LM8UU linear bearings

I recently got some lm8uu bearings for just a little over $1 each from suntek.com for use with 8mm O1 tool rods from McMaster.com and found that most of them are too tight and only a few of them slide freely. After about a day of occasionally sliding them around by had, they're still tight and don't slide smoothly. I can now also feel and see wear tracks on the rods. I measured the rods to be 8.00 mm +/- .02 mm. The error seems to be from my caliper. Since even on the same rods, different bearings behave differently, I think this means that the tolerance on the suntek bearings are poor. I suppose I can select only the ones that fit well and slide smoothly, but that means that the price for each would go up. The other option is to find a new source.

Can anyone recomment a good source?

Thanks.

depends where you are I guess VXB but expensive, Aliexpress but take about 3 weeks to deliver

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Experimenting in 3D in New Zealand

NelsonRap Wrote:
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> depends where you are I guess VXB but expensive,
> Aliexpress but take about 3 weeks to deliver


VXB rods are great, I love 'em. They can be a bit pricey though, and you can't get them cut to your specifications, so you may be better off ordering a Prusa rod kit. Ultimachine and Lulzbot sell nice ones.

Pointedstick Wrote:
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> NelsonRap Wrote:
> --------------------------------------------------
> -----
> > depends where you are I guess VXB but
> expensive,
> > Aliexpress but take about 3 weeks to deliver
>
>
> VXB rods are great, I love 'em. They can be a bit
> pricey though, and you can't get them cut to your
> specifications, so you may be better off ordering
> a Prusa rod kit. Ultimachine and Lulzbot sell nice
> ones.

Yes I make up my own to sell but only in NZ as the shipping to anywhere else would be too great

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Experimenting in 3D in New Zealand

are you sliding them by hand on a single rod? or are you sliding the carrige with the extruder on (Z axis?)

My - cheap chinese - lm8uu were not sliding very well on the rod until I gave them a couple of oil drops.

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Most of my technical comments should be correct, but is THIS one ?
Anyway, as a rule of thumb, always double check what people write.

I was testing each one separately. And yes, I put gobs of oil inside the bearings before sliding them in the rod just to be sure they were lubricated.

or you can get them here.

[www.ebay.com]

[forums.reprap.org]

I also have O1 rods from McMaster. They are quite rough, and I think you will have the same problem with other bearings. I also have a pair of hardened and polished rods and my bearings roll quite smoothly on them, compared to on the O1 rods. I used steel wool starting with coarse and working down to 0000, and the O1 rods are much smoother now

The O1 rods that I got from McMaster is quite smooth and they're not rough at all. They look smoother than the rods that I have on my working Mendel.

brnrd

If you have access to a lathe, drill press, or even just an electric drill, you may be able to put the rods in the chuck, and use sand paper running up to 400 or 600 grit to first reduce the diameter slightly, then smooth the rods again to make them less tight and still be able to use all the bearngs. If you have access to a leathe, be careful not to take too much off and make them loose fitting.

Mike

If I put the bearings on the same rod, some slide freely and some are tight. I think this tells me that the problem is with the bearings not all having the same inner diameter/clearance, not the rods.

I have had mixed results with cheap Chinese linear bearings too. I bought 18 from Suntek on ebay recently, 15 are fine, 1 is very stiff (like one race isn't turning), 2 are 'lumpy', like there's a ball bearing that isn't round in bearing. This is on Zapp Automation's SFC8 smooth rod (solid, induction hardened, chrome plated, very accurately 8mm). This isn't the first time; the Thames Valley RepRap Group ordered around 250-300 linear bearings, and have had a significant amount that exhibit similar problems. I think you just have to be lucky with what you receive.

I complained to Suntek, through ebay, regarding the 3 bearings. They offered to replace the bearings, but require £3 postage. Probably won't bother, as I have enough bearings for the current project now - I over-ordered.

I'm not entirely sure that you don't get exactly the same bearings if you buy locally, unless you buy brand bearings like SKF. But then they are £8 per bearing!

It definitely seems like the bearings have poor tolerances. However, you mentioned that the rod diameter measures +/- 0.02mm which is an incorrect tolerance for smooth rods. All smooth rods used as linear rails are supposed to have a tolerance of +0 mm/ -x mm where x is the lower deviation tolerance. The key here is that the actual diameter is never larger than the rated diameter of the rod as the tolerances on the linear bearings will only allow them to run smoothly on rods with a diameter less than or equal to the rated diameter. A while ago I accidentally ordered junk chinese smooth rods that measured up to 0.08mm over the rated diameter and the linear bearings would not even fit on the rods

Lesson to learn from this: ALWAYS check your tolerances!

Eric