According to the laws of dynamic motion, all objects generate centrifugal force when they rotate.


Figure 1 Centrifugal force caused by rotor imbalance mass

As shown in Fig. 1(a), if the rotor rotates at a constant speed around the axis and the angular velocity is ω, the random point A1 in which it is studied, as shown in the following figure, can be expressed as follows:

This centrifugal force is generated at a random mass in the rotor body as the rotor rotates. If the mass distribution of the entire rotor is completely uniform, there must be one corresponding particle. We set it to A2 (pictured). The centrifugal force generated by A2 and the centrifugal force of A1 are a pair of balancing forces, and their effect on the whole rotor. Zero. It can be inferred that the centrifugal force generated by the rotor with uniform mass distribution constitutes a balance force system, and dynamic pressure is not easily generated for the support.
If the rotor is rotating at a variable speed, the angular acceleration is assumed to be ε, as shown in Figure 1(b). Then the inertial force generated by the mass point A1 is:

and so

Similarly, the symmetry point of A1 will also be

It can be seen that when the rotor performs the shifting motion, the symmetrical mass point produces one force couple. This force is not easy to generate dynamic pressure. Therefore, regardless of whether the rotor is rotated at a constant speed or a variable speed, as long as the mass of the rotor is completely symmetrical with respect to the axis of rotation, dynamic pressure is not easily generated. We refer to such a rotor as a balanced rotor, otherwise it is an unbalanced rotor.