📌 Quick Answer
A synchromesh gear box is a constant-mesh gearbox fitted with synchroniser units that bring the gear and the shaft to the same speed before they lock together, so the gears engage smoothly without clashing.
It is the type of manual gearbox used in modern cars because the driver can change gear quickly and quietly without double-declutching.
🔹 Key Takeaways
- It is a development of the constant-mesh gearbox with added synchroniser (synchro) units.
- A friction cone on the synchroniser equalises the speeds before the dog teeth engage.
- This prevents gear clash and removes the need for double-declutching.
- Top gears are usually synchronised; on older boxes first and reverse sometimes were not.
What Is a Synchromesh Gear Box?
A synchromesh gear box is a manual transmission in which all the main-shaft gears are in constant mesh with the lay-shaft gears, and synchroniser units are used to engage the selected gear. The synchroniser first matches the speed of the freely rotating gear to the speed of the output shaft, and only then allows them to lock together. This is why gear changes are smooth and silent.
Construction
The gears on the main shaft rotate freely and are always meshed with the lay-shaft gears. Between each pair of gears sits a synchroniser assembly mounted on splines on the main shaft. Each synchroniser has a friction cone, a set of dog teeth, and a sliding sleeve held by springs and balls. The cone surfaces do the speed-matching; the dog teeth make the final positive drive connection.
Working: How Synchronisation Happens
When the driver selects a gear, the sliding sleeve moves towards the chosen gear. First the friction cone of the synchroniser touches the matching cone on the gear. Friction between the two cones quickly brings them to the same speed. Once the speeds are equal, the sleeve slides further and its internal teeth mesh with the dog teeth of the gear, completing a positive, clash-free engagement. Because the cones synchronise the speeds first, the gears never grind.
Advantages and Limitations
The main advantages are smooth, quiet and fast gear changes, no need for double-declutching, and less wear on the gear teeth. The limitations are a more complex and costly construction than sliding-mesh or plain constant-mesh boxes, and the synchroniser cones wear over time and can cause baulking when worn.
Frequently Asked Questions
What is a synchromesh gear box?
It is a constant-mesh manual gearbox fitted with synchroniser units that equalise the speed of the gear and shaft before engagement, giving smooth, clash-free gear changes.
How does synchromesh work?
A friction cone on the synchroniser contacts the gear and brings it to the same speed as the shaft; once the speeds match, the dog teeth engage to give a positive drive without clashing.
What is the difference between constant-mesh and synchromesh gearboxes?
Both keep the gears permanently meshed, but a synchromesh box adds synchroniser cones that match speeds before engagement, so it shifts smoothly without the skill or double-declutching a plain constant-mesh box needs.
Why is double-declutching not needed in a synchromesh gearbox?
The synchroniser cones automatically match the gear and shaft speeds, doing the job that double-declutching does manually in older gearboxes.
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