Here’s a detailed comparison of Constant Mesh vs Sliding Mesh Gearboxes, highlighting construction, operation, advantages, and disadvantages:

In this article:
1. Sliding Mesh Gearbox (SMG)
Construction
- Contains input shaft, layshaft (countershaft), and output shaft (main shaft).
- Gears are mounted directly on shafts and slide along the shaft to engage.
- No constant meshing; gear teeth slide to mesh when shifting.
- Usually 3–4 forward gears + 1 reverse.
Working Principle
- Driver operates clutch and gear lever.
- To engage a gear, gear slides along shaft to mesh with corresponding gear.
- Power flows: Engine → Clutch → Engaged Gear → Output Shaft → Wheels.
- Gear must be moving at nearly the same speed to avoid gear clash.
Advantages
- Simple and inexpensive design.
- Easy to understand and repair.
- Light in weight.
Disadvantages
- Gear grinding common if speeds are mismatched.
- Harder to shift at high speeds.
- Wear on gear teeth due to sliding action.
- Not suitable for modern vehicles with multiple gears.
2. Constant Mesh Gearbox (CMG)
Construction
- All gears on layshaft and main shaft are always meshed.
- Main shaft contains sliding collars (dog clutches) to engage gears.
- Typically 4–6 forward gears + 1 reverse.
- Can include synchromesh rings for smoother engagement.
Working Principle
- All gears rotate constantly (meshed).
- Driver moves gear lever → dog clutch locks desired gear to main shaft.
- Power flows: Engine → Clutch → Input Shaft → Layshaft → Engaged Gear → Main Shaft → Wheels.
- Other gears spin freely; gear clash is prevented.
Advantages
- Prevents gear grinding → smoother gear shifting.
- Longer gear life due to constant mesh.
- Can be combined with synchromesh → easier shifting.
- Flexible number of gear ratios possible.
Disadvantages
- Slightly heavier and more complex than sliding mesh.
- Requires lubrication and maintenance for dog clutches.
- More expensive to manufacture than sliding mesh.
Comparison Table: Sliding Mesh vs Constant Mesh Gearbox
Feature | Sliding Mesh Gearbox | Constant Mesh Gearbox |
---|---|---|
Gear Engagement | Sliding gears along shaft | All gears always meshed; dog clutch selects gear |
Gear Grinding | Common if speeds not matched | Rare; smooth shifting |
Complexity | Simple | More complex |
Durability | Lower due to sliding wear | Higher; gears last longer |
Ease of Shifting | Harder at high speeds | Easier; synchromesh improves it further |
Weight | Lighter | Slightly heavier |
Cost | Cheaper | More expensive |
Applications | Early vehicles, tractors | Modern cars, light commercial vehicles |
Conclusion
- Sliding Mesh Gearbox: Simple and cheap but prone to gear clash; outdated in modern vehicles.
- Constant Mesh Gearbox: Modern, smooth, durable, suitable for cars and trucks; prevents gear grinding and allows more gears in a compact design.
Also watch : Sliding vs constant mesh gearbox animation.
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