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Do i need to tension the belts on my 1 month old UM3 ?

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Posted (edited) · Do i need to tension the belts on my 1 month old UM3 ?


I have a brand new UM3 which has been active for approx. 5 days now (runtime).

In my latest 2 prints the layers are clearly visible and the printing on the x&y axis seem to be a bit irregular.

Do i need to retension the belts to resolve this to get a smoother vertical surface ?

The example below is printed using the default fine profile.



Edited by Guest

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Posted (edited) · Do i need to tension the belts on my 1 month old UM3 ?

I don't have a UM3, only a measly UMO.  I thought maybe an UM3 power user would speak up.  Some people are reporting problems with the site access, loading pages etc right now.  Maybe that's an issue.

It is hard to diagnose from one picture.  From what I can see your problem may be with the extrusion mechanics.  Since you asked about belts, I'll addressed that.  They are straightforward to check and tension.

If you still have problems after checking belts and pulleys, look into extrusion inconsistencies.  Having looked at the model of the UM3 extruder, I am not comfortable coaching you how to maintain it.  I would not personally be afraid to take it apart and check for problems.  It depends on your mechanical confidence.  What to check for would be loose set screws, clogged drive wheels, or that the spring is free to operate etc…

Problems in the drive train though are often loosened set screws in the drive pulleys – especially on new machines.  It’s not a bad idea to check them all.  The UM3 3D model shows there are 11 pulleys.  The 2 mounted to motors may be hard to reach, but you should be able to get to them without removing the inner covers.

It could be possible that the belts have loosened on your new machine.  It is as likely for the belts to have come loose on a new machine as one that has been in use.  It is easy to check.  If you suspect it, it should be investigated.  

The mechanics are in many ways the same between our units.  I opened a 3D model of the UM3, and I can explain how to check the belts.

If you have a belt tensioning issue with your UM3, it is more likely the short belts.  The X & Y slider blocks on the UM3 have torsion springs built into them applying a constant tension which if assembled correctly and are confirmed to have worked they should not be a problem going forward.  I cannot tell from the model exactly how the springs are assembled, but they are inside the slider blocks.  One spring per long belt.

Like I said, if you have a belt lash issue, it is most likely short belts connected to the motors.  These belts link each of your X and Y motors to the rotary/linear shafting.  The motors are mounted with screws in slots in the back of the print chamber and behind covers.  The screws are outside the print chamber (4) for each motor.  The belts should be snug, but not being stretched overly tight.  It is hard to explain in concrete terms.  Basically, it is a matter of feel.  I would slightly slacken the mounting screws on each of the 2 motors, hold a moderate down pressure (2-3lbs?) on the motor and re-tighten the mounting screws.  Check the belt feel again.  Print something that won’t take a lot of time or material, but still tests your issue.

If you want to check the long belts, pluck them like a guitar string with the print head parked in a corner.  If the built in tensioners are working properly, they should make a quiet low pitch twang.  Also, asses them by feel.  You should be able to easily tell if they are overly loose.  Another way to test is to hold one of the shafts at a pulley from turning (by hand not pliers or anything that would damage the shaft or pulley) and feel the backlash at the head by trying to shift it with the shaft held.  Backlash should be minimal.

Edited by Guest

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