Loose, over-tight or damaged belts: how to check and tension them

Symptom

Echoes or ripples appear next to edges (ghosting), circles come out oval, or layers shift. When plucked, the belt sounds slack, or the opposite: it's so tight that the axes move irregularly.

Likely causes, from most to least likely

1. Belts lose some tension over hours of printing.
2. The parts holding the belt or the tensioner have come loose.
3. On a CoreXY, the two belts have different tension or the gantry went out of square while tensioning.
4. An over-tightened belt, which causes irregular motion in X and Y.
5. A misaligned belt rubbing on a part, or a damaged belt.

Checks to tell which one it is

• Pluck the belt like a string: it should sound low, like a bass string, and give some resistance when you pinch both sides together.
• Print a round part: if it isn't perfectly circular, the belts need adjusting.
• If your printer has belt measurement (menu, wizard or an app that measures the frequency with the microphone), use it and compare with your manufacturer's values.
• On a CoreXY with Klipper and an accelerometer, compare both belts with TEST_RESONANCES AXIS=1,1 and TEST_RESONANCES AXIS=1,-1 and the graph_accelerometer.py plot.
• Look along the belt: it should run straight, not rub on plastic parts, with secure anchors and no visible damage.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Check anchors and belt path

   With the printer off, check that the parts holding the belt ends and the tensioner are secure, and that the belt runs straight without rubbing.

   Check: The belt doesn't slip in its anchors and doesn't touch any part.

   If that does not fix it: Adjust the tension with the tensioner.

2. Tension gradually with the tensioner

   Follow your printer's manual: sometimes you must first loosen the motor screws so it can move. Turn the tensioner a little at a time, check the tension after each turn, and retighten what you loosened when done.

   Check: The belt sounds low and firm along the whole axis.

   If that does not fix it: If the tensioner runs out of travel, manuals usually say to move the belt one or two teeth in its anchor; mark its position first.

3. On a CoreXY, tension both belts equally

   Before tensioning, check that the gantry is square. Tighten both tensioners alternately and by the same number of turns so it stays square.

   Check: Both belts end up similar (or within your manufacturer's range) and there is no gap at the gantry corners.

   If that does not fix it: Redo the gantry alignment following the manual.

4. Back off if it's too tight

   If after tensioning the axes move jerkily or the motors seem to strain, loosen a little: an over-tight belt also causes irregular motion.

   Check: Motion is smooth and the test part comes out right.

   If that does not fix it: Measure again with your manufacturer's method.

5. Replace a damaged belt

   If the belt has broken teeth or is frayed, tensioning won't fix it: replace it with a compatible spare following your manufacturer's guide.

   Check: The new belt runs straight with the right tension.

   If that does not fix it: Contact your manufacturer's support.

Prevention

• Check the tension at each routine maintenance and after many hours of printing.
• Check now and then that the parts holding the belt haven't come loose.
• Before removing a belt, mark its position in the anchor.

Still not fixed?

Tell usi3d's AI about your case: choose your printer, describe what you see and add photos if you like. It proposes checks step by step, with their sources.

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Adjusting belt tension (MK4/S, MK3.9/S, MK3.5/S, MK3/S/+) · Prusa Research
• Adjusting belt tension (CORE One/+, CORE One INDX, CORE One L/+) · Prusa Research
• Regular printer maintenance (XL) · Prusa Research
• Measuring Resonances (testing custom axes: belt tension on CoreXY) · Klipper