How smooth slide linear bearings?

Hi!

Despite the warning, that I shouldn't buy cheap linear bearings, I just bought some off ebay from Hong Kong. Unfortunately, I have no idea how good or smooth they should slide. When I push them on the rod, it does not take much force, but on the other hand it does kind of stutter. Since the printer is not ready yet to move on its own, I'm not able to say how prints turn out. But I can say, that if I move the printbed per hand, it does make some sound. Probably this helps: If I take an empty smooth rod, put a bearing on it and tilt the rod downwards, the bearing would stay on its place.

Is this normal? How smooth do your bearings slide? How do I know if a bearing is good or bad (on ebay)?

Thanks!

If it dont moves on its own weight it can be a very good one with preload or a bad one, it means nothing much by itself. Same for making some noise, the very best ones are mostly silent but the noise is usually from the return track so not a big deal.

What you can check is how smooth the motion is once it started and how constant is the force needed, but it is only meaningfull if you have good, grinded and hardened smooth rods. Motion should require always the same load to start too.

Actually, as long you dont have play, quality of motion will be more affected by the rods than the bearings i think. It is especially important rods are very hard, HrC 62 or so. Such rods are quite costly.

it depends on both the bearing and the smooth rods that you got. when you buystainless steel rods make sure u get a good type too (numbers on the stainless steel).
As for the bearing.. whether u buy them from US/EU or china.. they are mostly the same.. if u see bearings that cost 12 for 10$ from china or 12 for 20$ from USA - then its a pointer that its the same bearing lol.. if u want good bearings (no need really..) they will cost much more, more like 5$ for one bearing etc

buying stainless steel rods is the most common and most costly error you can make !

Stainless steel is not hard enough even in the best refs like 316L (eu denomination).

What you need is good chrome alloy like 100C6 or Cf53 tempered to 62-63 HrC and grinded not simply extruded.

Chrome alloy is costly, about 3x time the price of a standard steel rod, but guaranteed to be perfectly straight and still cheaper than stainless. The latter will also get grooves quickly, its hardness is no better than 49HrC.

good luck getting that chrome alloy ....

while its very true that normal stainless rod will develop grooves in it and relieve the preload on the bearing , its the kind of thing you only really worry about if you are mounting these things inside a machined hole

in our application there is two type of preload on the bearings, 1 is what the manufacturer puts in and the other is the mounted preload, in almost every 3d printed part that takes these the mounting holes are not perfect and that inperfection is what provides the second type of preload keeping the backlash out of the system, you will generally have more problems with belts and various other bits before the bearings give you issues

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I dont know elsewhere but in europa those rods are really easy to find, under the stubb moniker, and they make a strong difference, both in precision and lifetime.
Any industrial supplier will have them in centerless grounded form. In France, even harware stores for DIY often carry them.

this is what I'm speaking of, about 10€/m but you can find cheaper :

[shop.hpceurope.com]

The stainless version (inox) is 4x the price and only 54 HrC which is not enough for use with linear ball bearing. You can use igus bushing though.