The modern
automatic transmission consists of many components and systems that are
designed to work together in a symphony of clever mechanical, hydraulic and
electrical technology that has evolved over the years into what many
mechanically inclined individuals consider to be an art form. We try to
use simple, generic explanations where possible to describe these systems but,
due to the complexity of some of these components, you may have to use some
mental gymnastics to visualize their operation.
The main
components that make up an automatic transmission include:
Planetary
Gear Sets which are the mechanical systems that provide the various
forward gear ratios as well as reverse.
The Hydraulic
System which uses a special transmission fluid sent under pressure by
an Oil Pump through the Valve Body to control
theClutches and the Bands in order to control the planetary gear
sets.
Seals and
Gaskets are used to keep the oil where it is supposed to be and prevent it
from leaking out.
The Torque
Converter which acts like a clutch to allow the vehicle to come to a stop
in gear while the engine is still running.
The Governor and
the Modulator or Throttle Cable that monitor speed and
throttle position in order to determine when to shift.
On newer
vehicles, shift points are controlled by Computer which directs
electrical solenoids to shift oil flow to the appropriate component at the
right instant.
Planetary Gear Sets
Automatic
transmissions contain many gears in various combinations. In a manual
transmission, gears slide along shafts as you move the shift lever from one
position to another, engaging various sized gears as required in order to
provide the correct gear ratio. In an automatic transmission, however, the
gears are never physically moved and are always engaged to the same
gears. This is accomplished through the use of planetary gear sets.
The basic
planetary gear set consists of a sun gear, a ring gear and two or more planet
gears, all remaining in constant mesh. The planet gears are connected to
each other through a common carrier which allows the gears to spin on shafts
called "pinions" which are attached to the carrier .
One example
of a way that this system can be used is by connecting the ring gear to the
input shaft coming from the engine, connecting the planet carrier to the output
shaft, and locking the sun gear so that it can't move. In this scenario,
when we turn the ring gear, the planets will "walk" along the sun
gear (which is held stationary) causing the planet carrier to turn the output
shaft in the same direction as the input shaft but at a slower speed causing
gear reduction (similar to a car in first gear).
If we unlock
the sun gear and lock any two elements together, this will cause all three
elements to turn at the same speed so that the output shaft will turn at the
same rate of speed as the input shaft. This is like a car that is in third or
high gear. Another way that we can use a Planetary gear set is by locking the
planet carrier from moving, then applying power to the ring gear which will
cause the sun gear to turn in the opposite direction giving us reverse gear.
The
illustration on the right shows how the simple system described above would
look in an actual transmission. The input shaft is connected to the ring gear
(Blue), The Output shaft is connected to the planet carrier (Green) which is
also connected to a "Multi-disk" clutch pack. The sun gear is
connected to a drum (yellow) which is also connected to the other half of the
clutch pack. Surrounding the outside of the drum is a band (red) that can
be tightened around the drum when required to prevent the drum with the
attached sun gear from turning.
The clutch
pack is used, in this instance, to lock the planet carrier with the sun gear
forcing both to turn at the same speed. If both the clutch pack and the band
were released, the system would be in neutral. Turning the input shaft
would turn the planet gears against the sun gear, but since nothing is holding
the sun gear, it will just spin free and have no effect on the output shaft. To
place the unit in first gear, the band is applied to hold the sun gear from
moving. To shift from first to high gear, the band is released and the
clutch is applied causing the output shaft to turn at the same speed as the
input shaft.
Many more
combinations are possible using two or more planetary sets connected in various
ways to provide the different forward speeds and reverse that are found in
modern automatic transmissions.
Some of the
clever gear arrangements found in four and now, five, six and even seven and
eight-speed automatics are complex enough to make a technically astute lay
person's head spin trying to understand the flow of power through the
transmission as it shifts from first gear through top gear while the vehicle
accelerates to highway speed. On modern vehicles (mid '80s to the
present), the vehicle's computer monitors and controls these shifts so that
they are almost imperceptible.
Clutch Packs
A clutch pack
consists of alternating disks that fit inside a clutch drum. Half of the disks
are steel and have splines that fit into groves on the inside of the
drum. The other half have a friction material bonded to their surface and
have splines on the inside edge that fit groves on the outer surface of the
adjoining hub. There is a piston inside the drum that is activated by oil
pressure at the appropriate time to squeeze the clutch pack together so that
the two components become locked and turn as one.
One-Way Clutch
A one-way
clutch (also known as a "sprag" clutch) is a device that will allow a
component such as ring gear to turn freely in one direction but not in the
other. This effect is just like that of a bicycle, where the pedals will turn
the wheel when pedaling forward, but will spin free when pedaling backward.
A common
place where a one-way clutch is used is in first gear when the shifter is in
the drive position. When you begin to accelerate from a stop, the transmission
starts out in first gear. But have you ever noticed what happens if you release
the gas while it is still in first gear? The vehicle continues to coast
as if you were in neutral. Now, shift into Low gear instead of Drive.
When you let go of the gas in this case, you will feel the engine slow you down
just like a standard shift car. The reason for this is that in Drive, a one-way
clutch is used whereas in Low, a clutch pack or a band is used.
Bands
A band is a
steel strap with friction material bonded to the inside surface. One end
of the band is anchored against the transmission case while the other end is
connected to a servo. At the appropriate time hydraulic oil is sent to
the servo under pressure to tighten the band around the drum to stop the drum
from turning.
Torque Converter
On automatic
transmissions, the torque converter takes the place of the clutch found
on standard shift vehicles. It is there to allow the engine to continue
running when the vehicle comes to a stop. The principle behind a
torque converter is like taking a fan that is plugged into the wall and blowing
air into another fan which is unplugged. If you grab the blade on the
unplugged fan, you are able to hold it from turning but as soon as you let go,
it will begin to speed up until it comes close to the speed of the powered
fan. The difference with a torque converter is that instead of using air,
it uses oil or transmission fluid, to be more precise.
A torque
converter is a large doughnut shaped device (10" to 15" in diameter)
that is mounted between the engine and the transmission. It consists of
three internal elements that work together to transmit power to the
transmission. The three elements of the torque converter are the
Pump, the Turbine, and the Stator. The pump is mounted
directly to the converter housing which in turn is bolted directly to the
engine's crankshaft and turns at engine speed. The turbine is inside the
housing and is connected directly to the input shaft of the transmission
providing power to move the vehicle. The stator is mounted to a
one-way clutch so that it can spin freely in one direction but not in the
other. Each of the three elements have fins mounted in them to precisely direct
the flow of oil through the converter
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