Causes and solutions of shaft bending and thermal bending, shaft cracks, mechanical looseness/deviation and other faults

There are many common faults of rotating machinery, including imbalance, misalignment, shaft bending and thermal bending, oil film whirl and oil film oscillation, steam excitation, mechanical loosening, rotor blade breakage and falling off, friction, shaft crack, rotating stall and surge, mechanical deviation and electrical deviation.
out-off-balance
Unbalance is the most common fault in all kinds of rotating machinery.
There are many reasons for the unbalance of the rotor, such as unreasonable structure design of the rotor, machining quality deviation, assembly error, uneven material, and poor dynamic balance accuracy; Change of relative position of coupling during operation; Rotor parts are damaged, such as corrosion, wear, uneven scaling and falling off of medium during operation; The rotor is subject to fatigue stress, causing local damage and falling off of rotor parts (such as impeller, blade, shroud, lacing wire, etc.), resulting in flying out of fragments, etc.
Misalignment
Rotor misalignment usually refers to the inclination or offset degree between the shaft centerline of two adjacent rotors and the bearing centerline.
Rotor misalignment can be divided into coupling misalignment and bearing misalignment. The coupling misalignment can be divided into three situations: parallel misalignment, deflection angle misalignment and parallel deflection angle misalignment. The vibration frequency is twice the power frequency of the rotor in case of parallel misalignment. The misalignment of the deflection angle makes the coupling add a bending moment to try to reduce the deflection angle of the center lines of the two shafts.
Every time the shaft rotates, the direction of action of the bending moment changes once. Therefore, misalignment of the deflection angle increases the axial force of the rotor, causing power frequency vibration of the rotor in the axial direction. The misalignment of parallel deflection angle is the combination of the above two conditions, which causes radial and axial vibration of the rotor. Bearing misalignment actually reflects the deviation of bearing pedestal elevation and shaft center position.
Bearing misalignment redistributes the load of shafting. High order harmonic vibration may occur in bearings with large load, and the bearing with light load is easy to lose stability, which also changes the critical speed of the shafting.
Shaft bending and thermal bending
Shaft bending means that the centerline of the rotor is not straight. Rotor bending is divided into permanent bending and temporary bending.
Permanent bending of rotor refers to the permanent bow shape of rotor shaft, which is caused by unreasonable rotor structure, large manufacturing error, uneven material, improper long-term storage of rotor and permanent bending deformation, or failure to timely turn or improper turning during hot shutdown, poor thermal stability of rotor, and increased natural bending of shaft after long-term operation.
The temporary bending of the rotor is caused by large preload on the rotor, improper warm-up operation during startup, excessive speed rise, uneven thermal deformation of the shaft, etc.
Permanent bending and temporary bending of rotor are two different faults, but their fault mechanisms are the same. Whether the rotor is permanently bent or temporarily bent, it will generate a rotating vector exciting force similar to the mass eccentricity.
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Source: Xianji.com
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