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
Planetary Gear Sets which are the mechanical systems that provide
the various forward gear ratios as well as reverse. |
which uses a special transmission fluid sent
under pressure by an Oil Pump
through the Valve Body to
control the Clutches and the
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. |
Torque Converter which acts like
a clutch to allow the vehicle to come to a stop in gear while the engine is
still running. |
and the Modulator or
that monitor speed and throttle position in order to determine when to
|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 above 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
Some of the clever gear arrangements found in four
and now, five, six and even seven-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
newer vehicles, the vehicle's computer monitors and controls these
shifts so that they are almost imperceptible.
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
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.
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.