There are many common failures of rotating machinery, including unbalance, misalignment, shaft bending and thermal bending, oil film swirl and oil film oscillation.
Unbalance
Unbalance is the most common fault in various rotating machines.
The causes of rotor unbalance are many, such as unreasonable structural design of rotor, deviation of machining quality, assembly error, uneven material, poor dynamic balance precision; change of relative position of coupling in operation; defective rotor parts, such as: operation due to corrosion, wear, media uneven scaling, shedding; rotor by fatigue stress caused by the role of rotor parts (such as impeller, blade, surrounding belt, tension bar etc.) local damage, fall off, produce fragments flying out, etc.
Misalignment
Rotor misalignment usually refers to the degree of tilt or offset of the axis line and bearing centerline of two adjacent rotors.
Rotor misalignment can be divided into coupling misalignment and bearing misalignment. Coupling misalignment can be divided into parallel misalignment, off-angle misalignment and parallel off-angle misalignment. Parallel misalignment vibrates at twice the rotor frequency. Off-angle misalignment causes a bending moment to be added to the coupling in an attempt to reduce the deflection of the centerline of the two shafts.
The direction of the bending moment changes once for each week of shaft rotation, so the misalignment increases the axial force on the rotor and causes the rotor to vibrate in the axial direction. Parallel misalignment is a combination of the above two conditions, causing the rotor to vibrate radially and axially. Bearing misalignment actually reflects the deviation of the bearing seat elevation and shaft center position.
Bearing misalignment redistributes the load on the shaft system. Bearings with higher loads may experience high harmonic vibrations, bearings with lighter loads tend to destabilize, and also cause the critical speed of the shaft system to change.
Shaft bending and thermal bending
Shaft bending means that the centerline of the rotor is in an unstraight state. Rotor bending is divided into two types of permanent bending and temporary bending.
Rotor permanent bending refers to the rotor shaft is permanently bowed, it is due to the rotor structure is not reasonable, manufacturing errors, uneven material, rotor long-term storage improper and permanent bending deformation, or hot state parking is not timely pan or pan improper, rotor thermal stability is poor, long-term operation of the shaft natural bending increase and other reasons.
The temporary bending of the rotor is caused by a large pre-load on the rotor, improper warm-up operation when starting operation, too fast speed, uneven thermal deformation of the rotor shaft, etc.
Permanent bending of the rotor and temporary bending are two different kinds of failure, but the mechanism of failure is the same. Whether the rotor is permanently bent or temporarily bent, it will generate a rotating vector excitation force similar to the mass eccentricity situation.