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Linear bearings

Posted by Fri 
Fri
Linear bearings
September 08, 2012 06:57PM
I am trying to use linear bearings Lm8uu on my x axis smooth rods.
They slid very rough by themselves and are very hard to move when attached to the x axis base plate.
The rods are imp and measure 7.65mm not chromed.
Any suggestions how to make them slide smoother?
Re: Linear bearings
September 08, 2012 07:31PM
Try a drop of oil, I use precision 8mm rods without issue, if anything 5/16 rods should be loose.
They feel worse on the base plate because you get the combined resistance and it's unlikely the rods and bearings are accurately aligned.
Re: Linear bearings
September 08, 2012 09:02PM
LM8UU's will not ride properly on 5/16" rod. Even if everything is aligned properly. They require a properly sized rod for the ball bearings to seat properly in the channels.
Re: Linear bearings
September 09, 2012 12:21AM
You must use Metric Tool Steel Tight Tolerance Rod with these
bearings. (McMaster Item 88625K67 - 3' lengths)

In the US McMaster-Carr on www.mcmaster.com is a good source
for the Metric rod: pick a distribution point nearest you. I used eBay
for the bearings, with free shipping from China/Hong Kong.
Fri
Re: Linear bearings
September 10, 2012 09:00AM
Thanks for the help, I will try metric 8mm rods and post the findings.
Re: Linear bearings
September 10, 2012 08:54PM
ooh...did I purchase the wrong rod then? I didn't bother with the annealed variety, considering the low expected loading, and just purchased the McMaster part #3180T14 to save a few dollars. Bad choice, or negligible difference?

"You must"
Re: Linear bearings
September 10, 2012 11:36PM
xiando Wrote:
-------------------------------------------------------
> ooh...did I purchase the wrong rod then? I didn't
> bother with the annealed variety, considering the
> low expected loading, and just purchased the
> McMaster part #3180T14 to save a few dollars. Bad
> choice, or negligible difference?

If that's the part number you purchased, you spent a lot more than you really needed to. But the part number you listed will work perfectly fine, and is in fact higher tolerance than what most people use.
Re: Linear bearings
September 10, 2012 11:44PM
xiando,

looks like you have the more expensive, 'harder' version, of 8mm stainless rod. you should be fine. you could save a few dollars ordering the other part number.

biggest issue i would see is trying to use 5/16 with 8mm bearings. Even with 8mm rod, my ebay bearings aren't exactly smooth as silk.
Re: Linear bearings
September 11, 2012 12:28PM
"biggest issue i would see is trying to use 5/16 with 8mm bearings. "

I agree. there's too much play and the bearings can''t seat properly, leading to slide rather than roll. One could argue that he could get away with the printed bearings, but I don't want this to devolve into a specious debate.

"and is in fact higher tolerance than what most people use."

yup. I'm fully aware of the difference. Sorry, I was trying to be tongue-in-cheek regarding the "you must" comment accompanied with a very specific part number, but perhaps a bit too dry about it... There is no "must" aside from 8mm smooth rod (not btw limited to steel rod with certain restrictions) and a particular persons' expectation of performance and operating environment, in part only limited by available kitting.
Fri
Re: Linear bearings
September 22, 2012 09:59AM
Okay, got my new 8mm rods in.
It is worse now then with the 5/16" rods.
Any ideas?
Re: Linear bearings
September 22, 2012 12:16PM
Have you packed the bearings or otherwise lubricated them? If so, my guess would be that they're misaligned, the rods are misaligned, or both are.
Fri
Re: Linear bearings
September 27, 2012 10:44PM
Slauziere,
How are your prints?
With a rough x I don't get good precision.
Any thoughts on bronze / brass bushings
Re: Linear bearings
September 28, 2012 07:36PM
Take your caliper and measure between the rods at each end of the X, mine read 42.38mm & 42.25mm and it's SMOOTH as glass, I use Royal Purple Max Gear 75w90 as my lube... (it makes a difference for sure), if your not sure how to use a caliper correctly, you need to wiggle the caliper back and forth with pressure on it until you see the LOWEST measurement, that is the correct measurement. I would guess these cheap chineese bearings have quite a bit of "slop" so you can actually get away with quite abit of miss-alignment, so yours is either drastically bad, or your looking at a badly printed X carrage...
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