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 high load-bearing capacity, high rotational accuracy, and the lubricating film has impact resistance. Therefore, it has been widely applied in engineering.

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 fine cracks on the bearing surface. Under continuous operation, fatigue failure is eventually formed;

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

Sliding bearings have various faults, including excessive clearance between the bearing shells, oil film vortex and oil film oscillation, friction, as well as common faults such as bearing shell 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. We will not repeat them here. 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 film vortex - the phenomenon in which the wedge-shaped oil film drives the shaft to move around the center of the bearing according to the average flow velocity of the oil is called oil film vortex. Because its average velocity is half of the circumferential velocity of the shaft neck, it is also known as half speed vortex.

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 film oscillation - 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 first critical speed nk1 of the rotor, that is, 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 tightly biting the rotor - the first 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.

4. Diagnosis of vibration faults in sliding bearings

The factors related to vibration and sliding bearings themselves mainly include time gap vibration, thermal imbalance caused by high parameter equipment collision and wear, and instability caused by oil film. Other vibrations are almost always the response of the excitation of the rotor system on the bearing seat, and can refer to other mechanical fault diagnosis methods in the platform.

Regarding faults such as detachment, cracking, and low parameter common equipment wear and tear, the response is not sensitive to vibration, while oil analysis often achieves good results.

(1) When the bearing does not work properly or shows signs of friction, there may be no obvious response to vibration, but the temperature of the bearing will increase significantly. The changes in the local temperature gauge, cooling water temperature, and oil temperature can be observed, as well as the color changes of the lubricating oil.

(2) When the vibration is caused by excessive clearance or looseness of the bearing top assembly, the vertical vibration will significantly increase, and the ratio to the horizontal amplitude will decrease, approaching or even exceeding the horizontal amplitude. Vibration is generally only more pronounced on the bearing.

(3) If the device is equipped with non-contact sensors 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) The amplitude of oil film vortex suddenly increases when the speed reaches a certain value, and there is a significant change in the amplitude of oil temperature vibration.

(5) Oil film oscillation faults only occur on flexible rotors, with vibration frequencies close to half the speed. Light load bearings may experience oil film turbulence before reaching this speed. Heavy duty bearings may experience oil film oscillation directly during the acceleration process.

(6) When there is oil film vortex or oil film oscillation, increasing the lubricating oil pressure can sometimes cause significant changes in vibration.

(7) Oil film oscillation speed lag phenomenon. During the acceleration process, oil film oscillation occurs when the speed exceeds the instability threshold speed. However, after the oil film oscillation occurs, the vibration does not decrease when the unit slows down to the threshold speed. Only when the speed further decreases, will the vibration decrease. There is a difference in speed between the occurrence and disappearance of oil film oscillation during the acceleration and deceleration process, which is called speed lag phenomenon

(8) Vibration has the dual characteristics of large amplitude and suddenness. When approaching oil film oscillation, unstable low-frequency vibration components will appear, with amplitudes appearing and disappearing intermittently. Once oil film oscillation occurs, the vibration amplitude will sharply increase in a short period of time (a few seconds), and the vibration amplitude is much larger than that of ordinary forced vibration.

(9) The most effective method for detecting minor cracks and defects such as detachment in the bearing shell is the metal flaw detection method performed during shutdown.

(10) When the oil film eddies, there may be double or multiple ring axis trajectory characteristics, and the vibration waveforms that generate subharmonics have a winding characteristic.

(11) When the clearance is too large, the harmonic components of the frequency conversion are rich and significant, similar to the phenomenon of mechanical loosening. Sliding bearings with excessive clearance may cause small imbalances, misalignments, or other related forces to produce vibration spectra of multiple high-order harmonics. 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) When the oil film eddies, the half frequency component in the spectrum increases significantly, but the amplitude is generally less than the first harmonic amplitude. After the speed increases, the ranking of the half frequency and harmonic relationship still remains.

(13) Before oil film oscillation, the vibration is mainly composed of power frequency components. After a sudden change in vibration, the amplitude of the power frequency component will decrease, and the amplitude of the low-frequency component will greatly exceed that of the power frequency component, becoming the main frequency component.

(14) When oil film oscillation occurs, the vibration will suddenly increase, and even if the speed continues to increase, the amplitude will no longer change. The vibration frequency is always equal to the natural frequency of the rotor system and does not vary with changes in speed. The vibration phase of the bearings at both ends of the rotor is basically the same.

2026 April 3rd Week Marginal Product Recommendation

Material Specification Sheet–MG-6:

Material specification sheet – MG-6 is Ball and roller bearing steel according to EN ISO 683-17. Ball and roller bearing steel for balls and rollers of any dimension,rings and discs up to 30mm effective thickness.