Inoperative Conditions

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.

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.

FAILURE ANALYSIS
A successful failure analysis requires three steps:
1. Identify the type of failure.
2. Determine the cause of the failure.
3. Take corrective action to prevent a reoccurrence.

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.

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.

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.