Comparison of rigid and flexible rotors

2022-06-20 0 By

The rotor is divided into rigid rotor and flexible rotor.When the working speed is lower than the first order critical speed, the unbalanced centrifugal force is small, and the deflection caused by it is small and can be ignored, which is called the rigid rotor.On the contrary, the rotor whose working speed is higher than the first-order critical speed and deflection deformation cannot be ignored is called flexible rotor.The rotors of steam turbines and generators are basically flexible rotors, while auxiliary machines of power plants, such as fans, pumps and motors, are mostly rigid rotors.In the manufacturing sector, rigid rotors need only be balanced on low-speed balancing machines, flexible rotors need to be balanced on high-speed balancing machines.Both rigid and flexible rotors can be balanced in the field, but due to different balancing principles, the balancing methods are also different.The similarities are as follows: 1) Both rigid and flexible rotors achieve balance by reducing 1X vibration.So their measurement methods are basically the same, that is, they are all using the method of phase balance.The calculation method is the same, that is, the position of the corrected mass is calculated by the original vibration and the vibration after the trial mass addition.2) In the balancing process of these two types of rotors, unilateral weighting, symmetric weighting and anti-symmetric weighting are generally adopted.3) The orthogonal method is adopted in the balance.For rigid rotors, symmetric and antisymmetric weighting are used to realize the orthogonality of first-order rigidity and second-order rigidity.For flexible rotors, it is to achieve the orthogonality of flexible first order and flexible second order.The difference is that the balance of rigid rotor aims at eliminating static unbalance and coupling unbalance, while the balance of flexible rotor aims at eliminating mode unbalance.Due to the different balancing principles, the differences reflected in the balancing process are as follows: 1) the balance of rigid rotor needs only two calibration fronts, while the balance of flexible rotor needs N or N+2 calibration fronts (N is the highest order to correct the vibration mode imbalance).2) Rigid rotors need to balance only at one speed, while flexible rotors need to balance at several speeds.3) The rigid rotor balance has no lag Angle, while the flexible rotor has a lag Angle of 0~180 (Table 1).Table 1 Comparison of the two types of rotors It should be noted that “rigid” and “flexible” are only relative to the operating speed of the rotor.A rotor at low speed can be called rigid rotor, and at high speed can be called flexible rotor.The balance of steam turbine and generator rotor on low speed balance platform is called the balance of rigid rotor, and the balance method is based on the balance principle of rigid rotor.And the balance in the high-speed balancing machine or the field is based on the balance principle of the flexible rotor.