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MK3: something's gouging the upper X guide rail  

Brian Meek
Active Member
MK3: something's gouging the upper X guide rail

HI guys,

So I've got a kit built Mk3.  Built about a month ago.  It's got less than 100 hours on it, and I noticed in looking a the upper X guide rail, that something has taken a gouge out of the rail.  I'm a master machinist, and was very careful to orient the bearings so they don't line up with each other, and was gentle in loading them on.  Didn't even think of loosing any balls.  (How do you *do* that??)

The unit isn't catching on them yet, so no problem (yet) which is good, since it's in the middle of a print run.  But there are two linear grooves running about 5CM each down the length of the guide rod.  Deep enough to catch a fingernail in.  A group of two on the forward side of the rod, and another group of two on the rear of the rod.   Picture attached, but it's a little hard to read. 

I've worked with the same style of bearing rod before, and those things are *HARD*.  So what on earth could have taken a gouge out of them without apparently causing any noise or weirdness with the printer???

My plan is to just rotate the rods a few degrees, to get the grooves out of line with the bearing packs, to see if they get gouged again.

But my real question is to try to figure out what on earth's going on, to prevent it from happening again?  The bearing packs are rotated out of line with each other, and are *not* grabbed too tight.  So......????  (clean room with no abrasive dust.)

Any thoughts?



Posted : 21/09/2020 8:29 am
Patrick McNamara
Estimable Member
RE: MK3: something's gouging the upper X guide rail

Bearings are eating into the rod.  Rotating the rod will help, for a while, but eventually you will need to replace the rod and bearings.  Unfortunately its not an entirely uncommon problem.  Check your Y axis rods as well.  I eventually had to replace my X and Y rods.  The X carriage may not be "grabbing" on the grooves, but they will be impacting your print quality.  When I pulled mine apart, when the carriage was over the scored section, it was enough to prevent the rod from rotating. 

You should probably get in touch with support.  At 100 hours of printing time, there shouldn't be any wear.  Plus, it's harder to screw up the X bearings.  You can easily over tighten the Y bearings with the U-bolts.  X bearings, not so much.  You might get Prusa to send you a set of rods and bearings. 

Posted : 21/09/2020 12:00 pm
Brian Meek
Active Member
Topic starter answered:
RE: MK3: something's gouging the upper X guide rail

Thanks, that's about what I figured.

What I can't figure out is (A) how the bearing packs got screwed up in the first place, and (B) why it's only in a couple of places.

Normally, when those bearings get stuck, they gouge the entire length of whatever stroke they use.  This is only in a couple of spots.

I manufacture a product that uses those same linear bearings, in a slightly different size, and those things are *tough*.  In a much more abrasive, tougher application, I've never seen one of those bearing packs lock up like that, nevermind in under 100 hours.  So what happened with these?? 

You said this isn't uncommon?   Really?  What's going on?





Posted : 21/09/2020 7:29 pm
Active Member
Me Too!

I have around 30 hours on my Mark3s+ printer and I too have scoring on my upper X-axis rod!

I have 3 grooves at around 11:00, 2:00 and 6:00 o'clock near the middle of it's travel.

I cleaned and lubed all the bearings with the lube and injector tool provided in the kit and oriented bearings as per instruction.

I am also a machinist, mechanic and technologist, so it's not my first day at the rodeo!

Posted : 26/09/2021 10:09 pm
Dan Rogers
Noble Member
caused by

Couple of things cause this problem (replace rods, throw the linear bearings away you need new ones anyway).

1.  Not greasing the bearings before assembling them.  The oil they come packed in is for cleaning and transportation, it has very few lubricating properties.  When you get your replacement bearings, get extra grease so you can pack each bearing with grease before you assemble them on the rods.

2.  Then, the bearings are over-tightened in the bearing holders.  The casing flexes and the balls get wedged.  Since they were not lubricated with a real lubricant, they bind up and scratch the rods.  Don't over-tighten the bearing holders.  They need to be "just about to fall out" loose.  (Yes, they will develop ball rattle because you are using LMU bearings).

Something to consider - I did this in my last MK3S+ build - throw the linear bearings in the spares bag - they are more trouble than they are worth.  Buy a little bag of Drylins and use those instead of those messy bearings.

Posted : 27/09/2021 12:58 pm
Active Member
Good ideas!

Some good ideas there Dan.

Other things to consider are;

Rod diameter. I've heard there were some variations in past production runs, too large a shaft isn't the best thing.

Based on my experience, perhaps the most critical thing is bearing alignment. 

The upper X rods bearing holder looks very susceptible to alignment issues since the bearings are held in a plastic bearing half-shell very close together and held by zip-ties. Throw in the possibility of distortion and possibility of plastic bits in the way and you have an alignment challenge.

I paid close attention to these points and still had these issues! 

As an interim fix, I do plan to let the bearing housings float a bit more so they  can self align a bit more. Meanwhile I'll see if I can re-design the x-carriage so I can add an aluminum alloy sleeve. I will keep everyone posted!

Posted : 27/09/2021 5:31 pm