Bearing distress

Thursday, March 27, 2014

Tags :  bearings

I have read a report that says 55% of all bearing failures are attributable to the presence of dirt. Referred to more accurately in the report as “foreign body contamination” in this context, dirt is defined as being “any impurity, be that fine dust up to coarse casting sand or metallic particles resulting from machining swarf, iron, aluminium or copper”. It’s an interesting statistic, and one that engine manufacturers make every effort to minimise to the point where such failures occur at considerably less than one per 10,000 in warranty. If you then consider the amount of casting sand, machining swarf or simply airborne dirt that exists during the manufacturing process (casting, machining transporting and final assembly), and believe me there is a lot, this is indeed an impressive statistic – unless of course your engine is that one in 10,000!

However, the report went on to explain that second in the ranking of bearing failure, jointly with faulty or incorrect bearing assembly (another manufacturing issue), came “insufficient or unsatisfactory lubrication”. Described as the point at which the continuous oil film between the surface of the bearing and the journal is interrupted, the result is some kind of surface damage – polishing, wiping or smearing and scuffing, leading eventually to the engine’s demise. However, unlike the ingress of foreign matter, the lack of a continuous oil film can mean many things.

In the engine build process for instance, bearings have to be pre-lubricated with oils to ensure the existence of the oil film right from the moment the engine is cranked for the first time. There may be engine oil in the sump, but in the time taken to pump this oil through the system to the bearings for the first time the engine crankshaft may have revolved many times. Without any lubricant at this critical time, serious bearing damage will take place. During engine build therefore it is common to use special ‘build’ oils with a viscosity generally far greater than that usually specified for normal engine running. This extra viscosity ensures that the oil doesn’t drain away in the time between assembly and first fire, but also during the build process it can help centralise the bearing cap upon assembly – the other joint second issue highlighted in the report.

In the special case of the bearings, this oil should introduce no new additive technology to the engine other than that destined to be used in the engine sump. Since once the engine has started to rotate, the lubrication regime will be totally hydrodynamic, no additional oil additive technology is strictly necessary – save perhaps for anti-oxidants and possibly a corrosion inhibitor. Detergents, dispersants, friction modifiers and anti-wear additives are not really necessary, unless of course you want to use the same oil to lubricate the camshaft. In this case, certain aggressive camshaft designs may benefit from additional anti-wear technology to prevent scuffing during this initial cranking, so the best advice I can give is to use a product recommended for engine build by your engine sump oil supplier. This will most probably do the job and be compatible with the engine oil you eventually intend to run.

Once the engine is running, you should have no further issues with the engine bearings, provided of course you use the correct grade of oil specified by your engine designer and change it regularly. The engine manufacturer will spend a lot of time selecting the appropriate grade of oil to make sure you get that little bit of extra performance and yet protect the bearings for all eventuality, but that is a story for another time.

Fig. 1 - Big-end bearing showing signs, among other things, of foreign body contamination

Written by John Coxon

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