Types of rotor imbalance
Rotor unbalance can be divided into initial unbalance, gradual unbalance and sudden unbalance according to the occurrence process.
Rotor unbalance can be divided into static unbalance, even unbalance, and dynamic unbalance according to the position of the center of gravity.
1) The initial imbalance of the rotor
Initial unbalance refers to the amount of unbalance that exists on the rotor before balancing. It is caused by manufacturing errors, assembly errors, uneven materials, and unbalanced components themselves. The vibration amplitude is relatively large during the initial trial operation of the equipment, and the assembly error generally refers to the excessive clearance fit.
2) The gradual imbalance of the rotor
The gradual imbalance of the rotor is caused by the non-uniform fouling, corrosion, erosion, and wear of the rotor caused by the medium to the rotor. It is manifested in that the vibration value gradually increases slowly with the extension of the running time.
3) Sudden imbalance of the rotor
The sudden unbalance of the rotor is caused by the parts on the rotor falling off or the foreign matter enters and stuck or adheres, etc., which is manifested by a sudden and significant increase in the vibration value and approximately staying at a new level higher than the original vibration value. superior.
4) Static imbalance of the rotor
The static imbalance of the rotor refers to the imbalance in which the mass center line of the rotor deviates from the rotation center line and the two lines are parallel.
5) Even unbalance of the rotor
The even unbalance of the rotor refers to the unbalance in which the unbalanced masses at both ends of the rotor are 180 degrees out of phase, and the mass center line of the rotor and the rotation center line intersect at the center of gravity.
6) Dynamic unbalance of the rotor
The dynamic unbalance of the rotor refers to the random combination of the static unbalance and the even unbalance of the rotor, and the unbalance in which the center line of mass and the center line of rotation has no intersection in space.
The vast majority of actual rotors are dynamic imbalances with both static imbalance and even unbalance.
Dynamic unbalance is mainly reflected in the maximum amplitude at one frequency multiplication, and normal amplitude at other frequency multiplications.
Characteristics of rotor unbalance
1) Typical characteristics of vibration frequency spectrum: The problem of unbalance is usually higher frequency vibration dominates, and its frequency vibration component is generally greater than or equal to more than 80% of its pass frequency vibration.
2) The unbalanced force has a certain directionality. The centrifugal force is basically uniform in the radial direction. The motion track of the shaft and the supporting bearing is approximately a circle. However, because the vertical support stiffness of the bearing seat is greater than the horizontal direction, the normal shaft and The trajectory of the support bearing is elliptical, that is, under normal circumstances, the horizontal vibration is 1.5 to 2 times larger than the vertical vibration. If it exceeds this range, there may be other problems, especially resonance problems.
3) Comparing radial and axial vibrations, when the imbalance is dominant, the radial vibration (horizontal and vertical) is much larger than the axial vibration (except for cantilever rotors).
4) The directionality of the unbalance problem of the cantilever rotor. Under normal circumstances, the radial and axial vibrations are relatively large. It is the simultaneous existence of static unbalance and couple unbalance, so two planes are usually required for balance correction.
5) For the rotor with unbalanced vibration problem, its vibration phase is stable and repeatable.
6) The unbalance problem will increase the resonance amplitude value. If the working speed of the rotor is closer to the resonance point at the natural frequency of the system, a small amount of unbalanced vibration will increase by 10 to 50 times.
7) The phase performance of the rotor imbalance problem, the phase difference measured in the horizontal direction of the rotor input and output end bearings is basically equal to the phase difference measured in the vertical direction of the rotor input and output end bearings (+/-30°), otherwise The main problem is not dynamic balance.
