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TRANSMISSIONS
or powertrains are important links in the powertrain of any vehicle. Its
purpose is to use varying gears to give a mechanical advantage over the
driving wheels. Power from the engine is transferred through the clutch
to the input shaft of the transmission or transaxle. Gears in the transmission
or the transaxle alter torque and speed of power and pass it onto other
parts of the powertrain. The mechanical advantage provided by the gearing
is what enables the engine to generate sufficient torque. Without sufficient
torque it would not be possible to move the vehicle.
A crankshaft always rotates in the same direction so if an engine had
to transmit power directly to the drive axles the vehicle would be able
to travel in one direction only. It is the transmission or the transaxle
that provides the gearing to reverse the direction of the vehicle.
The powertrain propels the vehicle in one of three ways; through the front
wheels, the rear wheels or through all four wheels.
For many years a transmission was used for vehicles propelled through
the rear wheels. The gearing is housed within an aluminium or iron casing
called the transmission case assembly. The unit is attached to the rear
of the engine in the front of the vehicle. A drive shaft links the output
shaft of the transmission to the differential and drive axles located
at the rear of the vehicle. The differential splits the driveline power
and transfers it to the drive axles which in turn transfer the power to
the wheels.
Modern vehicles are mostly front wheel driven. The drivetrain design is
entirely different here. A transaxle is the special unit used to transfer
power to the front wheels. It combines the transmission gearing, differential
and drive axle connections into a single aluminium casing housed in the
front of the vehicle. One advantage is good superior traction due to the
weight of the powertrain components lying directly above the driven wheels.
In a four wheel drive system along with the transmission a transfer case
is employed as well. The transfer case mounts on the side or back of the
transmission. A chain or gears inside the transfer case receive power
from the transmission and transfers it to two different drive shafts.
One shaft is connected to the differential on the front wheels and the
other connects to the differential for the rear wheels. Most manual transmissions
are constant mesh synchronised units. Constant mesh means the gears within
the unit are constantly in mesh irrespective of the vehicle moving or
stationery. Fully synchronised means a unit that uses several brass rings
and clutches to bring rotating shafts and gears to the same speeds before
shift occurs. This ensures smooth shifts.
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August 2002