How are gears engaged in a sliding mesh gearbox?

Gears on the main shaft slide along splines to mesh with countershaft gears

Gears are always in mesh with dog clutches

A cone synchronizer matches speeds first

What is true of a constant mesh gearbox?

All gears are always in mesh and dog clutches lock a free gear to the shaft

It needs no declutching of any kind and has cone rings

What does the blocker ring do in a synchromesh?

It stops the sleeve sliding on until friction on the cone has equalised the speeds

It locks the gear to the shaft permanently

What happens after the speeds match in a synchromesh?

The sleeve slides over the dog teeth of the gear and locks it to the shaft

The blocker ring rotates the gear

Practice · 6 marks

Sliding mesh, constant mesh, synchromesh gearboxes

Short answer

Sliding mesh: spur gears slide into mesh, noisy, needs double declutching. Constant mesh: all helical gears stay in mesh, dog clutches engage, still needs double declutching. Synchromesh: adds cone synchronizers; the blocker ring friction-cone matches speeds before sleeve locks the gear, allowing silent, easy shifting without double declutching.

Differentiate between the sliding mesh, constant mesh and synchromesh gearboxes used in automobiles. Explain the working of the synchromesh device.

An automobile gearbox changes the torque-speed ratio between engine and wheels and gives reverse and neutral.

Comparison

Point	Sliding mesh	Constant mesh	Synchromesh
Gear engagement	Gears on the main shaft slide along splines to mesh with countershaft gears	All gears always in mesh; dog clutches slide to lock a free gear to the shaft	Constant mesh plus cone synchronizers that equalise speeds before engagement
Gear type	Spur (straight)	Helical (quiet)	Helical
Gear change	Needs double declutching and skill; noisy	Easier, but still needs double declutching	Very easy, no double declutching
Wear and noise	High; teeth damaged by clashing	Less wear, quieter	Least wear and noise
Cost	Cheapest	Moderate	Highest
Use	Old cars, some trucks	Heavy vehicles	Modern cars

Working of the synchromesh

 Gear (with cone)  Blocker ring  Hub/sleeve
     |\               |\            |
     | \_cone_______  | friction    | shift
     |  engages      |  cone       | fork

The driver moves the gear lever; the fork pushes the sleeve toward the selected gear.
A blocker ring (with a cone surface) is pressed on the cone of the gear. Friction brings the speeds of the shaft hub and the gear to the same value.
The blocker ring prevents the sleeve from sliding on until the speeds match, so there is no clash.
After speeds are equal, the sleeve slides over the dog teeth of the gear and locks it to the shaft.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Sliding mesh, constant mesh, synchromesh gearboxes

Comparison

Working of the synchromesh

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