Measurement method for fatigue crack progress speed of rolling element

Abstract

To provide a measurement method for the fatigue crack propagation speed of a rolling element having a stress concentration source such as peeling.SOLUTION: A method include: a processing step of forming, in advance, an artificial defect at least at one location among a central part or an edge of a raceway surface of a rolling element, or an end face of the rolling element; an assembly step of forming a rolling bearing by arranging the rolling elements freely rotatably between an inner ring and an outer ring; a testing step of propagating a fatigue crack originating from the artificial defect by rotating the rolling bearing while applying load to the rolling bearing; and a measurement step of determining a progress speed of the fatigue crack on the basis of a degree of fatigue crack propagation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for measuring the fatigue crack growth rate of rolling elements in a rolling bearing. [Background technology]

[0002] It is known that rolling fatigue flaking occurs on the raceway surface when rolling bearings are used for extended periods. When fatigue flaking occurs, it causes vibration, abnormal noise, and reduced positioning accuracy, leading to the bearing's end of life. However, for example, roller bearings with line contact are less likely to experience an increase in vibration, and when the bearing is unmanned, abnormal noises and other issues are difficult to notice, leading to continued use. When the bearing reaches its fatigue limit, there is a risk of cracks starting from the flaking. In particular, if the rolling elements are damaged, problems such as shaft locking can occur due to debris getting caught.

[0003] The fatigue failure limit of materials without stress concentration sources such as delamination can be predicted through experiments and calculations. However, to accurately determine the fatigue failure limit of materials with stress concentration sources such as delamination, it is necessary to measure the crack growth behavior of the material and make predictions based on this.

[0004] ASTM E647 is a well-known method for measuring fatigue crack growth rates. This method is performed using elementary specimens, such as compact tension (CT) specimens. The stress state of such elementary specimens is relatively simple and can be calculated based on previous knowledge, allowing the inherent crack growth resistance of a material to be measured. Specifically, the specimen has a notch. Force is applied in the tearing direction through holes on both sides of the notch, creating a precrack at the base of the notch. Then, the specimen is subjected to repeated stresses less than the force that caused the precrack. The propagation length of the crack after the precrack is observed, and the fatigue crack growth rate of the material is determined based on the degree of crack growth relative to the number of repeated stresses applied.

[0005] On the other hand, the stress state inside a rolling bearing is extremely complex, and it is necessary to conduct bearing tests to confirm whether cracks that occur in the rolling bearing exhibit crack growth behavior similar to that measured using CT test specimens, etc.

[0006] A known method for measuring the fatigue crack growth rate using rolling bearings involves creating artificial defects on the raceway surface of the rolling bearing and predicting the fatigue crack growth rate using a rotation testing machine (see, for example, Patent Document 1 and Non-Patent Document 1). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 3082307 [Non-patent literature]

[0008] [Non-Patent Document 1] Motion & Control No33(June 2022) Summary of the Invention [Problem to be solved by the invention]

[0009] The measurement method in Patent Document 1 is a method for measuring the strength of an inner ring that has a fatigue pre-crack on the raceway surface, while the measurement method described in Non-Patent Document 1 is a method for measuring the strength of an outer ring that has a notch machined by electrical discharge machining on the raceway surface, but neither method can test the strength of the rolling elements. Therefore, there is a need for a method for predicting the usability of a rolling bearing in which a stress concentration source such as flaking has occurred on the raceway surface of the rolling elements.

[0010] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for measuring the fatigue crack growth rate of a rolling element having a stress concentration source such as flaking. [Means for solving the problem]

[0011] Therefore, the above object of the present invention is achieved by the following configuration [1] relating to a method for measuring the fatigue crack growth rate of a rolling element. [1] A method for measuring the fatigue crack growth rate of rolling elements in a rolling bearing having an outer ring, an inner ring, and rolling elements, comprising: a processing step of forming an artificial defect in advance at least one location of the center of the raceway surface of the rolling element, the end of the raceway surface, or the end face of the rolling element; an assembly process in which the rolling elements are rollably disposed between the inner ring and the outer ring to form a rolling bearing; a testing step of rotating the rolling bearing while applying a load to the rolling bearing, thereby causing fatigue cracks to propagate from the artificial defects; and a measuring step of measuring a fatigue crack growth rate from the degree of fatigue crack growth. A method for measuring the fatigue crack growth rate of rolling elements. [Effects of the Invention]

[0012] According to the method for measuring the fatigue crack growth rate of a rolling element of the present invention, it is possible to measure the fatigue crack growth rate of a rolling element having a stress concentration source such as flaking. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a flowchart showing the steps of a method for measuring the fatigue crack growth rate of a rolling element. [Figure 2] FIG. 2 is a perspective view showing the positions where artificial defects are formed on a cylindrical roller serving as a rolling element. [Figure 3] FIG. 3 is a cross-sectional view showing an example of the shape of the artificial defect. [Figure 4] FIG. 4 is a front view of a rolling bearing in which the subject rolling element is incorporated. [Figure 5] FIG. 5 is a schematic diagram of a fatigue testing device for rolling bearings. [Figure 6] FIG. 6 is a schematic diagram showing a fatigue crack that has propagated from an artificial defect. [Figure 7]FIG. 7 is a diagram showing a schematic diagram of the relationship between the stress intensity factor range (ΔK) and the fatigue crack growth rate (da / dN). DETAILED DESCRIPTION OF THE INVENTION

[0014] The method for measuring the fatigue crack growth rate of a rolling element according to the present invention will be described in detail below with reference to the drawings. In the following description, a cylindrical roller will be used as the rolling element. Figure 1 is a flowchart showing the steps of the method for measuring the fatigue crack growth rate of a rolling element. As shown in FIG. 1, the method for measuring the fatigue crack growth rate of a rolling element according to the present invention includes a processing step S1, an assembly step S2, a testing step S3, and a measuring step S4. In the processing step S1, an artificial defect is formed in advance in at least one location on the rolling element. In the assembly step S2, the rolling element is arranged between the inner ring and the outer ring so that it can roll freely, and the rolling bearing is assembled. In the testing step S3, the rolling bearing is rotated while being loaded, causing a fatigue crack to propagate from the artificial defect. In the measurement step S4, the fatigue crack growth rate is measured from the degree of fatigue crack growth. Each step is explained below.

[0015] First, the processing steps will be described with reference to Fig. 1 to Fig. 3. Fig. 2 is a perspective view showing the positions at which artificial defects are formed in a cylindrical roller serving as a rolling element. Fig. 3 is a cross-sectional view showing an example of the shape of the artificial defects. In the processing step S1, in this embodiment, bearing steel (SUJ-2) is used, and a cylindrical roller 10 as a test specimen of a predetermined shape and dimensions is created by turning, heat treating, and grinding, and an artificial defect 11 is formed at the position where the fatigue crack growth rate is desired to be determined.

[0016] The artificial defect 11 may be formed at any position, but is preferably formed at the axial center 12a of the raceway surface 12, the axial end 12b of the raceway surface 12, the end face 13 of the cylindrical roller 10, or the like, as shown in FIG. 2, for example. The axial center 12a of the raceway surface 12 is the position where the maximum surface pressure occurs when there is no edge load, and is therefore a desirable position for providing the artificial defect 11 since there is a risk of breakage. The axial end 12b of the raceway surface 12 is considered to be the starting point of damage when an edge load is applied, and is therefore a suitable position for evaluating cracks after an edge load. The end face 13 of the cylindrical roller 10 is the position that comes into contact with the flange (not shown) of the raceway ring, and fatigue cracks from the end face 13 can be evaluated.

[0017] In this way, by forming artificial defects 11 at different positions on the cylindrical roller 10, it is possible to measure the fatigue crack growth rate at any position on the cylindrical roller 10. The number of artificial defects 11 formed on one cylindrical roller 10 may be one or more. If multiple artificial defects 11 are formed, it may be possible to measure the fatigue crack growth rate at multiple locations in a single test.

[0018] The artificial defect 11 is formed by die-sinking electrical discharge machining using an electrode that mimics the shape of a desired artificial defect.

[0019] Regarding the dimensions of the artificial defect 11, if the defect is small, such as 10 μm, a crack will not grow, but conversely, if the defect is too large, such as exceeding 1 mm, it will fall outside the region of uniform quality and residual stress on the outermost surface, and this is undesirable. For example, the shape of the artificial defect 11 is suitably a semi-elliptical slit with a length L of 100 μm to 5 mm, a width W of 0.035 mm, a depth D of 100 to 500 μm, and a tip angle of the peripheral edge 11a of 45°, as shown in FIG.

[0020] If the artificial defect 11 is a semi-elliptical slit, the degree of stress concentration can be increased and the fatigue crack growth rate can be measured effectively without changing the depth D of the artificial defect 11. Note that the semi-elliptical slit referred to here also includes a semicircular slit.

[0021] When a tensile stress of 100 to 300 MPa, a level that actually occurs as a residual stress, acts on such an artificial defect 11, the stress intensity factor (Kmax) acting on the tip of the artificial defect 11 is 1.3 to 8.7 MPa√m. As the crack growth limit of bearing steel is roughly 3 to 5 MPa√m, it is believed that adding such an artificial defect 11 is effective in measuring the crack growth limit and crack growth rate.

[0022] Next, the assembly step S2 will be described with reference to Figures 1 and 4. Figure 4 is a front view of a rolling bearing in which the rolling element, which is the subject, is incorporated. In the assembly process S2, a plurality of cylindrical rollers 10, including a cylindrical roller 10 with an artificial defect 11 formed therein, are rotatably assembled between the raceway surfaces of the outer ring 21 and the inner ring 22 to complete the cylindrical roller bearing 20, which is the bearing to be tested (see Figure 4).

[0023] Next, the testing step S3 will be described with reference to Figures 1 and 5. Figure 5 is a schematic diagram of a fatigue testing device for rolling bearings. In testing step S3, cylindrical roller bearing 20 is mounted on rotating shaft 31 of rotation testing machine 30 shown in Figures 4 and 5 and set in housing 23. Next, a radial load P is applied to housing 23, thereby applying a load to load zone 24 at the bottom of cylindrical roller bearing 20, and while lubricating by supplying lubricating oil filtered by filter 33 using pump 32, the bearing is rotated until it reaches the planned total number of rotations n, thereby causing fatigue cracks to propagate in cylindrical rollers 10. Note that, from the perspective of effective lubrication, it is preferable to circulate the lubricating oil while cooling it with oil cooler 34.

[0024] In the cylindrical roller 10 attached to the rotation testing machine 30 described above, as the rotation speed N of the cylindrical roller bearing 20 increases, the number of times that the artificial defect 11 passes through the load zone 24 also increases. For this reason, in the cylindrical roller 10, fatigue cracks start at the artificial defect 11 and propagate in proportion to the rotation speed N of the bearing.

[0025] Next, the measurement process will be described with reference to Figures 6 and 7. Figure 6 is an enlarged view of a fatigue crack that has propagated from an artificial defect. Figure 7 is a graph showing the relationship between the stress intensity factor range (ΔK) and the fatigue crack growth rate (da / dN). In measurement step S4 (Fig. 1), when the bearing rotation speed of cylindrical roller 10 reaches the planned bearing (inner ring) rotation speed N, rotation testing machine 30 is stopped, the test cylindrical roller 10 is removed and destroyed, and the fatigue crack surface is observed with an electron microscope or optical microscope to measure the depth a of fatigue progression portion 15. Fig. 6 is an enlarged view of the fatigue crack surface.

[0026] The length da of the fatigue crack that has progressed in the depth direction is determined by the difference between the depth a of the fatigue progressed portion 15 and the depth D of the artificial defect. Therefore, the fatigue crack growth rate can be calculated as da / dN, which is the length da that the fatigue crack has grown in the depth direction divided by the bearing rotation speed N. Similarly, the length dc of the fatigue crack propagating in the width direction can be calculated by the difference between the width c of the fatigue propagating part and the length L of the artificial defect. Therefore, the fatigue crack growth rate can also be calculated as dc / dN, which is the length dc of the fatigue crack that has grown in the width direction divided by the bearing rotation speed N.

[0027] Furthermore, by determining the stress intensity factor range (ΔK) from, for example, the tensile residual stress value or the crack dimensions, it is possible to plot the fatigue crack growth rate from one test as shown in Figure 7. In order to determine the stress intensity factor range (ΔK) value as accurately as possible, it is desirable that the depth a of the fatigue progression portion 15 be as short as possible, for example, on the order of several hundred microns.

[0028] Furthermore, by using multiple rolling elements made of the same material and measuring the fatigue crack growth rate using the above-mentioned measurement method while changing the size and shape of the artificial defect 11 and the applied stress, and then plotting the fatigue crack growth rate at multiple points to estimate an approximate curve, it becomes possible to evaluate the rolling element material and measure the stress of the rolling element during use, which has previously been extremely difficult. Furthermore, by comparing multiple types of materials with different residual stresses using this measurement method, it is possible to compare the strength of materials that contain residual stress.

[0029] The present invention is not limited to the above-described embodiment, and can be modified, improved, etc. as appropriate. For example, in the above embodiment, the method for measuring the fatigue crack growth rate of cylindrical rollers as rolling elements has been described, but the rolling elements are not limited to cylindrical rollers, and tapered rollers or spherical rollers may also be used. Furthermore, if an artificial defect is formed in the cage, the fatigue crack growth rate of the cage can also be measured.

[0030] As described above, the present specification discloses the following: (1) A method for measuring the fatigue crack growth rate of rolling elements in a rolling bearing having an outer ring, an inner ring, and rolling elements, comprising: a processing step of forming an artificial defect in advance at least one location of the center of the raceway surface of the rolling element, the end of the raceway surface, or the end face of the rolling element; an assembly process in which the rolling elements are rollably disposed between the inner ring and the outer ring to form a rolling bearing; a testing step of rotating the rolling bearing while applying a load to the rolling bearing, thereby causing fatigue cracks to propagate from the artificial defects; and a measuring step of measuring a fatigue crack growth rate from the degree of fatigue crack growth. A method for measuring the fatigue crack growth rate of rolling elements. This configuration makes it possible to measure the fatigue crack growth rate of a rolling element having a stress concentration source such as flaking, which has been extremely difficult to do in the past.

[0031] (2) The machining step forms the artificial defects in the rolling elements by performing die-sinking electrical discharge machining on at least one location of the center of the raceway surface of the rolling elements, the end of the raceway surface, or the end face of the rolling elements. (1) A method for measuring the fatigue crack growth rate of a rolling element. According to this configuration, an artificial defect of a desired shape can be formed at any position on the rolling element.

[0032] (3) The fatigue crack growth rate is calculated by dividing the depth (a) of the fatigue growth part by the bearing rotation speed (N) required to produce the depth of the fatigue growth part (da / dN). A method for measuring the fatigue crack growth rate of a rolling element according to (1) or (2). According to this configuration, the fatigue crack growth rate of the rolling element can be measured.

[0033] (4) The artificial defect is a semi-ellipse having a radius of 100 μm to 500 μm. A method for measuring the fatigue crack growth rate of a rolling element according to (1) or (2). This configuration allows the fatigue crack growth rate to be measured effectively.

[0034] (5) The artificial defect has a length of 100 μm to 5 mm. A method for measuring the fatigue crack growth rate of a rolling element according to (1) or (2). This configuration allows the fatigue crack growth rate to be measured effectively.

[0035] (6) The artificial defects have a depth of 100 μm to 500 μm. A method for measuring the fatigue crack growth rate of a rolling element according to (1) or (2). This configuration allows the fatigue crack growth rate to be measured effectively. [Explanation of symbols]

[0036] 10 Cylindrical roller (rolling element) 11 Artificial defects 11a Periphery 12 Raceway surface 12a Axial center 12b Axial end 13 End face of rolling element 15 Fatigue progression area 20 Cylindrical roller bearings (rolling bearings) 21 outer ring 22 Inner Circle 23 Housing 24 Load Zone 30 Rotational Testing Machine 31 Rotation axis 32 Pump 33 Filters 34 Oil cooler a Depth of fatigue progression da / dN Fatigue crack growth rate of rolling elements D Depth of artificial defect L length of the artificial defect N Bearing rotation speed W: width of the artificial defect

Claims

1. A method for measuring the fatigue crack growth rate of rolling elements in a rolling bearing having an outer ring, an inner ring, and rolling elements, comprising: a processing step of forming an artificial defect in advance at least one location of the center of the raceway surface of the rolling element, the end of the raceway surface, or the end face of the rolling element; an assembly process in which the rolling elements are rollably disposed between the inner ring and the outer ring to form a rolling bearing; a testing step of rotating the rolling bearing while applying a load to the rolling bearing, thereby causing fatigue cracks to propagate from the artificial defects; and a measuring step of measuring a fatigue crack growth rate from the degree of fatigue crack growth. A method for measuring the fatigue crack growth rate of rolling elements.

2. the machining step forms the artificial defects in the rolling elements by performing die-sinking electrical discharge machining on at least one location of a center of a raceway surface of the rolling elements, an end of the raceway surface, or an end face of the rolling elements; The method for measuring the fatigue crack growth rate of a rolling element according to claim 1.

3. The fatigue crack growth rate is calculated by dividing the depth a of the fatigue growth portion by the number of bearing revolutions required to produce the depth of the fatigue growth portion.

3. A method for measuring the fatigue crack growth rate of a rolling element according to claim 1 or 2.

4. The artificial defect is a semi-ellipse having a radius of 100 μm to 500 μm.

3. A method for measuring the fatigue crack growth rate of a rolling element according to claim 1 or 2.

5. The artificial defect has a length of 100 μm to 5 mm.

3. A method for measuring the fatigue crack growth rate of a rolling element according to claim 1 or 2.

6. The artificial defects have a depth of 100 μm to 500 μm.

3. A method for measuring the fatigue crack growth rate of a rolling element according to claim 1 or 2.

Images