In the practical production and manufacturing process, vibration and noise are a pair of problems that go hand in hand. In particular, the basic cause of mechanical noise is vibration, that is, to control mechanical noise should start from eliminating mechanical vibration.

The axial vibration of the end cover is one of the mechanical noise sources, which is mainly aroused by the bearing vibration, and is more important in small motors. The smaller the axial dynamic stiffness of the end cover, the greater the vibration speed and noise are likely to be aroused.

The vibration and noise of the brush device are caused by the poor appearance of the commutator, the large gap between the brush and the brush holder, the small brush pressure or improper pressure application that causes the brush to tilt, and the lack of rigidity of the brush holder, brush holder and brush rod.

When the DC motor is running, the sliding contact between the brush and the commutator, the cuprous oxide film formed on the commutator surface and the layer of graphite film and dust particles covered on the commutator not only affect the commutation performance of the motor, but also affect the vibration and noise.

It is theoretically proved that the above films are not easy to form due to the low temperature of the sliding contact surface at no load, and the dry friction between the brush and the commutator increases the noise. For example, when the no-load operation cycle of the rolling DC motor accounts for more than 50% of the total time, the noise under no-load condition is 6~10dB higher than that under load. This kind of vibration is different from the vibration caused by mechanical reasons on the commutator surface. The latter can be detected when the motor is running at low speed (touching the electric brush gently with your hand may cause vibration). The frequency spectrum of brush vibration noise generated by sliding contact is generally in the range of 1000~8000 Hz, and it does not change much when the motor speed changes, which is one of the characteristics that distinguish it from machinery.

The brush vibration noise generated by sliding contact is also related to the polarity of the brush. For example, the positive brush of the DC generator has less vibration than the negative brush; As the positive brush can separate the crystal of graphite and carbon, it absorbs water on the surface of the commutator to form a smooth film, while the negative brush can eradicate it. The composition of the sliding contact film is also related to the brand of the brush. The brand selection is first to ensure good commutation performance, but also to consider the vibration and noise of the brush.