Combating linear bearing slop

After noticing grooves being worn into my cheap O1 tool steel rods, I decided to upgrade my Prusa i3 to nice VXB linear motion rods. I bought a new batch of LM8UU's as well, since I lost a few bearings when I first put the printer together. After swapping in the new rods though, I noticed the bearings have significantly more play. Specifically, the x-carriage now wobbles and rattles against the smooth rod as it prints. I attribute this to the fact that linear motion rods are slightly undersized to accommodate linear bearings.

Unfortunately, this bearing slop translates to much bumpier prints and reduced print quality. What strategies are available to reduce bearing slop? Are IGUS bearings any better with regards to bearing play?

There's the LM8AJ type, which comes with a slit for play adjustment. It should be possible to make LM8AJs from LM8UUs with a hand saw.

Just as example: [www.isutami.com]

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

what is the tolerance of your rods ?
it must be H7 or even better H6.

Once I have tried H9 ... and it was very bad ... I was able to hear my y-carriage making click-click when I was shaking it with my hand.


mihai


jamesshuang Wrote:
-------------------------------------------------------
> After noticing grooves being worn into my cheap O1
> tool steel rods, I decided to upgrade my Prusa i3
> to nice VXB linear motion rods. I bought a new
> batch of LM8UU's as well, since I lost a few
> bearings when I first put the printer together.
> After swapping in the new rods though, I noticed
> the bearings have significantly more play.
> Specifically, the x-carriage now wobbles and
> rattles against the smooth rod as it prints. I
> attribute this to the fact that linear motion rods
> are slightly undersized to accommodate linear
> bearings.
>
> Unfortunately, this bearing slop translates to
> much bumpier prints and reduced print quality.
> What strategies are available to reduce bearing
> slop? Are IGUS bearings any better with regards
> to bearing play?

I would think that the belt tension would correct some of the slop.

Traumflug: that looks interesting... Not exactly sure how to make it smaller though -- if I clamp it once I've sawed through it, will it cold set and reduce in diameter?

Mihai: vxb doesn't actually list their specs anywhere. This is exactly what I bought: [www.vxb.com] the entire x carriage makes the click click when I shake it. It could easily be the fault of my bearings though, since I got cheap Chinese lm8uus

Jamesdanielv: my x tension isn't a problem. I have the new box version x end with the screw adjustments, and the belt is VERY tight.

measure them with a digital caliper.

here the tolerances are listed:

[www.toolingsolutions.com]

my H9 rods were 7.96 - 7.97 in diameter.
H6 have in the worst case 7.99

I also have cheap Chinese lm8uu ... the problem was with rods not with bearings.

regards,
mihai



jamesshuang Wrote:
-------------------------------------------------------
> Mihai: vxb doesn't actually list their specs
> anywhere. This is exactly what I bought:
> [www.vxb.com]
> ionSystems/kit11867 the entire x carriage makes
> the click click when I shake it. It could easily
> be the fault of my bearings though, since I got
> cheap Chinese lm8uus

Haha, I did just that but my cheap calipers are now too inexact! Depending on how much I push on them, I can get measurements between 7.98 and 8.02. These rods look pretty good...

looking good does not say anything about tolerance ... ask the provider for that info ...

mihai


jamesshuang Wrote:
-------------------------------------------------------
> Haha, I did just that but my cheap calipers are
> now too inexact! Depending on how much I push on
> them, I can get measurements between 7.98 and
> 8.02. These rods look pretty good...

The solution to the problem is that you need to load the bearings on one rail so that it is the guide rail and the other rail is just an idler. My CNC machine has THK high precision recirculating rails. They offer them in different preloads. You can buy them with no preload, the balls have a slight clearance. Light preload uses larger balls, and heavy preload uses even larger balls. No preload is easier to move and last longer but for greatest position accuracy you want the heavy preload.

In your case you need a preload on the bearings that is greater than the maximum force that pushes against the rail from the motors. Consider a third bearing between the first two on one rail. Use a spring to drive that bearing against the rail putting preload on the other two. Now you machine will run with zero backlash from the bearings.