Shaft Couplings: to flex or not to flex?

Zonker

Super Anarchist
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Canada
Is there a rule of thumb on eliminating the flex coupling based on unsupported shaft length?
Yeah. Bolt the engine to the beds without flexible mounts. Seriously if you have a flexibly mounted engine it is very wise to have a flexible coupling.
 
There are clamping non keyed coupler applications, I can only speak to what I used , the RND split coupling which required a keyed interface for our engine and drive line, 100hp engine with 1 1/2" shaft. If it is designed for the keyway to transfer load and there is no key it will slip under load and perform some expensive in place machining and welding. I have no idea if they have a product range that does not require the keyed interface, something you will need to get from the manufacturer.
 

Rasputin22

Rasputin22
13,920
3,471
Hello Occam,

When I delivered your beautiful cat from the VI to Newport many years ago, it was the shafts that gave us the most trouble. The shaft on one side came out of the coupling but the prop got hung on the rudder. I'm surprised we didn't lose the shaft! It was pretty nasty conditions and we were about a day out from landfall in Bermuda and it was no easy task getting that sorted out. Lots of vibration and the other side was almost as bad but I have blocked out a lot of that bad experience. Someone above mentioned shaft length and flex in the hull itself causing alignment problems and think that might be a good thing to look into. How to go about is another thing though.

I have installed BIG Centa thrust bearing/flex units on a big Gold Coast powercat with a big hollow primary shaft to the thrust bearing and then an an Aqualloy solid shaft. The clamping action on the smaller diameter solid shaft going to the propellor was very similar to a router collet and had no keyway. One can really torque down on those sort of collets and I can't imagine one ever failing. I have also installed the PYI's in far less demanding situations. CENTA and PYI staff are both excellent though. Good Luck!
 
There are clamping non keyed coupler applications, I can only speak to what I used , the RND split coupling which required a keyed interface for our engine and drive line, 100hp engine with 1 1/2" shaft. If it is designed for the keyway to transfer load and there is no key it will slip under load and perform some expensive in place machining and welding. I have no idea if they have a product range that does not require the keyed interface, something you will need to get from the manufacturer.
I called R&D Marine. Phone was answered right away by someone who knew stuff, so that felt good. Short version from R&D is that for lower power/smaller flanges/smaller shafts keying is optional but not standard. That included us at 50HP/4in flange/1in shaft. He also said some folks still go with no key even on the larger shafts to add a bit of safety factor - if something goes bump shaft spinning may be preferable to not.
 
Hello Occam,

When I delivered your beautiful cat from the VI to Newport many years ago, it was the shafts that gave us the most trouble. The shaft on one side came out of the coupling but the prop got hung on the rudder. I'm surprised we didn't lose the shaft! It was pretty nasty conditions and we were about a day out from landfall in Bermuda and it was no easy task getting that sorted out. Lots of vibration and the other side was almost as bad but I have blocked out a lot of that bad experience. Someone above mentioned shaft length and flex in the hull itself causing alignment problems and think that might be a good thing to look into. How to go about is another thing though.

I have installed BIG Centa thrust bearing/flex units on a big Gold Coast powercat with a big hollow primary shaft to the thrust bearing and then an an Aqualloy solid shaft. The clamping action on the smaller diameter solid shaft going to the propellor was very similar to a router collet and had no keyway. One can really torque down on those sort of collets and I can't imagine one ever failing. I have also installed the PYI's in far less demanding situations. CENTA and PYI staff are both excellent though. Good Luck!
Geez @Rasputin22 . I knew you had trouble on the trip down, but the trip home was that bad too?
 

JC522

Member
60
15
R&D coupling for 50hp and 1.25" shaft, deformed after 5 seasons use. R&D didn't have a good explanation and offered a big discount on a replacement. No thanks. They also said the coupling is primarily intended for impact protection, not for reducing vibration.

20220806_192745a.jpg
 
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Zonker

Super Anarchist
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5,723
Canada
well there's the flexible part.

Any chance your shaft is misaligned? (rubber engine mounts do sag over time)

Do you have enough room to slide the shaft coupling forward and see if it mates with the transmission flange properly?

I had an aluminum spacer made for just this purpose because I couldn't slide my shaft forward enough. Just a slice of aluminum round bar with the spigots machined into both faces for the female/male inner part of the output/prop flange. Like the green thing in the middle. Basically a hard coupling.

1669222050713.png




And it had the bolt holes machined in place so that if the flexible coupling ever failed then I could remove it and change to a hard coupling connection as a limp home mode.
 

hdra

Anarchist
653
147
I've just taken over running a 42' ex-fishing boat that's been converted into a research boat. It's got a detroit diesel 4-71 on soft mounts, and has a history of drive train issues - they've had to rebuild the transmission every 5 years for at least the last 10 years, maybe longer but don't have maintenance logs going back that far. It also seems to be chewing up cutlass bearing pretty quickly (500 hours or so), even with the engine perfectly aligned at the dock. The previous captain strongly suggested that we convert it to rigid engine mounts, but he wasn't able to get that done at the last haul-out for a variety of reasons. Currently there is not a flex coupling on the shaft. Is there any reason to think that putting a flex coupling in is a better option than rigid mounts, either in terms of cost or ease of use? I've only ever interacted with sailboats with flexible engine mounts, shorter shafts, and less torque-y engines, so the whole rigid engine mount thing is a totally new world for me. Are they something you normally just get fabricated, or do people sell them like they do fixed mounts?
 

Zonker

Super Anarchist
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5,723
Canada
Without a flexible coupling the engine is rocking around on the soft mounts - but the shaft is rigidly connected. Of course it eats up cutless bearings because the shaft is like a big lever with a prop on the end. I'd add a flexible coupling and see if your problems go away. Flexible couplings are quieter and should be used if you have soft mounts. Big ships use flexible couplings too if you want to keep them quiet.

Dynamically it's like this - small engine movements become big shaft movements.

1669241488962.png

If you go with rigid mounts you just bolt the engine to the beds. It will be louder inside the boat because all the engine vibrations will now become structure borne noise. There is a reason every car engine is on soft rubber mounts! TO ISOLATE THE ENGINE NOISE/VIBRATION FROM THE PEOPLE. Sorry for shouting, it was noisy on the boat now.

With rigid mounts - in the old days alignment was often done with machined steel shims. Now we use cast epoxy poured in a temporary dam at each mounting bracket, once the engine is at the right height and angle. (Chockfast is the trade name of the best known of this type of pourable chocking compound).

1669241413561.png
 

hdra

Anarchist
653
147
Thanks Zonker - appreciate it. Engine is very quiet right now (at least to my ear which is used to sailboats with crap engine room insulation) - would be nice to keep it that way. Are rigid engine mounts more common on fishing boats? Wondering why that was his go to idea instead of a flexible coupling.
 
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longy

Overlord of Anarchy
6,826
1,160
San Diego
On almost all engine installs the engine beds are not parallel to the shaft line. So shaft thrust will push the engine block forwards (or backwards) maybe not much, but alignment is done down to .003 or so. Since engine moves on a different plane than the shaft, mis-alignment gets worse. Harder engine mounts help a lot, but then more vibration gets transmitted to the hull. As Zonker details, one must still have some method to precisely align the coupling joint - without using the flex joint!. Most of the install's I've seen have just pushed the shaft aft to fit the flex unit with no thought about gap behind the strut.
 

Sailabout

Super Anarchist
I've just taken over running a 42' ex-fishing boat that's been converted into a research boat. It's got a detroit diesel 4-71 on soft mounts, and has a history of drive train issues - they've had to rebuild the transmission every 5 years for at least the last 10 years, maybe longer but don't have maintenance logs going back that far. It also seems to be chewing up cutlass bearing pretty quickly (500 hours or so), even with the engine perfectly aligned at the dock. The previous captain strongly suggested that we convert it to rigid engine mounts, but he wasn't able to get that done at the last haul-out for a variety of reasons. Currently there is not a flex coupling on the shaft. Is there any reason to think that putting a flex coupling in is a better option than rigid mounts, either in terms of cost or ease of use? I've only ever interacted with sailboats with flexible engine mounts, shorter shafts, and less torque-y engines, so the whole rigid engine mount thing is a totally new world for me. Are they something you normally just get fabricated, or do people sell them like they do fixed mounts?
rigid mounts on a gm are fine as its a 2 stroke, very smooth running
 

DDW

Super Anarchist
6,587
1,151
The shaft itself is one of the more flexible parts of the system. On my motorboat, the 2" shaft runs in two cutlass bearings so pretty rigidly held. Yet I can with one finger deflect it easily 1/8" at the coupling. It's own weight flexes it down that much. As a machinist you learn that everything behaves like rubber. Some of it pretty stiff, but still like rubber. A lot of the behavior depends on how far it is cantilevered, either inside or outside the boat.
 

Zonker

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A 4-71 is a 4 cylinder engine. 4 cylinder engines have inherent primary and secondary shaking forces right? (Sure 1,2,3 are worse but to say a 4 cylinder is a smooth running engine is not really true). It's the masses moving that cause these forces, not the power stroke (they just add to it)
 

Alaris

Super Anarchist
1,530
400
Maryland
R&D coupling for 50hp and 1.25" shaft, deformed after 5 seasons use. R&D didn't have a good explanation and offered a big discount on a replacement. No thanks. They also said the coupling is primarily intended for impact protection, not for reducing vibration.

View attachment 555352
Looks like it did its job of absorbing system flex. I don’t see the problem; replace it. And maybe your engine mounts and cutless bearing too.
 

hdra

Anarchist
653
147
Does it make any difference in this discussion that there's an intermediate shaft bearing that's about halfway between the cutlass bearing and the transmission? Not sure what's inside of it, but the outside is bronze, the alignment of it is adjustable, and it is greased heavily. If everything is aligned right, when the engine moves it on its flex mounts it should be wearing out that bearing as well, no?
 

slug zitski

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worldwide
Does it make any difference in this discussion that there's an intermediate shaft bearing that's about halfway between the cutlass bearing and the transmission? Not sure what's inside of it, but the outside is bronze, the alignment of it is adjustable, and it is greased heavily. If everything is aligned right, when the engine moves it on its flex mounts it should be wearing out that bearing as well, no?
Some kinda saddle bearing ….used to stabilize a long shaft

some of them are specified to handle thrust

you still use a flex coupling
 




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