Manufacturing Trend 2009/07-08, Technical Diagnostics Section

"Instead of firefighting and major repairs"

The task of bearings is to support and guide moving machine elements performing rotary or oscillating motion. Depending on the design, we fundamentally distinguish between rolling bearings, where the force transmission between moving surfaces is carried out by rolling elements, and plain bearings, where the supporting surfaces slide on each other (or on the lubricant layer between them). This chapter of our article deals with plain bearings operating with a lubricant layer, primarily oil-lubricated bearings.

In the case of plain bearings, we strive to completely separate the surfaces sliding on each other with a lubricant layer, so that they do not come into direct contact with each other metal-to-metal, instead creating a clean fluid friction. Depending on the method of lubricant layer formation, we distinguish between hydrostatic and hydrodynamic plain bearings. In hydrostatic plain bearings, the lubricant is pressurized into the moving surfaces sliding on each other, so in this case, the lubricant pressure is generated by a pump even before the lubricant enters the bearing.

In hydrodynamic plain bearings, the load-bearing lubricant film separating the moving surfaces is formed spontaneously during relative motion. In this version, at startup, the surfaces sliding on each other come into direct contact, then a mixed friction state is established during motion, and pure fluid friction only occurs above a certain relative speed. The pressure in the bearings is almost self-generated, meaning the bearing also functions as a pump.

Advantages of Plain Bearings	Disadvantages of Plain Bearings
- Extremely low friction resistance in case of clean fluid friction	- Relatively high lubricant consumption
- Simple construction, versatile in application	- Lubricant supply and maintenance require significant investment
- The lubricant layer has good vibration and noise damping effects	- Strict requirements regarding the quality of sliding surfaces
- Insensitive to shocks and vibrations	- Hydrostatic bearings require a separate oil pump
- Virtually unlimited lifespan with proper lubrication	- High friction factor at startup for hydrodynamic bearings
- Suitable for high loads and extremely high speeds

 

Formation of the Lubricant Film

In hydrostatic bearings, the formation of the lubricant film depends solely on the appropriate lubricant pressure; the separation of the surfaces sliding on each other is achieved even in a stationary position. However, the situation is different for hydrodynamic bearings. In the v=0 speed position (with a stationary shaft), the journal and the shaft come into direct metal contact. As the shaft starts to rotate, initially there is dry friction between them. With the increase in circumferential speed, the mixed friction state occurs first, then the lubrication state gradually improves (the friction factor curve drops suddenly). The pure fluid friction state begins at the minimum of the curve. After reaching the minimum, as the speed increases, the friction factor value increases to a certain extent, following a parabolic curve. The process of oil film formation is illustrated in the diagram by depicting the relative positions of the journal and the bearing.

 

The journal in the bushing is positioned with a certain degree of loose fit (emax). The loose fit results in the journal's positions shown in the partial diagrams from n=0 to n=∞. In the static state, the journal is positioned at the bottom of the bore as shown in figure "a": the journal rests at the bottom of the bore obeying the force of gravity. When the journal starts moving, due to the surface irregularities, it "climbs" slightly into the bearing bore as shown in figure "b," indicating the mixed friction state.

As the speed increases, more and more lubricant is introduced between the surfaces; at a certain speed, the surfaces separate, and the journal moves to the position shown in figure "c." This marks the beginning of pure fluid friction, where the gap formed under the journal - continuously narrowing - is the smallest. The friction factor is also at its lowest at this point, as the least amount of oil needs to be forced through the bearing surface. At higher speeds, the gap increases as shown in figure "d" because more and more oil enters, better separating the surfaces. The largest oil gap would occur at n=infinite speed, where the journal would be concentrically positioned in the bearing. The center of the journal - which is at the lowest position in figure "a" - moves in a curved path as the speed increases, reaching the highest position at infinite speed, when it moves to the center of the bearing. In reality, there are as many journal paths as there are bearings. In practice, for example, the path shown in the diagram can describe the journal's position from a stationary center to the operating speed.

Role of the Lubricant

It is obvious that the lubricant is an extremely important component of plain bearings. It can fulfill its task only if it has the appropriate characteristics and is supplied in the right quantity to the lubricating part, especially if it also acts as a coolant. The viscosity of the lubricating oil strongly depends on the temperature, decreasing as the temperature rises. However, the load capacity of the bearing, i.e., the applicable surface average load, is also a function of the oil viscosity, so for lightly loaded but rapidly rotating bearings, we use easily flowing, low-viscosity oil. For high loads, high-viscosity oil is required, or grease is used if the circumferential speed is low.

Since the viscosity of the oil in the bearing is about six to ten times higher in the cold state, i.e., at startup, than at the operating temperature of high-speed bearings, at startup, it can reach the state of pure liquid friction at lower speeds, specifically at 1/6 to 1/10 of the normal speed. However, during shutdown, with the low viscosity of warm oil, mixed friction can quickly occur with the decrease in operating speed, leading to wear. Therefore, in operations with interruptions, the proper selection of oil and the proper sizing of the bearing must be ensured so that pure liquid friction exists even at lower speeds.

Diagnosis of Plain Bearings

Correctly sized (and lubricated) plain bearings are significantly less sensitive to generating and transmitting vibrations compared to rolling bearings. In the former, the oil quantity between the surfaces forms a soft, highly internally damped cushion that significantly dampens vibrations caused by surface irregularities and defects, and suppresses incoming dense, low-amplitude vibrations. In contrast, irregularities of the rollers and raceways of rolling bearings easily generate vibrations. It is evident from the above that plain bearings involve relatively simple structures, so there should not be too many faults to look for. In the following, we discuss the typical faults of plain bearings and their vibration diagnostic methods.

Vibration Measurement of Bearings

Large bearing clearance or wear: If the bearing clearance (bearing play) is large due to manufacturing or wear, the shaft is not adequately supported (clamped) during operation in the bearing, and the shaft position becomes unstable. It will practically "wobble," and the vibration velocity spectra recorded from the bearing will indicate vibrations characteristic of mechanical looseness. Precisely filtering out large bearing clearances or other bearing installation looseness is extremely important because vibrations components can appear – or intensify – in the vibration spectra due to looseness, making us think of other serious vibration problems rather than suspecting bearing installation errors.

Incorrect alignment: The shaft axis and the bearing bushing are not parallel but form an angle. The error can occur if the adjusting bearing cannot move (adjust) in the direction of the load for some reason, or if it was poorly manufactured or assembled from the beginning. Edge running can occur, and in the worst case, the shaft and the bushing may even come into metallic contact.

In the axial direction, this error can cause significant axial vibrations even on non-load-bearing bearings. Vibration typically occurs at the rotation frequency, but in generators – due to the asymmetry of the rotating part – at twice the rotation frequency. If we only have a handheld vibration meter suitable for measuring broadband vibration levels, we should always suspect misalignment when measuring large axial vibrations on non-load-bearing bearings.

Inadequate lubrication

A sufficient thickness of lubricant film is not formed between the shaft and the bushing to establish pure liquid friction. This can lead to mixed or, in the worst case, dry friction, showing the well-known "worn out" pattern at higher frequencies in the vibration spectra. The phenomenon can be detected with a simple handheld device by measuring broadband vibration levels if the device is capable of measuring at least two vibration parameters (vibration displacement and velocity, or vibration velocity and acceleration). After recording both parameters, based on the first one (displacement or velocity) and knowing the speed, we calculate the second one (velocity or acceleration). If the calculated value is significantly lower than the measured one, we can be sure that higher frequency vibration components are present. However, this method is only indicative because the measured vibrations may also include other (even normal) vibrations from the machine's operation. Unfortunately, uncertainty can also exist in spectrum analysis.

Oil film resonance: It can be easily detected with spectrum analysis since it occurs at a very well-defined (unmistakable) frequency: 0.42–0.48×n (n = rotation frequency). The vibration usually occurs with a high amplitude, and there is no need to "search" for it in the spectrum.

Its detection with a handheld device is relatively simple: measure the broadband vibration velocity and displacement values at the same measuring point, then calculate the displacement value from the vibration velocity based on the known relationship (v=r/ω=v/2πf). If the measured displacement is significantly higher than the calculated one, we are almost certainly dealing with oil film resonance. However, in larger machines, even a handheld device is not necessary because the phenomenon is accompanied by unusually strong "thumping," which usually attracts the attention of the staff.

For machines with plain bearings, instead of measuring the vibration of the bearing, there is a much more effective method for detecting plain bearing installation and machinery faults, which is measuring relative shaft vibration. After the explanation of this method in the following, it will become apparent why it is more suitable for examining machines with plain bearings and what additional information it can provide.

András Szűcs, Eric Rahne (PIM Kft.) pim-kft.hu, gepszakerto.hu

 

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