What are the causes of oil film oscillation and oil film vortexing in sliding bearings, and how can they be resolved? ?

Oil film oscillation is generally affected by factors such as excessively high or low oil temperature, load changes, the viscosity of the lubricating oil, and the circumferential speed of the shaft journals

Oil film whirling is a subsynchronous vibration phenomenon in which the center of the rotor rotates around the center of the bearing. The main cause of oil film eddy currents and oil film oscillations is subsynchronous oscillation. This oscillation often occurs suddenly at a certain speed (above the critical speed). The solution is to ensure that the compressor’s speed remains below its critical speed. This is just my personal opinion, for reference only! This post was last edited by zhangya*ong on 2008-12-8 23:49]

Under normal circumstances, it will disappear once that speed is reached. Also, as mentioned above, factors such as your oil temperature being too high or too low, changes in load, the viscosity of the lubricating oil, and the circumferential speed of the shaft journals have an impact

There are many factors that affect the oil film, such as: 1 speed, 2 bearing load, 3 viscosity of the oil, 4 clearance between the journal and the shaft, 5 dimensions of the journal and the shaft, 6 oil temperature, 7 oil pressure, and 8 the diameter of the oil inlet holes in the bearings.

There are many factors that affect the oil film, such as: 1 speed, 2 bearing load, 3 viscosity of the oil, 4 clearance between the journal and the shaft, 5 dimensions of the journal and the shaft, 6 oil temperature, 7 oil pressure, and 8 the diameter of the oil inlet holes in the bearings.

In simple terms, oil film vibration is caused by the instability of the oil film; its vibration frequency is generally 0.4 to 0.6 times the fundamental frequency, but in actual observations on-site, frequencies of 0.3 to 0.7 times the fundamental frequency or even higher have also been observed； As mentioned above, there are many factors that affect the oil film, such as: 1. speed, 2. bearing load, 3. viscosity of the oil, 4. clearance between the journal and the shaft, 5. dimensions of the journal and the shaft, 6. temperature of the oil, 7. oil pressure, 8. diameter of the oil inlet holes in the bearings, etc. The width of the bearing bushes also plays a role. In addition to the impact of lubricant quality, the load on the bearing shells also plays a role; low loads on the bearing shells tend to cause oil film vortices, while excessive vibrations can also lead to oil film vortices ； Oil film oscillation is caused by resonance that occurs when the frequency of oil film vortices matches the rotor’s critical speed; therefore, the operating speed of such a rotor must be above its first-order critical speed. A characteristic of this phenomenon is that at startup, as the speed increases, a low-frequency vortexing frequency appears, and once a certain speed is reached, this vortexing frequency remains constant and does not increase further as the speed rises. In other words, the vibration frequency corresponds to the critical speed. Even if the speed is reduced below this frequency, the vibrations do not disappear immediately – it is necessary to reduce the speed even further for the vibrations to cease. When the oil film oscillates, the rotor becomes completely unstable, and a serious accident can occur at any moment; therefore, it is necessary to prevent this in advance ； In fact, all vibrations occur as the axis rotates around its center of rotation, and this center of rotation is not necessarily the center of the bearing shells; after all, it is well known that the axis is eccentric during operation ； The method to eliminate oil film oscillations is certainly not to operate the rotor below its critical speed; the operating speed is predetermined during design, and it’s not possible to change it to that of a rigid rotor ； It is possible to determine whether the axis is too high by looking at the axis position diagram, and this allows one to assess whether the bearing is under low load ； To address oil film vibration, one approach is to check the quality of the oil to ensure it meets the required standards and to keep the oil temperature within the designed range; another method is to increase the load on the bearing shells, for example by raising them or increasing the shaft-diameter ratio of the bearing shells.

Oil film vortexing: The phenomenon in which the wedge-shaped oil film moves around the center of the bearing shell at the average flow velocity of the oil is known as oil film vortexing. Since this average velocity is half of the circumferential velocity of the shaft journal, it is also referred to as half-speed vortexing. 　　Mechanism: When an oil-lubricated sliding bearing is in operation, the shaft journal is supported by a thin oil film. The oil film velocity on the surface of the bearing shell is zero (the bearing shell is stationary), while the oil film velocity on the surface of the shaft journal is the same as that of the shaft journal surface (the shaft journal rotates at high speed). Therefore, at any section on the circumference, the average velocity of the oil film is half of the circumferential velocity of the journal. 　　When the journal rotates at high speed, the thickness of the oil film changes due to the wedge effect, but the average flow velocity of the oil remains relatively constant. Due to the incompressibility of the oil, the excess oil will flow out from both ends of the bearing, or the wedge shape of the oil film will move around the center of the bearing bush at the average flow speed of the oil. 　　How to diagnose vibrations caused by oil film oscillation? 　　The diagnosis of oil film vibration can be determined from the following vibration characteristics: (1) The characteristic frequency of oil film vibration is slightly less than half of the rotor speed, and it increases as the speed rises; it is often accompanied by a first harmonic ； 　　(2) The vibration is relatively stable, and the amplitude of the harmonics increases as the operating speed rises ； 　　(3) Relatively stable phase ； 　　(4) The axis trajectory is a double-ring ellipse, and the precession direction is positive precession ； 　　(5) Sensitive to changes in the temperature, viscosity, and pressure of the bearing lubricating oil. 　　How to eliminate it? 　　In current production, oil film vibration can be eliminated through the following methods: (1) Structurally, ensure that the shaft journal operates with a significant degree of eccentricity with respect to the bearing shells ； 　　(2) Use bearings with good vibration suppression performance, such as tilting pad bearings ； 　　(3) When a problem occurs on site, lowering the lubricant temperature is also an effective measure as an emergency response. Oil film oscillation occurs in rotating equipment equipped with oil-lubricated sliding bearings. When the rotor is operating normally, the center of the shaft journal and the center of the bearing do not coincide; instead, there is an eccentricity e. When the load remains constant and the oil film is stable, this eccentricity e stays unchanged, allowing the equipment to operate stably, with the load W on the shaft journal being in balance with the oil film pressure. If an external force acts on the shaft journal, causing a displacement Δe in the position of the axis O1 to a new location, then the oil film pressure changes from p to p’. As a result, it is no longer in balance with the new load W’ (W’ – W). The combined force between these two is F; one of its components, F1, will push the shaft journal back to its original balanced position O1. Under the action of the other component, F2, the shaft journal not only rotates at an angular velocity ω but also undergoes vortical motion around O1 (the direction of this vortical motion is the same as that of rotation), with its vortical speed being approximately half of the angular velocity. This phenomenon is known as oil film vortillation (or half-speed vortillation). Once oil film vortexing occurs, it does not disappear; as the operating speed increases, its vortexing frequency rises as well, and its amplitude also increases. If the rotor’s speed continues to increase to twice the first critical speed, its vortex frequency becomes equal to that of the first critical speed, resulting in resonance. The amplitude of the vibrations increases sharply, and the vibration becomes very intense. The path of the axis suddenly changes to an irregular, spreading curve. The amplitude of the semi-frequency harmonics rises to levels close to or exceeding that of the fundamental frequency. If the speed is increased further, the rotor’s vortex frequency remains unchanged, staying equal to its first critical speed. This phenomenon is known as oil film oscillation. 　　When oil film oscillation occurs, its main characteristics are: a. During intense vibration, the amplitude increases suddenly, and the sound becomes abnormal. 　　b. The vibration frequency is a combined frequency; there are numerous harmonics, and the amplitude of the frequency equal to the rotor’s first critical speed is close to or exceeds the amplitude of the fundamental frequency ； 　　c. Severe vibration occurs only when the operating speed is more than 2 times the first critical speed; the oscillation frequency equals the rotor’s first critical speed and remains unchanged regardless of changes in the operating speed. Severe vibration disappears only when the operating speed is below 2 times the first critical speed ； 　　d. The axis trajectory is an irregular shape that diverges, with precession in the direction of positive precession ； 　　e. Changes in the temperature of the bearing lubricant have a significant impact on vibration; reducing the lubricant temperature can effectively suppress vibration.

5.1 Reducing the top clearance of bearing shells: Whether it is cylindrical shells, elliptical shells, or triple wedge shells, reducing the top clearance of bearing shells can significantly improve their stability. This approach is more effective than other measures such as increasing the specific pressure of the bearing shells or reducing the length-to-diameter ratio. To reduce the clearance at the top of the bearing shells on-site, the method of grinding the mid-surface of these shells is generally used; this transforms the cylindrical shells into elliptical ones, and further increases the degree of ellipticity of those shells. The three-oil- wedge type shells are then changed to a combination of three oil wedges and elliptical shapes. As a result, the oil film force acting on the upper shell increases, which reduces the elevation of the shaft journal and thereby enhances the stability of the bearing shells. The clearances between elliptical bearing shells and triple oil wedge bearings can be reduced to 1‰–1.3‰ of the journal diameter; for larger journal diameters, the upper limit should be used ； If the journal straightness is small, use the lower limit. Currently, true cylindrical bearing shells (with the top clearance equal to twice the side clearance) are rarely seen in practice; the so-called cylindrical shells are actually elliptical in shape, with the top clearance and side clearance being approximately equal. When such bearing shells experience self-excited vibration, their top clearance can be reduced to 1.2‰–1.5‰ of the shaft journal diameter. This is because the side clearance of such bearings is small, and the top clearance should not be too small, otherwise it will lead to an increase in the temperature of the bearing materials. 5.2 Using shaft bearings with better stability: Generally, an ellipse has two load-bearing areas, which is why it is also called a double-lip bearing; its stability is better than that of cylindrical bearings, but its load-carrying capacity is lower than that of cylindrical bearings. There is also another type called a three-oil-leaf bearing shell, which has three load-bearing areas; the upper part of this shell contains two oil wedges that generate two downward-directed oil film forces, resulting in better stability compared to elliptical-shaped bearings. However, its load-bearing capacity is significantly reduced, and it is generally used in bearings that operate at high speeds with light loads. Parallel to the oil-leaf bearings are the oil- wedge bearings. A true cylindrical bearing shell has only one oil wedge in the lower shell; if two additional oil wedges are added to the upper shell, this results in a three-oil-wedge bearing, which was used in 200MW units in China. The structure is shown in the diagram, where b represents the depth of the oil wedge, and a1 and a2 represent the oil-blocking edges as well as the top clearance between the oil wedge and the shaft journal. Generally, a1 is 1.2‰–1.7‰ of the diameter of the shaft journal. The dynamic stability of this type of bearing shell is far inferior to that of elliptical shells, as well as to that of cylindrical shells. From the 1980s to the early 1990s, oil film oscillation occurring frequently in domestically produced 200MW turbines with 6-watt and 7-watt bearings gave rise to what was known as the \"oil film oscillation problem\" at that time; in fact, this was due to poor stability of the three-oil- wedge bearings. Once elliptical bearings were used, oil film oscillation no longer occurred. In the imported 300 and 600 MW units put into operation later, although the linear speed of their shaft journals exceeded 65 m/s, film oscillation did not occur even after the use of elliptical or cylindrical bearing shells. In addition to the aforementioned cylindrical tiles, elliptical tiles, and triple-oil-wedge tiles, there is also a tilting tile type that is currently widely used in large-scale units in China. The principle of this bearing bush structure is shown in the figure. The bearing shells are composed of multiple individual segments, which can swing slightly around their pivot points in order to find the optimal working position. This allows each segment to form a converging oil wedge, thereby preventing the occurrence of any destabilizing forces. Additionally, each segment passes through both the pivot point and the center of the shaft journal, ensuring that all forces act at the same point relative to the external loads; this prevents the emergence of any tangential forces that could cause vortexing in the shaft journal. Theoretically, if the inertia of the bearing tile and the friction at its pivot are ignored, the tilting bearing tile will not cause self-excited vibration of the shaft bearing. However, its load-bearing capacity is low, so it can only be used on the rotors of high and medium pressure cylinders of turbines with low loads, as well as on the rotors of exciters. 5.3 Increasing the upper bearing width: For cylindrical bearings, elliptical bearings, and triple wedge bearings, the purpose of reducing the clearance at the top is to increase the oil film force exerted by the upper bearing. However, in some units in operation, the upper bearing has wide circumferential oil grooves in its central area, resulting in the upper bearing being divided into two oil film zones. Practice has shown that reducing the clearance on such bearing shells does not yield very significant results. To achieve better results, while reducing the top clearance, the upper bearing shell is widened or completely filled, which can significantly increase the oil film force on the upper bearing and raise the eccentricity of the shaft bearing.