Prop Shaft Worn Keyway

[Apollo]

Hola team,

I am doing some repairs on my engine and while decoupling my gearbox I noticed that one of the couplings on my prop shaft had quite a bit of wobble in it. The last coupling to the gearbox is one of those flexible ones with the spider so I don’t think any damage would have gone upwards to the gearbox or engine mounts from the vibration.

This being said I’ve read some alarming posts online about the kind of damage that can happen when keys go. It actually looks like most of the wrong fit comes from the keyways on the shaft and on the coupling. The edges have become tapered from probably years of wobble. Shafts and shaft couplings are expensive so I’m wondering wether:

- I can ignore the issue as if I tighten the center bolt hard enough the wobble goes away as my prop shaft is tapered.

- I can get the keyways on the shaft and the coupling re-machined for a bigger key and if that would be cheaper.

If you think I need to go for re-machining do you know anyone trustworthy?

Thank you for reading!

[Smoggy]

I'd use some fine grinding paste to lap the prop to the shaft and if the faces look even after lapping not worry about it, once tightened the taper should do the work not the keyway, it's maybe been loose at some point to do that.

[petersjoy]

I wouldn't rely on the taper only holding it, especially when there's sudden forward/ reverse loads involved.

Looking at that, any good engineering shop should be able to rebuild that and fit a new key.

[OldBerkshireBoy]

3 hours ago, Smoggy said:

I'd use some fine grinding paste to lap the prop to the shaft and if the faces look even after lapping not worry about it, once tightened the taper should do the work not the keyway, it's maybe been loose at some point to do that.

The key is the drive point not the taper as seen by the condition of the coupling keyway.

3 hours ago, petersjoy said:

I wouldn't rely on the taper only holding it, especially when there's sudden forward/ reverse loads involved.

Looking at that, any good engineering shop should be able to rebuild that and fit a new key.

What is the threaded section for?

Agree, I have made custom keys where one part is bigger than the other to suit shaft and coupling.

[Apollo]

4 hours ago, Smoggy said:

I'd use some fine grinding paste to lap the prop to the shaft and if the faces look even after lapping not worry about it, once tightened the taper should do the work not the keyway, it's maybe been loose at some point to do that.

Thanks, I’d love to get away with this and I would probably do that if I stayed on the canals but I’m planning on a channel crossing in summer so I need the drive train ship shape!

9 minutes ago, OldBerkshireBoy said:

The key is the drive point not the taper as seen by the condition of the coupling keyway.

What is the threaded section for?

Agree, I have made custom keys where one part is bigger than the other to suit shaft and coupling.

The threaded section has a thick washer and a bolt that push the coupling along the taper of the shaft as it gets tightened so both Smoggy and petersjoy are right. The taper creates a nice tight fit once tightened but as the bolt loosened a little the key did all the work and the wear happened…

[Vaughan]

7 hours ago, OldBerkshireBoy said:

What is the threaded section for?

This is for the nuts which fix the propellor onto the taper.  This shaft has been made with the taper at both ends, so that it can be turned and used again if it gets worn in the area of the stern gland.

So if you turned it, so that the worn keyway was at the prop end and then did the lapping job to make it fit better, would that put less strain on the worn keyway?

You mention only one nut on the thread that holds the coupling, but there should be a second, locknut, to prevent the first one coming loose.  Either that, or the nut and shaft are drilled through for a split pin, to stop the nut loosening.

Personally, I would like to be sure, and have it machined out for a larger keyway.  And then make sure it has a locknut!

[grendel]

looking at the shaft, the keyway will need machining out (on the shaft and in the other part), and the shaft will need cleaning up too as metal has pushed out into the taper (where I have circled in red), that metal will need to be removed before the taper will seat properly.

alternatively a new keyway of the same dimensions could be machined in at 180 degrees to the existing, the taper on the shaft would still need cleaning.

either option should be easily done by any competent machine shop.

[Apollo]

Thank you everyone. I’ll find a machining shop and get it sorted.