Necessity of Gear Box:-

• To vary the torque as per the requirement for driving the wheels.
• To shift the drive in a neutral position.
• To back the vehicle i.e. Reverse gear.

Working:-

• Constant mesh gear box consists of a drive shaft attached to a gear which is meshed with a gear mounted on a lay shaft.
• The lay shaft consists of various gears in mesh with the gears of the main shaft.
• The gears on the main shaft are not directly attached to the main shaft and can rotate freely around it.
• Dog clutches are used to engage a gear.
• Dog clutches are splined to the main shaft, thus it helps in rotating the main shaft.
• The dog clutches are provided with the friction material which helps them to engage with the gears on main shaft.
• When a driver moves the gear stick, the selector fork shifts the corresponding dog clutch to engage with a gear.
• As a result, the dog clutch rotates with the speed of the gear which is engaged with it.
• As a result, the main shaft rotates with the same speed as the selected gear.

Shifting gears:-

• In a constant mesh gear box, for smooth engagement of dog clutches, the speed of the main shaft gear and the dog clutch needs to be equal.
• Therefore to shift from lower gear (i.e. 2nd) to higher gear (i.e. 3rd), the speed of the clutch shaft and the lay shaft must be decreased.
• This is achieved by double declutching.
• First, the clutch is disengaged and brought to neutral.
• Then the clutch is engaged and the driver has to wait till the speed of the engine drops sufficiently for a smooth engagement. The driver can also apply brake gently to speed up the process.
• After this, clutch is pressed again and the gear is moved to the higher gear (3rd gear).
• As the speed of the clutch shaft is now same as the dog clutch, the dog clutch is engaged smoothly.
• In this way, we can shift from 2nd gear to 3rd gear in a constant mesh gear box.
• Similarly for changing it to lower gear, reverse effect is desired.
• The driver needs to put the gear in neutral position and then accelerate to increase the speed of clutch shaft and lay shaft.
• Then, the driver can shift to the required gear and the dog clutch can be engaged smoothly.