- Practice · 5+3 marks
Manual gearbox types and transfer case
Short answer
Constant mesh gearbox: all gears permanently meshed; synchromesh cones synchronise speeds before dog clutch engagement, enabling smooth, quiet shifts unlike sliding mesh which requires double-declutch. Transfer case splits drive to front and rear axles in a 4WD vehicle, offering high and low ranges and may include a centre differential.
(a) Explain the construction and working of a constant mesh gearbox with synchromesh devices and compare it with the sliding mesh type. (b) Write a short note on the transfer case.
(a) Manual gearbox
A gearbox gives different gear ratios so that torque and speed can be matched to the road load, and it provides neutral and reverse.
Sliding mesh: gears on the main shaft slide along splines to mesh with the countershaft gears. Straight spur gears are used, and they mesh smoothly only when their speeds are matched, so the driver must "double-declutch". Gear changes are noisy and cause wear.
Constant mesh: all gears on the main shaft are always meshed with the countershaft gears and spin freely on the main shaft. A dog clutch slides on the splined main shaft to lock the chosen gear to it. Helical gears are used, so the box is quiet and strong, but the dog clutch speeds must still be matched.
Synchromesh: a constant mesh box with a synchroniser (blocking ring with a cone) before each dog clutch.
Input -> [Constant gear] --> counter shaft
| | gears 1,2,3 (always meshed)
main shaft: gear 3 gear 2 gear 1
[sync ring + sleeve] [sync ring + sleeve]
Shift fork moves the sleeve
When the lever is moved, the sleeve pushes the cone of the blocking ring against the gear cone. Friction equalises the speeds, and only then does the sleeve slide over the dogs of the gear, so shifting is smooth and silent.
| Point | Sliding mesh | Constant mesh with synchromesh |
|---|---|---|
| Gears | Spur, slide | Helical, always meshed |
| Gear change | Difficult, double-declutching | Easy |
| Noise | Noisy | Quiet |
| Use | Old trucks | Almost all modern cars |
(b) Transfer case
A transfer case is a gearbox fitted after the main gearbox on a four-wheel-drive (4WD) vehicle. It takes power from the transmission and splits it between the front and rear propeller shafts through gears or a chain. Many have a high range (1:1) for road use and a low range (about 2.5:1) for extra torque multiplication off-road. Some have a centre differential or clutch to avoid wind-up of the drive line on hard ground.
✓ Answer checked by Claude Opus 5.5 (high effort)
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