Persistent belt wandering and squeaking traced to XY motor pulley migration

Bambu Lab X Series Troubleshooting
1

Hello everyone,

I am posting this as a technical observation and request for confirmation, and so that it may help others who have the same issue and where existing guidance does not help with a permanent solution.

This issue was observed with a X1 Carbon (mid-2024 purchase, moderate use, no abuse or drops, properly maintained), but may extend to the P1 series due to the shared kinetics and identical parts.

Summary

I experienced persistent XY belt wandering on the rear right tensioning idler, together with the associated squeaking of the belt rubbing against the tensioning idler flange. Officially documented belt tensioning and idler alignment procedures, as well as community guidance for slightly biasing the tensioning idler at a minimal angle did not result in a long term solution; the issue would recur every 48 hours of printing or so.

The root cause turned out to be axial migration and partial separation of the XY motor drive pulley on the stepper shaft, which created a destructive feedback loop that worsened belt tracking over time and eventually caused the belt to hop off the pulley.

This failure mode does not appear to be covered by existing Bambu documentation, despite being consistent with symptoms many users report.

Observed progression

  1. Belt began wandering up and down on the tensioning idler

  2. Squeaking developed, localized to one idler

  3. Re-tensioning and idler angle biasing temporarily helped but never fully stabilized.

  4. Belt debris accumulated, identified as such by the reflective fiberglass shreddings in the debris

  5. Eventually, the printer had a spaghetti failure of a special kind, when all regular gcode movement only resulted in diagonal movement at slightly different angles across the print bed, resulting in a very interesting failure.

  6. After a restart of the printer and an initial attempt to home all axes, it was clear that even linear X or Y movement resulted in actual movement just a few degrees off of 45º, suggesting a CoreXY failure with one axis decoupled but not moving entirely freely, either.

  7. With this in mint, I could clearly track it down to the belt having hopped off the drive pulley.

Key finding

Upon inspection, the XY motor drive pulley had migrated axially on the motor shaft and partially separated. In the below photos you can clearly see that the belt is no longer on the pulley, but also that the pulley appears to be a multi-part assembly where the bottom flange has separated from the toothed center.

IMG_3392
IMG_33922614×2903 843 KB

For contrast, the other motor which doesn’t have this issue:

IMG_3393
IMG_33931457×941 335 KB

There are no visible set screws; retention appears to rely on press-fit, the pulley assembly clicking together with retaining clips, and/or adhesive.

This explains why:

  • Belt wandering could not be corrected via idler bias alone

  • Abrasion and debris increased over time

  • The problem worsened despite following all recommended maintenance steps

I was able to temporarily restore function by carefully pressing the pulley back into position (although i was unable to press it all the way down; there remains a minimal gap between the toothed part and the bottom flange), cleaning all debris, and reassembling. This is clearly not a confidence-inspiring or permanent fix but will let me keep printing until I can order a replacement belt and motor.

Why I believe this matters

  • Official guidance correctly addresses minor misalignment via idler angle bias

  • However, there is no escalation guidance when biasing exceeds a reasonable range

  • There is no mention of inspecting motor pulley seating anywhere, which I believe is a crucial gap.

  • Continued mitigation without diagnosis will likely result in premature belt destruction and eventual motion failure like I experienced.

  • I believe this creates a feedback loop:

    • Once the belt comes off the pulley just a bit, it will introduce a bias that can be countered by slightly angling the tensioning idler
    • once the belt is no longer properly seated on the pulley (either partially off, or hitting one flange regularly), the belt will impose an axial force on the pulley, which is aided by calibration vibrations as well as actual printing motion over time
    • pulley migration will continuously worsen engagement with the belt, and thus the bias, and thus the force on the pulley
    • Eventually, the pulley is pushed too far and the belt loses traction, resulting in a failure of the motion system.

I found only one community post by user ReignOfTerror on Reddit, buried in a response to another post, describing an almost identical root cause where the pulley adhesive failed and the pulley moved on the shaft, causing belt dust and wandering. That user however seems to have caught it before the motion system failed.

Questions for Bambu and the community

  1. Is XY motor pulley migration a known issue?

  2. Is there official guidance that I missed that calls for inspecting (and replacing or repairing) the motor pulley in cases of persistent belt wandering?

  3. Are there revised pulleys or motors that address this?

  4. Should documentation be updated to recommend pulley inspection when belt wandering persists beyond minor idler bias correction?

My goal here is to verify the correct long-term fix before disassembling the printer farther, and help prevent others from chasing symptoms while the root cause worsens.

Thanks!

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