Sliding bearings, commonly known as flat bearings, have a simple form and a large contact area. If well lubricated, they have excellent wear resistance and a long bearing life. Sliding bearings have a high load-bearing capacity, high rotational accuracy, and the lubricating film has impact resistance, making them widely used in engineering. This article shares the causes, vibration mechanisms, and vibration methods of sliding bearing vibration faults.
Sliding bearings can be divided into thrust sliding bearings and radial sliding bearings according to the direction of the load they bear; According to the different principles of lubricant film formation, it can be divided into dynamic pressure sliding bearings and static pressure sliding bearings; According to different structural forms, it can be divided into integral sliding bearings and sectional sliding bearings.
1. Causes of sliding bearing failure
The main causes of sliding bearing failures are:
Improper design and installation of bearing shells;
Overspeed, overloaded operation, or impurities in lubricating oil;
Under high temperature, high speed, and high load operating conditions, the journal and bearing material undergo thermal expansion, the bearing clearance disappears, and direct contact occurs between metals;
Under the action of alternating loads, the bearing surface generates reciprocating tensile stress, compressive stress, and shear stress, resulting in the formation of fine cracks on the bearing surface. Under continuous operation, fatigue failure occurs after Z;
The detachment caused by long-term operation under larger amplitudes;
Sub synchronization instability caused by misalignment of couplings, improper operation, and other reasons.
2. Types of vibration faults in sliding bearings
There are various manifestations of vibration faults in sliding bearings, including excessive clearance between the bearing shells; Oil film vortex, oil film oscillation, and friction; And common faults such as bearing wear, burning, and fatigue induced detachment cracks.
3. Vibration mechanism of sliding bearing faults
There are many reasons that can cause vibration in sliding bearings, most of which are caused by other mechanical problems such as rotor imbalance, misalignment, stiffness issues, etc. The vibration caused by sliding bearings themselves is mainly due to the decrease in stiffness caused by improper fit clearance, as well as oil film problems caused by improper design and installation.
Oil Whirl
Oil film vortex is a type of vortex generated by the oil film force of radial sliding bearings. When the rotor journal runs stably in the sliding bearing, the oil film force R of the bearing is balanced with the load W, and the rotor axis is at a certain equilibrium position O1. If the rotor is disturbed and moves away from the equilibrium position to point O2, the resultant force F of the changed oil film force R 'and load W is no longer zero or collinear.
The resultant force F can be decomposed into two components in the radial and tangential directions. The radial component Fr is opposite to the displacement direction of the journal and strives to push the journal back to its original equilibrium position O1, which is an elastic restoring force; The tangential force Fu is perpendicular to the direction of the journal displacement, and it pushes the journal to continue rotating around the equilibrium position O1, generating vortices. This type of vortex is called oil film vortex, and Fu is called vortex force.
If the vortex force acting on the journal is less than the oil film damping force, the axis trajectory formed by the axis vortex will converge and the vortex will decrease; If the vortex force is equal to the oil film damping force, the axis trajectory will no longer expand and become a closed shape, and the vortex will be stable; If the vortex force exceeds the damping force, the axis trajectory is divergent and the vortex is unstable. When the turning direction of the vortex is the same as the rotation direction of the rotor, it is positive precession; On the contrary, it is anti precession.
Theoretical calculations indicate that the rotational frequency Ω of oil film vortex is equal to half of the rotor rotational frequency Ω, i.e. Ω=ω/2. Therefore, oil film vortex is theoretically also known as half speed vortex. In practice, the vibration frequency of oil film vortex is about 0.42~0.48 speed frequency, that is, Ω=(0.42~0.48) ω.
Oil Whip
With the continuous increase of the rotor rotation frequency ω (i.e. speed n), the vortex frequency Ω of the oil film vortex also increases. When the speed n reaches twice the critical speed nk1 of the rotor, that is to say, when the frequency of the oil film vortex is equal to the natural frequency of the rotor bearing system, i.e. Ω=ω k1, the rotor bearing system will undergo strong resonance, which is called oil film oscillation.
After the occurrence of oil film oscillation, even if the speed continues to increase, the vortex frequency no longer increases according to the constant law of vortex ratio (Ω/ω), and remains at ω k1, which means that the natural frequency of the rotor tightly biting the critical speed no longer changes.
Oil film vortex and oil film oscillation are self-excited vibrations, which means that the energy required to maintain the vibration is generated by the rotor bearing system (including lubricating oil) during its own rotation. It can continuously provide great energy without being affected by the outside world. So, oil film oscillation also has the characteristics of severity, suddenness, and sometimes intermittent roaring sounds.
Theoretical calculations show that for the tilting pad bearings commonly used in large units, the cross stiffness of the tilting pad bearings is zero, and it is impossible to generate oil film vortex and oil film oscillation without considering the mass of the pad and the frictional force of the fulcrum. Because its tiles can sway freely, the oil film force can automatically adjust to pass through the axis, thus being collinear with the load, eliminating the tangential oil film force and fundamentally eliminating the vortex driving force.
However, in practical use, there are often situations that do not comply with the design conditions, such as friction at the fulcrum, improper bearing tension, excessive lubricating oil viscosity, etc., so tilting pad bearings may also experience oil film oscillation. As for other types of bearings, such as cylindrical bearings, elliptical bearings, multi oil wedges, multi oil blades, etc., as long as they belong to high-speed light loads, oil film eddies and oil film oscillations may occur.
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