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Discussion Starter · #1 ·
Hello,
I recently had the output coupling from my Yanmar 2QM15 transmission fall out while underway (we didn't hit anything), disconnecting the prop shaft as a result. I'm in the process of undergoing repair, but I'm concerned that I may only be addressing the symptoms without isolating the cause. Does anyone out there have experience regarding common causes of transmission decoupling that might help my investigation?

The transmission output coupling consists of a circular flange with one side slipping onto the output spline of the transmission (long pointy gear), and the other side bolted flush-faced to the prop shaft coupling. There is a threaded locking nut in the hollow between these two coupling pieces that is supposed to hold the transmission flange onto the spline, but it appears to have bucked its "calking" (keyed into a groove on the spline with a cold chisel) and spun off.

The repair will likely require replacement of the output coupling flange because the teeth on the inside of the flange appear to have been worn down and are only 1-2mm long anymore. The only diagrams and pictures I've found appear to show the teeth being at least an inch long, though I can get the flange to stay on the spline if I really crank down on the locking nut (sorry, lots of jargon). I will probably replace the locking nut too for good measure.

The last time we changed the transmission oil we used 25W instead of 30W, and the oil was pretty dark when we checked it this time. I'm unable at this point to determine whether there is a leak. I've also recently become paranoid about our cutless bearing, which has a little bit of play when I grab the prop shaft in the water. Add to that an engine that just ran 400 hours in 5 months, and there are probably all kinds of things that could be out of whack enough to wear down that flange.

What do you think? Could it just be a one-off occurrence? A slow boil built on years of use that wore down this flange? Could there be a larger problem that would cause this to happen again in the short term? I have a mechanic coming today to look at it, but the collected wisdom of this forum has yet to let me down and I'd love your opinion please, if you have one.

Many thanks.
 

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So.. you're saying the flanges didn't separate.. but the entire coupling assembly came away from the gearbox output shaft? Did this happen when in reverse gear?

If it's a spline connection and was allowed to get loose, then over time it's possible that the spline teeth are worn excessively by the free play that may have been there, and by shifiting reverse/forward over time. It would be nice to have the original spline specs to be able to compare.

The spline section of the trannsmission coupling half may well be damaged as well, so you probably are looking at a new coupling half at least - changing the output shaft is another level of grief. Is drilling and pinning the existing output shaft an option? It would be difficult, probably, to do in place - if you have to pull the gearbox I suppose you may as well fix it proper.

Pictures????
 

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There should be..as I recall..(2) set screws on each side of the coupling collar. Where they missing or just loose? Also they should have been safety wired..

Does it look something like this..?

http://boatinfo.no/lib/yanmar/manuals/2qm15.html#/162

Your loose/worn cutless bearing may have been a factor as to why the shaft coupler departed ways as it will possibly cause a mild to significant vibration..
 

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Discussion Starter · #4 ·
Faster: That's correct, the two coupling faces did not separate. The entire coupling came off the back of the transmission via the output spline, after the locking nut backed off.

I agree that the transmission-side flange will need to be replaced, and I hope it's like you say, that the nut just got loose over time and allowed the teeth on the flange to be eaten away by the spline on the transmission output. The spline still appears to be in pristine shape, but your point about the specs on the spline is a good avenue to double check, thanks.

I can't envision how I'd drill and pin the flange to the spline, given how worn the existing flange's teeth are. Once I get a new flange and nut (whenever that will be, being in Mexico), the reassembly should hopefully be straightforward.

aa3jy: The coupling you're describing with the set screws sounds like the prop shaft side of the 2-part coupling. Fortunately that part is still attached to the prop shaft and in fine shape with the set screws still in.

You're probably right about the vibration being a major contributor, but in the four years I've owned this boat I've never known it to *not* have some amount of vibration. It's a diesel after all, right? Do people out there actually have smooth diesels?

Here are pictures of the problem at hand:


-- The output coupling flange that fell out of the transmission. The side facing the camera goes into the transmission. Notice the tiny teeth way back there. From what I can gather, these are supposed to go all the way up what is now the smooth inner shaft of the flange. As it is now, the teeth barely engage the spline on the transmission when cranked all the way down with the . . .


-- Locking nut. This nut requires a special wrench to cinch down, and requires about 68lbs of torque. Supposedly these nuts are easy to find at industrial supply shops. Once spun down, the end of the nut needs to be "calked" into the keyway on the spline, as shown in the next photo.


Here is the output flange reassembled the first time. I was able to make an initial wrench out of a notched PVC pipe, followed by a sturdier one made out of pipe. At the time I didn't know about keying in the end of the locking nut, which is why you can see that gap, but I don't think I can truly reassemble it without replacing the flange due to the worn teeth. The nut worked itself back off after about 20 minutes of running in forward. There might have been some dragging on the packing gland nut or somewhere else on the shaft that provided enough resistance to help the nut come off. Dear God I hope not.

Also pictured is the other half of the coupling, which hugs the prop shaft and bolts onto the transmission side flange.
 

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Is it me or does that locking nut appear to be turned around incorrectly on that last pix... That collar on the nut should be inboard.

Do you sail with the transmission in gear or free wheel the prop?
 

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Discussion Starter · #6 ·
Is it me or does that locking nut appear to be turned around incorrectly on that last pix... That collar on the nut should be inboard.

Do you sail with the transmission in gear or free wheel the prop?
This is how it looks in the schematics for the transmission, plus you have to have it this way in order to do the calking.

We used to keep the transmission in neutral under sail, but on the ocean we started noticing that the prop would never fold back up after shutting down the engine, and we would still hear it spin unless we put it into forward. After the prop folds, we're usually 50/50 about whether we put it back into neutral or not. Why do you ask?
 

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I'm guessing he is asking because with a fixed prop you are supposed to keep a Yanmar in neutral. With a folding prop it does not matter.
 

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Mack Boring, distributor for Yanmar, recommends leaving the transmission in neutral due due to the stresses it puts on the transmission if locked in gear.

They recommend a shaft lock such as...

SHAFT LOK INC.

..but with a folding prop all bets are off..not sure
 

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This is how it looks in the schematics for the transmission, plus you have to have it this way in order to do the calking.
These nuts are not designed to reused.
There is a part of the nut missing in your picture, indicating that this nut has been reused.
Whan calking the nut you should not use any sharp tool that can cut into the ring.


It should look like this when installed (caulked or staked)


Does not look like teeth has been worn of, looks to smooth in the picture.
Would also expect visible damages to the spline on the shaft if this was the case.
Someone could have installed an incorrect machined flange previously?


Is there anything that prevents the flange going all the way onto the shaft?
Do you have a picture of the output shaft w/o the flange installed.
 

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Agree with knute that the 'smooth' area inside coupling half is too 'machined' to be a worn down spline pattern.. but those spline teeth look awfully small/unsubstantial for the job.
Does the keyway on the output shaft have any role then? That might be an easier way to go, if you can get a keyed coupling half to match (doesn't look like the spline goes full length of the output shaft)
EDIT: belay that... the nut wouldn't go on then would it.....'doh...

A picture of the exposed shaft would be great!
 

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Discussion Starter · #11 ·
  • This is the same nut that originally failed. I reused it by filing off the older calked spot. The threads look good on the nut still, nearly perfect with minor scoring. I plan to replace this nut now that we're back to civilization and a better repair is clearly needed. However, couldn't a person just chisel in a new calking in the old nut in a new location around the rim?
  • I agree that the inside of the flange looks and feels suspiciously smooth. The thing is, the schematic drawing, while tiny, does show longer teeth. Also, I found a machinist here who had an old rusted out flange that was unusable but did have fuller teeth. It would make my month if I didn't need a new flange, but the evidence so far seems to point the other way.
  • From what I can tell, the flange does go all the way onto the spline. When the nut is torqued as far as I can get it, turning the flange turns the gears in the transmission. However, those tiny teeth seem such a tenuous hold on an awfully long spline, and if the calking on the nut is the only thing keeping this whole show together, it seems like an awfully weak link.

Here's a picture of the spline. Apologies, I'm not near the photographer that my wife is.
 

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The spline on the shaft looks serviceable still.. that's the good news, I think. But it's very hard to believe that the mating spline on the original coupling wouldn't have been similarly matched for full engagement so now one has to wonder if the damned thing didn't machine itself... But it wouldn't happen overnight and I'd have thought you'd have noticed some slippage or unusual behaviour/noises prior to failure.. it's still hard to credit.

Odd problem. Would there be an original part number to use on an image search?
 

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  • This is the same nut that originally failed. I reused it by filing off the older calked spot. The threads look good on the nut still, nearly perfect with minor scoring. I plan to replace this nut now that we're back to civilization and a better repair is clearly needed. However, couldn't a person just chisel in a new calking in the old nut in a new location around the rim?
  • I agree that the inside of the flange looks and feels suspiciously smooth. The thing is, the schematic drawing, while tiny, does show longer teeth. Also, I found a machinist here who had an old rusted out flange that was unusable but did have fuller teeth. It would make my month if I didn't need a new flange, but the evidence so far seems to point the other way.
  • From what I can tell, the flange does go all the way onto the spline. When the nut is torqued as far as I can get it, turning the flange turns the gears in the transmission. However, those tiny teeth seem such a tenuous hold on an awfully long spline, and if the calking on the nut is the only thing keeping this whole show together, it seems like an awfully weak link.

Here's a picture of the spline. Apologies, I'm not near the photographer that my wife is.
Agree that the flange and shaft seems an odd pair.

Looks like a "bodge job"..

I had a prop shaft flange split along the keyway once.
First we found that the key was just a bit of scrap iron, did not fit at all:hothead

Had to get the original drawings from the manufacturer to get a new flange made.

We then discovered that the nut should be conical fitting into a conical recess on the flange locked with a set screws.
But some genius had modified the flange to take a custom made hex nut where only the corners had contact with the flange and no set screw at all :hothead:hothead

So the old flange had been sitting on the cone only (probably a long time).
When it started to slip the scrap iron piece split the flange hub.
 

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Hopefully this is clear enough:



Not well drawn but it looks like the spline on the shaft and in the coupling half are meant to be matched.. certainly there's no detail showing a small ridge of teeth to engage the shaft.. mysteriouser and mysteriouser...
 

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Discussion Starter · #15 · (Edited)
Odd problem. Would there be an original part number to use on an image search?
Here's the best I could come up with using the part number from my Yanmar guy in Portland.



This syncs up with the picture in my maintenance manual .

I'm liking where this is going though -- the problem was with the "bodge job" somebody did in the past when machining the flange. This was just fated to fail, nothing I did wrong through poor maintenance, right? Right? :)

I guess for now I just stay the course and try to find a way to get a new flange and nut to La Paz. Nobody here seems to be inclined to recondition the teeth on the existing flange (way difficult machining IMHO so I don't blame them). Then I guess we wait and see if it breaks again.

Thanks all for the help so far.

-----

PS - Faster, I think I finally understand what you meant when you suggested a drill and pin option. You're suggesting drilling through the flange and the spline crossways, then fitting and securing some kind of bolt or key into that hole right? Would that hold up to the strain on the engine? I feel like it'd just warp and/or crack the spline over time.
 

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PS - Faster, I think I finally understand what you meant when you suggested a drill and pin option. You're suggesting drilling through the flange and the spline crossways, then fitting and securing some kind of bolt or key into that hole right? Would that hold up to the strain on the engine? I feel like it'd just warp and/or crack the spline over time.
That is what I meant, but very difficult to do in situ.... and at best a stopgap measure to get you 'home'...

Your latest image surely shows full length spline teeth in the coupling half..
 

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the flanges always a softer metal than the output shaft./check output shaft seal!
looking at picture three there appears to be damage or wear on the boss output shaft coupling. Take great care to fit both couplings together to see if the boss aligns both couplings ( output shaft coupling/shaft coupling any play here could cause your problems.
get new output shaft coupling and nut remove shaft coupling machine both couplings at a machine shop to fit.
Install output shaft coupling and nut.
Install shaft coupling.
Slide shaft forward to transmission coupling.
Using a feeler gauge to check all around circumference of shaft couplings.
A- line even as necessary to make these distances equal.
tighten all bolts and nuts.
And turn the shaft in neutral by hand to see if there is any binding. CaptG
 

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Looks like Cabo is closest..
 
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