The Case of the Battered Bearing

My father was a genius at coaxing the last year or so of service from old tractor, truck, and car engines. He could distinguish between the usual squeaks and rattles and the much more serious "pecks" of a loose connecting rod bearing, or worse yet the "pounding" of loose main bearings, also known as journals. Main bearings serve to keep the crankshaft aligned and free to rotate in the engine block. Trouble with the mains often indicated the impending end of the engine.

Scene of the Crime

Some 20 years ago routine maintenance of a near-new, two-engine airplane showed particles of bearing babbitt in the crankcase oil. Such particles are a sign of serious bearing deterioration and possible engine failure.

The owner claimed that the engine had a manufacturing defect. The manufacturer claimed that the problem was due to oil starvation and rejected the claim. The battle was on!

The engine was of the six-cylinder, horizontally opposed type wherein three pistons reciprocated in one direction, the other three in the opposite. (The old Volkswagen beetle engines had a similar layout.) The engine had four sets of main bearings, one on each end, and two at intermediate positions. The bearings are in halves, with each half resembling a bent playing card. The inner surface is babbitt, a soft metal that conforms to the contours of the crankshaft, while a steel backing provides support.

The Investigation

Disassembly of the engine showed all the bearings save one to be in good order. The number three main, located between the number four and five connecting rods had failed. The figure at right shows that the babbitt is missing over much of the surface, and the bearing has fractured through the oil hole. The bottom part of the bearing has been removed for testing.

I know from painful experience how quickly oil starvation will wreck an engine. But, oil starvation or degradation would not affect just one bearing. Other common causes of bearing failure, such as corrosion, foreign particles, overheating, and overloading would similarly have affected more than one bearing.

The scanning electron microscope (SEM, for short) revolutionized the field of fractography. It has depth of focus and magnification capabilities that allow for examination of fracture surfaces at a level impossible in a light microscope. An SEM study in this case showed tell-tale fatigue striations on the fracture surface. Study of the failed bearing surface showed that in some cases the soft metal had broken completely away from the steel backing. There were also signs of rewelding of babbitt particles to the bearing surface. The back of the #3 bearing (and that bearing only) showed signs of sliding during engine operation. But what could cause such a peculiar damage pattern?

The Smoking Gun

The answer was fretting fatigue, a relatively rare phenomenon. I had heard of it only once, in connection with fatigue of chain links due to extended rubbing, or fretting during use.

Engines are assembled at about 25C, but heat up considerably during use. Thermal expansion coefficients vary from alloy, and it is necessary to insure that the main bearings are firmly fixed in the engine block during operation. This is accomplished by making the bearing a little bit oversized, so that there is what is called a press fit, or "crush" on tightening. In the absence of adequate crush, the bearing may work loose and slide back and forth during use. The result is fretting fatigue. The damaged bearing in this study could serve as a textbook example of fretting fatigue.

My lawyer-client settled the case before trial, but in fact he settled a bit too soon. Shortly thereafter the manufacturer issued an engine recall because of the fretting fatigue problem. I suspect that a more generous settlement would have been possible after the recall.