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The principle of static balance of rotating parts: For disc-shaped rotating parts whose ratio of axial size to radial size is less than 0.2, such as gears, flywheels, pulleys, etc., it can be approximately considered that all its masses are distributed in the same plane of rotation. When it rotates, the centrifugal inertial force generated by these masses forms a plane intersection force system that intersects at the center of rotation. If the force system is unbalanced, its resultant force is not equal to zero. According to the equilibrium conditions of the plane intersection force system, in order to make it balanced, a balance mass mB should be added to the rotation surface (correction plane) so that the vector sum of the centrifugal inertia force FB generated by it and the centrifugal inertia force generated by the original masses mi is equal to zero, thus becoming a balanced force system and allowing the rotating parts to be balanced. Principle of dynamic balancing of rotating parts For rotating parts whose axial size to radial size ratio is greater than 0.2, such as turbine rotors, generator armatures, etc., the mass distribution can no longer be assumed to be concentrated in the same plane of rotation, but should be regarded as distributed in different planes of rotation perpendicular to the axis of rotation. When the rotary part rotates, the centrifugal inertial force generated by each eccentric mass is no longer a plane intersection force system, but a spatial force system. The unbalanced masses m1 and m2 are distributed in two planes of rotation separated by l, and the vector radius of their center of mass is r1 = -r2. When m1 = m2, m1 r1 + m2r2 = 0, which satisfies the static balance condition. The total center of mass of the rotary part falls on the axis of rotation. But when the rotating parts rotate, although the centrifugal inertial force F1=-F2 they generate, because they are located in two rotating planes separated by l, form an unbalanced inertial couple moment with a size of F1l, the rotating parts are still in an unbalanced state. We call the phenomenon of unbalanced inertial couple moment generated after the rotating part rotates called dynamic unbalance.