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  #1  
Old 04-24-2013
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Prop Shaft Coupling

Could someone explain how a prop shaft coulpling stays connected to the prop shaft?

Specifically I am looking at yanmar prop shaft couplings, but I assume they are generally the same as other suppliers.

Yanmar makes 3 types of prop shaft couplings. A straight, a taper and a spit. Looks like the straight and the taper depend on a set screw to resist the longitudinal forces on the shaft (forces that cause boat to go forward or backward). The split coupling has no set screw and looks to resist longitudinal forces by its clamping action.

All couplings have a key and keyway to resist torsional forces.

I ask this question because I have a split couple and the shaft slid forward on the coupling (I have a V-drive so nothing to stop shaft if it slips) about 3/4 inch until the prop started to rub the shaft strut. My coupling does have a set screw threaded hole- but no set screw was ever installed, and yanmar does not show the set screw hole or screw on their drawings or parts list. Wondering why my coulpling/shaft slipped. Were the clamping bolts not tight enough? I will be having a look at the clamping bolts in next few days, but to date, have not tried to tighten them.

The other question is I have seen many times it is difficult to get a coupling of the shaft and you can use a socket between the trans coupling and shaft coupling to force it off. For one thing, this seems like a good way to damage transmission (with all forces involved), but the question is, how would you get this coupling back onto the shaft? I saw somthing on youtube where they heated the coupling in home oven , the slipped it on to get an interference fit. Is this what you do with a sail boat coupling?

Reason I ask is that in the near future, I will be taking my drive line apart, just trying to figure out how it is supposed to be connected.

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Last edited by casey1999; 04-24-2013 at 01:56 PM.
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Old 04-24-2013
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Re: Prop Shaft Coupling

Casey,
Using the socket wrench cylinder to push apart the shaft and coupling can damage the output flange on the transmission. This is why I first suggested that you bite the bullet and cut your shaft in half, making removal a relative breeze.
As you know, your shaft should not have been able to move once in the coupling. Either due to corrosion or a bad initial press/interference fit your shaft moved. This suggests that either the coupling or the shaft (or both) are flawed.
Once you remove the coupling (hopefully the easy way) I'd take it to a good prop shop in Hono and have them check it out.
When we replaced our drive train I had our prop shop face & fit our old coupling for re-use when they were making up our new shaft. Old couplings are not always re-usable and in your case I'd consider getting a new one that will work better. When I picked up our new shaft & coupling the shop had machined it such that the interference fit in the coupling was fairly easy to achieve without tons of pounding.
Use of heat (expanding) & cold (shrinking) can also facilitate getting the interference fit more easily.
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Old 04-24-2013
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Re: Prop Shaft Coupling

Quote:
Originally Posted by CalebD View Post
Casey,
Using the socket wrench cylinder to push apart the shaft and coupling can damage the output flange on the transmission. This is why I first suggested that you bite the bullet and cut your shaft in half, making removal a relative breeze.
As you know, your shaft should not have been able to move once in the coupling. Either due to corrosion or a bad initial press/interference fit your shaft moved. This suggests that either the coupling or the shaft (or both) are flawed.
Once you remove the coupling (hopefully the easy way) I'd take it to a good prop shop in Hono and have them check it out.
When we replaced our drive train I had our prop shop face & fit our old coupling for re-use when they were making up our new shaft. Old couplings are not always re-usable and in your case I'd consider getting a new one that will work better. When I picked up our new shaft & coupling the shop had machined it such that the interference fit in the coupling was fairly easy to achieve without tons of pounding.
Use of heat (expanding) & cold (shrinking) can also facilitate getting the interference fit more easily.
I am just trying to figure out what is actually holding the coupling to the shaft. Is it a set screw, interference fit or in the case of a split coupling- clamping force. I think my coupling will come off easily, as it seems to have slipped. One thing I did a couple years ago it to spray wd40 on the shaft coupling to keep it (and bolts) from rusting- could this have penetrated the couple and made it slip? Maybe I can remove the coupling, clean the oil off, and tighten up the clamping bolts to specified torque.

I will be replacing the shaft (and probably coupling), engine mounts, cutlass bearing, packing gland, and prop if required in 6 months when I plan to do haul out- if I can fix the existing shaft slip.
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Old 04-24-2013
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Re: Prop Shaft Coupling

It is easy to make a proper removal tool if you have access to a drill press. Do this instead of the socket approach. Here is a photo of one that I made out of 1/4" plate steel:


The 4 bolts are attached (with nuts) to the shaft coupling. The 1/2" bolt in the center is tightened (the plated is threaded for it) and pushes the shaft out of it. This took about 20 minutes to make.
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Old 04-24-2013
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Re: Prop Shaft Coupling

Quote:
Originally Posted by CalebD View Post
When I picked up our new shaft & coupling the shop had machined it such that the interference fit in the coupling
Is that all that is holding your coupling to the shaft? You have no set screw (I would assume you have a key/keyway)?
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Re: Prop Shaft Coupling

Look at these couplings:

walter shaft couplings, buck couplings, propeller shaft coupling

Both the split and straight have a set screw. Is this what should be taking the longitudinal forces?
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Old 04-24-2013
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Re: Prop Shaft Coupling

Found some real good information on all types of shaft couplings. Looks like I should tighten the clamp screws and install a grub screw:

Couplings
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Old 04-24-2013
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Re: Prop Shaft Coupling

In a perfect world it would have a double taper with both ends like the prop which can be removed easy and NEVER SLIP

In this world we put up with the 1/2 azzed light press and lock screw and safety wire
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Old 04-24-2013
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Re: Prop Shaft Coupling

Quote:
Originally Posted by casey1999 View Post
Is that all that is holding your coupling to the shaft? You have no set screw (I would assume you have a key/keyway)?
I have a solid coupling (not split) and yes, there is a set screw which has to be wired so it does not decide to pull out. So my shaft is held in place at the coupling by the press fit and the set screw.
The key/keyway is on the prop end of my shaft.

See if you can torque down the split sides together. It may work.

Obviously I've never used this shop:

Marine Propellers Hawaii
(808) 537-6772
1100 Bishop St, Honolulu, HI 96813


but it would seem near the Alu Wai and perhaps convenient.
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Last edited by CalebD; 04-24-2013 at 07:51 PM.
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Old 04-24-2013
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Re: Prop Shaft Coupling

A split coupling is held in place by the clamping action and a proper "fit". Good ones will also have a set screw which should always be "spotted" or dimpled into the shaft.

With split couplings it is very important to tighten the clamping bolts evenly, like lug nuts, or you can create an alignment issue. Contrary to popular misconceptions about "splits" they still require a fit & face when installed.
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