Tham khảo tài liệu 'gas turbine engineering handbook 2 episode 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Balancing 585 done. Rotor mass unbalance from dissymmetry nonhomogeneous material distortion and eccentricity can be corrected so that the rotor can run without exerting undue forces on the bearing housings. In balancing procedures only the synchronous vibrations vibration in which the frequency is the same as the rotor rotating speed are considered. In a real rotor system the amount and location of unbalances cannot always be found. The only way to detect them is with the study of rotor vibration. Through careful operation the amount and the phase angle of vibration amplitude can be precisely recorded by electronic equipment. The relation between vibration amplitude and its generating force for an uncoupled mass station is F i 17-1 Y i 17-2 A ----- 1 - F K 17-3 17-4 where Ỹ t vibration amplitude F generating force A amplification factor Ộ phase lag between force and amplitude From Equation 17-4 one will find that the phase lag is a function of the relative rotating speed and the damping factor . See Figure 17-1. The force direction is not the same as the maximum amplitude. Thus for maximum benefit the correction weight must be applied opposite to the force direction. 586 Gas Turbine Engineering Handbook Figure 17-1. Typical phase lag between force and vibration amplitude chart. Figure 17-2. Distribution of unbalance in a rotor. Balancing 587 The existence of unbalance in a rotor system may be in continuous form or discrete form as shown in Figure 17-2. Ascertaining an exact distribution is an extremely difficult if not impossible task by today s techniques. For a perfectly balanced rotor not only should the center of gravity be located at the axis of rotation but also the inertial axis should coincide with the axis of rotation shown in Figure 17-3. This condition is almost impossible to achieve. Balancing may be defined as a procedure for adjusting the mass distribution of a rotor so that the once-per-revolution vibration motion of the journals or forces on the .
