Clear all

Belt Tension Issues - Prusa I3 MK3S  

New Member
Belt Tension Issues - Prusa I3 MK3S

Hi Everyone,

Got my printer pre-assembled from Prusa and printed a couple of benchy's which were fine but was getting crash detected but the printer carried on, on my most recent print the printer paused on a crash so decided to do some troubleshooting.

I flashed the latest firmware and ran a belt test (which was stated on the crash detected guide from Prusa) here - and got results just under 230 for both X and Y, I adjusted the X Axis by one notch on the belt and the adjusted the Y axis with the screw to loosen but still get the same results when running the belt test.

I read on another forum post on here that the number from the belt test doesn't update until you run a self test so have done that this morning and the results still are only marginally above 230 for both.  The self test comes back ok.

I thought I had spotted in another post about something about rods / bearings being too tight but can't find it again now - given mine was pre-assembled I hope that wouldn't be the issue but unsure what I'm supposed to be checking.  I guess if the motor is measuring torque for the test and the bearings aren't flowing freely enough then could this impact the results of the test?

I've looked at the guide for adjusting the X-Axis screw (for smaller adjustments) and it doesn't make sense to me about what the top screw is doing next to the X-Axis motor.  There is a very real chance that I'm being somewhat thick here but I've read it several times and don't understand.  Nothing on YouTube from Prusa about belt tensioning which I thought would be in the setup video 🙁

Any help or assistance appreciated!



Posted : 21/10/2020 10:23 am
RE: Belt Tension Issues - Prusa I3 MK3S

Hi MDX, 

what you are supposed to do is covered in this assembly guide

starting at Step 48... 

Please Be aware that the X motor has VERY restricted rotation capability AND the screw on top of the end of the X motor mounting Bracket, IS NOT a tensioning Jack    If you try and crank the  Belt tension up using this screw, the chances are that you will exceed the rotational capability og the X motor screws in the plastic housing, and end up breaking the housing...

Please follow the instructions in the assembly guide.

NOTE, there are user comments at the end of each section in the assembly guide...

Please read these before attempting to adjust the X belt... 

Once the X belt is adjusted and the motor screws are tightened up, the top screw can be advanced until it touches the left hand motor mounting screw, to prevent the motor rotating back in use, and loosening the belt tension. 

Best of luck, 


I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK

Posted : 21/10/2020 9:21 pm