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Old 09-06-2012
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SPLIT-HALF COUPLING - replacing old keyway'd coupling

I have a Volvo MD2B with a MS transmission. Currently there is a keyway'd solid coupling connecting a 1.25" prop shaft. The keyway has been damaged (long story)... so I'd like to replace the coupling, and simply use the same shaft. I have no reason to believe it's bent, and it's in fairly good shape otherwise.

I'm looking at an R&D Marine Split Half Coupling (the clamping type). They make them to fit the Volvo trans and I can get the right bore for the shaft.

The question is... are there issue with these types of couplings? Will it work with my current prop shaft (does the keyway in the prop shaft matter)? Should I add a 'flexible coupling' also?

Please help me out. Thanks!

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Old 09-06-2012
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Re: SPLIT-HALF COUPLING - replacing old keyway'd coupling

If the rest of the present coupling is in good shape, why not consider simply to cut another keyway in the 'old' coupling ... at 180° from the buggered keyway? You probably have setscrews, etc. that were inadequate to stop the coupler from errantly rotating on the shaft .... so ADD more set screws, etc. so you dont bugger the new keyway. Any competent local machinist shop can do this.

Note: a keyway is usually NEVER used to stop rotation, only to serve as a 'backup' if the primary means of restraint (set screws, 'clamps', interference/forced fit, etc.) fails.
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Old 09-06-2012
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Re: SPLIT-HALF COUPLING - replacing old keyway'd coupling

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Originally Posted by RichH View Post
If the rest of the present coupling is in good shape, why not consider simply to cut another keyway in the 'old' coupling ... at 180° from the buggered keyway? You probably have setscrews, etc. that were inadequate to stop the coupler from errantly rotating on the shaft .... so ADD more set screws, etc. so you dont bugger the new keyway. Any competent local machinist shop can do this.

Note: a keyway is usually NEVER used to stop rotation, only to serve as a 'backup' if the primary means of restraint (set screws, 'clamps', interference/forced fit, etc.) fails.
Maybe, but the prop shaft keyway is damaged also.
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Re: SPLIT-HALF COUPLING - replacing old keyway'd coupling

Your suppose to have a safety wired screw dimpled into the prop shaft to prevent it from pulling out
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Re: SPLIT-HALF COUPLING - replacing old keyway'd coupling

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Originally Posted by tommays View Post
Your suppose to have a safety wired screw dimpled into the prop shaft to prevent it from pulling out
I do. In fact I have two.
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Re: SPLIT-HALF COUPLING - replacing old keyway'd coupling

Okay, so I guess you don't want to divulge the "long story"?

As long as the diameter of the prop shaft in the area that the coupling will clamp to is true, then the split coupling should be fine, as long as it's rated for the torque. Looking at R&D's page, the part listed for Volvo and 1.25" shaft is keyed, so I would check with them. In any case, you nee to check the shaft end with a micrometer or calipers, as whatever damaged the keyway could easily have also damaged the circumfrence of the shaft, If it's not true, you would need to get the coupling end of the shaft machined and order the solid coupling.

If the shaft end is still true, another option is to take it to a machine shop, and have them cut a new keyway, and use a standard coupling. Sounds like you might be trying to avoid pulling the shaft, but if that end isn't still parallel and concentric, the split type will fail, and Murphy will make sure it fails at the worst possible time.
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Re: SPLIT-HALF COUPLING - replacing old keyway'd coupling




Been there and done even faced it at work because the budget was getting tight and it looked fine BUT under a full load it was not and made the 2011 season Needlesly nerve wracking



Correctly the second time and it was just the wrong place to save
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Re: SPLIT-HALF COUPLING - replacing old keyway'd coupling

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Originally Posted by Brewgyver View Post
If the shaft end is still true, another option is to take it to a machine shop, and have them cut a new keyway, and use a standard coupling. Sounds like you might be trying to avoid pulling the shaft, but if that end isn't still parallel and concentric, the split type will fail, and Murphy will make sure it fails at the worst possible time.
How would the split-type fail? Not grip well enough because it was out of round? Or?

And yes, this option is to do the fix without pulling the shaft/taking the boat out of the water. But that is the way I'll go if this doesn't work, or isn't a good idea.
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Re: SPLIT-HALF COUPLING - replacing old keyway'd coupling

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Originally Posted by Brewgyver View Post
Looking at R&D's page, the part listed for Volvo and 1.25" shaft is keyed, so I would check with them.
The R&D part I'm looking at is 202-255. It says 1.25" bore is available. Or I'll buy the .75" which I've seen online and have a local shop increase the bore.

Here's the R&D PDF I'm looking at...
http://www.randdmarine.com/downloads/RandD_Steel.pdf
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Re: SPLIT-HALF COUPLING - replacing old keyway'd coupling

It looks like a good solid unit to me. I think it would work very well, as long as your shaft isn't bent. I would install the coupling just a little snug, and center the coupling on the transmission flange. Then use feeler gauges all the way around to determine if the engine alignment is okay. If it is badly out, then try rotating the shaft 180 degrees, centering it and checking again. If the out of alignment has moved as well the shaft is bent. If not then the motor and transmission are out of alignment and you should fix that before bolting it to the flange. A rigid coupling with misalignment or a bent shaft is likely to damage your transmission.

Gary H. Lucas
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