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Shimming shafts to fit bearings - problem
Mr Whippy - 5/3/24 at 09:10 AM

Hi,

On the scrounge for ideas. I have two 1.5m long, 6mm OD hollow carbon propshafts that are to rotate inside miniature roller bearings. The problem I have is the shafts are very slightly (at a guess about 0.2mm) less than the id of the bearings and I’m wondering how I can shim this up, so the bearings and shaft are a nice snug fit. I also must be able to remove the bearings in the future for maintenance, so simply gluing them in is not an option. My only idea so far is wrapping some thin baking foil round the shafts as a shim. Oh, and the shafts are under high load, spinning around 5k rpm & the bearings are sitting in oil, as we don’t want to make life too easy.

Suggestions welcome. Thanks.

rusty nuts - 5/3/24 at 11:02 AM

Shim stock.

nick205 - 5/3/24 at 11:15 AM

Not a problem I've encountered, but think I'd go with "rusty nuts" and aim for using Shim Stock.


Curiosity...

Did you purchase the shafts as "designed to fit the bearings"?

If so, any possibility of having the shafts changed for ones of the correct dimension?

jps - 5/3/24 at 11:59 AM

quote:
Originally posted by Mr Whippy
1.5m long, 6mm OD

My suggestion would be to machine bearings that fit but presume that’s not an option you have.

Can’t help but ask, are those dimensions correct? If so, what machinery is it?

[Edited on 5/3/24 by jps]

coyoteboy - 5/3/24 at 12:12 PM

Thats a very noodly shaft to be taking....any load at 5k rpm.

You'll never shim it accurately. Depends what balance requirements you have.

Option1 - turn down the shaft to the correct diam for the next bearing down if the load on the shaft isn't huge.
IF you're going to shim it, make a fixture to hold everything in place while it's epoxied. Don't expect it to remain in place during lots of flexing and oil bathed. Seems like a strange design, whatever this is - model boat prop shaft?

Mr Whippy - 5/3/24 at 12:52 PM

quote:
Originally posted by rusty nuts
Shim stock.


Thanks, I suppose for a shaft only 6mm in diameter then kitchen foil would be much the same idea?

As for the shafts themselves, this my solution to not having to spend £200 on come custom made propshafts I can't justify buying (more than the cost of the rest of the ship). There are some (5 per shaft) lip seals which I was thinking would actually help center the shaft too and help dampen out any vibration. Not sure yet, it's all very experimental

obfripper - 5/3/24 at 01:31 PM

Use a cheap feeler gauge set as source of shim stock, you'll have a good choice of sizes for pennies. The foil will mush up fairly quickly and push out of the gap.

You could also use a bearing fit compound like loctite 638 which will take up a 0.25mm gap, and use a couple of wire strands to centralise the bearing on the shaft while curing.

Dave

gremlin1234 - 5/3/24 at 01:57 PM

silly question, but do you maybe have a 1/4 inch i/d baring rather than 6mm?

PAULD - 5/3/24 at 02:00 PM

Bind the shaft with fine cotton stuck on with superglue then sand down to fit.

Mr Whippy - 5/3/24 at 10:58 PM

Thanks good suggestions, I'll have a play as I have about a foot spare to experiment with. As for whether the bearings are the correct size, they are as their marked but its a good question. I think it is just down to manufacturing tolerances of the carbon shaft. I have been looking for a supplier of 6mm stainless rod who can cut an accurate M4 thread on the end (the prop is M4).

HowardB - 6/3/24 at 07:54 AM

I would talk to my local friendly bearing shop - they may have odd bearings that could fit and if your 6mm shaft is actually smaller - might it be an imperial size?
If it is critical to make it an exact fit to the bearings,. how about overwinding the shaft at the bearing point and then machining back?

coyoteboy - 6/3/24 at 10:50 AM

quote:
Originally posted by PAULD
Bind the shaft with fine cotton stuck on with superglue then sand down to fit.


Actually this is a nice thought - instead of sticking the cotton on, push the shaft through it and slide the bearing on, then apply epoxy to the cotton and it will wick through.