Simply put, a transmission allows power to be sent from a power source, most often an engine or motor, to a drive mechanism. Transmissions use gears and a clutch to convert the speed of the power source into torque. A simpler transmission is often referred to as a gearbox since it is basically a box containing a configuration of gears.
most common example of a transmission is that found in an automobile.
There are two types of automobile transmissions, manual and automatic.
Both accomplish the same function in turning engine speed or revolutions
per minute (rpm) into torque (measured in pounds/feet). They also allow
the drive mechanism to shift from forward into reverse without the need
to shut off one engine and reverse the direction of the crankcase with a second engine running in the opposite direction.
In the case of a manual transmission, the flywheel,
which is connected to the crankcase of the engine, runs at a constant
speed. Through the use of a manually operated clutch and a shifter,
engine power is reduced and increased significantly through the
engagement or disengagement of a series of larger and smaller gears.
This shifting of gears will avoid a correspondingly significant,
inefficient and potentially damaging increase of engine speed. These
gears are running at different speeds because they are of differing
sizes. Larger gears convert higher engine rpm into higher torque or
energy at lower drive wheel speed by spinning
more slowly than the crankcase; smaller gears, conversely, convert
lower engine rpm into higher speed and efficiency by spinning faster
than the crankcase.
Automatic transmissions, called transaxles in
front wheel drive vehicles, accomplish exactly the same function through
speed-torque conversion. A torque converter
automatically engages and disengages the correct gears in the proper
ratio using a system of bands engaging and disengaging the hydraulically
operated clutch pack. This replaces the manually operated clutch. While
manual transmission gearing is in line, the gears in an automatic transmission are in a "planetary" arrangement, meaning they revolve around a stationary "sun" gear.
All of this is enabled through the use of gear
reduction where larger gears that rotate more slowly are exchanged via
the clutch or the torque converter for smaller, more rapidly rotating
gears to increase the speed of the drive wheels. The reverse of the
gearing exchange occurs with a need to decrease speed. Thus the speed of
the power source, the engine or motor, is dampened or enhanced by the
manual clutch or the torque converter and clutch pack to increase the
efficiency of, as well as decrease the wear and tear on, the engine while providing the needed torque to the drive shaft.
Even such simple power sources as windmill
vanes or the pedals on a three speed bicycle utilize a primitive
transmission to transfer and partition the energy of the power source
rpm into driving torque.