There are many reasons for the vibration of the sliding bearing, most of which are caused by other mechanical problems, such as rotor imbalance, misalignment, and stiffness problems. The vibration caused by the sliding bearing itself is mainly caused by the rigidity reduction caused by improper matching of the gap, and the oil film problem caused by improper design and installation.
Oil film whirl
Oil film whirl is a kind of whirl caused by the oil force of the radial sliding bearing. When the rotor journal is stably operated in the sliding bearing, the oil film force R of the bearing and the load W are balanced with each other, and the rotor shaft is at a certain equilibrium position O1. If the rotor is disturbed from the equilibrium position and moved to the O2 point, the resultant force F of the oil film force R' and the load W is no longer zero and collinear.
The resultant force F can be decomposed into two component forces in the radial direction and the tangential direction. The radial component force Fr is opposite to the direction of displacement of the journal, and it is an elastic restoring force to push the journal back to the original equilibrium position O1. The tangential component force Fu is perpendicular to the direction of the journal displacement. It pushes the journal to continue to circulate around the equilibrium position O1, that is, vortex is generated. This whirl is called oil film whirl, and Fu is called vortex power.
If the vortex force received by the journal is smaller than the oil film damping force, the axial trajectory formed by the axial vortex is converged, and the whirl is reduced; if the vortex dynamic is equal to the oil film damping force, the axial trajectory is no longer expanded and becomes The closed vortex is stable; if the vortex power exceeds the damping force, the axial trajectory is divergent and the eddy is unstable. When the whirl steering is the same as the rotor rotation direction, it is positive precession; otherwise, it is reverse precession.
Theoretical calculations show that the rotational frequency Ω of the oil film whirl is equal to half of the rotor rotational frequency ω, that is, Ω = ω/2, so the oil film whirl is theoretically called half-speed whirl. In practice, the vibration frequency of the oil film whirl is about 0.42~0.48, which is Ω=(0.42~0.48)ω.
Oil film oscillation
Along with the continuous increase of the rotor rotation frequency ω (ie, the rotational speed n), the eddy frequency Ω of the oil film whirl is also continuously increased, and when the rotational speed n rises to twice the first critical rotational speed nk1 of the rotor, that is, when the oil film vortex When the frequency of the motion is equal to the natural frequency of the rotor bearing system, that is, Ω = ωk1, the rotor bearing system will have a strong resonance, which is the oil film oscillation.
After the oil film oscillation occurs, even if the rotation speed continues to rise, the whirl frequency does not rise according to the law of the vortex ratio (Ω/ω), and is still ωk1, that is, the natural frequency of the rotor is tightly bitten - first Critical speed - no longer changes.
Oil film whirl and oil film oscillation are self-excited vibrations, that is, the energy to maintain vibration is generated by the rotor bearing system (including lubricating oil) in its own rotation, which can continuously provide great energy, and The outside world has nothing to do. Therefore, the oil film oscillation is also characterized by seriousness, suddenness, and sometimes intermittent squeaking.
For the tilting pad bearing used in large units, the theoretical calculations show that the cross-stiffness of the tilting pad bearing is zero, and the oil film whirl and oil film oscillation are impossible to occur, ignoring the tile mass and the fulcrum friction. Because the tile can swing freely, the oil film force can be automatically adjusted to pass through the axis, thus collinear with the load, eliminating the tangential oil film component, and fundamentally eliminating the whirl force.
However, due to the fact that some of the design conditions are inconsistent in actual use, such as friction at the fulcrum, improper bearing tension, excessive viscosity of the lubricating oil, etc., the oil film may also oscillate. As for other types of bearings, such as cylindrical tiles, elliptical tiles, multi-oil wedges, multi-oil blades, etc., as long as it is a high-speed light load, oil film whirl and oil film oscillation may occur.