Adjusting belt tension (CORE One) | Prusa Knowledge Base

On the Prusa CORE One, adjusting the belt to the optimal tension is crucial to achieving good quality on the prints. A loose belt can cause Layer shifting, Ghosting, or other print abnormalities, like getting an irregular shape instead of a perfect circle. An over-tightened belt can cause an irregular movement in the X and Y axes.

Alignment

Before adjusting the belt, ensure the proper alignment of the CoreXY structure.

Ensure the stepper motors are disabled. This can be done by switching the printer off, or through the LCD Menu -> Control -> Disable Motors.
By hand, move the Nextruder to the frontmost Y-axis position, and to the center of the X-axis.
Press the gantry corner against the front of the CoreXY, and make sure there is no play. Do this procedure on both sides. If there is any play in either side, perfect alignment is not achieved yet.
Slightly loosen both the belt tensioner bolts. Loosen the bolt on the corner with play less than the other.
Repeat steps 3 and 4 until a perfect alignment is reached. Do not worry if the belt is loose at this stage, adjusting the belt tension is done at a later step.

Belt tuner

We have our own belt tuner, incorporated in our Prusa App, or available on our belt tuner webpage.

The application uses your device's microphone and determines the belt tension by measuring the frequency of the vibration generated by strumming it.

Using a mobile device, open the Prusa App, and navigate to Menu >Belt Tuner. Select CORE One, then select XY-belt, and follow the in-app instructions. When ready to measure the belt frequency, tap on Start listening to use your device's microphone.

Adjusting the belt

Ensure the stepper motors are disabled. This can be done by switching the printer off, or through the LCD Menu -> Control -> Disable Motors.
By hand, move the Nextruder to the rearmost Y-axis position, and to the center of the X-axis.
On the left and the right sides, strum the belts in the middle. Place your device next to the belts during this step, to measure the frequency in Hz of the belt sound.
The strumming sound frequency is typically between 80-90 Hz, with the ideal value at 85 Hz.
Adjust the belts by tightening or loosening the belt tensioner screws. A tighter bolt causes a higher frequency, and vice versa.
It is fundamental for belts to have ideally the same frequency, or at most a very small difference, up to 3 Hz.

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31 comments

pa1ntman

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After a couple of hours messing with this, I got inconsistent results from the prusa tuner application, its response was iffy at best, and frequently couldn’t sense the belt frequency until it was way to tight.I got an app called gates carbon drive, which is for tuning gates drive belts, it actually can hear the belt being plucked unlike the prusa app, reads in htz and was able to to actually achieve 85 htz on each side .

Just for kicks the d string on my bass registers at 74 htz in both prusa app and gates app. Oddly enough there’s a prusa belt app via the Webb and it gives good results unlike the version in the prusa app?

Nagatobimaru

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In case anyone else gets derailed by Prusa's belt tuner: the initial pluck may give you way higher notes (Hz) than what your actual sustained frequency is. This initial pluck goes as high as ~175Hz, which then gets a sustained ~87Hz - which is what you actually need. I just tuned my guitar string to 87 (since it had a much more reliable built-in tuner) and checked the initial pluck, applied greatly to the belts. It'll take a bit of fiddling around, but I hope this helps someone save some time.
I recommend using Spectroid (available on Android) instead of Prusa's tuner if you can, and zoom into the view so you only see up to 200Hz (since anything above should be irrelevant).

BenG

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Well I've been playing with this for several hours now and I've decided that it's a bit of a fools errand. There's no way that adjusting the tension is a good solution for the fact that the gantry brackets are just too soft and easily bendable we just need better gantry brackets!
I have had several PRUSA printer kits and upgrades, and think they're great. This is the first time that it feels like I've been given an experimental prototype.....

hyiger

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Prusa, it would be much easier and save people a lot of grief if you just include a belt tension gauge with these kits. Something like this: https://www.printables.com/model/1312851-prusa-core-one-belt-tension-gauge

Basically the app won't register the frequency if the belts are too loose or too tight. The tensioner gauge gets it in the ballpark and then the app will properly register the frequency.

Jeff Lastofka

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I think I have it working now, so maybe another idea for people to try... Lower tension. I was going through this several (many...) times and not getting repeatable results. I went back to totally slack and checked squareness of frame more than once. I tried 85 Hz, I tried 90, 95. Re-did all this a few times. Sometimes it worked, sometimes not. Then I went through it all again and tried 77 Hz and it seems to be working great. The Prusa phone app works but I have a hard time knowing when it's taken a reading and just getting the same number, or if it's just not reading. They should have some indicator blink every time a new measurement is made, in case it's just the same number. The way it is, we're left wondering - is it the same frequency or has it just not made a reading yet? They do a great job on so much of their stuff, but this belt thing needs more attention.