In this type of gearbox, all the gears are in constant mesh with the corresponding gears on the layshaft. 

The gears on the main shaft which is splined, are free. 

The dog clutches are provided which are free to slide on the main shaft. 

The gears on the layshaft are, however, fixed.

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 drive. 

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.

Double declutching :-

In the constant mesh box, for the smooth engagement of the dog clutch. 

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, layshaft 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 that is the driver has to wait with the gear in neutral till the main shaft speed is decreased sufficiently for a smooth engagement of the gear.

Advantages :-

Compared to the sliding mesh type, the constant mesh gearbox 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 quiter in 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.