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Cool and Clean
How would you like to slide 300,000 km. a year, most of it at a boring 40 km.h? Fatigued? Yes, you would be. But that's about the distance the surface of the main bearing of a marine engine has to travel, irrespective of engine size, and do it for each year of its life, say 3 to 4 years. That ride, although cushioned by the oil film, is not very smooth, clearing the bearing surface by a minimum of 2 micron in a 200mm bore engine to a maximum of 0.5mm in a 600mm bore engine.
Take, for instance, an engine with a bore of 350mm having a main bearing diameter of say 300mm. The combined forces from the rotating, reciprocating and cylinder gas loads give the crankshaft journals a bumpy ride over the bearing surface. In the worst bearing case the clearances can vary between 3 micron to 0.25mm.
Maintaining these minimum clearances throughout the life of a bearing without incurring premature wear or damage caused by dirt, overheating, fatigue, cavitation etc. requires not only good initial engine design and operation within its performance parameters, but that the lube oil is kept in good condition. The engineering crew must adhere to the filtration and cooling regimes laid down by engine and equipment manufacturers.
Forget to change the filters and the pressure drop increases across them. Higher pressure drop - lower oil flow and increased oil temperature! A rise in oil temperature has the biggest influence on oil film thickness in the bearings, far greater than a reduction in oil pressure or changing oil grades. Forget to clean the coolers and up goes the oil temperature (Yes, there are thermostats, but they have a limit). Changing up an SAE grade or increasing oil pressure by 0.5 bar may increase oil films by 5% (don't operate outside the design spec.), but an increase in oil temperature by 10 deg. C may decrease oil film thickness by 30%, that's down to about 1.5 micron on a generator engine and 5 micron on a 600mm bore engine.
Filter mesh size is about 10 to 15 micron. How does the dirt go through the bearing? How do the bearings work? They do, and you have to help by keeping the oil cool and clean.
Oh, and don't forget the big end bearings. They can run safely with smaller oil films, but that's another cool clean story.
Take, for instance, an engine with a bore of 350mm having a main bearing diameter of say 300mm. The combined forces from the rotating, reciprocating and cylinder gas loads give the crankshaft journals a bumpy ride over the bearing surface. In the worst bearing case the clearances can vary between 3 micron to 0.25mm.
Maintaining these minimum clearances throughout the life of a bearing without incurring premature wear or damage caused by dirt, overheating, fatigue, cavitation etc. requires not only good initial engine design and operation within its performance parameters, but that the lube oil is kept in good condition. The engineering crew must adhere to the filtration and cooling regimes laid down by engine and equipment manufacturers.
Forget to change the filters and the pressure drop increases across them. Higher pressure drop - lower oil flow and increased oil temperature! A rise in oil temperature has the biggest influence on oil film thickness in the bearings, far greater than a reduction in oil pressure or changing oil grades. Forget to clean the coolers and up goes the oil temperature (Yes, there are thermostats, but they have a limit). Changing up an SAE grade or increasing oil pressure by 0.5 bar may increase oil films by 5% (don't operate outside the design spec.), but an increase in oil temperature by 10 deg. C may decrease oil film thickness by 30%, that's down to about 1.5 micron on a generator engine and 5 micron on a 600mm bore engine.
Filter mesh size is about 10 to 15 micron. How does the dirt go through the bearing? How do the bearings work? They do, and you have to help by keeping the oil cool and clean.
Oh, and don't forget the big end bearings. They can run safely with smaller oil films, but that's another cool clean story.
