JP7699692B2 - Calculation method - Google Patents

Description

The present invention relates to a crack growth test method, a method for calculating allowable defect dimensions, an inspection method, and a method for manufacturing bearings.

The outer and inner rings that make up a rolling bearing are subjected to repeated stress from the rolling elements. This causes fatigue in the material that makes up the outer and inner rings, and eventually cracks occur and grow. This type of fatigue failure phenomenon occurs not only in rolling bearings, but also in other metallic materials. Therefore, understanding the fatigue of metallic materials and the associated crack initiation and growth is important for estimating the fatigue life of the material and using it safely.

For example, JP 2015-28441 A (Patent Document 1) proposes a test method in which rolling fatigue is applied to a test piece with a small hole formed therein, and the length of the crack from the edge formed at the bottom of the small hole is confirmed. Furthermore, JP 2015-28441 A discloses a fatigue crack growth rate diagram showing the relationship between the stress intensity factor range and the crack growth rate, which allows the fatigue life of the bearing to be estimated according to the operating conditions.

JP 2015-28441 A

In the crack propagation test disclosed in JP 2015-28441 A, the test specimen is cut and the cross section is observed to confirm the crack length. That is, in JP 2015-28441 A, the test specimen is cut when measuring the crack length. Therefore, once the crack length is measured, it is impossible to use the test specimen for a test to further propagate the crack. Therefore, in JP 2015-28441 A, multiple test specimens are prepared that have different numbers of times that a load is applied for rolling fatigue, and cracks of different lengths are generated for each of the multiple test specimens to evaluate the cracks.

However, the crack growth rate is affected by the depth of the micro-hole in the test specimen, the stress field at the crack tip, and differences in the structure around the crack. Therefore, if evaluations of cracks of different lengths are all performed using separate test specimens, the measurement results will be affected by the variations in the above parameters that exist for each test specimen, and the measurement results obtained will simply take into account the effects of variations in each parameter other than the number of times the rolling fatigue load is applied.

In other words, it would be ideal if all evaluations of different crack lengths were done using the same, i.e., single, test specimen, in order to eliminate the effects of variability between the test specimens. To achieve this, a technique is required in which the length of the crack that forms each time a load is applied to the test specimen a certain number of times is measured without cutting the specimen, and then the same test specimen is used to apply further load to cause the crack to grow while measuring the crack length again.

Measurement using ultrasonic waves is one method for measuring the crack length without destroying the test piece. However, in the past, measurements were performed using ultrasonic waves with low frequencies of about several tens of MHz, making it difficult to measure the crack length accurately. In other words, even if ultrasonic waves with such low frequencies were used, it was not possible to repeatedly treat the crack growth and measure the crack length as described above.

The present invention has been made in consideration of the above problems. Its purpose is to provide a highly accurate crack growth test method, a calculation method for allowable defect dimensions, an inspection method, and a manufacturing method for bearings that are not affected by variations between test pieces.

In the crack propagation test method according to the first example of the present disclosure, a test piece with a small hole formed therein is prepared. A rolling element is rotated multiple times on the test piece to impart rolling fatigue to the test piece. The length from the small hole to the crack that has developed in the test piece to which rolling fatigue has been imparted is measured. The above measurement process is performed using high-frequency ultrasonic waves of 100 MHz or more.

In the crack propagation test method according to the second example of the present disclosure, a test piece with a small hole formed therein is prepared. A rolling element is rotated multiple times on the test piece to impart rolling fatigue to the test piece. The length from the small hole of a crack that has developed in the test piece to which rolling fatigue has been imparted is measured. After the above measurement process, a re-fatigue process is performed in which the test piece whose crack length has been measured is again imparted with rolling fatigue. After the above re-fatigue process, a re-measurement process is performed in which the crack length of the test piece is measured again.

Based on the above, it is possible to provide a highly accurate crack growth test method, a method for calculating allowable defect dimensions, an inspection method, and a manufacturing method for bearings that are not affected by variations between test pieces.

2 is a flowchart showing a crack growth test method according to the present embodiment. FIG. 2 is a schematic diagram showing an aspect of a test piece and rolling elements used in a crack growth test according to the present embodiment. 3 is a schematic cross-sectional view of a portion taken along line III-III in FIG. 2. 1 is a graph comparing the results of measuring the length of a crack formed from a small hole using the crack growth test method of the present embodiment with the results of measurement using a SEM (Scanning Electron Microscope). 10 is a flowchart showing a method for calculating a range of a stress intensity factor of a crack and an allowable defect size of a test piece in embodiment 2. 5 is a schematic diagram of the same part as FIG. 3, showing a manner in which a calculation method according to a second embodiment is performed. FIG. FIG. 11 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. FIG. 11 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. FIG. 11 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. 13 is a flowchart showing an inspection method for checking whether or not there is a defect in a race in the third embodiment.

(Embodiment 1)
The crack propagation test method according to the present embodiment will be described below with reference to the drawings. Fig. 1 is a flow chart showing the crack propagation test method according to the present embodiment. Referring to Fig. 1, a test piece having a small hole to be used in the crack propagation test is first prepared (S10).

Figure 2 is a schematic diagram showing the configuration of the test piece and rolling elements used in the crack growth test according to this embodiment. With reference to Figure 2, a test piece 1 is prepared, for example, made of high carbon chromium bearing steel SUJ2, which constitutes the outer and inner rings of a general rolling bearing. In Figure 2, the test piece 1 is, for example, disk-shaped. However, the shape of the test piece 1 in Figure 2 is merely one example, and is not limited to this, and any other shape may be used.

A microhole 2 is formed on the principal surface 1a of the test piece 1, which may be circular, for example. The microhole 2 is formed, for example, by drilling. The microhole 2 formed in the test piece 1 has a circular or elliptical planar shape with a diameter of 0.02 mm or more, or a shape close to this. Here, the diameter means the maximum dimension of the formed microhole 2 in a planar view (for example, the length of the major axis if the shape is elliptical), in other words, the diameter of an imaginary circle circumscribed by the planar shape of the microhole 2.

A rolling element raceway 3 is provided on the main surface 1a. The rolling element raceway 3 is a raceway along which the rolling elements 4 constituting the rolling bearing for performing the rolling fatigue test rotate multiple times on the main surface 1a to apply a load to the test piece 1. The rolling element raceway 3 passes through the micro-holes 2, for example, and has a circular ring shape with a smaller radius than the circular main surface 1a. However, the rolling element raceway 3 is not limited to this and can have any other shape. In any case, the rolling element raceway 3 passes through the area in which the micro-holes 2 are formed.

3 is a schematic cross-sectional view of a portion along line III-III in FIG. 2. Referring to FIG. 3, in the process of preparing the test piece 1, an initial crack 5 of 5 μm or more is formed starting from the micro hole 2. The initial crack 5 is generated, for example, by rolling fatigue of about 10 ⁴ times. In FIG. 3, the inner wall surface of the micro hole 2 is shown in a schematic manner with the inner wall surface extending in the vertical direction of the figure and only the bottom part having a pointed shape, but this is merely one example, and the cross-sectional shape of the micro hole 2 is not limited to this. For example, the micro hole 2 may be hemispherical. In FIG. 3, the inner wall surface of the micro hole 2 is the bottom part of the part extending in the vertical direction of the figure, and the initial crack 5 is generated so as to extend in a substantially horizontal direction starting from the bent part at the boundary with the part where the inner wall surface is inclined so as to be pointed. In FIG. 3, the extension length of the initial crack 5 is a'.

Referring again to FIG. 1, next, a rolling fatigue life test is performed on the test piece 1 prepared as shown in FIG. 2 and FIG. 3 (S20). That is, as shown in FIG. 2, for example, the rolling element 4 on the main surface 1a of the test piece 1 is rotated multiple times along the rolling element track 3, thereby giving the test piece 1 rolling fatigue.

Next, after the rolling of the rolling element 4 has been repeated a certain number of times, the rolling of the rolling element 4 is stopped once, and the length of the crack in the test piece 1 is measured (S30). In other words, the length of the crack that has formed in the test piece 1 to which rolling fatigue has been imparted, extending from the initial crack 5, is measured. Here, the length of the crack from the (outer edge of) the microhole 2 is measured. This crack length is measured using high-frequency ultrasonic waves of 100 MHz or more, without cutting the test piece 1. It is more preferable that the measurement be performed using high-frequency ultrasonic waves of 150 MHz or more, and even more preferable that the measurement be performed using high-frequency ultrasonic waves of 200 MHz or more.

Next, after the crack length is measured in the above (S30), the test piece 1 whose crack length has been measured is again subjected to rolling fatigue by a rolling fatigue test (S20). Here, the exact same process as the previous (S20), that is, the process of rotating the rolling element 4 multiple times to cause rolling fatigue on the test piece 1, is performed again on the same test piece 1 as the one that was previously treated in (S20). This re-fatigue process causes the crack that had formed in the test piece 1 to grow further and become longer.

Next, a re-measurement step is performed on the test piece 1 after the rolling fatigue life test is performed again to measure the crack length of the test piece 1 (S30). Here, as in the above, the crack length is measured by using high-frequency ultrasonic waves of 100 MHz or more without cutting the test piece 1. It is more preferable that the measurement be performed by using high-frequency ultrasonic waves of 150 MHz or more, and even more preferable that the measurement be performed by using high-frequency ultrasonic waves of 200 MHz or more.

As described above, in this embodiment, (S20) and (S30) are repeated for the same (single) test piece 1. This allows the length of the crack formed when various loads are applied to be determined for the single test piece 1.

Figure 4 is a graph comparing the results of measuring the crack length formed from a microhole using the crack propagation test method of this embodiment with the results of measurement using a 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 using the crack propagation test method of this embodiment using an SEM. The vertical axis shows the results of measuring the crack length using the measurement method using ultrasound of this embodiment and the measurement method using an optical microscope as a comparative example. In the measurement method using ultrasound of this embodiment, high-frequency ultrasound of 200 MHz was used to measure the crack. In the measurement method using an optical microscope as a comparative example, the length of the crack extending from the microhole was measured by cutting the test piece and observing its cross section. Note that the measurement results using the SEM are also the result of cutting each test piece and observing its cross section, but here, these measurement results are considered to be highly accurate. In other words, the accuracy of the measurement results using each method is verified by considering the difference between the measurement results using the SEM and the measurement results using each method. In addition, x in the formulas in the graph indicates the horizontal axis and y indicates the vertical axis.

Referring to Figure 4, the slope of the line obtained by plotting the data is closer to 1 when using "ultrasound", i.e., the method of this embodiment, than when using "optical microscope", i.e., the method of the comparative example. In other words, it can be seen that the method of this embodiment can measure the crack length with higher accuracy than the method of the comparative example. Also, ^R2 , which indicates the value of the dispersion of the individual data for the obtained straight line, is larger when using the method of this embodiment than when using the method of the comparative example. In other words, it can be seen that the method of this embodiment reduces the dispersion compared to the method of the comparative example.

Based on the results in Figure 4, the effects of this embodiment are explained below.

In the crack propagation test method according to the first example of the present disclosure, a test piece 1 having a small hole 2 formed therein is prepared. A rolling element 4 is rotated multiple times on the test piece 1 to impart rolling fatigue to the test piece 1. The length from the small hole 2 of a crack that has developed in the test piece 1 to which rolling fatigue has been imparted is measured. The above measurement process is performed using high-frequency ultrasonic waves of 100 MHz or more.

In the crack propagation test method according to the second example of the present disclosure, a test piece 1 having a small hole 2 formed therein is prepared. A rolling element 4 is rotated multiple times on the test piece 1 to impart rolling fatigue to the test piece 1. The length from the small hole 2 of a crack that has developed in the test piece 1 to which rolling fatigue has been imparted is measured. After the measuring process, a re-fatigue process is performed in which the test piece 1 whose crack length has been measured is again imparted with rolling fatigue. After the re-fatigue process, a re-measurement process is performed in which the crack length of the test piece 1 is measured again.

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 by using ultrasonic waves of 100 MHz or more, for example, as in the first example. In this way, in both the first and second examples, a measurement result with higher accuracy can be obtained compared to the case where ultrasonic waves of a low frequency of about several tens of MHz are used for measurement. Therefore, when measuring a longer crack and measuring the length of the crack thereafter, the unbroken test piece 1 can be reused. Therefore, according to this embodiment, a highly accurate measurement result can be obtained that does not include the variation (which affects the measurement result) that occurs between each test piece 1 when measuring using a plurality of test pieces 1, for example.

As a result of the above, the crack growth limit and crack growth rate can be more accurately confirmed by testing. Specifically, first, regarding the crack growth limit, if a load, i.e., rolling fatigue, is applied to a test piece a certain number of times and the crack growth formed in the outer ring and inner ring is small, it has been determined that the crack does not grow and remains stationary. However, until now, it was not possible to confirm whether the crack truly did not grow and remain stationary. This is because the length of the initial crack was unknown. However, according to this embodiment, it is possible to measure the crack length before and after applying a load to one test piece 1 any number of times. Therefore, it is possible to confirm whether the crack did not grow and remain stationary by freely measuring and comparing the crack length before and after the rolling fatigue life test.

Furthermore, up until now, the crack growth rate has been estimated from the results of tests carried out on the length of cracks formed by varying the load applied to each of multiple test pieces, i.e., the number of times the test pieces are subjected to rolling fatigue. However, according to this embodiment, it is possible to non-destructively repeat the formation of cracks and the measurement of their lengths multiple times on a single test piece 1. This makes it possible to carry out a crack growth test on a single crack while checking its growth. This makes it possible to confirm the crack growth rate more accurately than ever before.

In the crack propagation test method according to the first example of the present disclosure, the measuring step is preferably performed using high-frequency ultrasonic waves of 150 MHz or more. In the crack propagation test method according to the first example of the present disclosure, the measuring step is preferably performed using high-frequency ultrasonic waves of 200 MHz or more. This allows the crack length to be measured with even greater accuracy.

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 may be performed in which the test piece 1, whose crack length has been measured, is subjected to rolling fatigue again, and after the re-fatigue step, a re-measurement step may be performed in which the crack length of the test piece 1 is measured again. The effect of this is as described above. Note that this re-measurement step is also preferably performed using high-frequency ultrasonic waves of 100 MHz or more, more preferably 150 MHz or more, and even more preferably 200 MHz or more.

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

In the crack propagation test method according to the first and second examples of the present disclosure, it is preferable that the micro-holes 2 formed in the test piece 1 in the process of preparing the test piece 1 have a diameter of 0.1 mm or less. This is because the size of non-metallic inclusions actually contained in the material of the test piece 1 is 0.1 mm or less, and because the influence of the micro-holes 2 at the contact area between the rolling element 4 and the rolling element raceway 3 is prevented from becoming too great. In this way, the crack can be allowed to propagate with minimal influence other than that of the applied load, and the reliability of the measurement results can be further improved.

In the crack propagation test method according to the first and second examples of the present disclosure, 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 microhole 2. In this way, the crack can be allowed to propagate with minimal influence other than that of the applied load, and the reliability of the measurement results can be further improved.

(Embodiment 2)
In this embodiment, a method for calculating the range of the stress intensity factor of the crack in shear type fatigue crack growth called mode II and the allowable defect size of the test piece from the crack formed in the test piece is described. FIG. 5 is a flowchart showing a method for calculating the range of the stress intensity factor of the crack and 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 a mode in which the calculation method of embodiment 2 is performed. With reference to FIG. 5 and FIG. 6, the process of the crack growth test method similar to the process shown in embodiment 1 (FIG. 1) is performed. That is, first, a test piece having a micro hole 2 to be 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 dimension in plan view when the micro hole 2 is not circular (for example, the length of the major axis if the micro hole 2 is elliptical). The center of the microhole 2 in plan view is the position of the center of gravity of the microhole 2 in plan view, and if the microhole 2 has a circular or elliptical planar shape, the center of the microhole 2 and the center of gravity will be the same. A rolling fatigue life test is performed on the prepared test piece 1, and the rolling element 4 is rotated multiple times (S20). The length a1 of the crack 5A formed from the microhole 2 in the test piece 1 to which rolling fatigue has been applied is measured (S30). The length a1 of the crack 5A is measured using high-frequency ultrasonic waves of 100 MHz or more.

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

Next, based on the result of the first test, i.e., the length of the crack 5A that is longer than the diameter of the microhole 2, a range ΔK _II of stress intensity factors at the crack tip 5P, which is the tip of the crack 5A having a dimension a1 longer than the diameter D of the microhole 2, is calculated (S40). The following formula (1) is used for this calculation.

The unit of the stress intensity factor range ΔK _II is (MPa√m). The stress intensity factor range ΔK _II in formula (1) is a value indicating the change in the stress intensity factor when the rolling element 4 rolls on the rolling element track 3 of the test piece 1. That is, for example, when the rolling element 4 rolls on the rolling element track 3 shown in FIG. 3 in the direction indicated by the arrow M in FIG. 6 (to the right), 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 stresses (present at the apex of the parabola) is the maximum surface pressure indicated by Pmax. Since this stress distribution moves from the left side to the right side in FIG. 6, for example, the stress intensity factor at the crack tip 5Q of the crack 5B on the left side has a different value when the rolling element 4 approaches the crack tip 5Q, when it passes through, and after it passes through. The same is true for the stress intensity factor at the crack tip 5P of the crack 5A on the right side. The difference between the maximum and minimum values of the different stress intensity factors is represented by the range of stress intensity factors ΔK _II . The coefficient 0.136 in formula (1) corresponds to the difference between the coefficient values for determining the maximum and minimum values of the stress intensity factor. At least a in formula (1) is the distance from the center of the microhole 2 in a plan view to the tip of the crack 5A (crack tip 5P). That is, a indicates the radius of the crack 5A including half D/2 of the diameter D of the microhole 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 microhole 2 to the crack tip 5P (not including the diameter D of the microhole 2). The length a1 is longer than the diameter D of the microhole 2. Therefore, as shown in FIG. 6, a and a1 are different values, and a1<a. ΔK _II can be obtained by substituting the values of Pmax and a into formula (1). Here, for the purpose of explanation as an example, the value of the calculated range of stress intensity factors ΔK _II is set to 3 MPa√m.

After the step of calculating the range of stress intensity factors ΔK _II , the rolling fatigue life test is performed again on the same test piece 1 as that used in each step up to this point (S50). That is, the rolling element 4 is rolled on the same test piece 1 as that used in each step up to this point under the previously calculated ΔK _II or higher conditions. The value of ΔK _II can be changed, for example, by changing the value of the maximum surface pressure Pmax. Then, the length a1 of the crack 5A of the test piece 1 is measured again (S60), and the presence or absence of the propagation of the crack 5A of the test piece 1 is confirmed by comparing it with the previously measured length a1. The steps up to this point (S50) and (S60) are regarded as the second test.

For the sake of explanation, as an example, assume that a test is performed under the condition that the value of ΔK _II is 5 MPa√m in step (S50), and the growth of the crack 5A is confirmed in step (S60). Based on this result, the value of the range of the stress intensity factor ΔK _II is changed, and the second test is repeated until the growth of the crack 5A stops. Specifically, if the growth of the crack 5A is confirmed when the value of ΔK _II is 5 MPa√m, the rolling fatigue is applied again under the condition that the value of ΔK _II is 4 MPa√m, for example, and the crack is measured again. If the growth of the crack 5A progresses under the condition that the value of ΔK _II is 4 MPa√m, the value of the range of the stress intensity factor ΔK _II is further reduced, and the second test consisting of steps (S50) and (S60) is further performed. If the growth of the crack 5A stops by setting ΔK _II to 4 MPa√m, the ΔK _II at that time, 4 MPa√m, is determined as the crack growth lower limit ΔK _IIth of the crack 5A (S70). Alternatively, if the growth of the crack 5A is stopped by setting ΔK _II to 4 MPa√m, conversely, ΔK _II may be increased to, for example, 4.5 MPa√m to similarly check for the growth of the crack 5A. In this way, by repeating the test while changing (for example, lowering) the value of the range of stress intensity factors ΔK _II , the maximum value of ΔK _II at which the crack 5A stops growing can be obtained as ΔK _IIth . In other words, the boundary value between ΔK _II at which the crack 5A grows and ΔK _II at which the crack 5A does not grow can be obtained as ΔK _IIth . In other words, the crack growth lower limit value ΔK _IIth is the maximum value of the range of stress intensity factors ΔK _II that can prevent the crack 5A from growing in the test piece 1. When the value of the range of stress intensity factors ΔK _II exceeds the value of the crack growth lower limit value ΔK _IIth , the crack 5A grows.

From the obtained crack growth lower limit ΔK _IIth , the allowable defect size a _th of the test piece 1 is calculated (S80). Specifically, the value of a that is uniquely obtained by substituting ΔK _IIth obtained in step (S70), for example, 4 MPa√m, and the maximum surface pressure Pmax applied during the test into ΔK _II in the above formula (1), is obtained as the allowable defect size a _th .

Figure 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 Figure 7, the test piece 1 may be, for example, a raceway of a radial bearing 101. The radial bearing 101 has an outer ring 1A and an inner ring 1B. These raceways may be used as the 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 at intervals in the circumferential direction by a cage 15.

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 the second embodiment. Referring to FIG. 8, the test piece 1 may be, for example, a raceway of a thrust bearing 102. The thrust bearing 102 has a shaft raceway 1C and a housing raceway 1D. These raceways may be used as the test piece 1. A plurality of rolling elements 4 as rollers roll between the shaft raceway 1C and the housing raceway 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 in which grooves are not provided on the raceway surface where the shaft raceway 1C and the housing raceway 1D as raceways come into contact with the rolling elements 4. In this way, the raceway may be flat.

Figure 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 Figure 9, test piece 1 may be, for example, a raceway of thrust bearing 103. In thrust bearing 103, rolling elements 4 as a plurality of balls roll between shaft race 1C and housing race 1D. In thrust bearing 102 in Figure 9, grooves are provided on the raceway surface where shaft race 1C and housing race 1D as raceways come into contact with rolling elements 4. Thrust bearing 103 differs from thrust bearing 102 in the above points, but is similar in other points, so description will not be repeated here.

As described above, in this embodiment, the test piece 1 may be the raceway of either the thrust bearings 102, 103 or the radial bearing 101. It is preferable that these test pieces 1 (raceways) used in the calculation method of this embodiment are made of the same material as the raceways of the product that is planned to be inspected for quality later. This can further increase the reliability of the quality assurance of the actual product, which will be described later, using the calculation results.

Next, we will explain the effects of this embodiment.

The calculation method of this embodiment is a calculation method using the crack growth test method described in embodiment 1. The length a1 of the crack 5A generated in the test piece 1 by the rolling contact fatigue application process from (the outer edge of) the microhole 2 is longer than the diameter D of the microhole 2. Based on the results of the first test by the crack growth test method described in embodiment 1, the range ΔK _II of the stress intensity factor at the crack tip 5P, which is the tip of the crack 5A longer than the diameter of the microhole 2, is calculated (S40).

The range of the stress intensity factor ΔK _II is given by the following equation, where a is the length from the center of the small hole 2 of the crack 5A obtained in the first test in a plan view, and P is the maximum surface pressure applied by the rolling element 4 to the test piece 1:
It can be calculated using the following formula (1).

However, when the micro-hole 2 is formed by drilling, stress concentration occurs around the micro-hole 2. Therefore, 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 in the test piece 1 with the micro-hole 2 formed therein than in the test piece 1 without the micro-hole 2 formed therein. On the other hand, the above formula (1) is a generally known formula, but this formula is based on the premise that it is applied to a so-called disk crack model without the micro-hole 2. Therefore, when the range ΔK _{II of the stress intensity factor at the crack tip 5P in the system with the micro-hole 2 formed by drilling is calculated, an error occurs between the range ΔK II} and the actual numerical value. As described above, the crack 5A of the test piece 1 with the micro-hole 2 generates a larger stress than the crack 5A of the test piece 1 without the micro-hole 2, and an error occurs by the amount of the larger stress. Therefore, it was difficult to apply the above formula (1) as it is to a model with the crack 5A generated from the micro-hole 2 in the test piece 1. Furthermore, a formula for calculating with high precision the range ΔK _II of the stress intensity factor for a model having a minute hole 2 has not yet been established.

Therefore, in this embodiment, as described above, the length a1 of the crack 5A from (the outer edge of) the microhole 2 is made longer than the diameter D of the microhole 2. According to Saint-Venant's principle, if the length a1 of the crack 5A from (the outer edge of) the microhole 2 is made longer than the diameter D of the microhole 2, the crack tip 5P is not affected by the stress concentration due to the microhole 2. Therefore, in a system in which a1 is larger than D, the value of the range ΔK _II of the stress intensity factor at the crack tip 5P can be calculated with high accuracy using the known formula (1) assuming a disk crack model. Note that, in this embodiment, it is more preferable to make the length a1 of the crack 5A from the outer edge of the microhole 2 longer than twice the diameter D of the microhole 2. That is, this embodiment has been derived through extensive research as a method that enables the value of the range ΔK _II of the stress intensity factor to be calculated using the known formula (1) assuming no microhole 2 for a model having the microhole 2.

In the above calculation method, after the step (S40) of calculating the stress intensity factor range ΔK _II , the method further includes a step (S50) of subjecting the test piece 1 to rolling contact fatigue again as a second test, and a step (S60) of measuring the length a1 of the crack 5A again to confirm the presence or absence of the growth of the crack 5A in the test piece 1. Based on the result of the step of confirming the presence or absence of the growth, the second test is repeated while changing the value of the stress intensity factor range ΔK _II until the crack 5A no longer grows, thereby determining the crack growth threshold ΔK _IIth of the crack 5A (S70). The allowable defect size a _th of the test piece 1 is determined from the crack growth threshold ΔK _IIth .

Since ΔK _II can be calculated with high accuracy and without error as described above, the crack growth threshold ΔK _IIth calculated based on this can also be calculated with high accuracy and without error. As a result, the value of the allowable defect size a _th calculated from ΔK _IIth calculated with high accuracy can also be calculated with high accuracy and without error. The crack growth threshold ΔK _IIth is the maximum value of the range of stress intensity factors ΔK _II that can prevent the crack 5A from growing in the test piece 1. This is because the allowable defect size a _th can be calculated as the value of a when the crack growth threshold ΔK _IIth is substituted for ΔK _II in the following formula (1).

In addition, as described later, the allowable defect size a _th makes it possible to judge whether a product is good or bad in the quality assurance inspection of an actual product by checking the size of the defect contained in the product, thereby simplifying the inspection process and increasing the reliability of the inspection process.

(Embodiment 3)
Fig. 10 is a flow chart showing an inspection method for checking the presence or absence of defects in a raceway ring in the third embodiment. With reference to Fig. 10, in this embodiment, a method for inspecting defects contained inside a raceway ring (e.g., an outer ring or an inner ring) constituting an actual bearing product is provided, using the value of the allowable defect dimension ath determined in the _second embodiment as a reference value. 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 the object to be inspected, is prepared.

Next, the object is inspected for defects exceeding the allowable defect size (S25). This inspection is preferably performed by ultrasonic flaw detection using high-frequency ultrasonic waves of 10 MHz or more. Alternatively, this inspection may be performed by radiographic testing. In this way, it becomes possible to inspect all products by non-destructive testing.

In step (S25), it is inspected whether the largest defect contained in the object is equal to or smaller than the allowable defect size a _th obtained in embodiment 2. If the largest defect contained in the object is equal to or smaller than the allowable defect size a _th , the product passes, and if it exceeds the allowable defect size a _th , the product fails.

Next, we will explain the effects of this embodiment.

In the inspection method of the present embodiment, an object is prepared, and the object is inspected for defects exceeding the allowable defect size. In the inspection process, it is inspected whether the defect contained in the object has a size equal to or smaller than the allowable defect size a _th obtained by the calculation method described in the second embodiment.

By inspecting an actual product based on the value of the allowable defect size a _th determined with high accuracy by the second embodiment, the quality of the product can be judged. Therefore, the actual product can be inspected simply and reliably using a parameter that is easy to detect, such as the defect size, without using a parameter that is complicated to calculate, such as the crack growth threshold value ΔK _IIth of the actual product. In this way, if the range of stress intensity factors ΔK _II is determined with high accuracy, the crack growth threshold value ΔK _IIth and the allowable defect size a _th can be determined with high accuracy therefrom, which can be applied to quality assurance of highly reliable products. Therefore, the manufacturing method of a bearing using the above inspection method can provide a high-quality raceway ring that does not have a defect that causes a crack to grow, and a high-quality bearing that includes the raceway ring.

The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is indicated by the claims rather than the above description, and is intended to include all modifications within the meaning and scope of the claims.

1 Test piece, 1a Main surface, 1A Outer ring, 1B Inner ring, 1C Shaft raceway, 1D Housing raceway, 2 Micro hole, 3 Rolling element raceway, 4 Rolling element, 5 Initial crack, 5A, 5B Crack, 5P, 5Q Crack tip, 15 Cage, 101 Radial bearing, 102, 103 Thrust bearing.

Claims (6)

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. 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. 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. 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. 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. 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.

The calculation method according to any one of claims 1 to 5 , wherein the calculation is performed by :

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