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what's in a transmission?
• A modern automatic transmission is an assembly
of electronic, hydraulic, and mechanical components
that come together to change the output speed of the
drive train. Its primary components include:
• Torque converter
• Planetary gears
• Valve body
what is Torque converter?
A Torque converter or "stall converter" is a fluid
coupler that allows the engine to turn while the
transmission is stopped. If you picture two fans
placed face to face and you turn one on, it can spin
while you hold the free fan but if you let go of the free
fan it will start to spin faster and faster because of the
air moving across it. A torque converter operates in
a similar way but with fluid instead of air. Modern
torque converters also contain a clutch to lock the
two "fans" together, this locking of the torque
converter results in better fuel efficiency during
highway driving. When a torque converter fails it
begins to shed friction material from this clutch. This
material clogs the filter and can result in overheating
other components in the transmission.
What are planetary gears?
A planetary gear set is the component that changes
the output speed of the drive train, transmissions can
contain as many as 4.
They contain 3 gears, an outer large ring gear, 3 to 6
small planet gears and a central sun gear. They work
by holding one of the three and turning one of the
three the remaining gear will rotate at a different rate
than the drive gear.
The difference in circumference of each gear results
in a different ratio of rotation between any two gears
in the set. As illustrated in the lower image.
When these gear sets fail they break teeth off of one
or more gears resulting in damage to the entire set
and sometimes other components in the unit.
What are Clutches and Bands?
Clutches and bands are used to change which gear
in a planetary gear set is being held to the case or
driven by the engine.
Clutches are arranged in stacks of alternating steel
plates and plates coated with a friction material
similar to brake pads. When theses plates are
stacked in a drum together they are known as a
"clutch pack" these plates have teeth on there inside
or outside edge that engage other components or
the case. When they are applied hydraulic pressure
pushes on a piston that squeezes the plates together
locking the the inner teeth to the outer teeth and
holding the two components together.
Bands are used in a similar manner to hold drums to
the case, they are made of a flexible steel coated in
the same kind of friction material. when they are
applied, hydraulic pressure forces a servo piston pin
against one end of the band while the other is
anchored to the case this force squeezes the band
around the drum locking it to the case.
What is a valve body and what are
Valve bodies are the brain of the transmission, they
are made from aluminum castings that contain valves
and form the pathways for pressurized transmission
fluid to travel to different components. The channels
in a valve body are like "U" shaped pipes with the
tops sliced off. When you bolt it to the case the plate
in between becomes the other half of the pipe. If the
casting is warped or damaged these "pipes" can
leak pressure into places it should not be or jam
valves damaging components. We check all valve
bodies for flatness and surface them if necessary.
the valves in a valve body are known as spool valves
and can be driven by hydraulic force, spring force or
with the use of a solenoid. Solenoids are valves that
are moved electronically by the computer using an