Analysis of Inoperative Conditions

If the axle fails to operate, there will be broken or jammed-up parts. Analyze why the parts have broken to prevent the condition from recurring.

BROKEN AXLE SHAFT
Inspect a broken axle shaft for a twist in the spline next to the break.

Breaks that occur away from the spline or that are not accompanied by a twist in the spline may be caused by forging flaws or fatigue of the axle shaft.

Conditions that can cause a broken axle shaft include forging flaws or excessive impact load to the axle shaft as a result of a collision or other factors.

BROKEN PINION STEM
The drive pinion stem can break at any time if the differential pinion bearing preload is not set up properly. If there is no preload, the differential pinion bearing will turn relative to the axle shaft. This galls the roller shaft and generates heat which can draw the temper away from the shaft.

Too much preload also generates bearing heat with the same result. The break normally occurs between the two differential pinion bearings.

BROKEN WELDS
Axle housing welds, if completely broken, require replacement of the rear axle housing.

WHEEL BEARING MALFUNCTIONS
Because of the severe loads, rear wheel bearings may require replacement at high mileage. If a rear wheel bearing fails at low mileage, the cause is often overloading.





Rear wheel bearing noise might be mistaken for differential pinion bearing noise. Look at the rear wheel bearing carefully before tearing down the axle.





Rear wheel bearings are pressed into the axle housing tubes, making them more difficult to check. However, the axle shaft is the inner race for the bearing. If the bearing is damaged, the roller surface on the axle shaft may also be damaged. The rollers run on approximately the center of the polished surface.

AXLE LOCK-UP
Lock-up or seizure of the rear axle is usually associated with a low lubricant condition. The lubricant prevents friction and helps cool the parts as they heat up. Without enough lubricant, the heated parts get soft and may eventually weld together. Check for lubricant leaks after servicing.

SHOCK FAILURES
Shock failure is another common type of carrier failure. This failure results from a rapidly applied load or force that is severe enough to exceed the strength of the carrier component, and cause it to crack or fail instantly.

If the failed part separates into two or more pieces, the operator will usually immediately realize the fact. But if the part is only cracked, the operator may not be aware of the damage until some time later. The crack will be a point of origin for a fatigue failure and can progress until the part fails. The failure could happen while the vehicle is being operated under normal conditions.

There are a number of operating conditions which can result in a shock failure:
^ Hitting dry pavement with a spinning wheel
^ Missing a shift
^ Popping the clutch

HITTING DRY PAVEMENT WITH A SPINNING WHEEL
This can induce a severe shock load in the carrier and drivetrain. When the wheel is spinning, the carrier components are rotating at high speed. As the wheel contacts a dry surface or one with greater traction, it slows down very rapidly. If the deceleration is great enough, forces sufficient to exceed the strength of the carrier may result, and cause a failure.

MISSING A SHIFT
A missed shift is especially damaging when the operator is downshifting on a down grade. The impact which occurs to the coast side of the tooth can cause a failure.

POPPING THE CLUTCH
A certain amount of torque is required at the driving wheels to start a vehicle moving. If the wrong transmission gear is selected there may not be enough torque available at the wheels. In this situation the driver may speed up the engine and rapidly release the clutch, rather than shifting to a lower gear. This action induces a rapidly applied load in the drivetrain, and can result in a shock failure.

LUBRICATION FAILURES
The fourth common cause of axle carrier failures originates with the lubricant, or with lubricant change practices. The lubricant that protects the axle components has three key functions:
^ to reduce friction between parts
^ to carry heat away from parts
^ to carry dirt and wear particles away from parts

When a lubrication failure occurs, it is generally the result of improper maintenance and had its roots in one of three basic problem areas:
^ low lubricant level
^ improper type of lubricant or lubricant with depleted additives
^ contaminated lubricant

LOW LUBRICANT LEVEL
When the lubricant level in an axle carrier is too low, the friction between the parts generates heat and causes temperatures to increase considerably. If the temperatures become high enough, the parts may be galled and the lubricant burned.