During the operation of sliding bearings, the dynamic behavior of the oil film directly affects the stability of the bearing. Oil film vortex and oil film oscillation are two common unstable phenomena, but their physical mechanisms, manifestations, and effects differ significantly.
Oil Whirl
Oil film vortex is a rotational flow phenomenon formed by the centrifugal effect of lubricating oil in the bearing clearance. When the shaft rotates, the oil film is subjected to centrifugal force to generate circumferential flow, forming a motion pattern similar to vortex. This type of flow is usually subsynchronous (with a frequency lower than the rotational frequency of the shaft) and has a small amplitude, belonging to the local instability dominated by fluid dynamics.
Oil Whip
Oil film oscillation is the "self-excited vibration" caused by the development of oil film eddies to a critical state. When the rotational speed of the shaft approaches "twice" (or higher) the natural frequency of the system, the oil film loses its damping ability, causing severe synchronous vibration between the shaft neck and the bearing (with a frequency close to the rotational frequency of the shaft). At this point, the vibration amplitude significantly increases, which may trigger mechanical resonance.
The fluid dynamics effect of oil film vortex is dominated by uneven distribution of oil film pressure and centrifugal force. It usually begins to appear when the speed reaches 30% to 50% of the critical value. If the system damping is sufficient, eddies may be suppressed.
Oil film oscillation is triggered by system resonance, and when the vortex frequency couples with the natural frequency of the shaft, the system enters an unstable state. Usually occurs when the speed reaches "twice" the critical value (known as "half speed vortex"). The nonlinear characteristics of oil film stiffness and damping lead to energy accumulation, resulting in self-excited vibration.
Engineering response measures:
Oil film vortex:
-Optimize bearing design (such as increasing bearing clearance ratio and using tilting pad bearings).
-Increase the viscosity of lubricating oil or adjust the oil supply pressure.
Oil film oscillation:
-Avoid areas where the speed approaches twice the critical speed.
-Increase system damping (such as installing squeeze film dampers).
-Adjust the bearing support stiffness or use multi oil wedge bearings.
Oil film vortex, also known as forward precession, is caused by the oil film action (liquid friction) inside the bearing. It is more common than shaft shaking caused by dry friction.
As shown in the figure, if the shaft rotates counterclockwise, due to the lack of concentricity or imbalance of the shaft itself, a centrifugal force will act on the shaft, which always points radially towards the bearing and never balances with the pressure of the oil on the shaft neck. The resultant force of oil pressure on the shaft neck is P, which forms an angle with the line connecting the bearing center O and the shaft center O1. As a result, a component force Psin ∅ causes the shaft neck to generate and maintain a rotational motion in the same direction as the shaft rotation direction Ω, which is known as oil film vortex. The frequency of oil film vortex is slightly less than half of the rotational speed, and the vortex frequency w can be expressed as w ≈ (0.43-0.48) Ω.
Schematic diagram of oil film vortex
As the working speed of the shaft increases, the neck vortex velocity also increases. After the oil film vortex is generated, it does not disappear. For high-speed rotating machinery such as steam turbines, turbo compressors, etc., when the working speed of the shaft is Ω≥ 2gn (where wn is the critical speed), the vortex frequency is exactly equal to the natural frequency of the shaft system, thereby exciting the resonance of the entire shaft system, which is called oil film oscillation.
Oil film vortex case sharing
Event sequence: The main fan unit of the 200000 t/a heavy oil catalytic unit was put into trial operation in August 2020. During the start-up process, it was found that the gearbox had resonance with a motor power of around 3800kW, with a maximum vibration amplitude of 144um. The resonance disappeared when the motor power decreased to below 3000kW. After the manufacturer's after-sales service personnel disassembled and inspected the gearbox, it met the design standards. Based on the analysis of on-site operational data, it is believed that the vector resultant force of gear meshing force and the weight of the large gear falls in the oil film vortex area of the low-speed shaft cylindrical bearing.
Rectification measures:
(1) Control the motor power range during normal production, avoid long-term operation within the range of 2000~4000kW, and avoid areas with high vibration values mentioned above.
(2) Adjust the temperature and pressure of the lubricating oil appropriately, increase the stiffness of the oil film, reduce the factors that cause oil film vortex formation, and achieve the goal of reducing vibration.
(3) By changing the bearing form to solve the problem of vibration fluctuations, the gearbox manufacturer is responsible for rectifying the bearing shell, replacing it from round to misaligned, and replacing it during the device maintenance period.
Misalignment bearing: a pair of sliding bearings installed radially with an offset distance smaller than the radius gap.
(From GB/T 2889.1-2020 Sliding Bearings - Terminology, Definitions, Classification and Symbols - Part 1: Structure, Bearing Materials and Their Properties)
1. The core difference between round tiles and misaligned tiles
1. Round tile (cylindrical tile)
(1) Simple structure, symmetrical cylindrical inner surface.
(2) The oil film pressure distribution has a single peak value and a low bearing capacity.
(3) Oil film oscillation or instability is prone to occur at high speeds.
2. Misaligned tiles (such as elliptical tiles, tilting tiles)
(1) Asymmetric structure (such as half misalignment or adjustable tile blocks).
(2) Form multiple oil wedges to improve oil film pressure distribution and load-bearing capacity.
(3) Suppress vibration, enhance rotor stability, suitable for high-speed and heavy-duty working conditions.
2. Improvement advantages of misaligned tiles
2.1 Enhance bearing capacity: The multi oil wedge structure disperses the load, for example, an elliptical tile can generate two dynamic pressure oil films, increasing the bearing capacity by about 30% -50%.
2.2 Enhance stability: Misalignment design reduces the risk of oil film oscillation and avoids instability (especially in high-speed or variable load conditions).
2.3 Optimize lubrication and temperature control: Strengthen cooling in different areas of the oil wedge to reduce the risk of local high temperature.
3. Prioritize the use of misaligned tiles in the working conditions
3.1 High speed rotating machinery (steam turbine, centrifugal compressor).
3.2 Equipment with variable load or impact load.
3.3 The original circular tile has problems with unstable oil film or high temperature rise.
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