Shaft Coupling and shaft vibration

Engines, Drive trains, Propellers, Steering, Ground Tackle and other mechanical system
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philipl
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Joined: Mon Jul 27, 2009 3:12 pm
Location: Manchester by the Sea, MA

Shaft Coupling and shaft vibration

Post by philipl »

I have a F38 with a Yanmar 3GM30F. While motoring to my mooring yesterday the shaft key ( securing the prop coupling to the prop shaft ) slipped out, so that the shaft and tranmission coupling turned but the prop shaft did not. I no longer had power to the prop shaft. I noticed some prop vibration during the week. I think this caused the coupling to loosen on the shaft and eventually the key slide down the shaft. The prop shaft itself slipped about 1 inch as far as I can tell. I plan to disconnect the coupling ( from the transmission coupling) , slide it down the shaft and put the key back in position. I have also pushed the prop shaft back in from underwater. I will probably do an engine alignment at the same time. My question is the following. The key will ensure that the coupling and shaft rotate together. However I assume that the coupling needs to be secure on the shaft so that the prop shaft doesn't slide out. The boat is on the water and I don't want to haul with only one or two months of the season left. Can I use a cold weld or other adhesive to secure the coupling to the shaft? I will probably use the engine about one or two hours for the balance of the season.
Philip Leahy
F-38 ex Manchester, MA.

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philipl
Posts: 102
Joined: Mon Jul 27, 2009 3:12 pm
Location: Manchester by the Sea, MA

Re: Shaft Coupling and shaft vibration

Post by philipl »

Would Loctite Stud and Bearing Fit ( or equivalent) work?
Philip Leahy
F-38 ex Manchester, MA.

peaceandfreedom
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Joined: Wed Jul 29, 2009 8:51 pm
Location: F33 CK & F38 CK Milford ct

Re: Shaft Coupling and shaft vibration

Post by peaceandfreedom »

In addition to the key there should be a set screw in the coupling. Both the set screw and the key are needed to keep the shaft secured to the coupling. I think the Loctite would be more useful on the set screw.
Happy sailing

Jim D

bad
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Re: Shaft Coupling and shaft vibration

Post by bad »

Having just done a propeller shaft coupling replacement on my engine... The key will keep the coupling from rotating on the shaft, the set screws (if it is a solid coupling) or the clamping screws (if it is a split coupling) should hold the shaft in the coupling axially.

When connecting the coupling to the engine/transmission, there should be a male ring (also called spigot or register) on one side, and a female counterpart on the other side of the two faces being bolted together. These align/center the two parts. But this only works if the shaft coupling is centered on the shaft... After you refasten your coupling on the prop shaft, use a dial indicator on the face or flange of the coupling and spin the shaft to determine if one is centered on the other. I was able to take a reading on the male register on the coupling face. If the shaft/coupling are not centered with respect to each other, then it will not be possible to properly align the engine with the shaft. I think solid couplings are suppose to have an interference it on the shaft. If you have a solid coupling that easily slides along the shaft, then the set screws are going to off-set it from the center-line of the shaft, and the shaft will not align with the center-line of the transmission coupling. My shaft was badly galled and the original solid coupling was worn by 30 mil. A new split coupling took care of it and was easy to install, but did take lifting the engine up six inches.

If you decide the coupling is sufficiently centered on the shaft, aligning it to the engine seems to take patience more than anything else. There is good info on the internet on this. I found that the prop shaft, due to its weight, is a little bit below its neutral position before bolting it to the engine so I slightly lifted it when mating it to the engine side. I first adjusted the engine height (pitch) on the engine mounts. BTW, it was much easier to use a scissor jack to lift the engine up a little and then spin the nuts on the engine mounts by hand rather than trying to get a wrench in there. Then I bolted the engine tight to the mounts and then loosened the screws holding the engine mounts to the engine bed to adjust the yaw. I moved engine side to side using a piece of pipe. Feeler gauges between the the coupling faces were helpful, so was using the dial indicator on the propeller shaft to measure run-out. I did get the prop shaft run-out down to less than 4 mils which seems to be some sort of standard. When the engine is running now, there is no discernible movement of the prop shaft.

Hope this helps and is not too lengthy

Erik

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philipl
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Location: Manchester by the Sea, MA

Re: Shaft Coupling and shaft vibration

Post by philipl »

Thank you Jim and Erik for the advice. I have a split coupling. I will take it apart this weekend. Hopefully I can just reinsert the key with some lockite and get through the rest of the season. Otherwise...
Philip Leahy
F-38 ex Manchester, MA.

bad
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Joined: Sat Jul 09, 2011 11:14 am

Re: Shaft Coupling and shaft vibration

Post by bad »

To get my split coupling onto the shaft, my local yard was kind enough to lend me a set of metal wedges (6" long, 1/2" thick, about 1"wide) that I used to gently pry the split coupling apart. Then it was very easy to slide on the shaft and move into the desired position. I tapped out the wedges and it fit tight. Taking it off was the same in reverse. The bolts to clamp it - four - have lock nuts to hold the tension once properly torqued.

It is probably worth some head scratching to see what caused the coupling to come loose in the first place and how to prevent it

Best of luck and report back how it went :)

Erik

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philipl
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Location: Manchester by the Sea, MA

Re: Shaft Coupling and shaft vibration

Post by philipl »

Final Report

Well I tried to balance the engine twice. The vibration continued and seemed to get worse. Finally I had the boat hauled. The strut was bent off center and the cutless bearing was gone. The coupling was also worn. I think the strut may have been bent by an object. That caused the prop shaft to enter the hull off center and caused the vibration. Paul Dennis also advised new engine mounts, apparently Yanmar advise new engine mounts every 5 years or so. So new prop coupling, new cutless bearing, new engine mounts and a straightened strut....and then an engine alingment. And everything is back to normal.
Philip Leahy
F-38 ex Manchester, MA.

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