Using the principle
of mechanical advantage, transmissions provide a torque-speed
conversion from a higher speed motor to a slower but more
forceful output.
Automotive basics
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 outputs
approximately in the middle of its range, while often the
greatest torque is required when the vehicle is moving from
rest or travelling 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
applications are contained in a cast iron case, though sometimes
aluminum is used for lower weight. There are three shafts:
a mainshaft, a countershaft, and an idler shaft.
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.
Automobile includes manual, automatic or semi-automatic
transmission.
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