1. Hammer installation wrong order --- rearrange
2. Corresponding weight difference between two sets of hammer is too large --- balance weight does not exceed 5 grams
3.Individual hammer did not throw off --- make the hammer rotate flexible
4. Other parts of the rotor unbalance --- balance other parts
5. Spindle bending --- calibration or replacement of the spindle
6. Bearing damage --- replace the bearing
7. Anchor bolts loose --- tighten the anchor bolts
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