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Go Back   SailNet Community > On Board > Gear & Maintenance
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Old 06-27-2008
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Shaft Coupler to Shaft attachment

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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Last edited by dhornsey; 06-27-2008 at 04:29 PM.
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Old 06-27-2008
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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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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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So do you think this was an oversight at the machine shop? Does anybody else have a setup like this?
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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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A little more information. Yes, they certainly knew they were machining this for use on a boat. Apparently it is called a "split coupling". We have a keyway installed at the prop end of things, as well as a shaft saver between the transmission and the coupling.

It looks exactly like this: http://www.betamarine-service.eu/sho...20COUPLING.jpg

So it sounds like the machine shop forgot something?
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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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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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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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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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