Diagnosis and analysis of various faults in sliding bearings! Causes, mechanisms, characteristics, and solutions of vibration

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A sliding bearing is what is commonly referred to as a plain bearing; it has a simple design and a large contact area. If lubrication is maintained properly, it exhibits excellent wear resistance and a long service life. Sliding bearings have a high load-carrying capacity and high rotational precision, and their lubrication film provides impact resistance; therefore, they are widely used in engineering. This article shares the causes of vibration faults in sliding bearings, the vibration mechanism, and vibration detection methods. Sliding bearings can be classified into thrust sliding bearings and radial sliding bearings depending on the direction of the load they bear ； Based on the different principles of lubricant film formation, they are divided into hydrodynamic sliding bearings and hydrostatic sliding bearings ； Based on their structural design, they can be divided into integral sliding bearings and split sliding bearings. 1. Causes of sliding bearing failures The main causes of sliding bearing failures include improper design and installation of the bearing bushes ； Operating at excessive speeds, with excessive loads, or with impurities in the lubricating oil ； Under operating conditions of high temperature, high speed, and high load, the materials of the journal and bearing shells undergo thermal expansion, the bearing clearance disappears, and direct contact between the metals occurs ； Under alternating loads, reciprocating tensile stresses, compressive stresses, and shear stresses are generated on the bearing surface, resulting in tiny cracks on it; over time, as the bearing continues to operate, fatigue failure eventually occurs ； Delamination caused by long-term operation at high amplitudes ； Subsynchronous instability caused by factors such as misalignment of the couplings and improper operation. 7 @# S3 k" ?' ` 2. Types of vibration faults in sliding bearings There are various manifestations of vibration faults in sliding bearings, including an excessive clearance between the bearing shells ； Oil film eddy currents and oil film oscillations, friction ； As well as common faults such as bearing wear, bearing seizure, and cracks resulting from fatigue. 3. Vibration mechanism of sliding bearing failures – There are many reasons for vibrations in sliding bearings, and most of them are caused by other mechanical problems, such as rotor imbalance, misalignment, and stiffness issues. The vibrations caused by the sliding bearing itself are primarily due to reduced stiffness resulting from improper fit clearance, as well as oil film issues arising from inadequate design and installation. Oil film whirl is a type of whirling motion caused by the oil film forces in radial sliding bearings. When the rotor journal operates stably in a sliding bearing, the oil film force R of the bearing balances the load W, and the rotor axis is at a certain equilibrium position O1. If the rotor is perturbed and moves out of its equilibrium position to point O2, the resultant force F of the altered oil film force R′ and the load W is no longer zero or collinear. The resultant force F can be broken down into two components: a radial component Fr and a tangential component. The radial component Fr acts in the opposite direction to the displacement of the journal, attempting to push the journal back to its original equilibrium position O1; it is a type of elastic restoring force ； The tangential force Fu is perpendicular to the direction of journal displacement; it drives the journal to continue rotating around the equilibrium position O1, thereby generating vortices. Such vortices are known as oil film vortices, and Fu is referred to as the vortexing force. http://p1.pstatp.com/large/3b160000b4946692d02b If the vortex force acting on the journal is less than the damping force of the oil film, then the trajectory formed by the shaft vibration is convergent, and the vibration will decrease ； If the vortex force is equal to the oil film damping force, the axis trajectory stops expanding and becomes a closed shape; the vortex motion is stable ； If the vortex force exceeds the damping force, the axis trajectory is divergent, and the vortex motion is unstable. When the direction of the vorticity is the same as the rotation direction of the rotor, it is positive precession ； Conversely, it is counter-precession. Theoretical calculations show that the rotational frequency Ω of the oil film vortex is equal to half of the rotor’s rotational frequency ω, that is, Ω = ω/2; therefore, oil film vortices are theoretically also known as half-speed vortices. In practice, the vibration frequency of oil film vortexing is approximately 0.42~0.48 times the rotational frequency, that is, Ω = (0.42~0.48)ω. As the rotor’s rotation frequency ω (i.e., the speed n) increases, the vortex frequency Ω of the oil film vortexes also increases. When the speed n reaches around twice the first critical speed nk1 of the rotor, that is, when the frequency of the oil film vortexes equals the natural frequency of the rotor bearing system, i.e., when Ω = ωk1, the rotor bearing system experiences severe resonance – this is what is referred to as oil film oscillation. After oil film oscillation occurs, even as the rotational speed continues to increase, the vortex frequency no longer rises according to the constant vortex ratio (Ω/ω); it remains at ωk1, thus remaining tightly fixed at the rotor’s natural frequency—the first critical speed—and does not change. Oil film eddy currents and oil film oscillations are a type of self-excited vibration; in other words, the energy that maintains these vibrations is generated by the rotor bearing system (including the lubricating oil) as it rotates, and this system can continuously supply a large amount of energy, independent of external influences. Therefore, oil film oscillation also features severity, suddenness, and sometimes intermittent roaring sounds. For tilting-pad bearings, which are widely used in large-scale units, theoretical calculations show that, when the mass of the pad elements and the friction at the support points are ignored, the cross-stiffness of such bearings is zero; as a result, oil film vortices and oil film oscillations cannot occur. Since its tiles can swing freely, the oil film force can be automatically adjusted to pass through the axis, thereby aligning with the load and eliminating the tangential component of the oil film force, thus fundamentally removing the driving force behind vortexing. However, since in actual use certain conditions may arise that do not conform to the design specifications, such as friction at the pivot points, improper bearing tension, or excessive lubricant viscosity, oil film oscillation can also occur in tilting pad bearings. As for other types of bearings, such as cylindrical bearing shells, elliptical bearing shells, multi-oil- wedge bearings, and multi-foil bearings, oil film whirling and oil film oscillation can occur in them as long as they are used under high-speed and light-load conditions. 4. Vibration fault diagnosis of sliding bearings: The factors related to sliding bearings themselves include clearance vibration, thermal imbalance caused by rubbing in high-parameter equipment, and instability issues arising from the oil film. Most other types of vibration are essentially responses of the rotor system to disturbances, and for such cases, other mechanical fault diagnosis methods available within the platform can be referred to. As for faults such as delamination, cracks, and wear caused by ordinary equipment with low parameters, these do not show significant responses in terms of vibration, while oil analysis often yields good results. Vibration fault diagnosis methods for sliding bearings: 1. When the bearing shells are not functioning properly or show signs of friction, there may be no obvious vibration effects, but the temperature of these bearing shells will increase significantly. It is possible to monitor changes in the temperature indicated by on-site thermometers, as well as the temperature of the cooling water and oil, and also to observe any changes in the color of the lubricating oil. 2. When the vibration is caused by an excessive or loose bearing clearance, the vertical vibration will increase significantly, and the ratio to the horizontal amplitude will decrease, approaching or even exceeding the horizontal amplitude. Vibration is generally more noticeable only on that bearing. 3. If the equipment is equipped with contactless sensors for measuring shaft vibration, bearing wear is usually accompanied by a significant increase in the DC gap voltage, which indicates the degree of wear of the bearing relative to the position of the sensor. 4. When the oil film vibrates, its amplitude increases suddenly at a certain rotational speed, and there is a significant change in the vibration amplitude as the oil temperature changes. 5. Oil film oscillation faults occur only on flexible rotors, with their vibration frequency being close to half the rotational speed. At speeds below this value, light-load bearings may first experience oil film whirling. Heavy-duty bearings may experience oil film oscillation directly during the acceleration process. 6. When oil film vortices or oil film oscillations occur, increasing the lubricating oil pressure can sometimes result in a noticeable change in the vibrations. 7. Delay in the rotational speed of oil film oscillations. During the speed-up process, oil film oscillation occurs once the rotational speed exceeds the instability threshold speed. However, after oil film oscillation occurs and the unit slows down to that threshold speed, the vibration does not decrease. Vibration will only decrease when the rotational speed is further reduced. During acceleration and deceleration, there is a difference in speed between the occurrence and disappearance of oil film oscillations; this phenomenon is known as speed lag. 8. Vibration has the dual characteristics of large amplitude and suddenness. As oil film oscillation approaches, unstable low-frequency vibration components appear, with their amplitude fluctuating intermittently. Once oil film oscillation occurs, the vibration amplitude increases sharply within a short period of time (several seconds), and this amplitude is much larger than that of ordinary forced vibrations. 9. For defects such as minor cracks and delamination in bearing shells, the most effective method is still metal flaw detection performed during shutdown. 10. When the oil film vibrates, axisymmetric trajectory patterns of two or more rings may appear, and the vibration waveforms resulting from harmonics all exhibit a sinuous characteristic. 11. When the clearance is too large, the frequency conversion harmonic components are abundant and quite significant, similar to the phenomenon of mechanical looseness. Sliding bearings with excessive clearance may cause small imbalances, misalignments, or other related forces to generate a vibration spectrum with higher harmonic frequencies. In this case, the bearing is not the source of the failure. However, if the bearing clearance meets the specified requirements, the vibration amplitude will not increase. 12. When the oil film vibrates, the semi-frequency component in the spectrum increases significantly, but its amplitude is generally less than that of the fundamental frequency; as the rotation speed increases, the relationship between the semi-frequency and the fundamental frequency remains unchanged. 13. Before oil film oscillation, the vibration is dominated by the power frequency component. After the vibration disturbance, the amplitude of the power-frequency component decreases, while the amplitude of the low-frequency component **exceeds that of the power-frequency component, becoming the dominant frequency component. 14. When oil film oscillation occurs, the vibration amplitude increases suddenly, and this remains unchanged even if the rotational speed is increased further. The vibration frequency always equals the natural frequency of the rotor system and does not change with the speed. The vibration phases of the bearings at both ends of the rotor are essentially the same.