Excessive Unbalance after Motor Magnet Bonding? Check Adhesive Distribution and Runout First

When excessive unbalance, rising vibration or abnormal noise occurs on motor rotors after magnet bonding, priority shall be given to checking magnet mass distribution, bond-line thickness, circumferential squeeze-out, rotor concentricity and cured assembly position, instead of immediately concluding insufficient structural bond strength. For robot joint motors, industrial servo motors and high-speed brushless motors, magnet bonding must deliver structural reliability while preventing mass distribution changes caused by inconsistent adhesive dosage and assembly deviation. Permanent magnet motor rotors generally consist of rotor core, magnets, shaft and required retention features. Once magnets are installed with adhesives or hybrid fastening methods, bond-line thickness and magnet placement affect rotor geometry, and local adhesive variation alters rotating mass distribution. Therefore, acceptable bonding and acceptable dynamic balance are two separate engineering judgements. Bond strength tests evaluate joint reliability, but cannot replace rotor runout, mass distribution and dynamic balance tests. 1. Why Does Unbalance Occur after Magnet Bonding? Under high-speed rotation, uneven mass distribution generates periodic centrifugal excitation. Deviations in magnet installation position, bond-line thickness or rotor geometry may shift the centre of mass away from the rotational axis and worsen vibration. During magnet assembly, inconsistent adhesive volume under individual magnets or excess adhesive squeezed and accumulated at a certain circumferential position changes cured mass distribution. For precision motors with small rotor diameter and high rated speed, even minor local mass variation must be assessed against dynamic balance requirements. Unbalance is not necessarily caused by the adhesive itself. Magnet mass tolerance, shaft eccentricity, core dimensional deviation, fixture error and bearing condition all affect test results. Phased inspection is required to locate the root cause. 2. How to Distinguish Issues from Adhesive Volume, Magnet Assembly and Rotor Geometry? It is recommended to record the baseline geometry of the rotor core before magnet mounting, inspect position and clearance after magnet…