Smooth rods wearing out...
Had my MK2s for about 3 months. Already seeing black grooves along Y axes smooth rods 🙁 Feels like they were scratched by linear bearings... It works a lot but not 24x7 (maybe 20 hours a week) and I occasionally apply lithium grease on all smooth rods (maybe once every 2-3 weeks)...
Has anyone else experienced this? Is this normal?
Re: Smooth rods wearing out...
I'm also noticing lines on my Y axis rods, though it's been a few hundred hours of printing on this machine. I'm not sure what best practices are for lubing/maintenance...
Re: Smooth rods wearing out...
The rods should definitely not show grooves after that short period of time.
Since the MK2S version, Prusa is using hardened smooth rods, which should last a very long time, especially since there is hardly any load on the bearings if the system is assembled correctly.
Possible reasons for damage:
bearings are not aligned correctly or otherwise somehow pre-tensioned, leading to too high pressure on the balls or worse stuck balls. possible cause for this: U-bolts too tight, bearings dual bearings not aligned or rods are not parallel over the whole length.
IMHO the most likely issue is overtigthening of the U-bolts leading to bent LM8UU bearings. (see below)
Other possibility would be wrong or too much grease. Too much and to viscous grease can prevent the balls from rolling freely in the rails inside the bearing, if that happens they will change from rolling to sliding and that will certainly scratch the rods over time. Check bearing manufacturer's recommendations for lubrications.
FIX:
I'd recommend getting new rods and bearings, since it's very liekly that when the rods show grooves, also the bearings will have been damaged. Maybe you can still use the rods and just change the bearings, turning them in a way that the balls are running outside the grooves, but I don't think it's worth the hassle. When assembling, check the movement of the Y-axis that it's smooth and even throughout the range. Play with the U-bolt tension if you decide to use them.
If you decide to use th U-bolts, take a lot of care to tighten them JUST enough. I found them to be extremely sensititve - a tiny little bit too much and the bearings seize, and a little too loose and you get play, or as I experienced, they vibrated loose after some time.
I printed these bearing holders https://www.thingiverse.com/thing:1848700 on my MK2S before selling it and am planning to use them on the MK3 assembly. They snap on to the bearings and distribute any forces evenly to the shell. I'll report on the MK3 forum when I have the kit.
Re: Smooth rods wearing out...
Thanks a lot for the advise!
I just got replacement bearings and hardened rods from Mitsumi. They are amazing, I can feel difference in sliding right away. You are right it's difficult to judge how tight I should adjust U-bolts. I tightened mine just enough until I can no longer move bearings by hand. Replaced m3 nylock nuts too just in case.
Whole process was pretty much easy to do. From now on I'll be only only using 3 in 1 oil to lubricate these...