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Old 06-02-2017, 17:57   #1
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Flex couplings

I am at a yard in Thailand and having an alignment issue of some kind with my shaft. Not sure what the problem is yet. Shaft, coupling, propeller and strut is at the shop right now to check to see if their is a problem there. There is a problem somewhere just not sure where. No vibration. Just a hum. Has worn my new cutlass bearing and scored my shaft. Done in Fiji. There is good talent here. They will figure it out I hope. However, they immediately recommended a flex coupling. For them it is just an add on at this time. They haven't checked my old one yet. Wondering what others have experienced with them. Pros and Cons

Bob
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Old 07-02-2017, 05:33   #2
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Re: Flex couplings

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Originally Posted by rnjpinz View Post
I am at a yard in Thailand and having an alignment issue of some kind with my shaft. Not sure what the problem is yet. Shaft, coupling, propeller and strut is at the shop right now to check to see if their is a problem there. There is a problem somewhere just not sure where. No vibration. Just a hum. Has worn my new cutlass bearing and scored my shaft. Done in Fiji. There is good talent here. They will figure it out I hope. However, they immediately recommended a flex coupling. For them it is just an add on at this time. They haven't checked my old one yet. Wondering what others have experienced with them. Pros and Cons

Bob
Bob--

We added a Flex Coupling to our boat (Perkins 4-108/Hurth) about 10 years ago after a friend of ours had an unfortunate event with a mooring line on his boat that resulted in some costly repairs. Better a broken coupling that can easily be replaced. While we've never needed it for that type eventuality, I did find that the coupling seemed to reduce drive train noise/vibration somewhat and the "thunk" when shifting from forward to reverse. The coupling will make up for some misalignment of the shaft but not much so that needs be carefully calibrated. Overall, however, I think the investment, although not that great, was worth it (at least to us).

FWIW...
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Old 07-02-2017, 05:54   #3
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Re: Flex couplings

A normal flex coupling is a fuse, some are called drive savers, idea of course being they shear before any damage is done if you wrap a line or something.
they are not really intended to accept and misalignment.
Until very recently the only real couplings that I am aware of that could accept misalignment were basically a constant velocity driveshaft, work great I'm sure but they require a thrust bearing and that usually means glassing in another bulkhead to mount the thrust bearing, although I'm sure a very good solution, but complex and expensive.

I haven't fitted it yet, but Brunton make what they call a Sigma drive, it is a flex coupling that will allow a 3 degree driveshaft offset and will accept the thrust from the prop too. I got it in the mail yesterday and it looks to be good kit.
It can be installed without pulling the shaft, it clamps onto the shaft, no keys and bolts to the output flange of the transmission.
Looking at the design, I see nothing new, it appears to be an Rzeppa joint, but they are extremely robust with a proven record. They are the U joint in the bigger Military truck front axles, Deuce and a half and bigger.
https://www.bruntons-propellers.com/...igmadrive.html
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Old 07-02-2017, 11:34   #4
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Re: Flex couplings

I've always used R&D flex couplings on my boats up to 300 HP. One thing I like is the ability to do the alignment with the coupling bolted in place. I've only had one break and that was when a friend used my boat and ran it aground, replaced the coupling and realigned with no other damage.
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Old 07-02-2017, 12:43   #5
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Re: Flex couplings

Four a properly aligned drive train (<0.001" per inch of coupling dia) a drive saver or flexible coupling really should not be required or desired. The presence of one can actually make accurate alignment impossible.

Best bet is to make sure your strut and shaft log are in good alignment, then move to engine to match. If the strut is misaligned you'll be fighting cutlass wear even if the engine alignment measures good.
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Old 07-02-2017, 13:11   #6
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Re: Flex couplings

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Four a properly aligned drive train (<0.001" per inch of coupling dia) a drive saver or flexible coupling really should not be required or desired. The presence of one can actually make accurate alignment impossible.

Best bet is to make sure your strut and shaft log are in good alignment, then move to engine to match. If the strut is misaligned you'll be fighting cutlass wear even if the engine alignment measures good.
This is not always the case.

If you have a rigidly attached stuffing box (I know, rare now. The sort that has a flange bolting it to deadwood, rather than the more common sort that is attached to the hull by a flexible hose.) and intend to use flexible engine mounts you must also use a flexible coupling to make up for the fact that the engine can wiggle but the shaft cannot.
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Old 07-02-2017, 15:40   #7
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Re: Flex couplings

Very good point, besides it's virtually impossible in a fiberglass or wood hull to get perfect alignment. David Pascoe wrote a very good article about this subject, I apologize I have no link.
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Old 07-02-2017, 15:54   #8
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Re: Flex couplings

To my mind a flex coupling is un needed unless your engine has an inherent vibration obvious in neutral.
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Old 07-02-2017, 16:01   #9
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Re: Flex couplings

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To my mind a flex coupling is un needed unless your engine has an inherent vibration obvious in neutral.
In theory. Problems is hulls can be more flexy than we like so if we have a smooth engine with good alignment, we may not have good alignment once there's load on the boat.
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Old 07-02-2017, 16:19   #10
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Re: Flex couplings

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In theory. Problems is hulls can be more flexy than we like so if we have a smooth engine with good alignment, we may not have good alignment once there's load on the boat.
Yeah, I'm assuming by "flex coupling" we just mean a urethane plate or etc. They have a bit of "softness" to them, but are really quite rigid, to my mind don't really solve mis alignment much.
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Old 07-02-2017, 16:41   #11
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Re: Flex couplings

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Yeah, I'm assuming by "flex coupling" we just mean a urethane plate or etc. They have a bit of "softness" to them, but are really quite rigid, to my mind don't really solve mis alignment much.
I assumed OP meant something like a sigma drive coupling.
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Old 08-02-2017, 14:47   #12
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Re: Flex couplings

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Yeah, I'm assuming by "flex coupling" we just mean a urethane plate or etc. They have a bit of "softness" to them, but are really quite rigid, to my mind don't really solve mis alignment much.
Exactly.

They are a better "fuse" than vibration reducer, and they're not that great at being a fuse...

Had one on my boat. Didn't solve a darned thing, but it did make alignment troublesome. Took it off and the vibes were no worse.
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