Belt tension is how tightly the timing belts are pulled between the stepper pulley and the idler. The right amount is enough that the belt never slips or goes slack when the print head reverses, but not so much that it loads the bearings and motor shafts. There is no universal number, because the frequency a belt rings at depends on its length, width and the mass it moves. Use your maker’s figure: Prusa gives 90 to 98 Hz for the CORE One and CORE One+ (85 to 95 Hz for the CORE One L), and the Voron docs suggest about 110 Hz on a 150 mm span as a starting point for A/B belts.
This guide covers the signs of loose belts, how to measure tension with a phone app or a printer’s built-in tuner, how to set it on Prusa, Bambu Lab and Voron-style machines, and what to redo afterwards. For printers without a published figure we say so and fall back to pluck and slack tests. If you only see a single shifted layer, start with layer shift fixes.
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Belt tension in 6 steps
- Check the mechanics first: pulley set screws tight on the motor shaft, idlers spinning freely, belts clean and not rubbing.
- Measure: pluck the belt over the span your maker specifies and read the frequency, or run the printer’s own tuner.
- Adjust in small steps (Prusa uses half a turn at a time) and, on CoreXY, change both sides by the same amount.
- Move the axes through their full travel by hand, then measure again. Tension often shifts after the first move.
- Re-run input shaping or vibration compensation, because tension changes the printer’s resonance.
- Print a ringing test and compare it with your earlier print.
Signs of loose belts and what else looks similar
Prusa lists layer shifting, ghosting and out-of-round cylinders as the results of loose belts. Bambu Lab adds that circles can come out as ovals. Klipper’s documentation puts it more generally: ringing usually has mechanical origins, such as a frame that is not rigid enough or belts that are not tight or are too springy.
| Sign | What you see | What to check |
|---|---|---|
| Ghosting or ringing | Faint echoes of corners and holes on flat walls | Belt tension, then frame and speed; see ringing and ghosting |
| Layer shift | A sudden, permanent offset in X or Y | Belt slipping, loose pulley set screw, motor current; see layer shift fixes |
| Dimensional errors | Circles that print as ovals, cylinders out of round | Tension on both axes, belt wear, squareness of the gantry |
| Slack on reversal | The belt moves before the motor shaft does | Prusa’s slack test: hold the motor shaft by its flat with pliers and push the carriage by hand; no belt slack should show |
| Worn belt | Fraying, cracks, missing teeth, shiny rubbed spots | Replace the belt. Adjustment cannot fix a damaged belt |
Too tight is also a fault. Bambu Lab warns that over-tightening the tensioner screws can cause damage and does not give better results, and Prusa’s CORE One article says to re-measure after every adjustment because a tighter bolt raises the frequency.
Target values by printer
| Printer | How it is checked | Target | Source |
|---|---|---|---|
| Prusa CORE One / One+ | Pluck, Prusa belt tuner or the printer’s Manual Belt Tuning wizard (firmware 6.4.0 or later) | Upper belt at most 98 Hz, lower belt at least 92 Hz (the wizard text says 90 Hz); keep the two within 8 Hz of each other | Prusa |
| Prusa CORE One L | Same tools | 85 to 95 Hz for both belts | Prusa |
| Prusa MK3/S/+ | LCD: Support, then Belt Status | About 250 for X and 275 for Y, plus or minus 15. Prusa says loosen below about 240 and tighten above about 290. These are Prusa’s own units, not Hz | Prusa |
| Prusa MK4/S, MK3.9/S, MK3.5/S | Pluck by ear or use the belt tuner | No Hz figure in the article. The belt should sound like a low bass note, and pinching the two sides together should take a little resistance | Prusa |
| Voron (A/B belts) | Pluck a 150 mm span, read the lowest peak in a spectrum app | About 110 Hz as a starting point (roughly 2 lb of tension, on the low side of the range). Voron 2 Z belts: about 140 Hz | Voron docs |
| Bambu Lab X1 and P1 series | Spring tensioner; the X1 notices drift during its calibration sweep | No Hz figure. Loosen, move, retighten (see below) | Bambu Lab Wiki |
| Any other printer | Pluck and slack tests | No verified number here. Aim for both axes feeling and sounding alike, with no slack on reversal | – |
These frequencies are not interchangeable. Bambu Lab’s wiki explains why: the gantry and belt behave like a mass on a spring, with a natural frequency of f = sqrt(k/m), so a looser belt rings lower. Belt length and moving mass differ between printers, which is why Voron specifies a 150 mm span and Prusa gives different ranges for different models.
How to measure tension
- Phone app. The Voron docs name Sound Spectrum Analysis (iOS) and Spectroid (Android), which show a graph, and Gates Carbon Drive (with the motorcycle option), which shows one number and is harder to read but easier to interpret. Pluck the belt and adjust until the lowest frequency peak sits at your target.
- Prusa belt tuner. Open it at belt.connect.prusa3d.com or in the Prusa App, pick the printer and the XY belt, and it listens through the microphone while you pluck.
- Built-in wizard. On the CORE One family the Manual Belt Tuning wizard drives the motors at controlled frequencies, and you turn the encoder until the belt shows a steady, extended resonance.
- Pluck technique (Prusa CORE One). Disable the motors, move the toolhead to the front and right, then strum the middle of the upper belt and the middle of the lower belt from the front, with the device next to the belt.
- No tools. Pluck for a low, string-like tone, pinch the two sides together to feel resistance, and do the slack test from the table above.
How to set the tension
- Switch the printer off or disable the motors so nothing moves while you work.
- Check the pulley set screw sits on the flat of the motor shaft (Prusa) and clear dust and filament debris from belts and pulleys.
- On CoreXY, make sure the gantry is square first. Prusa warns that misalignment can stop you reaching the target and can distort wizard readings.
- Turn the tensioners in small, equal steps. On the Prusa CORE One the left screw tensions the upper belt and the right screw the lower belt, half a turn at a time. Re-measure after each step.
- On Voron-style CoreXY, remember that the A and B belts affect each other: tightening one tightens the other, so adjust back and forth until they match. Then move the X extrusion back a few centimetres and forward again and re-check.
- Secure any locking screws without over-torquing, and re-measure once more.
Printer-specific notes
- Prusa MK4/S, MK3.9/S, MK3.5/S, MK3/S. For slight changes use the 2.5 mm Allen key: the rear screw of the X-end motor for X, and the idler screws behind the LCD module for Y. For large changes Prusa moves the belt 1 to 2 teeth in its mount.
- Bambu Lab X1 and P1 series. Power off, loosen the screws of the belt tensioner without removing them, move the toolhead gently back and forth several times and finish at the rear, then tighten the screws again. The spring in the tensioner sets the tension. Bambu stresses not to overtighten the screws.
- Other printers. Follow your maker’s guide for how the tensioner works. If a belt will not reach the right tension at the end of the adjuster, move it one or two teeth in its clamp, as Prusa does.
After tensioning: recalibrate and test
Changing tension changes the printer’s resonance. Bambu Lab says both belt tensioning and carbon rod cleaning can change the toolhead’s resonance characteristics, so recalibrate vibration compensation afterwards. On Klipper, the Resonance Compensation page lists tightened belts among the changes that can shift ringing frequencies, so measure again before trusting an old shaper. See Klipper input shaping for the procedure. Then print a ringing test and compare it with your earlier result.
If tension is right and ringing or shifts remain, look elsewhere: frame bracing for a flexing frame, the lubrication guide for rough rails, and stepper driver problems for skipped steps and overheating drivers.
How often to check
Prusa’s maintenance list puts a check of axes, belts and fans at every 200 hours of printing time, and asks you to remove debris from belts and pulleys when you see it. It also tells you to check that the pulley set screw sits firmly on the flat of the motor shaft. Bambu’s X1 monitors belt tension through the frequency sweep during calibration, and tells you when it is time to tighten. For other printers, check after a move, a crash, or when ringing or shifts appear.
Common mistakes
- Tightening belts to cure ringing without measuring. More tension is not better, and Bambu says it can cause damage.
- Copying another printer’s Hz value. Span, belt and moving mass change the number.
- Adjusting only one CoreXY belt. A and B interact, so tune them as a pair.
- Ignoring the pulley set screw. A loose pulley mimics a slack belt and causes layer shifts.
- Measuring once and printing. Move the axes through full travel, then measure again.
- Skipping the recalibration of input shaping or vibration compensation.
Frequently asked questions
How tight should 3D printer belts be?
Tight enough to show no slack when the head reverses and to ring at your maker’s target frequency, for example 90 to 98 Hz on a Prusa CORE One or about 110 Hz on a Voron A/B belt over 150 mm. Where no number is published, aim for a low string-like note and equal feel on both axes. More tension than the target does not improve prints.
Can I use a phone app to check belt tension?
Yes. Voron’s documentation recommends spectrum apps such as Sound Spectrum Analysis on iOS and Spectroid on Android, and Prusa has its own belt tuner that listens through the microphone. Pluck the same span your maker specifies, and compare the lowest peak with the target for your model.
How often should I check belt tension?
Prusa’s maintenance schedule includes belts in a check every 200 hours of printing time. Also check after moving or crashing the printer, or whenever you see ghosting, oval circles or layer shifts.
Can over-tight belts damage a printer?
Bambu Lab warns that over-tightening the tensioner screws can cause damage and will not give better results. Prusa’s guidance is to move in half-turn steps and re-measure, so you stop at the target instead of tightening until the belt feels hard.
Sources
- Prusa Knowledge Base: Adjusting belt tension (CORE One, CORE One L)
- Prusa Knowledge Base: Adjusting belt tension (MK4/S, MK3.9/S, MK3.5/S, MK3/S/+)
- Voron Documentation: Secondary printer tuning (belt tension)
- Bambu Lab Wiki: X1/P1 series belt tensioning procedure
- Klipper documentation: Resonance compensation
