Gear box

Ch-3 Gear Box
By, Deep Shah
Sub: ATM (3330202)

Introduction of Powertrains
A Transmission system uses a clutch, gear box, propeller shaft and a
differential gear to transmit power from engine to the road wheels
The power may be transmitted to rear or front or all four wheels
depending on the drive used
The clutch and gear box varies the ratio of torque output to torque
input
The propeller shaft transmits final torque to the rear axle from gear
box
A differential gear equally distributes the final torque between the
road wheels.

Functions of Transmission System
It disconnects engine from driving wheels when
required
The engine is connected to driving wheels when
required
It changes ratio of torque output to torque input, as
desired
It turns the drive through a right angle

Gear Box
The gear box is necessary in the transmission system to
maintain engine speed (or torque) at the most economical
value under all conditions of vehicle movement.
An ideal gear box would provide an infinite range of
gear ratios, so that the engine speed should be kept at or
near that the maximum power is developed whatever the
speed of the vehicle.

Purpose of Gear Box
Provides speed and torque conversions because of the limitations of internal
combustion engines.
Also facilitates change of direction of output shaft for reversing.
Automotive gearboxes are used to reduce load on the engine by manipulating
torque and speed.
They have the option to select one of several different gear ratios.
Once the engine has reached a number of revolutions per minute, it is advisable to
increase the gear to reduce the engine rpm to reduce wear on the engine, allow
more control, and greater speeds, better acceleration, and better fuel economy.
Most gearboxes are used to increase torque & reduce the speed of a output shaft.
This produces a mechanical advantage
Automotive gearbox also have the provision to do the opposite i.e. provide an
increase in output shaft speed with a reduction of torque (overdrive).

Types of Gear Box
Sliding Mesh type gear box
Constant Mesh type gear box
Synchro Mesh type gear box
Planetary type gear box

Types of Gear Box
Manually Operated Selective type
Sliding Mesh type gear box
Constant Mesh type gear box
Synchro Mesh type gear box
Planetary type gear box
Over-Drive
Semi-Automatic
Automatic Transmission

Sliding Mesh type Gear Box
The power comes from the engine to the clutch shaft and then
to the clutch gear which is always in mesh with gear on the lay
shaft.
All the gears on the lay shaft are fixed to it and as such they are
all the time rotating when the engine is running and the clutch is
engaged.
Three direct and one reverse speeds are attained on suitably
moving the gear on the main shaft by means of selector
mechanism.

Constant Mesh type Gear Box
In this type of gear box, all the gears are in constant mesh with the
corresponding gears on the lay shaft. The gears on the main shaft
which is splined are free.
The dog clutch are provided which are free to slide on the main shaft.
The gears on the lay shaft are, however, fixed.
When the left dog clutch is slide to the left by means of the selector
mechanism, its teeth are engaged with those on the clutch gear and we
get the direct gear.
The same dog clutch, however, when the slide to right makes contact
with the second gear.
Similarly movement of the right dog clutch to the left result in low
gear and towards right in reverse gear.

Synchro-Mesh type Gear Box
This type of gear box is similar to the constant mesh type in that all
the gears on the main shaft are in constant mesh with the
corresponding gears on the lay shaft.
The gears on the lay shaft are fixed to it while those on the main
shaft are free to rotate on the same.
Its working is also similar to the constant mesh type, but in the
former there is one definite improvement over the latter.
This is the provision of synchromesh device which avoids the
necessity of double declutching. The parts which ultimately are to be
engaged are first brought into frictional contact which equalizes their
speed, after which these may be engaged smoothly.

Synchronizers
A device used to bring two adjacent members to the same speed
before allowing the sleeve to engage them.
The two elements are friction clutch and toothed clutch.
Lock the positive engagement until speeds are Synchronized.
Establish the positive engagement and power flow.
Synchronizer is splined on the shaft Cone on the gear (blue) fits into
cone-shaped area in the collar.
Friction between the cone and collar synchronize the collar & gear.
The outer portion of the collar (sleeve) then slides so that the dogteeth
engage the gear.

Epicyclic/Planetary Gear Box
It consists of two, three or even four epicyclic or planetary gear
sets. A simple gear set has a sun gear, about which planets turn
around. These planet gear are carried by a carrier and a shaft
and are also in mesh internally with a ring gear, which is also
called annulus or internal gear sometimes.
Different torque ratios i.e. speed ratios are obtained by making
of the part stationary. Similarly by locking two parts with each
other, a solid drive i.e. direct gear obtain.

Epicyclic/Planetary Gear Box
Driving Member Driven Member Stationary
Forward Very Slow Sun Planetary Ring
Forward Slow Ring Planetary Sun
Forward Fast Planetary Ring Sun
Forward Very Fast Planetary Sun Ring
Reverse Slow Sun Ring Planetary
Reverse Fast Ring Sun Planetary

Different Types of Gears (Spur Gear)
Advantages
Simple and Rigid
Spacer is not required at the end
of shaft
Can join two parallel shafts
Disadvantages
Produce noise during working
Worn out fast/quickly

Different Types of Gears (Helical Gear)
Advantages
Load can be divided on more then
one teeth.
Light in weight
Can rotate without vibration at
high speed
Works easily and noiseless
Doesn’t wear out fast
Disadvantages
Due to inclined tooth, spacer is
required at shaft end
Possible that gear may come out
of meshing

Different Types of Gears (Double Helical Gear)
Advantages
Spacer is not necessary to provide
at shaft end due its double form
Gears isn’t coming out of the
meshing
Can rotate at high speed without
vibration
Function is simple
Service life is more
Disadvantages
Produce little noise during
working
Construction is complex
Production cost is more

Gear Shifting (Selector) Mechanism
The gear shift lever can be located either on steering or on the
steering column, for using it for shifting the gears.
The steering column gear shift mechanism is used on cars
while footboard type is used in commercial vehicles, jeeps,
medium vehicles and vans.
The gear shifting mechanism is used for changing gears easily
and selecting required gear train and bringing the selected
gears into operation.

For engaging the slots on the gear box it is consisting of fork
on the gearbox.
Into a slot at the top of the boss of the selector fork, the tip of
the gear shift lever is fitted.
In neutral position it is provided with free sideway movement
in either way. In case of steering column gear changes
arrangement, the free movement is in the up and down
direction.
The floor type shifting mechanism is less costly as it requires
less maintenance.

The steering column type shifting mechanism can be operated
easily, as driver has to move his hand through a short distance
and the front compartment space of the vehicle is more.
It is difficult to learn to operate the steering column shifting
mechanism, it is complex, need more maintenance and is
costly.

Torque Converter
Internal combustion engines create power by burning fuel that forces the
pistons to turn the crankshaft located at the bottom of the engine. This
rotational force is transferred to the transmission by the fluid pressure
inside the torque converter.
Inside of the torque converter cover lives a series of propeller-like blades
called the pump. This assembly spins in unison with the engine
crankshaft, forcing transmission fluid onto another blade assembly called
the impeller. This second set of blades is connected to the transmission
input shaft. The amount of hydraulic pressure that it creates inside the
transmission dictates the gear and ultimately, the speed of the vehicle.

Torque Converter
The impeller’s speed is regulated by the engine side of this
hydrodynamic circuit (i.e. speed of the pump blades). When the vehicle is
stationary, or the driver applies the brakes, the impeller will slow
considerably, while the pump continues to spin. This allows the torque
converter to act like the clutch in a manual transmission – it allows the
engine to continue running while the vehicle is at a complete stop.
Once the transmission fluid has been hurled onto the impeller blades, it
has to return to the pump in order to keep the cycle going. Since the fluid
is now flowing in a different direction than the pump, it has to be
reversed to avoid slowing down (and stalling) the engine.

Torque Converter
To do this, a third finned wheel called the stator is located between the
two turbines on the transmission pump shaft. Its blades are precisely
angled so that when the transmission fluid hits them, it reverses direction
and gets channeled back to the pump. When the vehicle stops, its built-in
one-way clutch causes it to stop spinning, breaking the hydrodynamic
circuit.

Advantages of Torque Converter
It multiplies the torque by the movement of oil.
All parts are remain sinked in oil hence their wear is less.
It transmits power without noise & shocks at different speed and torque
ratio.
Operation is completely automatic & reliable.
Generally servicing of torque converter is not necessary.
Construction of torque converter is simple, wear is less due to less effect
of slip.

Disadvantages of Torque Converter
The torque available to start the vehicle from rest in first gear is reduces
with the increase in speed of the vehicle.
It becomes fluid coupling on increasing speed of vehicle, hence the
torque multiplication stops thereafter.
There is no torque multiplication in third and fourth gear.
Reverse gear arrangement of mechanical type is to be used with it.
To prevent torque converter from overrunning, a free wheel device is
required.

Over Drive
Overdrive is a device to step up the gear ratio in the car. It is fitted in between
transmission and the propeller shaft. It enables a high cruising speed to be attained with
a comparatively low engine speed (up to 20 – 25%) on long journeys.
This results in less wear of the engine and decreases vibration and noise. As the friction
lows at lower speeds is less, there is a saving of fuel also with the overdrive.
Overdrive is generally fitted on top gear only. But in some sport cars, over drives are
also fitted on gears other than the top gear which increases the torque ratios available.
For examples, when overdrive is fated on top, third and second gear, seven forward
speeds or torque ratios are available.
The overdrive may be operated either manually or automatically at a predetermined
speed.

Over Drive
It consists of an epicyclic gear train in which the sun gear is free to rotate on the
input shaft, while the carrier can move on splines, on the input shaft. A freewheel
clutch is also fitted on the input shaft splines. The output shaft is connected to the
ring. When the sun gear is locked with the casing i.e., it becomes stationary, of the
output shaft is increased i.e., overdrive is engaged.
When the sun gear is locked to the carrier or to the ring, solid drive through n is
obtained. Thus depending on the locking of the sun gear with ring gear or with
carrier the overdrive or the normal direct drive is obtained. There is another
possible control of the mechanism i.e., when the sun wheel is kept free to rotate on
the input shaft. In this case there is direct drive through the freewheel clutch when
the engine develops power.

Advantages of Over Drive
When car is moving ahead at constant speed, then is doesn’t require more power to
maintain that speed. Hence to maintain the speed of the car, engine speed can be reduced
using the over drive.
High speed running of vehicle at low engine speed can be obtain.
Fuel consumption decrease.
The wear of engine & auxiliary units decreases.
Due to reduction in engine RPM considerable saving of fuel is achieved.
For required vehicle speed, engine is sunning slowly hence its service life increase.
The overdrive can be controlled electrically or hydraulically from dash-board or a switch
provided on steering column.

Gear box

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