TRANSMISSIONS
INTRODUCTION
Transmission is
the mechanism which is used to transfer the power developed by engine to the
wheels of an automobile. The transmission system of an automobile includes
clutch, gear box, propeller shaft axle and wheels, etc.
CLUTCH
Clutch is used
to engage or disengage the engine to the transmission or gear box. When the
clutch is in engaged position, the engine power or rotary motion of engine
crankshaft is transmitted to gear box and then to wheels. When clutch is
disengaged, the engine power does not reach to gear box and to wheels although
engine is running.
PRINCIPLE OF CLUTCH
When two
surfaces are brought in contact and are held against each other due to friction
between them, they can be used to transmit power. If one is rotated, then other
also rotates. One surface is connected to engine and other to the transmission
system of automobile.
MAIN PARTS OF A CLUTCH
(a) a driving member,
Driving
member has a flywheel which is mounted on the engine crankshaft. A disc is
bolted to flywheel which is known as pressure plate or driving disc.
(b) a driven member, and
The
driven member is a disc called clutch plate. This plate can slide freely to and
from on the clutch shaft.
(c) an operating member.
The operating member consists of a
pedal or lever which can be pressed
to
disengaged the driving and driven plate.
TYPES OF CLUTCHES
(a) Friction Clutch:
(b) Centrifugal
clutch.
(c)
Semi-centrifugal clutch.
(d) Hydraulic
clutch.
(e) Positive
clutch.
(f) Vacuum
clutch.
(g)
Electromagnetic clutch.
SINGLE PLATE CLUTCH
A single plate
is commonly used in cars and light vehicles. It has only one clutch plate which
is mounted on the splines of the clutch shaft. A flywheel is mounted on the
crankshaft of the engine. A pressure plate is connected to the flywheel through
the bolts and clutch springs.
It is free to slide on the clutch shaft with
the movement of clutch pedal. When clutch is in engaged position, the clutch
plate remains gripped between flywheel and pressure plate. Friction linings are
provided on both the sides of clutch plate. clutch plate is in touch with
flywheel and on other side with pressure plate. Due to friction on both sides,
the clutch plate revolves with engine flywheel. Clutch transmits engine power
to clutch shaft. Clutch shaft is connected to transmission or gear box of
automobile. Thus, clutch transmits power from engine to transmission system
which in turn rotates wheels of engine.
When the clutch
plate is to be disengaged, the clutch pedal is pressed. Because of this
pressure plate moves back and clutch plate is disengaged from flywheel. Thus,
clutch shaft stops rotating even if engine flywheel is rotating. In this
position, power does not reach the wheels and vehicle also stops running.
MULTIPLATE CLUTCH
Multi-plate
clutch consists of more than one clutch plates contrary to single plate clutch
which consists of only one plate. Friction surfaces are made in case of
multi-plate clutch. Due to increased number of friction surfaces, a multi-plate
clutch can transmit large torque. Therefore, it is used in racing cars and
heavy motor vehicles which have high engine power. The clutch plates are
alternatively fitted with engine shaft and the shaft of gear box. The plates
are firmly held by the force of coil springs and they assembled in a drum. One
plate slide in the grooves on the flywheel and the next plate slides on spines
provided on pressure plate. Thus, each alternate plate slides in grooves on the
flywheel and the other on splines of pressure plate. If we take two consecutive
plates, then one has inner and other has outer splines.
When the clutch
pedal is pressed, the pressure plate moves back against the force of coil
spring, when the clutch plates are disengaged and engine flywheel and gear box
are decoupled. However, when clutch pedal is not pressed the clutch remain in
engaged position and the power can be transmitted from engine flywheel to the
gear box. This type of clutch has been shown in Figure 4.2.
CLUTCH PEDAL FREE-PLAY ADJUSTMENT
Clutch remains
in engaged position when clutch pedal is not pressed. Free play adjustment is
required to maintain a given free play of the pedal after the clutch is
engaged. Before making this adjustment, correct floorboard clearance or clutch
pedal travel must be adjusted.
Floorboard
clearance adjustment is made to prevent touching of floor by pedal when clutch
is engaged.
Clutch pedal
travel adjustment is done to ensure total clutch disengagement when the clutch
pedal is pressed.
(a) To provide
high torque at the time of starting, vehicle acceleration, climbing up a hill.
(b) To provide
more than forward speeds by providing more than one gear ratios. In modern
cars, five forward gears and reverse gear is provided. For given engine speed,
higher speed can be obtained by running in higher 4th and 5th gears.
(c) Gear box
provides a reverse gear for driving the vehicle in reverse direction.
TYPES OF GEAR BOXES
(a) Selective
type gear boxes:
(i) Sliding
mesh gear box
(ii) Constant
mesh gear box
(iii)
Synchromesh gear box
(b) Progressive
type gear box
(c) Epicyclic
type gear box.
SLIDING MESH GEAR BOX
It is simplest
type of gear box out of the available gear boxes. In this type of gear box,
gears are changed by sliding one gear on the other. This gear box consists of
three shafts; main shaft, clutch shaft and a counter shaft. In a four-speed
gear box, the counter shaft has four gears which are rigidly connected to it.
Clutch shaft has one gear and main shaft has two gears. The two gears on the
main shaft can slide in the horizontal direction along the splines of the main
shaft. However, the gears on the counter shaft cannot slide. The clutch gear is
rigidly fixed to the clutch shaft. It is always connected to the countershaft
drive gear.
The two gears
on the main shaft can be slide by the shifter yoke by operating the shift lever
These two gears are second gear and low/reverse gear respectively. These gears
can be meshed with corresponding gears on the countershaft with the help of
shifter yoke and shift lever. Shift lever is operated by hand in four wheelers
for changing the gears. A reverse idler gear is mounted on another shaft and is
always in mesh with reverse gear on countershaft.
NEUTRAL POSITION
The engine is
in running condition, clutch remains engaged and clutch gear drives the
countershaft drive gear. The direction of rotation of countershaft is opposite
to that of clutch shaft. In this position Ist, IInd and IIIrd and reverse gears
are free. Thus, main transmission shaft does not rotate and automobile wheels
do not rotate.
FIRST GEAR
When first gear
position is selected by the shift lever, first gear on the main shaft slides
and is connected to first gear on the countershaft. The direction of rotation
of main shaft is same as that of clutch shaft. In first gear, small gear of
countershaft meshes with larger gear on main shaft, speed reduction in the
ratio 3: 1.
SECOND GEAR
When second
gear is selected by the shift lever, second gear on countershaft meshes with
second gear on the main shaft. The direction of main shaft is same as that of
clutch shaft. Speed reduction of the order of 2: 1 is obtained in second gear.
THIRD GEAR
In third gear,
the main shaft is slides axially towards the clutch shaft so that main shaft is
directly connected to the clutch shaft. In this position, the main shaft
rotates at the speed of clutch shaft. Thus, a speed ratio of 1: 1 is obtained.
It can be noted
that the clutch gear is directly connected to engine crankshaft and main shaft
is connected to the wheels through propeller shaft.
REVERSE GEAR
When the shift
lever is operated to engage the reverse gear, of the main shaft meshes with the
reverse idler gear. Reverse idler gear is always connected to reverse gear on
countershaft. The reverse idler gear between countershaft reverse gear and main
shaft larger gear changes the direction of rotation of main shaft. Thus, the
direction of main shaft becomes opposite to that of clutch shaft. Therefore,
wheels of the automobile start moving in backward direction.
CONSTANT MESH GEAR BOX
In this gear
box, all gears on the main transmission shaft are constantly connected to
corresponding gears on countershaft or lay shaft. In addition, two dog clutches
are provided on the main shaft. One dog clutch is between the second gear and
cutch gear and another is between the first gear and reverse gear. Splines are
out on main shaft so that all the gears are feed on it.
ADVANTAGE OF CONSTANT MESH GEAR BOX
Since all the
gears are in constant mesh, wear and tear of gears and any possible damage of
gears do not occur in engaging and disengaging gears. Also, any sound is not
generated in engaged/disengaged.
GEAR TRAINS
A combination
of two or more gears, which mesh in such a way that power is transmitted from
driving shaft to drive shaft, is known as gear train.
TYPES OF GEAR TRAINS
There are three
types of gear trains:
(a) Simple gear
train,
(b) Compound
gear train, and
(c) Epicyclic
gear train.
SIMPLE GEAR TRAIN
If the axes of
all the gears remain fixed relative to each other, the gear train is known as
simple gear train. A simple gear train is shown in Figure 4.5.
COMPOUND GEAR TRAIN
There is more
than gear on the shaft (generally intermediate shaft) in a compound gear train.
Two gears are moved on intermediate shaft; therefore, both the gear s have same
speed. A compound gear train is shown in Figure 4.6. Gears 2 and 3 will rotate
at same speed as they are mounted on same shaft.