Since drive axles and manual transmissions both use gears and shafts to transmit power, their failure modes are quite similar. And, as in gearbox failures, the root cause almost always boils down to misapplication, improper operation or poor maintenance.
Don’t Be A Burden
Perhaps not surprisingly, fatigue failures caused by overloading are most common. Starting up on steep hills or off-road operation with a drive axle not designed for the application only exacerbates the problem. Fatigue can show up first as pitting, then spalling in bearing rollers and races, as shown in Figure 1. That means that the extra weight can push these surfaces together with excessive force, creating heat and breaking down their lubrication film.
“Usually, you’ll start to see surface metal chunking out of the component,” noted Charles Allen, national service director, ArvinMeritor, Inc. Left unchecked, the bearing can fail completely.
Overloading also can cause excess pressure between gear teeth, again resulting in metal removal. It even can cause axle shafts to endure a bending moment they weren’t designed for. At first glance, the shaft in Figure 2 looks like a shock failure since it displays a gray, granular texture.
However, the small, polished beach marks along the outer circumference indicate gradual fatigue, at least initially. The shaft was bending with every revolution, causing hairline surface fractures, which deepened with time. Finally, when there wasn’t enough metal left at the core to withstand even a moderate shock, the shaft snapped.
Fatigue damage can show up on virtually any internal component. The beach marks on the gear teeth in Figure 3 are typical of fatigue damage caused by overloading.
When looking at failed components, “It helps to know the vehicle’s history,” advised Lee French, technical support manager/Ontrac crew chief, ArvinMeritor, Inc. In other words, knowing how a vehicle was being operated can go a long way in determining the root cause(s) of failures.
Shock Damage Failure
As with manual transmissions, shock damage to drive axles is all too common. It occurs when a vehicle is stopped suddenly under load or when a wheel spinning on a slick surface suddenly gains traction. It also can happen when a driver pops the clutch at elevated rpm on a loaded vehicle. In any case, one wheel usually is stopped, and another wants to go right now.
With nowhere for the energy to go, something invariably breaks.
What’s left of the differential spider in Figure 4 shows it was the victim of such an event. The gray, crystalline appearance at the breaks indicates sudden, catastrophic failure.
Finally, since shock failures can be transmitted upstream, it is highly recommended that when one occurs in a drive axle, it should be standard practice to check for evidence of other drivetrain component damage, such as twisted drive shafts, fractured U-joints and transmission troubles.
Don’t Go For A Spin
Even if it doesn’t suddenly gain traction, a spinning wheel can be a wheel of misfortune. That’s because, with one wheel stopped, the spinning wheel, differential spider and spider gears are spinning at twice drive shaft, or speedometer-indicated, speed. Spun long enough at this higher-than-normal speed, centrifugal force causes the assembly to throw off its lubrication.
The result is often scoring and galling between the wheel disc spider arm and the wheel disc side pinion bores, as shown in Figure 5. In many cases, cautioned Bob Crepas, Dana service specialist, spinout damage can be initiated months before a part actually fails. “Spinout damage is very common,” he said, “especially in colder climates. Drivers really need to take it easy when they get stuck.”
Secondary failures of this type include spider-gear thrust washers that display heat discoloration and deep circumferential scoring, often leaving metal embedded in the thrust seat, as in Figure 6. Eventually, the thrust washers can self-destruct and seize the assembly. Figure 7 shows totaled thrust washers.
Maintaining For Life
Failures stemming from poor maintenance practices are almost always lubrication issues. Low lube level, the wrong type of lube and lube contamination are the most common maintenance faux-pas.
Low lube or the wrong type of lube often result in failures that mimic some overloading failures, since a result of each can be heat discoloration, spalling, galling and accelerated wear.
The gear teeth in Figure 8 display crow’s feet, or metal removal resulting from lubrication failure. When there’s not enough lube, or the wrong type is used-such as a lube that doesn’t have sufficient extreme-pressure (EP) capability-the lube film is insufficient to prevent metal-to-metal contact.
This happens first on sliding surfaces, then on rolling surfaces. The extra friction creates heat, which breaks down what film is left, which creates more heat, and so on.
In extreme cases, the heat can begin to melt the metal until the component is nearly unrecognizable. The pinion gear in Figure 9 has undergone such a transformation, according to ArvinMeritor’s Allen, probably from a low- or no-lube condition.
Oil also can be contaminated, either with dirt or metal particles, which produce scoring on rotating components (Figure 10), or with water, which causes rust on non-working bearing surfaces. “But if you check the oil for water, it might not show up,” said French. That’s because when the axle heats up, the water can evaporate.
When it comes to drive-axle maintenance, “Lubrication is number one,” said Crepas. “Change the fluid according to the manufacturer’s recommendations, and check the seals for leakage.”
“Use the correct lube, and have a good maintenance program in place,” agreed Allen. “And don’t forget to occasionally check the breather threads for leakage.”
Allen and French also recommended: maintaining proper driveline angles by keeping ride height at spec to prevent accelerated wear from torsional vibrations, and carefully matching tire sizes to prevent overworking differentials.
Analyzing component failure, axle or otherwise, can be like putting together a large puzzle. Knowing how the pieces fit together through careful deduction can help prevent future failures or more serious problems.
For More Information
While this overview provides a sampling of causes of drive axle failures, interested readers can access exhaustive publications on the subject at www.meritorhvs.com and www.roadranger.com.
