Reason:
1. Axis parallelism is not accurate.
2. Insufficient rigidity of bearings.
3. Bending of pulley retaining edge.
4. The diameter of the pulley is smaller than the width of the leather band.
Countermeasures:
1. Correct and locate the pulley.
2. Increase the rigidity of the bearing and fix it firmly.
3. Modify or replace the shield.
4. Inspection design.
