In this type of gearbox, all the gears are in constant mesh with the corresponding gears on the layshaft. The gears on the most shaft that is splined, are free (fig). 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 that of the selector mechanism, its teeth area unit engaged with those on the clutch gear and that we get the direct gear. The same dog clutch, however, once slide to right makes contact with the gear and gear is obtained. Similarly movement of the right dog clutch to the left results in low gear and towards the right in reverse gear.
the constant mesh box, for the smooth engagement of the dog, clutches it's necessary that the speed of the main shaft gear and therefore the sliding dog should be equal. Therefore to get 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 therefore the gear is dropped at neutral. Then the clutch is engaged and the accelerator pedal pressed to extend the speed of the most shaft gears, After this, the clutch is once more disengaged and therefore the gear moved to the specified lower gear, and therefore the clutch is once more engaged. As the clutch is disengaged doubly during this method, it is called double-declutching. For changing to a higher gear, however, the 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 of the gear.
Constant Mesh Gear Box Advantages
Compared to the sliding mesh type, the constant-mesh gearbox has the following advantages :
- As the gears got to stay always in mesh. It is now not necessary to use straight spur gears.' instead, voluted gears area unit used that is quieter running.
- 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.