Gear Box
Gear Box
Function of gear box
An
automobile is able to provide varying speed and torque through its gear box. Various
functions of a gear box are listed below;
1.
To provide high torque at the time of starting, vehicle acceleration, climbing
up a hill.
2.
To provide more than forward speeds by providing more than one gear ratios. In modern
cars, five forward and reverse gear is provided. For given engine speed, higher
speed can be obtained by running in higher 4th and 5th
gears.
3.
Gear box provides a reverse gear for driving the vehicle in reverse direction.
Types of gear boxes
1. Selective type gear boxes:
(i)
Sliding mesh gear box
(ii)
Constant mesh gear
box
(iii)
Synchromesh gear box
2. Progressive type gear box
3. Epicyclic type gear box.
Sliding mesh gear box
It is simplest type of gear box out of the available gear
boxes. In this type of gear box, 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. In a four speed gear box (which includes one reverse gear), the
counter shafts has four gears which are rigidly connected to it. Clutch shaft
has one gear and main shaft has two gears. The two 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. The clutch gear is rigidly fixed
to the clutch shaft. It is always connected to the countershaft drive gear.
The two gears on the main shaft can be slided by the
shifter yoke by operating the shift lever. These two gears are second gear and
low\reverse gear respectively. These gears can be meshed with corresponding
gears on the countershaft 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 shaft and is always in mesh with reverse gear on counter shaft.
Neutral position:
In this
position, the engine is in running condition, clutch remains engaged and clutch
gear drives the countershaft drive gear. The direction of rotation of counter shaft is opposites to that of clutch shaft. In third position ist, IInd
and IIIrd and reverse gears are free. Thus, main shaft does not rotate and
automobile wheels do not rotate. So vehicle remains stationary.
First gear
When
first gear position is selected by the shift lever, first gear on the main
shaft slides and is connected to first gear on the counter shaft. The direction
of rotation of main shaft meshes with larger gear on main shaft, speed reduction
in the ratio 3: 1 is obtained.
Second gear
When
second gear id selected by the shift lever, second gear on countershaft meshes with
second gear on the main shaft. The direction of main shaft is same as that of
clutch shaft is same as that of clutch shaft. Speed reduction of the order of
2:1 is obtained in second gear.
Third gear
In third
gear, the main shaft is slided axially towards the clutch shaft so that main shaft
is directly connected to the clutch shaft. In this position, the main shaft
rotates at the speed of clutch shaft. Thus, a speed ratio of s1:1 is obtained.
It can
be noted that the clutch gear is directly connected to engine crankshaft and
main shaft is connected to the wheels through propeller shaft
Reverse gear
When
the shift lever is operated to engage the reverse gear, the larger (reverse)
gear of the main shaft meshes with the reverse idler gear. Reverse idler gear
is always connected to reverse gear on counter shaft. The reverse idler gear
between counter shaft gear and main shaft larger gear changes the direction of
rotation of main shaft. Thus, the direction of main shaft becomes opposite to
that of clutch shaft. Therefore, wheels of the automobile start moving in
backward direction.
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