Re: Scratched rods from linear bearings
I just did some maintenance to my MK2S and noticed that my X-axis lower rod is scratched badly, deep deep grooves all the way through coating. 😯
Check with a magnet.. Is that rod different to the others...
Seems to be all the same.. All attracts magnet. Weird. And I double checked, only lower on X-axis (one with single bearing) have scratchs...
Re: Scratched rods from linear bearings
One solution is to remove the rods:
https://learn.watterott.com/3d-printer/mk3-y-upgrade/
😀
PRUSA3D MK3S
Re: Scratched rods from linear bearings
One solution is to remove the rods:
https://learn.watterott.com/3d-printer/mk3-y-upgrade/
😀
Had me until I looked at the price... 🙂
Re: Scratched rods from linear bearings
One solution is to remove the rods:
https://learn.watterott.com/3d-printer/mk3-y-upgrade/
😀
Had me until I looked at the price... 🙂
hiwin.. are spendy.but soo smooth
I have a Prusa,therefore I research.
RE: Scratched rods from linear bearings
So after about five months of use I can add scratched rods to the list of problems I have with the Mk3 that I assembled myself:
As the manual says, I did nothing but lubricating the rods (which are magnetic = hardened steel) about once a week with a high quality bike oil (Oil of Rohloff), but seems that wasn't helping.
Prusa support was kind enough to send me new ones plus bearings, but I will probably order a set of Misumi bearings to go with the new rods. Still I would like to lubricate them properly.
I've read through the whole thread, but couldn't find a definite answer:
Is there a reliable low viscosity grease without nasty additives? (preferably one that I can buy in Germany) 😋
RE: Scratched rods from linear bearings
Is there a reliable low viscosity grease without nasty additives? (preferably one that I can buy in Germany) 😋
Divinol Lithogrease 000/150 works great on the linear bearings of my Mk3.
RE: Scratched rods from linear bearings
The very same scratching on the lower X rod happened on my printer after 2 months of printing. These scratches happen when a bearing is sliding rather than rolling on the rod. A bearing that caused a scratch like this is shot and needs to be replaced, the balls are damaged in a similar way.
There are a few contributing factors:
- The No-name LM8UU bearings are far from precise and the balls aren't as round as one would wish. So they have a bigger propensity to slide rather than roll than more expensive bearings (Misumi or if you go even higher, SKF or INA).
- The Prusa (newer, hardened) rods are also lacking precision. The diameter of the rods varies piece to piece and on some the bearings are gripping too much, and on others sliding too much. Some people are lucky to get a perfect set, I had one rod which was a tad too thick.
- The oil on the bearings as you unpack them is transport, rust-protection oil, not lubrication oil. It's best to remove it first, before applying appropriate grease.
- And finally, the I think MK3 R3 X carriage, particularly the X carriage back part has an error in it. The lower bearing cutout is 1mm higher than it should be. That results in a bearing axis distance of 44.5mm when you tighten the X carriage well. Where the rods are 45mm apart (center to center). While some preload on the bearings is good and prevents slippage and removes slop, this is way too much and it changes as the carriage moves to the ends of the axis.
If you go Misumi, I suggest you also get the Misumi rods that go with the bearings or at least check the ones you got from Prusa with a micrometer. If they're not the right diameter, even Misumi will eventually scratch them. And I also suggest that you check that the X carriage moves absolutely smoothly and is not bending the rods. If it is bending the rods, loosen the screws on the carriage, that helps. Or print a different carriage, like the Bear. It's possible the Mk3s R4 fixes the misalignment issue, but I haven't checked yet.
RE: Scratched rods from linear bearings
One solution is to remove the rods:
https://learn.watterott.com/3d-printer/mk3-y-upgrade/😀
I could imagine the Hiwins to make sense on the Z axis, where lateral stiffness is an issue. I wonder if they do any good on the Y.
RE: Scratched rods from linear bearings
- And finally, the I think MK3 R3 X carriage, particularly the X carriage back part has an error in it. The lower bearing cutout is 1mm higher than it should be. That results in a bearing axis distance of 44.5mm when you tighten the X carriage well. Where the rods are 45mm apart (center to center). While some preload on the bearings is good and prevents slippage and removes slop, this is way too much and it changes as the carriage moves to the ends of the axis.
That... explains a lot, including some observations made while mucking with bearings... :-/
RE: Scratched rods from linear bearings
That... explains a lot, including some observations made while mucking with bearings... :-/
Would you care to share those observations? If anyone is interested in testing, I can try preparing corrected SCAD/STL files. I myself have switched to a custom part aluminum part printed X carriage that works great and that I would like to avoid disassembling for a while.
RE: Scratched rods from linear bearings
That... explains a lot, including some observations made while mucking with bearings... :-/
Would you care to share those observations? If anyone is interested in testing, I can try preparing corrected SCAD/STL files. I myself have switched to a custom part aluminum part printed X carriage that works great and that I would like to avoid disassembling for a while.
Of course - I'd experimented with PLA bushings by buying some cheap LM8s and pulling the guts out to replace them with a printed sleeve. After doing this I'd noted that despite moving very well in the middle of the axis, It had a tendency to stick/be sluggish at the extremes.
In addition, my top rod was very scored after about two months of printing; I had it replaced and went back to the stock bearings (Support assured me if it moved smoothly it was fine, but evidence suggests that's not the case...) and in the brief period between my experimental PLA bearings failing after a long PETG print session and getting the Vesconite option, there are already faint scratches on my rods again - almost certainly caused by the design problem you mentioned. Even if the bearing is fine, if it's over-tensioned you're going to get premature failure of it or the bearing surface at the point of contact...
I'm currently running the Butterworth design modified Mk3 "indirect" extruder with Vesconite bushings. I haven't done any concrete investigation but it does seem like it doesn't "click" home as well anymore. There used to be a solid, sharp 'tick' from it hitting the left endstop but now it is a more muffled "thump" if that explanation makes any sense.
Is it just the very back plate (with the "fin") that's the problem? When I built my Mk3 I did not use stock and instead went for a modified design that offered proper belt access from the get-go. When 3S was released I swapped since the design/holes were different; Quite possible that was what precipitated the above observation.
I'd be game for some updated back carriage STLs if you are willing to make them - I have a 3S upgrade kit on the way so will need to reprint some different parts and teardown anyway.
RE: Scratched rods from linear bearings
Of course - I'd experimented with PLA bushings by buying some cheap LM8s and pulling the guts out to replace them with a printed sleeve. After doing this I'd noted that despite moving very well in the middle of the axis, It had a tendency to stick/be sluggish at the extremes.
That's exactly what led me to investigate. I took a caliper and measured the rod distance next to the carriage and at the ends and found the difference.
In addition, my top rod was very scored after about two months of printing; I had it replaced and went back to the stock bearings (Support assured me if it moved smoothly it was fine, but evidence suggests that's not the case...) and in the brief period between my experimental PLA bearings failing after a long PETG print session and getting the Vesconite option, there are already faint scratches on my rods again - almost certainly caused by the design problem you mentioned. Even if the bearing is fine, if it's over-tensioned you're going to get premature failure of it or the bearing surface at the point of contact...
Indeed. And bending the rods isn't particularly helpful either.
I'm currently running the Butterworth design modified Mk3 "indirect" extruder with Vesconite bushings. I haven't done any concrete investigation but it does seem like it doesn't "click" home as well anymore. There used to be a solid, sharp 'tick' from it hitting the left endstop but now it is a more muffled "thump" if that explanation makes any sense.
Same experience. With the aluminum X carriage (and aluminum X ends) it's the 'tick' again and I don't hear the stepper struggling to overcome the force of the rods. Another observation with new Misumi bearings and the R3 plastic parts was that the X carriage would slide back to the center by itself if let go (without the belt or the electric cable attached). That was definitely weird.
Is it just the very back plate (with the "fin") that's the problem?
I believe so. The front of the extruder seems correct. When I was making my the plastic parts for my alu/printed x-carriage, I started by putting the front and the back of the x-carriage together in OpenSCAD and was surprised they didn't seem to fit together properly. But given the weird shapes and given how each of the source files is approaching the part differently, I couldn't outright prove this is a bug and not intentional.
When I built my Mk3 I did not use stock and instead went for a modified design that offered proper belt access from the get-go. When 3S was released I swapped since the design/holes were different; Quite possible that was what precipitated the above observation.
I haven't checked any of the modified designs. I suppose many fixed the issues, but there will likely be some that started with the R3 design and didn't modify this particular aspect.
I'd be game for some updated back carriage STLs if you are willing to make them - I have a 3S upgrade kit on the way so will need to reprint some different parts and teardown anyway.
I'll sit down with OpenSCAD and try to create them in the evening.
RE: Scratched rods from linear bearings
+1 for PR design being open-source 🙂
Another observation with new Misumi bearings and the R3 plastic parts was that the X carriage would slide back to the center by itself if let go (without the belt or the electric cable attached). That was definitely weird.
I'd always attributed that to the tension from the extruder umbilical cord... but in light of this, the offset almost certainly has something to contribute to the issue...
RE: Scratched rods from linear bearings
So I went to OpenSCAD to fix the part, and while there is an inexplicable misalignment of the front and back (the back is 1mm shorter than it should be), it doesn't affect the spacing of the bearings: I was wrong, the part uses the correct 45mm offset.
It's not an error in the source: It's the part deforming when tightened, particularly the two bottom screws.
This is the part in OpenSCAD, with the bearings inserted and then cut in half:
As you can see, they fit pretty much perfectly, but because the lower bearing isn't supported from all directions, it tends to move towards the other bearing when the bottom screws are tight. You can also see how the bottom ends (left on the picture) of the part don't align correctly. That is what threw me off initially thinking there is an alignment problem also for the bearings.
RE: Scratched rods from linear bearings
Interesting. So should we consider using a washer so the screws can be tightened without causing the problematic deformation?
RE: Scratched rods from linear bearings
Interesting. So should we consider using a washer so the screws can be tightened without causing the problematic deformation?
I've had success relieving the stress on the rods by being very gentle on those bottom screws, tightening them just enough to hold the extruder barely together. I didn't know back then what is the mechanism of the stress, just observed the X rods moving together when I tightened the screws (I really like to tighten everything well) by measuring with a caliper.
RE: Scratched rods from linear bearings
Just to verify for myself that I'm not imagining things, I put together the original Prusa R3/B7 parts that I've removed from my printer. I measured 36.98mm inside between the rods at the ends and 36.62 mm inside between the rods next to the carriage. Tightening/loosening the screw has a huge impact on how easily the bearings move.
RE: Scratched rods from linear bearings
Another possibility would be anisotropic shrinkage of PETG in the X/Y vs Z dimensions while printing. Parts that I printed myself had the same issue as the ones received from Prusa. Can someone else verify the differences on their printer?
RE: Scratched rods from linear bearings
I'll try to remember to measure tonight when I get home.
FWIW my extruder parts were printed in an enclosure; I have had zero issue with parts warping/lifting from the bed since I started doing that.
Said warp/lift could definitely be confounding the issue.
RE: Scratched rods from linear bearings
@vojtech:
Thanks for all the suggestions ans explanations! The grease looks promising... But I suppose, there's no way of buying less than a whole kilogramm of that stuff?
I'll think about getting the rods from Misumi as well. I'm a little tired from constantly trying to fix this machine and keeping up with its flaws. A little tinkering is fine, but sometimes I wonder what I spent 800€ for.
I haven't even thought about pressure from the X-carriage and it's good you made the effort to align the parts in OpenSCAD. Are those the Mk3 or Mk3s parts?
Maybe one day I'll upgrade to the Bear design or something similar completely, but that's too much work, time and headache for me at the moment. 😐