Adjusting belt tension (XL) | Prusa Knowledge Base

On the Original Prusa XL, 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.

The Original Prusa XL geometry is CoreXY: it has two belts for the XY-axes.

Alignment

It is crucial to have perfect alignment between the right and left sides. The X-axis should reach both sides of the front printed parts at the same time.

Tightening the right Y-axis rail reduces the space between the CoreXY Front Left and the X-axis on the right side but creates space on the left side, and vice versa.

For the procedure, 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 toolchanger or Nextruder to the frontmost Y-axis position, and to the center of the X-axis.

Test the alignment by squeezing the CoreXY gantry and the corner printed part, first on one side and then on the other. There should be no play between the parts. With perfect alignment, the squeezing test on both sides feels like pressing two solid objects together.

If there is misalignment, then the squeezing test reveals play in the front corner. In that case, loosen the belt on both sides slightly until the gantry is aligned. Loosen the tensioning screw of the corner with more play less than the other.

Repeat the procedure and squeezing tests until perfect alignment is achieved.

The right side image shows the mechanism behind the tensioning screw. Tightening the screw pulls the tensioner towards the front, resulting in a more tensioned belt on that side.

Belt tuner

We have our own belt tuner. The application determines the belt tension by measuring the frequency of the vibration generated by strumming it. Follow the instructions in the video below to adjust the belt.

Homing tower

We prepared a specific homing tower, to visualize possible issues with the belt tension and have the possibility to correct them on the spot.

In case the belt is loose, the tower will have an inconsistent finish, with scattered material on the walls instead of a smooth finish. Without interrupting the print, tighten the screws on both Y-axis rails by half turn.

If the print finish does not fully improve, repeat the procedure once again, until the homing tower has a smooth, consistent finish.

An over-tightened belt can result in layer shifting.

Once the tower has a smooth finish, loosen the same screws very slightly, and observe if this further improves the tower. Repeat the procedure until the tower has an ideal finish.

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38 comments

bruhnstv

•

I think I have the belt tension thing nailed now, but it turned out to be quite a frustrating experience. It pretty much boiled down to having two adjustment screws to set up three parameters: upper belt resonance (tension), lower belt resonance and gantry squareness. By my way of thinking, there should be an actual adjustment for the squareness, and since the two tensoning screws do not individually and independently set the two belt tensions, there should be perhaps a table for each of four desired outcomes: both up, both down, lower belt up and upper belt down, and lower belt down and upper belt up. - At least it was an opportunity to insure I had properly assembled and lubricated the belt tensioning system! ;-)

Scratcher's Guy

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A comment in the description of the tower that the G Code causes homing resets at each layer would be helpful.

Also advice to use a glossy PLA would be a helpful tip to start. I shouldn't have to dig through comments to figure these things out.

I don't know if it is possible to post more pictures about what could go wrong. I'm doing a lot of hit-or-miss experimentation to see what happens. It's pretty random. It would be helpful if there was more explanation about how tight and loose belts could produce different results and what "optimal" should be.

Atle Dreier

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I can't get the app happy. With the gantry square, I cannot get the belt tuning right.

Any advice? Core One, prebuilt.

bruhnstv

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I had a lot of the same trouble. It may help to think: if you tighten either tensioning screw, BOTH belts get tighter (higher pitch/frequency), but if it was the lower belt screw, the lower belt moves a little more than the upper. So for example, if lower belt is too low and upper is about right, tighten lower and loosen upper by the same amount, then recheck. Also, if you are using the new built-in app that generates a known pitch vibration, I found it worked well to set that pitch to the center of the range (95Hz for me), and use the tensioning screws to move the belt resonances. Say both belts are at too low a resonance, and lower is quite a bit lower than the upper. Tighten the lower belt screw to move both belts past that 95Hz. If the upper belt hit resonance first and the lower after more tightening, then move to loosening the _upper_, again watching both of them pass the 95Hz. Keep doing that, and the belt resonances will come closer together. If you "get" how that's working, you should be able to figure how to move things to the right places. Then check the gantry, and pray it's still OK! If not, I suggest loosening both tensioning screws the same amount, as closely as you can, do the straightening, tighten the screws the same amount you loosened them. And do not do what I did. I managed to break BOTH the tensioning pully carriers. :-(

michanisani

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The enable mic do not work with samsung M52 phone. Last android 13

Chris T

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I was getting very frustrated trying to use the belt tuning app. The app seems very unresponsive BUT if you do it from the website via a browser it works much better.

I kept getting constant readings of 120 Hz and telling me to loosen the belt. Well in order for the tuning to work appropriately the belt needs to be decently tightened before it will actually register appropriately. Once I got it to that point making adjustments to the appropriate frequency was much easier and consistently sounds much better.

Running a homing tower now to see if everything looks good.

3D Medic Vince

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I just had to redo the alignment of the axis as the right side wasn't touching the front like the left side. I went to retune and the app wouldn't work but if I put my phone in airplane mode it would. I assume there's probably an app that's blocking the Prusa app from using the mic but after airplane mode worked great.