1. 1
Manish RK Sahu
Asst. Professor
Department of Mechanical Engg.
Automobile
Engineering
Shri Shankaracharya Institute of
Professional Management And Technology
UNIT - 03
3. Functions of Gear Box
Gear Box - Introduction
To provide variation in speed and torque.
To provide neutral condition to the
vehicle.
To provide reverse movement of vehicle
without changing the direction of rotation
of crankshaft.
4. 1. Air Resistance
Resistance to vehicle motion
It depends upon speed of vehicle, speed and direction of wind and body
profile of the vehicle. It is taken to proportional to the square of the vehicle
speed. when a body moves, the air around it generates a resistance in the
opposite direction of the movement. For vehicles, air resistance affects the
comfort of the passengers, the fuel consumption, the stability, and many
other performance factors.
grraeroaccel FFFFF
5. 2. Rolling Resistance
Resistance to vehicle motion
Composed primarily of
1. Resistance from tire deformation (90%)
2. Tire penetration and surface compression (
4%)
3. Tire slippage and air circulation around wheel
( 6%)
4. Wide range of factors affect total rolling
resistance
6. The magnitude of this force is Approximated as:
Rolling resistance of a vehicle is proportional to the component of
weight normal to the surface of travel
gVMC=P rrrr
MgFrr MgCF rrrr
VMC)10(2.72=P
VMC
3600
9.81
=P
rr
3-
rr
rrrr
where:
Prr = power (kW)
Crr = coefficient of rolling resistance
M = mass (kg)
V = velocity (KpH)
7. 3. Gradient Resistance
Resistance to vehicle motion
It depends upon slope of the road and mass of the vehicle. This
remains practically constant at all speed of the vehicle.
θg mg
Fg
θg
Composed of
◦ Gravitational force acting on the vehicle
gg mgF sin
10. Vehicle Speed vs. Engine Speed
o
crank
G
irN
V
60000
12
V = velocity , km/hr
r = wheel radius, m
Ncrank = crankshaft rpm
i = driveline slippage
GO = Overall gear reduction ratio
11. Tractive Effort
The force available at the contact between the
drive wheel tire and road is known as ‘tractive
effort’. The ability of the drive wheels to
transmit this effort without slipping is known
as ‘traction’. Hence usable tractive effort never
exceeds traction.
17. Sliding Mesh Gear Box
An obsolete type of
transmission or gearbox in
which the gears on the layshaft
are fixed to the shaft rigidly,
whereas the gears on the main
shaft can slide on it by means of
splines but are otherwise in
permanent rotational mesh
with the shaft.
18. Construction of Sliding Mesh Gearbox:
• The clutch shaft is connected to the engine output and rotates
when the engine rotates. A gear is mounted on the clutch shaft
which is connected with a gear of lay shaft.
• The lay shaft has several gears, one of which is connected to gear
of clutch shaft and others gears connect with different gear of
main shaft to obtain different gear ratio. Also, one gear in lay
shaft is reverse gear and has and idler gear which is placed
between the lay shaft gear and main shaft gear when operated.
• The main shaft has several gears and these gears can slide over
the main shaft to mesh with different gears of main shaft.
19. Working
• At first, the clutch shaft is driven by engine. It carries the engine output
and rotates in the same direction as that of engine. The gear connected
to the clutch shaft also rotates.
• As gear of clutch shaft rotates, the lay shaft gear which is connected to
the clutch shaft gear also rotates but in opposite direction.
• So the lay shaft rotates due to rotation of lay shaft gear that is rigidly
fixed in the lay shaft. Due to rotation of lay shaft other gears of lay shaft
also rotates as all the gears in lay shaft are rigidly fixed including the
reverse gear.
• The gears of main shaft are internally splined and the main shaft is also
splined, so the gears of main shaft can slide over it. The gear of main
shaft are shifted and meshed with different gears of lay shaft to obtain
different gear ratios required to face different road problem.
20.
21. Advantages
1 Since only one gear is in mess in sliding mesh gearbox so less
fluctuating loads on shafts causing less vibration and noise unlike
the constant mesh gearbox in which all gears are in constant mesh.
2. Its efficiency is more than constant gearbox as only one gear is in
mess unlike the constant mesh gearbox in which all gears are in
constant mesh.
3. Its manufacturing is easy as compared to constant mesh gearbox.
4. Its mechanism is simple
22. Disadvantages
1 Only spur gears can be used as gears are not in constant mesh like
constant mesh gearbox in which helical or herringbone gears can be used.
2 More effort is required to engage the gear as the gear has to be slided in
sliding mesh gearbox unlike constant mesh gearbox where only dog clutch
has to be slided for engagement of different gears.
3 Less life of gear as more wear and tear of gear is caused in sliding mesh
gearbox due to friction.
4 It takes more time and money to replace the gears if the gearbox fails but
in constant mesh gearbox only dog clutches are to be replaced at failure
which takes less time and money.
23. Constant Mesh Gear Box
Constant mesh gearbox is a type of
Transmission in which all or most of
the gears are always in mesh with one
another, as opposed to a sliding-gear
transmission, in which engagement is
obtained by sliding some of the gears
along a shaft into mesh. In a constant-
mesh manual gearbox, Gear ratios are
selected by small Clutches that connect
the various gear sets to their shafts so
that power is transmitted through
them.
24.
25.
26. Dog Clutch: -
The dog clutch couples the lay shaft and main shaft by interference and not by
friction.
Dog clutches are used to transmit appropriate gear ratio to the main shaft or output
shaft by coming in interference with pair of gears with suitable gear ratio.
There are usually two dog clutches in a Constant Mesh Gear Box.
27. Working
Constant gear mesh gearbox employed
helical gears for power transmission. The
gears are rigidly fixed in the lay shaft. The
gears in output shaft rotates freely
without engaging with shaft, thus not
transmitting power. The gears in both
shafts are always meshed together.
To engage the gears with output shaft dog
clutch is used. the dog clutch is shifted by
the selector fork moved by gear lever. To
provide reverse gearing a idler gear is
used . It involves “Double Declutching”.
28. Double Declutching
With this method, instead of pushing
the clutch in once and shifting directly to
another gear, the driver first engages the
transmission in neutral before shifting to the
next gear.
29.
30. Advantages
• The first and foremost benefit of the constant gear mesh is
the utilization of helical gears. The double helical gears and
the helical gears are extremely beneficial owing to their
quieter operating capabilities.
• There are various conditions which might cause harm. In
the case of constant mesh gearbox, any harm is suffered
entirely by the dog clutch teeth. The teeth belonging to the
gear wheels remain intact. This is not the case for sliding
mesh gear box.
• The other gear boxes are noisy and create an unwanted
friction.
31. Disadvantages
• It is less efficient than the others due to higher mesh teeth. Skill is
required for it.
• The double clutch mesh is required. This is required to have the
spinning movements of the shaft.
32. Synchromesh Gear Box
Synchromesh transmissions are a
further refined version of the
constant mesh system, although less
common. How it improves the
system is by adding another stage to
the process of connecting the gears
to the drive shaft through the dog
clutch.
Synchromesh gear box consists of a synchronizer
which resembles a doc clutch, the only variation
being that it first matches the speed of the driving
and driven gear and then meshes both the gears
33. A new component
was added to the
gears themselves
– the synchronizer
cone - and a
further moving
part called the
synchronizer ring
was introduced
that surrounded
the cone.
It splits the dog clutch in two – a gear fixed to the drive shaft called the synchronizer hub,
and a collar around the outside of it that could slide back and forth called the shift sleeve.
34. Salient features :-
The clutch shaft and main shaft are in-line
while the lay shaft is parallel to them.*
The main shaft gears are free to rotate , while
lay shaft gears are fixed to it.*
The system incorporates a synchronizing
mechanism which allows the member first
brought into friction contact
35. Synchronizers
In automobiles, a synchronizer is part of
a synchromesh manual transmission that
allows the smooth engagement of gears.
36. Working
The synchronizer is
placed between two
gears. So, we can use one
unit for two gears. G1
and G2 are the ring-
shaped members which
are having the internal
tooth that fits onto the
external teeth. F1 and F2
are the sliding members
of the main shaft. H1, H2,
N1, N2, P1, P2, R1, R2 are
the friction surface
37. First gear
When the driver push or pull the gear lever in order to select the
first gear which gives the maximum torque and minimum speed
and is used to move the vehicle from its initial state ,the
synchromesh device attached with the pair of meshed gears
having biggest gear of the main shaft and smallest gear of the lay
shaft equalizes the speed of the shafts by making frictional
contact with the pair and finally the first gear is obtained.
Second gear
This is the gear having lower torque and higher speed than first
gear and is obtained when the pair of gears having second largest
gear of the main shaft and second smallest gear of the lay shaft is
meshed by the corresponding synchromesh device.
38. Third gear
This gear having higher speed and lower torque than
second gear is obtained when the corresponding
synchromesh device attached to the pair of gear having
intermediate size gear of main shaft and intermediate
size of gear of lay shaft makes contact.
Fourth gear
It is the second highest speed gear which is obtained
when the corresponding synchromesh device attached
to the pair of meshed gears having second smallest
gear of main shaft and second largest gear of the lay
shaft makes contact
39. Reverse gear
It is the gear that reverses the direction of the output
shaft which in turn reverse the direction of the vehicle
with the help of the idler gear which is usually fit in the
middle of the lay shaft and main shaft and is obtained
when the idler gear makes contact with the gears on
the main shaft and lay shaft.
Note – The reverse gear does not have any
synchronizer mechanism, so the gearbox shaft rotation
is completely stop before engaging the reverse gear.
40. Advantages:
Smooth and Noise free shifting of gears which is
most suitable for cars.
No loss of torque transmission from the engine to
the driving wheels during gear shifts.
Double clutching is not required.
Less vibration.
Quick shifting of gears without the risk of
damaging the gears.
41. Disadvantages:
It is extortionate due to its high manufacturing cost
and the number of moving parts.
When teeth make contact with the gear, the teeth
will fail to engage as they are spinning at different
speeds which causes a loud grinding sound as they
clatter together.
Improper handling of gear may easily prone to
damage.
Cannot handle higher loads.
42. Automatic Transmission System
An automatic transmission, also
called auto, self-shifting
transmission, n-speed
automatic (where n represents its
number of forward gear ratios),
or AT, is a type of motor
vehicle transmission that
automatically changes the gear
ratio as the vehicle moves, meaning
that the driver does not have to shift
the gears manually.
43. Epicyclic Gear Set
An Epicyclic gear train (also known
as planetary gear) consists of
two gears mounted so that the center of one
gear revolves around the center of the other.
A carrier connects the centers of the two
gears and rotates to carry one gear, called
the planet gear or planet pinion, around the
other, called the sun gear or sun wheel. The
planet and sun gears mesh so that
their pitch circles roll without slip. A point on
the pitch circle of the planet gear traces
an epicycloid curve. In this simplified case,
the sun gear is fixed and the planetary
gear(s) roll around the sun gear.
44. Mode Role Gears
1.Forward Fast
Output Speed
Driving P
Driven R
Stationary S
2.Forward Very
Fast Output
Speed
Driving P
Driven S
Stationary R
3.Forward slow
Output Speed
Driving R
Driven P
Stationary S
4.Forward Very
slow Output
Speed
Driving S
Driven P
Stationary R
45. Mode Role Gears
5.Reverse Slow
Output Speed
Driving S
Driven R
Stationary P
6.Reverse Fast
Output Speed
Driving R
Driven S
Stationary P
46. Torque Converter
• A torque converter is a
type of fluid coupling
used for transferring
rotating power to the
transmission from the
motor.
• It is achieved in an
automatic transmission
by a hydraulic clutch.
47. • Its main functions are:
• 1.It transfers the power from engine to the
transmission input shaft.
• 2. It drives the front pump of the transmission.
• 3. It isolates the engine from the load when the
vehicle is stationary.
• 4. It multiplies the torque of the engine and
transmits it to the transmission. It almost doubles
the output torque.
Functions of Torque Converter
48. Working of Torque Converter
It has three stages of operations –
1. Stall: The engine uses power to the impeller when the vehicle is stall
(stop), but the turbine can not rotate. This is when the car is stationary
and the driver has kept his foot on the brake paddle so that it does not
move. Maximum torque multiplication takes place during this condition.
The impeller moves faster as the driver removes the base from the brake
paddle and releases the accelerator paddle, allowing the motor to roll.
There is a greater difference in the pump speed from the turbine in this
situation. The speed of the rotor is significantly higher than that of the
turbine.
49. 2. Acceleration: During acceleration, the speed of the turbine continues to
rise, but the impeller and turbine speed also differ greatly. The torque
multiplication is decreased as the turbine speed increases. The torque
multiplication is less than understanding conditions when the engine is
accelerated.
3. Coupling: This happens when the turbine reaches a speed of about 90%
of the impeller, which is known as the coupling point. The torque
multiplication is null and negative and the torque converter is compared
to a simple fluid relation. The locking clutch comes in at the coupling
point and locks the turbine to the converter’s impeller. This enables the
turbine and drives to move at the same speed. Only when the point is
reached is the clutch locked up. The stator also begins to rotate towards
the rotation of the rotor and of the turbine during the coupling.
50. Numericals
A
B C
DF
E
G
H
78 mm
𝐺𝑒𝑎𝑟 𝑅𝑎𝑡𝑖𝑜 𝐺 =
𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑓𝑖𝑟𝑠𝑡 𝑑𝑟𝑖𝑣𝑒𝑟 𝑔𝑒𝑎𝑟
𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑎𝑠𝑡 𝑑𝑟𝑖𝑣𝑒𝑛 𝑔𝑒𝑎𝑟
=
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑒𝑒𝑡 𝑜𝑛 𝑙𝑎𝑠𝑡 𝑑𝑟𝑖𝑣𝑒𝑛 𝑔𝑒𝑎𝑟
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑒𝑒𝑡 𝑜𝑛 𝑓𝑖𝑟𝑠𝑡 𝑑𝑟𝑖𝑣𝑒𝑟 𝑔𝑒𝑎𝑟
𝑚 =
𝑑
𝑇
51. Numericals
A Sliding mesh type of gear box with forward speed only is to be
designed. The gearbox should have the following gear ratio
available approximately 1.0, 1.5, 2.5, 3.9. The Centre distance
between the lay shaft and main shaft is 78 mm and smallest gear is
to have at least 16 teeth with a module of 3.25mm. Calculate the
number of teeth of various gears and exact gear ratio.
A
B C
DF
E
G
H
78 mm
52. A
B C
DF
E
G
H
78 mm
rA + rB = rH+ rG = rF + rE = rD + rC = 78
𝑚 =
𝑑
𝑇
𝑚 =
2𝑟
𝑇
=
2𝑟𝐴
𝑇 𝐴
rA =
m x TA
2
TA + TB = TH+ TG = TF + TE = TD + TC =
78 𝑥 2
𝑚
TA + TB = TH+ TG = TF + TE = TD + TC = 48
53. For First Gear (A-B-C-D)
G1 = 3.9 =
𝑁 𝐴
𝑁 𝐷
=
𝑇 𝐷
𝑇 𝐶
x
𝑇 𝐵
𝑇 𝐴
For Second Gear (A-B-E-F)
G2 = 2.5 =
𝑁 𝐴
𝑁 𝐹
=
𝑇 𝐹
𝑇 𝐸
x
𝑇 𝐵
𝑇 𝐴
For Third Gear (A-B-G-H)
G3 = 1.5 =
𝑁 𝐴
𝑁 𝐻
=
𝑇 𝐻
𝑇 𝐺
x
𝑇 𝐵
𝑇 𝐴
A
B C
DF
E
G
H
78 mm
54.
55. Numericals
A Sliding mesh type of gear box with forward speed is to be designed. The
gearbox should have the following gear ratio available approximately 1.0,
1.5, 2.5 And in reverse speed as 3.2. The Centre distance between the lay
shaft and main shaft is 84 mm and smallest gear is to have at least 12 teeth
with a module of 4 mm. Calculate the number of teeth of various gears and
exact gear ratio.
A
B G
H
F
E
C
D
84 mm
56. rA + rB = rF + rE = rD + rC = 48
𝑚 =
𝑑
𝑇
𝑚 =
2𝑟
𝑇
=
2𝑟𝐴
𝑇 𝐴
rA =
m x TA
2
TA + TB = TF + TE = TD + TC =
84 𝑥 2
𝑚
TA + TB = TF + TE = TD + TC = 42
A
B G
H
F
E
C
D
84 mm
57. For Second Gear (A-B-C-D)
G1 = 3.9 =
𝑁 𝐴
𝑁 𝐷
=
𝑇 𝐷
𝑇 𝐶
x
𝑇 𝐵
𝑇 𝐴
For First Gear (A-B-E-F)
G2 = 2.5 =
𝑁 𝐴
𝑁 𝐹
=
𝑇 𝐹
𝑇 𝐸
x
𝑇 𝐵
𝑇 𝐴
For Third Gear
G3 = 1.0
A
B G
H
F
E
C
D
84 mm
For Reverse Gear
GR = 3.2=
𝑇 𝐹
𝑇 𝐺
x
𝑇 𝐵
𝑇 𝐴
58. From Eqn. 3
2.5 =
𝑁 𝐴
𝑁 𝐹
=
𝑇 𝐹
𝑇 𝐸
x
𝑇 𝐵
𝑇 𝐴
𝑇 𝐹
𝑇 𝐸
=1.875
From Eqn 1
TF = 27
TE = 15
