Seized Nuts foil Core ONE+ upgrade
I proudly got into the first round of Core ONE machines released this year, and I have been very happy with it for the most part. I did run into an issue where the nuts used to adjust the belt tension both seized up on me, and I had to replace them (at my small expense), but I figured that it was a one-off issue and everything has worked fine since replacing them up to this point.
I just tried to replace the printhead left cover by removing two simple screws that aren't even under tension, and the lower one seized up on me, causing the nut to tear its hole into a circle. I had to tear it out of the tool head since I also could not screw it back in again. That is three screw/nut combinations that have seized up on me following Prusa instructions. Fixing this would require taking apart the whole toolhead, possibly discovering even more seized nuts.
What is going on here? I've had Prusas since the MK2, and I have never had an issue like this where simply maintaining the machine would cause these failures. What is going on here?
RE: Seized Nuts foil Core ONE+ upgrade
Steel-on-steel fastener combinations are prone to "galling" -- a very tight connection, like a cold weld, forms between the two parts. Several people have reported that this happened to their belt tensioners.
For the belt tensioners, Prusa now recommends in the kit instructions to lubricate the screws before assembly. There are also community-designed modified tensioner blocks which use a brass insert instead of the square steel nuts.
But this is the first time I read about other screw/nut pairs in the Core One being affected. For that printhead cover, the screw can't have been very tight -- otherwise you would mangle the plastic cover, I'd assume. Maybe the constant vibrations can still cause galling?
RE: Seized Nuts foil Core ONE+ upgrade
It certainly was not a tight nut and was not under tension. It seized only after unscrewing it about 2/3 of the way by hand, so I would assume that the galling was from that action, not the print-head movement. It could just be a bad batch of nuts, but that doesn't make maintaining my printer any easier. I'm at a loss as to the best remedy.
RE: Seized Nuts foil Core ONE+ upgrade
Fortunately there are not that many of the square nuts -- most screws in the Core One simply use self-tapping threads in plastic, or precut threads in steel sheet material. 🙄
Nevertheless, it is not an attractive proposition to locate all those square nuts to replace the nuts or/and grease the screws. Personally, I would just leave everything as it is, hoping that the nut on the cover was a fluke. The belt tensioner screws are the exception in my opinion, since they have been reported to cause problems quite often. If you have not done so yet when you had to replace them, I would take these out, lubricate and reinstall.
(Then again, maybe the belt tensioner screws have only been found problematic more often because users regularly adjust them, while most other screws are left alone? I dunno...)
RE: Seized Nuts foil Core ONE+ upgrade
I have plans to re-print the tensioners with heat-set threaded inserts to prevent issues with that part of the system, but I shouldn't have to be afraid of upgrading my machine, and I know that is not what Prusa wants, either.