What is Gear Box
The road conditions are not uniform
whenever we are in travel in the vehicle.Moreover we do not drive the vehicle always at the same speed. The vehicle must be operated at lower speed when it needs more pulling power. Moreover based on the operating conditions, the vehicle
requires different speed and torque. To perform this, a box containing different gears, shafts and bearings is required. This box is called as the gear box. It can be also called as teeth wheel box. By using this box, the driver can achieve different speeds and torques for a vehicle. Gear box is the one which comprises of various teethed gears for different vehicle speed conditions.
Gear box location
Gear box is located at the second
place of the vehicle’s transmission system.In the vehicle after the engine, clutch is located. After the clutch gear box is located. It is located in between the clutch and the propeller shaft.
Needs 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.
Principle of gear box mechanism
Gear box works on the principle
of lever mechanism.
Types of gear box
The following are the gear boxes
used in automobiles.
1. Sliding mesh gear box
2. Constant mesh gear box
3. Synchromesh gear box
4. Epicyclic gear box
5. Automatic gear box
There are many types of gear boxes
available. The important gear boxes are discussed below.
Sliding mesh gear box
Sliding mesh gear box was used
in earlier day’s vehicles. Image
Represents the details of the sliding mesh gear box. In this gear box spur gears are present. This box is suitable for heavy vehicles. By sliding the gears this type of gearboxes move the gears and mesh them and hence it is called as sliding mesh gear box.
Construction
In the gear box, lay shaft or counter
shaft is present. This shaft is supported by the bearings in the gearbox. The gear teeth are fixed as stationary in this lay shaft. In the main shaft, splines are made in such a way that the gears are free to move on the slots. The gear teethed wheel
(1) in the clutch shaft is always in mesh
with the gear in the lay shaft wheel (2).The gear (7) in the lay shaft is always in mesh with the idler gear (8). A three speed sliding mesh gear box is shown in the image
Working principle
a) Neutral position
The gear teeth (1) in the input
(clutch) shaft is in mesh with the counter shaft gear 2. Hence the counter shaft rotates. However the gears (3 or 4) are not in mesh with the output shaft gears (5 or 6) as shown in the image. Hence the power will not be transmitted to the output shaft. This condition is called as neutral position.
b) First gear
The gear (1) in the clutch shaft is
in mesh with the gear in the counter shaft (2). As the gears 3 and 4 are in the counter shaft they also rotate. When the gear shift fork is moved towards left as shown in the image. to engage the gear 5 in the output shaft with the gear 4 in the lay shaft,then the output shaft rotates. The rotary
power is now transmitted to the output shaft from gear 1-2-4-5. Now we can get first gear ratio 3:1.
c) Second gear
During second gear, the gear shift
fork is moved right as shown in image to mesh the gear 6 in the output
shaft with gear 3 in the lay shaft gear 1, 2,3 and 6 will get contact and rotate. Now the power is transmitted from the gear 1-2-3-6. Now the speed becomes more than the first gear and the gear ratio of 2:1 is achieved.
d) Third gear
Dog Clutch is located on the gear
1 in the input (clutch) shaft and the
gear 6 in the output shaft as shown in the image . Now by moving the gear
shift fork on the left as shown in Figure the dog clutch is allowed to mesh with the dog clutch in the clutch shaft.Now the output shaft rotates at the speed same as the clutch (main) shaft. This is the maximum speed and the gear ratio in this condition is 1:1. The rotational power is now directly transmitted from the clutch shaft to the output shaft.
e) Reverse gear
For reverse gear, the gear shift fork
is moved towards the right direction as shown in the image. Hence the
gear 5 in the output shaft is in mesh with the gear 8 in the lay shaft which is the idler gear. Now the power transmission is from gear 1-2-7-8-5 to the output shaft.
Advantages of sliding mesh gearbox
1. Simple in construction
2. Simple in maintenance
3. Suitable for heavy vehicles
4. Low cost
5. Higher mechanical efficiency
6. Production cost is low
Disadvantages
1. Gear changing is difficult
2. Noisy in operation
3. Experience is required for changing
gear
4. The edges of teeth wear rapidly.
Constant mesh gear box
In constant mesh gear box the
teeth in the main shaft and lay shaft are always in constant mesh with each other.Hence it is called as constant mesh gear box. Helical gears are used in this type of gear box. The gear arrangements in the constant mesh gear box can be seen
in this image.
Construction
In the constant mesh gear box the gears 6, 7 and 8 in the output shaft are
always in mesh with the gears 5, 4 and 3 in the lay shaft. In the clutch shaft and in the lay shaft there are gears 1 and 2 respectively which are always in mesh with each other. The gears in the main shaft are supported by bushes. Gear 5 in the lay shaft is in mesh with an idler gear.After the clutch gear in the clutch shaft and before the reverse gear in the main
shaft there are dog clutches (D2 and D1 respectively) present. These dog clutches are allowed to move on the slots made on the main output shaft. All the shafts in the gear box are supported by the bearings in the housing.
Working principle
Neutral position
During neutral condition both the dog clutches (D1 and D2 ) are not in mesh with any gears as shown in Figure.All gears in the shafts 1,2 & 3,4 & 5,6 &
7,8,9 rotate. However the main output
shaft does not rotate as the dog clutches are not in engagement with any of the gears. Hence rotational power is not transmitted.
First gear
By actuating the gear shift lever,the dog clutch D1 is moved towards left side to engage gear 7 in the main shaft
as shown in Figure. Now the rotational power is transmitted to the main shaft through the dog clutch.Now the speed is low. The power transmission from the clutch shaft to the output shaft is by gear1-2 is means rotational power is transmitted to main shaft by 1-2-4-7-D1
.
Second gear
For obtaining the second gear, by actuating the gear shift lever, the dog clutch D2 is moved towards right as shown in Figure to engage the gear 8 in the output main shaft. Now the rotational power is transmitted to the main shaft through the dog clutch D2
.Now the speed of the main shaft is higher than the speed of the first gear. The power transmission from the clutch shaft to the output shaft is through the gears1-2-3-8-D
Third gear
To achieve the third gear the dog
clutch D2 is moved left to engage with
the clutch shaft gear directly. Now the
output shaft rotates at the speed same
as the clutch shaft speed. The rotational power is transmitted to the main shaft directly through the gear1-D2
Reverse speed
To obtain the reverse gear, the dog
clutch D1 is actuated to move right towards the reverse gear 6 to engage as shown in Figure. As the idler gear is placed between the gears 5 in the lay shaft and 6 in the main shaft the idler gear changes the direction of rotation of the gear 6 in the main shaft. Now the vehicle moves in the
reverse direction. The power transmitted from the cluth shaft to the main shaft is through gears 1-2-5-Idler-6-D1
Advantages of constant mesh gear box
1. Gear changing is simple
2. Less noise
3. Less chance for the teeth of the gears
to break up.
4. Smoothness in operation
5. Low maintenance
Disadvantages
1. For changing gears from one speed to
other speed, double de clutching has
to be done.
2. More wear
3. Possibility of wear in dog clutch.
Synchromesh gear box
Synchromesh gear box is similar in construction as constant mesh gear box but differs slightly. Instead of dog clutch used in constant mesh gear box, synchronizing unit is used in synchromesh gear box. In the synchromesh mesh gear box the gears 6, 7 and 8 in the output shaft are always in mesh with the gears 5, 4 and 3 in the lay shaft. In the clutch shaft and in the lay shaft there are gears 1 and 2 respectively which are always in mesh with each other. The gears in the main shaft are supported by bushes. Gear 5 in the lay shaft is in mesh with an idler gear. After the clutch gear in the clutch shaft and before the reverse gear in the main shaft there are synchronizers (S2 and S1 respectively) present. These synchronizers are allowed to move on the slots made on the main output shaft. All the shafts in the gear box are supported by the bearings in the housing.
Working principle
The layout of the synchromesh gear
box is shown in Figure. The operation of the synchromesh gear box for different vehicle speed is explained below
.Neutral position
During neutral condition both the synchronizers (S2 and S1) are not in mesh with any gears as shown in Figure.
All gears in the shafts 1,2& 3,4 & 5,6 & 7,8,9 rotate. However the main output shaft does not rotate as the dogclutches are not in engagement with any of the gears. Hence rotational power is not transmitted.
First gear
By actuating the gear shift lever,the synchronizer S1 is moved towards left
side as shown in Figure 3.5.3.2 to engage gear 7 in the main shaft.Now t he rot at i ona l p owe r is transmitted to the main shaft through the dog clutch. Now the speed is low.The power transmission from the clutch shaft to the output shaft is by gear 1-2 is means rotational power is transmitted to main shaft by 1-2-4-7-S1.
Second gear
For obtaining the second gear, by actuating the gear shift lever, the synchronizer S2 is moved towards right to engage the gear 8 in the output main shaft. Now the rotational power is transmitted to the main shaft through the synchronizer S2.Now the speed of the main shaft is higher than the speed of the first gear. The power
transmission from the clutch shaft to the output shaft is through the gears 1-2-3-8-S2
Third gear
To achieve the third gear the synchronizer S2 is moved left to engage with the clutch shaft gear directly. Now the output shaft rotates at the speed same as the clutch shaft speed. The rotational power is transmitted to the main shaft directly through the gear 1-S2
Reverse Speed
To obtain the reverse gear, the synchronizer S1 is actuated to move right towards the reverse gear 6 to engage as shown in Figure. As the idler gear is placed between the gears 5 in the lay shaft and 6 in the main shaft the idler gear changes the
direction of rotation of the gear 6 in the main shaft. Now the vehicle moves in the reverse direction. The power transmitted from the clutch shaft to the main shaft is through gears 1-2-5-Idler-6-S
Synchronizing unit
In synchronizing unit two cone shaped parts are present. One cone is a part of the collar and the second cone is a part of the gear. Both the cones rotate at different speeds. An exploded view of the synchronizer unit is shown in Figure.When the cone 2 is rotated, cone 1 start to rotate and move towards the cone 2 and be in contact with the cone 2.Because of this action friction occurs in the contact area of the cones and hence it
increases the speed of gear. Now the both cone speeds come to the same. Further moving the collar makes the outer drum to contact with the dog clutch in the gear.Now the rotational power is transmitted to the main haft. By the same way it moves on left hand side and contacts with the another gear and transmits the power to the main shaft. The cone 2 is locked
under different states by the ball and
spring in the cone 1. It is connected to
the gear changing fork in the slot which is located in the outer sliding sleeve.
Uses of synchromesh unit
1. For operating the gears smoothly by
synchronizing two different speeds of
the gears.
2. For changing gears easily
3. No need of double de clutching
4. For transmitting power efficiently
5. Gears will not wear rapidly
6. Less noise in operation
De merits of synchromesh unit
1. Low cost
2. Maintenance is difficult
3. Difficult repair
