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.
