Relevant for
On the Prusa CORE One, CORE One+, CORE One INDX, and CORE One L/+, adjusting the belt to optimal tension is crucial for achieving high-quality 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 following check applies exclusively to CORE One and CORE One+ kits. Pre-assembled CORE One, CORE One+, and CORE One L have this step done at the factory.
During the assembly process, we instruct you to apply a small amount of Prusa lubricant to the tips of both M3x30 screws for the belt tensioner parts. If this was done during the assembly of the kit, there is no need to redo it. Proceed directly to the next steps.
However, early CORE One kits did not have this step recommended in our manual. If this is the case, and other steps have not worked, the belt tensioner right/left and their attached belt tensioner pulleys might need to be replaced.
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For printers equipped with INDX upgrade, skip to the next paragraph Alignment (CORE One INDX).
Before adjusting the belt, ensure the proper alignment of the CoreXY structure.
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1. Fully loosen both tensioner bolts; this will allow free gantry movement.
2. Move the gantry halfway back along the rails.
3. Grip the gantry firmly on both ends and apply a gentle counter-force in the opposite direction of the tilt until it straightens.
4. Check the alignment with the belts still loose, and carefully push the gantry forward. Since the loose belts will not pull it all the way, you may need to guide it by hand. Verify that there is no gap on either side at the front position.
5. Tighten both tensioner bolts gradually, alternating between them, tightening each by the same amount to keep the gantry straight. You may hold the gantry in place with your hand while tightening. After tightening both belts, make sure no gap has reappeared.
Note: Apply force carefully to avoid damaging belts or bearings.
Before adjusting the belt, ensure the proper alignment of the CoreXY structure.
Quick gantry alignment
If there is a gap on either side of the gantry, you can try to force it in the opposite direction manually:
1. Fully loosen both tensioner bolts; this will allow free gantry movement.
2. Move the gantry halfway back along the rails.
3. Grip the gantry firmly on both ends and apply a gentle counter-force in the opposite direction of the tilt until it straightens.
4. Check the alignment with the belts still loose, and carefully push the gantry forward. Since the loose belts will not pull it all the way, you may need to guide it by hand. Verify that there is no gap on either side at the front position.
5. Tighten both tensioner bolts gradually, alternating between them, tightening each by the same amount to keep the gantry straight. You may hold the gantry in place with your hand while tightening. After tightening both belts, make sure no gap has reappeared.
Note: Apply force carefully to avoid damaging belts or bearings.
Starting from firmware 6.4.0, a belt tuning wizard is available in the printer menu, navigating to LCD Menu -> Settings -> Manual Belt Tuning. The printer uses controlled belt excitation combined with stroboscopic feedback to visualize belt resonance.
The wizard begins by prompting the user to check that the gantry is properly aligned, as misalignment can affect measurement accuracy. Follow the in-screen instructions to verify the gantry alignment, and in case of misalignment, follow the steps in the previous alignment paragraph.
The wizard then measures the vibration frequency of both belts and recommends whether to tighten or loosen them. Follow the in-screen instructions to turn the encoder to vary the vibration frequency, and at the same time, observe the vibrating movement made by the selected belt as the frequencies are changed.
Look for a slow, extended, and steady resonating movement of the selected belt. Once you see that movement on the selected belt, select the current frequency on the printer screen.
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| Upper belt resonating movement | Lower belt resonating movement |
On CORE One, the ideal target frequency range is a maximum of 98 Hz for the upper belt and a minimum of 90 Hz for the lower belt.
On CORE One L, the ideal target frequency range is a maximum of 95 Hz for the upper belt and a minimum of 85 Hz for the lower belt.
Once the resonance test is completed, the printer dialog shows the found resonating frequencies and the suggested adjustment.
Important: always tighten both tensioner bolts evenly and alternately. This ensures that the gantry stays square during adjustment.
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We have our own belt tuner, available on our belt tuner webpage. The belt tuner can also be accessed in our Prusa App, navigating to >Menu >Belt Tuner.
The belt tuner uses your device's microphone and determines the belt tension by measuring the frequency of the vibration generated by strumming it.
Select CORE One, then select XY-belt, and follow the instructions shown on your device. When ready to measure the belt frequency, tap on Start listening to use your device's microphone.
On CORE One, the ideal target frequency range is a maximum of 98 Hz for the upper belt and a minimum of 92 Hz for the lower belt.
On CORE One L, the ideal target frequency range is a maximum of 95 Hz for the upper belt and a minimum of 85 Hz for the lower belt.
If your gantry is out of alignment, you may not be able to reach these exact values. In some cases, the lower belt can even show a slightly higher frequency than the upper belt. This is acceptable as long as the difference between the two does not exceed 8 Hz, and both values remain within the 90–98 Hz range (CORE One) and within the 85-95 Hz range (CORE One L).
If you cannot achieve this, you will need to square the gantry manually before adjusting the belt tension. See the instructions in “Quick Gantry Alignment” above or “Extensive Adjustment of gantry and belts” at the bottom of this article.
Important: Always tighten both tensioner bolts evenly and alternately. This ensures that the gantry stays square during adjustment.
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The belt tensioners should be able to solve most belt issues. However, if you need to make a larger adjustment because of a really out-of-alignment gantry or a belt that has slipped from the holder, follow the steps below to access the belt.
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97 comments
The main problem is that it mixes two different concepts: a **target frequency** and an **acceptable frequency range**. A target frequency is normally a single nominal value, whereas a frequency range must be defined by lower and upper limits. Therefore, wording such as *“the ideal target frequency range is a maximum of 98 Hz”* is ambiguous and does not clearly tell the user what value should actually be targeted.
Based on values commonly mentioned in technical discussions and user reports, the intended target frequencies appear to be approximately **98 Hz for the upper belt** and **92 Hz for the lower belt**. If acceptable tolerances around these values also exist, then two separate ranges should be specified: one for the upper belt and one for the lower belt.
I would therefore recommend reformulating the documentation so that it clearly distinguishes between the **two nominal target frequencies** and the **two acceptable frequency ranges**. For example:
**“For the CORE One, the recommended target belt frequencies are approximately 98 Hz for the upper belt and 92 Hz for the lower belt. Each belt also has its own acceptable frequency range: [upper-belt range] and [lower-belt range].”**
Prusa has a strong reputation for technical competence, which is precisely why this sentence stands out. In its present form, the terminology is imprecise and could easily confuse users attempting to tune the belts correctly. I would suggest revising it using standard technical terminology and explicitly separating nominal target values from acceptable ranges.
Also for the wizard... Give some option for... Values are in range -finish OR... with xyz turning you can reach the perfect spot... So the user can decide if he/she will invest more time to calibrate