Split Shaft Prop Coupling Fit

[sailorboy1]

This comes into threads once in a while, but I searched and didn't really see what I am looking for in the topic:

It seems that common forum answers about removing a prop shaft coupling is that you have to replace the coupling after. And the new coupling needs to be fit to the shaft, so the shaft needs to be removed. Well removing the shaft on my, and I bet lots of others, boat is a pretty serious thing to do (if yours isn't that is great and there is no need to thread drift about it).

The last time I needed to do shaft seal work 9 years ago I ended up just reusing the coupling. There was no rust inside the coupling and after clean up the coupling just barely slid onto the shaft and even a light coat of oil resulted in having to "tap" it on. This is called a clearance fit and just a little research showed this as a common method of lower power (and in this case lower power is like less than 500-1,000HP) couplings of a shaft that also uses a key.

But everything you come across in forums talks about fitting the coupling to the shaft so you have to tap it to get it on. That is call a transition fit and since the shaft OD could be undersize or oversize within standard tolerance and the couplings come with the ID undersized, you would have to "fit" the two together. But of course you would need to get the shaft out of the boat. There is probably no doubt that a transition fit is better that a clearance fit if possible to do.

Lots of people like/recommend that a new coupling be a split coupling to made it easier to remove. But these couplings still need to be fit and it appears on boat forums it still gets recommended to use a transition fit. If so it becomes just like any other coupling in my mind in that you are going to need a puller/press to remove (and no I am not going to use a socket between the the shaft and transmission coupling method).

So now to the decisions:

I need to replace my shaft seal so need to remove the shaft coupling. The coupling is probably 21 years old, has been off at least once and reused the the last time, it isn't super rusted up. I wouldn't know how "stuck" on the shaft it is till next week till I try to remove so don't really know what the inside surfaces may be like, but expect it is going to come off fairly easy. But I went ahead and decided to get a new coupling anyway and got a split coupling, which it going to need to be fit. The fitting is the question to resolve.

Can it just be bored to a standard shaft size (in this case 1.25")? This may result in a clearance fit or a transition fit. If it is a clearance fit does it really even matter as the key transmits the power anyway and tightening up the split is going to lock it down anyway (it has been a clearance fit at least 9+ years)? In measuring the shaft behind the coupling it appears the shaft is/may be on the upper end of ASME tolerance so a bored coupling is most likely going to end up being a transition fit (i.e. will need to be "tapped" on).

And finally since this whole "fitting" all has to do with alignment and "wobble" and since a boat is flexing and bouncing and I feel in operation shafts are never really aligned, does it even matter? BTW I am also installing a R&D flex coupling as part of the project as from my thread a few months ago I decided the only downside was the cost of the coupling and it probably helps in exactly this issue.

I have 5 days to decide. Post/discuss as you wish, but please vote on the poll.

[PippaB]

Pretty sure I will be in the very tiny minority here, but our boat had a keyless split coupling when we bought her, connected to a Volvo D2-40. It also has Volvo shaft seals that need replacement about every 5 years. I've had the coupling off to replace the seals three times, only at the last one did I replace the coupling. And since it had worked I replaced with the same thing.

[sailormed]

Installed Sigma drive with own circumferential pressure coupling.

[deblen]

Originally Posted by PippaB

Pretty sure I will be in the very tiny minority here, but our boat had a keyless split coupling when we bought her, connected to a Volvo D2-40. It also has Volvo shaft seals that need replacement about every 5 years. I've had the coupling off to replace the seals three times, only at the last one did I replace the coupling. And since it had worked I replaced with the same thing.

I have a similar setup. Volvo split coupling & Volvo Seal. Replaced the seal & re-used the coupling no problem. I did check face to face alignment before tightening the flange bolts-it was perfect.
I did replace the 6 original allen clamp screws with new SS allen screws,as 2 of the original screws snapped while trying to unscrew them.
After all,the engine is mounted on rubber mounts....
Cheers/Len

[deblen]

The recommended proceedures:


https://marinehowto.com/a-new-prop-shaft/

[deblen]

Couplings.pdf

[sailorboy1]

I have read all that stuff way before deciding to post this thread.

Given how many people here I bet have a PSS shaft seal that you need to replace the bellows regularly, which means they had to remove the coupling half, just how few seem to to able to answer the poll.

[sailorboy1]

so far only 1 in 6 have have a transition fit that everyone says is the "proper" way

interesting

[Cheechako]

Never had a split coupling, never had an issue ... except getting the damn thing apart if I need to!

[sazarac]

Push your motor sideways hard. Even new mounts allow a surprising amount of movement. I have to remove my engine and pull the shaft inboard. Takes me about an hour to remove my 3gm30. Rhodes Bounty.

[sailorboy1]

Originally Posted by sazarac

Push your motor sideways hard. Even new mounts allow a surprising amount of movement. I have to remove my engine and pull the shaft inboard. Takes me about an hour to remove my 3gm30. Rhodes Bounty.

Great

I definitely am not doing that.

[Ballsnall]

Originally Posted by sailorboy1

It seems that common forum answers about removing a prop shaft coupling is that you have to replace the coupling after.

If the coupling is undamaged there is no need to replace it.

And the new coupling needs to be fit to the shaft, so the shaft needs to be removed.

The new coupling needs to be the correct size for the shaft, you don't need to remove the shaft to fit it.

But everything you come across in forums talks about fitting the coupling to the shaft so you have to tap it to get it on. That is call a transition fit and since the shaft OD could be undersize or oversize within standard tolerance and the couplings come with the ID undersized, you would have to "fit" the two together.

"Transition fit" is not an actual fit, it just means the fit is somewhere in the transition zone. If you tap the coupling on then it is size for size or clearance fit.

But of course you would need to get the shaft out of the boat. There is probably no doubt that a transition fit is better that a clearance fit if possible to do.

No, you would just machine the coupling to suit the shaft size with 1 to 2 thou clearance and then assemble. I can't imagine why you would want to remove the shaft unless it was an interference fit and needed to be pressed on.

Lots of people like/recommend that a new coupling
be a split coupling to made it easier to remove. But these couplings still need to be fit and it appears on boat forums it still gets recommended to use a transition fit.

Split couplings are much easier to use which is why they are popular. They do not need to be "fit" to shaft. They are essentially sized as a clearance fit but when tighten actually become an interference fit which is why they can transmit power without keyways.

But I went ahead and decided to get a new coupling anyway and got a split coupling, which it going to need to be fit. The fitting is the question to resolve.

Can it just be bored to a standard shaft size (in this case 1.25")?

The shaft should be already bored to suit your shaft size + clearance. If you bought a blank coupling then the 2 halves need to be appropriately spaced and then bored to your shaft size. If you don't space the 2 halves then it will not tighten onto the shaft.

[fourlyons]

Sailorboy, are you confusing "fitting" a coupling to a shaft and "facing"? When new, a split coupling will fit on the shaft that it's designed for in terms of diameter for the shaft, but the face has to be trued on a lathe while installed on the shaft. Around here that's called "fitting and facing". Once that is done, a coupling can be removed and reinstalled many times without issue. Forgive me if you know all this already.

Sent from my Z5157V using Cruisers Sailing Forum mobile app

[sailorboy1]

Originally Posted by fourlyons

Sailorboy, are you confusing "fitting" a coupling to a shaft and "facing"?

no I am not, and no my brand new coupling will not fit on the shaft till it gets fit/bored

[CAR54]

I ordered a split coupling from general propeller. They only make them custom order. They have excellent customer support and after giving them the specifications they required, they shipped me one. It costs a little over twice as much as a interference coupling but saved me having to drop the rudder to do a fit and face which, would have required two trips of the travel hoist (one to remove and one to install the rudder.)

General Propeller said the coupling didn’t require a key way or fit and face. Using two steel wedges tapped into the slots of the coupling it slide on easily. Then I torqued the bolts in the pattern and torque steps recommended by the manufacturer.

30 minutes of operation later there was no excess vibration, but I decided to rechecked the torque values on the bolts and nuts and picked up 1/2 to 3/4 turn on each nut. After calling customer support with my findings they seemed concerned, but also they said they had never had a customer recheck the torque before. After another 1/2 hour of operation I rechecked them again and found the torque to be at specifications and updated them. They sounded greatly relieved and were great full for the update.

Three years later and about 175 hours, all is well and I highly recommend this coupling from this manufacturer to anyone worried about using a split coupling.