Shimming with foil

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yadnom1973

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I have a design I’ve been working on for some time and part of it is supported on 4 rollers. The rollers are polyurethane on aluminum hubs with 19mm x 8mm bearings. The Hubs are mounted on four long M8 socket head cap screws.

But the unthreaded part of the bolts is about 0.2-0.3mm shy of 8mm on the diameter so my bearings are loose. 0.2mm, it’s not the end of the world. I could use some Loctite 680 retaining compound that I’ve got knocking about. That stuff is good up to 0.25mm it says on the tin.

Then I thought how about wrapping the shaft of the bolt with aluminum foil? To test it I wet a carefully cut rectangle of foil and rolled it onto the bolt and sure enough, it made the bearing a snug fit. To try it for real I’ll wipe the shaft with the Loctite 680 then I’ll roll the foil onto it, surely this will be a better shim and fit than just the compound?

Can anyone see any problems with this idea? I’ve never heard of someone shimming by wallpapering foil onto a shaft with Loctite. The 680 is good by itself but I’m thinking that if it is built up between layers of aluminum wrapped around the shaft it should be even stronger. Something feels wrong about this though.
So I thought I’d ask here to see if anyone knows why I should or shouldn't be doing this. Any help appreciated.
 

petertha

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I have see some engine plans where plain bearing surfaces were called for in this manner - attached to bearing supports. I cant say aluminum specifically. Brass comes in all kinds of shim stock thickness starting around 0.001". Whether it wears better or worse or forming/installation is much different I cant say. Bronze sheet is quite expensive & I haven't seen it thinner than about .0625" but I didn't look to hard.
 

kiwi2

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Hi,
Ideally it would be better to start with some 8mm round stock and thread the ends while leaving the middle intact to fit the bearing.
If this isn't possible, you could try loctiting a nut on the bolt and turning it down to 8mm. There obviously won't be much of the nut left. If strength isn't an issue, it would be better to use a 6mm bolt and nut which would leave much more of the nut after turning to 8mm.
Another thought just occurred to me. Can you get a longer bolt with a plain shank near the head which should be the full 8mm and cut it down to fit?
Hope this helps,
Alan
 

yadnom1973

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Thanks for the reply guys. I don't really know anything about shim stock certainly not that it came that thin, I'll look into it. Maybe better to buy the right stuff if they have it in the right thickness.

I am using the plain shank near the head on a longer bolt, not the threaded part. Unfortunately, all bolts and screws are slightly below the designated diameter of the bolt on the unthreaded part of the bolt, so M8's are 7.8, M10's are 9.8 and so on. Turing down the nut is an interesting idea, your right there would not be much left but if it's on there with one of the 600 series Loctite's I don't think that will matter. I'll have to test this.

The lathe I'm using cant thread. The lead screw nut is too worn I think. These bolts are in stainless, M10 and M8 with 30mm of thread, I tried with a die, even built a floating die holder and got some special tapping grease for the stainless but the results are not too good. The force required to turn the die on an M8 on stainless in dramatic never mind the M10, I couldn't even turn that one and the threads that came out the M8 were chipped and broken.
 

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