Driveline components usually fail under three conditions:
- Shock Load
- Lubrication failures
A shock load is a sudden and powerful force that exceeds the strength of the driveline components. The shock load can either destroy components instantly or crack them so that they fatigue and fail at a later time.
There are three common operating conditions, which can result in a shock failure of a driveline component:
- Backing under a trailer with excessive force
- Hitting dry pavement with a spinning wheel
- Popping the clutch or rough clutch engagement.
Instantaneous shock load failures to driveline components are most easily identified by the rough, crystalline finish that is found where the parts separate from one another (Fig. 44). A twisted drive shaft tube is the only part of the driveline that deviates from this rule.
This universal joint show a rough crystalline finish typical of most shock loads
Shock loads to driveshafts usually do not break or crack the shaft, but rather twist it like this.
As mentioned in the first section, a fatigue failure is caused by numerous excessive load applications to a component. A single application of these loads is not great enough to cause an immediate part failure, but repeated load applications gradually weaken the part to the point of failure,
Fatigue failures of drivelines are caused by two conditions:
- Exceeding the rating of the vehicle
- Poor vehicle operation
There are three types of fatigue that occur most often in a driveline system:
- Torsional fatigue
Spalling occurs when the metal surface fatigues to the point where pieces of metal flake away. In the driveline, this type of fatigue is most commonly found on the turnions of universal joints. Spalling of trunnions is caused by repeated overloads or by using improper lubricants.
This trunnion has spalled due to repeated overloads.
Brinelling is a condition in which the needle rollers wear grooves in the surface of the trunnion and, occasionally, in the bearing cap. Sometimes the trunnion appears to brinelled but, when felt with a fingernail, does not exhibit the characteristic grooves of brinelling. This is known as “false brinelling.” In the case of false brinelling, the needle rollers have actually polished their mating surface so that it appears to be worn, but is not. The universal joint is still usable if the bearing surface has no detectable grooves.
There is no question that this trunnion has brinelling. The roller bearings have worn deep grooves that are easily detectable by touch.
In the driveline, torsional fatigue is found most often on the splined shaft. When this occurs, the fractured area will usually have a somewhat conical star shape.
Lack of lubricant affects the universal joints, center bearings, and slip yokes of the driveline. A lack of lubricant to any of these components can cause scoring, galling, or premature wear. Commonly, the U-joint is most easily damaged from lack of lubrication because it has polished bearing surfaces that are placed under great pressure. Like all high pressure bearing surfaces, the U-joint needs a high quality lube with extreme pressure (E.P.) additives.
This universal joint shows the damage that can happen from lack of lubricant. The friction and heat created by lack of lubricant caused