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Discussion Starter #1 (Edited)
I recently bought a new prop, shaft, and coupling. Our old shaft and coupling were drilled right through, with a bolt right through the entire thing to prevent any kind of slip.

The new coupler simply has 4 bolts, which tighten the grip of the coupler on the shaft. There is no keyway, no setscrews, nothing other than pressure holding the coupler to the shaft.

I don't have any experience in this stuff, so I just wanted to ask if this is a normal arrangement? Is the force of friction alone considered strong enough to hold a shaft to a coupler?

Thanks!
 

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The usual arrangement is a key and keyway to prevent the coupling from spinning on the shaft and a couple of setscrews to prevent it from slipping off. Setcrews should be wired to prevent loosening from vibration.
 

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Swansea, MA
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That is totally scary,
No keyway, no bolt counter sinks' into the shaft, just bolt pressure on the outside of the shaft?

Throw in reverse someday and out goes the shaft.

As a mechinical engineer - I would not do it. I would put in a keyway and a key.
 

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Discussion Starter #4
So do you think this was an oversight at the machine shop? Does anybody else have a setup like this?
 

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Swansea, MA
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I would assume since they were machining the shaft that they Knew it was a prop shaft and coupler for a boat.
I have never seen a set up like that, I would not use that for power transmission for a machine ever.

The key is made from a softer material to transfer the torque but to shear if a head impact load is encountered, rather than bending the shaft or ripping the strut out of the boat.

I have seen the bolts on the coupling screwing into counterbores on the shaft that have a greater diameter than the bolts outside diameter. This makes the four bolts the coupling for positive torque transfer, without large decrease the cross section of the shaft. But never just jack bolts on a convex surface. That has only four lines of contact in friction only. unbelivable.
 

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Discussion Starter #6

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Swansea, MA
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Split coupling! You had me totally scared. I pictured four bolts radial to the shaft, so that the the ends of the bolts were pushing on the shaft.

If if were my boat I would have positive locking as well. What I did on mine when I replaced the shaft, coupling and Packing to a PSS system was:
I had a brass key way machined into shaft,
two locking bolts radially installed 90 degrees from the key in which I counterbored 3/8 deep into the shaft for the bolts to bottom out into.

This way if the key sheared or got loose and fell out, or the press fit between the coupler failed, or the bolts broke the wire and starting backing out that the bolts would stop any movement aft of the shaft.
Saftey factor 5.

I don't know the specifications for the split coupling you have so I can not say for certain what it rated for. But I assume that you did let the shop know the HP, and gear reduction of the power plant? So that they matched it properly.

Split is definately easer to install, I would not use one though. Since the clamping force of the bolts is going to take the whole load of the motor.
 

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Swansea, MA
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One possibility, on the inside of the split coupling is there cut out for a caputred key? since you can load the coupling on side ways there should be enough room to have a blind key cut in both the coupling and the shaft. That would be kind of nice.
 

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Discussion Starter #9
There's definitely room for a keyway - but currently there's no cut out on the shaft or the coupling.

Do split couplers normally have keyways installed? Thanks for the info.
 

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Swansea, MA
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Not that I have seen, but generally I have not used them for critical high load applications. Only used them in low load applications in which space or access was an issue, i.e. can't get to the end of the shaft.

Again I don't know the specification on this coupling.
Just make for sure that everything is clean (no oils) and burr free when assembling, and torque the bolts to the proper values. From the pictures it appears it has lock nuts as well.
 

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Coupling

From the picture there is no keyway evident. Perhaps so that customer has it broached to his size requirement. You could have this done and keyway put in shaft by any competent machine shop.
 

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Keyway, plus a couple of "grub screws" going into dimples or actual tapped holes in the shaft is the way to go, plus the grub screws are best drilled through their heads and wired (moused), and if you want to go completely belt and suspenders, put a hose clamp on that part of the shaft between the coupling and the stuffing box. That way, if the coupling shattered into eight pieces, the shaft could not spin out of the boat, leaving you with a one inch or better hole far below the waterline.
 

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I think I would have a machine shop drill and tap (tap on one side only) a hole through the center of the coupling. Then, back at the boat, drill straight through the hole in the coupling, and through the shaft so a bolt can be passed through both the coupling and the shaft. You could either tap and thread the bolt, or forget tapping, just used a washer and nut on the opposite side of the head of the bolt. This will mimic your old setup, and I think it should be plenty strong. The bolt size would be an important, critical choice.
 

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Nah, I personally wouldn't go with that arrangement. I agree with pfatyol, you need a mechanical lock between coupling and shaft and the keyway method is the ticket. In fact, never seen any other method in a marine application....different ways of fail-safeing, but always the keyway. Drilling a hole thru the coupling and the shaft would be one way but IMHO, the only hole I would drill thru the shaft would be at the prop end for securing the prop nut. It looks like the coupling in your post pic (#6) has a provision for a key.?? I see a machine shop in your future. :rolleyes: Must admit, the split coupling looks like an easy install...and I like the 4 self locking bolts/nuts. Good luck and let us know of the result.

Bob
 

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I suppose a designer could say "Well, this way if you foul the shaft it pulls free without any collateral damage" and that's certainly a valid point of view.

Or, the four bolts that you have, could be very hard metal with a conical point--not flat ends--so that when torqued down, they bit into the softer shaft and ensured positive grip. That would work too, plenty of allen screws on small shafts are designed that way.

But I think I'd rather see four divots drilled into the shaft, to trap those bolt tips, or four flats, with matching flat bolt tips. Or, at least on shear pin that went all the way through the shaft and was, again, designed to shear and stop collateral damage if the shaft was fouled.

I don't think this is a "it has to be one way" deal, there is some logic for and against each of the possible approaches.

But the way you have it now...just seems like it is aching for a chance to loosen up and drop the shaft.
 

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Normally is rusts on there so solidly that it will not move by itself.
Any time I have had to remove a coupling from the shaft, it had to be heated like crazy and hammered off.
Given the choice, I would key it on there. My own is keyed and it has a big bolt on there opposite to the key with the shaft dimpled to accept it.

The shaft is reduced to enter the coupling, so going ahead, it is not going anywhere. Going astern would tend to pull the shaft out of the stuffing box, so I liked the idea of the big hose clamp on there, ultra-tight. Get ready for corrosion below the clamp though... de-oxygenated seawater normally eats stainless and it's rapid.
 

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Rockter, another method I've seen to keep a loose shaft from leaving the sterntube is to bolt on a shaft zinc between the cutlass bearing and the stern tube exit. If the shaft came loose, the zinc itself would stop it.

Conversely, if you have the usual stainless shaft, you could put a stainless hose clamp between the cutlass bearing and the shaft zinc. Same idea.

I have heard of exactly one case of a shaft leaving a boat due to a failed or sheared coupling or keyway or something of that nature. It's pretty rare, but not many people have a tapered plug for that particular potential hole in the boat.
 

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Good idea, Valiente. I'd have to think that if a shaft came loose, a nice fat zinc on it would do less damage to the shaft log than a "sharp" stainless hose clamp though. And it makes a good place to keep a spare zinc, since the chandlery never has the one you need at launch time, right?<G>
 

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Split collar

I would use a split collar. They are available in common shaft sizes & are split into and fastened togeather with 2 socket head capscrews (allen head bolts). Very strong and you have a thicker surface all around the shaft not just the screw of a hose clamp.
 
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