Constant mesh gear box-Everything you need to know »

Here’s a comprehensive explanation of the Constant Mesh Gearbox, a type of manual transmission system:

Constant Mesh Gearbox :

A constant mesh gearbox is a type of manual transmission in which all the gears on the main shaft and layshaft are always meshed with each other, but only one gear pair is engaged at a time to transmit power to the output shaft.

This design prevents gear grinding and provides smoother shifting compared to older sliding-mesh gearboxes.

1️⃣ Purpose

Transmit engine power to the wheels at different speed ratios.
Allow smooth gear shifting without damage to gears.
Prevent gear clash during engagement.

2️⃣ Main Components

Input Shaft (Primary Shaft)
- Receives torque from the engine via the clutch.
Layshaft (Countershaft / Secondary Shaft)
- Always rotates and carries gears meshed with main shaft gears.
Main Shaft (Output Shaft)
- Connected to the driveshaft and wheels.
- Contains sliding collars (dog clutches) that engage gears.
Gears
- Fixed gears: Permanently splined to the layshaft and always meshed.
- Free gears: Mounted loosely on the main shaft; they rotate freely until engaged via dog clutches.
Dog Clutch / Sliding Collar
- Moves along the main shaft to lock a specific gear to the shaft, engaging that gear.
Reverse Idler Gear
- Used to reverse the direction of rotation for reverse gear.

3️⃣ Working Principle

The constant mesh gearbox operates on dog clutch engagement:

All gears are always meshed (hence “constant mesh”) → reduces wear.
Power is transmitted only through the engaged gear:
- Driver moves gear lever → sliding collar (dog clutch) locks the desired gear to the main shaft.
- Torque flows from input shaft → layshaft → selected gear → main shaft → driveshaft → wheels.
Other gears remain free → no power transmitted through them.

Reverse Gear

Uses an idler gear to reverse rotation.
Sliding collar engages main shaft gear with idler → vehicle moves backward.

4️⃣ Advantages

Prevents gear clash → smoother shifting.
Longer gear life due to constant mesh.
Can be combined with synchromesh mechanism for easier engagement.
Suitable for modern manual transmissions in cars and light trucks.

5️⃣ Disadvantages

More complex than sliding mesh gearbox.
Slightly heavier due to additional dog clutches.
Requires lubrication and maintenance for sliding collars.

6️⃣ Applications

Passenger cars and light commercial vehicles (manual transmissions).
Motorcycles sometimes use a similar principle in gearbox design.

7️⃣ Summary

Constant mesh gearbox = always-meshed gears + dog clutches to engage desired gear.

All gears rotate constantly on the layshaft.
Main shaft gears only transmit power when engaged by dog clutch.
Prevents gear grinding and improves shift smoothness.

Also read: constant mesh gear box animation.

FAQ Section:

1. What is a constant mesh gearbox?

A constant mesh gearbox is a type of manual transmission in which all gears are always in mesh with each other, but only one gear pair is engaged at a time through dog clutches to transmit power.

2. How does a constant mesh gearbox work?

In this gearbox, all gear pairs are constantly meshed, but they rotate freely on the shafts until the driver selects a gear. When a gear is selected, a dog clutch locks the required gear to the shaft, transmitting power.

3. What are the main components of a constant mesh gearbox?

Key components include:

Input shaft (clutch shaft)
Countershaft (layshaft)
Output shaft (main shaft)
Constantly meshed gears
Dog clutches
Gear selector mechanism

4. Why is it called a “constant mesh” gearbox?

It’s called “constant mesh” because all the gears on the main shaft and countershaft are constantly in mesh, regardless of which gear is engaged.

5. What is the purpose of the dog clutch?

The dog clutch connects the freely rotating gears to the shaft when a particular gear is selected, allowing torque transmission without sliding the gears themselves.

6. How does it differ from a sliding mesh gearbox?

In a sliding mesh gearbox, gears are physically slid into mesh to engage, causing wear and requiring skillful gear shifting.
In a constant mesh gearbox, all gears remain meshed, and engagement is achieved through dog clutches—resulting in smoother and faster gear shifting.

7. What are the advantages of a constant mesh gearbox?

Less wear and tear on gears
Quicker and smoother gear shifting
Reduced noise during operation
Longer gearbox life

8. What are the disadvantages of a constant mesh gearbox?

More complex and expensive design
Requires proper synchronization while shifting
Slightly higher maintenance cost

9. What is the function of the synchronizer in a constant mesh gearbox?

In advanced constant mesh gearboxes, synchronizers are added to match the speed of gears before engagement, preventing gear clash and enabling smooth shifting (as seen in modern synchromesh gearboxes).