The document discusses different types of automobile transmission systems, including manual transmissions like sliding mesh gearboxes, constant mesh gearboxes, and synchromesh gearboxes, as well as automatic transmissions. It provides details on the construction and operation of each type of manual transmission, focusing on how they allow the driver to change gears in order to adjust the vehicle's speed and torque output based on driving conditions. The synchromesh gearbox is highlighted as the most advanced type of manual transmission that uses synchronizers to smoothly engage gears that are rotating at different speeds.

1. AUTOMOBILE ENGINEERING
Unit-III- Transmission system of
Automobile
(Gear Box)
By,
Dr.S.SURESH,
Assistant Professor,
Department of Mechanical Engineering

2. WHAT IS GEAR BOX
 When we drive the vehicle, we do not drive the
vehicle always at the same speed. The vehicle must
be operated at a lower speed when it needs more
pulling power.
 Moreover, based on the operating conditions, the
vehicle requires different speeds and torque.
 To perform this, a box containing different gears,
shafts, and bearings is required. This box is called the
gear box.

3. NECESSITY OF GEAR BOX
1. For reducing the engine speed and increasing the
torque.
2. For reducing the engine torque and increasing speed.
3. For the selection of speed levels according to the
driver’s requirement.
4. For pushing the vehicle in the backward direction.
5. For neutralizing the vehicles when there is no need to
transmit the engine power

4. TYPES OF GEAR BOX
 Manual Transmission
Sliding Mesh Gear box
Constant Mesh Gear box
Synchromesh Gear box
 Automatic Transmission

5. 1. SLIDING MESH GEAR BOX
 It is simplest type of gear box
 spur 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 (or) lay shaft
 In a four speed gear box (which includes one reverse gear), the
counter shaft has four gears which are rigidly connected to it.
 Clutch shaft has one gear and main shaft has two gears.
 The 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.

6. CONT..
 The clutch gear is rigidly fixed to the clutch shaft. It is
always connected to the counter shaft drive gear.
 The gears on the main shaft can slide by the shifter
yoke by operating the shift lever . These gears can be
meshed with corresponding gears on the counter shaft
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 (third) shaft and is always in mesh with
reverse gear on counter shaft.

7. LINE DIAGRAM OF SLIDING MESH GEAR BOX

9. First gear
First Gear is used at the time when vehicle starts its movement in forward
direction.
Provides maximum torque and minimum speed.
The smallest gear on the layshaft meshes with the biggest gear in the main
shaft.
Second gear
Second Gear is obtained when the second-largest gear of the second smallest
gear of layshaft meshes with the middle size gear of the main shaft.
Provides lower torque and higher speed than First Gear.
Third gear
The third gear is the last gear or top gear of the Sliding Mesh Gearbox. This
gear is obtained when the biggest gear of layshaft meshes with the smallest
gear of the main shaft.
Provides maximum speed and minimum power.
Reverse gear
Reverse Gear is used when the vehicle needs to move in the opposite
direction.

10. 2. CONSTANT MESH GEARBOX
 All the gears on the layshaft and main shaft are in constant
mesh with each other and the dog clutch decides the
transmission of the required gear to be engaged.
 The speed of main shaft gears and the dog clutch must be
equal for smooth engagement
 Components
1.Shafts
Clutch shaft
Lay Shaft
Main Shaft
2. Gear Lever
3.Dog clutch
4.Gears

11. LINE DIAGRAM OF CONSTANT MESH GEARBOX:

12. *The vehicle moves slowly in the first
gear & acquires maximum torque.
*Driving shaft gear (layshaft) is small
compared to the gear of the driven shaft
(Main Shaft).
*The dog clutch(D1) slides towards its
left on the main shaft and meshes with
the 1st gear.
* The Torque acquired is smaller and the
speed acquired is larger than the first gear.
* The dog clutch (D2) slides towards its right
and engages with the 2nd gear on the main
shaft.

13. *provides Maximum speed and
Minimum Torque.
*The dog clutch (D2) slides
towards its left and engages with
the 3rd gear on the main shaft
which is in direct contact with the
clutch shaft.
*Idler gear is to be placed in
between the bigger diameter gear
of the main shaft and smaller
diameter gear (G) of the layshaft.

14. 3. SYNCHROMESH GEARBOX
 Latest version of Constant mesh type.
 It is a manually operated transmission in which change
of gears takes place between gears that are already
revolving at the same speed.
 In this type of gearbox, gears can rotate freely or it is
locked on layout shaft.
 Synchromesh is an improvement on dog clutch.
 The synchronizer is the main part of this gearbox that
stabilizes the speed.
 A synchronizer is a kind of clutch which lets
components turning at different speeds.
 To synchronize the speeds cone friction is used.

15. PRINCIPLE:
 In a gearbox, there is always a difficulty in engaging the stationary
gear with the gears already rotating at a high speed.
 The principle states that ”Before engaging the gears they are
brought in frictional contact with each other and after
equalizing the speed the engagement is done.”
Construction:
 The synchronizer is placed between two gears. So, we can use one
unit for two gears.
 G1 and G2 are the ring-shaped members who 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.

16.  Main shaft Gears
 Lay Shaft Gears
 Clutch Shaft
 Cone Synchromesh
 Synchronizers

18.  Advantages
* The noise problem during the engagement of Gears has
been minimized in the Synchromesh Gearbox.
* Due to the presence of Synchronizers, the quick shifting of
gears is possible.
* During transmission, there is no loss of Torque due to the
presence of synchronizers.
 Disadvantages
* Improper gear changing will lead to the failure of gears.
* It is expensive compared to the other gearbox due to the
usage of Synchronizers.