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Old 18-07-2020, 20:01   #1
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I keep breaking set screws in prop shaft coupling

In 2016 when I bought the boat one of the two set screws was broken. See below. i drilled it out and re-assembled during refit. During 2018 and 2019 i did not put too many miles on the engine but at haul out in October 2019 one of the two set screws was broken again. That time I had put a lot of red thread locker and re-aligned the engine after replacing two aft isolation mounts. Things seemed fine. Sailed to FL last fall and back this spring putting about 2000NM + on the boat. On a way back in may, I did noticed that one of the front isolation mounts was broken. So i replaced it and I adjusted the mounts to try and I thought i aligned the engine pretty good. This morning as I was anchoring and baking up, shaft slipped out about an inch and keyway is now loose. So I lost propulsion. My space is limited down below and PSS below is already compressed a lot. So I have no room between the PSS collar and shaft coupling.

I took off the seizing wire and one of the screw was broken again, but this time right at the dimple that is located on the shaft.

I think what protected prop shaft from coming out altogether was the PSS collar.
Is there a better way to couple shaft to that coupling besides these set screws? Am I just having too many vibrations and thats what is breaking the set screws? To me, it looked like engine was aligned pretty good.

I will haul out yet again but I really need to find the root cause of why this is happening.
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Old 18-07-2020, 20:24   #2
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Re: I keep breaking set screws in prop shaft coupling

My first two questions would be is the shaft straight and is the shaft coupling face exactly perpendicular to the shaft?

Both are very hard, if not imossible, to verify with 100% certainty with the shaft/coupling assembly in the boat.
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Old 18-07-2020, 20:57   #3
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Re: I keep breaking set screws in prop shaft coupling

It sounds like you have play between the shaft and coupling. Couplings are supoosed to be an interference fit, meaning they will NOT just slide together wihout being tapped lightly tapped together.

The key needs to be a snug fit on the slot on both the coupler and shaft as well. Technically, youre not supposed to reuse a solid coupler, although it can be 'gotten away with' if relatively new. Sounds like youve had this one off a couple of times already....

If you have play, the shaft can torque slight when enagaging gears, breaking those set screws.

In any case, id toss the coupler and have a new one mated properly to the shaft at a prop shop.

Matt

By the way, a split coupler will pinch the shaft with hefy bolts.
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Old 18-07-2020, 21:04   #4
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Re: I keep breaking set screws in prop shaft coupling

Jim bunyard - purpendicular to shaft is actually easy to check - take feeler gauge readings with the couplers pressed together, then rotate one 180 degrees and remeasure.

If you had..05 on the top and. 10 on the bottom, a purpendicular coupling will maintain that., whereas a crooked one will change the numbers

Matt
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Old 19-07-2020, 03:06   #5
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Re: I keep breaking set screws in prop shaft coupling

If you’re continually breaking set screws I reckon it’s because of excessive clearance on the key or the keyway on the shaft and/or coupling. It’s also possible (if you haven’t already had the shaft completely out), that there is no key installed and the shaft is depending on the set screws to turn it.

The key should be a snug to tight fit in the keyways on both the shaft and the coupling.

It’s not about coupling/transmission alignment (IMHO)
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Old 19-07-2020, 05:23   #6
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Thumbs up Re: I keep breaking set screws in prop shaft coupling

The coupling looks to be quite short ,rule of thumb is five diameters of the shaft ,a real snug fit is a must,the key must be firm ,make shire the coupling is not riding on the key and that there is top clearance,if the horse power is high the best way is to double end the shaft I e taper and key both ends the same,coupling full length of the taper ,keyed and nutted ,do it once ,all good .
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Old 19-07-2020, 09:08   #7
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Re: I keep breaking set screws in prop shaft coupling

this may sound simple- but be sure the snug coupler/shaft and key way.
I have come across vessels with a metric coupler and standard shaft- particularly European built vessels that came to the US and had shafts replaced.

The discussion of a square face on the shaft is paramount. Your mention of realigning the engine makes me suspect you assume the issue is alignment and not a bent shaft or coupler issue. Try checking as mentioned above and if even begin to suspect problem then if pulling boat out of water- pull the entire shaft and coupler and take to machine shop and check- return to fit properly, If they are fine , check engine side of coupler face.
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Old 19-07-2020, 09:13   #8
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Re: I keep breaking set screws in prop shaft coupling

Quote:
Originally Posted by mlydon View Post
It sounds like you have play between the shaft and coupling. Couplings are supoosed to be an interference fit, meaning they will NOT just slide together wihout being tapped lightly tapped together.

The key needs to be a snug fit on the slot on both the coupler and shaft as well. Technically, youre not supposed to reuse a solid coupler, although it can be 'gotten away with' if relatively new. Sounds like youve had this one off a couple of times already....

If you have play, the shaft can torque slight when enagaging gears, breaking those set screws.

In any case, id toss the coupler and have a new one mated properly to the shaft at a prop shop.

Matt

By the way, a split coupler will pinch the shaft with hefy bolts.
Best advise, IMHO.
I've seen shafts thru bolted as well.
I don't recommend that though.
Coupler is loose or worn out.
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Old 19-07-2020, 09:31   #9
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Re: I keep breaking set screws in prop shaft coupling

These are excellent Welcome to Aquadrive
Your engine is perhaps 80hp?
John
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Old 19-07-2020, 10:07   #10
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Re: I keep breaking set screws in prop shaft coupling

Whenever I hear someone say they aligned it “pretty good”. That usually they did it by “eye”. Engine and prop shaft alignment is not one of those things you can do by eye or feel. If you are not a presidium type of person and willing to do things the right way with micrometer and feeler gauges and be able to look at your hardware with a critical eye the this is. EST turned over to someone else. There are dozens of YouTube on the matter.
If your engine mount broke it was either bad and out of support for a long time or you hit something very hard.

I suspect by your history of repeated breaking of set screws you let the canary in the coal mine keep on dying. It’s like someone who keeps replacing fuses rather than fixing the short.
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Old 19-07-2020, 12:07   #11
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Re: I keep breaking set screws in prop shaft coupling

Quote:
Originally Posted by johnn33 View Post
These are excellent Welcome to Aquadrive
Your engine is perhaps 80hp?
John
They look good. I have a Sigma drive, which look similar. I am very happy with mine. They do a great job at compensating for all manner of sins.

But as mentioned above the underlying things need to be checked and correct-
Shaft straightness,
Coupling tightness on the shaft,
Coupling concentricty,
Coupling angular trueness,
Engine alignment,
Mounts condition,
Worn coupling bolt holes,
Worn coupling bolts
Etc

If these things arent upto snuff then one of these Couplings may help in the short term. But longer term the coupling will wear faster absorbing additionsl vibrations and fail sooner.

Most people use feeler gauges to check engine alignment. Mostly gets close. I prefer Dial gauges, as for example feeler gauges cant show you Coupling eccentricity. They are also faster and easier to measure shaft bend and other things you cant measure with feeler gauges.

By the way your profile doesnt say much.
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Old 19-07-2020, 15:04   #12
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Re: I keep breaking set screws in prop shaft coupling

If you have flexible engine mounts and are not using a paired universal coupling or a constant velocity drive system you are asking for trouble.

A rigid prop drive requires perfect alignment perfection and with rigid engine mounts. Those rubber bushed coupling things are not proper universal drives--they are best used as shock absorbers for the propeller.
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Old 19-07-2020, 15:54   #13
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Re: I keep breaking set screws in prop shaft coupling

Thank you all. engine is Lehman NE 90HP and yes I did not do a filler gauge.

Coupling is probably original from the 90s. I also have this old down angle drive made by walter. Engine sits strait, has the down angle drive and then borg warner with I think 1.91:1 reduction. see pics. I also have PSS. So PSS collar is snug against the coupling and bellow is very compressed. I have no room whatsoever in there to do anything. And the entire thing has no support besides the aft isolation mounts. That is why when I bought the boat in 2016, I found one aft isolation mount broke. All were 5/8". I replaced the aft ones with 3/4" in 2018. Front ones are impossible to up size from 5/8" to 3/4" without getting the engine out. So I went with new 5/8" isolation mounts for the front. And I got one done about a month ago with one more to replace.

I am going to have to get a rebuilt downangle transmission and get rid of borg warner and the rest of the stuff. And I will probably need and new shaft. since walter down angle drive is about 6".
I had the max prop off in 2018 and it was easy to come off. Hopefully I can keep that. Shaft is 1.5"
in the pic below, arrow are is pointing to the set screws that keep braking.

Since shaft moved aft about an inch, and I think key way is now inside the coupling, PSS collar set screws might have been what saved the shaft from fully coming out.
So after that the inner o-rings on PSS collar were no longer sealing and 1/4" keyway cutout was allowing water to come in. I had do get quite creative to get the water to stop. Included getting in the water and sealing the outside with Teflon tape and zip ties. My strut is in the hull.
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Old 20-07-2020, 05:21   #14
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Re: I keep breaking set screws in prop shaft coupling

Hi Phorvati, hey man I did had this same problem for many years until I remove the shaft with the hub and put them in a lathe and found out it was not align. I discover the hole in the hub was drill off center. I have a friend redone it to center it and since never had to touch it for last 18 years. Good luck.
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Old 20-07-2020, 08:13   #15
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Re: I keep breaking set screws in prop shaft coupling

Quote:
Originally Posted by mlydon View Post
It sounds like you have play between the shaft and coupling. Couplings are supoosed to be an interference fit, meaning they will NOT just slide together wihout being tapped lightly tapped together.

The key needs to be a snug fit on the slot on both the coupler and shaft as well. Technically, youre not supposed to reuse a solid coupler, although it can be 'gotten away with' if relatively new. Sounds like youve had this one off a couple of times already....

If you have play, the shaft can torque slight when enagaging gears, breaking those set screws.

In any case, id toss the coupler and have a new one mated properly to the shaft at a prop shop.

Matt

By the way, a split coupler will pinch the shaft with hefy bolts.
So Now that I am thinking about it, I think coupling was sliding on the shaft relatively easy. almost too loose. And the keyway might not be a snug fit. This is probably the biggest contributor to my issue.
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