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We learnt about the bearings used in marine diesel engines in our previous article. Bearings tend to undergo wear and tear during their normal operation and can develop several types of defects during their lifetime. It is important for a marine engineer to know and identify these defects so that faulty bearings can be isolated and changed.
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Types of Defects
Wear – the relatively softer surface of the bearings rub against the harder surface to which they are providing the bearing surface. Hence when two metals of different hardness rub together, the one with the lesser hardness with wear more and this wear is accelerated by the fact that impurities and abrasive particles can enter between these surfaces thus speeding up the wear.
Corrosion – bearings are lubricated with lub oil from the engine systems and this lub oil can turn acidic in due course of time with effect of combustion products, temperature and so on. This acidic lub oil can cause corrosion of the bearings especially the white metal parts of the bearings.
Overloading/Overheating – a bearing can get damaged due to local concentration of stress or temperature and certain regions which in turn can be due to several factors. This can lead to various types of defects such as galling, scuffing, scoring and so forth.
Given below are a few pictures which show different types of defects which have occurred due to wear, loss of lubrication etc. This should give you an idea about the manner in which a defective bearing appears from outside without using any measuring tool.
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Checking for Defects
Since the main engine bearing is made out of two shells they need to be inspected for wear and tear. The bottom shell of the bearing is more difficult to remove and the sketch below shows two methods to do this. Firstly you can use a jack arrangement or you can use oil pressure which makes the shell come out of its seat.
A bridge gauge is also used to take the reading while the bottom shell is in its genuine position plus taking another reading after removing the shell and seeing the difference between them.
If the bearing is found to be too much worn or having other defects and needs replacement, the same should be done by removing the bottom shell, and fitting a new shell of similar dimensions after scraping it with a mandrel cutter or such tool to ensure proper alignment of the crankshaft journal.
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Defective Bearings: NB COJINETES DE FRICCIÓN, Spain
Bottom Shell Removal: Principles and Practice of Marine Diesel Engines by D.K. Sanyal