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Grease is the most common type of lubricant used for rolling element bearings today. About 90 percent of all bearings are lubricated in this manner. It is important to select the right grease for specific requirements and to calculate the lubricant life cycle. For accurate calculation of grease service life, it is necessary to know and apply limiting factors. The correct calculation will enable minimum quantity lubrication (MQL).
Ball and cylindrical roller bearings used in electric motors are an example of rolling bearings with MQL. If, however, these bearing types are subjected to negative influences, the effective grease lifetime can be rapidly reduced and bearing damage can occur. This article discusses some of these negative influences and their effects based on practical examples. Practical implication will be introduced through the issue of electric continuity (bearing currents or bearing spark erosion) and the effect on the grease and rolling bearings.
Grease and Grease Lubrication of Rolling Bearings
Lubricating grease for rolling element bearings consists of thickener, oil and selected additives to improve desired properties. The actual lubricant for a rolling bearing is oil, which can be a mineral oil, fully synthetic or a blend of the two. Different types of additives are added to these oils to influence the corrosion resistance properties and/or build layers that protect the metal surface under extreme conditions. Additives also improve the viscosity behavior at different temperatures.
The task of the thickener is to absorb the oil and release it in small quantities to the bearing element over a long period.
In practice, only a few grams of grease are used for lubricating rolling element bearings and this quantity typically lasts a long time. An exact calculation of the grease service life is consequently of particular importance.
Calculating Grease Service Life
The grease service life for rolling bearing lubrication depends on the selection of grease, type of bearing, working conditions and environmental influences.
The basis for the grease service life calculation can be seen in the generally accepted diagram (Figure 2).
This diagram contrasts what is often called a “general-purpose grease” (a mineral oil-based lithium grease, Grease A) with the grease service life curve of a high-quality, polyureathickened, synthetic ester oil-based grease (Grease B).
The advantages of polyurea-thickened, synthetic oils increase at higher temperatures. They can easily achieve grease lifetimes which are 20 times higher than standard greases, depending on the temperature. This means the user may be able to increase the safety margin for lubricant-related bearing damage and simultaneously increase relubrication intervals.
The so-called bearing type value (kf) assumes the actual design of the bearing to be lubricated. This factor can have values between 0.9 and 10 for kinematically simple ball bearings. For kinematically complex bearings (like axial cylindrical roller bearings with high sliding friction) the kf factor can reach values up to 90. The larger numbers suggest greater surface areas and the greater total stress applied to the oil and thickener matrix. Spherical roller bearings as a category tend to apply the greatest stress on greases.
The speed factor, n*dm (RPM * mean diameter of the bearing), is a classification number for the rotational speed of the rolling bearing and is dependent on operating conditions. In this way, one can already read the available service life of the specific grease type used, although this is only a theoretical value. In the following calculation, influencing factors of the actual application must be observed and their importance evaluated.
tfq = tf* f1* f2* f3* f4* f5* f6
tfq … grease service life in hours in practice
tf … grease service life from Figure 2
f1 … f6 … influence factors
These factors reflect known negative influences on the service life of roller bearing grease, which shorten grease life, per the values shown in Figure 2.
The influence of contamination (f1), vibration (f2), increased bearing temperature (f3), high bearing load (f4), and air circulation (f5) on or around the bearing must be considered. The values can easily vary between 0.1 and 1 (no influence), meaning the result of the actual calculation is strongly influenced by the experience level of the person estimating factor values. Structural factors (f6) may also significantly reduce the grease service life. For example, the assembly direction of the bearing (horizontal, vertical or angular) is important for the relubrication interval. Due to the different influences of the centrifugal forces on the grease, the driven race way of a bearing (IR or OR rotating) must be considered.
The reduction factor ranges must be selected from a range. As the conditions become more severe the factor value becomes smaller, which shortens the grease life calculation. Experience plays a key role in accurate estimation.
f1 = Environmental media, degree of pollution (0.1 to 0.9)
f2 = Load dynamics, impacts (0.1 to 0.9)
f3 = Bearing temperature (0.1 to 0.9)
f4 = Bearing load (0.1 to 1.0)
f5 = Airflow (0.1 to 0.7)
f6 = Type of mounting, centrifugal energy (0.5 to 0.7)
While the reduction factors 1, 2, 5 and 6 are based on empirical values, the bearing temperature (3) and load (4) can be attributed to chemical physical coherences and are grease-type dependent.
For the standard grease (lithium soap and mineral oil), thermal aging increases disproportionately following any increase in temperature above 140°C. Grease service life shrinks to almost zero when it reaches its dropping point at approximately 190°C. One could expect increased oil separation and, due to the increased circulation, an appreciable increase in oxidation rate. As the grease reaches its dropping point, irreversible and spontaneous oil bleeding occurs and the grease loses its properties. Grease service life also degrades with extremely low temperatures, but this cannot be measured with the same test stand configuration. Consequently, it is possible to determine grease life factors based on performance within a range of temperatures.
Grease-lubricated Bearings in Electric Motors
A greased roller bearing in an electric motor is offered to demonstrate the possible grease service life. In general, the suspension of rotors with grease-lubricated roller bearings is a widely used and well-known application, and is a good example for a bearing subjected to various influencing factors.
With the advent of modern frequency converting techniques, an additional negative influence on bearing life time has been discovered and continues to cause failures: bearing currents. Normally, rolling bearings in electric machinery are minimally loaded with the typical load being between P/C=0.05 and C/P=20. The load, in relation to the carrying capacity of the bearing, is so minimal that reaching the maximum endurance range should be possible. In reality, bearing failures still occur after 15,000 to 20,000 hours with this type of bearing. With correct relubrication, the grease service life can be matched to the optimum bearing service life and thus easily achieve 100,000 hours and longer.
In planned preventive maintenance strategies, electric motors are often replaced after only two to three years operating time. A variety of factors drives the interval, but generally this has to do with previous application lifecycle experiences. Motor rebuilds require time, are costly and present increased risk with each new installation.
In new machinery, modern frequency converting techniques, such as high-frequency variable speed motors, the regulation of motor speed, increased speeds and lengthened operating hours all have different effects that reduce service life (see sidebar). Higher speed capability of an electric motor will lead to elevated bearing temperatures, exposing the grease to stronger centrifugal forces. These centrifugal forces remove the oil from the contact surfaces at the time that it is most vital to the bearings function and survival. This may result in premature aging (oxidation and stiffening), due to overstressing of the performance capabilities in general-purpose greases. Extreme bearing temperatures 212°F (100°C) can cause oil evaporation, condensation and stability issues for the grease and the bearing. In recent years, increased failures due to electrical arcing (highfrequency alternating current passing between the rotor and the frame through the bearing) in high-frequency drives have added to these issues.
By switching the square wave voltage, harmonics in the MHz-range are produced, which cannot be isolated with common insulation materials. Conventional measures used by bearing manufacturers (insulation of the surface of the bearing ring with a ceramic layer approximately 100 microns thick), are no longer successful. These methods are effective only when working with direct current (DC) or low-frequency alternate current (AC). It is speculated that there is so much energy left in these high-frequency currents that grounding occurs through the lubricant film, and the element and grease are damaged. This influence is not taken into consideration by today’s conventional calculations and has, in turn, led to bearing damage on modern machines using frequency-converting techniques for the speed regulation.
Recognizing the environmental influences (f1 and f3) and selecting appropriately reduced lifecycle factors can contribute to overcoming the arc induced stress on the element. The equipment owner may help offset the effect of the pollution and temperature contaminants that will be present under these circumstances by reducing the quantity in increasing the frequency of the replenishment of the in-service lubricant.
One can observe strong oxidation and hardening of the grease that occurs following high-temperature stress, which is produced through electrical grounding (arcing). Loss of lubricant health produces mixed friction and wear in the roller contact area. The fact that a bearing cannot be easily relubricated from the outside plays a crucial role in eventual element failure. The newly added grease cannot displace the hardened and oxidized lubricant already present, and it makes an exchange of grease impossible. With normal relubrication intervals, bearing failure is inevitable (Figures 3 through 8).
Figure 3. Aged Grease Between Cage and IR
Figure 4. Relubrication is Not Possible
Figure 5. Mixed Lubrication in a CRB
Figure 6. Damage from Bad Lubrication
Figure 7. Water from Outside
Figure 8. Condensed Water Problem
As mentioned, there is a noticeable increase of electrical current damage due to highfrequency alternate current (AC). Dull brown-colored raceways and tracks on the ball or rollers are typical (Figures 9 through 14).
Figure 9. CRB-Outer Ring is Brown
Figure 10. Ball with Brown Bands
Figure 11. Deep-groove Ball Bearing Damage
Figure 12. SEM Picture Showing Race
Figure 13. Damaged CRB Due Current
Figure 14. Damaged Grease
As shown in Figure 12, the actual electric current crater is small and can be identified only under an SEM. Today, the typical diameter of the nearly circular craters present in most common failures ranges from 1 to 4 µm. Practical experience shows that bearing surfaces will be damaged, even with a minimal load. These arcs also lead to a catastrophic oxidation-induced aging of the grease in the rolling contact area, which dramatically shortens grease life (Figures 13 and 14). At the roller contact points, the deteriorated grease can no longer lubricate effectively, while the outer portions of the bearing retain fresh grease. This condition is sometimes characterized as underlubrication, which may be an accurate depiction of a secondary failure mechanism but is not necessarily the fundamental contributor to failure. Corrective measures are usually not successful when the actual cause is not correctly identified and amended.
Figure 15. Typical Fluting Pattern
(Courtesy of MH Electric Motor and Control Corp.)
The last stage is characterized by the typical fluting pattern as a result of bearing currents (Figure 15).
Figure 16. DuoMax 160
Lubricating roller bearings with grease is a common practice for long-term lubrication. To achieve the expected operating life, special attention must be paid to the correct grease service life calculation. By addressing a number of influencing factors, grease service life can be greatly reduced. Modern electric motors with frequency converters for regulating rotating speed suffer increased problems due to bearing currents at the rolling contact points. These currents lead to rolling bearing surfaces that are damaged by micro-craters after the grease is thermally destroyed at the metal contact points by small electric arcs. This particular reduction in grease service life has not yet been considered in conventional grease life calculations. Failure due to bearing currents continues to increase with the frequent use of modern drive technology for motor control.
Insulated gate bipolar transitors (IGBT) came onto the scene in the 1990s. These represented a huge improvement in drive technology, increasing the switching frequency to 20 kHz, reducing harmonics and audible noise.
Recently though, it has become apparent that these improvements have been bought at a price: IGBT technology has resurrected bearing problems due to electrical discharge, creating a new challenge to manufacturers of electric motors.
The inverter switching mechanism also creates what is called common-mode voltage.
Due to the high switching frequencies of IGBT inverters, parasitic capacitances between stator winding and stator, and between rotor and stator winding become relevant.