The application and development of high speed machine tool require the spindle speed to be improved. But the machine tool spindle group parts in the manufacturing process, inevitably will cause the center of gravity to deviate from the center of rotation because of the uneven material, the shape asymmetry, the machining assembly error, causes the machine tool to produce the vibration and the vibration force, causes the machine tool noise, the bearing heating and so on. As the rotational speed increases, the vibration caused by the imbalance becomes more and more intense. Because the deformation of the spindle assembly of machine tool is very small, in order to simplify the calculation, it is regarded as the balance method of rigid rotor. The rotor is regarded as an absolute rigid body, and it is assumed that the rotating shaft under unbalanced centrifugal force will not deform significantly. Therefore, many complex unbalanced states of rigid rotor under these conditions can be reduced to force system imbalance, which can be balanced by adding or subtracting two dynamic equilibrium forces equivalent to Ud1, and Ud2 on any selected two planes. Rigid rotor dynamic balance is generally low speed dynamic balance, generally choose the first critical speed of less than 1/3. Unbalanced: the mass distribution state of the rotor that causes vibration or motion on the bearing due to centrifugal force. residual imbalance U： the imbalance left after equilibrium treatment. relative unbalance e： unbalance divided by the value obtained by the rotor mass, which is equal to the displacement of the centrifugal force to the center of the shaft. Equilibrium degree G： is the product of relative imbalance and specified angular velocity. Balancing treatment: the operation of adjusting the rotor mass distribution in order to keep the vibration and force acting on the bearing in sync with the rotation speed within the specified limit. Full key: a key or equivalent used for the final assembly of rotating shafts and mating parts with keyways. Half-key: The key used for balancing the rotating shaft with a keyway or for matching parts. This imbalance is equivalent to the key used in the final assembly (the imbalance buried in the keyway of the rotating shaft or the mating part).