2. SEMIAUTOMATIC TRANSMISSION
Semi-automatic transmissions are much
more similar to manual transmission
system. A gearbox that works semi-
automatically (also known as automated
manual) requires the driver to make gear
changes, like with a manual gearbox. The
key difference is that, while a manual
transmission gearbox requires the use of a
clutch pedal, but semi-automatic removes
this part from the driver's reach and
completes its function electronically. Since,
electronic equipment takes care of the
clutch, the gearbox can time its gear
changes to thetorque of the engine and
make each gear change fast, precise and
smooth.2.19.1
4. OVERDRIVE :
Overdrive is the unit fitted between gearbox and propeller shaft to permit
propeller shaft faster speed than that in top gear. At Overdrive permit
engine to operate about 70% of propeller shaft speed, when vehicle is
operating in higher speed range. Most of the overdrives have an epicyclic
gear arrangement that includes a hydraulically operated cone clutch.
When the overdrive is not in operation, the clutch which is attached to
central sun wheel is locked by spring pressure to the annulus or ring is
connected to outputshaft.
The planet carrier connected to the gearbox shaft turns the whole unit to
give direct drive. When the driver selects overdrive, the clutch locks to the
outer casing and stops sun wheel turning. The planet carrier is then driven
around the stationary sun wheel and in turn drives ring gear at slightly
higher speed than itself. The effect. s that, the output shaft turns fasterthe
input shaft. Overdrives are controlled electrically or hydraulically by dash
board or steering column switch. The change can be made without using
the clutch, but some designs require.
5. ADVANTAGES OBTAINED BY USE OF OVERDRIVE:
1. Extended engine life.
2. Lesser fuel consumption.
3. Reduced vibration and noise.
4. Generally, overdrive is fitted to operate only in
top gear but in some sports cars, it can operate
in second, third and top gear.
6. FLUID FLYWHELL OR FLUID COUPLING
Fluid Flywheel is also known as hydraulic coupling or fluid coupling. It is a
hydrodynamic device which is used to transfer rotational motion or power
from one shaft to another shaft by means transmission fluid. It is used in
automotive transmission system, marine propulsion system and in
industries for power transmission. It is used as an alternative for the
mechanical clutch.
It consists of three main components:
1. Housing or shell: It has oil tight seal around the drive shaft. Ite len
nentnete tha impeller and turbine from outside damage.
2. Impeller or pump: It is a turbine shaft and called as impeller. It is also
known as pump becausewhich is connected to the iit acts as a
centrifugal pump.
8. WORKING OF FLUID COUPLING:
As the prime mover starts rotating, it rotate the impeller of the coupling. The
impeller acts as a centrifugal pump and it throws the fluid outward and directs it
towards the turbine blade.
As the high moving fluid strikes on the turbine blades, it change the direction of
the fluid and it moves towards the impeller blade again. The blades of turbine
are designed such a way that it can easily change the direction of the fluid, as
changing of direction of the fluid that makes the turbine to rotate.
As the impeller speed increases, the speed of the turbine also increases. After
sometime the speed of impeller and turbine becomes equal. In this way, power is
transmitted from one shaft to another by means of fluid coupling.
9. TORQUE CONVERTER:
Most cars with automatic transmission use
a form of fluid drive known as
torqueconverter. As name implies, it
converts the torque of engine into higher
torque required at low speed. A torque
converter is a device which performs a
function of gearbox with the help
ofhydraulic means. It provides maximum
torque at starting of gradually decreases as
vehicle gains speed. In torque converter, a
fluid drive is used in conjunction with
transmission.
11. TORQUE CONVERTER:
Torque converter consists of,
1. Pump or impeller, which is driving member connected to engine shaft.
2. Stator, which is one-way clutch or stationarymember. Due to stator it is
possible to obtain a change of torque between input and output shaft.
Stator is mounted on free wheel.
3. Turbine or rotor, which is driven member connected to propeller shaft.
4. Free wheel mechanism, which allows the stator to run free when the
pump and turbine run at same speed.
12. EPICYCLIC GEARBOX:
The conventional gearbox has problems like noise and wastage of time
while waiting for synchronization. The epicyclic gearboxovercomes these
problems and is suitable with automatic transmission.
Epicyclic gearbox consists of
1. Sun wheel
2. Planet gear
3. Planet carrier on which planet gears aremounted.
4. An annulus or ring gear having internal gear, it surrounds and meshes
with the planet gears. An external friction brake band is placed over the
annulus.
14. EPICYCLIC GEARBOX:
Applications of Epicyclic
1. Cars having automatic transmission.
2. Army tanks.
3. Wrist watches/wall clocks.
Epicyclic gears are widely used in automatic transmission because,
(1) They are always constantly in mesh.
(2) Smooth and quick.
(3) Considerable variation in gear ratio both forward and reverse can be obtained
through it.
