Using the principle of mechanical
advantage, transmissions provide a speed-torque conversion (commonly
known as "gear reduction" or "speed reduction") from a high speed motor
to a slower but more forced output or vice-versa.
Most modern gearboxes and other
truck parts either
reduce an unsuitable high speed and low torque of the prime mover output
shaft to a more stable lower speed with higher torque, or do the
opposite and provide a mechanical helpful to allow higher forces to be
generated. 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 are used to slow down
the output speed of the engine and increase torque. The highest
gears may be "overdrive" types that increase the output speed.
Early transmissions included the
right-angle drives and other gearing in windmills, horse-powered
devices and steam engines, in support of pumping, milling, and
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, while the car's wheels rotate between
0 rpm and around 1800 rpm.
In addition the engine provides its
highest torque outputs approximately in the middle of its range, while
often the greatest torque is required when the vehicle is moving from
rest or traveling slowly. 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.
Most transmissions and gears used
in automotive and truck parts applications are contained in a cast iron case,
though sometimes aluminium is used for lower weight.
The mainshaft extends outside the
case in both directions: the input shaft towards the engine, and the
output shaft towards the rear axle. The shaft is suspended by the main
bearings, and is split towards the input end. At the point of the split,
a pilot bearing holds the shafts together. The gears and clutches ride
on the mainshaft, the gears being free to turn relative to the mainshaft
except when engaged by the clutches.
Types of automobile transmissions
include manual, automatic or semi-automatic transmission.
Many applications require the
availability of multiple gear ratios. Often, this is to ease the start
and stop of a mechanical system, though another important
need is that of maintaining good fuel economy.
Manual is also classified in two
* a simple but rugged
sliding-mesh or unsynchronized system, where straight-cut spur gear sets
are spinning freely, and must be synchronized by the operator matching
engine revs to road speed, to avoid noisy and damaging "gear clash",
* and the now common
constant-mesh gearboxes which can include non-synchronised, or
synchronized / synchromesh systems, where diagonal cut helical gear sets
are constantly "meshed" together, and a dog clutch is used for changing
gears. On synchromesh boxes, friction cones or "synchro-rings" are used
in addition to the dog clutch.
The former type is commonly found
in many forms of racing cars, older heavy-duty truck partss, and some
Manual transmissions dominate the
car market outside of North America. They are cheaper, lighter, usually
give better performance, and fuel efficiency. It is customary for new
drivers to learn, and be tested, on a car with a manual gear change. In
Malaysia, Denmark and Poland all cars used for testing have a manual
transmission. In Japan, the Philippines, Germany, Israel, the
Netherlands, New Zealand, Austria, the UK, Ireland, Sweden, France,
Switzerland, Australia, Finland and Lithuania , a test pass using an
automatic car does not entitle the driver to use a manual car on the
public road; a test with a manual car is required.
Manual transmissions are much more common than automatic transmissions
in Asia, Africa, South America and Europe.
Transmissions are also used in
agricultural, industrial, construction, mining and automotive truck
parts. Including ordinary transmission equipped with gears, such
equipment makes extensive use of the hydrostatic drive and electrical
There are commercial applications
engineered with designs taking into account that the gear shifting will
be done by an experienced operator. They are a manual transmission, but
are known as non-synchronized transmissions. Dependent on country of
operation, many local, regional, and national laws govern the operation
of these types of vehicles. This class may include commercial, military,
agricultural, or engineering vehicles. Some of these may use
combinations of types for multi-purpose functions.
An example would be a
PTO, or power-take-off gear. The non-synchronous transmission type
requires an understanding of gear range, torque, engine power, and
multi-functional clutch and shifter functions. Also see
Double-clutching, and Clutch-brake sections of the main article at