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There are many common faults of rotating machinery, including unbalance, misalignment, shaft bending and thermal bending, oil film whirl and oil film oscillation, steam excitation, mechanical looseness, broken rotor blades and falling off, friction, shaft cracks, rotational stall and surge, mechanical deviation and electrical deviation, etc. 1 Unbalance Unbalance is the most common fault in various rotating machinery. There are many reasons for rotor imbalance, such as unreasonable rotor structural design, machining quality deviation, assembly error, uneven material, and poor dynamic balancing accuracy. ； Changes in relative position of couplings during operation ； Defective rotor parts, such as: Due to corrosion, wear, uneven medium scaling and shedding during operation, ； The rotor is subject to fatigue stress, causing parts of the rotor (such as impellers, blades, shrouds, ties, etc.) to be partially damaged and fall off, causing pieces to fly out, etc. 2 Misalignment Rotor misalignment usually refers to the degree of inclination or offset between the axis center lines of two adjacent rotors and the bearing center line. Rotor misalignment can be divided into coupling misalignment and bearing misalignment. Coupling misalignment can be divided into three situations: parallel misalignment, angular misalignment and parallel angular misalignment. When parallel misalignment occurs, the vibration frequency is twice the power frequency of the rotor. Angle misalignment adds a bending moment to the coupling in an attempt to reduce the deflection angle of the center lines of the two shafts. Every time the shaft rotates once, the direction of the bending moment alternates. Therefore, the misalignment of the deflection angle increases the axial force of the rotor, causing the rotor to produce power-frequency vibration in the axial direction. Parallel declination 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 the shaft center position. Bearing misalignment redistributes the load on the shaft system. Bearings with larger loads may experience high-order harmonic vibrations, while bearings with lighter loads may become unstable, and may also change the critical speed of the shaft system. 3-axis bending and thermal bending Axis bending means that the center line of the rotor is not straight. Rotor bending is divided into two types: permanent bending and temporary bending. Permanent bending of the rotor means that the shaft of the rotor has a permanent bow shape. It is caused by unreasonable rotor structure, large manufacturing errors, uneven materials, permanent bending deformation of the rotor due to improper long-term storage, failure to turn the wheel in time or improper turning during hot parking, poor thermal stability of the rotor, natural bending of the axle after long-term operation, etc. Temporary bending of the rotor refers to the large preload on the rotor, improper warm-up operation during startup and operation, excessive speed increase, uneven thermal deformation of the rotating shaft, etc. Permanent bending and temporary bending of the rotor are two different types of failures, but their failure mechanisms are the same. Regardless of permanent or temporary bending of the rotor, a rotation vector excitation force similar to mass eccentricity will be generated. 4. Oil film whirling and oil film oscillation. Oil film whirling and oil film oscillation are self-excited vibrations in sliding bearings caused by the dynamic characteristics of the oil film. Oil film whirling is generally caused by excessive bearing wear or clearance, inappropriate bearing design, changes in lubricating oil parameters and other factors. It is easy to identify oil film whirl based on the vibration spectrum. The vibration frequency when it occurs is close to half of the rotational speed frequency. As the rotational speed increases, the ratio of the fault characteristic frequency of oil film whirl to the rotational speed frequency remains at a constant value, which is often called half-speed whirl. Oil film vortex and oil film oscillation are two different concepts. They are both different and closely related. Oil film oscillation occurs when oil film whirl occurs in the machine and the oil film whirl frequency is equal to the natural frequency of the system. Oil film oscillation may only occur when the machine operating speed is greater than twice the critical speed of the rotor. When the speed rises to twice the critical speed, the whirl frequency is very close to the critical speed of the rotor, so resonance occurs and causes large vibrations. Usually once oil film oscillation occurs, no matter how high the speed continues to rise, the whirl frequency will always remain at the first-order critical speed frequency of the rotor. When oil film oscillation occurs in the rotor, it generally has the following characteristics:: ①The time waveform is distorted and appears as an irregular periodic signal, usually a low-frequency signal with a large amplitude superimposed on the power frequency waveform. ； ②In the spectrum diagram, the amplitude of the frequency component at the rotor’s natural frequency ω0 is the most prominent. ； ③Oil film oscillation occurs when the operating speed is greater than twice the first-order critical speed. After that, even if the operating speed continues to increase, the characteristic frequency of the oscillation basically remains unchanged. ； ④The occurrence and disappearance of oil film oscillation is sudden and has an inertia effect. That is to say, the speed at which oil film oscillation occurs during acceleration is higher than the speed at which oil film oscillation disappears during deceleration. ； ⑤When the oil film oscillates, the whirl direction of the rotor is the same as the direction of rotation of the rotor, which is positive precession. ； ⑥When the oil film oscillates violently, the oscillation stops as the oil film is destroyed. After the oil film recovers, the oscillation occurs again. If this continues, the journal and the bearing will continue to rub against each other, causing impact sounds, and the oil film pressure in the bearing will fluctuate greatly. ； ⑦When the oil film oscillates, its axis trajectory is irregular and divergent. If friction occurs, the axis trajectory will be petal-shaped. ； ⑧The smaller the bearing load or the smaller the eccentricity, the easier it is for oil film oscillation to occur ； ⑨When the oil film oscillates, the vibration phases of the bearings at both ends of the rotor are basically the same. 5. Steam excitation There are usually two reasons for steam excitation. First, due to the opening sequence of the regulating valve, the high-pressure steam generates a force that lifts the rotor upward, thereby reducing the bearing specific pressure and causing the bearing to become unstable. ； Second, due to the uneven radial clearance of the blade tip, the tangential component force is generated, and the tangential component force generated when the gas flows in the end shaft seal causes the rotor to produce self-excited vibration. Steam-induced vibration generally occurs on the high-pressure rotor of a high-power steam turbine. When steam oscillation occurs, the main characteristic of the vibration is that the vibration is very sensitive to the load, and the frequency of the vibration coincides with the first-order critical speed frequency of the rotor. In most cases (steam excitation is not too severe) the vibration frequency is dominated by the half-frequency component. When steam oscillation occurs, sometimes it is useless to change the bearing design. The problem can only be solved by improving the design of the steam seal flow part, adjusting the installation gap, significantly reducing the load, or changing the opening sequence of the main steam inlet regulating valve. 6 Mechanical looseness There are usually three types of mechanical looseness. The first type of looseness refers to the structural looseness of the base, platen and foundation of the machine, or insufficient cement grouting and deformation of the structure or foundation. The second type of looseness is mainly caused by loosening of the fixing bolts of the machine base or cracks in the bearing seat. The third type of looseness is caused by improper fit between components. The looseness at this time is usually the looseness of the bearing pad pillow in the bearing cap, excessive bearing clearance, or looseness of the impeller on the rotating shaft. This loose vibration phase is very unstable and varies widely. The vibration during loosening is directional. In the loosening direction, the vibration amplitude will increase due to the decrease of the binding force. 7 Broken rotor blades and falling off The fault mechanism of broken rotor blades, parts or scaling layer falling off is the same as that of dynamic balancing failure. Its characteristics are as follows: ①The frequency amplitude of the vibration suddenly increases in an instant ； ②The characteristic frequency of vibration is the operating frequency of the rotor ； ③The phase of power frequency vibration will also undergo sudden changes. 8 Friction When the rotating parts of the rotating machine come into contact with the fixed parts, radial friction or axial friction between the dynamic and static parts will occur. This is a serious malfunction that may result in complete damage to the machine. When friction occurs, it is usually divided into two situations:: The first is partial friction, in which the rotor only accidentally contacts the stationary part, while maintaining contact only for a fraction of the entire precession cycle of the rotor. This is usually relatively less destructive and dangerous for the entire machine. ； The second type, which is more serious for the destructive effects and dangers of the machine, is the entire circumferential annular friction, sometimes also called "total friction" or "dry friction", which are mostly produced in seals. When annular friction occurs throughout the circumference, the rotor maintains continuous contact with the seal. The friction generated at the contact can cause a drastic change in the precession direction of the rotor, from forward forward precession to backward reverse precession. Friction is very harmful, and even short-term friction between the rotating shaft and the bearing bush will cause serious consequences. 9-axis crack Rotor cracks are mostly caused by fatigue damage. If the rotor of a rotating machinery is improperly designed (including improper material selection or unreasonable structure) or processing method, or it is an old unit that has been running for a long time, micro-cracks will occur at the location of the rotor's original induction point due to stress corrosion, fatigue, creep, etc. In addition, due to the continuous action of large and changing torque and radial load, the micro-cracks will gradually expand and eventually develop into macro-cracks. The original induction point usually appears in places where the stress is high and the material is defective, such as stress concentration points on the shaft, tool marks and scratches left during processing, and parts with minor defects in the material (such as slag inclusions, etc.). In the early stages of cracks in the rotor, their expansion speed is relatively slow and the amplitude of radial vibration is relatively small. However, the crack expansion speed will accelerate as the crack depth deepens, and correspondingly, the amplitude will increase rapidly. In particular, the rapid rise of the second-octave frequency amplitude and its phase changes can often provide diagnostic information for cracks. Therefore, the changing trends of the second-octave frequency amplitude and phase can be used to diagnose rotor cracks. 10 Rotating Stall and Surge Rotating stall is the most common instability phenomenon in compressors. When the compressor flow rate decreases, due to the increase in attack angle, boundary layer separation will occur on the back of the blade cascade, and the flow channel will be partially or completely blocked. In this way, the stall zone will propagate in the opposite direction of the cascade motion at a certain speed. Experiments show that the relative speed in the stall zone is lower than the absolute speed of the blade cascade rotation. Therefore, we can observe that the stall zone moves along the rotation direction of the rotor at a speed lower than the power frequency, so the rotational motion of the separation zone relative to the blade cascade is called rotational stall. Rotational stall worsens the flow conditions in the compressor, reduces the pressure ratio, and causes flow and pressure fluctuations over time. At a certain speed, when the inlet flow rate decreases to a certain value, the unit will produce a strong rotational stall. Strong rotational stall will further cause a more dangerous unstable aerodynamic phenomenon in the entire compressor unit system, namely surge. In addition, the compressor blades are subject to a periodic excitation force during rotational stall. If the frequency of rotational stall coincides with the natural frequency of the blades, strong vibration will be caused, causing fatigue damage to the blades and causing accidents. Severe rotational stall can lead to surge, but the two are not the same thing. Surge is not only related to the gas flow inside the compressor, but also closely related to the working characteristics of the pipeline network system connected to it. The compressor always works in conjunction with the pipe network. In order to ensure a certain flow rate through the pipe network, a certain pressure must be maintained to overcome the resistance of the pipe network. The outlet pressure of the unit during normal operation is balanced with the pipe network resistance. But when the flow rate of the compressor decreases to a certain value, the outlet pressure will drop quickly. However, due to the large capacity of the pipe network, the pressure in the pipe network does not decrease immediately. Therefore, the gas pressure in the pipe network is greater than the outlet pressure of the compressor. Therefore, the gas in the pipe network flows back to the compressor until the pressure in the pipe network drops below the compressor outlet pressure. At this time, the compressor begins to supply air to the pipeline network again, and the flow rate of the compressor increases, returning to normal working condition. But when the pressure in the pipe network returns to the original pressure, the flow rate of the compressor decreases again, and the fluid in the system flows back again. This cycle produces strong low-frequency pulsation of gas - surge. Identification characteristics of surge faults: ①The objects that cause surge faults are gas compressor units or other gas-powered machinery with long pipelines and containers. ； ②When surge occurs, the inlet flow of the unit is less than the minimum flow at the corresponding speed. ； ③During surge, the amplitude of vibration will fluctuate greatly. ； ④During surge, the characteristic frequency of vibration is generally within 1 to 15 Hz. ； Inversely proportional to the volume of the pipe network and container connected behind the compressor ； ⑤The unit, its attached pipes and other attachments, as well as the ground, all undergo strong vibrations. ； ⑥The outlet pressure fluctuates greatly ； ⑦The flow rate of the compressor fluctuates greatly. ； ⑧The motor current of a motor-driven compressor unit changes periodically. ； ⑨Surge is accompanied by periodic roaring sounds, and the size of the roaring sound is proportional to the molecular weight and compression ratio of the compressed gas. 11 Mechanical deviation and electrical deviation The reason why mechanical deviation and electrical deviation occur in vibration signals is determined by the working principle of non-contact eddy current sensors. Cutting imperfectly machined shaft surfaces (elliptical or non-axial) produces an indication of sinusoidal dynamic motion with a frequency consistent with the rotational frequency of the rotating component. The cause of an imperfect cutting surface is usually due to worn bearings on the last machine being machined, blunt cutting tools, feeds that are too fast or other defects in the machine, or wear on the lathe's ejector pin. Unsmoothness or other defects on the journal surface, such as scratches, pits, burrs, rust scars, etc., will also produce deviation output. The simplest way to check this error state is to use a dial indicator to check the runout value of the journal. The fluctuation value of the dial indicator will confirm the presence of errors on the measured surface observed by the non-contact eddy current sensor. The measured surface of the journal should be carefully protected like the journal surface of the sliding bearing. When hoisting, the cable used should avoid the surface area measured by the sensor. The support frame for storing the rotor should ensure that it will not cause scratches, dents, etc. on the journal surface. Generally speaking, eddy current sensors work satisfactorily in the presence of a magnetic field as long as the magnetic field is uniform or symmetrical. If a certain surface area on the shaft is highly magnetic, while the rest of the surface is non-magnetic or has only very low magnetism, electrical misalignment may occur. This is because when the magnetic field from the eddy current sensor acts on the journal surface, it causes a change in sensor sensitivity. In addition, uneven coating and uneven rotor materials can also cause electrical deviations, which cannot be measured and confirmed with a dial indicator.