Due to various reasons, the center of mass or inertial principal axis of the slider coupling does not coincide with the axis of rotation, and the phenomenon of unbalanced centrifugal inertia force, centrifugal inertial couple force and dynamic deflection (vibration shape) will be generated during operation, which is called the phenomenon of rotor Unbalance phenomenon, this kind of unbalance phenomenon will inevitably cause the vibration of the shaft system, which will affect the normal operation and service life of the machine, so it is important to pay attention to it. The degree of unbalance (unbalance U) is usually measured by the mass m and center of mass of the rotor. The product mr of the distance r from the axis of rotation of the rotor is expressed, which is called the mass-diameter product. It is also expressed by the mass-diameter product per unit mass, which is called the eccentricity e (not geometrically eccentric.) The mass-diameter product mr is a and The relative quantity related to the rotor quality, and the eccentricity e is a quantity that has nothing to do with the rotor quality. The former is relatively intuitive and is often used for the balancing operation of a specific given rotor, and the latter is used to measure the pros and cons of the rotor balance or to detect the balance accuracy. The shaft balance and other standards are evaluated according to e. For flexible rotors, the mode eccentricity (n-order mode) en=Un/mn, Un and mn are the n-th mode and the first-order mode quality respectively.