Separate prop shaft coupling from gearbox

mogmog2

mogmog2

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My Centaur gearbox situation (monoshift gearbox on MD2B) moved on a bit last night when I managed to undo the bolts holding the gearbox to the engine (a bit fiddly, but only took about half an hour: 8mm hex socket but driven by an 8mm ring spanner on it as there was no room for any ratchet/extension combo I could devise).

I had already undone the prop shaft coupling. I undid the stern gland nuts and tried a number of moderately gentle methods to slide it back: hit/tap with hammer the rotate, hit, rotate etc, drifting it back with a hammer & cold chisel, and striking a ring spanner laterally whilst it's on the coupling through-bolt head.

Pulling in the prop from outside the boat is a possibility, albeit unappealing as the mud is grim. And cold.

Does anyone have any tips for separating the coupling-with-shaft from the gearbox? Have I got to get brutal?
 
You'll probably find that the coupling is seated on a flange on the gearbox output, so applying sideways force won't do anything. And it may be rusted too. So you'd need to try to force the parts apart in some way. Alternatively, might it be easier to release the shaft from the coupling?
 
I was thinking more shock/impact than sideways force, to break the (almost certain) rust bond.

There is a through bolt, which may not be too bad, but also a Woodruff key which I'll need to investigate. I may rather get out into the mud and whach the prop hub than deal with getting the shaft out of the coupling - from what I've read on here alone, many have struggled with this.
 
If you hit the propshaft endways you may damage the gearbox. Maybe, drill a hole in the joint between the couplings, and drive a tapered round bar, eg punch in. You would need a dumb weight on the opposite side when you hit it. take care.
 
fisherman said:
If you hit the propshaft endways you may damage the gearbox. Maybe, drill a hole in the joint between the couplings, and drive a tapered round bar, eg punch in. You would need a dumb weight on the opposite side when you hit it. take care.
Click to expand...

It appears that the coupling mating face is shrouded by the gearbox casting. I'm not sure about this, other than I couldn't see a join and the bolts go into threaded sockets (ie. there is no mating flange-with-nuts). I've realised that the length of the bolts should help indicate where the other face is.
I've got food for thought for my next visit.
Thanks.
fisherman said:
Aha, the correct way would be to drill the shaft coupling face, tap the holes and drive bolts in, that have the last 10mm of thread ground off.
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Would the tapped bolts be for attaching a slide hammer? If so, that force would be pretty much the same as hitting the prop hub sternwards?
 
When I had a similar problem I had a chat with a mechanic working on the boat next door and he told me not to waste time and just cut through the shaft with an angle grinder. I didn't want to sacrifice the shaft so I spent the next three days struggling with pullers, hammers and blowlamp and finally got it apart, but wrecked the half coupling in the process. The shaft turned out to be worn out so I had to get a new shaft anyway, which was surprisingly cheap. The new half coupling was surprisingly expensive!
 
SlowlyButSurely said:
When I had a similar problem I had a chat with a mechanic working on the boat next door and he told me not to waste time and just cut through the shaft with an angle grinder. I didn't want to sacrifice the shaft so I spent the next three days struggling with pullers, hammers and blowlamp and finally got it apart, but wrecked the half coupling in the process. The shaft turned out to be worn out so I had to get a new shaft anyway, which was surprisingly cheap. The new half coupling was surprisingly expensive!
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A bit like my experience when i couldn't separate the shaft from the coupling.

After cutting the shaft in two, I found it was well worn in the cutless and gland area and would have needed replacing anyway.
 
I'm trying to get the coupling separated from the gearbox, not the coupling from the shaft which I understand is a task and a half.

I'm not sure how cutting the shaft would help me pull the thing off the gearbox? If anything it gives me fewer opportunities.

This season I need to replace as little as possible as we're planning on replacing the engine soon (but not this season) which will mean new everything anyway.

If, and I may be wrong pending closer inspection, the coupling flange is partially shrouded by the gearbox casting, I couldn't cut that anyway. But we'll see. I've found that ASAP list a 1" coupling (not checked if suitable for The Penta MS gearbox) and that is £77, which could be worse.

I'm prepared to exhaust less destructive methods first though.
 
mogmog2 said:
My Centaur gearbox situation (monoshift gearbox on MD2B) moved on a bit last night when I managed to undo the bolts holding the gearbox to the engine (a bit fiddly, but only took about half an hour: 8mm hex socket but driven by an 8mm ring spanner on it as there was no room for any ratchet/extension combo I could devise).

I had already undone the prop shaft coupling. I undid the stern gland nuts and tried a number of moderately gentle methods to slide it back: hit/tap with hammer the rotate, hit, rotate etc, drifting it back with a hammer & cold chisel, and striking a ring spanner laterally whilst it's on the coupling through-bolt head.

Pulling in the prop from outside the boat is a possibility, albeit unappealing as the mud is grim. And cold.

Does anyone have any tips for separating the coupling-with-shaft from the gearbox? Have I got to get brutal?
Click to expand...

I did this job some months ago and managed to separate the coupling from the gearbox using big flat screw drivers as a wedge and a BIG hammer.

This slide hammer on the end of the prop shaft may also help.

1823l.jpg


 
fisherman said:
Drilling and tapping the coupling is so the bolts push the halves apart, not slide hammer.
Click to expand...

Ok thanks. I keep nearly buying a tap set. Maybe now's the time. I'll have a try of other methods this afternoon. I also think, from my research, that the flange faces may actually be accessible to try and split them, although I feel like I read something about being very wary of levering against the flange though.
 
fisherman said:
yerbut, a car driveshaft doesn't have thrust bearings.....
Click to expand...

Thay do actually by virtue of either deep groove ball bearings, angular contact ball bearings of even taper roller bearings.

The thrust in my PRM gearbox is taken by a pair of taper roller bearings, F4 an F10 below

PRM160_outshaft.JPG
 
mogmog2 said:
Ok thanks. I keep nearly buying a tap set. Maybe now's the time. I'll have a try of other methods this afternoon. I also think, from my research, that the flange faces may actually be accessible to try and split them, although I feel like I read something about being very wary of levering against the flange though.
Click to expand...

It will be laborious, with the shaft in the way you will only get a half turn of the tap at a time. Sharpened screwdrivers opposite sides, use a good deadweight/dolly hammer opposite where you hit.
 
fisherman said:
It will be laborious, with the shaft in the way you will only get a half turn of the tap at a time. Sharpened screwdrivers opposite sides, use a good deadweight/dolly hammer opposite where you hit.
Click to expand...

Have to say I've gone off the hitting away approach somewhat.

Firstly going to lump it one laterally once or twice with a club hammer & another opposite as dead weight to see if it happens to let go. Then #2 approach is driving things between faces (if accessible) I'm just getting the tools together and looking for screwdrivers to sacrifice.

I did wonder if the prop was under tension (winched from boat on the other pontoon) might help efforts in breaking the joint?

By the time I've succeeded or failed, the tide will be right & I'll hopefully blag a sail with someone :-)
 
It gave up without a struggle. A couple of knocks sideways and the joint started to open & I gently drove wedges into the gap.
Thanks for your input.
 
Hi to all, I reduced the output flange from 60mm to 40mm to fit my actual coupling prop shaft 40mm. And now I hear a bump everytime I put farward or reverse . Can that bump caused after reducing the output flange ? Many thanks to all
 
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