Main Bearing Check

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Max Oiltemp
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Main Bearing Check

Postby Max Oiltemp » Wed Feb 08, 2012 2:23 pm

If, during a partial overhaul, one was to drop a single main bearing on a 9 cylinder medium speed genset engine of 1800 Kw to check for condition, which position should it be?

Beer may be riding on your responses! Thanks

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Big Pete
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Re: Main Bearing Check

Postby Big Pete » Fri Feb 10, 2012 1:42 am

I suppose that all depends, do you want to find the one in worst condition, or the one in the best condition?

Most Engine makers suggest thay you open up one or two mains at random, to check the condition, and if they are OK no need to open up the rest, If they are in poor condition then you have to open up all the bearings to check / renew them.

If the engine is misaligned, there will be a lot more force on the drive end main bearing and that will become more worn than the rest, the overhanging mass of the flywheel can also cause more wear in the lower bearing supporting it. The "bend" in the shaft caused by the weight of the flywheel or misalignement can also cause excess wear on the opposite face of the next bearing.

If the chocking or supporting structure for the engine has failed causing excessive crankshaft deflections, the deflections may also indicate which bearings are likely to become damaged.

The tension on a chain drive for a camshaft, (remember them?) can also put more load on the adjacent bearings and cause them to wear more.

Some engine makers may advise you, either in Service Bulletins, or if you email the Technical Department, if any particular bearing number is prone to problems.

Some makers specify that the bearings should not be refitted if they have done more than a certain No of hours because they say the bearing will no longer conform to the journal after it has been disturbed. I sailed with Bergens, the mains were supposed to be good for I think 24,0000 hours but if you opened a bearing after about half that you had to scrap it.

I hope you get a few more comments, and let us know who won the beer!!

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Re: Main Bearing Check

Postby JFC » Fri Feb 10, 2012 9:51 am

I have always inspected the Thrust Bearing, to get as much information as possible with one bearing.......Not always a pretty site, but usually an indication of a few things in the history of the machine.....


Max Oiltemp
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Re: Main Bearing Check

Postby Max Oiltemp » Fri Feb 10, 2012 12:53 pm

I suppose that all depends, do you want to find the one in worst condition, or the one in the best condition?

I do believe that may have been the reason the shoreside mechanic protested so vigorously against my suggestion that we look at the drive end bearing, preferring instead to remove one at the centre of the crankshaft. So vigorously, in fact, that I began to wonder if my reasoning (as so well expressed by Big Pete and JFC) was still valid in these days of 1000 rpm diesels on rubber-mounted integrated generator/engine foundations and flex couplings. Both bearings looked fine. I may have trouble actually collecting the beer though - he's gone back to Europe already.

Thanks for the responses,

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Re: Main Bearing Check

Postby JollyJack » Wed Feb 15, 2012 5:22 am

I'd look at the drive end too, and take a look at the thrust bearing at the same time. For the reasons stated, I belive this is the area in which the greatest wear takes place.
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Re: Main Bearing Check

Postby jimmys » Tue Feb 21, 2012 9:06 am

I usually look to drop a bearing with a crank at each side if it. Tend to be the heaviest loaded. I also look to see if continuous chocking if not drop in the gap of the chock. Tend to vibration, always a chock at the end.
Last failure I attended was a Doosan Pielstick PC2-5 Vee 16. Main bearing failed under load at full speed. Not in international waters. Main bearing failure was in middle of engine. The bearing was a steel backed thin wall lead bronze with a tin lead overlay, tin lead gone, lead bronze held and crankshaft just needed a polish. The engine did not trip the engineers stopped it and put a new main in at sea. Steamed in at reduced speed and was promptly detained. Hours on bearing around 25,000. The vessel was a single engine tanker. Class had no program for mains opening at these hours. All mains were removed for inspection and around half were not satisfactory, too much visual lead bronze. The thrust was at the aft coupling face and was ok. I have seen a lot of failures most in the engine middle. A bucket job to remove the bits in a passenger ship engine in Scotland, right in the middle of the engine, it nearly always is.
PS the hours for passenger ship medium speed main bearings change was around 15000hrs would not go to 25,000hrs
This engine had an upgrade around the crankshaft to increase oil film thickness and increase bearing life.This engine is best forgotten about.

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