“A
gearbox is any device which converts speed and torque, whereas a transmission
is a type of gearbox that can be "shifted" to dynamically change the
speed:torque ratio, such as in a vehicle.”
The
most common use is in motor vehicles, where the
transmission adapts the output of the internal
combustion engine to the drive wheels. Such engines need to operate at a
relatively high rotational
speed, which is inappropriate for starting, stopping,
and slower travel. The transmission reduces the higher engine speed to the
slower wheel speed, increasing torque in the process. Transmissions are also used on pedal
bicycles, fixed machines, and anywhere else rotational speed and torque needs
to be adapted.
Often,
a transmission will have multiple gear ratios (or
simply "gears"), with the ability to switch between them as speed
varies. This switching may be done manually (by the operator), or
automatically. Directional (forward and reverse) control may also be provided.
Single-ratio transmissions also exist, which simply change the speed and torque
(and sometimes direction) of motor output.
In
motor vehicle applications, the transmission will generally be connected to the crankshaft of the engine. The output of the transmission is
transmitted via driveshaft to one or more differentials, which in turn drive the wheels. While a differential may
also provide gear reduction, its primary purpose is to permit the wheels at
either end of an axle to rotate at different speeds (essential to avoid wheel
slippage on turns) as it changes the direction of rotation.
Most
modern gearboxes are used to increase torque while
reducing the speed of a prime mover output shaft (e.g. a motor crankshaft).
This means that the output shaft of a gearbox will
rotate at a slower rate than the input shaft, and this reduction in speed will
produce a mechanical
advantage, causing an increase in torque. A gearbox can be set up to do
the opposite and provide an increase in shaft speed with a reduction of torque.
Some of the simplest gearboxes merely change the physical direction in which
power is transmitted.
Many
typical automobile transmissions
include the ability to select one of several different gear ratios. In this case, most of the gear ratios (often simply
called "gears") are used to slow down the output speed of the engine
and increase torque. However, the highest gears may be "overdrive" types that increase the output speed.
The need for a transmission in an automobile is a consequence of the
characteristics of the internal
combustion engine. Engines typically operate over a range of 600 to about 7000 revolutions per minute (though this varies, and is typically
less for diesel engines), while the car's wheels rotate between 0 rpm and
around 1800 rpm.
Furthermore, the engine provides its highest torque and power
outputs unevenly across the rev range resulting in a torque band and a power band. Often the greatest torque
is required when the vehicle is moving from rest or traveling slowly, while
maximum power is needed at high speed. Therefore, a system that transforms the
engine's output so that it can supply high torque at low speeds, but also
operate at highway speeds with the motor still operating within its limits, is
required. Transmissions perform this transformation.
