Mechanical And Industrial Engineering Department
Vehicle Transmission Drives (Gear Box)
1. Introduction
The gearbox is a mechanical device used to increase the output torque or to change the speed (RPM)
of a motor. The shaft of the motor is connected to one end of the gearbox and through the internal
configuration of gears of a gearbox, provides a given output torque and speed determined by the gear
ratio
High torque is required to start the vehicle from rest, accelerating, hill climbing, pulling a load and
facing other resistances. But the IC engine operates over a limited effective speed range which
produces a comparatively low torque. In such a situation, the engine is responsible for the stall and
the vehicle rests if the speed falls below the limit.
The torque developed by the engine is increasing within limits with the increase of engine speed and
reaches a maximum value at some predominant speed. If the engine directly connects to the driving
axle, the engine speed may reduce.
Due to the variable nature of the vehicle resistance resulting in load and gradient changes, it requires
that the engine power should be available over a wide range of road speeds. Hence, for this reason,
the engine speed maintain by using a reduction gear resulting in the road wheels rotating at a proper
speed suited to the operating conditions of the vehicle.
Therefore, a single torque multiplication in the rear axle must be interposed and a variable
multiplication factor in the gearbox is provided for this purpose.
2. Objectives
 To design gear box which uses simple mechanism to reduce/increase speed, increase torque
and change the direction of rotation easily.
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3. Types of gear box
There are two main types of Gearbox in an Automobile:
3.1.Manual transmission and
3.2. Automatic transmission
But for this discussion, we only focussed on the manual transmission.
3.1.
Manual transmission
In a manual transmission type of gear system, the driver gets total control and chooses all gear
manually operating both a movable gear selector and a driver-operated clutch. This type of
transmission is also known as a stick shaft or a standard transmission.
This manual transmission allows the driver to either drop a gear to make the faster process or choose
an increased gear to conserve fuel.
Following are the types of gearbox used in modern vehicles:3.1.1. Sliding mesh type gearbox
3.1.2. Constant-mesh type gearbox
3.1.3. Synchromesh gearbox
3.1.4. Epicyclic gearbox
3.1.1. Sliding Mesh Gear Box
It is the simplest type of gearbox. The arrangement of gears is in a neutral position. The gear housing
and bearing are not shown. The clutch gear is fixed to the clutch shaft. It remains always connected to
the drive gear of the counter-shaft.
Three other gears like first speed, second speed, and reverse speed gear are also rigidly fixed to the
countershaft or also known as lay shaft. Two gears mounted on the splined main shaft can be slid by
the shifter yoke when the shift lever is operated.
The gears are connected to the corresponding gears of the countershaft. A reverse idler gear is fixed
on another shaft and remains connected to the reverse gear of the countershaft
The main parts of the sliding mesh gearbox are:3.1.1.1.Shafts
 Main Shaft-It is the shaft used as an output shaft in a sliding-mesh over which the sets
of gears with internally splined grooves are arranged in an organised fashion.
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 Clutch Shaft-It is the shaft that is used to carry engine output to the transmission box
with the help of engaging and disengaging clutch which is mounting at the engine
end, gear or a pair of gear is mounted over this shaft which is used to transmit
rotational motion to the lay shaft.
 Lay Shaft-It is the shaft having gears mounted over its outer surface and is in
continuous rotation with the clutch shaft as one gear of this shaft is always in contact
with a gear on the clutch shaft, it is used as an intermediate shaft that provides the
meshing of the gears of the main shaft in order to transmit appropriate output to the
final drive.
3.1.1.2.Gears
Two types of gear were usually used in sliding mesh gearbox.
 Spur gear- This is the type of gears having straight-cut teethes over its surface, straight
teethes proving maximum meshing area.
 Helical gear- This is the type of gear in which the teethes are cut in an angular fashion,
unlike spur gears this type of gears are smooth and less noisy
3.1.1.3.Gear lever
 It is the selecting mechanism operated by the driver in order to select the appropriate gear
ratio, this liver is connected to the main shaft along with the selector forks.
3.1.2. Constant Mesh Gearbox
In this type of gearbox, all the gears of the main shaft are in constant mesh with the corresponding
gears of the countershaft (lay shaft). In this case sliding two dog clutches are provided on the main
shaft. The one sliding dog clutch is placed in between the clutch gear and the second gear, and the
other is placed in between the first gear and reverse gear. All gears are free on the splined main shaft.
Dog clutch slides on the main shaft to rotate with it. All the gears on the countershaft are fixed with it.
When the left-hand dog clutch is made to slide to the left through the gearshift lever, it meshes with
the clutch gear and the top speed gear is achieved.
When the left-hand log clutch meshes with the second gear, the second speed gear is obtained.
Likewise, by sliding the right-hand dog clutch to the left and right, the first gear and reverse gear are
obtained.
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In this type of the gearbox, all the gears are in constant mesh, they are safe from being damaged and
unpleasant grinding sound does not occur while engaging and disengaging them.
The main components of constant mesh gear box
3.1.2.1.Shafts – There are 3 shafts.
 Main Shaft (output shaft):- It is the splined shaft over which the dog clutches along with
gears are mounted. Gears on this shaft are free to rotate.
 Lay Shaft or Counter Shaft: -It is an intermediate shaft between the Main Shaft and Clutch
Shaft. The gears of counter shaft are in constant mesh with gears of main shaft. Also the gears
of counter are shaft are not free to rotate as they are directly connected to the Counter Shaft.
 Clutch Shaft:-The clutch shaft carries the engine output to the gearbox but act as input for the
gearbox.
3.1.2.2.Dog Clutch- The dog clutch couples the lay shaft and main shaft by interference and not by
friction.
3.1.2.3. Gears- Gears of constant mesh gearbox come in pairs. All gears of lay shaft or counter shaft
are
always
paired
with
gears
of
main
shaft
or
output
shaft.
Two type of gears are used in constant mesh gearbox: Helical Gears- These gears have angular cut teeth over cylindrical cross-section metal
body.
 Bevel Gears- These gears have angular cut teeth over conical cross-section metal body.
3.1.2.4. Gear Lever-it is the lever used for shifting or siding the dog clutches over the main shaft and is
operated by the diver.
3.1.3. Synchromesh Gearbox
Modern cars use helical gears and synchromesh devices in the gearboxes, that synchronize the
rotation of gears that are about to mesh. This eliminates clashing of the gears and makes gear shifting
easier.
This type of gearbox is similar to the constant mesh gearbox. The synchromesh gearbox is provided
with a synchromesh device by which the two gears to be engaged are first taken into frictional contact
which adjusts their speed after which they are engaged easily.
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In most vehicles, the synchromesh devices are not fitted to all the gears. They are fitted only on the
top gears. Reverse gear, and in some cases the first gear, do not have synchromesh devices. Because
they are intended to be engaged when the vehicle is stationary.
When the gear lever is moved the synchromesh cone meets with a similar cone on the pinion. Due to
friction the rotating pinion is, made to rotate at the same speed as the synchromesh unit. To give a
positive drive further movement of the gear lever enables the coupling to override several
springs loaded balls and the coupling engages with the dogs on the ride of the pinion. Since both
pinion and synchromesh units are moving at the same speed, this engagement is necessary before
engaging the dog teeth so that the cones have a chance to bring the synchronizer and pinion to the
same speed
Synchromesh Gearbox consists of the following key components
3.1.3.1. Engine Shaft
3.1.3.2.Lay Shaft
3.1.3.3. Output Shaft
3.1.3.4. Gears on Lay shaft
3.1.3.5. Gears on Output shaft
3.1.3.6.Cone
3.1.3.7.Synchronizer Hub
3.1.3.8.Synchronizer Ring
3.1.4. Epicyclic Gearbox
In an ordinary gearing, the axes of the various gears are fixed, the motion of the gears being simply
rotations about their own axes. In epicyclic gearing, at least one gear not only rotates, about its own
axis but also rotates bodily about some other axis.
These types of gearbox are the most widely used automatic transmission system. In an automatic
transmission system, there is only an accelerator and brake will be provided. So there will not be any
clutch pedal or gear lever available on the vehicle
Epicyclic Gearbox Parts
The three basic components of the epicyclic gear are:3.1.4.1 Sun gear:- it is the central gear
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3.1.4.2 Planet carrier:- Holds one or more peripheral planet gears, of the same size, meshed
with the sun gear
3.1.4.3 Annulus or Ring Gear:- An outer ring with inward-facing teeth that mesh with the
planet gear or gears
 In many epicyclic gearing systems, among the three basic components, one component is
held stationary; one component is input, provide power to the system and last component
is output, receive the power from the system. The ratio of input rotation to output rotation
is dependent upon the number of teeth in each gear, and upon which component is held
stationary.
4.
Working mechanism
The working of a manual transmission system contains a set of gears along with a pair of shafts
which is the input and output shafts. The gear on the first shaft engages with those on the other
shaft. The ratio between the selected gear on the input shaft and the engages on the output shaft
determines the overall gear ratio for that gear.
Gears are engaged in manual transmission system by moving the shift lever. The engagement is
done by the linkages that control the movement of the gears along the input shaft. Cars with four
gears or speed have two linkages and cars with five or six speeds have uses three linkages. These
linkages changes by moving the shift lever left and right.
Clutch plays an important role in the working of the manual transmission as disconnect the
engine from the input shaft of the transmission when pressed. It frees the gears on the input shaft
causing it to easily move as the engine is sending torque through the input shaft. This caused the
engagement. The clutch is said to be disconnected when the clutch lever is not press. Once the
clutch disconnects the power from the engine to the transmission, the driver easily selects the
gear and release the clutch. Releasing the clutch allowed engine power to be reengaged to the
input shaft which makes the car to move at the selected gear ratio.
A gearbox contains gears of different sizes. The first gear is the biggest in a gearbox and provides
maximum torque output while producing minimum speed. All the gears between 1st and last gear vary
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in size, in a decreasing ratio. Thus, it provides a varying combination of pulling ability and speed. The
gearbox, basically, improves the vehicle’s driveability in all conditions.
Generally, there are two sets of gears in a conventional gearbox – the input & the output. The input
gears are fixed on the countershaft, making them single units. It drives the individual gears on the
main shaft, which rotates freely on the bearings. Thus, the gearbox passes the drive to the wheels
depending upon the gear which engages on the main shaft. Furthermore, when you push the shiftersleeve towards the desired gear, that gear locks onto the main shaft and rotates it. Thus, the main shaft
rotates at the engaged gear’s speed and provides the output according to the engaged gear’s ratio.
THE RECOMMENDATION: It is better the working mechanism be sequential like the input
power from the engine drives the initial gear(gear mounted on the
motion is transferred to the lay shaft
clutches shaft )and this
by the gear mounted on it. due to this gear the lay
shaft rotates ,the rotation of lay shaft is transferred to the main shaft .the
output motion is
depending on the gears that mesh with the gear in the lay shaft using synchronizer or slider.
5.
Main components of gear box
5.1. Clutch shaft/driving shaft/input shaft
A clutch shaft is a shaft that takes power from the engine to supply another shaft. The clutch shaft or
driving shaft is connected through the clutch and when the clutch is engaged, the driving shaft also
rotates. Only one gear is fixed on the clutch shaft and this engine rotates with the same speed as the
crankshaft. In addition, the driving shaft and main shaft are in the same line.
5.2. Countershaft/lay shaft
The counter shaft is a shaft that connects directly to the clutch shaft. It has gear which connects it to
the clutch shaft as well as the main shaft. It can be run at engine speed or below engine speed
according to gear ratio.
5.3. Main shaft /output shaft
The main shaft or output shaft that rotates at different speeds and also provides the necessary torque
to the vehicle. The output shaft is a splined shaft, so that the gear or synchronizer can be moved to
engage or disengage.
5.4. Bearings
The bearings are required to support the rotating part and reduce friction. The gear box has both a
counter and main shaft which is supported by the bearing.
5.5. Gears
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Gears are used to transmitting the power from one shaft to another shaft. The amount of torque
transmitted through the gears depends on the number of teeth and the size of the gears. Higher the
gear ratio, higher the torque / acceleration and lower the speed. All gears except those on the main
shaft are fixed to their respective shafts; they can slide in any of the directions along the shaft.
5.6. Gear selector fork
Gear selectors are simple devices that use a lever that selects gears to engage in disengages
mechanisms. The motion of the lever slides the engaging part on the shaft. It depends on the type of
gearbox whether the lever slides the gear or synchronizer that is already forged along the main shaft.
6.
Advantages and disadvantages of each type of gear box
6.1. A) Advantages of Sliding Mesh Gearbox
 Since only one gear is in mess in the sliding mesh gearbox so less fluctuating loads on
shafts causing less vibration and noise
 Unlike the contact mesh gearbox in which all gears are in constant mesh.
 Its efficiency is more than a constant gearbox as only one gear is in mess, unlike the
contact mesh gearbox in which all gears are in constant mesh.
 Its manufacturing is easy as compared to a constant mesh gearbox.
 The mechanism of this gearbox is simple compared to Constant Mesh Gearbox and
Synchromesh Gearbox.
6.1. B) Disadvantages of Sliding Mesh Gearbox

Only spur gears can be used as gears are not in a constant-mesh gearbox in which helical
or herringbone gears can be used.

More effort is required to engage the gear as the gear has to be slide in sliding mesh
gearbox, unlike constant mesh gearbox where only dog clutch has to be a slide for
engagement of different gears.

Less life of gear as more wear and tear of gear is caused in sliding mesh gearbox due to
friction.

It takes more time and money to replace the gears if the gearbox fails

The Mechanical Efficiency was very low in the case of Sliding Mesh Gearbox.

The driver requires considerable skill for changing the gears because the gears of main
shaft and lay shaft are away from each other.
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6.2. A) Advantages of Constant Mesh Gearbox
 There is no need for straight spur gears because the gears were always in mesh in the case
of constant mesh gearbox. Instead, helical gears will be used which are quiet running
 Helical gears are used in this system to avoid vibrations during the engagement.
 During engagement and disengagement, the wear of dog teeth is reduced because all the
teeth of dog clutches are involved in the meshing when compared to the sliding gears
where only 2 or 3 teeth were under the mesh.
 The noise level was very low and Mechanical Efficiency was very high compared to
Sliding Mesh Gearbox.
 Since the gears are engaged by dog clutches, if any damage occurs while engaging the
gears, the dog unit members get damaged and not the gear wheels
6.2. B) Disadvantages of Constant Mesh Gearbox
 To avoid little vibrations taking place during the engagement of dog clutch with the gears
on the main shaft, it is advised to use synchronizers which can reduce the effect of the
vibrations.
 Double declutching was the biggest disadvantage of Constant Mesh Gearbox
 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
6.3. A) Advantages of synchromesh gearbox
 It provides smooth & quite shifting of gears due to use of Synchronizer Ring and Sliding
sleeve.
 No need of skill to operate.
 It has more power transmission capacity as compared to constant mesh gearbox.
 No Double Declutching is required.
 The biggest disadvantage of Constant Mesh Gearbox has been solved by the
Synchromesh Gearbox i.e. Double-Declutching.
 The noise problem during the engagement of Gears has been minimized in the
Synchromesh Gearbox.
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 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.
 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
6.3. B) Disadvantages of synchromesh gearbox
 It is Bulky gearbox.
 It is Expensive.
 It requires more space
 Improper gear changing will lead to the failure of gears.
 It is expensive compared to the other gearbox due to the usage of Synchronizers.
 Where cost is the consideration, synchronizers can only be used to higher gears whereas
dog clutches are used for lower gears and Reverse gears.
 When teeth make contact with the gear, the teeth will fail to engage as they are spinning
at different speeds which cause a loud grinding sound as they clatter together.
 Improper handling of gear may easily prone to damage.
 Cannot handle higher loads
6.4. A) Advantages of Epicyclical Gearbox
 High reduction ratios
 Compact and lightweight with high torque transmission
 High radial loads on the output shaft
 It is quieter in operation
 Uniform distribution of load over all gears having greater tooth contact.
 All gears are constantly in mesh, so a change of one gear to another is possible without
any loss.
 It Exist with higher gear ratio
 It increase the driver comfort
 It is also used in bicycle for controlling power during pedalling
6.4. B) Disadvantages of Epicyclical gear systems
 Design complexity
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 Assembly of gears is limited to specific teeth per gear ratios
 Efficiency calculations are difficult
 Driver and driven equipment must be in line to avoid additional gearing
 The cost of manufacturing is high.
 Constant lubrication is required
 High bearing loads
 Continuously lubrications is required
7. Design procedure for Gear Box
One has to follow a procedure to complete a task and the design of an important part of an automobile.
Follow these procedures to have an idea about how to design a complete Gear Box with each
component's individual design.
7.1. Determine the maximum and minimum speeds of the output shaft. This will determine the
complete speed ratio. Calculate the number of steps or speed reduction stages for this range. This
depends on the application as well as space optimization. Higher reduction stages require more
space because of more number of gears and shafts requirements.
7.2. Select the type of speed reducer or gearbox based on the power transmission requirements, gear
ratio, positions of the axis, space available for speed reducer. Also, make sure that for low gear
ratio requires single-speed reduction. Select worm gears for silent operation and bevel gears for
intersecting axis.
7.3. Determine the proggression ratio which is ratio maximum speed and minimum speed of output
shaft of Gear Box. The nearest progression ratio should be a standard one and is taken either from
R 20 or R 40 series.
7.4. Draw the structural diagram and kinematic arrangement indicating various arrangement
possibilities during speed reduction or increment.
7.5. Select materials for gears so that gear should sustain the operating conditions and operating load.
Normally cast iron is chosen for housing and cast steel or other alloy can be selected as per the
load requirements.
7.6. Note down the maximum power output in Horse Power (H.P) or transmission power and
revolution per minute of short i.e. rpm of each shaft.
7.7. Determine the centre distance between the driven and driver shaft based on the surface
compressive stress.
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7.8. Determine the module of gear by beam strength as well as fix the number of teeth required.
7.9. Calculate the diameter of the shafts by torque requirements and bending moment considerations.
7.10. Calculate the key size, shape or type of transmission keys for each gear.
7.11. Select appropriate fit and tolerance for mating parts like shaft and gear.
7.12. Select bearings types based on the loading and operating conditions. Also, make sure to include
consideration of maximum speed and expected life of gears and gearboxes. Normally ball
bearings and roller bearings are used for the small size of gearbox because they have lower thrust
requirement due to low-speed reduction.
7.13. Make the shaft stepped or provide collar to prevent axial displacement of bearing and gear.
7.14. Provide suitable clearance between gears and walls of the housing of a gearbox and based on
these considerations design the casing/housing of the gearbox.
7.15. Complete the design of the casing. One should provide fins if necessary to have increased heat
transfer by convection and conduction. Put inspection hole/manhole as well as a drain hole to
drain lubricating oil. Also, provide an oil level indicator to have the proper amount of oil during
operation, if not put, this will lead to failure of gear and shaft due to overheating or due to
frictional failure. Provide transportation hooks for transporting the gearbox from one place to
another or during installation and maintenance. Also, design gear changing lever accordingly.
7.16. Draw neat and clean working drawings in suitable software like AutoCAD, Pro-Engineer etc.,
indicating required details during manufacturing or assembly.
7.17. One can also perform finite element analysis of the complete Gear Box after it completely
designed
8.
Conclusion
Generally,
9.
References
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