What would be normal shaft vibration? - SailNet Community

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  #1  
Old 04-28-2008
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What would be normal shaft vibration?

Had to replace the two front engine mounts on our Yanmar 2gm20f while on the hard--boat was splashed the other day. We had to get it out of the boatyard and back to our marina the next day, so we gave the boat nearly 24 hours to take a normal shape before aligning.

Complication is, PO put a drive saver donut (orange, maybe 2" thick) between shaft flange and trans flange. Do you test with feeler gauge on both sides of the donut? I did check the back side, detected some difference (could get the gauge in one side, but not other), screwed around with mounts (much sweating, cursing, and maybe even some crying), and got it to where things seemed even. When done, shaft seemed fairly smooth at 2400, but of course there is some vibration from the engine and all. How solid should it be? I could dangle an open wrench end (probably not recommended) against the top of the shaft at 2400 and just feel a tiny bit of chatter as it spun.

Ran it 2 hours under load up to marina and it felt fine. Do I need to do more?

Tom
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Old 04-29-2008
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Sorry--giving this a bump in hopes of getting some help on this.

Tom
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Old 04-29-2008
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With the donut bolted to the trans flange, it simply becomes an extension of that flange, so working on the aft end vs the shaft coupling half makes sense to me.

It's a bit difficult to separate engine vibration from shaft wobble, esp at low rpms and with 1 or 2 cyl engines. If things look/feel/sound OK at normal revs and you have no significant gaps on a feeler gauge check, you should be OK. Did you "center" the shaft in the shaft log before you started? If so you should be fine.
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Did I center the shaft in the shaft log before starting? I'd probably have to say no--not sure what that is. Are you referring to the 2 "frankenstein" bolts on either side of the shaft?

Tom
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Old 04-30-2008
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Find out what brand of "donut" you have and go to their website to get instructions on how to align the shaft. I had one that had an orange painted bolt that was used to check with a feeler gauge at 12,3,6, and 9 o'clock. Time consuming and cussworthy no doubt but doable.
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Old 04-30-2008
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Something to keep in mind is that the spacer makes it harder to align the engine. That's because by design it gives, so when you tighten the nuts or move the engine mounts you can compress the spacer, which could give you false readings with a feeler gauge that it is even all the way around. That said, the beauty of those things is that they compensate for an alignment that's not exactly perfect (that's part of the reason for using them, in addition to saving the tranny in the event of an accident). There is a thought that the only way to get a perfect alignment when using a spacer is to substitute in a doughnut made of steel exactly the same size, do the alignment, then swap out the steel doughnut for the flexible one.

If the engine runs smooth and isn't jumping up and down when you're in gear at various RPM, you don't see or feel any wobble in the prop shaft, and you aren't getting excessive drip from your stuffing box (assuming it's not a dripless), you likely are fine.
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Old 04-30-2008
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You can probably count on the shaft saver having parallel sides. So, If you have the freedom to move the shaft a bit, remove the shaft saver, align the engine and shaft, re-install the shaft saver. This is actually pretty easy if the bolts are not corroded in place.
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Quote:
Originally Posted by arf145 View Post
Did I center the shaft in the shaft log before starting? I'd probably have to say no--not sure what that is. Are you referring to the 2 "frankenstein" bolts on either side of the shaft?

Tom
When the coupling is disconnected, the shaft will tend to drop to the bottom of the tube that the shaft comes through (there's enough slack in the rubber "hose" that supports the shaft gland for this to occur) We usually try to find the limit of play (vertically and laterally) and temporarily support the shaft in the center and align from there.

On some boats this drop can be as much as 1/4 to 1/2 inch, depending on shaft OD, stern tube ID and the stiffness of the hose section. Theoretically if you're centered in the stern tube you should also be in line with the cutlass bearing axis too.

I like gc1111's idea of aligning without the shaftsaver once the shaft is centered and then inserting it afterwards.
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Quote:
Originally Posted by gc1111 View Post
You can probably count on the shaft saver having parallel sides. So, If you have the freedom to move the shaft a bit, remove the shaft saver, align the engine and shaft, re-install the shaft saver. This is actually pretty easy if the bolts are not corroded in place.
Not so easy. First, you need to be able to pull the shaft sufficiently far forward to connect it to the tranny. Depending on the boat you may or may not have that much shaft. Even if the shaft is long enough, it's not so easy to do on most boats (pulling the shaft in and out like that). Also, I'm not so sure the shaft savers are machined that perfectly even all the way around. They might be, I'm just not sure they're sufficiently even to the point of a fine feeler gauge. All that said, I think your idea is a really good one, if it works on that boat.
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Old 05-01-2008
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OK, I didn't center in the entry tube--didn't even think of that. I might undo the aft side bolts on the drivesaver and double check things. I admit I'm probably not willing to remove the whole drivesaver and try to pull in the shaft. Somehow I could picture messing up my packing box seal with a rough portion.

By the way, when working the feeler gauge, are you supposed to have the two sides bolted together, or just held together?

Tom

PS--Thanks for the suggestions--much appreciated.
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