CONSTANT MESH GEARBOX:

In this gear box, all gears on the main transmission shaft are constantly connected to corresponding gears on counter shaft or lay shaft. In addition, two dog clutches are provided on the main shaft. One dog clutch is between the second gear and clutch gear and another is between the first gear and reverse gear. Splines are out on main shaft so that all the gears are feed on it.w





Dog clutches can also slide on main shaft and rotate with it. However, all the gears on counter shaft are giddily fixed to it. Different gear ratios (speed ratios) are obtained as follows:

For Three Forward and One Reverse Gear
Top or 3rd speed gear is obtained when the left dog clutch is slide to left to mesh with clutch gear by using the gear shift lever. In this case, main shaft rotates at the same speed as that of clutch gear or engine crankshaft speed which is the maximum speed. Speed ratio obtained is 1: 1.

Second gear is obtained when dog clutch (left side) meshes with second gear. In this condition clutch gear rotates the drive gear on counter shaft and counter shaft drives the second gear on the main shaft. All other gears on main  shaft are free, so they do not move.

In the same manner, first gear is obtained when right hand side dog clutch meshes with first gear. Reverse gear is obtained when right side dog clutch meshes with reverse gear on main shaft.

In this type of gearbox, all the gears of the main shaft are in constant mesh with corresponding gears of the counter shaft. The gears on the main shaft which are bushed are free to rotate. The dog clutches are provided on main shaft. The gears on the lay shaft are, however, fixed.

When the left Dog clutch is slid 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. The same dog clutch, however, when slid to right makes contact with the second gear and second gear is obtained. 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 inconstant mesh gearbox for smooth and noiseless operation.

Double De-clutching
In the constant mesh box, for the smooth engagement of the dog clutches it is necessary that the speed of main shaft gear and the sliding dog must be equal. Therefore to obtain lower gear, the speed of the clutch shaft, lay shaft and main shaft gear must be increased. This is done by double declutching. The procedure for double declutching is as given below:

The clutch is disengaged and the gear is brought to neutral. Then the clutch is engaged and accelerator pedal pressed to increase the speed of the main shaft gears. After this the clutch is again disengaged and the gear moved to the required lower gear and the clutch is again engaged.
As the clutch is disengaged twice in this process, it is called double declutching. For changing to higher gear, however, reverse effect is desired i.e., the driver has to wait with the gear in neutral till the main shaft speed is decreased sufficiently for a smooth engagement ofthe gear.

Advantages
Compared to the sliding mesh type, the constant mesh gear box has the following advantages:
1. As the gears have to remain always in mesh, it is no longer necessary to use straight spur gears. Instead, helical gears are used which are quieter running.
2. Wear of dog teeth on account of engaging and disengaging is reduced because here all the teeth of the dog clutches are involved compared to only two or three teeth in the case of sliding gears.