This is a powerful guide that helps you adjust the belt tension and alignment on your Prusa XL perfect evenly.

Introduction

I was one of the first to receive a Prusa XL Multitool printer. From the start, I had problems tightening the belt equally on both sides and still aligned parallel so that there is no gap on either side.

When I did this, the alignment of the carriage to the front stops was not parallel. The best I could achieve was 77 Hz on the left and 86 Hz on the right. As far as alignment goes, I had no gap on the left and a gap of 1-2mm on the right. (To close the gap on the right, I would have needed around 95 Hz on the right, which is obviously outside the target range.) With this setting, I was at least able to print well.

It’s also possible that the frequency measured with the tuner app doesn’t reflect the actual belt tension. (More on this at the end)

How it should be

Under no circumstances should there be a gap on the left. Otherwise the printer cannot XY homing precisely. This can lead to layer shifts. For this reason, you should always print an XY-homing-tower after adjusting the belt-tension. If there are no layer shifts there, everything should be fine. However, this can lead to dimensional accuracy deviations. For this reason, you should always tension both belts equally, especially with a core-XY printer. The belt tension on the XL should be between 78 Hz and 85 Hz. I’m a fan of slightly more tension, as this results in sharper edges. That’s why I set my printer to around 82-85 Hz. Too much isn’t good either, as this can lead also to layer shifts.

Unfortunately, despite hours of trying, I couldn’t resolve the issue using this Prusa guide. Prusa Support couldn’t help me either. Even after returning the printer for this and other issues, Prusa didn’t resolve the problem. I received a new replacement printer for various reasons. But it had the same issue. Here, I’ve described the complete issue with my first Prusa XL in detail.

Since I’ve spent a lot of time researching the problem, I know many people have this problem. So I tried a lot of things and have finally found a working solution!

With the help of a spacer and a little patience, you can do this too!

What do we need

• Printable Spacer Tool 2mm, 3mm, 4mm
• Printable Measurement Position Tool
• Screw Clamp
• Prusa Belt Tuner App (Not recommended)
• BIQU Belter (Recommended)

Before we start

To adjust the belt, all covers on the printer must be securely attached to the CoreXY part. If you are using an enclousure, this should also be fully installed beforehand. This can cause the belt tension to change, and it will then need to be checked and readjusted if necessary.

It is advisable to disconnect the XY motors beforehand. The frequent movement of the XY axis, which is necessary to adjust the belt evenly, can induce electric power through the motors and thus can damage the circuit boards.

Now we can begin. As already mentioned, our goal is to achieve the most even tension possible for both belts and to align the X-carriage parallel. There must be no gaps between the front parts and the XY carriage on the left and right.

Quick Guide Overview

• After each adjustment, the entire carriage must be rotated 1-2 times completely back and forth and completely left to right and back to the belt to evenly distribute the tension across the entire belt. Only then measure again.
• The adjustment position is always in the very center of the front.
• The measurement position is in the center of the left and right as shown.
• The alignment check position is in the front left and right.
• If there is a gap on the left, the spacer should be right.
• If there is a gap on the right, the spacer should be left.
• Always alternate between a maximum of 0.5 to 1 turns per side and a maximum of 2 full turns per adjustment procedure.

Detailed instructions

1. Distribute the belt tension evenly across the entire belt

In this video you can see how to move the tool changer after adjusting and before each measurement of the belt tension. This is important to ensure the belt tension is evenly distributed across the entire belt. Otherwise, reliable measurement results will not be obtained.

2. Align the tool changer correctly for measuring

Here you can see how to align the XY carriage to measure the belt tension. On the Y-axis, the front of the carriage must be flush with the rear part of the Z-axis mount. Just use the positioning tool I designed.

3. Set the two belt tensioners to the starting position

We’ll first try adjusting the belt tension evenly without the spacers.

4. Presetting the belt tension

Now we tighten each side alternately by half a turn. Using the Prusa Belt Tuning app (or the Belter from Biqu), we repeat the process until we have about 80 Hz on both sides.

5. Checking the alignment

6. Install of the compensation spacer

However, if there is a gap on the left, we use the spacer on the right. If there is a gap on the right, we use the spacer on the left. The 2 mm spacer works well in most cases. Only in some cases are the 3 mm and 4 mm spacers necessary.

I’ll now show what we need to do if there’s a gap on the left side. If you have a gap on the right side, simply mirror this process.

7. Attaching the screw terminal clamp

8. Adjusting the belt tension

Now we first loosen the tension on the right side by 1 turn. Then we repeat this on the left side. Now we loosen 0.5 turns on the right side, then again on the left side. And again, so we that we have loosened 2 full turns on each side.

Now we first tighten the left side by 0.5 turns, then 0.5 turns on the right side. We repeat this until we have tightened 2 full turns on each side.

Now we remove the screw clamp and the Spacer and move the entire carriage back, forth, left, and right across the entire track so that the belt tension can equalize over the entire belt length. (like in the video of step 1 above)

Now we measure the tension on each side (As described in step 2 above). If the belt tensions deviate from each other, we must first adjust them again. We first reinsert the spacer on the same side (right side). We do this by moving the XY carriage all the way forward and centering the tool changer. If necessary, we press the carriage firmly back onto the front using the screw clamp on the front left. Now we roughly adjust the tension on both sides.

9. Final advice

Now we measure the tension again to ensure it’s correct. If it’s now correct, you can check the alignment. If not, simply repeat from step 4. To do this, remove the spacer and push the scale all the way forward again. If there’s no gap on either side, you’ve done it. If there’s still a gap on the left side, repeat the entire process, using a thicker spacer if necessary. However, if there’s now a gap on the right side, repeat the entire process with the 2mm spacer on the left side. To get closer to the final sweet spot, 0.25-1 turn per side is sufficient. In some cases, this process may need to be repeated several times to achieve the desired result.

It may take a few tries to get it perfectly aligned. Just take your time and work carefully and with concentration, and you’ll get it right.

10. Important Note

Caution! When using the Prusa Belt Tuner app, problems can arise that can be misleading. It’s possible that even though the belts are actually equally tensioned, the app displays different frequencies. This isn’t an error with the app, but rather with the belt’s vibration. If, like me, you’ve adjusted the belt tension with the BIQU Belter and then noticed a significant deviation with the tuner app, it’s because In this case, the frequency does not reflect the actual belt tension. This finding can also be easily verified: Align the Y-Carriage correctly for measuring and place the tool changer exactly in the middle instead of left or right. Now check the frequencies on both sides again. You’ll notice that the belt tension is now balanced in terms of frequency:

Now we place the tool changer at the center instead of on the right or left.

So if you go by the Tuner App and the measuring position as Prusa specifies, you may never achieve balance.

I therefore recommend using the BIQU Belter.

The Biqu Belter (Belt Tension Gauge)

Some people aren’t so keen on the Prusa Belt Tuning app. Especially in noisy environments, it’s almost impossible to get accurate readings. Biqu offers a “Belter,” a digital measuring device that can accurately measure belt tension. I’m very satisfied with it. And at just €20, it’s not an expensive purchase. You can purchase it here on aliexpress.