Principle and operation of Constant mesh gear box

Introduction:

  

•  Gears on the main shaft are rigidly fixed

•  Gears on the counter shaft also fixed rigidly

•  The main shaft gears and counter shaft gears always meshed with each other

•  Engaging of the gears are done through the dog clutches

•   As the gears are always meshed, this type is called  as constant mesh gear box 

 

 

Construction of Constant Mesh Gear Box:

   

• Main shaft gears rotate freely

• Clutch gear is rigidly fixed to the clutch shaft and always remain connected to the counter

  shaft

• Gears on counter shaft are rigidly fixed

• Two slidable dog clutches are provided on the main shaft 

• One is in between clutch gear and second gear 

• Another between first gear and reverse gear

• Dog clutches are moved by the yoke 

• Dog clutches can slide on the main shaft and rotates with main shaft 

• The yokes are connected to shifting lever 

• Yoke is moved by the gear shifting lever

• Idler gear is mounted on the another shaft 

• Idler gear is provided for obtaining the reverse gear 

• Idler gear remains meshed with the reverse gear of the counter shaft

 

 Working Principle:

 

• Right hand dog clutch is slided to wards left  side by gear shifting lever 

• By this movement first speed is obtained 

• Left hand side dog clutch movement to right side gives the second speed  

• Left hand side dog clutch movement to left side gives the top speed 

• At this speed main shaft gear directly engages with clutch gear

 

Advantages:

• 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. 

• 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 in the case 

  of sliding gears