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Propshaft coupling problems


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Got problems with the propshaft and coupling. We towed our friends 70' narrowboat to the drydock to be blacked after their skintank failed and the extra strain on the drivetrain has caused the shaft to slip in the coupling. the coupling is now quite worn but the shaft seems to have lost little in the way of diameter but is rather scuffed. I would quite like to fit a centaflex coupling, would it clamp up alright on the scuffed shaft? R & D marine do a bobbin which would allow me to cut the damaged portion of the shaft off, but would this together with said centaflex put too much strain on the gearbox (PRM 120) output bearing? The boat is a 56' Liverpool/New Boat Company model with an Isuzu 35, PRM120 gearbox and Vetus 30mm shaft and stern gear. Any advice would be much appreciated, thanks.

CD70D7CC-1537-46E3-8C26-0E3A09097910.jpeg

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Looks like the locking stud has sheared off or is that just the mark of a bolt end that was pinched onto the shaft? It should have been a pointed screw in a drilled dimple.

 

I reckon a Centaflex would lock onto the remains of that shaft, they have a lot of wind up in the collet.

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Thanks for the reply. The locking stud is one I fitted a couple of years ago. I drilled and tapped the coupling and drilled a dimple in the shaft. Since taking the pic and posting the original post I've found that the end of the grub screw had sheared off and that is what can be seen in the photo. I've since got it out.

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I've been looking at the Centaflex  flexible couplings and it looks like I need the M127 for a 30mm shaft. Anyone with experience of these? I take it they're pretty straightforward to fit. Be good to hear from anyone that's put one on a 2005 ish Liverpool boat with Isuzu 35 and PRM 120 gearbox. Does it make the transmission much smoother?

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4 minutes ago, Larkboy said:

I've been looking at the Centaflex  flexible couplings and it looks like I need the M127 for a 30mm shaft. Anyone with experience of these? I take it they're pretty straightforward to fit. Be good to hear from anyone that's put one on a 2005 ish Liverpool boat with Isuzu 35 and PRM 120 gearbox. Does it make the transmission much smoother?

Depends what you rate as smoother. It is only a rubber cushion so it will absorb some low frequency vibration. You still have to have the shafts perfectly aligned.

 

If you have no fixed plumber block bearing to take the end thrust the rubber will absorb some of the shock from the propeller.

 

Be careful with a PRM120 mechanical gearbox, they are good but have more take up shock than say a PRM150 hydraulic and can wear the cone clutch quite quickly if abused. Keep the engine tick over low and never go from forward to reverse or visa versa without pausing in neutral.

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Thanks again for the reply. I am pretty gentle on the gearbox and always pause in neutral before changing direction and our engine tickover is about 750-800 rpm. I change the gearbox oil yearly and find hardly any residue on the sump plug magnet. From what you said about the Centaflex it uses a collet arrangement to clamp the shaft which would be far more efficient than the existing split clamp.

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2 minutes ago, Larkboy said:

Thanks again for the reply. I am pretty gentle on the gearbox and always pause in neutral before changing direction and our engine tickover is about 750-800 rpm. I change the gearbox oil yearly and find hardly any residue on the sump plug magnet. From what you said about the Centaflex it uses a collet arrangement to clamp the shaft which would be far more efficient than the existing split clamp.

You don't find any of the bronze/brass worn from the cone clutch on the magnet..................................................................

 

Yes, they are a good collet.

As an ace welder you could build that shaft up a bit and file/grind it back if you felt the need. Most boat "engineers" wouldn't bother.

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There is next to nothing removed from the diameter, although I can't check it properly right now as my verniers are at work, it's just got surface scuffing. I think it's going to be Centaflex here we come...🙂 Thanks for your helpful input.

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Any number of years ago I put my boat astern only for the shaft to pull out of the half coupling.  It was 'secured' by a grub screw into a dimple.  Short term I drilled and tapped the threaded part of the half coupling one size up and used a bolt. When I looked at it some time later there were signs of the bolt 'worrying ' in the dimple.

I  simply drilled right the way through the half coupling and the shaft  and put a nut and bolt through the lot. Held for years before I fitted a Sigma Drive using the same shaft.

Edited by Slim
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I doubt if towing another boat would have put any significant extra strain on the drive train compared with those occasions when you have revved the engine for other reasons.

 

If the coupling can slip like that the most likely cause is that the clamp bolts on the coupling weren't tight enough, or that the two halves of the coupling were in contact with each other so that they didn't clamp on the shaft. You don't have a metric coupling and an imperial shaft or vice versa do you? The connection should rely on friction from the clamp bolts. The grub screw is just extra security and should not be relied on to transmit load (or it will shear, as it has in this case).

Edited by David Mack
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Originally these split couplings had a key way so that they could not spin on the shafts.  Another good idea dumped in the interest of economy.

It was not tight and has probably been fretting for some time before the pinch bolt sheared and it spun.

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I figured it didn't have a keyway or locking screw so that if the prop hit anything solid it would spin the shaft in the coupling to protect the gearbox?

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2 minutes ago, Larkboy said:

I figured it didn't have a keyway or locking screw so that if the prop hit anything solid it would spin the shaft in the coupling to protect the gearbox?

I would doubt that. The gearbox would likely stand a stall, the engine would stop before any serious damage occurred. Its more likely you would tear the engine mountings apart.

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2 minutes ago, Larkboy said:

I figured it didn't have a keyway or locking screw so that if the prop hit anything solid it would spin the shaft in the coupling to protect the gearbox?

 

I would suspect the clutch in the gearbox would slip rather than spin the shaft in a properly fitted coupling. Especially the multiplane clutches in the PRM hydraulics. If a mechanical box uses cone clutches plus dog clutches like a vehicle synchromesh that they would not slip.

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48 minutes ago, Larkboy said:

I figured it didn't have a keyway or locking screw so that if the prop hit anything solid it would spin the shaft in the coupling to protect the gearbox?

 

 

On my Tractor mounted PTO powered equipment they have a shear-pin thru the shaft - if the (say) mower jams the shear pin breaks protecting the tractor and the mower from secondary damage.

 

The Honda outboard (for my Tender) has a shear pin thru the prop so if it fouls, the pin 'shears' (at a known force) rather than destroying the gearbox.

 

It has always surprised me that NBs do not have them.

 

From my outboard manual :

 

 

 

Screenshot (1240).png

Edited by Alan de Enfield
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11 hours ago, David Mack said:

You don't have a metric coupling and an imperial shaft or vice versa do you?

 

This was my first thought too, but the OP says it is a Liverpool boat sold by New Boat Company, so built way after imperial shafts will have ceased being manufactured.

 

Besides, the difference between 1 1/4" and 30mm is 1.75mm, a huge difference meaning imperial and metric items on that size of shaft would obviously not fit.

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9 minutes ago, MtB said:

 

Besides, the difference between 1 1/4" and 30mm is 1.75mm, a huge difference meaning imperial and metric items on that size of shaft would obviously not fit.

I must admit that when I wrote that I was thinking at a Big Woolwich scale - I could just about see someone fitting a 2 inch (50.8mm) coupling onto a 50mm shaft and tightening up the clamp screws enough to get a sort of grip...

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1 hour ago, David Mack said:

I must admit that when I wrote that I was thinking at a Big Woolwich scale - I could just about see someone fitting a 2 inch (50.8mm) coupling onto a 50mm shaft and tightening up the clamp screws enough to get a sort of grip...

 

 

Interestingly a quick look on the Midland Chandlers site suggests that even now, propeller shafts, R&D couplings etc are only available in imperial sizes! 

 

 

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I would like to thank everyone for their input on this, much appreciated. although the shaft is scuffed it appears to have lost next to nothing in diameter so I think I will order a Centaflex coupling with its collet clamping system. Out of interest could I use an R & D bobbin to bolt onto the gearbox and then bolting the coupling to that which would allow me to cut 80mm from the end of the propshaft? I'm not sure of the loadings that might create on the gearbox output shaft.

6 minutes ago, MtB said:

 

 

Interestingly a quick look on the Midland Chandlers site suggests that even now, propeller shafts, R&D couplings etc are only available in imperial sizes! 

 

 

ASAP supplies sell R & D couplings to fit metric shafts.

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When the cast iron coupling between Fulbourne's gearbox and shaft failed in 2008, a standard new coupling wouldn't fit. If I remember correctly we had to order a R&D 50mm coupling with the flange undrilled, then one of our number with the requisite engineering facilities bored it out to 2", cut an imperial width key slot, drilled the bolt holes in the flange on the appropriate pitch circle to match the Brunton gearbox output flange, and then turned a register to match the brass locating ring of the original.

101473716.Y46cKEdr.IMG_0238.JPG

More pictures of the installation at https://pbase.com/timlewis/mending_boat

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15 hours ago, Larkboy said:

I would like to thank everyone for their input on this, much appreciated. although the shaft is scuffed it appears to have lost next to nothing in diameter so I think I will order a Centaflex coupling with its collet clamping system. Out of interest could I use an R & D bobbin to bolt onto the gearbox and then bolting the coupling to that which would allow me to cut 80mm from the end of the propshaft? I'm not sure of the loadings that might create on the gearbox output shaft.

ASAP supplies sell R & D couplings to fit metric shafts.

I may be stating something you are already aware of. The coupling in the photo is definitely metric. On the R&D coupling the line you can see machined around it indicates it is metric. An imperial coupling woud not have this.

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On a normal boat,commonly the prop has a brass key in the taper,and this shears  when the prop hits a log or sandbar.........the prop spins on the taper,which ruins taper both on shaft and in prop.........however ,the tapers are easily recut in both ,and a slight shortening of the shaft may be OK,or in case of a rich owner ,a new prop and shaft.................the coupling between gearbox and propshaft would normally have two keyways at 90deg,as well as clamp bolts ...........as to the OPs shaft......I would bet that the coupling reapplied to the scored shaft would fail in short order.....................take out the shaft ,turn down the scored part to suit a new coupling (since the old one is split,its possible the weld up and bore out to suit),and I d cut a keyway or two in the coupling and shaft.

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