Shaft to flange connection...

[ProjectManaia]

Hello everyone,
We were trouble free with our propulsion setup for 2 years now until recently I changed from idle neutral to reverse and nothing happened. After lots of digging it turned out the connection from the flange to the shaft had given in.

Looking into it it appeared as if only a bolt (no key, just a bolt) was holding it in place for a long time. So in agreement with a local mechanic we took out the old bolt, put a new one in (after pushing the shaft back in) and seemed happy.

Two weeks later same thing. From idle neutral to idle reverse, and nothing - so the same thing happened. The bolt is still in but seems to have snapped.
Question is: Why is this happening and how to best fix it?
The flange is not designed for a key - the shaft has a fitting (cutout) for it. Taking of the flange is near impossible without lifting the boat out and we need a good (and quick) fix...
Picture of the setup attached for clearing the image.
The issue only occured when going into reverse (guess it ahs to do with the pull of the shaft when in reverse) but not entirely sure how to stop that either....

Any ideas are more than welcome!

[DougR]

From the looks of the photo, there is a serious amount of misalignment between the gearbox output flange and the prop shaft flange. This misalignment will cause constant flexing in the flex coupling and constant working of the propshaft flange on the shaft, fatiguing the pin and everything else. Likewise if the bolt isn’t a high strength bolt with a tight fit in both the shaft and coupling it will shear.

It ‘s time to have a skilled tech check the engine alignment and the prop shaft to shaft coupling interface for issues.

[ProjectManaia]

Originally Posted by DougR

From the looks of the photo, there is a serious amount of misalignment between the gearbox output flange and the prop shaft flange. This misalignment will cause constant flexing in the flex coupling and constant working of the propshaft flange on the shaft, fatiguing the pin and everything else. Likewise if the bolt isn’t a high strength bolt with a tight fit in both the shaft and coupling it will shear.

It ‘s time to have a skilled tech check the engine alignment and the prop shaft to shaft coupling interface for issues.

That is already in Process, he will be here tomorrow morning. Still hoping to be able to fix this without hauling out (again) - In respect of the hardened bolt: My thinking was the bolt should be the "weak link" in the system since you don't want the shaft to go bang, right? Or am I getting something very wrong here?
Thanks again!

[ProjectManaia]

Also the alignment looks good it real life (guess it is lens distortion making the look here)
Anyways, here another picture from a different angle also showing the "bolt"
Which really is just a galvanised bolt with the three ground off on the first centimetres to slot into the key slot...

[kmacdonald]

There should be a key in the shaft and coupling. The bolt prevent the shaft from backing out of the coupling in reverse. It's not for torsional loads.

Did someone extend the shaft with a second coupling and flange and short piece of shaft?

[kmacdonald]

Could be a bad damper plate shock loading the shaft when put in gear.

[DougR]

So if I understand you correctly, there is a keyway in the shaft, but no keyway in the shaft coupling....and you have ground the threads off of the end of a standard 8.8 grade bolt to fit into the shaft keyway and function like a key. And no doubt also to act like a set screw and keep the shaft from backing out of the coupling?

As Kmacdonald mentions, the bolt doesn’t have the strength to transmit the torque required, that’s what a key and keyway is for. And a bolt used as a set screw to take thrust needs to be a hardened bolt fitting into a dimple on the shaft, not a soft 8.8 socket head bolt.

[ProjectManaia]

Originally Posted by DougR

So if I understand you correctly, there is a keyway in the shaft, but no keyway in the shaft coupling....and you have ground the threads off of the end of a standard 8.8 grade bolt to fit into the shaft keyway and function like a key. And no doubt also to act like a set screw and keep the shaft from backing out of the coupling?

As Kmacdonald mentions, the bolt doesn’t have the strength to transmit the torque required, that’s what a key and keyway is for. And a bolt used as a set screw to take thrust needs to be a hardened bolt fitting into a dimple on the shaft, not a soft 8.8 socket head bolt.

Thank you guys for this, that would explain why the "fix" only lasted 2 weeks. btw it was not me, it was a so called professional doing this.
So my gameplay right now would be:
Take out the coupling, grind in a slot for a shaft key to fit. Get it back in, put the key in, add the screws to stop the shaft from backing out and go...
sounds reasonable?

[wingssail]

Originally Posted by ProjectManaia

Thank you guys for this, that would explain why the "fix" only lasted 2 weeks. btw it was not me, it was a so called professional doing this.
So my gameplay right now would be:
Take out the coupling, grind in a slot for a shaft key to fit. Get it back in, put the key in, add the screws to stop the shaft from backing out and go...
sounds reasonable?

That is the right answer however grinding may be difficult. It may need to be machined.

However, we are curious why the rig lasted for two years before failure then lasted only two weeks with the replacement bolt. Possibly a bolt with less strength than the previous one? But something else may be going on, such as a flex plate which is not cushioning the impact of the shift? Or could it be that your engine rpm at shifting is higher than it used to be?

In any rate, adding the key is correct, but I'd also try to figure out why the failures started after two years.

[kmacdonald]

The shaft would have to be removed and a keyway machined. I think you need a new longer shaft and get rid of the extension. I'm guessing it was rigged originally and therefore the lack of a keyway.

[ProjectManaia]

Originally Posted by wingssail

That is the right answer however grinding may be difficult. It may need to be machined.

However, we are curious why the rig lasted for two years before failure then lasted only two weeks with the replacement bolt. Possibly a bolt with less strength than the previous one? But something else may be going on, such as a flex plate which is not cushioning the impact of the shift? Or could it be that your engine rpm at shifting is higher than it used to be?

In any rate, adding the key is correct, but I'd also try to figure out why the failures started after two years.

I assume (very well aware that assumption is the mother of all mess-ups) that the replacement bolt put in 2 weeks ago was weaker than the original. I know the replacement was simple galvanised steel...
I am currently tied up right next to a machine shop so we are in the right place to get those things done.... my big hope however is being bale to sort things out without lifting the boat out (crane looks dodgy as something else) .... so fingers crossed and hope for the best

[ProjectManaia]

Originally Posted by kmacdonald

The shaft would have to be removed and a keyway machined. I think you need a new longer shaft and get rid of the extension. I'm guessing it was rigged originally and therefore the lack of a keyway.

I think the shaft length is fine since the shaft now slots all the way into the flange. so it can't go much further really. Plus we have a couple of cm to spare on the outside if needed... But we shall see.
Hope to be able to get back with more insight tomorrow....
As always it is of course a bastard of a place to work at, since it is way down in the bilges, below gearbox and engine.... what to do....

[kmacdonald]

Is it a v-drive?

[ProjectManaia]

Originally Posted by kmacdonald

Is it a v-drive?

Yes, a V-Drive with a velvet hydraulic gearbox, so even IF the rpms would be higher (which they don't appear to be) the "hit" should be very smooth...

[kmacdonald]

Now for the bad news. The rudder may have to be removed to remove the shaft. The keyway and key must be machined to close tolerances. You cant "grind them".