied to the sun
gear, the planet gears are forced to go around the sun gear. This
makes the pinion gears revolve more slowly around the inside gears,
and drive from the carrier will have lower speed and increased torque.
To reverse the torque's direction:
If the planet gear carrier is stopped, and torque is applied to the
sun gear, the planet gears are forced to turn by the sun gear. This
makes the ring gear revolve, but more slowly than the ring gear,
which increases the torque, and in the opposite direction as the sun
gear, giving reverse.
If two members of the gear set are locked together, planetary action
is stopped and the gear set turns as one unit. When this happens,
there are no increases or decreases in torque transmission.
In order to have more than 2 forward speeds, two sets of epicyclic
gears are needed. By changing the number of teeth (size) of one set
of planetary gears, 4 forward speeds can be produced.
The clutches within the transmission are used to connect the input
torque, and the brake bands are used to lock the sun gear or the rear
planet carrier. One way bearings serve to allow power flow in certain
directions only, working as clutches. All of the clutches and brake
bands are powered by hydraulic pressure, and regulated by the logic
circuit which is connected to the governor and/or directly to a
computer-controlled valve assembly. The transmission senses gas pedal
position and drive selector position, and engages the proper clutches
and bands for you to "Get out of Dodge".
The details of automatic transmission functions are vast, and
different designs are introduced by the many automakers with great
regularity. Some common principles shared by virtually all automatics
are: fluid clutches, brake bands, one way bearings (one way
clutches), and epicyclic gears. This crazy diagram is a simplified
version of but one design among many, and if you think it's hard to
understand, don't feel bad. It is!
In this type of transmission, to give first gear, the forward drive
clutch (C) locks the turbine shaft to the front ring gear. At the
same time, the second planet carrier brake band (D) locks the rear
planet carrier in place. The power from the turbine shaft flows
through the front ring gear, which turns the front carrier, which
turns the sun gear. This reduces the RPM's and increases torque one
time. The second reduction/multiplication happens when the sun gear
turns the rear planet gears, each of which rotate within their
stationary carrier. This causes the second ring gear to turn. The
second ring gear transfers its torque to the output shaft through the
second one-way clutch. (Does your brain hurt yet?)
Second gear is accomplished by engaging the sun gear brake band (B)
and the forward-drive clutch (C). This gives one reduction in RPM.
Third gear (Drive) is engaged by locking the reverse-high clutch (A)
and the forward-drive clutch (C). This gives a 1:1 (direct) ratio
between the input and output shafts.
When reverse is selected, the reverse-high clutch (A) and the second
carrier brake band (D) are locked. This reverses the torque
direction, and reduces the ratio (twice) for use in backing up the
car.
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