Gearbox to prop shaft connection - lose bolts?

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Ben998

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Hi All,
Ive have got a sun odyssey fitted with a 37hp Yanmar diesel. The gearbox is connected to the prop shaft via a couple of flanges that are bolted together with four stainless steel nuts and bolts.
Whilst doing some work on the boat today I noticed that all of the nuts and bolts on the 'connecting flanges' are lose, so much so that I can spin them by hand. The nuts are nyloc locking nuts so they arent about to fall off but should the nuts and bolts be tight, or are they lose to allow for any movement between the gearbox and propshaft?

cheers,
Ben
 
Erm - I believe they should not be loose! Loose fittings only lead to premature wear of parts that are not designed for it!

If you can spin Nyloc nuts by hand then they are knackered ... replace with new ones or some glue when you do it up tight!
 
Thanks for the reply. The nyloc nuts are fine, they just aren't done up tight. I thought they might have been done like that to allow for some movement in the join - in a sort of similar fashion to one of the Vetus flexible couplings but without the rubber cushioning part.
Out with the spanners tomorrow morning then!
cheers,
Ben
 
Firstly, the nuts/bolts should be tight, but as an engineer I can tell you that I can't give you a reason why, BUT even when you use all new nuts/bolts for some reason they always seem to work loose but when you re-tighten them they seem to stay put.

I recommend that you re-tighten them as hard as possible using standard spanners and then re-check them on a regular basis. The frequency of the check can be determined by you IE you found them loose so check after (say) 10 hrs running time, if they are still tight check them again after (say) 20 hrs and so on until you get to a point of checking them maybe once or twice a season.

If the problem persists in as much as you keep finding them loose then consider using Loctite "nut-lock" as well with the nyloc nuts.

Peter.
 
[ QUOTE ]
Firstly, the nuts/bolts should be tight, but as an engineer I can tell you that I can't give you a reason why,

[/ QUOTE ] .... good grief! /forums/images/graemlins/shocked.gif

If they are loose then the facing plates will move as load comes on, applying unfair shear loads to the bolts. Also in reverse the coupling will be pulled apart and the bolts take the stress load sideways. We all know how easy it is to bend and break bolts.

I would start by replacing the nuts and bolts as they are almost certain to be damaged - stress cracks are often invisible in Stainless before they fail. Nylocs are 'single use', and are supposed to be renewed every time they are loosened, though they will often hold reasonably well in re-use.

Properly tightened down the faces of the coupling are forced together and most of the engine torque load is carried by the friction between the coupling faces, very little by the actual bolts. Thats why some makers specify bolt torque.

If bolts are working loose: 1. They are not being tightened enough in the first place, 2. there may be alignment problems overloading the coupling bolts.
 
Just a thought, but on an all-steel engine, wouldn't it be prudent to have 'normal' steel bolts in this situation as opposed to stainless?
 
An aircraft mechanic of my acquaintance was very disparaging of both stainless steel bolts and standard nyloc nuts. The former because they tended to work loose and anyway had poor sheer strength and the nuts because the inserts tend to degrade quite quickly. The blue nyloc type are better but locking nuts with a split in them are even better.

The difficulty is in getting engineering quality components in the UK. Screwfix stuff is cheap and not really up to that sort of job. You really need good HT bolts, locking nuts and Loctite as well. Lord knows where to get them wherever you are but Namrick - "The Nut and Bolt" shop in Brighton (well Hove, actually) are knowledgeable and do a mail order service www.namrick.co.uk
 
Yep, I agree that normal mild steel aren't the answer. I prefer high tensile bolts with Nylocs BUT also add a locking nut to each. Since we never get water inside the boat the question of rust hasn't arisen.

I have used this method for over thirty years and never had a single bolt slacken off.
 
Also try this company
http://www.bbnonline.co.uk/online/index2.aspx

Use them a fair bit and no complaints,

As a footnote remember if its threaded all the way up to the head its a 'screw' and if theres an unthreaded part then its a 'bolt',
Speaking of which, ben are they nuts and bolts or screws that are coming undone? Hi tensile bolts should be used, these will also resist stretch better and as others have said they should not undo if correctly tightened.

Mike
 
Oldharry
Maybe I didn't explain it very well in my post. What I was trying to say is that I have lined up loads of engines and most within a few thou' and using new nuts & bolts I've tightened them. BUT for reasons that I can't explain after a few weeks I've often found them starting to loosen - all I do is re-tighten them and after that they stay tight. I don't re-align the engine - just do'em up.

[ QUOTE ]
2. there may be alignment problems overloading the coupling bolts.

[/ QUOTE ]

As an engineer I find the question of alignment on a modern boat a bit of a laugh when almost everything is on the "float".

Engine sits on soft rubber mounts.
Most have a flexible coupling between the shaft & g/box.
The inboard bearing is normally a floating type.

The only part of the shaft that is anywhere near supported is the out-board or cutlass bearing after that everything is allowed to move.

If anyone doubts that - the next time your underway (1/2 - 3/4 throttle setting) take a quick squizz at the flexible coupling and watch it "hunt" backwards and forwards and generally move about.

However, I do still try and get everything in line even if the engine is at a standstill - at least it gives you a fighting chance.

Peter.
 
OK - I agree about the joke of trying to align rubber mounted or 'floating' engines - they go all over the place even not running. I reckon the only answer has to be a flexible coupling.

But I am puzzled why you are having bolts undoing - managing a fleet of Trip boats checking engine mounts, couplings etc was part of the regular ongoing safety and maintenance routines, and while I regularly had engine mounts coming adrift or undoing themselves - interesting vibes with 2-300hp power units - I do not remember ever having coupling bolt problems, although I had a schedule to replace nylocs every time they were released, and the bolts every third time they were disturbed.

Having said that IIRC we used to take them up to within 10% of the highest specified torques recommended by the bolt manufacturers/ suppliers. We used HT bolts, as stainless was reckoned to give less warning of failure if it was flexing or over stressed - not that I ever saw one fail in service!

The only explanations I can think of are either that the coupling is bedding in in some way, or the bolts themselves have slightly rough or irregular threads which again are bedding in or stretching slightly in service but neither of these sounds very likely somehow.

As far as Ben998's bolts are concerned though, I would think they were never torqued down properly in the first place after the engine was aligned, if he can spin them, and would almost certainly have been over stressed by taking all the engine torque.
 
Hi Ben I have a SO34 with a Yanmar 3GM30 (27hp) the nut holding the inboard connecting plate to the gear box spindle came adrift as we were raising the anchor in the River Guadiana between Portugal and Spain which was flowing at 4kn,The connecting plates stayed together and fortunately we were able to get a tow to a jetty where we found a liveaboard couple who had the right size socket set to effect a repair.It could have been a lot worse but someone was looking after us that day.
 
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