WCBF standard telescopic flange coupling does not coincide with its main axis of mass or inertia due to various reasons. During its operation, unbalanced centrifugal inertia force, centrifugal inertia couple force and dynamic deflection (vibration mode) will occur, which is called unbalanced phenomenon of rotor. This unbalanced phenomenon will inevitably cause vibration of shaft system. Therefore, the normal work and service life of the machine are affected, and attention is paid to it. The degree of unbalance (unbalance U) is usually expressed by the product M r of the mass m of the rotor and the distance from the center of mass to the axis of rotation of the rotor, which is called mass-diameter product. It is also expressed by the product of mass and diameter per unit mass, which is called eccentricity e (not eccentricity in geometric sense). The mass-diameter product MR is a relative quantity related to the mass of the rotor, while the eccentricity e is a quantity independent of the mass of the rotor. The former is more intuitive, often used for the balancing operation of a given rotor, while the latter is used to measure the balance of the rotor or to detect the balance accuracy. The balance grade standard of coupling is evaluated according to E. For flexible rotors, mode eccentricity (the nth mode) en = Un/mn is used. Un and Mn are the nth mode shape and mode mass, respectively.