A question for bearing and rail experts

Over the past 3,000 hours or so of printing, my beloved Prusa Mk3s has been diligently digging a hole in one of the x axis rods.
The internet is full of people who are happy to give advice - mostly telling others replace the bearings with something decent, lubricate them, etc etc. And mostly they are just wrong.

So I had a chat with the Prusa chat person, who didn’t seem fussed and suggested that if it’s printing properly, why not leave well alone until it’s not?

There’s no apparent binding or scratching on the bearing - it’s only happening for that 35mm (inch and a half) or so - so what could it be?

I will probably dismantle the little sucker, clean and repack the bearings and turn them a little, but am happy to take advice from you lot!

Weird. Is it possible the steel there isn’t as hard? Why would it only be there? Can you rotate it to move that spot to the other end and see if it follows the rod or starts making a new groove in the same place?

Me too.

If one of the balls in the linear bearing were bad, and if the bad ball were to rub and scrape instead of roll, then you might expect it to dig a groove in the rail. Even though it doesn’t have a hard constraint to hit the same spot every time, it might be close enough to hit nearly the same spot as the balls circulate, like the rolling plotter build.

If this were the case you might expect a series of scratches along the rail where the period depends on the length of the cycle of the balls. I’m not sure if those bearings have multiple separate loops of balls or if it is one continuous loop that cycles through all the rows. In the former case you might expect multiple gouges along a straight line with a period of about 4x the bearing length. In the latter case you might expect gouges at a different location rotated around the rail, again with a lengthwise period of approximately 4x the bearing length.

That’s just a theory. Inspect the rail closely to see if there is any evidence of other scrapes elsewhere. If there are no other scratches, then it’s probably not what I’ve described. :slight_smile:


You’re lucky, the bearings that came with my a8 had balls missing, and they scratched the rods from day one. They made a horrible squealing noise too. I got some misumi rods and IGUS bushings (stock rods were jacked up from the scratches), and they’ve been great for years.

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You could also rotate, so the weight would then be on what is now the bottom. What I would do :slight_smile:

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That’s what I don’t get @jamiek Jamie, It’s just in that one spot - I also like to mix up the positions of my models on the build plate so there’s no chance that only that length of rod has been used.

Anything’s possible I suppose - but why aren’t the bearings that caused it binding or making an awful click every time they jump in and out of that spot?

I defies my binary thought processes - I think dismantling is inevitable. Oh well, I was going to change the nozzle anyway. :wink:

Is it exactly mid span ?
Could it be due to minor shaft flex from the kind of mid span dead zone ?
I would try and use a black sharpie and do marker that spot and just before and after.
Then run the machine and see if you can see the bearing contact points and if they change from contacting on top to on the bottom in that exact spot.

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It’s possible if it’s ball bearings in there that one has a flat spot and that one only contacts when the bearing block is loaded a certain way.

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That all makes a lot of sense, thank-you!

I’ve gone all measury-feely as a result of your questions -
It’s not quite mid span, but it’s pretty much the right hand half of the bearing when it is dead centre. Shaft flex seems improbable to my tiny brain given the diameter of the rod and the the small span, but there are PETG fittings at either end so it’s not out of the question.

I will attempt to try the black sharpie mark and rotate the bearing slightly to see what’s up. A flat spot on a ball sounds feasible, too. It’s curious that it doesn’t appear to be affecting print quality at all.

No to get around to it!


It only takes a little things to make a snow ball affect.
Usually ball bearings are very hard but they will wear away.
I have to deal with bearings a lot.

I hope you get it sorted. They could just be at their service life, some never really fully rotate and only ride one way.
To prolong life you could spin the bearing pack 90deg every 600 hrs or so

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Your’s has been exactly the reply I was hoping for. For once I’m going to believe something I just read on the internet - I think I’ll just order a pair of new bearings, clean and repack the rest and then take your advice from here on!

Thanks again.

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