Due to various causes of the coupling or centroid principal axis of inertia and rotor misalignment, in operation will produce the unbalanced centrifugal inertia force, centrifugal force and inertia coupling dynamic deflection (mode) of the phenomenon, known as the rotor imbalance, the imbalance will cause vibration of shaft system, thus the normal work the influence of the machine and service life, so the attention should be paid to. The degree of unbalance (unbalance quantity U) is usually expressed as the product of the mass of the rotor m and the centroid of the rotor to the axis of rotation of the rotor. The product MR is called the product of R. It is also expressed in terms of the product of the mass of the unit mass, called the eccentricity e (not the geometric eccentricity) The mass product MR is a relative quantity related to the rotor mass, whereas the eccentricity e is an absolute quantity independent of the rotor mass. The former is more intuitive, often used in specific given rotor balancing operations, the latter used to measure the balance of the rotor or check the balance accuracy, the balance of the coupling level standards, according to e to assess. For flexible rotors, the vibration mode, eccentricity (n order of vibration), en=Un/mn, Un and Mn are the order n modes and the modal mass respectively.
In order to correct the imbalance or minimize the amount of coupling, should choose the appropriate balance grade according to the needs, and in the manufacture of products completed and installed in the machine, in the specified equilibrium coupling (correction) plane, by increasing or decreasing the quality of the appropriate method, so as to achieve a balanced level. This process is referred to as balance correction, referred to as balance.