Transfer Case/ box
• A transfer case is a part of the drive
train of four-wheel-drive, all-wheel-
drive, and other multiple powered
axle vehicles. The transfer case
transfers power from the
transmission to the front and rear
axles by means of drive shafts. It also
synchronizes the difference between
the rotation of the front and rear
wheels, and may contain one or more
sets of low range gears for off-road
• Transfer case working principle:
• The transfer case is an auxiliary transmission mounted in
back of the main transmission. This is used in four wheel
• The transfer box enables the driver to drive in two wheel
drives on highway or shift to four wheel drives for Off
Road, to drive in high gear or low gear as required.
• The input shaft is connected to the gear box and carries
on it a neutral having axial teeth. Two input shaft gears
are free to rotate on the shaft. Each of these gears have
bosses on the side which have axial teeth of the same
pitch as the central member on the input shaft.
• Depending upon the movement of the transfer box gear
level the central member and thereby the input shaft
may be connected either to the small gear or to the big
• There are two output shafts, one going to the front axle
and the second going to the rear axle. The front output
shaft is smaller in diameter & is supported inside the rear
output shaft, which is directly connected to the output
1. It transmits rotary motion and power from gear box to
the differential at varied angle.
2. It accommodates change in length when the rear axle
moves up and down.
3. It absorbs the shocks coming on the transmission
system when the vehicle starts from rest.
Working of Different components,
1) Universal joint: To Transmit the power in different
angular position between transmission shaft to
propeller shaft and from propeller shaft to Differential.
2) Sliding joint: This serves to adjust the length of the
propeller shaft when demanding from the axle
3) Tubular Shaft: Withstand mainly torsional load.
Need of differential:
1. When vehicle is taking turn, then outer wheel
will have to travel greater distance as compared to
2. The vehicle has a solid rear axle only and no
other device, there will be tendency to skid.
3. Hence wheel skidding is avoided by
incorporating by mechanism i.e. differential.
4. Differential reduces the speed of inner wheel
and increases the speed of outer wheel when
vehicle is taking turn, at the same time keep the
speed of rear wheel same when going straight
1. When vehicle moves in a straight line:
The power comes from propeller shaft to the bevel pinion
which drives the crown wheel. Then it is carried to the
differential cage in which a set of planet pinions and sun gears
are located. From the sun gear it is transmitted to the road
wheels through axle half shafts. In this case, the crown wheel,
differential cage, planet pinions and sun gears all turn as a
single unit and there is no any relative motion between the sun
gear and planet pinion. The planet pinions do not rotate about
their own axis. The road wheels, half shafts and sun wheels
offer the same resistance to being turned and the differential
gearing does not therefore operate. Both the road wheels turn
at the same speed
2. When Vehicle takes a turn:
The inner wheel experiences a resistance and tends to rotate in
opposite direction. Due to this the planet pinions starts rotating
about their own axis and around the sun gear and transmit
more rotary motion to the outer side sun gear. So that outer sun
gear rotates faster than the inner sun gear. Therefore the outer
road wheel runs faster than the inner road wheel and covers a
more distance to negotiate a turn safely.
Introduction and Purpose of Gear Box
•Provides speed and torque conversions because of the limitations of
internal combustion engines.
•Also facilitates change of direction of output shaft for reversing
•Automotive gearboxes are used to reduce load on the engine by
manipulating torque and speed. They have the option to select one of
several different gear ratios.
•Once the engine has reached a number of revolutions per minute, it is
advisable to increase the gear to reduce the engine rpm to reduce wear
on the engine, allow more control, and greater speeds, better
acceleration, and better fuel economy.
•Most gearboxes are used to increase torque & reduce the speed of a
output shaft. This produces a mechanical advantage
•Automotive gearbox also have the provision to do the opposite ie provide
an increase in output shaft speed with a reduction of torque (overdrive).
Types of Gear Box
A. Selective gear transmission
1. Sliding mesh gear box
2. Constant mesh gear box
3. Synchromesh gear box
a. Three speed
b. Four speed
C. Six speed
B. Planetary gear transmission
1. Epicyclic gear type
2. Automatic Transmission
a. Torque Converter
b. Electric type
Constant Mesh Gearbox
•All the gears are always in mesh
•Gears on counter shaft are fixed to it
•Gears on main shaft are free to rotate
•Dog clutches can slide on the main shaft and rotate with it
•Dog clutches engage with gears on the main shaft to obtain desired speed
Advantages over Sliding mesh Gearbox:
•Helical and herringbone gear can be used in these gearboxes and therefore,
constant mesh gearboxes are quieter.
•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
1. In this type of gearbox, all the gears of the main shaft are in constant
mesh with corresponding gears of the countershaft.
2. The gears on the main shaft which are bushed are free to rotate.
3. The dog clutches are provided on main shaft.
4. The gears on the lay shaft are, however, fixed.
5. When the left Dog clutch is slide to the left by means of the selector
mechanism, its teeth are engaged with those on the clutch gear and
we get the direct gear
6. The same dog clutch, however, when slide to right makes contact with
the second gear and second gear is obtained.
7. Similarly movement of the right dog clutch to the left results in low
gear and towards right in reverse gear. Usually the helical gears are
used in constant mesh gearbox for smooth and noiseless operation
Constant Mesh Gearbox
Double declutching is a method of shifting gears used primarily for vehicles with
an unsynchronized manual transmission, such as commercial trucks and specialty vehicles.
Double clutching is not necessary in a vehicle that has a synchronized manual transmission.
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. The clutch
is pressed and released with each change.
The double clutching technique involves the following steps:
1. The accelerator is released, the clutch pedal is pressed, and the gearbox is shifted into
2. The clutch pedal is then released, the driver matches the engine speed to the gear speed
either using the accelerator (when changing to a lower gear) or waiting for engine speed to
decrease (when changing to a higher gear) until they are at a level suitable for shifting into
the next gear.
3. At the moment when the revs between engine and gear are closely matched, the driver
then instantly presses the clutch again to shift into the next gear. The result should be a
very smooth gear change.
1. This type of gearbox is similar to the constant mesh type gearbox.
Instead of using dog clutches here synchronizers are used.
2. The modern cars use helical gears and synchromesh devices in
gearboxes, that synchronize the rotation of gears that are about to
1. The gears on the lay shaft are fixed to it while those on the main shaft are free to
rotate on the same.
2. Its working is also similar to the constant mesh type, but in the former there is one
definite improvement over the latter.
3. This is the provision of synchromesh device which avoids the necessity of double-
4. The parts that ultimately are to be engaged are first brought into frictional contact,
which equalizes their speed, after which these may be engaged smoothly.
5. Figure shows the construction and working of a synchromesh gearbox. In most of
the cars, however, the synchromesh devices are not fitted to all the gears as is
shown in this figure.
6. They are fitted only on the high gears and on the low and reverse gears ordinary
dog clutches are only provided.
7. This is done to reduce the cost.
1. In figure A is the engine shaft, Gears B, C, D, E are free on the main shaft
and are always in mesh with corresponding gears on the lay shaft.
2. Thus all the gears on main shaft as well as on lay shaft continue to rotate
so long as shaft A is rotating.
3. Members F1 and F2 are free to slide on splines on the main shaft.
4. G1 and G2 are ring shaped members having internal teeth fit onto the
external teeth members F1 and F2 respectively.
5. K1 and K2 are dogteeth on B and D respectively and these also fit onto
the teeth of G1 and G2. S1 and S2 are the forks.
6. T1 and T2 are the balls supported by spring. These tend to prevent the
sliding of members G1 (G2) on F1 (F2). However when the force applied on
G1 (G2) slides over F1 (F2).
7. M1, M2, N1, N2, P1, P2, R1, R2 are the frictional surfaces.
1. For direct gear, member G1 and hence member F1 (through spring-
loaded balls) is slide towards left till cones M1 and M2 rub and friction
makes their speed equal. Further pushing the member G1 to left causes it
to overdrive the balls and get engaged with dogs K1.
2. Now the drive to the main shaft is direct from B via F1 and the splines.
3. Similarly for second gear the members F1 and G1 are slide to the right so
that finally the internal teeth on G1 are engaged with L1. Then the drive to
main shaft will be from B via U1, U2, C, F1 and splines.
4. For first gear, G2 and F2 are moved towards left. The drive will be from B
via U1, U2, D, F2 and splines to the main shaft.
5. For reverse gear, G2 and F2 are slid towards right. In this case the drive
will be from B via U1, U2, U5, E, F2 and splines to the main shaft.
Manual transmission cars use a clutch, which completely disconnects the engine from the transmission.
Automatic transmission cars use a torque converter.
A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the
transmission. If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque
passed through the torque converter is very small, so keeping the car still requires only a light pressure on the
there are four components inside the very strong housing of the torque converter:
4. Transmission fluid
The housing of the torque converter is bolted to the flywheel of the engine, so it turns at whatever speed the
engine is running at.