The Flying Flywheel

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swashbuckler

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In a classic example of confidence far exceeding any mechanical competency, a recent attempt to clear the cooling passages on my Bukh DV10 went spectacularly wrong after failing to re-attach the flywheel to the crankshaft with sufficient torque.

Seeing the flywheel shoot across the cabin on starting the engine was a highlight of the weekend! An engineer has since extracted the remains of the bolts from the crankshaft and reattached the flywheel, retightening the bolts in excess of the Bukh recommended torque setting.

We have now run the engine for about 4 hours without a problem but I'm concerned this might happen again as the tops of the holes into the crankshaft are elliptical in shape (running tangentially to the perimeter of flywheel). I estimate that the 6 threaded bolts keeping the flywheel attached to the crankshaft are about 5mm diameter and the holes in the top of the crankshaft are exaggerated to a size of about 8-10mm wide for the top 3-5mm.

There doesn't seem to be any play between the flywheel and crankshaft and if anything, the engine seems to run quieter since the flywheel was reattached.

The engineer suggested getting the crankshaft replaced or machined after a season or two. Is this likely to be necessary if the flywheel stays attached for the first season and bolts are kept tight with a torque wrench ?

Has anyone else experienced a similar problem? Any advice gratefully received...


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If I understand correctly, it's the threaded hole that's been damaged?

So, I'd say that if the correct (and more) torque was achieved, then there's enough metal there for this time. Next time might not be so lucky, so I'd budget for fixing it next time the flywheel come off.

<hr width=100% size=1>my opinion is complete rubbish, probably.
 
I agree.....

...and a further point to consider is:-

If you think it sounds quieter now, then isn't it possible that some of the screws were loose previously allowing the flywheel to "nod"?

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion
 
Re: I agree.....

Somewhere in the depth of my memory is the rule that the maximum holding strength of a bolt is achived with 7 threads - after that the CSA comes in to play

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Re: I agree.....

Blimey, I knew the Child Support Agency poked their noses in everywhere but I didn't know they also fixed the holding power of bolts. You live and learn /forums/images/icons/wink.gif

Tony C.

<hr width=100% size=1>There are 10 kinds of people, those who understand binary and those who don't.
 
I"m inclined to think the fretting of the holes occured over time, and not just on start up. Regardless, "overtightening" is a no-no, and nobody who claims engineer status should ever do it. I'd be inclined to find another engineer and see about helicoiling the crank to eliminate the problem. Most unlikely you'll find anyone who'll do it with the crank in situ, but you may be lucky - it really needs to be jigged in a mill while the holes are drilled.

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Personally I wouldn't consider helicoiling the crank. The load on these fixings is enormous, and helicoiling isn't really up to it...

I do agree with the earlier comment, that if torque can be acheived then you are probably OK.... one thing to do would be to check them after a few hours running to see if they have held the original torque seeting.... if they have 'fretted' loose then get the crank replaced..... its not really possible to repair such a high load area.... also one other suggestion.... when you take the boat out of the water next time, do yourself a favour, and take the crank to a specialist to be tested for both straightness, and also crack tested (often a magnaflux test)... I don't know about your engine type, but some marine engines are balanced with both the flywheel and crank, and them being badly out of balance (ie the flywheel falling off!) could cause some serious damage if continued to be used.... serious enough to require a new engine....

To end on a positive note.... if you've re-attached it, and its running happily, then your probably alright!

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Steve wrote:
>Is thread lock not an option as well ?

The real problem here is that the threaded holes are elongated in their upper section - thread lock is appropriate to stop bolts coming loose in their threads, but not to replace lost thread. Someone posted that the helicoils I suggested are not the go, however with this engine they are entirely appropriate. The flywheel carries only the loads of storing energy while running, and holding the ring gear, being on the front of the engine.

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Good to see the YBW forum being used to bolster confidence and attempt to repair dented egoes (if not toes...)

Personally, I think there can be no better life enhancing experience than consistently attempting to fix complex problems with little or no experience and only rudimentary tools only to watch the fruits of your labour go south beaten only by the deceleration in your bank account.

Lesser mortals have been known to blow crazy money (often well in excess of a hundred pounds) and waste precious tinkering time being tutored in the basic principles of boat maintenance.

I salute you for having the courage to ignore your obvious incompetency and attack these jobs with such vigor. I am confident that with enough ££££ behind you, and notwithstanding the smirks of resident engineers (who, after all, enjoy nothing more than clearing up after your botched attempts),you will soon move on to bigger and more spectacular screw ups.

Happy sailing, may you always enjoy a broad reach, a welcoming smile and a hot pasta and tuna bake on your return.

Next time, use more 'no more nails' and try whacking the 3 inch long allen key a bit harder to try to achieve the 80Nm of torque required



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It is common practice in performace tuning of car engines to remove the top few threads to prevent streses at the joint surface.

Also, it may be worth tapping the holes with a plug tap all the way to the bottom and making bolts to fit the entire depth of the holes. Thus using all the available threads.
PM me if that makes no sense.

If it aint broke, don't fix it.

D.

<hr width=100% size=1>Last weekend was shopping darling........ so this weekend is boating. (Duck!)
 
Re: I agree.....

Hi Tross,

You are almost right, it is 70% of the bolt diameter.

What it means in practise, even if you make a nut 5 times longer than standard, the bolt will stretch somewhat and this part carries all the force. Further away from the force, there is (after 70% diameter) no more laoding.

Hope you understand what I am saying.

regards ongolo


<hr width=100% size=1>So what......... it floats
 
How about wireing the heads together? This will need a small hole drilled in each head though - very easy to see if a bolt unscrews as the wire breaks.

<hr width=100% size=1>dickh
I'd rather be sailing... :-) /forums/images/icons/smile.gif
 
Re: I agree.....

I've always worked on 3 full threads engagement = full load capacity, which on an M10 coarse amounts to 6mm (near your 70%..). But then being a stress engineer first & foremost I tend to allow much lower loads on a bolt than many seem to get away with.

The best way of getting more engagement working is to use a nut with a stiffness equal or less than the bolt - like the sort of skinny little nuts you see on aeroplanes that seem to be little more than a threaded bulge that goes around the bolt. This type of joint generally costs more as it should really be analysed in each case.


Next thing - waisting of cylinder head bolts.
There are quite a few misconceptions about this running about in the wild, which maybe I can help to nail down. Cylinder head bolts are frequently waisted, for two (and usually only two) reasons:-
1) If you waist the shank just below the head it's possible to get a smoother stress distribution between the shank and the head. This means that the stresses are lower than at a sharp corner between the shank and head, so the head's less likely to break off at a given torque.
2) It can improve the fatigue life. The idea is that: If the bolt is more flexible than the joint it's holding together, when a load is applied across the joint, the joint load is preferentially reduced with minimum change in the pre-load in the bolt. The greater the stiffness of the joint and the less the stiffness of the bolt the closer the bolt will approach to zero stress change. With no stress change, the bolt can't fatigue. Waisting of the bolt reduces it's stiffness at the expense of increasing the overall bolt stress and making it more susceptible to fatigue (smaller cyclic variation does more damage as there is more energy in the system to propagate cracks) - it's a fine balance and carelessly waisting a bolt can make things worse, just as carefully doing it can improve the situation. Once again this is a case where individual analysis is necessary, and I wouldn't recommend it without the analysis.

Regards

Richard.


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