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Old 12-03-2020, 06:41   #1
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Busted shaft and repair C30

Welcome, thanks for reading. I hope this will be educational for many of us.




In late September (end of northern sailing) we busted the shaft on our 1990 Catalina 30TR.



We'd left another port, headed for home, when I heard an unusual sound. Now, it was not a loud sound, but having owned this boat for 26 years I was able to pick out some tiny little extra noise.


I put the boat in neutral, checked belts, etc. All looked good. On re-engaging the tranny, the boat did not move. Back below again, to find that the shaft had busted (inside the coupler) and backed out a bit. I put vice grips on the shaft so it wouldn't back out (it shouldn't the zinc is forward of the shaft log) but just in case...


This shaft is original to the boat. Here is the damage:











I should note this has always been a freshwater boat. It's out of the water for six plus months each year, stored indoors. There was no indication that the engine had slipped out of alignment, and all the engine mounts are in very good condition.


The rust on the coupler for the most part is from a damp bilge. The stuffing box needed to be stuffed again (though drip only under way) and water does leak in from the lazarettes during heavy rain.


Any thoughts thus far?


No matter how hard I try the remnant of the shaft will not come out of the coupler. I could use a 5 ton press, but I'm concerned it may warp the coupler. I didn't think the shaft was pressed in the coupler- is it?





Now, I can get another shaft from Catalina Direct for just over $300. New coupler is about $100. So no big deal.



Catalina Direct recommends a new coupler (why?) and says the shaft needs to be refaced to suit the coupler- why? Does the shaft get pressed in? I could ask CD this, but figured if I post here it may be good info for others as well.


Appreciate any questions and insights.
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Old 12-03-2020, 07:18   #2
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Re: Busted shaft and repair C30

As the coupler ages the internal bore rusts. The original fit was very tight to prevent the shaft from moving in the coupler, add the rust and it becomes an interference fit.

A new coupler needs to be “fit and faced” to a new shaft. This involves first Honing the coupler to just barely fit the shaft, then one fit, skimming the face of the coupler with the shaft in the lathe chuck to be sure the face is true to the axis of the shaft. Skipping these steps can make an installation that vibrates or is impossible to align.
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Old 12-03-2020, 08:30   #3
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Re: Busted shaft and repair C30

It is critical that the face of the coupling be exactly perpendicular to the shaft. Without machining the two of them together this is very unlikely to happen.

If the front of the coupling is not perpendicular to the shaft, you can do a perfect job of aligning the two flanges, but the shafts are not aligned. With a crooked flange, it is impossible to really align the shaft to the transmission flange.
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Old 12-03-2020, 09:36   #4
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Re: Busted shaft and repair C30

This failure appears to be a progressive fatigue failure, where the shaft has been in the process of failure for some time. The dark surfaces of the break have been broken for some time and the bright surfaces are the more recent and final failures. The failure appears to have started from the set screw dimples and spread across the shaft from there. This could have taken years to develop and could have been caused by lengthy periods of misalignment or excessive vibration, such as that caused by worn flex mounts.

The remaining shaft should press out of the coupling after the set screws are removed, but it could be tight as the result of corrosion.
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Old 13-03-2020, 11:03   #5
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Re: Busted shaft and repair C30

Any welding/ machine shop should be able to get the key and remnants of the shaft out for a few bucks. p
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Old 17-03-2020, 07:49   #6
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Re: Busted shaft and repair C30

If it were my boat, I wouldn't reuse that coupler.
I agree with Doug about the cause, but would also suggest that the set screw was over-torqued and thus began the slow process.

Quote:
I put vice grips on the shaft so it wouldn't back out (it shouldn't the zinc is forward of the shaft log)
??? Forward of the log??? Bet you didn't have to replace it very often!

Or did you mean forward of the cutlass bearing?
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Old 19-03-2020, 06:00   #7
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Re: Busted shaft and repair C30

A bronze shaft that is that small in diameter with two set screws set deep as those and with a key, was not going to last forever.

A solid coupling needs to be carefully matched to a shaft. The two cannot be assembled if they have an interference fit. Until rust sets in, the stress at the connection ends up being at the key and the set screws. Which is what you had here and why it broke.

Replace with stainless shaft and a split, clamp style, coupling. It will normally not be required that the shaft be matched to the coupling and it will make it a hell of a lot easier to get them apart again in the future. Put that mess behind you.
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