The belt pulley is worn loose due to assembly or prolonged strain operation. Here’s the solution
1. Unscrew two screws from the back of the machine and remove the upper cover
2. take off the corresponding slack belt
3. use the M2 hex wrench synchronous wheel black screws (if the screw does not move, can use hot air / lighter black screw heating and then unscrew the pulley assembly because, in order to prevent the screw screw screw relaxation, sticky glue, and the glue needed to remove the screws to screw in order to melt heating).
The screw type is “243 medium strength screw glue””
4. the pulley back to the factory location (belt just don’t touch the Y axis crossbeam), then special screw glue will black screw fastening
5. Tighten the black screws and wait for 5 hours to secure the screws and then tighten the belt again
6. install the upper cover