Vibrating shaft at slightly high revs.

[Hoohaa]

Here is a shot of the coupling brought together. There was still plenty of meat left on the shaft so i think it allows me to take accurate measurements.

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[jimbunyard]

Originally Posted by GILow

Interesting assertion. I wonder how you can be so confident of this?

The OP's pictures look a LOT more like a polyflex coupling than what you have pictured here, and given the engine is mounted on polyflex engine mounts, (according to the OP) it seems probable both were from the same manufacturer. Finally, the OP is in Australia, and polyflex is very, very commonly found on boats in Aus. A quick Google failed to find a distributer of the Globe Rubber Works Drive Saver in Australia, but I concede it could be imported.

Either way, the device you have pictured, and the original polyflex coupling both lack the apparent damage on the outer rim, and neither have the captive nuts even remotely as close to the edge of the polymer as that pictured by the OP.

Anyway, this thread is an interesting read, and I am learning there are many ways to skin this particular cat. (With apologies to the cat lovers out there. )

Matt

You are of course correct, it appears that it is manufactured by Polyflex, though I can't be sure either way without looking through the entire line of drivesavers and polyflex couplings.

However, the point is the same, these type couplings are not meant to allow for and absorb misalignment, as traditional flexible couplings are. For drivesavers, it is explicit in their literature that the primary function is impact absorbtion, for the polyflex, who knows, I couldn't get any technical information to load, but based on what they look like I will assume the function is identical.

Some flex couplings as 'traditionally' described.

[Three Sisters]

Originally Posted by Hoohaa

Here is a shot of the coupling brought together. There was still plenty of meat left on the shaft so i think it allows me to take accurate measurements.

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The two couplings should be joined flat against one another, cleaned of debris. Unless a specific reason exist for the washers between the two couplings in the pic. And from here they look like plain old zinc washers.

Using this method as opposed to the 1" gap method, indicated earlier, requires feeler gauges.

[GILow]

Originally Posted by jimbunyard

You are of course correct, it appears that it is manufactured by Polyflex, though I can't be sure either way without looking through the entire line of drivesavers and polyflex couplings.

However, the point is the same, these type couplings are not meant to allow for and absorb misalignment, as traditional flexible couplings are. For drivesavers, it is explicit in their literature that the primary function is impact absorbtion, for the polyflex, who knows, I couldn't get any technical information to load, but based on what they look like I will assume the function is identical.

]

Well Polyflex call them simply "flexible disc couplings" on their web site and there's no literature on them that I could find either. In talking to them recently when replacing my coupling, they were at pains to point out that alignment still needed to be spot on so I assume they are not intended to manage alignment issues, just vibration and shock loads as you say.

Either way, I reckon the OPs coupling is buggered. I relegated a much better condition coupling to the spares when I changed the engine from hard mounted to soft mounted a year or two back.

On that point, Hoohaa, you might want to check if Vetus makes a suitable mount for your engine. Their prices were excellent and they were able to calculate and supply a suitable set of mounts for our 450 kg, 96 HP monster engine, which cost me considerably less than what you are being quoted. I think it was $75 per mount, or maybe only $55. Either way, not $180.

Matt

[a64pilot]

Align the mounts and bolt the flanges together without the coupling and take her out and see.
If it's a bad coupling, problem will be gone, if it's due to excessive prop overhang, problem will be gone, you will never know which it was, but will you care as long as the problem is gone?

[OceanSeaSpray]

Originally Posted by Hoohaa

I originally said that the wobble occurred at higher revs but i had a better look at it and its always wobbling at the stuffing box.
Summarizing.
The system has always wobbled ( not vibrated) at the stuffing box since I bought the boat.
1 year ago on advice from a shipwright, Surveyor. I replaced the cutlass bearing and rebuilt the skeg. New coupling and new shaft.
The problem is still there.
Its possible that before I purchased the boat that the prop hit something, sustained some damage thats not readily visible and bent the shaft in the process.
I think all things considered its either shaft alignment or the prop?

I have looked back through the whole thread to find the shaft length, diameter, configuration and distances between support points and it is nowhere to be found...
There is no point trying to troubleshoot an installation if there happens to be something fundamentally wrong in its design.

So, starting from the beginning:

• What is the shaft length?
• What is the longest unsupported length?
• Where are the support points?
• What is the shaft diameter?
• Specifically, is the shaft supported or floating at the seal?
• What is the shaft maximum RPM? (Or engine max RPM and reduction ratio)

I have a spreadsheet in my design files to calculate critical speeds, so if you give me the info, I will have a look.

What you see as a wobble at the seal at low speed may just be the engine moving on its mounts and taking the shaft with it.

[Cheechako]

Originally Posted by OceanSeaSpray

I have looked back through the whole thread to find the shaft length, diameter, configuration and distances between support points and it is nowhere to be found...
There is no point trying to troubleshoot an installation if there happens to be something fundamentally wrong in its design.

So, starting from the beginning:

• What is the shaft length?
• What is the longest unsupported length?
• Where are the support points?
• What is the shaft diameter?
• Specifically, is the shaft supported or floating at the seal?
• What is the shaft maximum RPM? (Or engine max RPM and reduction ratio)

I have a spreadsheet in my design files to calculate critical speeds, so if you give me the info, I will have a look.

What you see as a wobble at the seal at low speed may just be the engine moving on its mounts and taking the shaft with it.

Good thoughts. As I said early on, based on the OP's video, "I'm not sure I'm seeing anything unusual"

[jimbunyard]

Originally Posted by OceanSeaSpray

I have looked back through the whole thread to find the shaft length, diameter, configuration and distances between support points and it is nowhere to be found...
There is no point trying to troubleshoot an installation if there happens to be something fundamentally wrong in its design.

So, starting from the beginning:

• What is the shaft length?
• What is the longest unsupported length?
• Where are the support points?
• What is the shaft diameter?
• Specifically, is the shaft supported or floating at the seal?
• What is the shaft maximum RPM? (Or engine max RPM and reduction ratio)

I have a spreadsheet in my design files to calculate critical speeds, so if you give me the info, I will have a look.

What you see as a wobble at the seal at low speed may just be the engine moving on its mounts and taking the shaft with it.

You did see the video, where the engine end appears to be rock solid while the seal end wobbles merrily back and forth?

It'll be interesting to find out what the numbers say though...

https://www.youtube.com/watch?v=fUkl...ature=youtu.be

[senormechanico]

Was that video shot at a dock in flat water or out in waves? Possibly yet another variable.

[Hoohaa]

Underway on Sydney Harbour in smooth water. Weather was bright and sunny at about 27 deg. Beer in hand.

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[OceanSeaSpray]

Originally Posted by jimbunyard

You did see the video, where the engine end appears to be rock solid while the seal end wobbles merrily back and forth?

It'll be interesting to find out what the numbers say though...

Look at the shadow of the shaft/steel coupling against the white background paint after 0:36 and you will see that the engine is shuddering just as well.

If you shake a spinning shaft that is not being held perfectly straight and true at its end - courtesy of the wonderful flex coupling - you can easily cause it to start whirling out of control from a certain speed up.
The seal is at the end of a rubber hose, so it offers no support at all and I would bet anything that the shaft is also floating in the tube at the inboard end.

So back to where I had started, this installation should NOT be using a flex coupling, because the engine is rubber mounted and the shaft is floating, only supported by the cutless bearing at the far end.
Make a hard spacer with a precise spigot, get rid of the junk coupling and it will all start running smoothly.

Alternatively, because there is such a long length of exposed shaft behind the engine, add a second cutless bearing at the inboard end of the tube, before the seal, and then you should leave the flex coupling in to absorb the motion of the engine.
If the unsupported shaft length was excessive (which we still don't know), then this would be the pathway to follow.

Either way, the installation is incorrect as it is, so there is no point trying to tune it. The better you align, the worse it will get actually.

[Three Sisters]

Originally Posted by OceanSeaSpray

partial quote

So back to where I had started, this installation should NOT be using a flex coupling, because the engine is rubber mounted and the shaft is floating, only supported by the cutless bearing at the far end.

The engine mounts are pictured in post #70 and appear to be poly

unless Hoohaa can verify otherwise.

[Hoohaa]

The mounts are poly. And i have not noticed any rough movement of Engine.

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[OceanSeaSpray]

Enjoy your problem then.

[Hoohaa]

Originally Posted by Hoohaa

I spent today trying a few different things.

1.Removed the flexible coupling and loosened the metal coupling and slid it up so that the two flanges could come together.
I put a couple of bolts in it and checked the gaps with feeler gauges. Made some adjustments and then idled the motor in gear. Wobble was still there.

I idled the motor in gear with the flanges bolted together with the flexible coupling removed. It made no difference, the wobble was still there.
The wobble is not there when I turn it by hand but is there once she is put in gear under load. Any load.
I am in the process of finding out from the people that did the work, if and or what bearings where replaced and why they put the flexible coupling into the system.
I will hopefully have some answers tomorrow.



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