Replacing a worn or broken belt and tensioning it properly

Symptom

The belt has broken teeth, cracks or frayed edges, or it has snapped; or, even after tensioning it, the layer shifts or the noises when the axis moves come back.

Likely causes, from most to least likely

1. Wear with hours of use: Bambu Lab replaces the belt when it is worn or broken, and a belt with broken teeth or fraying cannot be fixed by tensioning it.
2. An over-tight belt: Elegoo warns that it increases the motor load, and a loose belt can skip steps.
3. Filament debris caught in the belt or pulley, or a belt rubbing against a part (Prusa).
4. A replacement that is not the same as the original or is cut wrong: Creality warns that on the Ender-5 Max the X and Y belts have different lengths.

Checks to tell which one it is

• With the printer off, go along the whole belt and check the teeth, the edges and the ends.
• Check that the replacement is the same type as the original (tooth pitch and width) and, if you buy it by the metre, that you have enough.
• Before removing the old one, note or photograph its route, which way the teeth face and how each end is held.

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. Switch off and unplug

   Switch the printer off and unplug it before touching anything: Bambu Lab and Creality ask for it, and Bambu Lab explains that this avoids short circuits when removing covers and toolhead cables. Have the Allen keys your model's guide asks for at hand (Bambu Lab uses a 2.0 mm one on the A1).

   Check: The printer is disconnected from the mains.

   If that does not fix it: Do not go on with the printer plugged in.

2. Loosen the tensioner and remove the old belt

   Loosen the axis tensioner, remove any covers over the belt and release the ends from their holders (sleeves or clamps). Pull the belt out along its route without forcing pulleys or cables; if it resists, flatten it to pass it through the holes, as Creality suggests.

   Check: The old belt comes out in one piece.

   If that does not fix it: If it will not come out, follow your model's disassembly guide.

3. Fit the new one along the same route

   If you buy it by the metre, cut a piece the same as the old one: measure it before throwing it away. Thread it along the same route, with the teeth facing the way they did and on the same side of each pulley, and fix the ends as they were (Bambu Lab, Creality and Elegoo mark the direction and route in their guides).

   Check: The belt goes round all the pulleys without slipping off or rubbing.

   If that does not fix it: Check the route against your photo or your model's guide.

4. Tension it

   Tension it with the tensioner as your maker says: Bambu Lab, on the A1, loosens the tensioner screw one turn, moves the toolhead from side to side three times and tightens it again; Creality gives 5.5-6.5 N for the K1's X and Y belts; Prusa wants it to sound like a low bass string when plucked. Neither loose nor too tight (Elegoo).

   Check: The toolhead moves smoothly and without play.

   If that does not fix it: If the axis feels stiff or the motor strained, loosen it a little: too tight loads the motor (Elegoo).

5. Reassemble, switch on and calibrate

   Refit the covers and cables, plug in and run the tests your printer asks for: Creality suggests the printer's self-test and Bambu Lab the vibration compensation calibration after replacing the belt.

   Check: The test finishes fine and the test print comes out without shifts.

   If that does not fix it: If the shifts come back, check the pulleys and their grub screws.

Prevention

• Check the belts at every maintenance: teeth, edges and tension.
• Remove filament debris from belts and pulleys as soon as you see it (Prusa).
• Check every now and then that the parts holding the belt have not worked loose (Prusa).

Still not fixed?

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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:

• X-belt Replacement Guide for A1 · Bambu Lab
• Replace Ender-5 Max Belt · Creality
• XY Axis Belt Tension (K1 series) · Creality
• The X-Axis Timing Belt Replacement (Neptune 4 Pro) · Elegoo
• Layer Shift (FDM printer troubleshooting) · Elegoo
• Layer shifting (belt tension and debris) · Prusa Research