Shaft Coupling

[Bob Cofer]

Close call today on Ebbtide.

Started the engine and was warming in reverse. Engine room access was open and I was checking for leaks and just looking around. All of the sudden there was a bang and the lines went slack. What the heck!

Turns out the shaft pulled out of the coupling from the stress of being in reverse. At some point in the past the shaft must have been removed and when reinstalled the set screws were not in the dimples just tightened to the shaft. The key fell out into the bilge pan.

Luckily it only pulled out of the coupling about an inch and a half and was not damaged.

Pulled the coupling off the transmission and managed to get it back on the shaft and properly secured. Blue lock tight on the set screws. A bucket of sweat later and I managed to get it all realigned and bolted back together.

I do a thorough pre underway check but this was not on my list. Do yourselves a favor and take a look at your shaft couplings once in a while.




Bob

 

[Ski in NC]

I like using split type couplings. Solid couplings like you have are either too tight or too loose. Shaft should also be dimpled where set screws go to give better bite. Screws are normally square head with lock wire holes to keep them from backing off.

You might be able to swap with a split and not change anything else. On the split, once you dog the clamp bolts it has a heck of a hold on the shaft.

 

[dwhatty]

After a rough weather, hair raising (if I had hair) maiden delivery voyage on our boat, I was doing an engine room check and found 4 out of 6 of the shaft flange to tranny flange bolts lying in the bilge. Two of them sheared off. I now regularly check those bolts, as well as all of the set screws on the shaft and dripless box religiously.

 

[Blue Heron]

Great thread.

Here is my shaft coupling

 

[Northern Spy]

Another thing I've found is that the set screws themselves are not even set screws and the material is often too soft. I swapped mine with a knurled cup point set screw made out of 17-4 precipitation hardened SS.

 

[Nomad Willy]

I dimple about 1/16" and use lock tight. Then safety wire.

One should use a center punch exactly the right size so as to not cut part of the threads off in the coupling. And de burr the shaft well.

 

[Bob Cofer]

Dimples were already there and about 3/16" deep. The set screws were not in the dimples, just tightened to the shaft and no locktite. Properly installed now!

 

[markpierce]

Congratulations. ... My propeller shaft came apart, luckily when coasting into my berth.

Thank goodness the boat was properly blessed.

 

[Duvie]

Shaft to coupler fit

Last year I noticed that my coupler was not tight on the shaft when I was tightening up the stuffing box. Stopped by my prop shop as I was concerned about the fit. It seemed a little sloppy to me. They told me that any time those come loose like that after being in service for some time that it is necessary to replace them and that the new one needs to be fitted to the shaft. Hauled the boat and pulled the shafts. Shafts were pretty worn in the area of the stuffing box. Ended up replacing both shafts, couplers, cutlass bearings and rebuilt stuffing boxes. If you just reset your coupler I would keep a close eye on it as fit seems to be pretty critical.

 

[Electro]

I just had mine in the yard for paint and what not.... long list.... and the guy tells me that there are only 2 bolts left hold my stbd shaft ! Now checking them is part of my normal engine room inspection

 

[bligh]

Great thread.

Here is my shaft coupling


View attachment 28609

Is that some sort of adapter or vibration damper in there?

 

[Capt.Bill11]

Is that some sort of adapter or vibration damper in there?

Yes, that is a misalignment/stress relief adapter. Looks sort of like a [FONT=Arial,Helvetica,Geneva,Swiss,SunSans-Regular]R & D Marine Flexible Coupling. [/FONT]

 

[Nomad Willy]

Capt.bill11,
For about the last 14 years I've used the R&D flex coupling. The picture isn't perfect but I think that is an R&D.

But Bill I don't think it should be installed to alleviate or make up for alignment problems. Alignment should be done to the same standard as w/o the flex coupling. I view it as a poor man's Aqua Drive.

One needs to be careful w that coupling as getting one not big enough to handle the thrust of the prop can result in serious gear damage. Being a flexible coupling it is indeed flexible. Too much thrust will push the bolt heads fwd and hammer the transmission output flange. I had this problem and lost a lightweight Kanzaki gear. I could clearly see the hammering marks on the output flange.

Otherwise I really like the coupling and think that ideally there needs to be some kind of flexibility in the shaft to engine coupling unless your engine is rigidly mounted to the engine beds. With rubber or other flexible mounts used lots of forces are transferred from the engine to the shaft and then to the boat through structures that aren't designed to absorb vibration.

The flexible drives like Aqua Drive and Python Drive address this problem completely but are not very easy to install and cost several thousand dollars. They are ideal but certainly not necessary on most boats. I'd sure have one if I could afford it.

 

[Capn Craig]

Duvie,

I concur with the idea that a coupler should be fitted to the shaft. I got this lesson when I was ordering a new shaft for my old cruiser. The guy asked do want a new shaft coupler with the new shaft? I asked why do I need one? He said they are relatively cheap and he could fit a new one before he shipped the new shaft. I asked what does fitting mean? He explained, the new coupler is installed to the shaft and the set screw centerlines marked to the shaftt. This can be done with a very accurate pointed set screw or done with a centering punch as used in the tool and die trade to transfer dowel holes. The coupling is then removed and the dimple holes drilled. The coupling is then replaced on the shaft and the set screws locktited, torqued, and safety wired. The whole assembly is then mounted in a lathe to have the face of the coupler dressed so that it is exactly perpendicular to the shaft centerline. This only removes a few thousandths from the face, but makes a difference if you want a smooth running shaft. Is this necessary? Probably not, but quality workmanship is never a bad deal. When I installed the shaft and realigned the engine I was amazed at how much difference it made. As I recall it cost me about 50 bucks, money well spent in my book.

 

[Capt.Bill11]

Capt.bill11,
For about the last 14 years I've used the R&D flex coupling. The picture isn't perfect but I think that is an R&D.

But Bill I don't think it should be installed to alleviate or make up for alignment problems. Alignment should be done to the same standard as w/o the flex coupling. I view it as a poor man's Aqua Drive.

I wasn't implying that it should be installed to alleviate alignment issues. Just that some of them, and I think the R&D is advertized as one of them, that can handle minor misalignment's.

 

[meridian]

When I had my new shaft and coupling made I didn't realize how close they make the interference fit. When I slid the shaft in the hull and tried to get the coupler on it wouldn't move. It needed a slide hammer that screwed on the prop end of the shaft. It still took a long time to seat the shaft to the coupling.

 

[Nomad Willy]

Capt.Bill11,
Absolutely. Perhaps I misread your post.

meridian,
Really. Imagine the slide hammer activity it will take to remove it after a few years in the bilge of a boat. To install one could chill the shaft and warm the coupling but I don't think the fit should be that tight. Been 12 years or so since I've messed w them. And my couplings were probably all the split type that clamp onto the shaft by bolts on the outer edge of the coupling body.

Regarding the installation of R&D flex couplings the shaft is usually or always removed and shortened the 1 1/2" or so to accommodate the new coupling. There are other possible options. One could of course cut the end of the shaft off w a Sazall but lengthening the key way would require shaft removal. But perhaps one could get a short key in what's left of the key way. One or more set screws could be installed or even a through bolt or two. Would anything like that be secure enough to employ?

Of course shaft removal is easy in many boats but very difficult in others. I drilled a hole in my rudder w a hole saw so as long as I'm hauled it's not a huge job. But some will want to install them in the boat in the water and I don't see any good method I'd recommend to my friends. And lesser methods of secure attachment would depend on the power applied to the shaft and other variables. My own boat w 40hp and a 1 1/4" shaft may be acceptably secure w several good set screws as outlined in Capt Craig's previous post. Coupled to an engine w three times the power a key way would probably be the only secure attachment.

It seems to me I've installed a shaft to shaft coupling of the split type that had more than two socket head clamping screws and I don't remember any key. Any opinions?

 

[Ski in NC]

You guys with solid couplings should consider getting split types. Much easier to install and remove, fit up more tolerant, and with clamp bolts squeezed hard, very little risk of it slipping. Not relying on setscrews, which is cheesy at best.

I use splits exclusively for smaller shafts. Big ones get tapers on both ends, but even tapers have issues.