A semi-floating shaft carries the vehicle's weight as well as the torque; a full-floating shaft transmits torque only, with the axle tube carrying the weight.

On a semi-floating axle the wheel bearing runs on the shaft itself, so the shaft is both a drive member and a structural one.

That makes it simpler and cheaper, and it is standard on light vehicles and light commercials.

On a full-floating axle the hub runs on two bearings on the outside of the axle tube, and the shaft only drives.

A full-floating shaft can be withdrawn by undoing its flange, without jacking the vehicle or removing the wheel - a substantial advantage on a loaded commercial vehicle.

Critically, if a full-floating shaft breaks the wheel stays supported; if a semi-floating shaft breaks the wheel can come off.

Shock loading and fatigue, concentrating at the splines and at the flange radius.

Torque applied suddenly - wheelspin that suddenly grips, dropping the clutch, or a locked differential on hard ground - imposes loads far above the steady-state figure.

Splines are the usual victim, twisting or stripping, and a partially twisted shaft can run for a long time before finally failing.

The radius where the shaft meets its drive flange is a stress concentration, and fatigue cracks initiate there and grow under repeated loading.

Corrosion pitting and previous overload both accelerate it.

A shaft that has been overloaded once should be regarded with suspicion even if it survived, because fatigue damage accumulates and is invisible.

By whether the symptom is a clunk on take-up with no drive loss, or a noise that varies with speed and load.

Worn axle shaft splines give a clunk as drive is taken up or as the vehicle changes between drive and overrun, without any noise while running.

A stripped spline gives no drive at all to that wheel while the other wheel spins, which on an open differential means no drive at all.

Differential problems more typically produce a whine that varies with road speed, or noise specifically on cornering.

With the shaft removed, the splines can be inspected directly at both ends, which is conclusive.

Checking the shaft for straightness on rollers or vee-blocks also reveals a bend from a kerb strike or an overload.

Yes - it is accessible only at that moment, it is inexpensive, and a leaking one puts oil onto the brake linings.

The seal runs on the shaft or on the hub and keeps axle oil inside the casing.

Removing the shaft frequently damages the seal lip even when it was serviceable.

A leaking axle seal contaminates the brake shoes or pads on that corner, which cannot be cleaned and must be replaced.

So an oil leak that is ignored becomes a braking defect and a much larger bill.

Check the seal running surface on the shaft or hub too - a groove worn by the old seal will leak past a new one unless a repair sleeve is fitted.

No. It is a highly stressed rotating component, and straightening leaves residual stress with no way to verify the result.

Shafts bend from kerb strikes, severe potholes and overload, and the bend is often small enough to be invisible.

The symptom is a vibration or a wobble that tracks road speed, sometimes with uneven brake or tyre wear on that corner.

Straightening work-hardens material that has already been overloaded and does nothing about the fatigue damage already present.

A shaft that has bent has by definition exceeded its yield strength, so its remaining fatigue life is unknown.

Check runout against specification with a dial gauge, and replace anything outside it.

Spline condition at both ends, the seal and its running surface, the bearing, the circlip or retainer, and the correct endfloat where the design specifies one.

The differential side gear splines wear too, and fitting a new shaft into worn side gear splines simply moves the problem.

Semi-floating shafts are usually retained by a C-clip inside the differential or by a bearing retainer plate, and the correct method must be followed.

Endfloat is specified on many axles and is set with shims; too much lets the shaft move axially, too little preloads the bearings.

Check the axle tube for damage and the breather for blockage while access is available.

Fill the axle to the correct level afterwards with the specified oil, including any friction modifier a limited-slip unit requires.

Not the shaft itself, but the axle oil, the breather and the seals that protect it all do - and hub temperature after a run is a useful check.

The shaft runs in axle oil and needs no service of its own.

Axle oil changes at the specified interval protect the bearings and the gears the shaft works with.

A blocked breather pressurises the casing and pushes oil past the axle seals onto the brakes, which is the most common way a healthy axle causes a defect.

On full-floating axles, hub bearings are separately packed and adjusted and belong on the inspection schedule.

Checking hub temperature by hand after a run is a quick and genuinely informative check on any commercial vehicle or trailer - a hot hub is a bearing or a dragging brake.