Constant mesh gearbox: Definition, Diagram, Construction, Working, Advantages, Application, Double declutching [with Pdf]

What is constant mesh gearbox?

1] Constant mesh gearbox is one of the types of manual transmission gearbox.  in which lay shaft gears constantly mesh with output shaft gears.

2] In this gearbox, helical and double helical gears are used for the power transmission.

3] The dog clutch is used in this gearbox, As per the action of the gear pedal, this dog clutches slides on the main shaft and meshes with desired output shaft gears.

Constant mesh gearbox diagram:

Constant mesh gearbox construction:

Constant mesh gearbox is consists of the following key components:-

• A) Engine shaft
• B) Lay shaft
• C) Output shaft
• D) Clutch gear
• E) Lay shaft gears ( gear 1, gear 2, gear 3, gear 4, gear 5)
• F) Output shaft gears ( gear A, gear B, gear C, Gear
• G) Dog Clutch
• H) Gear Shifting mechanism
• I) Gear pedal
• J) Idler gear

A) ENGINE SHAFT:-    Engine Shaft has Clutch gear fixed on its one end. Another end of the engine shaft is connected to the Clutch.

When clutch engages, power receives at Engine shaft from Engine.

Clutch gear meshes (engages) with gear 5 on lay shaft, hence power transmits from the engine shaft to lay shaft.

B) LAY SHAFT :-   Lay shaft has 5 fixed gears (gear 1, gear 2, gear 3, gear 4, gear 5).   Gear 5 meshes with clutch gear & takes power from the engine shaft.

Lay shaft gears are constantly meshed with output shaft gears. Hence power transmits from the engine shaft to output shaft gears through lay shaft gears.

C) OUTPUT SHAFT:–    It is a spline shaft. It has four gears ( gear A gear B, gear C, gear D ).   These gears freely rotate on the output shaft.

There are two dog clutches on the output shaft. These dog clutches has internal splines hence they can easily slides on output shaft and rotate with it.

• Read here: Synchromesh gearbox

The gear shifting mechanism is used in the gearbox to moves dog clutches on the output shaft.   when the dog clutches meshes with output shaft gears, power transmits from engine shaft to output shaft through lay shaft.

D) CLUTCH GEAR:-

Clutch gear is fixed at one end of the engine shaft. Clutch gear meshes (engages) with gear 5 on lay shaft, hence power transmits from the engine shaft to lay shaft.

E) LAY SHAFT GEARS:-

These five gears (gear1, gear2, gear3, gear4, gear5 ) are rigidly fixed on the lay shaft.

These gears Constantly mesh with output shaft gears, hence power transmits from the engine shaft to the output shaft through lay shaft.

F) OUTPUT SHAFT GEARS:-

These gears freely rotate on the output shaft.   when the Dog clutch meshes with gears, power transmits from the engine shaft to the output shaft through lay shaft.

The Gear pedal is connected to dog clutches on the output shaft through a gear shifting mechanism.

The gear shifting mechanism is used to move dog clutches (either on the left side or right side) on the output shaft. This means when the driver shifts the gear pedal to achieve the desired gear, through the gear shifting mechanism, the dog clutches on the output shaft slides to mesh (engage) with gears.

The gear shifting mechanism is used to move dog clutches (either on the left side or right side) on the output shaft. This means when the driver shifts the gear pedal to achieve the desired gear, through the gear shifting mechanism, the dog clutches on the output shaft slides to mesh (engage) with gears.

G) DOG CLUTCH:-

The output shaft has two dog clutches. One is between gear A and gear B and another one is between gear C and Gear D.

Dog clutches are internally splined, hence they can slide on the output shaft and rotate with it. When the dog clutch meshes with gears on the output shaft, power transmits from the engine shaft to the output shaft through lay shaft.

H) GEAR SHIFTING MECHANISM:-

Gear shifting mechanism is used to move dog clutches (either in left side or right side) on output shaft.

One end of this mechanism is connected to the gear pedal and another end is connected to dog clutches.

Therefore, when the driver shift the gear pedal to achieve the desire gear, through the gear shifting mechanism, the dog clutches on the output shaft slides to mesh (engage) with output shaft gears.  

• Read here: Sliding mesh gearbox

I) GEAR PEDAL:-

The Gear pedal is connected to dog clutches through a gear shifting mechanism.   when the driver shift the gear pedal to achieve desire gear, through the gear shifting mechanism, the dog clutches on output shaft slides to mesh (engage) with gears on the output shaft.

J) IDLER GEAR:-

It is Separate Gear use in this gearbox. The idler gear is used in the gearbox, to achieve reverse gear.

When Gear on lay shaft meshes (engage) with gears on the output shaft, both rotate in opposite directions, which means the vehicle runs frontward.

Therefore Idler gear is used between the lay shaft gear and output shaft gear therefore both gears (lay shaft gear & output shaft gear) rotate in the same direction, which means the vehicle runs backwards.

Constant mesh gearbox working:

In constant mesh gearbox, as gears constantly mesh so with the help of  2 dog clutches we can obtain 4 gears.

In the neutral position, dog clutches are not meshes with any gear, hence output shaft gears freely rotate on the output shaft.  

As per movement of dog clutches on the output shaft, four gears can obtain are as follows

1) FIRST GEAR:- When dog clutch 1 move towards the left and meshes with gear A, the Dog clutch starts to rotate with gear A and it also rotates the output shaft.

Therefore In first gear The power transmits from  Engine shaft -> Clutch gear-> gear 5->lay shaft->gear 4->gear A->output shaft

• Read here: How power transmitted from engine to wheels?

2) SECOND GEAR:- When dog clutch 1 moves towards the right and meshes with gear B, the Dog clutch starts to rotate with gear B and it also rotates the output shaft.

Therefore In second gear The power transmits from  Engine shaft -> Clutch gear-> gear 5->lay shaft->gear 3->gear B->output shaft

3) THIRD GEAR:- When dog clutch 2 moves towards left and meshes with gear C, Dog clutch starts to rotate with gear C and it also rotates the output shaft.

Therefore In third gear The power transmits from  Engine shaft -> Clutch gear-> gear 5->lay shaft->gear 2->gear C->output shaft

4) REVERSE GEAR:- When dog clutch 1 move towards the left and meshes with gear D, the Dog clutch starts to rotate with gear D and it also rotates the output shaft.

Therefore In reverse gearThe power transmits from  Engine shaft -> Clutch gear-> gear 5->lay shaft->gear 1->Idler gear -> gear D->output shaft

Constant mesh gearbox working video:

Double declutching in constant mesh gearbox:

In Constant mesh gearbox for smoothly engagement of dog clutch and output shaft gears, speed of both ( dog clutch and gear ) should be the same, so this is done by double declutching.

Double declutching means to change/shift gear, the driver needs to press the clutch pedal two times.

Steps to change/shift gear from 3rd to 4th (Increase):-

1) First clutch pedal is pressed.

2) Shifting the gearbox from 3rd gear to a Neutral position.

3) Clutch pedal is Release.

4) Driver needs to wait till output shaft gear speed reduces to sufficient to mesh with dog clutch.

5) Again clutch pedal is pressed then gear shift from Neutral to 4th.

6) hence again clutch pedal is Release. In such a way, gear shifts from 3rd to 4th by double declutching.

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Steps to change/shift gear from 4th to 3rd (Decrease):-

1) First clutch pedal is press.

2) Shifting gearbox from 4th gear to Neutral position.

3) Release the clutch pedal.

4) Press the accelerator to increase the speed of output shaft gears to math with the speed of the dog clutch.

5) Again press the clutch pedal then gear shift from neutral to 4th.

6) hence again release the clutch pedal. In such a way, gear shifts from 4th to 3rd by Double declutching.

Advantages of constant mesh gearbox:

The advantages of constant mesh gearbox are as follows:-

• It is compact in size as compared to sliding mesh gearbox.
• Smooth engagement.
• Silent in operation due to use of helical and double helical gears.

Application of constant mesh gearbox:

Constant mesh gearbox was used in the following vehicles

• Farmer trucks
• Two wheeler
• heavy duty machinery

FAQ’s:

Media credits:

• Video of “How Does a Constant Mesh Gearbox Work” by India Tractors