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.

8 comments

RFT2112

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Looking at the pics of the position of the head I can see tow belts on either side. this tells me that the head is in the formost position or closest to the door, Am I seeing this wrong?
Prusa really just needs to make a video of this for clarity.

ljhms

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It's apparent from the images, but for clarity, you should point out in the text that when aiming for 85 Hz, it's the innermost section of the belt on either side you are checking. It's not entirely obvious how the belts run (because CoreXY isn't immediately intuitive), so better to be explicit. More importantly, when ensuring equality between the left side and the right side, accidentally plucking different halves between the two sides (e.g. the outermost half on the left and the innermost one on the right) would give you a big discrepancy.

Also, while it says that the X-position of the extruder should be in the middle, there is no exact reference you can push it up against to ensure it's actually dead center. Would it cause problems if it's off-center too much? Looking at the belt diagram in the assembly manual, I think the answer is no, because the length of the two innermost sections of either belt looks like it only depends on the Y-position of the gantry. Is this correct? So placing the extruder roughly in the center of the X-axis is just to get it out of the way, then?

ljhms

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The same ambiguity about the inner/outer sections of the belts is present in the instructions in the tuner webpage itself too. Even less clear there since there aren't any images and it refers to "_the_ left and right belt" as if there is only one section of belt on either side.

MacLemon

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I had much more reliable results with the website than the app, which is kinda odd.

The app picks up much more resonant higher frequencies, making it hard to find the desired dominant frequency, since the display will mostly show the resonances instead of the belt frequency. (Same location and surrounding noise floor as with my MK4S.)

IMHO another clear advantage of the website is, that the display will rotate and always be readable, opposed to the app which will always be hard to read since it forces portrait orientation. To get the microphone as close as possible to the belt I need to rotate the phone though., meaning I'm holding it in landscape orientation. The website also has a much larger font size, or can be adjusted easily.

GBA

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After performing the alignment , adjusting the belt tension ruins the alignment from the previous step . How can I ensure the alignment is still correct and the belt tension is right?

Alessandro Pantaleo

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Hi! Once we perform the alignment, we should adjust both belt tension bolts by the same amount. This way, we keep the alignment while tensioning the belts.

Of course, if there are still issues with your printer, do not hesitate to contact our Technical Support, ideally with a video, we'll check it in more detail and help you solve it! https://help.prusa3d.com/article/customer-support_2287

Shushuda

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1. Then why is it not said in the instructions it needs to be the same amount?
2. But if we do that, then the belts won't have the same frequency. It's either the alignment or equal belt tension.

Jan Kratochvíl

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Hello. This is a conflict between the squareness of the CoreXY and the belt tensions. The squareness is more important, but only slightly. So, first, tighten the belts, then try to make the CoreXY perfectly square by slightly tightening the screw where you see the gap and slightly loosening the screw on the other side.

Sebex

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the instructions are very unclear if this is the correct explanation.

After aligning (Core XY square), I tuned the belts. In order for them to be in tune there is a difference of 3/4 of a full bolt turn at the tensioners. Is that acceptable?

Stony3225

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For me this procedure would result in a difference of 17 Hz between the two belts. The left belt would be at 90 Hz, the right one at 107 Hz.

If I try to bring both belts to ~85 Hz, the squareness of the CoreXY is not given. On the right side I would have a significant gap when pulling the gantry all the way forward doing like step 3 under "Alignment" describes.

ljhms

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I read through the long thread about it on the Prusa forum: https://forum.prusa3d.com/forum/prusa-core-one-assembly-and-first-prints-troubleshooting/failed-y-calibration-of-a-new-coreone/

To summarize the findings in the thread, steps 1-4 of the Alignment section described above should be used only to _verify_ the alignment is correct (ideally by also loosening the belts almost completely first). But following step 5 to then use the belt tension to square up the XY-gantry is really just a crutch and will always lead to one of the belts being tighter than the other. In extreme cases, this difference would mess up the relative behavior (e.g. backlash) between the two belts, which could lead to things like VFA. A skewed gantry, on the other hand, would lead to skewed prints, so really both do need to be corrected. The trick with adding uneven belt tension might work for very small misalignments, but I certainly hope they don't recommend you end up with a difference of tens of Hz between the two.

According to the thread, provided that the skew is not caused by some obstruction in the printer (e.g. filament tube or cables tugging on the gantry), it could be that the brackets that hold the X-axis aren't square. There are two solutions to this:

Bending the brackets to square, see here for a post with instructions from Prusa support: https://forum.prusa3d.com/forum/postid/738308/ with some additional tips here: https://forum.prusa3d.com/forum/postid/746960/

Adding shims between the bracket and the X-axis to correct for the angle: https://forum.prusa3d.com/forum/postid/737892/ with more detailed step-by-step instructions here: https://forum.prusa3d.com/forum/postid/738263/

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Hopefully the instructions can be updated to point out up to what point it's acceptable to use belt tension to do skew adjustments and when needing too much adjustment could be a sign that you need to do additional troubleshooting. The belt tuning section mentions ±3 Hz: Maybe it should instruct you to go back to the Alignment section if you are out of this range? And the Alignment section is written with the assumption that the belts are already pre-tensioned like in the assembled models, e.g. "loosen one belt" (not very instructive for kit builders). I'd want to know how much extra tightening of one of the screws is alright, starting from a slack belt. Half a turn? Two turns? Ten?

nilslorand

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Hey, could you please include an image of where exactly to strum the belts? You answered a comment about it but I am still not 100% sure where to strum them exactly. Thank you :)