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Bearing wear

Even after more than 30 years in the engine business, the bearing systems in modern internal combustion engine still amaze me. Made from a mixture of aluminium and tin on a steel backing (for bi-metallic examples) or a mixture of copper, silicon and nickel topped off with an aluminium-tin overlayer, even after many hours of running, after stripping the engine I have seen bearings as good as new. And of course this is what the designer is aiming for because in theory, once rotating, the bearing remains in a hydrodynamic (or elasto-hydrodynamic) lubrication regime with no contact between crankshaft and bearing again until the engine stops. So long as the oil film layer in the bearing supports the combustion and dynamic loads generated, and the oil film does not break down, then job done.

But bearing failures do occur, and sometimes the designers do not get it right, but more likely these days, some other event in the engine build process goes awry. Thus when a bearing exhibits, say, fretting, fatigue, or failure due to cavitation then the fault generally lies in the engine design process. In other cases, however, when for instance the failure is wear-related, things are not always so clear-cut.

In theory, all bearings will be subject to some degree of wear. At initial cranking, when the surface asperities of journal and bearing are interlocked, any relative movement is bound to generate friction and a bit of wear. Once the crank is moving, however, and the 'wedge' of oil is generated by the relative movement of the two surfaces, so long as the loads can be supported by the oil film then - again, in theory - no further wear can take place. I say 'in theory' simply because engineering components are not perfect things, and bearings journals are never perfectly round, or in some cases the bearing housing is not totally rigid so that bearing needs to adjust itself to accommodate these slight imperfections. Once 'accommodated' though, any wear will gradually reduce to a stable geometry. In the past we would have called this 'running-in' or 'bedding-in' - a process very familiar to those over a certain age.

But other forms of wear can be more serious, and they will all have much to do with either the cleanliness of the build environment or that of the lubricant used.

The most common form of bearing damage is often referred to as 'embedded debris' damage. Much as the term suggests, foreign particles become embedded in the soft overlayer, often surrounded by a ridge or 'halo' where the displaced bearing material has been forced up and then smoothed over by the passing of the crankshaft surface.

A result of poor cleanliness during build or poor filtration, most of the material found originates from the drillings in the crankshaft, oil galleries or casting sand when the quality of the end-of-line wash leaves much to be desired. Dust or dirt in the atmosphere settling on the bearings during build can also cause similar but less critical debris damage. Modern bearings, because of their lack of soft lead overlays, are perhaps less tolerant towards this kind of dirt.

Other forms of damage can occur, for instance, debris is caught between the bearing and its housing. Pressed into the inside diameter of the housing, the resulting distortion will feed through onto the running surface, resulting not only in accelerated wear at the point where it touches the rotating journal, but a localised increase in the temperature caused not only by friction but the reduced capability of transmitting the heat away and into the structure of the crankcase or cylinder block.

It may be obvious, and it may be like telling your granny to suck eggs, but complete cleanliness during the engine build process - while essential - is in practice quite difficult to achieve.

bearings

Fig. 1 - Main bearing after only a few hours' running. Notice the extreme wear on the right-hand side and the scoring as the engine finally failed

Written by John Coxon

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