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Service life of super precision bearings

by:JNSN     2021-12-25
Super-precision bearings must be able to withstand loads at extremely high speeds and have high precision. Their selection mainly considers the following factors: Accuracy, Rigidity, Running performance. These performances can only be achieved when there is no wear during the entire life cycle. This depends on the formation of a hydrodynamic lubricant film in the rolling contact area. Under these conditions, rolling bearings can reach their maximum life in various applications. But this reliable life is usually limited by the life of the lubricant (as shown in 5). In this respect, the Hertzian stress at the contact point and the dynamic performance of the bearing play a decisive role. It is best to use a special calculation program to determine the layout of each bearing, especially for high-performance bearing units. In actual work, since super-precision bearings will not fail due to fatigue, the method of determining the service life of the bearing based on the rating life L10 calculated according to DIN ISO 281 is no longer appropriate. Bearing static load For ultra-precision bearings, the static load, that is, the load when the bearing is not rotating, is rarely checked. The stress coefficient fs used to measure the static load can be obtained by the following formula: fs u003d C0/P0fs u003d static stress coefficient C0 u003d rated static load [kN] P0 u003d equivalent static load [kN] The equivalent static load is based on the axial and The radial load is calculated (see below). When there are several bearings on the shaft, the distribution of the external load on a single bearing is shown in Figure 8. In each case, the load carrying capacity of the bearing with the highest load needs to be tested. Spindle bearing contact angle α u003d 15° P0 u003d Fr [kN] When Fa/Fr 1.09 P0 u003d 0.5 · Fr + 0.46 · Fa [kN] When Fa/Fr 1.09 Contact angle α u003d 25° P0 u003d Fr [kN] When Fa/Fr 1.31P0 u003d 0.5 · Fr + 0.38 · Fa [kN] When Fa/Fr 1.31 bearings are in contact with each other or are positioned with spacers, the axial and radial forces calculated based on the external load. In order to ensure the accuracy of the bearing, the static stress index should be greater than 3.0. Only in a very short time and under the action of central axial load (unloading force), fs 1 can be used for hybrid ball bearings. The stress coefficient of double-direction angular contact thrust ball bearing P0 u003d Fa should be higher than 2.5. Floating displacement bearings and cylindrical roller bearings P0 u003d Fr static stress coefficient should be higher than 3.0. Endurance strength should be tested for endurance strength. The static stress coefficient fs* u003d C0/P0*P0* can be calculated according to the following formula; the equivalent static load formula can be used for calculation; but the calculation uses the same dynamic load as the equivalent load. The stress factor is used to determine whether the bearing will fail under specific application conditions. If the factor fs* 8, the bearing is considered to have an infinite life. It is more accurate to use the calculation program to calculate the Hertzian contact stress and check the dynamic performance of the bearing (see appendix, page 226). If there is a continuous lubricating film (κ 2) and extremely high cleanliness conditions, there is no need to calculate the rated life. The cleanliness level of lubricating oil is the basis for evaluating whether circulating particles will reduce the life of the bearing. It can be judged by the lubricating oil samples provided by the filter manufacturer and academic institutions. If all the oil can flow through the filter within a few minutes, the required cleanliness level has been reached. To ensure extremely high cleanliness, the bearing must be cleaned before it runs. For example, the filtration ratio β3 200 means that only two hundredths of particles with a diameter of 3 μm can pass through the filter in the so-called multiple filtration test. In order to avoid damage to other components of the lubricating oil circulation system, filters with a particle size exceeding β3 75 should not be used. If the above conditions are not met, the revised life calculated by hand calculation according to DIN ISO 281 Appendix 1 or the computer-aided program according to DIN ISO 281 Appendix 4 can be used to evaluate the influence of lubrication and contamination on the service life of the bearing. Cleanliness is very important for precision bearings, because the cleanliness of general light-load bearings has a great impact on the service life, and pollution will increase the degree of wear. For oil-lubricated bearings, the reference value of lubricant cleanliness is taken from the hydraulic field and can be obtained from Table 9. In actual applications, if the bearings are greased and sealed by the manufacturer, extreme cleanliness can be ensured. The inner and outer rings of ultra-precision bearings at operating temperature can ensure geometric stability when the temperature is not higher than 150 °C. If this value is not exceeded, there is no need to consider the effect of temperature on material properties. Attention must be paid to the limit temperature of the cage, seal and lubricant (see Table 10). For the application of ultra-precision bearings at higher temperatures, please consult the Schaeffler Group Industrial Application Department.
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