Vibration and noise are the most common problems in motor testing and operation. There are many causes of vibration and noise, and it is not easy to distinguish them completely.

In many cases, we will attribute vibration and noise caused by vibration to the rotor dynamic balancing link, and in many cases, the exact cause cannot be found from the rotor itself.

In the final analysis, the theoretical analysis and the actual situation will exist a certain degree of variability. Only by ensuring that every link involved meets the requirements can we make a truly fine motor.

The mechanical vibration and noise of the motor include the vibration and noise generated by the imbalance on the motor rotor, the noise generated by the bearing vibration, the axial vibration and noise of the end cap generated by the bearing excitation, the frictional vibration and noise between the brush and commutator (or collector ring), etc. Today, I am involved in sharing the vibration and noise problems generated by the mechanical imbalance of the rotor.[1]

Dynamic and static imbalance of the rotor

Usually, the unbalance of the motor rotor can be divided into static unbalance, dynamic unbalance, and mixed unbalance. The centrifugal force caused by static unbalance produces vibrations of equal magnitude and phase on both supports. The force couple of centrifugal force caused by dynamic unbalance produces vibration of equal size and opposite phase on the two supports.

X-TEAM brushless motor

In practice, the most commonly encountered is mixed unbalance, which is the residual static unbalance centrifugal force and dynamic unbalance centrifugal force couples acting together on the two supports to produce unequal size and different phase vibration. The mechanical imbalance of the rotor can be eliminated by calibration balancing.

Unbalanced vibration of the rotor

Residual imbalance at the time of manufacture.

-Much dust adhesion due to a long period of operation.

-Shaft bending due to thermal stress during operation.

-Unbalanced load due to thermal displacement of rotor parts.

-Deformation or eccentricity caused by the centrifugal force of rotor parts.

-Shaft bending caused by an external force (belt, gear, poor straight junction, etc.).

-Bending of the shaft or internal deformation of the bearing caused by poor installation of the bearing (accuracy of the shaft or locking).

Suppression of rotor unevenness.

-Maintain within the allowable unevenness.

-Improvement of excessive tightness of shaft and core.

-Improvement of design for anisotropy of thermal expansion.

-Improvement of strength design or assembly.

-Correction of shaft strength design, change of shaft coupling type, and correction of the straight junction to the center.

-Prevention of misalignment between bearing end face and shaft attachment or locking nut.

Abnormal vibration and noise of bearings

Injuries inside the bearing.

-Abnormal vibration in the shaft direction of the bearing, the vibration of the vibration system consisting of the spring constant, and the rotor mass in the shaft direction.

-Friction sound: cylindrical rolling bearings or large-diameter high-speed ball bearings produce poor lubrication and bearing clearance cause.


Bearing replacement.

-Appropriate shaft direction spring preload to the bearing clearance changes.

-Choose soft grease or low-temperature excellent grease, residual clearance to make small (must pay attention to the temperature rise problem).

Rotor balancing method

In the dynamic balancing machine of the rotor for dynamic balance measurement, according to the need for the rotor can be weighted method and de-weighting method of the rotor balance processing: the so-called weighting method, that is, in the opposite direction of the imbalance with the correction of the weight block.

Commonly used methods are welding, soldering, riveting, screwing, and weighting blocks. The de-weighting method is to remove a certain amount of weight in the direction of the imbalance. Commonly used methods are boring, drilling, chiseling, milling, grinding, etc.[2]

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