Calculation method

Abstract

To provide an accurate method for testing a crack advance, a method for calculating an acceptable defect size, a method for inspection, and a method for manufacturing a bearing which are not affected by variations among test pieces.SOLUTION: A test piece 1 with a minute hole 2 formed therein is prepared. A rolling body 4 is rolled multiple times on the test piece 1 and rolling fatigue is applied on the test piece 1. A length from the minute hole 2 of a crack generated in the test piece 1 applied with the rolling fatigue is measured. A measuring step is performed by high-frequency ultrasonic waves of 100 MHz or more.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a crack propagation test method, a method for calculating allowable defect dimensions, an inspection method, and a method for manufacturing a bearing.

Background Art

[0002] The outer ring and inner ring constituting a rolling bearing are repeatedly stressed by rolling elements. As a result, the materials constituting the outer ring and inner ring fatigue, and eventually cracks occur and propagate. Such a fatigue failure phenomenon occurs not only in rolling bearings but also in other metal materials. Therefore, it is important to know about the fatigue of metal materials and the generation and propagation of accompanying cracks in order to estimate the fatigue life of the material and use it safely.

[0003] For example, Japanese Patent Application Laid-Open No. 2015-28441 (Patent Document 1) proposes a test method in which rolling fatigue is applied to a test piece having micro holes, and the length of a crack from an edge portion formed at the bottom of the micro holes is confirmed. Further, Japanese Patent Application Laid-Open No. 2015-28441 discloses a fatigue crack propagation rate diagram showing the relationship between the stress intensity factor range and the crack propagation rate, and thereby, the fatigue life is estimated according to the use conditions of the bearing.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the crack growth test disclosed in Japanese Patent Application Laid-Open No. 2015-28441, in order to confirm the crack length, the test piece is cut and the cross-section is observed. That is, in Japanese Patent Application Laid-Open No. 2015-28441, the test piece is cut when measuring the crack length. For this reason, once the crack length is measured, it is impossible to conduct a test to further grow the crack using that test piece. Therefore, in Japanese Patent Application Laid-Open No. 2015-28441, a plurality of test pieces with different numbers of load applications for rolling fatigue are prepared, and cracks of different lengths are generated for each of the plurality of test pieces, and the cracks are evaluated.

[0006] However, the crack growth rate is affected by the depth of the micro-holes provided in the test piece, the stress field at the crack tip, and the difference in the structure around the crack. Therefore, when the evaluation of cracks of different lengths is made using separate test pieces for each, since there are variations in the above-mentioned parameters for each test piece, the measurement results are affected by them, and measurement results in which the variations in the parameters other than the number of rolling fatigue load applications are taken into account are obtained.

[0007] That is, it is ideal for the evaluation between different crack lengths to be made using the same, that is, a single test piece, because it can eliminate the influence of the variations for each test piece. For this purpose, it is required to use a method in which the length of the crack formed each time a load is applied to the test piece is measured without cutting the test piece, and then the length of the crack is measured again while applying a further load to grow the crack using the same test piece as the original one.

[0008] As a method for measuring the crack length without destroying the test piece, measurement using ultrasonic waves can be mentioned. However, conventionally, since the measurement was made using ultrasonic waves with a relatively low frequency of about several tens of MHz, it was difficult to accurately measure the crack length. That is, even when using ultrasonic waves with such a low frequency, it was not possible to repeat the crack growth treatment and the measurement of the crack length as described above.

[0009] The present invention has been made in view of the above problems. An object thereof is to provide a highly accurate crack propagation test method, a method for calculating allowable defect dimensions, an inspection method, and a method for manufacturing a bearing, which do not include the influence of variations between test pieces.

Means for Solving the Problems

[0010] In the crack propagation test method according to the first example of the present disclosure, a test piece having a micro hole formed therein is prepared. A plurality of rolling elements are rotated on the test piece a plurality of times to apply rolling fatigue to the test piece. The length of a crack generated in the test piece subjected to rolling fatigue from the micro hole is measured. The above measuring step is performed by high-frequency ultrasonic waves of 100 MHz or higher.

[0011] In the crack propagation test method according to the second example of the present disclosure, a test piece having a micro hole formed therein is prepared. A plurality of rolling elements are rotated on the test piece a plurality of times to apply rolling fatigue to the test piece. The length of a crack generated in the test piece subjected to rolling fatigue from the micro hole is measured. After the above measuring step, a re-fatigue step of applying rolling fatigue again to the test piece in which the crack length has been measured is performed. After the above re-fatigue step, a re-measurement step of measuring the crack length of the test piece again is performed.

Effects of the Invention

[0012] According to the above, it is possible to provide a highly accurate crack propagation test method, a method for calculating allowable defect dimensions, an inspection method, and a method for manufacturing a bearing, which do not include the influence of variations between test pieces.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Mode for Carrying Out the Invention

[0014] (Embodiment 1) Hereinafter, the crack propagation test method according to this embodiment will be described with reference to the drawings. FIG. 1 is a flowchart showing the crack propagation test method of this embodiment. Referring to FIG. 1, first, a test piece having a micro-hole used in the crack propagation test is prepared (S10).

[0015] FIG. 2 is a schematic diagram showing the mode of the test piece and the rolling element used in the crack propagation test according to this embodiment. Referring to FIG. 2, for example, a test piece 1 made of high-carbon chromium bearing steel SUJ2 constituting the outer ring and the inner ring of a general rolling bearing is prepared. The test piece 1 is, for example, disk-shaped in FIG. 2. However, the shape of the test piece 1 in FIG. 2 is only an example, and it is not limited to this and can be any other arbitrary shape.

[0016] On, for example, the circular main surface 1a of the test piece 1, minute holes 2 are formed. The minute holes 2 are formed, for example, by drilling. The minute holes 2 formed in the test piece 1 have a circular or elliptical planar shape with a diameter of 0.02 mm or more and a shape close to this. Here, the diameter means the maximum value of the dimensions in the plan view of the formed minute holes 2 (for example, the length of the major axis in the case of an ellipse), in other words, the diameter of the virtual circle circumscribing the planar shape of the minute holes 2.

[0017] A rolling element track 3 is provided on the main surface 1a. The rolling element track 3 is a track for a plurality of rolling elements 4 constituting a rolling bearing for performing a rolling fatigue test to rotate on the main surface 1a a plurality of times and apply a load to the test piece 1. The rolling element track 3, for example, passes through the minute holes 2 and has an annular shape with a radius smaller than that of the circular main surface 1a. However, it is not limited to this, and the rolling element track 3 can have any other shape. In any case, the rolling element track 3 passes through the region where the minute holes 2 are formed.

[0018] FIG. 3 is a schematic cross-sectional view of a portion along line III-III in FIG. 2. Referring to FIG. 3, in the step of preparing the test piece 1, an initial crack 5 of 5 μm or more is formed starting from the minute hole 2. The initial crack 5 is generated, for example, by about 10 4 cycles of rolling fatigue. In FIG. 3 schematically shows the inner wall surface of the minute hole 2 extending in the vertical direction of the figure and having a pointed shape only at the lowermost part, but this is only an example, and the cross-sectional shape of the minute hole 2 is not limited to this. For example, the minute hole 2 may be hemispherical. In FIG. 3, at the lowermost part of the portion where the inner wall surface of the minute hole 2 extends in the vertical direction of the figure, starting from the bent portion at the boundary with the portion where the inner wall surface is inclined to be pointed, the initial crack 5 is generated so as to extend substantially horizontally. In FIG. 3, the length of the extension of the initial crack 5 is designated as a'.

[0019] Referring to FIG. 1 again, next, a rolling fatigue life test is performed on the test piece 1 prepared as shown in FIGS. 2 and 3 (S20). That is, as shown in FIG. 2, for example, the rolling elements 4 on the main surface 1a of the test piece 1 are rotated a plurality of times along the rolling element track 3, whereby rolling fatigue is applied to the test piece 1.

[0020] Next, after the rolling of the rolling element 4 has been repeated a certain number of times, the rolling of the rolling element 4 once ends, and the length of the crack in the test piece 1 is measured (S30). That is, the length of the crack formed so as to extend from the initial crack 5 in the test piece 1 subjected to rolling fatigue is measured. Here, the length of the crack is measured from the micro hole 2 (outer edge). The measurement of the length of this crack is performed by high-frequency ultrasonic waves of 100 MHz or more without cutting the test piece 1. It is more preferable that the measurement is performed by high-frequency ultrasonic waves of 150 MHz or more, and among these, it is even more preferable that the measurement is performed by high-frequency ultrasonic waves of 200 MHz or more.

[0021] Next, after the length of the crack has been measured in (S30) above, rolling fatigue is again applied to the test piece 1 in which the length of the crack has been measured by a rolling fatigue test (S20). Here, the same process as the previous (S20), that is, the process of rotating the rolling element 4 a plurality of times to apply rolling fatigue to the test piece 1, is again performed on the same test piece 1 as the test piece 1 on which the process of the previous (S20) was performed. By this re-fatigue process, the crack formed in the test piece 1 further progresses and its length becomes longer.

[0022] Next, a re-measurement process of measuring the length of the crack in the test piece 1 is again performed on the test piece 1 on which the rolling fatigue life test has been performed again (S30). Also here, as described above, the measurement of the length of the crack is performed by high-frequency ultrasonic waves of 100 MHz or more without cutting the test piece 1. It is more preferable that the measurement is performed by high-frequency ultrasonic waves of 150 MHz or more, and among these, it is even more preferable that the measurement is performed by high-frequency ultrasonic waves of 200 MHz or more.

[0023] As described above, in this embodiment, (S20) and (S30) are repeated for the same (single) test piece 1. Thereby, the length of the crack formed in each case when various numbers of loads are applied is obtained by a single test piece 1.

[0024] Figure 4 is a graph comparing the results of measuring the crack length formed from a micro-hole using the crack propagation test method of the present embodiment with the measurement results by SEM (Scanning Electron Microscope). The horizontal axis shows the results of measuring the crack length of a test piece prepared under the same conditions as the test piece by the crack propagation test method of the present embodiment using SEM. The vertical axis shows the measurement results of the crack length by the measurement method using ultrasonic waves of the present embodiment and the measurement method using an optical microscope as a comparative example. In the measurement method using ultrasonic waves of the present embodiment, high-frequency ultrasonic waves of 200 MHz were used for crack measurement. In the measurement method using an optical microscope as a comparative example, the length of the crack extending from the micro-hole was measured by observing the cross-section by cutting the test piece. The measurement results by SEM are also the results of observing the cross-section by cutting each test piece, but here this measurement result is regarded as having high accuracy. That is, by examining the difference between the measurement results by SEM and the measurement results by each method, the accuracy of the measurement results by each method is verified. Also, in the formula in the graph, x represents the horizontal axis and y represents the vertical axis.

[0025] Referring to Figure 4, the value of the slope of the straight line obtained by plotting the data is closer to 1 when using "ultrasonic waves", that is, the method of the present embodiment, than when using "optical microscope", that is, the method of the comparative example. That is, it can be seen that the crack length can be measured with higher accuracy by the method of the present embodiment than by the method of the comparative example. Also, R 2 indicating the variation value of each data with respect to the obtained straight line is larger by the method of the present embodiment than by the method of the comparative example . That is, it can be seen that the variation is smaller by the method of the present embodiment than by the method of the comparative example.

[0026] Based on the results of Figure 4, the effects of the present embodiment will be described below.

[0027] In the crack propagation test method according to the first example of the present disclosure, a test piece 1 in which a micro hole 2 is formed is prepared. The rolling element 4 is rotated a plurality of times on the test piece 1 to apply rolling fatigue to the test piece 1. The length of the crack generated in the test piece 1 to which rolling fatigue has been applied from the micro hole 2 is measured. The above measuring step is performed by high-frequency ultrasonic waves of 100 MHz or more.

[0028] In the crack propagation test method according to the second example of the present disclosure, a test piece 1 in which a micro hole 2 is formed is prepared. The rolling element 4 is rotated a plurality of times on the test piece 1 to apply rolling fatigue to the test piece 1. The length of the crack generated in the test piece 1 to which rolling fatigue has been applied from the micro hole 2 is measured. After the measuring step, a re-fatigue step of applying rolling fatigue again to the test piece 1 in which the crack length has been measured is performed. After the re-fatigue step, a re-measurement step of measuring the crack length of the test piece 1 again is performed.

[0029] A crack of a certain length generated in the test piece 1 by the rolling fatigue life test is measured without destroying the test piece 1 using ultrasonic waves of 100 MHz or more, for example, as in the first example above. In this way, in both the first example and the second example, higher-precision measurement results can be obtained compared to the case where measurement is performed using ultrasonic waves with a low frequency of about several tens of MHz. Therefore, even when measuring a longer crack and measuring the crack length thereafter, the undamaged test piece 1 can be reused. Therefore, according to the present embodiment, for example, when measuring using a plurality of test pieces 1, high-precision measurement results can be obtained without including the variation (affecting the measurement results) occurring between each test piece 1. High-precision measurement results can be obtained.

[0030] As described above, it is possible to more accurately test and confirm the crack growth lower limit and the crack growth rate. Specifically, first, regarding the crack growth lower limit, when a certain number of loads, i.e., rolling fatigue, is applied to the test piece and the crack growth formed in the outer ring and the inner ring, etc., is small, it has been judged that the crack has not progressed and has remained stationary until now. However, until now, it has not been possible to confirm whether the crack has truly not progressed and has remained stationary. This is because the length of the initial crack was unknown. However, according to this embodiment, for one test piece 1, the crack length before and after applying an arbitrary number of loads can be measured. Therefore, it is possible to confirm whether the crack has not progressed and has remained stationary by freely measuring and comparing the crack lengths before and after the rolling fatigue life test.

[0031] Also, regarding the crack growth rate, until now, it has been estimated from the results of tests on the crack lengths formed by changing the load, i.e., the number of times of rolling fatigue, applied to each of a plurality of test pieces. However, according to this embodiment, for one test piece 1, the formation of the crack and the measurement of its length can be repeated non-destructively a plurality of times. Therefore, for one crack, a crack growth test can be carried out while confirming its progress. As a result, the crack growth rate can be confirmed more accurately than before.

[0032] In the crack growth test method according to the first example of the present disclosure described above, the measuring step is preferably performed by high-frequency ultrasonic waves of 150 MHz or more. In the crack growth test method according to the first example of the present disclosure described above, the measuring step is preferably performed by high-frequency ultrasonic waves of 200 MHz or more. Thereby, the crack length can be measured with higher precision.

[0033] In the crack propagation test method according to the first example of the present disclosure described above, after the measuring step, a re-fatigue step of applying rolling fatigue to the test piece 1 whose crack length has been measured may be performed again, and after the re-fatigue step, a re-measurement step of measuring the crack length of the test piece 1 may be performed again. The effects thereof are as described above. Note that also in this re-measurement step, it is preferably performed using high-frequency ultrasonic waves of 100 MHz or more, more preferably 150 MHz or more, and still more preferably 200 MHz or more.

[0034] In the crack propagation test method according to the second example of the present disclosure described above, the above-mentioned measuring step and the above-mentioned re-measurement step are preferably performed using high-frequency ultrasonic waves of 100 MHz or more, more preferably 150 MHz or more, and still more preferably 200 MHz or more. The effects thereof are as described above.

[0035] In the crack propagation test method according to the first and second examples of the present disclosure described above, in the step of preparing the test piece 1, the micro-hole 2 formed in the test piece 1 preferably has a diameter of 0.1 mm or less. This is because the size of the non-metallic inclusions actually contained in the material of the test piece 1 is 0.1 mm or less, and to prevent the influence of the micro-hole 2 from becoming large at the contact portion between the rolling element 4 and the rolling element track 3. By doing so, the crack can be made to progress with as little influence as possible other than the applied load, and the reliability of the measurement result can be further enhanced.

[0036] In the crack propagation test method according to the first and second examples of the present disclosure described above, in the step of preparing the test piece 1, it is preferable that an initial crack 5 of 5 μm or more is formed starting from the micro-hole 2. By doing so, the crack can be made to progress with as little influence as possible other than the applied load, and the reliability of the measurement result can be further enhanced.

[0037] (Embodiment 2) In this embodiment, the range of the stress intensity factor of the crack in the shear-type fatigue crack growth called Mode II and the method for calculating the allowable defect size of the test piece are described starting from the crack formed in the test piece. FIG. 5 is a flowchart showing the range of the stress intensity factor of the crack and the method for calculating the allowable defect size of the test piece in Embodiment 2. FIG. 6 is a schematic diagram of the same part as FIG. 3 showing the mode in which the calculation method of Embodiment 2 is performed. With reference to FIGS. 5 and 6, the steps of the crack growth test method similar to the steps shown in Embodiment 1 (FIG. 1) are carried out. That is, first, a test piece having the micro hole 2 used in the crack growth test is prepared (S10). The diameter D of the micro hole 2 is preferably 0.1 mm or less as in Embodiment 1. The diameter D of the micro hole 2 is the diameter when the micro hole 2 has a circular planar shape, and is the maximum value of the dimensions in the plan view (for example, the length of the major axis in the case of an ellipse) when it is not circular. Also, the center of the micro hole 2 in the plan view is the position of the centroid of the micro hole 2 in the plan view, and if the micro hole 2 has a circular or elliptical planar shape, the positions of the center and the centroid coincide. A rolling fatigue life test is carried out on the prepared test piece 1, and the rolling elements 4 are rotated a plurality of times (S20). The length a1 of the crack 5A formed from the micro hole 2 in the test piece 1 subjected to the rolling fatigue is measured (S30). The measurement of the length a1 of this crack 5A is carried out by high-frequency ultrasonic waves of 100 MHz or more.

[0038] In this embodiment, in measuring the length of crack 5A, it is evaluated whether the length a1 of crack 5A from micro-hole 2 is longer than the diameter D of micro-hole 2. If the length a1 of the crack 5A is longer than the diameter D of the micro-hole 2, the first test consisting of the above steps ends. If the length a1 of crack 5A is less than or equal to the diameter D of micro-hole 2, the rolling fatigue life test (S20) and the measurement of the length of crack 5A (S30) are performed again using the same test piece 1 in the same manner as above. The steps (S20) and (S30) for the same test piece 1 are repeated until the length a1 of crack 5A becomes longer than the diameter D of micro-hole 2. Note that the length from micro-hole 2 to the crack tip 5Q of crack 5B formed from micro-hole 2 may or may not be equal to a1 above. When a plurality of cracks are formed, it is sufficient if the length of at least one of the cracks becomes longer than D. The above steps are referred to as the first test.

[0039] Next, based on the result of the first test above, that is, the length of crack 5A that has become longer than the diameter of micro-hole 2, the range ΔK of the stress intensity factor at crack tip 5P, which is the tip of crack 5A having a dimension a1 longer than the diameter D of micro-hole 2 II is calculated (S40). The following formula (1) is used for this calculation.

[0040]

Equation

[0041] The range ΔK of the stress intensity factor II has the unit of (MPa√m). The range ΔK of the stress intensity factor in formula (1) II is a value indicating the change amount of the stress intensity factor when the rolling element 4 is rolled on the rolling element track 3 of the test piece 1. That is, for example, when the rolling element 4 rolls in the direction indicated by arrow M in FIG. 6 (rightward) on the rolling element track 3 shown in FIG. 3, the rolling element 4 applies a stress having an approximately parabolic distribution shown in FIG. 6 to the rolling element track 3 (main surface 1a). The maximum of the stress (existing at the vertex of the parabola) is the maximum surface pressure indicated by Pmax. This stress distribution is shown in FIG. 6 In order to move from the left side to the right side, for example, the stress intensity factor at the crack tip 5Q of the crack 5B on the left side has different values when the rolling element 4 approaches, passes through, and passes by the crack tip 5Q. The same applies to the stress intensity factor at the crack tip 5P of the crack 5A on the right side. The difference between the maximum value and the minimum value among these different stress intensity factor values is the range ΔK of the stress intensity factor II which is represented by. The coefficient 0.136 in Equation (1) corresponds to the difference in the values of the coefficients for obtaining the maximum and minimum values of the stress intensity factor. Also, at least a in Equation (1) is the distance from the center of the micro hole 2 in plan view to the tip of the crack 5A (crack tip 5P). That is, a indicates the radius of the crack 5A including half of the diameter D of the micro hole 2, D / 2. On the other hand, the above length a1 is the length of only the crack 5A, that is, the length of the crack 5A as the distance from the outer edge of the micro hole 2 to the crack tip 5P (excluding the diameter D of the micro hole 2). The length a1 is longer than the diameter D of the micro hole 2. Therefore, as shown in FIG. 6, a and a1 have different values, and a1 < a. By substituting the values of Pmax and a into Equation (1), ΔK is obtained. Here, for the sake of explanation as an example, the range ΔK II of the calculated stress intensity factor is set to a value of 3 MPa√m. After the step of calculating the range ΔK II of the stress intensity factor, a rolling fatigue life test is performed again on the same test piece 1 as the test piece 1 used in each of the steps so far (S50). That is, for the same test piece 1 as the test piece 1 used in each of the steps so far, the rolling element 4 is rolled under the previously obtained ΔK

[0042] or more conditions. The change in the value of ΔK II is made, for example, by changing the value of the maximum surface pressure Pmax. Then, again, the length a1 of the crack 5A of the test piece 1 is measured II (S60), and the presence or absence of the progress of the crack 5A of the test piece 1 is confirmed by comparison with the previously measured length a1. Each of the steps (S50) and (S60) so far is regarded as the second test. II The value of is changed, for example, by changing the value of the maximum surface pressure Pmax. And then, again, the length a1 of the crack 5A of the test piece 1 is measured (S60), and the presence or absence of the progress of the crack 5A of the test piece 1 is confirmed by comparison with the previously measured length a1. Each of the steps (S50), (S60) up to here is taken as the second test.

[0043] For purposes of explanation by way of example, in step (S50), ΔK II is tested under the condition that the value is 5 MPa√m, and it is assumed that the propagation of crack 5A is confirmed in step (S60). Based on this result, while changing the value of the stress intensity factor range ΔK II , the above-described second test is repeated until crack 5A stops propagating. Specifically, if the propagation of crack 5A is confirmed when the value of ΔK II is 5 MPa√m, then next, for example, rolling fatigue is applied again under the condition that the value of ΔK II is 4 MPa√m, and the crack is measured again. And if the propagation of crack 5A proceeds under the condition that the value of ΔK II is 4 MPa√m, the value of the stress intensity factor range ΔK II is further decreased and the second test consisting of step (S50) and step (S60) is further performed. If the propagation of crack 5A stops by setting ΔK II to 4 MPa√m, then 4 MPa√m at that time, which is ΔK II , is obtained as the crack propagation lower limit value ΔK IIth for crack 5A (S70). Alternatively, if the propagation of crack 5A stops by setting ΔK II to 4 MPa√m, conversely, ΔK II may be increased to, for example, 4.5 MPa√m and the presence or absence of the propagation of crack 5A may be examined in the same manner. In this way, by repeating the test while changing the value of the stress intensity factor range ΔK II (for example, while decreasing it), the maximum value of ΔK II when crack 5A stops propagating is obtained as ΔK IIth . In other words, the boundary value between when ΔK II at which crack 5A propagates and when ΔK II at which crack 5A does not propagate is obtained as ΔK IIth . More specifically, the crack propagation lower limit value ΔK IIth is the maximum value of the stress intensity factor range ΔK II that enables crack 5A not to propagate in test piece 1. When the value of the stress intensity factor range ΔK II exceeds the value of the crack propagation lower limit value ΔK IIth , crack 5A propagates.

[0044] The required lower limit value of crack propagation ΔK IIth From this, the allowable defect size a of test piece 1 th is calculated (S80). Specifically, for ΔK in the above formula (1) II substitute the ΔK IIth obtained in step (S70), for example, 4 MPa√m, and the maximum surface pressure Pmax applied during that test The value of a uniquely obtained by doing so is the allowable defect size a th and is obtained as such.

[0045] FIG. 7 is a schematic cross-sectional view showing a first example of a bearing including a test piece used in the calculation method of Embodiment 2. Referring to FIG. 7, test piece 1 may be, for example, a raceway ring of a radial bearing 101. The radial bearing 101 has an outer ring 1A and an inner ring 1B. These raceway rings may be used as test piece 1. A plurality of rolling elements 4 roll between the outer ring 1A and the inner ring 1B. The plurality of rolling elements 4 are arranged by a cage 15 so as to have intervals in the circumferential direction.

[0046] FIG. 8 is a schematic cross-sectional view showing a second example of a bearing including a test piece used in the calculation method of Embodiment 2. Referring to FIG. 8, test piece 1 may be, for example, a raceway ring of a thrust bearing 102. The thrust bearing 102 has a shaft raceway plate 1C and a housing raceway plate 1D. These raceway rings may be used as test piece 1. A plurality of rolling elements 4 as rollers roll between the shaft raceway plate 1C and the housing raceway plate 1D. The plurality of rolling elements 4 are arranged by a cage 15 so as to have intervals in the circumferential direction. The thrust bearing 102 in FIG. 8 is a so-called flat raceway ring in which the shaft raceway plate 1C and the housing raceway plate 1D as raceway rings are not provided with grooves on the raceway surfaces in contact with the rolling elements 4. Thus, the raceway ring may be flat.

[0047] FIG. 9 is a schematic cross-sectional view showing a third example of a bearing including a test piece used in the calculation method of Embodiment 2. Referring to FIG. 9, the test piece 1 may be, for example, a raceway ring of a thrust bearing 103. In the thrust bearing 103, rolling elements 4 as a plurality of balls roll between a shaft raceway plate 1C and a housing raceway plate 1D. In the thrust bearing 102 of FIG. 9, grooves are provided in the raceway surfaces where the shaft raceway plate 1C and the housing raceway plate 1D as raceway rings come into contact with the rolling elements 4. Although the thrust bearing 103 is different from the thrust bearing 102 in this respect, since other aspects are the same, the description will not be repeated here.

[0048] As described above, in the present embodiment, the test piece 1 may be a raceway ring of any of the thrust bearings 102 and 103 and the radial bearing 101. These test pieces 1 (raceway rings) used in the calculation method of the present embodiment are preferably made of the same material as the raceway rings of the products that are scheduled to be inspected for quality later. Thereby, the reliability of the quality assurance of the actual products described later using the calculation results can be further enhanced.

[0049] Next, the effects of the present embodiment will be described.

[0050] The calculation method of the present embodiment is a calculation method using the crack propagation test method described in Embodiment 1. The length a1 of the crack 5A generated in the test piece 1 from the micro hole 2 (outer edge) due to the step of applying rolling fatigue is longer than the diameter D of the micro hole 2. Based on the result of the first test by the crack propagation test method described in Embodiment 1 above, the range ΔK of the stress intensity factor at the crack tip 5P which is the tip of the crack 5A longer than the diameter of the micro hole 2 II is calculated (S40).

[0051] The range ΔK of the stress intensity factor II is obtained by the following formula (1), where a is the length from the center in plan view of the micro hole 2 of the crack 5A obtained by the first test, and Pmax is the maximum surface pressure applied by the rolling element 4 to the test piece 1. It is obtained by the following formula (1).

[0052]

Equation

[0053] By the way, when the micro-hole 2 is formed by drilling, stress concentration occurs around the micro-hole 2. For this reason, the crack tip 5P, which is the tip of the crack 5A formed from the micro-hole 2, is affected by the stress concentration. Specifically, a larger stress acts on the crack tip 5P of the test piece 1 in which the micro-hole 2 is formed than on the test piece 1 in which the micro-hole 2 is not formed. On the other hand, although the above formula (1) is a generally known formula, this is a formula premised on being applied to a so-called disk crack model that does not have the micro-hole 2. Therefore, when obtaining the range ΔK of the stress intensity factor at the crack tip 5P in the system where the micro-hole 2 is formed by drilling, an error occurs between the actual value and the calculated value. As described above, a larger stress is generated in the crack 5A of the test piece 1 having the micro-hole 2 than in the crack 5A of the test piece 1 not having the micro-hole 2, resulting in an error corresponding to the magnitude of the larger stress. For this reason, it has been difficult to directly apply the above formula (1) to a model having the crack 5A generated from the micro-hole 2 in the test piece 1. Also, a calculation formula for accurately obtaining the range ΔK of the stress intensity factor for a model having the micro-hole 2 has not yet been established. II Therefore, in the present embodiment, as described above, the length a1 of the crack 5A from the micro-hole 2 (outer edge) is made longer than the diameter D of the micro-hole 2. According to the San-Bunan principle, if the length a1 of the crack 5A from the micro-hole 2 (outer edge) is made larger than the diameter D of the micro-hole 2, the crack tip 5P is not affected by the stress concentration due to the micro-hole 2. For this reason, in a system where a1 is larger than D, the range ΔK of the stress intensity factor at the crack tip 5P can be calculated with high accuracy using the known formula (1) based on the disk crack model. In the present embodiment, it is more preferable that the length a1 of the crack 5A from the outer edge of the micro-hole 2 is made longer than twice the diameter D of the micro-hole 2. That is, in the present embodiment, for a model having the micro-hole 2, the range ΔK of the stress intensity factor is obtained using the known formula (1) on the premise of not having the micro-hole 2. II has not yet been established.

[0054] Therefore, in the present embodiment, as described above, the length a1 of the crack 5A from the micro-hole 2 (outer edge) is made longer than the diameter D of the micro-hole 2. According to the San-Bunan principle, if the length a1 of the crack 5A from the micro-hole 2 (outer edge) is made larger than the diameter D of the micro-hole 2, the crack tip 5P is not affected by the stress concentration due to the micro-hole 2. For this reason, in a system where a1 is larger than D, the range ΔK of the stress intensity factor at the crack tip 5P can be calculated with high accuracy using the known formula (1) based on the disk crack model. hole 2 (outer edge) is made larger than the diameter D of the micro-hole 2, the crack tip 5P is not affected by the stress concentration due to the micro-hole 2. For this reason, in a system where a1 is larger than D, the range ΔK of the stress intensity factor at the crack tip 5P can be calculated with high accuracy using the known formula (1) based on the disk crack model. II In the present embodiment, it is more preferable that the length a1 of the crack 5A from the outer edge of the micro-hole 2 is made longer than twice the diameter D of the micro-hole 2. That is, in the present embodiment, for a model having the micro-hole 2, the range ΔK of the stress intensity factor is obtained using the known formula (1) on the premise of not having the micro-hole 2. IIAs a method enabling the calculation of the value, it has been derived through dedicated research.

[0055] In the above calculation method, after the step (S40) of calculating the range ΔK of the stress intensity factor II a step (S50) of applying rolling fatigue again to the test piece 1 as a second test and a step (S60) of measuring the length a1 of the crack 5A again are further provided, and a step of confirming the presence or absence of the progress of the crack 5A in the test piece 1 is included. Based on the result of the step of confirming the presence or absence of the progress, while changing the range ΔK of the stress intensity factor II the second test is repeated until the crack 5A stops progressing, and the crack growth lower limit value ΔK of the crack 5A IIth is obtained (S70). From the crack growth lower limit value ΔK IIth the allowable defect size a of the test piece 1 th is obtained.

[0056] As described above, since ΔK II is obtained with high precision without error, the crack growth lower limit value ΔK IIth obtained based on this is also obtained with high precision without error. As a result, the allowable defect size a IIth obtained from the ΔK th obtained with high precision is also obtained with high precision without error. The crack growth lower limit value ΔK IIth is the maximum value of the range ΔK of the stress intensity factor that enables the crack 5A not to progress in the test piece 1. The allowable defect size a II is obtained as the value of a when the crack growth lower limit value ΔK th is substituted into ΔK in the following formula (1) II for the above reason. IIth

[0057]

Equation

[0058] Also, the allowable defect size a th ​Thus, as will be described later, in the inspection for quality assurance of actual products, it becomes possible to determine whether the product is a good product or a defective product by examining the dimensions of the defects included in the product, and the inspection process can be simplified. In addition, the reliability of the inspection process can be enhanced.

[0059] (Embodiment 3) FIG. 10 is a flowchart showing an inspection method for examining the presence or absence of defects in a raceway ring in Embodiment 3. Referring to FIG. 10, in the present embodiment, based on the value of the allowable defect dimension a th obtained in Embodiment 2 as a reference value, an inspection method for defects included inside a raceway ring (for example, an outer ring or an inner ring) constituting an actual bearing product is provided. Specifically, first, an object is prepared (S15). Here, as described above, a raceway ring such as an outer ring or an inner ring of an actual bearing product, which is an object of inspection, is prepared.

[0060] Next, the presence or absence of defects exceeding the allowable defect dimension in the object is inspected (S25). This inspection is preferably performed by an ultrasonic flaw detection method using high-frequency ultrasonic waves of 10 MHz or more. Alternatively, this inspection may be performed by a radiation transmission test. In this way, it becomes possible to inspect all products by non-destructive inspection.

[0061] In step (S25), it is inspected whether the largest of the defects included in the object is th equal to or smaller than the allowable defect dimension a obtained in Embodiment 2. If the dimension of the largest of the defects included in the object is equal to or smaller than the allowable defect dimension a th , the product is qualified, and if it exceeds the allowable defect dimension a th , the product is unqualified.

[0062] Next, the effects of the present embodiment will be described.

[0063] In the inspection method of this embodiment, an object is prepared, and the presence or absence of a defect exceeding the allowable defect dimension in the object is inspected. In the inspection step, whether the defect included in the object is the allowable defect dimension a obtained by the calculation method described in Embodiment 2 th is inspected to see if it is the following dimension.

[0064] The allowable defect dimension a accurately obtained according to Embodiment 2 th Based on the value of, by inspecting the actual product, the quality of the product can be determined. Therefore, for example, the crack propagation lower limit value ΔK of the actual product IIth Without using a parameter with a complicated calculation like this, it is possible to simply and reliably inspect the actual product by using a parameter such as the dimension of the defect that is easy to detect. In this way, if the range ΔK of the stress intensity factor II is accurately obtained, from this, the crack propagation lower limit value ΔK IIth and the allowable defect dimension a th can be accurately obtained and applied to the quality assurance of highly reliable products. Therefore, the bearing manufacturing method using the above inspection method can provide a high-quality raceway ring without defects in which cracks progress, and a high-quality bearing including the raceway ring.

[0065] Each of the embodiments disclosed this time should be considered as illustrative in all respects and not restrictive. The scope of the present invention is shown not by the above description but by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

Explanation of Reference Numerals

[0066] 1 Specimen, 1a Main surface, 1A Outer ring, 1B Inner ring, 1C Shaft raceway plate, 1D Housing raceway plate, 2 Micro hole, 3 Rolling element raceway, 4 Rolling element, 5 Initial crack, 5A, 5B Cracks, 5P, 5Q Crack tips, 15 Retainer, 101 Radial bearing, 102, 103 Thrust bearing.

Claims

1. A calculation method using a crack growth test method, the crack growth test method comprising: Preparing a test piece having a microhole formed therein; a step of rotating a rolling element on the test piece a number of times to cause rolling fatigue to the test piece; and measuring the length of a crack generated in the test piece to which the rolling contact fatigue has been applied from the small hole, The measuring step is carried out using high-frequency ultrasonic waves of 100 MHz or more, the step of applying rolling fatigue and the step of measuring are repeated on the test piece until the length of the crack generated in the test piece from the microhole by the step of applying rolling fatigue becomes longer than the diameter of the microhole, and when the length of the crack becomes longer than the diameter of the microhole, the first test repeated by the crack growth test method is terminated; The calculation method further comprises a step of calculating a range of stress intensity factors at the tip of the crack longer than a diameter of the small hole based on the results of the first test.

2. a re-fatigue process in which, after the measuring process, the test piece whose crack length has been measured is subjected to rolling contact fatigue again; The calculation method according to claim 1 , further comprising a re-measuring step of measuring the length of the crack in the test piece again after the re-fatigue step.

3. A calculation method using a crack growth test method, the crack growth test method comprising: Preparing a test piece having a microhole formed therein; a step of rotating a rolling element on the test piece a number of times to cause rolling fatigue to the test piece; measuring the length of a crack generated in the test piece to which the rolling contact fatigue has been applied from the small hole; a re-fatigue process in which, after the measuring process, the test piece whose crack length has been measured is subjected to rolling contact fatigue again; and a re-measuring step of measuring the length of the crack in the test piece again after the re-fatigue step, the step of applying rolling fatigue and the step of measuring are repeated on the test piece until the length of the crack generated in the test piece from the microhole by the step of applying rolling fatigue becomes longer than the diameter of the microhole, and when the length of the crack becomes longer than the diameter of the microhole, the first test repeated by the crack growth test method is terminated; The calculation method further comprises a step of calculating a range of stress intensity factors at the tip of the crack longer than a diameter of the small hole based on the results of the first test.

4. The calculation method according to any one of claims 1 to 3, wherein in the step of preparing the test piece, an initial crack having a size of 5 µm or more is formed starting from the small hole.

5. The calculation method according to any one of claims 1 to 4, wherein the microhole formed in the test piece in the step of preparing the test piece has a diameter of 0.1 mm or less.

6. The range ΔK of the stress intensity factor is given by: a is the length from the center of the small hole in a plan view of the crack obtained in the first test; and P is the maximum surface pressure applied by the rolling element to the test piece. [0010] The calculation method according to any one of claims 1 to 5, wherein the calculation is performed by:

Images