Engine vibration mystery...

[rays]

Last season I noticed a vibration when in gear (smooth as silk in neutral). This is a Volvo MD22L with a shaft drive and a volvo rubber shaft seal. Engineer checked engine/shaft alignment when afloat at beginning of last season. Perfect. Likewise engine mountings. The vibration continued. Worst at 1500-1900 revs. Got nasty above that, so had to curtail revs, not exactly conveinient! Made arrangement to have prop and shaft removed for checking whilst ashore this February. Prop was checked and found to have two blades unbalenced. Now been machined and declared perfect (certainly looks like new!) Once the prop was back on I spun it around by hand and we noticed the prop shaft moving from side to side ( about 1/8" maybe a little more) at the point at which it enters the hole in the hull. Engineer and I remove shaft which has now been checked by a well-known and respected engineering company in Ipswich (East Coasters will know who I mean). They have just told me that the shaft is only 1/5000" out of true. I can't believe that and can't believe it would cause a vibration of the magnitude we suffered previosly. Also, not consistent with our eye balled out of true movement, albeit at the middle of the shaft.

Has anyone got any idea what could be going on here?

 

[npf1]

Does the shaft run true just behind the coupling? The reason I ask is I found on mine that the shaft to coupling fit is a bit slack, hence when the keyway retaining grub screw is tightened, the shaft is moved out of centre.

 

[Rowana]

I would agree that if, as you observed, the shaft is running "out of true", and has been checked, then the next thing to look at is the coupling.

What type is it? It could be that the face of either half is not exactly perpendicular to the shaft/gearbox. I'd get a clock guage on it personally.

 

[matelot]

A practical test for shaft alignment is to sail the boat with the engine in neutral and see if the rotation of the prop and shaft causes the engine to wobble. If it does then the alignment is poor.

If the shaft moves about in the stern tube and the shaft is straight then that also suggests alignment.

You can get vibration due to one of the four engine mounts going soft or alternatively to the engine not putting the proper share of weight onto each engine mount. Way to check is to disconnect the shaft, undo the engine mount nuts, and put a crowbar under each end of the engine in turn. The mounts should lift clear of their base at the same moment

Last thing - the engine is firing on all cylinders isnt it?

 

[PuffTheMagicDragon]

Another thing to check with the mountings is whether one of them is slightly twisted. This sometimes happens when the lock nut is being tightened and not enough care is taken to ensure that the stud does not rotate. If it does - even by a small amount - the twist in the rubber will be enough to make that particular mounting harder than the others, giving you vibration. Happened to me on the SilentBlocs of my Lombardini.

 

[robmcg]

I would put money on the engine mountings being shot. Visual inspection no use whatsoever. Our Moody has a Volvo 2003. It had a similar issue to what you are describing. Between 1800 -2200 rpm the vibration was unbearable and audible - bit like a grinding type noise. Engine mountings looked A1. Changed all 4 anyway and hey presto problem solved. The rubber in the mountings can go hard but it still looks like they are OK. Not a cheap fix mind........ouch!!!! /forums/images/graemlins/shocked.gif

 

[Bilgediver]

What type is it? It could be that the face of either half is not exactly perpendicular to the shaft/gearbox. I'd get a clock guage on it personally
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I don t know how in a modern machine shop this can happen but I found this on a friends boat when he was installing a new yanmar and shaft etc and friend couldn t understand why he couldnt get alignment with the shaft at all 4 ninty degree positions. If he had run the engine he would have had your problem plus broken coupling bolts at a later date.

If the coupling halves appear perfectly aligned then rotate the shaft 90 degs at a time and recheck flanges are still face to face at all four positions do not turn the engine coupling half.

I also second the question about wether all cylindrs are firing properly as a duff injector can give these symptoms as one other poster observes.

 

[pampas]

If you can move the shaft side ways I would suspect the bearing inside the hull is worn. You did say where the shaft comes out of the hole, implying that you have no external stuffing box. Worth a look.

 

[rays]

Thanks to all respondents.
1) Re: engine firing on all cylinders. Yes, it is. Runs very smoothly until put in gear and revs increased. If you then look at the shaft seal it is moving from side to side/up and down.
2) The coupling. This sounds like it's worth further investigation. I assumed the shaft and coupling were run up together. I'll check again.

 

[Bilgediver]

2) The coupling. This sounds like it's worth further investigation. I assumed the shaft and coupling were run up together. I'll check again

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You implied that when you spun your shaft in the lathe there was no run out. Therefore if the shaft in way of the seal is throwing it around then the shaft seems to have become bowed and one way of this happening is to have a faulty half coupling where the face of the coupling is NOT at 90 to the axis of the bore and so bends the shaft what the coupling bolts are tightened up. This should not happen but it does.

This assumes your cutless bearing IS unworn and the propeller is running true.

Continued running like this will cause excessive cutless wear and probably fatigue failure of the coupling bolts.

WHen checking on alignment dont just make one check at the gearbox coupling, rememebr to do 4 checks turning the shaft half of the coupling 90 degrees between each check. All readings should be the same and mean the flange is square to the shaft however if the readings vary then the flange is not true.