(1) Fatigue pitting corrosion: When the bearing is working, the force acting on the shaft is transmitted to the frame through the inner ring, rolling element and outer ring of the bearing, so that the contact surface between the rolling element and the raceway of the inner ring and outer ring generates contact stress. Due to the relative movement of the inner and outer rings, the rolling body rolls along the raceway, and the contact stress on the contact surface changes according to the law of pulsating cycle. When the number of stress cycles reaches a certain value, the surface metal on the roller or the raceway of the inner and outer rings will peel off, that is, fatigue pitting will be formed, which will cause vibration and noise of the bearing, reduce the rotation accuracy and affect the normal work of the machine. Fatigue pitting corrosion is the main failure form of rolling bearings.
(2) Plastic deformation: When the bearing speed is very low (n < 10r/min) or oscillates intermittently, fatigue pitting generally does not occur. At this time, the bearing often bears excessive static load or impact load, which makes the local stress at the contact between the inner and outer raceways and the rolling elements exceed the yield point of the material, resulting in plastic deformation and uneven pits, which makes the bearing invalid.
2. Bearing life and life calculation
(1) Bearing life: The life of a rolling bearing refers to the total number of revolutions of the bearing or the total working hours at a certain speed before any rolling body or raceway in the inner and outer rings of the bearing suffers from fatigue pitting.
For a batch of bearings with the same model and size, because the materials, machining accuracy, heat treatment and assembly quality cannot be exactly the same, even if they work under the same conditions, the life of each bearing is different, and the difference between the longest and shortest life can reach dozens of times, so it is difficult to predict the specific life of a single bearing. In order to ensure the reliability of bearing work, the basic rated life is stipulated as the calculation basis in the national standard.
The basic rated life of bearings refers to a batch of identical bearings working under the same conditions, in which 10% of the total number of revolutions when fatigue pitting occurs is expressed by L 10.
The load that the bearing can bear when the basic rated life is 106r is called the basic rated dynamic load, which is expressed by c. Under the basic rated dynamic load, the reliability of the bearing working in 106r without fatigue pitting is 90%. For radial contact bearing C, it is radial load, axial contact bearing C is central axial load, and centripetal angular contact bearing C is radial component of load. C values of various types and sizes of bearings can be found in the mechanical design manual.
(2) Life calculation: The calculation formula of basic rated life of bearing is:
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Where L 10 is the basic rated life of the bearing,106r; ; FP is equivalent dynamic load, see equivalent dynamic load calculation in this section; ε is the life index, with ball bearing ε = 3 and roller bearing ε≈ 10/3.
In practical calculation, people are used to taking time Lh(h) as the life of bearings. If the bearing speed is n (rpm), another expression for calculating the bearing life is
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When the working temperature of the bearing is higher than 120℃, the bearing life will be reduced and the basic rated dynamic load will be affected. The impact and vibration in the work will increase the actual working load of the bearing, so the temperature coefficient ft (Table 2- 1 1) and the load coefficient fp (Table 2- 12) should be introduced in the calculation to correct the C value and Fp value respectively. At this point, the bearing life calculation formula is:
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Table 2- 1 1 temperature coefficient ft
Table 2- 12 Load Factor fp
3. Calculation of equivalent dynamic load
Equivalent dynamic load is an imaginary load. Under this load, the life of the bearing is the same as that under the actual load.
For cylindrical roller bearings that can only bear radial load, the equivalent dynamic load is the radial load Fr of the bearing, that is
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For thrust ball bearings that can only bear axial load, the equivalent dynamic load is the axial load Fa of the bearing, i.e.
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For deep groove ball bearings, self-aligning bearings and centripetal angular contact bearings that can bear radial and axial loads at the same time, the calculation formula of equivalent dynamic load is
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Where: Fr is the radial load on the bearing; Fa is the axial load on the bearing; X is the radial load factor, as shown in Table 2- 13; Y is the axial load factor, as shown in Table 2- 13.
When looking up table 2- 13, for deep groove ball bearings and 7000C angular contact ball bearings, it is necessary to calculate Fa/C0 first, find out the value of E, and then calculate Fα/Fr and compare it with E before determining the values of X and Y..
Table 2- 13 radial load factor x and axial load factor y
Note: 1. C0 is the basic rated static load of the bearing. Please refer to the mechanical design manual.
2.e is the boundary value of the applicable range of Fa/Fr when the coefficients x and y are different.
3. For the intermediate value of Fa/C0, its E and Y values can be obtained by linear interpolation.
4. Calculation of axial load of centripetal angular contact bearing
As shown in Figure 2-9, due to the contact angle α of the centripetal angular contact bearing, when the bearing bears radial load, the normal force FQi of each rolling element in the bearing area can be decomposed into radial component FRi and axial component FSi. The sum fs (fs = ∑ ifsi) of the axial components of each rolling element will separate the outer ring and the inner ring of the bearing in the axial direction, so such bearings should be installed in pairs in opposite directions.
Figure 2-9 Internal axial force of centripetal angular contact bearing
FS is the axial force generated under radial load, which is usually called internal axial force. Its size is calculated according to the formula given in Table 2- 14, and the direction (for the shaft) points from the wide side of the bearing outer ring to the narrow side in the axial direction.
The actual axial load Fa of centripetal angular contact bearings when used in pairs is not only related to the external axial load FA, but also influenced by the internal axial force FS.
Table 2- 14 Internal axial force FS of centripetal angular contact bearing
Note: Check the Y value in the mechanical design manual.
Fig. 2- 10 shows two installation methods of angular contact ball bearings, fig. 2- 10a shows that the narrow sides of two outer rings are opposite, and fig. 2- 10b shows that the wide sides of two outer rings are opposite. FA is the external axial load, and FS 1 and FS2 are the internal axial forces of bearings 1 and 2 respectively. According to the force balance condition, the actual axial load on the two bearings can be obtained.
Fig. 2- 10 axial load analysis of angular contact bearing
For bearing 1: Since the direction of FS2 is opposite to that of FA, the axial load on the bearing should be determined by comparing the sizes of FS 1 and FS2-FA.
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For bearing 2: Because the direction of FS 1 is the same as that of FA, the axial load on the bearing should be determined by comparing the sizes of FS2 and FS 1+FA.
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If the direction of external axial load FA is opposite to that shown in the figure, (-FA) should be substituted into the formula for calculation.
5. Static load calculation of rolling bearing
The purpose of bearing static load calculation is to prevent bearing from producing excessive plastic deformation.
Under a certain load, if the contact stress between the bearing and the raceway of the inner and outer rings reaches: ball bearing -4200mpa (self-aligning ball bearing -4600MPa) and roller bearing -4000mpa, this load is called the basic rated static load, which is represented by C0. Practice shows that the bearing can work normally when the load does not exceed this load. Therefore, the basic rated static load is the calculation basis of bearing static load. For radial contact bearings, C0 is the radial load; For a centripetal angular contact bearing, C0 is the radial component of the load; For axial contact bearing C0, it is the central axial direction.
Load. When the bearing is working, if it bears both radial load and axial load, it should be calculated as equivalent static load. Equivalent static load is an imaginary load. Under this load, the sum of plastic deformation of the roller and the raceway of the inner and outer rings at the maximum stress is equal to the sum of plastic deformation under the actual load. For radial contact bearings and centripetal angular contact bearings, the equivalent static load is radial load; For axial contact bearings, the equivalent static load is axial load. The equivalent static load is expressed as FP0, and its relationship with the actual load is as follows
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Where: Fr is the radial load on the bearing; Fa is the axial load on the bearing; X0 is the static radial load coefficient, as shown in Table 2-15; Y0 is the static axial load factor, as shown in Table 2- 15.
Table 2- 15 Static radial load coefficient X0 and static axial load coefficient Y0
When the calculation result fp0 < fr, fp0 = fr should be taken.
The strength conditions for static load calculation are as follows
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Where C0 is the basic rated static load of the bearing, refer to the mechanical design manual; S0 is the safety factor, as shown in Table 2- 16.
Table 2- 16 safety factor S0