It lets the two wheels on an axle turn at different speeds while both are driven, and it carries the final drive reduction.

Through a corner the outer wheel travels further than the inner one. Rigidly connected, one of them would have to scrub.

The differential gears allow that speed difference while still transmitting drive to both sides.

The crown wheel and pinion also turn the drive through ninety degrees and reduce the speed, multiplying torque - the final drive ratio.

That ratio determines the relationship between engine speed and road speed, which is why axles are specified by ratio as well as by capacity.

Mixing axles of different ratios on a multi-axle vehicle causes driveline wind-up and rapid damage, so the ratio must be matched.

Because it delivers equal torque to both wheels, so the wheel with the least grip sets the limit for both.

Torque is split evenly by design, and a wheel on a slippery surface can only accept a small amount before it spins.

Since the other wheel receives the same amount, the vehicle has almost no drive even though one tyre has full grip.

One wheel lifted or on ice therefore stops progress entirely, which on site and agricultural work is a common and expensive situation.

Traction control systems address it by braking the spinning wheel, which raises the torque the differential can deliver to the other side.

Mechanical answers are a limited-slip or locking differential, chosen according to how often the vehicle meets the problem.

Limited-slip for road-going vehicles that occasionally lose grip; locking for genuine off-road and site work, where it must be disengaged on the road.

A limited-slip differential uses clutches, gears or a viscous coupling to resist speed difference, transferring some torque to the wheel with grip while still allowing normal cornering.

It works continuously and needs no driver action, which suits road use.

A locking differential rigidly connects both wheels, giving maximum traction and forcing both to turn at the same speed.

That fights every corner, so it must be disengaged on hard surfaces or it causes driveline wind-up, tyre scrub and eventual damage.

Clutch-type limited-slip units usually need an oil containing a friction modifier; using plain gear oil causes the chatter on turns that is so often reported.

A whine that changes with road speed and varies between drive and overrun points at the crown wheel and pinion; a clunk on take-up points at backlash or the differential gears.

A whine that appears under acceleration and disappears on the overrun, or vice versa, indicates an incorrect contact pattern or wear on one flank of the gear teeth.

A whine present under both conditions usually means worn pinion bearings.

A rumble that changes with road speed and not with load points at the carrier bearings or the wheel bearings.

A clunk on taking up drive or changing direction is excessive backlash, worn spider gears or worn splines.

A noise only on cornering points at the differential gears themselves, which is where the speed difference is accommodated.

Removal and refitting yes; setting a crown wheel and pinion is specialist work with measured pinion depth, preload and backlash verified by contact pattern.

Pinion depth is set by shims behind the pinion head and determines where the teeth mesh along their profile.

Bearing preload is set by a crush sleeve or shims and measured as rolling torque, not by feel.

Backlash between crown wheel and pinion is measured with a dial gauge and adjusted by moving the crown wheel laterally.

The result is checked with marking compound and read against reference patterns - a skill that takes experience rather than a procedure that can be followed.

For most fleets an exchange unit or a specialist rebuild is the right answer, with the workshop doing removal, refitting and the oil change.

Always. They are lapped together at manufacture and are never mixed between assemblies.

The pair is run together during production to establish the final tooth finish, and their numbers are etched on both parts.

Fitting a crown wheel from one set with a pinion from another gives a mesh that cannot be corrected by adjustment and will be noisy and short-lived.

The etched matching number is checked before assembly, along with the ratio marking.

Both parts are always replaced together even when only one appears damaged.

Ratio must also match the other axle on a multi-axle vehicle and the vehicle's speedometer calibration.

Oil changes at the specified interval with the correct specification, plus a clear breather and attention after wading or heavy duty.

Axle oil degrades and accumulates wear metal, and the interval shortens substantially under severe duty, towing or high loads.

Specification matters: hypoid gears need extreme-pressure additives, and limited-slip units usually need a friction modifier as well.

The breather is routinely overlooked - a blocked one pressurises the housing as it heats and pushes oil past the seals.

Wading is the classic case: a hot axle entering cold water draws water in through the breather, so oil is checked and changed after deep water.

Inspect the drained oil. Metallic glitter or a strong burnt smell tells you more about the axle's condition than any external check.