Constant Mesh Gearbox

Constant Mesh Gearbox: A to Z Description
The constant mesh gearbox is an improvement over the sliding mesh gearbox and represents an intermediate stage in gearbox evolution. In this system, all the gears on the main shaft and countershaft are constantly in mesh, but power transmission is controlled by dog clutches.
Key Components
Input Shaft: Receives power from the engine via the clutch.
Countershaft (Layshaft): Contains gears that are in constant mesh with gears on the main shaft.
Main Shaft: Transmits power to the output, with gears that are free to rotate when not engaged.
Dog Clutches: Sliding sleeves with internal teeth that engage gears on the main shaft for power transmission.
Gear Lever and Selector Forks: Mechanisms for moving the dog clutches to engage the desired gear.
Working Principle
The constant mesh gearbox works by engaging dog clutches rather than sliding gears directly. This simplifies the gear-shifting process and reduces wear on the gears.
1. Constant Mesh of Gears
All gears on the countershaft and main shaft are constantly meshed with each other.
This eliminates the need to align and engage gears manually during shifting.
2. Neutral Position
In neutral, the gears are in mesh, but the dog clutches are not engaged.
No power is transmitted to the output shaft.
3. Engaging a Gear
The gear lever moves a selector fork, which slides the dog clutch.
The dog clutch connects the desired gear to the main shaft, allowing power transmission.
4. Changing Gears
The clutch pedal is pressed to disengage the engine from the gearbox.
The dog clutch is disengaged, and a new gear is selected by sliding the corresponding dog clutch into position.
The clutch pedal is released to resume power transmission.
5. Reverse Gear
The reverse gear uses an idler gear that reverses the rotation direction of the output shaft when engaged.
Gear Ratios
Like other gearboxes, the constant mesh gearbox offers different speed and torque ratios:
First Gear: Low speed, high torque.
Second Gear: Moderate speed and torque.
Third Gear: High speed, low torque.
Reverse Gear: Changes the direction of rotation.
Advantages
Durability: Reduced wear and tear on the gears because they remain constantly meshed.
Improved Shifting: Dog clutches simplify the shifting process compared to sliding gears.
Noise Reduction: Less noisy than sliding mesh gearboxes during operation.
Versatility: Handles higher speeds and torque loads effectively.
Disadvantages
Skill Requirement: Requires precise clutch operation and gear shifting, especially without synchromesh.
Noise: Though reduced compared to sliding mesh gearboxes, noise is still significant in older designs.
Complexity: More components than a sliding mesh gearbox, making it costlier and heavier.
Applications
Early motor vehicles (before synchromesh gearboxes became standard).
Heavy machinery and trucks where robustness and torque handling are prioritized.
Agricultural and construction equipment.
Advancements Over Time
Addition of Synchromesh
Synchromesh devices replaced dog clutches to make gear shifting smoother and easier.
Synchronizers match gear speeds, eliminating the need for double declutching.
Helical and Herringbone Gears
Introduction of helical gears for smoother operation and noise reduction.
Herringbone gears further minimized axial thrust.
Automatic and Semi-Automatic Systems
Automated versions of constant mesh systems are used in advanced machinery.
Computer-Controlled Shifting
Modern heavy vehicles integrate computer systems for optimized gear selection.
Comparison with Other Gearboxes
Feature Sliding Mesh Constant Mesh Synchromesh
Gear Engagement Direct gear sliding Dog clutch engagement Synchronizer engagement
Ease of Shifting Difficult Moderate Smooth
Durability High Higher High
Noise High Moderate Low
Modern Usage Rare Limited Common
Diagram of Constant Mesh Gearbox
A schematic typically includes:
Input shaft with drive gear.
Countershaft with fixed gears.
Main shaft with free gears and dog clutches.
Selector forks and reverse idler gear.