Transmission output flange fell off Yanmar 2QM15 - Theories re: why? - SailNet Community
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Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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????

Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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..

Last edited by aa3jy; 01-30-2013 at 12:42 PM.
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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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?

Last edited by aa3jy; 01-30-2013 at 02:38 PM.
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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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Originally Posted by aa3jy View Post
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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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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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.

Last edited by knuterikt; 01-30-2013 at 03:34 PM.
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Re: Transmission output flange fell off Yanmar 2QM15 - Theories re: why?

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!

Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
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