X-Axis Suddenly Started Skipping (Replaced bearings, belt tight but still no luck)
 

X-Axis Suddenly Started Skipping (Replaced bearings, belt tight but still no luck)  

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MintPrint
(@mintprint)
Active Member

Hi,

I have a MK3S that has been working great with around 150 days of print time on it. The other day it started doing a noticeable skip on the x-axis always in the same spot towards the center of the axis, but only when moving right. It also fails for short x-axis length during the belt test, but only when the z-axis is almost at bed level. At higher heights it passes. This is causing globs on my prints but not a noticeable layer shift. I've read a bunch of threats on here and other sites that recommend replacing the bearings. 

So far I've:

Lubricated the original bearings with Lucas White Lithium Grease NLGI #2. I built this printer 2 years ago and didn't lubricate the bearings then, because I didn't know better. I made sure to pack the bearings to get the grease on the balls. I checked them and couldn't see any obvious damage. It felt like they moved smoothly on the rods. My rods don't have any noticeable gouges. Still the same skip in the same location after lubrication. My X and Y are perpendicular. 

Made sure the belts are tight. X reads ~255 and Y is 265. I checked the pully isn't touching the motor. I checked holding the belts with pliers at the location to see if it skips, and it doesn't. The belt feels tight at all locations and the extruder doesn't skip when I try to move it by hand. 

I then replaced the Prusa bearings with these bearings from Amazon. https://www.amazon.com/LM8UU-Linear-Bearings-Length-Printer/dp/B087WPGQ8T/ref=cm_cr_arp_d_product_top?ie=UTF8. I now understand that I probably shouldn't have done this, because they aren't as good as the Prusa bearings or Misumi bearings. I lubricated these the same way with white lithium grease. These bearings skip on X going right at the same spot

Does the fact that the new bearings are skipping at the same spot when going right indicate the problem is with the stepper motor itself? I'm lost as to what is causing this. I'd really appreciate any suggestions. Thank you!

This topic was modified 2 months ago by MintPrint
Posted : 05/01/2021 6:24 pm
MartyS
(@martys)
Trusted Member

Hard to imagine it's the motor, with all the work you did replacing the bearings it would be amazing if you got the belt on the motor pulley in the exact same spot, but it would be easy to check, just loosen the belt enough so you can turn the pulley half a turn without moving anything else, then tighten everything back up and see if it still skips in the same spot.  Actually while the belt is off the motor pulley move the extruder back and forth by hand and feel for any binding.  On our MK3 at work it started binding in one direction and I had to loosen the center screw on the back plate a little to get it smoothed out, I think when tight it was turning one of the bearings just slightly, so I could tighten the screw again I put a thin washer inside between the 2 parts on that center screw.

Did you check the grub screws on the pulley?  They can come lose over time, especially if one isn't properly seated on the flat of the motor shaft.

 

 

Posted : 05/01/2021 11:38 pm
Snympi liked
MintPrint
(@mintprint)
Active Member

Thank you. I'll try this and report back. Also the stringing and globs occur on the same spot rear on a lamp I've been printing. It's been this way on 3 of the same object with 2 different brands of PETG filament (Duramic and Sunlu). So I think the skip is causing that. 

Stringing on bed

Stringing on onject

 

This post was modified 2 months ago by MintPrint
Posted : 06/01/2021 6:20 pm
MintPrint
(@mintprint)
Active Member

@martys

I removed the belt completely then turned the motor and reinstalled it. It is still skipping at the same spot, although playing around with the tightening makes the skip less severe, but it's still there. I also didn't feel any binding at the spot it skips. Does this point to the stepper motor then?

Posted : 06/01/2021 7:38 pm
JasonG
(@jasong)
Active Member

I have the same issue.  I have not tried re-packing the bearings, but it its failing.  I can hear this happen, and the printer doesnt detect it, even though it should.  Any recommendations other than what was already noted would be amazing.

Posted : 07/01/2021 8:33 pm
MartyS
(@martys)
Trusted Member

@mintprint

If it's skipping in the same spot after changing where the belt engages the pulley then it's unlikely to be the motor, if it was the motor it would now be skipping in a different spot.

By skipping do you mean the stepper motor is skipping or there is a spot on the print with a hole but overall the shape is OK?   The term skipping usually refers to the stepper motor skipping steps and that shifts the print at that layer.  Holes in the print can have many causes even if everything is mechanically fine on the printer.

 

Posted : 07/01/2021 11:21 pm
MintPrint
(@mintprint)
Active Member

Thank you. There's not super noticeable print artifacts, but the printer is crashing more than normal (it never used to crash). The layer lines look more visible but not massive layer shifts (overall worse print quality). I'm also getting a lot more globs and stringing. By skipping I mean the motion going right at the one location in the X center is not smooth. At the spot it skips it appears to move more much more quickly, like it's jumping horizontally. I will try to catch it on video. I don't notice a spot on the print where it happens. It doesn't make an audible noise I notice. 

Posted : 08/01/2021 1:17 am
jsw
 jsw
(@jsw)
Noble Member

If a stepper motor is not performing all steps, one of the first things to check is the wiring, as in reseating the connection to the board and inspecting for pinches and breaks.

When you say 'crash', do you mean like a physical nozzle crash, or a software crash, like a freeze-up, spontaneous reboot, abnormal end of a process, etc?

Posted : 08/01/2021 3:26 am
hli
 hli
(@hli)
Eminent Member

Maybe look at the belt. Maybe it rubbing somewhere on the extruder assembly, or the Z-axis parts. Maybe its dirty or damaged and somehow sticks a little bit to the pulley.

Anything to do with the motor or the pulleys would happen multiple times on the X-axis (as the motor makes multiple turns for the whole distance). So if the rails and bearings are OK (as your test implies), this leaves the belt as the next culprit.

Posted : 08/01/2021 12:20 pm
MintPrint
(@mintprint)
Active Member

I have figured out the problem, kind of. I have 2 MK3S, and after watching the other printer, I noticed it has the same skip in the same spot, but no decline in print quality or crashing. 

I checked the E3D silicone sock and realized it has PETG build up and slightly sagged (hard to see unless looking at a certain angle). I replaced the sock and there's no more stringing, globs, or crashes. Sorry for this post. The solution was stupid but I learned something new. 

I still don't know what is causing this skip (or maybe a better term would be jerk- change in acceleration). It doesn't seem to be causing problems and since both printers do it, I don't think there is a problem with the stepper motor. 

Posted : 09/01/2021 10:43 pm
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