Plain bearings, commonly known as plane bearings, are characterized by their simple design and large contact area. When well lubricated, they exhibit excellent wear resistance and have a long service life. Plain bearings have high load-bearing capacity, high rotational accuracy, and a lubricating film that provides shock resistance. Therefore, they are widely used in engineering applications. Sliding bearings can be classified according to the different directions of load bearing: thrust sliding bearings and radial sliding bearings; according to the different principles of lubricating oil film formation: dynamic pressure sliding bearings and static pressure sliding bearings; according to the different structural forms: integral sliding bearings and split sliding bearings
1. Causes of sliding bearing failure
The main causes of sliding bearing failure include improper design and installation of the bearing shell;
Operating at excessive speed or overload, or with impurities in the lubricating oil; under high-temperature, high-speed, and high-load operating conditions, thermal expansion occurs between the shaft neck and bearing pad materials, causing the bearing clearance to disappear and direct contact between metals;
Under the influence of alternating loads, reciprocating tensile stress, compressive stress, and shear stress are generated on the bearing surface, leading to the formation of microcracks. With continuous operation, fatigue failure ultimately occurs;
Long-term operation at high amplitude leading to detachment; sub-synchronous instability caused by misalignment of the coupling, improper operation, and other factors.
2. Types of vibration faults in sliding bearings
Sliding bearings can experience various malfunctions, including excessive clearance between the bearing shell and the shaft, oil film whirl and oil film oscillation, friction, as well as common issues such as bearing shell wear, bearing burnout, and fatigue-induced cracks in the bearing shell.
3. Vibration mechanism of sliding bearing failure
There are many reasons for the vibration of sliding bearings, most of which are caused by other mechanical issues such as rotor imbalance, misalignment, and stiffness problems, which will not be repeated here. The vibration caused by the sliding bearing itself is mainly due to reduced stiffness caused by improper fit clearance, as well as oil film problems caused by improper design and installation.
Oil film whirl - The phenomenon where the wedge-shaped oil film drives the shaft to move around the center of the bearing pad at the average flow velocity of the oil is called oil film whirl. Because its average velocity is half of the circumferential velocity of the shaft neck, it is also known as half-speed whirl.
If the vortex force acting on the shaft neck is less than the oil film damping force, the trajectory formed by the shaft center vortex motion is convergent, and the vortex motion will decrease; if the vortex force equals the oil film damping force, the trajectory of the shaft center will no longer expand and become a closed figure, indicating that the vortex motion is stable; if the vortex force exceeds the damping force, the trajectory of the shaft center is divergent, and the vortex motion is unstable.
When the direction of vortex motion is the same as the direction of rotor rotation, it is called positive precession; otherwise, it is called negative precession.
Theoretical calculations indicate that the rotational frequency Ω of oil film whirl is equal to half of the rotor's rotational frequency ω, that is, Ω = ω/2. Therefore, oil film whirl is also theoretically known as half-speed whirl. In practice, the vibration frequency of oil film whirl is approximately 0.42 to 0.48 times the rotational frequency, that is, Ω = (0.42~0.48) ω.
Oil whip—As the rotor's rotational frequency ω (i.e., the speed n) continues to rise, the whirl frequency Ω of the oil film whirl also increases. When the speed n approaches twice the first critical speed nk1 of the rotor, that is, when the frequency of the oil film whirl equals the natural frequency of the rotor-bearing system, i.e., Ω = ωk1, the rotor-bearing system will undergo strong resonance, which is known as oil whip.
After the occurrence of oil film oscillation, even if the rotational speed continues to rise, the whirl frequency no longer increases according to the constant whirl ratio (Ω/ω), but remains at ωk1, which tightly clings to the natural frequency of the rotor - the first critical speed - and does not change.
Oil film whirl and oil film oscillation are a type of self-excited vibration, which means that the energy required to sustain the vibration is generated by the rotor-bearing system (including lubricating oil) during its own rotation. This system can continuously provide significant energy, independent of external factors.
Therefore, oil whipping is characterized by its severity, suddenness, and occasional intermittent howling noise.
For tilting pad bearings, which are commonly used in large units, theoretical calculations indicate that, neglecting the mass of the pads and the friction force at the fulcrum, the cross stiffness of the tilting pad bearing is zero, making it impossible for oil film whirl and oil film oscillation to occur.
Because its tile can swing freely, the oil film force can automatically adjust to pass through the axis, thereby aligning with the load, eliminating the tangential oil film component force, and fundamentally removing the driving force of whirl.
However, due to situations that often arise in practical use that do not align with design conditions, such as friction at the fulcrum, improper bearing tightness, excessive viscosity of lubricating oil, etc., oil film oscillation may also occur in tilting pad bearings.
As for other types of bearings, such as cylindrical bearings, elliptical bearings, multi-oil wedge bearings, multi-oil leaf bearings, etc., as long as they are high-speed and light-load, oil film whirl and oil film oscillation may occur.
4. Vibration fault diagnosis of sliding bearings
Factors related to vibration and sliding bearings primarily include clearance-induced vibration, thermal imbalance caused by rubbing in high-parameter equipment, and instability issues arising from oil films. Almost all other vibrations are responses of the rotor system to excitation on the bearing pedestal. For reference, other mechanical fault diagnosis methods within the platform can be consulted.
Regarding faults such as disengagement, cracks, and wear and tear on low-parameter common equipment, the vibration response is not sensitive, while oil analysis often achieves better results.
(1) When the bearing bush is not working properly or shows signs of friction, there may not be a noticeable reaction in terms of vibration, but its temperature will increase significantly. It is advisable to observe changes in the local temperature gauge, cooling water temperature, and oil temperature, as well as changes in the color of the lubricating oil.
(2) When the vibration is caused by excessive assembly clearance or looseness of the bearing top clearance, the vertical vibration will increase significantly, and the ratio of vertical amplitude to horizontal amplitude will decrease, approaching or even exceeding the horizontal amplitude. The vibration is generally more pronounced only on that bearing.
(3) If the equipment is equipped with a non-contact sensor to measure shaft vibration, bearing wear is usually accompanied by a significant increase in DC gap pressure, indicating the degree of wear of the bearing relative to the sensor position.
(4) During oil film whirl, the amplitude suddenly increases when the rotational speed reaches a certain value, and there is a significant change in the vibration amplitude when the oil temperature is altered.
(5) Oil whipping faults only occur on flexible rotors, with a vibration frequency close to half the rotational speed. For lightly loaded bearings, oil whirl may occur first before reaching this speed. For heavily loaded bearings, oil whipping may directly occur during the speed increase process.
(6) When oil film whirl or oil film oscillation occurs, increasing the lubricating oil pressure can sometimes lead to a noticeable change in vibration.
(7) Oil film oscillation speed lag phenomenon. During the speed increase process, oil film oscillation occurs when the speed exceeds the instability threshold speed. However, after the occurrence of oil film oscillation, the vibration does not decrease when the unit speed is reduced to this threshold speed. The vibration only decreases when the speed is further reduced. There is a difference between the speed at which oil film oscillation occurs and disappears during the speed increase and decrease processes, and this phenomenon is called the speed lag phenomenon
(8) Vibration exhibits dual characteristics of large amplitude and suddenness. As oil film oscillation approaches, unstable low-frequency vibration components emerge, with amplitudes appearing intermittently. Once oil film oscillation occurs, the vibration amplitude increases sharply within a short period of time (a few seconds), and the vibration amplitude is much larger than that of ordinary forced vibration.
(9) For defects such as minor cracks and detachment in bearing shells, the most effective method is still metal flaw detection conducted during machine downtime.
(10) During oil film whirl, double-ring or multi-ring shaft centerline trajectory characteristics may appear, and the vibration waveform generated by subharmonic waves exhibits a meandering characteristic.
(11) When the mating clearance is too large, the rotational frequency harmonic components are abundant and relatively significant, resembling the phenomenon of mechanical looseness. A sliding bearing with an excessively large clearance may generate a vibration spectrum with high-order harmonics due to small imbalances, misalignments, or other related forces. In this case, the bearing is not the source of the fault. However, if the bearing clearance meets the specified requirements, the vibration amplitude will not increase.
(12) During oil film whirl, the half-frequency component in the spectrum increases significantly, but its amplitude is generally less than that of the fundamental frequency. As the rotational speed increases, the relationship between the half-frequency and the fundamental frequency remains unchanged.
(13) Before oil whip, the vibration is primarily composed of fundamental frequency components. After a sudden change in vibration, the amplitude of the fundamental frequency components decreases, and the amplitude of the low-frequency components greatly exceeds that of the fundamental frequency components, becoming the dominant frequency component.
(14) When oil film oscillation occurs, the vibration will suddenly increase, and even if the speed continues to rise, the amplitude will not change. The vibration frequency is always equal to 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 basically the same.
