Sheared Shaft Key? Coupling Replacement? Help

[SeacretSailor]

I got a problem, I think I sheared the key on my prop shaft in the shaft coupling and my shaft backed out (moved aft) about an inch. I think I need to replace the shaft coupling and key. Help!

Background: I've got a Yanmar 2QM15 (C&C29) with a PSS shaft seal. Earlier this summer I remember hitting a log or line while motoring as evident by a small nick in my 14" bronze prop. Last weekend, while motoring back from a trip, my PSS shaft seal developed a serious leak. A temp fix got me back to dock. Today I examined the issues and found that it appears my 1" shaft has slipped back about 3/4", compressing the PSS bellows.

Here's the main problem/indicator: I can rotate the tranny & shaft couplings 3/4" left and right before it starts to spin the prop shaft. I know that's not good. I'm sure the couplings should be positively attached to the shaft with NO leeway or slippage. The set screws on the shaft coupling are still wired and don't appeared to have backed out.

I think I snagged something with the prop, got the nick in the prop and the strike sheared the shaft key in the shaft coupling. This loosened the shaft in the coupling, allowed it to slide out and aft a little, caused the leak in the PSS seal and that's why there's play between the couplings and the shaft.

Any thoughts, ideas, suggestions? I have to fix this DIY as money's tight. Since the coupling's loose, with play/slippage, it should come off OK so I can replce the key. Should I also replace the coupling? Mine is not a split coupling but a solid one.

Thanks
Kevin

 

[Ted]

Sheared Shaft Key

The shaft key will not prevent the shaft from backing out of the shaft coupling. There should be two set screws in the shaft coupling. Your shaft should have shallow holes drilled in it which the set screws will fit into. It's the set screws that will prevent the shaft from backing out. To a lesser extent, they will help prevent the shaft from spinning within the coupling. The main function of the key will be to prevent the shaft from spinning on the collar. If your shaft has shifted aft and the set screws have not loosened up, they may not have been properly seated in the holes in the shaft prior to your incident. If the shaft is turning within the coupling, the key may have sheared as you suggested but I would take a close look at the keyway in the shaft as that may have been damaged bad enough to cause the condition you have described. It's an easy DIY project as long as you can get the coupling off the shaft.

 

[kloudie1]

Sorry to hear that, Kevin. Sounds like ya have a couple of problems in addition to a sheared key. Here is a quick and dirty that can keep ya in the water until you can push a few dollars into a pile.. first, the coupling bore sounds like it is too loose. Soon, you'll need a new coupling half that goes onto the shaft. It also sounds like you have some misalignment that has worked the shaft in the loose fit.. Short term repair .. if the shaft key groove is long enough, you can probably get a short key into the groove and up into the coupling. you'll have to cut the safety wires and loosen the set screws first. Once you get some keystock back into the coupling, and move the shaft to where it was, kinda move it around to find the places where the screws were originally tightened. You should be able to feel that place as a place where the screws are more loose as you move the shaft into the coupling.. you may be able to see a line on the shaft where the coupling hub used to fit.. Once you have the screws back in place, tighten them down really tight. Then run the engine and engage forward and reverse for a few minutes each .. stop the engine and again retighten as much as you can. Run the engine again for a few minutes in forward and aft and retighten.. then install safety wire to hold em. now check and align the engine and shaft as best you can .. Should last for long enough to get the parts together to replace the coupling half the next time you are out getting a bottom job.. but check after ya get back to the dock to see if the screws need tightning. As Ted notes, the shaft should be dimpled under where the screws hit it .. when you install the new tight fitting coupling half, make sure this is done..MaineSail posted some very good notes on couplings a while back.. that should be a guide to get the new one on correctly.. Good Luck..

 

[Don12364]

It always seemed risky to me that the only thing preventing the shaft pulling backwards if the prop hit something solid were two little dimples in the shaft and any clamping force from the coupling. If the shaft pulls out "glug glug glug", the leak is tremendous and difficult to get to to block. Also it might prevent the rudder from turning.
I worry.
On my previous Nicholson (a better class of old time boat builder) I was pleased to discover a high tensile 3/8" bolt tapped lengthwise into the end of the shaft with a thick washer under the head which fitted into a recess in the shaft coupling. Thus the force required to pull the shaft out of the coupling would need to break the bolt first.
So this was already a known type of failure.
They don't build 'em like they used to!

 

[alexco38]

Missing torque bolt

Some boat yard (engine shaft installer), forgot to put a torque bolt on the coupler. The shaft has to transmit the torque to the transmission. When going forward, it pushes against the end of the coupler. When reverse, it is only held by the little dimple. That is not good.

The proper way is to have a 3/8 high-tensile bolt threaded thru the coupler into a blind hole drilled 1/2 way onto the shaft. (That's how my boat, and a few others that I worked on, came from the factory). The 3/8 bolt also has a nut as compression lock nut. Furthermore, it has a hole on the head for security seizing wire.

The key may not have been loose. The shaft just backed out of the key way. Don't continue to use the boat until this is resolved. It is dangerous.

It always seemed risky to me that the only thing preventing the shaft pulling backwards if the prop hit something solid were two little dimples in the shaft and any clamping force from the coupling. If the shaft pulls out "glug glug glug", the leak is tremendous and difficult to get to to block. Also it might prevent the rudder from turning.
I worry.
On my previous Nicholson (a better class of old time boat builder) I was pleased to discover a high tensile 3/8" bolt tapped lengthwise into the end of the shaft with a thick washer under the head which fitted into a recess in the shaft coupling. Thus the force required to pull the shaft out of the coupling would need to break the bolt first.
So this was already a known type of failure.
They don't build 'em like they used to!

 

[SeacretSailor]

Thanks. Hitting a submerged object a few months ago & getting the nick in my prop's blade has to be connected with the shaft now being loose in the coupling. Don't you agree that the shaft's 3/4" slippage/play in the coupling has to mean that the key's sheared?

Just to be safe, I've already ordered a new coupling and will add a R&D Flexible coupling adaptor. I'll replace the coupling while in the water since because it's loose on the shaft right now (and not frozen tight) it should come off easily. Also, I think I'm going to drill the set screws dimples 1/3 deeper into the shaft so there's more of a lock onto the shaft. The tricky part will be backing the shaft aft/backwards a few inches while I replace the coupling and keeping the seal on the PSS shaft seal so I don't get tons of water in.

All this happened last weekend while sailing / motoring for 14 hours back from Ship Island, MS back to Mandeville (Lake Pontchartrain) Louisiana. So I'm lucky that everything held until I got back to the dock!

Does it sound like I'm on the right track? Please let me know if I'm missing anything? I'm hoping Mainesail gives me the "thumbs-up" on my plan...

 

[Maine Sail]

Does it sound like I'm on the right track? Please let me know if I'm missing anything? I'm hoping Mainesail gives me the "thumbs-up" on my plan...

I will post more later when I have a better 3G, not 1X, connection. We are off sailing and internet is spotty..

 

[Allan12210]

Can you see a key? Is the end sticking out from the keyway? I just wondered if there wasn't one. Pull the screws and since you can rotate the shaft relative to the coupling you will pass by the keyway. Look in there and see if there is half of a key. Also look at the points where the screws impacted the shaft. Do they look like they spun to the current location (i.e., all scratched)? Maybe no key and a couple of loose screws?

Just asking.

 

[Maine Sail]

Some input on couplings.

1) If you are removing a coupling that is more than say two years old, depending on bilge moisture, you should not re-use it. This is a mistake many yards, DIY's and mechanics make. Solid straight couplings are a one time use after they have become "made on" as my machinist calls it! Why is this?

When you remove an older coupling you will break a layer of rust free. You have a PSS and it was likely "added" after the fact. This rust layer was your tolerance. A shaft and straight coupling are meant to fit together very, very snugly with an interference or light press fit.. In all my years of boating & working on boats I have only had one or perhaps two couplings go back on and maintain the proper fit. One of those boats only had a few months of service on her. Re-installing a used coupling can ruin your shaft. Shafts are expensive couplings are cheap in comparison!

The machine shop I use fits the coupling to the shaft for a light tap fit. On a properly machined shaft, without any anti-corrosive agent like Tef-Gel, it will slide on about a just a tad of the way before you experience some serious resistance. This initial fit is often referred to as "lead" meaning they give you a little bit of leeway to get the shaft started into the coupling. The coupling and shaft are machined & fitted to exacting tolerances so there can be NO wobble or play between the shaft and coupling. Any wobble will cause excessive shaft, key and coupling wear and could eventually lead to a failure.

2) The shaft is often fine and can be cleaned and re-used unless damaged by the wiggling coupling. It is the steel coupling that gives up surface material, and this lost layer of rust means you can't re-use it. If the coupling just "slides" back on it is far too loose and you will eventually have an issue!

3) When reinstalling a new coupling to an old shaft you should always have it fitted and faced by a machine shop or prop shop. Most shops charge between $65.00 & $85.00 for a fit and face. While you are at it have the checked for true. Shaft truing is more costly than a just a fit and face but a lot cheaper than a new shaft and it will get rid of any annoying vibrations.

4) When re-installing the shaft you should get it started with the machined in "lead" then lightly tap it home with either a copper, lead or brass mallet or an oak or maple block protecting the shaft and a hammer. For this job need two people or many trips up and down the ladder. Pound it in while looking in the coupling holes until you see the dimples for the set screws. Don't over do it cause backing it off is more of a paint than driving it in. Shafts should ALWAYS be "spotted" or dimpled for the coupling set screws. This is an ABYC requirement.

5) Shaft keys should fit snugly but not so snug that they are "key bound". If the key has any play have a machine shop make you a new one.

6) If you can, I can't I have a v-drive, don't replace the coupling with a solid coupling and instead replace it with what's called a "split coupling". This will make future removal and re-install much easier. Even with a split coupling you should still have it fitted and faced after removal. The $65.00 +/- is well worth it.


At the least you need a new coupling properly fitted & faced at the worst you need a new shaft and coupling.

Never use a hammer to pound directly on a coupling that is connected to the gear box. You can damage the bearings and brinell them.

 

[SeacretSailor]

Mainsail,
Thanks for the guidance/advice and thoughts. I will definitely replace the shaft coupling with a new one. I agree with you and want to go with a split cpl and not replace it with a solid cpl.

** Question: If the set screws on the existing and other solid cpls are at the 10 and 2 position, is it OK that on the split cpls there's a single set screw at the 6 o'clock position?

Do to upcoming work travel, I'm planning this project for Thanksgiving week. So I'm spending this time ordering parts (all new!) and doing the research.

Where can I get this Tef-Gel?

Kevin

 

[SeacretSailor]

*** UPDATE***

So with the boat remaining in the water at my dock, I started the repair job of replacing the shaft coupler and broken key by:
1 - Removing the solid coupler's 2 set screws and 4 flange bolts (to the transm)
2 - Backing the shaft aft 2" and out of the shaft coupler (while monitoring the in-flow of water from the PSS shaft seal.)
3 - Removing the coupler, taking it to my boatyard and ordering a new Buck-Algonquin split coupler and an R&D Flexible Coupling

What I found was that the key absolutely sheared, but I found no remnants of it inside the coupler or in the shaft's keyway. It was empty! The only thing spinning my shaft was the 2 set screws!!

Also, the key-slot in the coupler was bent and likewise in the shaft, i.e. the keyway on the shaft has a beveled edge from shearing the key. So on my return trip from Gulfport, MS to Pake Pontchartrain, I was lucky I made the motor-sail trip back.

To solve this, I'll be able to cut about 1.5" of the shaft off (eliminating the damaged part of the shaft's keyway slot) because by adding the R&D Flex Cpl, and a 1" spacer I'll add, this would set the shaft 1.5" more to the aft once re-seated. So this will give me an 1.5" of the damaged shaft I can remove, then I can lengthen the 1/4" keyway slot for the new key.

Finally, once put back together, the split coupler should bite down on the shaft sufficiently, along with the set screw. The new key and lenghten keyway slot should work well and the R&D flex coupling should prevent this happening again if I hit a log iin the bayou again.

Will do all this Thanksgivng week, so hopefully my boat will be back and running for Turkey Day!

Please let me know if anybody thinks I'm missing anything?

Kevin

 

[Maine Sail]

*** UPDATE***

So with the boat remaining in the water at my dock, I started the repair job of replacing the shaft coupler and broken key by:
1 - Removing the solid coupler's 2 set screws and 4 flange bolts (to the transm)
2 - Backing the shaft aft 2" and out of the shaft coupler (while monitoring the in-flow of water from the PSS shaft seal.)
3 - Removing the coupler, taking it to my boatyard and ordering a new Buck-Algonquin split coupler and an R&D Flexible Coupling

What I found was that the key absolutely sheared, but I found no remnants of it inside the coupler or in the shaft's keyway. It was empty! The only thing spinning my shaft was the 2 set screws!!

Also, the key-slot in the coupler was bent and likewise in the shaft, i.e. the keyway on the shaft has a beveled edge from shearing the key. So on my return trip from Gulfport, MS to Pake Pontchartrain, I was lucky I made the motor-sail trip back.

To solve this, I'll be able to cut about 1.5" of the shaft off (eliminating the damaged part of the shaft's keyway slot) because by adding the R&D Flex Cpl, and a 1" spacer I'll add, this would set the shaft 1.5" more to the aft once re-seated. So this will give me an 1.5" of the damaged shaft I can remove, then I can lengthen the 1/4" keyway slot for the new key.

Finally, once put back together, the split coupler should bite down on the shaft sufficiently, along with the set screw. The new key and lenghten keyway slot should work well and the R&D flex coupling should prevent this happening again if I hit a log iin the bayou again.

Will do all this Thanksgivng week, so hopefully my boat will be back and running for Turkey Day!

Please let me know if anybody thinks I'm missing anything?

Kevin

Kevin,

Be sure to use a stainless key on the coupling end. You can fit it with wet sand paper but make sure you get a tight fit. Sometimes the BA split couplings only ship with a set screw with no hole in it. Drill a hole and mouse the bolt around the coupling with monel or stainless wire. Be sure to "spot" the shaft so the set screw sets down into the shaft. with a slight dimple..

 

[Don12364]

Disturbing that the key parts had disappeared. (Almost??) unknown whether sheared or not.
I had a key come out of my Autopilot rotary drive and, like your prop shaft, the grub-screw was not man enough to hold the sprocket to the shaft. Of course it happened at the worst possible time!

Though a much smaller key than yours, I made a new one with a raised part at one end - see attachment. The sprocket boss prevents the key moving forwards out of the slot and the length of keyway prevents it moving along the shaft towards the motor drive unit.
If your keyway is long enough you might think of doing same, or even with a raised part at each end if the coupling recess will allow.

BTW: If you have a rope cutter such as 'Stripper' or similar; moving the shaft back and forwards will disengage the drive dog for the stationary part of the cutter - so the shaft job would need to be done with the boat ashore to enable reenegagement.

 

[SeacretSailor]

Repair Job Successful!

GOT IT FIXED!!! After 3 long days of cuts/bumps/aches and $300 in parts, I finally got my damaged prop shaft repaired & re-assembled with a new coupling! The boatyard wanted $1,200.

I replaced the solid coupling with a new split coupling after sawing off the last 1.25" of the damaged shaft. I lengthned the keyway slot, added an R&D Flexible coupling and re-assembled the shaft and parts with anti-seizing gel (Thanks MainSail!). Then added the 1.5" wide Flex coupling (which allowed me the space to saw off the damaged part of the shaft) and re-connected it. Next, I re-wired the set screw and re-seated the PSS sahft seal. Tested it and it ran nicely! No funds for a boatyard visit, so my DIY adventure paid off. And all while in the water, without hauling the boat out! My thanks to everybody for your guidance & advice.

 

[kloudie1]

Loooks great, Kevin.. Well done !