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Old 09-10-2018, 01:27   #1
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Damaged shaft fit to coupling help?

Hi, we are trying to fit a new solid coupling to our shaft but the shaft seems to be damaged? The old coupling was really hard to get off, (not rusted). The shaft has an old unused keyway in it that we are not using and measuring with a vernier, it quite uneven. We have our new coupling freshly machined but we can't get it on because I think there are high spots on the shaft from past abuse.

The shaft cannot come out of the boat without lifting the engine, which i do not want to do

Can some wise person suggest the best way to deal with the uneven surface on the shaft end?
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Old 09-10-2018, 03:31   #2
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Re: Damaged shaft fit to coupling help?

I assume the shaft has some high spots?

If that is the case I'd get a good quality file and file the shaft forward and backwards to try to find the high spots and then file the high spot. I would make use of my high quality vernier gauge as well. That's pretty agricultural but what else?
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Old 09-10-2018, 03:59   #3
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Re: Damaged shaft fit to coupling help?

Thanks Coopec, a good flat file, youre right, that will help. I've done my best with the vernier but strugling to find the high spots just by measuring. I'm scared I'm just going to make it worse.

I've hit it with a wite wheel in a grinder but not making any progess...
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Old 09-10-2018, 04:32   #4
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Re: Damaged shaft fit to coupling help?

Just be aware that if you donít have a coupling face that is normal (perpendicular) to the shaft, you arenít going to be able to align your drive assembly. After attacking the shaft with hand tools to make it fit, you are going to feel pretty silly when you go back with a .005 feeler gauge in your hand.

Removing some nubs or burrs is one thing, trying to reshape your shaft in situ is another.
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Old 09-10-2018, 04:33   #5
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Re: Damaged shaft fit to coupling help?

Be careful here. If the couple does not fit squarely on the shaft, youíll never be able to align the shaft to the engine.
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Old 09-10-2018, 04:39   #6
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Re: Damaged shaft fit to coupling help?

If you get a 25mm id bearing and carefully use it you should be able to find your high spots. If you slip it on the shaft until it stops at a high spot & then feel the shaft at the point where it stops you might feel the high spot which you can then gently file off or use Emery cloth. If you can't feel the high spot then put never seize in the bearing & gently tap it on the tight spot a short distance then remove the bearing. There should be a shiny spot on the high spot which you can then remove. It is very important that you keep the shaft & bearing clean of any filings or grit when you do this. If a small foreign particle gets between the bearing and shaft it can gall & seize on the shaft. Before testing with the bearing wipe the shaft & inside the bearing with a clean cloth & put a film of clean oil on both mating parts. If you get some 2" wide Emery cloth about 120 grit that will clean off small ridges as well. The file should be used to take off the bigger marks down close to the shaft. By taking a 10" long piece of the Emery cloth you can wrap it around the shaft like a basket and pull each end back & forth over the offending spot. The bearing is hardened so will not pick up as easily as your soft coupling hub but you still don't want to use to much force putting it on to test. Remember it has to come off again. Your finger tips can pick up small ridges with practice. I think you said the shaft is 25mm. A 25mm bore ball bearing will end in 05 ex. 6205. With patience careful use of force you can come out with a still round shaft. Good luck.
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Old 09-10-2018, 04:42   #7
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Re: Damaged shaft fit to coupling help?

Once the bearing goes on then you have to do the same to the coupling bore until it goes on the shaft.
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Old 09-10-2018, 09:33   #8
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Re: Damaged shaft fit to coupling help?

I would stay the hell away from the shaft with a file other than possible burs on the end. If I understand you correctly, I cannot see how a shaft would develop a high spot. You might try heating the flange evenly with a heat gun or stick it in the oven. Get your oven mitts out.
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Old 09-10-2018, 10:38   #9
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Re: Damaged shaft fit to coupling help?

Once the coupling is on the shaft, try using PLASTIGAUGE to help align the two flanges perfectly. PLASTIGAUGE is available from Amazon or any machine supply shop. The plastic strips change color when compressed. When the couplings are properly mated, all of the PLASTIGAUGE strips come out the same color.
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Old 09-10-2018, 12:31   #10
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Re: Damaged shaft fit to coupling help?

Thanks a real lot everyone. I've got a bad feeling about this. I don't know why the shaft is damaged.

I will have a crack with the file today, the bearing idea is great but the shaft is machined down to what looks non standard to me. It varies between 31.8 and 31.9 mm. If I get it on somehow today, if I cannot align it, I'm going to have to lift the engine a foot to get the shaft out.
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Old 09-10-2018, 12:31   #11
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Re: Damaged shaft fit to coupling help?

I don't think filing the shaft by hand will give you the precision you will need for any long term use of the shaft. As a "get home" procedure and keeping the Revs low you might make it work. removing pressed bearings and such without the proper presses ( and this is similar) just ruins the pieces and can lead to failures of the equipment attached - transmission bearing, etc. Lots of things can be "shade tree" engineered but knowing which can not is important.
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Old 09-10-2018, 12:52   #12
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Re: Damaged shaft fit to coupling help?

Quote:
Originally Posted by Warby12 View Post
Thanks a real lot everyone. I've got a bad feeling about this. I don't know why the shaft is damaged.

I will have a crack with the file today, the bearing idea is great but the shaft is machined down to what looks non standard to me. It varies between 31.8 and 31.9 mm. If I get it on somehow today, if I cannot align it, I'm going to have to lift the engine a foot to get the shaft out.
There's no possible way to go out aft past the skeg????
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Old 09-10-2018, 13:20   #13
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Re: Damaged shaft fit to coupling help?

Pull the vessel out of the water and turn the shaft by hand, while holding a gauge against the on-board end of the shaft to see if there are any irregularities. Bearing blue will often get wiped off where they are if one can clamp a rest against a rib or frame.

Personally, I would see if I could remove the shaft from the stern tube while the vessel is out of the water if one does not wish to raise the engine. A misaligned shaft, even to the smallest degree--is going to be a source of vibration, leaky seals and grief until it IS properly aligned and trued--so one might as well grasp the nettle and do it properly at the outset.

The other thing to do is pull the shaft and replace it with one known to be correctly machined. It may be necessary to remove the rudder blade to do so.
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Old 09-10-2018, 13:33   #14
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Re: Damaged shaft fit to coupling help?

I really think you have to pull that shaft which means hauling anyways - many boat builders do offset the shaft just enough for it to squeak by the skeg for just this situation.

Any number of things could be wrong with the shaft - including bent. A good shop can fix it - or tell you that your old shaft is junk. It happens a lot. I just had a bent shaft straightened. The bend was invisible to the eye until it was put on the shop's special table - no idea when it happened but over time it wore out the cutlass bearing and damaged the stuffing box.

Here's a good posting by MainSail. The whole thing is much more complicated than you'd expect

A New Prop Shaft Photo Gallery by Compass Marine How To at pbase.com
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Old 09-10-2018, 13:38   #15
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Re: Damaged shaft fit to coupling help?

Did I totally misread the original post. It appeared getting a flange off was hard and getting a new one on was the problem.
Trying to machine a shaft with a file is Ludacris. Heat the flange hit the shaft with a blast of CO2
Sorry if I misread it but that's machine shop 101.
New coupling freshly machined. I would suspect it prior to the shaft. JMHO
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