Engine parting company with gearbox. Help!

[Deleted User YDKXO]

Sorry, not a Volvo problem this time
Three of the bolts fixing the gearbox housing to the port (Sabre 275L) engine in my boat have sheared within as many months, each one individually and in different locations. No shaft alignment problems, no noise/vibration problems and no evidence of over torquing the bolts. Each bolt has sheared cleanly at the interface between the housing and the rear of the engine which tends to suggest housing is trying to rotate relative to the engine rather than move laterally or longtidunally. Starboard engine all OK. Engines about 900hrs operation
Mechanic and I are mystified. What does the panel think?

 

[Col]

Hmmm, no vibration eh!
It then points to missing locating dowels, most engines have 1 either side, I would check there first.

 

[kimhollamby]

Bring back the old Sumianda?

First step I would have thought was to renew the balance of the old bolts and then see what happens next, provided you can get at them of course. Whilst I cannot quite figure out an incident in the past that may have stressed them, if the new ones survived it might give you a pointer?

kim_hollamby@ipcmedia.com

 

[Guest]

Suggest replace with Volvo Penta. Far more likely to break in understandable fashion, and far sooner than having to wait 900hrs...

 

[jfm]

Tighten the bolts even more!

Sounds strongly like the bolts are not tightened enough. The failure sounds like fatigue. I assume each time you put a new bolt in, that was ok, it was another old one that bust? You say they have not been overtightened, but that is the problem, they're too loose. Torque them up loads more. Under tightening bolts in these situations (I mean vibrating machinery) is what causes fatigue failure, not overtightening. Undertightening the bolts reduces the absolute stress applied to the metal in the bolt (obviously) but it increases the stress amplitude, I mean the cyclic variation in stress which the bolt suffers as the machinery vibrates. Tightening the bolts increases the absolute stress but reduces the stress amplitude. Fatigue is caused by too much stress amplitude, not by too much stress. Therefore you should always make sure these bolts are torqued up to the maximum torque specification.

Also could be a slightly duff batch of bolts, hence one side went and not the other?

You say they sheared, suggesting the parts tried to rotate, but I think that analysis may be incorrect. When you bolt together two flanges (say) and apply a torsional load, the bolts are not subjected to any SHEAR load at all. At least they shouldn't be. The bolts should be highly loaded in TENSION, and the torsional load is transmitted entirely by the friction of the two flange faces. For similar reasons, the comment about dowels in another post isn't correct. The dowels only align the parts, they should take zero shear loading, all the shear loading is handled by friction between the mating faces

I think

 

[KevL]

Re: Tighten the bolts even more!

Spot on there jfm. But I'd look up the torque settings for the bolts before "torque them up loads more" that is assuming you don't like extracting sheared bolts. It might be worth considering upping the bolt spec if poss.


--
Tides - Never there when you need one.
Wind - Always there when you don't.

KevL

 

[jfm]

Re: Agree, should read the manual

Yes, was a bit tongue in cheek. What i meant was, bolts usually fail from fatigue by under torquing, not over torquing. A bit counter-intuitive, but true

Yes, upping the bolt spec could be a good idea. The spec is denoted by the number codes stamped on the bolt head, can't remember the code off hand. But if you put higher tensile bolts in, you must increase the torque as higher tensile bolts can be more susceptible to fatigue and prefer less stress amplitude

Forgot to mention in other post Deleted User, was corrosion a contributor to stress cracks too? Or were they pefickly clean? If suspect corrosion, def replace em, obviously

 

[Guest]

are they the original bolts or do they look like replaced ones, ie same paint on head. If not change to a high shear stength bolt. see a local engineering factors

 

[jfm]

.......tensile, not shear

 

[Deleted User YDKXO]

Re: Agree, should read the manual

Excellent advice as usual, jfm. Tomorrow, I shall buy super new spanner and extra long scaffold pole to get max tightening effect
No sign of corrosion causing shearing as breaks were all clean

 

[Deleted User YDKXO]

Already thought about renewing all old bolts except my 'man wot does' says he will have to cut hole in hull to reach bottom ones but certainly worth doing what can be reached.

 

[jfm]

Re: calling mick carburettors

I'm still waiting for Mick's (with tugboat) contribution to this thread. Expecting his advice to be "forget the bolts, weld the b@gger up" <BG> (Apols mick )

 

[hlb]

My boat breaks engine mountings for fun. Cant find nuffing wrong. Just stick some new bolts in and see how it goes on.

No one can force me to come here-----------
----- I'm a Volunteer!!!

Haydn

 

[mtb]

Re: YES I\'m here

I agree with you but also , if the bolts have been tightened to much in the past they will have stretched which could of course weaken em.
Also is the gearbox bellhousing distorted hence alowing one side to be loose as aposed to the other .

I would assume new spring washers were used .

One last but very unlikely possibility what's the fly wheel bush like .

Oh and weld the b####### up !!

No No only kidding you could do the old fashioned way wire the bolt heads ( yes I know you would need to drill em but it is in my view better than Loctight.
Mick

http://homepage.ntlworld.com/boats
I want a big steel ex trawler / tug v / cheap or swap for tug

 

[sonarbell]

Maybe wrong here, but should have 8.8 stamped on the head for High tensile.

 

[jfm]

Re: tensile strength grades

Yes correct I think. That's the "standard" HT grade, there are higher ones than that. I seem to recall that, although the numbers are axshully metric, you get near enuf the right answer by multiplying. so 8.8 = 64 tons per square inch tensile strength (as opposed to 23 for mild steel). But not sure, it's bin a long time.... can you remeber better?