Laminated spring

Abstract

This laminated spring is formed by laminating a plurality of spring members, and includes: a first spring member serving as a base; one or a plurality of second spring members laminated on the first spring member; and a fastening member for fastening the first and second spring members. A hole through which the fastening member is inserted is formed in the first and second spring members. The second spring members have formed therein: a first surface modification part that is a portion of a surface on a side subjected to compressive stress during operation and that is obtained by performing surface modification on a region including a portion of an opening edge of the hole; and a second surface modification part that is provided on a portion of an inner peripheral surface forming the hole and that is continuous with the first surface modification part. When an extension length from the opening edge of the hole of the second surface modification part is D1, and a plate thickness of the second spring member is T1, 0.3 × T1 ≤ D1 is satisfied.

Description

Laminated springs

[0001] The present invention relates to a laminated spring.

[0002] Conventionally, suspension systems have been known that are provided on vehicles and have a buffering function to prevent vibrations caused by road surface irregularities from being transmitted to the vehicle body via the wheels, thereby improving the vehicle's ride comfort, steering stability, etc. Among such suspension systems, leaf spring type suspension systems are configured using leaf springs (see, for example, Patent Document 1).

[0003] Patent Document 1 describes a laminated spring formed by stacking multiple spring members. Each spring member is fastened with fastening members (bolts and nuts) through holes formed in the center of the spring member's longitudinal direction. Generally, the surface of the spring member that receives tensile stress is subjected to surface modification by shot peening.

[0004] Japanese Patent Application Laid-Open No. 2015-121262

[0005] However, spring members may have a curvature that curves away from other spring members that they abut against during stacking. In this case, the spring members are fastened together to suppress the curvature. When the leaf spring operates, tensile stress is generated around the hole on the surface that is subjected to compressive stress. Conventionally, strict tolerance control was required during manufacturing to prevent this tensile stress from exceeding a certain level, and improving the durability of spring members to improve yields was a challenge.

[0006] The present invention has been made in view of the above, and an object of the present invention is to provide a laminated spring formed by stacking a plurality of spring members, which can improve durability.

[0007] In order to solve the above-mentioned problems and achieve the object, the laminated spring of the present invention is a laminated spring formed by stacking a plurality of spring members, and comprises a first spring member as a base, one or more second spring members stacked on the first spring member, and a fastening member for fastening the first and second spring members, wherein the first and second spring members are formed with holes through which the fastening members are inserted, and the second spring member is formed with a first surface-modified portion formed by surface-modifying a part of the surface that receives compressive stress during operation, the second spring member including a part of the opening edge of the hole, and a second surface-modified portion that is provided on a part of the inner surface that forms the hole and is continuous with the first surface-modified portion, wherein the extension length from the opening edge of the hole of the second surface-modified portion is D1 and the plate thickness of the second spring member is T1, and the laminated spring satisfies 0.3 × T1 ≦ D1.

[0008] Furthermore, in the laminated spring according to the present invention, the second surface modified portion has a compressive residual stress of 150 MPa or more.

[0009] Furthermore, the laminated spring according to the present invention is characterized in that, in the above invention, the second surface modified portion is imparted with compressive residual stress to a depth of 0.05 mm or more from the opening edge of the hole.

[0010] Furthermore, in the laminated spring according to the present invention, in the above invention, the second surface modified portions are provided as a pair facing each other in the width direction of the leaf spring.

[0011] In addition, the laminated spring according to the present invention is characterized in that, in the above invention, the first surface modified portion extends in the width direction of the second spring member.

[0012] Furthermore, in the laminated spring according to the present invention, the first surface modified portion extends to both widthwise ends of the second spring member.

[0013] In addition, the laminated spring of the present invention is characterized in that, in the above invention, the formation range of the first surface modified portion in the longitudinal direction of the second spring member is smaller than the diameter of the hole formed in the second spring member.

[0014] In addition, the laminated spring of the present invention is characterized in that, in the above invention, the formation range of the first surface modified portion in the longitudinal direction of the second spring member is larger than the diameter of the hole formed in the second spring member.

[0015] Furthermore, in the laminated spring according to the present invention, in the above invention, the formation range of the first surface modified portion forms a circular ring shape concentric with a hole formed in the second spring member.

[0016] Furthermore, the laminated spring according to the present invention is characterized in that, in the above invention, the first surface modified portion passes through the longitudinal center of the second spring member, and a central axis extending in the width direction passes through the center of a hole formed in the second spring member.

[0017] According to the present invention, it is possible to achieve an effect of improving the durability of a laminated spring formed by laminating a plurality of spring members.

[0018] FIG. 1 is a diagram showing the configuration of a laminated spring according to an embodiment of the present invention. FIG. 2 is a partial cross-sectional view showing the configuration of a main part of the laminated spring shown in FIG. 1. FIG. 3 is a view showing an example of the shape of a spring member before fastening. FIG. 4 is a diagram showing the configuration of a portion of a spring member included in a laminated spring according to an embodiment of the present invention. FIG. 5 is a cross-sectional view taken along line A-A shown in FIG. 4. FIG. 6 is a view for explaining a surface treatment method for a spring member included in a laminated spring according to an embodiment of the present invention. FIG. 7 is a diagram showing the configuration of a portion of a spring member included in a laminated spring according to a first modified example of the present invention. FIG. 8 is a cross-sectional view taken along line B-B shown in FIG. 7. FIG. 9 is a diagram showing the configuration of a portion of a spring member included in a laminated spring according to a second modified example of the present invention. FIG. 10 is a cross-sectional view taken along line C-C shown in FIG. 9. FIG. 11 is a diagram showing the configuration of a portion of a spring member included in a laminated spring according to a third modified example of the present invention. FIG. 12 is a cross-sectional view taken along line D-D shown in FIG. 11.

[0019] Hereinafter, embodiments for carrying out the present invention (hereinafter referred to as "embodiments") will be described with reference to the accompanying drawings. Note that the drawings are schematic, and the relationship between the thickness and width of each part, the thickness ratio of each part, etc. may differ from the actual ones, and the drawings may also include parts with different dimensional relationships and ratios.

[0020] (Embodiment) Fig. 1 is a diagram showing the configuration of a laminated spring according to one embodiment of the present invention. The laminated spring shown in Fig. 1 is provided, for example, on the body of a vehicle and used as a leaf spring type suspension device that supports the axles that support the wheels. This leaf spring type suspension device has a buffer function that prevents vibrations caused by road surface irregularities from being transmitted to the vehicle body via the wheels, thereby improving the ride comfort and steering stability of the vehicle. The laminated spring 1 is formed, for example, using a metal material (e.g., spring steel), a resin material, or fiber reinforced plastics (FRP).

[0021] The laminated spring 1 is formed by stacking a plurality of spring members. In this embodiment, an example in which the laminated spring is formed by stacking two spring members will be described. The laminated spring 1 includes a first spring member 11 serving as a base, a second spring member 12 stacked on the first spring member 11, washers 13 and 14, and a fastening member 15.

[0022] The first spring member 11 is formed by bending both ends of a strip-shaped member, and is curved in an arc shape from one end to the other end in the longitudinal direction.

[0023] The second spring member 12 is formed by bending a strip-shaped member into an arc from one end to the other in the longitudinal direction. Hereinafter, the direction perpendicular to the longitudinal direction and thickness direction of the spring member will be referred to as the width direction.

[0024] Fig. 2 is a partial cross-sectional view showing the configuration of a main part of the laminated spring shown in Fig. 1. Fig. 2 shows a cross-section taken along a plane parallel to the longitudinal direction and the plate thickness direction at the center of the longitudinal direction of the laminated spring 2. The fastening member 15 includes a bolt 151 consisting of a head and a shaft, and a nut 152 that screws onto the shaft of the bolt 151.

[0025] Here, the first spring member 11 and the second spring member 12 are formed with holes 11a and 12a, respectively, through which the shank of the bolt 151 is inserted.

[0026] The laminated spring 1 is fabricated by laminating a first spring member 11 and a second spring member 12, disposing a washer 13 on the side of the hole 11a opposite to the second spring member 12 side, and disposing a washer 14 on the side of the hole 12a opposite to the first spring member 11 side. Thereafter, a bolt 151 is inserted through the holes 11a and 12a, and a nut 152 is screwed onto the shaft of the bolt 151, thereby fabricating the laminated spring 1.

[0027] FIG. 3 is a diagram showing an example of the shape of the spring members before fastening. During operation, the laminated spring 1 has a side that receives compressive stress and a side that receives tensile stress due to deformation caused by the operation. Specifically, in the laminated spring 1 shown in FIG. 1 , the lower side is the side that receives compressive stress during operation, and the upper side is the side that receives tensile stress during operation. Hereinafter, the surface that receives compressive stress is referred to as the CS surface, and the surface that receives tensile stress is referred to as the TS surface. Although individual differences exist, the first spring member 11 and the second spring member 12 may have warpage that separates holes from each other when stacked in a natural state where no load other than gravity is applied. In this embodiment, the second spring member 12 is described as having warpage, and the holes are separated from each other when stacked, as shown in FIG. 3 .

[0028] FIG. 4 is a diagram showing the configuration of a portion of a spring member included in a laminated spring according to one embodiment of the present invention. FIG. 5 is a cross-sectional view taken along line A-A in FIG. 4. The second spring member 12 has a surface-modified portion 12b formed by surface modification using shot peening on a portion of the CS surface. The region in which the surface-modified portion 12b is formed has a longitudinal length shorter than the width of the second spring member 12 and extends to both widthwise ends. The width of the surface-modified portion 12b, specifically, the extension range in the widthwise direction from the hole 12a, is set, for example, based on the results of an analysis of stress concentration around the opening edge of the hole 12a. In this case, the longitudinal length of the second spring member 12 in the surface-modified portion 12b is shorter than the diameter of the hole 12a. The surface-modified portion 12b is imparted with compressive residual stress. Preferably, the surface-modified portion 12b has a compressive residual stress of 300 MPa or more. In this case, the surface modified portion 12b extends in the width direction, and the central axis N1 passing through the central portion 121 in the longitudinal direction of the second spring member 12 passes through the central portion 121 of the hole 12a. In other words, the surface modified portion 12b is formed in a region extending the same length in the longitudinal direction, with the central portion 121 of the hole 12a as the boundary.

[0029] Furthermore, the second spring member 12 has a surface-modified portion 12c formed on the inner circumferential surface forming the hole 12a (see FIG. 5). The surface-modified portion 12c extends from the opening edge of the hole 12a to the surface-modified portion 12b. The extension length D1 (depth D1) of the surface-modified portion 12c from the opening edge of the hole 12a is set in the range of 0.3 × T1 ≦ D1, where T1 is the plate thickness of the second spring member 12. From the viewpoint of improving durability against delayed fracture, the surface-modified portion 12c preferably has a compressive residual stress of 150 MPa or more. In this case, it is preferable that the compressive residual stress be imparted to the inner circumferential surface of the surface-modified portion 12c to a depth of 0.05 mm or more from the opening edge of the hole 12a.

[0030] The TS surfaces of the first spring member 11 and the second spring member 12 are entirely subjected to a surface modification process by shot peening.

[0031] The second spring member 12 is stacked with the TS surface facing the CS surface of the first spring member 11, and is fastened by the fastening member 15. At this time, the surface modified portion 12b on the CS surface of the second spring member 12 is in pressure contact with the TS surface of the first spring member 11.

[0032] An example of a method for forming the surface-modified portions 12b and 12c will now be described with reference to FIG. 6 . FIG. 6 is a diagram for explaining a surface treatment method for a spring member included in a laminated spring according to an embodiment of the present invention. The surface-modified portions 12b and 12c are formed by shot peening, which involves the collision of shot material projected from a nozzle 100. During this process, the nozzle 100 ejects the shot material from a direction inclined with respect to the axis N2 of the through-hole 12a. This results in shot peening of the surface of the second spring member 12, as well as shot peening of a portion of the surface of the second spring member 12 that is not covered by the hole 12a. In this manner, the surface-modified portions 12b and 12c are formed on the second spring member 12. In addition to shot peening, the surface-modified portions 12b and 12c may be formed by machining (burnishing or reaming) to impart residual stress.

[0033] In the embodiment of the present invention described above, the second spring member 12 stacked on the first spring member 11 is provided with a surface-modified portion 12b extending in the width direction on a portion of the CS surface including the hole 12a through which the fastening member 15 is inserted, and a surface-modified portion 12c extending from the opening edge of the hole 12a to the surface-modified portion 12b is provided. By surface-modifying the periphery of the hole 12a and a portion of the inner peripheral surface of the CS surface, compressive residual stress is imparted around the hole 12a, improving durability against delayed fracture. This embodiment improves durability in a laminated spring formed by stacking multiple spring members.

[0034] Furthermore, according to this embodiment, the surface-modified portions 12b, 12c can be formed by spraying shot material from a direction perpendicular (axis N2 direction) or from an inclined direction relative to the surface of the second spring member 12 (here, a plane perpendicular to the axis N2 of the through-hole 12a), and this can be performed by a simple shot peening process. From the viewpoint of imparting deeper compressive residual stress to the surface-modified portion 12c, the inclination angle is preferably 45 degrees or less, and more preferably 30 degrees or less. Furthermore, when performing shot peening from an inclined direction, it is preferable to perform shot peening not only from one direction but also from opposing directions.

[0035] (Variation 1) Next, Variation 1 of the embodiment of the present invention will be described with reference to Figures 7 and 8. Figure 7 is a diagram showing the configuration of a portion of a spring member included in a laminated spring according to Variation 1 of the present invention. Figure 8 is a cross-sectional view taken along line B-B shown in Figure 7. The laminated spring according to Variation 1 differs from the embodiment in the manner in which the surface modified portion of the second spring member 12 is formed. The other components have the same configuration as the embodiment, and therefore description thereof will be omitted.

[0036] The second spring member 12 has a surface-modified portion 12d formed by surface modification by shot peening on a portion of the CS surface. The surface-modified portion 12d is formed in a region that includes the hole 12a and extends from one end to the other in the width direction. The region in which the surface-modified portion 12d is formed has a longitudinal length that reaches both ends of the second spring member 12 in the width direction. In this case, the longitudinal length of the second spring member 12 in the surface-modified portion 12d is longer than the diameter of the hole 12a. Compressive residual stress is imparted to the surface-modified portion 12d.

[0037] Furthermore, the second spring member 12 has a surface modified portion 12c formed on the inner circumferential surface of the hole 12a.

[0038] The second spring member 12 according to the first modification is stacked with the CS surface facing the TS surface of the first spring member 11, as in the embodiment, and fastened by the fastening member 15. At this time, the surface modified portion 12d on the CS surface of the second spring member 12 is in pressure contact with the TS surface of the first spring member 11.

[0039] In the above-described modified example 1, similar to the embodiment, in the second spring member 12 stacked on the first spring member 11, a surface modified portion 12d extending in the width direction is provided on a part of the CS surface including the hole 12a through which the fastening member 15 is inserted, and a surface modified portion 12c is provided extending from the opening edge of the hole 12a to the surface modified portion 12d, thereby improving the durability of the laminated spring formed by stacking multiple spring members.

[0040] (Variation 2) Next, Variation 2 of the embodiment of the present invention will be described with reference to Figures 9 and 10. Figure 9 is a diagram showing the configuration of a portion of a spring member provided in a laminated spring according to Variation 2 of the present invention. Figure 10 is a cross-sectional view taken along line CC shown in Figure 9. The laminated spring according to Variation 2 differs from the embodiment in the manner in which the surface modified portion of the second spring member 12 is formed. The other components have the same configuration as the embodiment, and therefore description thereof will be omitted.

[0041] The second spring member 12 has a surface-modified portion 12e formed by surface modification by shot peening on a portion of the CS surface. The surface-modified portion 12e is formed in a region that passes through the hole 12a in the width direction and extends from one end to the other end in the width direction. The region where the surface-modified portion 12b is formed has a longitudinal length that is shorter than the width (width) of the second spring member 12 and does not reach both ends in the width direction. In this case, the longitudinal length of the surface-modified portion 12e of the second spring member 12 is shorter than the diameter of the hole 12a. Compressive residual stress is imparted to the surface-modified portion 12e. In this case, the surface-modified portion 12e extends in the width direction, and its central axis passing through the longitudinal center of the second spring member 12 passes through the center of the hole 12a.

[0042] Furthermore, the second spring member 12 has a surface modified portion 12c formed on the inner circumferential surface of the hole 12a.

[0043] The second spring member 12 according to the second modification example is stacked with the TS surface facing the CS surface of the first spring member 11, as in the embodiment, and fastened by the fastening member 15. At this time, the surface modified portion 12e on the CS surface of the second spring member 12 is in pressure contact with the TS surface of the first spring member 11.

[0044] In the second modification example described above, similar to the embodiment, in the second spring member 12 stacked on the first spring member 11, a surface modified portion 12e extending in the width direction is provided on a part of the CS surface including the hole 12a through which the fastening member 15 is inserted, and a surface modified portion 12c is provided extending from the opening edge of the hole 12a to the surface modified portion 12e, thereby improving the durability of the laminated spring formed by stacking multiple spring members.

[0045] (Variation 3) Next, Variation 3 of the embodiment of the present invention will be described with reference to Figs. 11 and 12. Fig. 11 is a diagram showing the configuration of a portion of a spring member included in a laminated spring according to Variation 3 of the present invention. Fig. 12 is a cross-sectional view taken along line D-D shown in Fig. 11. The laminated spring according to Variation 3 differs from the embodiment in the manner in which the surface modified portion of the second spring member 12 is formed. The other components have the same configuration as the embodiment, and therefore description thereof will be omitted.

[0046] The second spring member 12 has a surface-modified portion 12f formed by surface modification using shot peening on a portion of the CS surface. The surface-modified portion 12f is an annular region that extends over the entire circumference of the hole 12a. The region in which the surface-modified portion 12f is formed is a circle centered at the center of the opening edge of the hole 12a, with its diameter greater than the diameter of the hole 12a and smaller than its width. That is, the surface-modified portion 12f is an annular region concentric with the circle formed by the opening edge of the hole 12a, and the diameter of the circle formed by the outer periphery is shorter than the width of the second spring member 12. In this case, the surface-modified portion 12f extends in the width direction, and its central axis passing through the longitudinal center of the second spring member 12 passes through the center of the hole 12a. Compressive residual stress is imparted to the surface-modified portion 12f.

[0047] Furthermore, the second spring member 12 has a surface modified portion 12c formed on the inner circumferential surface of the hole 12a.

[0048] The second spring member 12 according to the third modification is stacked with the TS surface facing the CS surface of the first spring member 11, as in the embodiment, and fastened by the fastening member 15. At this time, the surface modified portion 12f on the CS surface of the second spring member 12 is in pressure contact with the TS surface of the first spring member 11.

[0049] In the third modified example described above, similar to the embodiment, in the second spring member 12 stacked on the first spring member 11, a surface modified portion 12f extending in the width direction is provided on a part of the CS surface including the hole 12a through which the fastening member 15 is inserted, and a surface modified portion 12c is provided extending from the opening edge of the hole 12a to the surface modified portion 12f, thereby improving the durability of the laminated spring formed by stacking multiple spring members.

[0050] Although the embodiments for carrying out the present invention have been described above, the present invention should not be limited to the above-described embodiments. The surface modified portion formed on the second spring member 12 is not limited to the region shown in the above-described embodiments and modifications. In addition, various methods of shot peening, such as air or ultrasonic, can be used depending on the required performance of the spring member, and conditions such as the particle material, shape, size, injection speed, and temperature can also be set appropriately.

[0051] In addition, although the above-described embodiment and modified examples have been described with respect to a configuration having one second spring member, a configuration having multiple second spring members may also be used. In the case of a configuration having multiple second spring members, each second spring member is formed with a surface modified portion as described above. Furthermore, the laminated spring may be configured without a washer.

[0052] As such, the present invention may include various embodiments not described here, and various design changes may be made within the scope of the technical idea specified by the claims.

[0053] Hereinafter, examples of the laminated spring according to the present invention will be described, but the present invention is not limited to these examples.

[0054] Example 1 In Example 1, a delayed fracture test was conducted using a test specimen with a hole formed in the center of a plate-shaped member. <Test Specimen> A plate-shaped member was used, made of spring steel (SUP10) with a width of 70 mm, length of 180 mm, and thickness of 22 mm and a hardness of 500 HV. A φ12.9 mm hole was formed in the center of this plate-shaped member by hot punching. Furthermore, shot peening (SP) was performed on the entire TS surface of the plate-shaped member and around the hole on the CS surface to prepare a test specimen. Shot peening was performed using an air-type shot peening system, firing shot material at a 90-degree angle relative to the member surface with a width of 8 mm and a duration of 7 seconds. In Example 1, shot peening was performed using a line SP, in which the projection nozzle scanned in one direction while firing shot material at a 90-degree angle relative to the member surface. In Example 1, shot peening was performed on the CS surface in the region shown in Figure 4 to impart compressive residual stress. The shot peened area of ​​the CS surface was given a compressive residual stress of approximately 900 MPa at most. <Delayed fracture test> An arbitrary bending stress was applied to the test piece by three-point bending, and this test piece was immersed in 10 L of a 1% ammonium thiocyanate aqueous solution at room temperature for 100 hours, and the presence or absence of breakage in the test piece after immersion was confirmed. Those that broke in the test at a stress of 1000 MPa or more were rated as passing (◯), and those that did not break were rated as failing (×).

[0055] The SP construction conditions and delayed fracture test results for Example 1 are shown in Table 1.

[0056] Example 2 is the same as Example 1, except that the CS surface was shot peened in the region shown in Figure 7 to impart compressive residual stress, and the treatment width was set to 30 mm. The treatment conditions of SP in Example 2 and the delayed fracture test results are shown in Table 1.

[0057] Example 3 Example 3 is the same as Example 1, except that shot peening was performed using an air shot peening system, in which shot material was emitted at a 45-degree angle to the component surface, and the application time was 3.5 seconds. In Example 1, shot peening was performed using an inclined SP in which shot material was emitted at a 45-degree angle to the component surface from opposite directions (bidirectional) relative to the central axis (central axis N1) passing through the center of the hole, sequentially for 3.5 seconds each. The application conditions of the SP for Example 3 and the results of the delayed fracture test are shown in Table 1.

[0058] Comparative Example 1 Comparative Example 1 was the same as Example 1 except that shot peening was not performed on the CS surface. Table 1 shows the SP processing conditions and delayed fracture test results for Comparative Example 1.

[0059] As shown in Table 1, Examples 1 to 3, which were subjected to the shot peening of the present invention, had good resistance to delayed fracture, while Comparative Example 1, which was not subjected to shot peening, had poorer resistance to delayed fracture than Examples 1 to 3.

[0060] As described above, the laminated spring according to the present invention is suitable for improving the durability of a laminated spring formed by stacking a plurality of spring members.

[0061] REFERENCE SIGNS LIST 1 laminated spring 11 first spring member 11a, 12a hole 12 second spring member 12b, 12d to 12f surface modified portion (first surface modified portion) 12c surface modified portion (second surface modified portion) 13, 14 washer 15 fastening member 151 bolt 152 nut

Claims

1. A laminated spring formed by stacking a plurality of spring members, comprising: a first spring member that serves as a base; one or more second spring members stacked on the first spring member; and fastening members that fasten the first and second spring members together; wherein the first and second spring members are formed with holes through which the fastening members are inserted, and the second spring member is formed with a first surface-modified portion that is part of the surface that receives compressive stress during operation and that includes part of the opening edge of the hole, and a second surface-modified portion that is provided on part of the inner surface that forms the hole and is continuous with the first surface-modified portion; and wherein, where D1 is the extension length from the opening edge of the hole of the second surface-modified portion and T1 is the plate thickness of the second spring member, the laminated spring satisfies 0.3 x T1 ≦ D1.

2. The laminated spring according to claim 1, wherein the second surface modified portion has a compressive residual stress of 150 MPa or more.

3. A laminated spring as described in claim 2, characterized in that the second surface modified portion has compressive residual stress imparted to it from the edge of the opening of the hole to a depth of 0.05 mm or more.

4. The laminated spring according to claim 3, wherein the second surface modified portions are provided in pairs facing each other in the width direction of the leaf spring.

5. The laminated spring according to claim 1, wherein the first surface modified portion extends in the width direction of the second spring member.

6. The laminated spring according to claim 2, wherein the first surface modified portion extends to both widthwise ends of the second spring member.

7. The laminated spring according to claim 1, characterized in that the formation range of the first surface modified portion in the longitudinal direction of the second spring member is smaller than the diameter of the hole formed in the second spring member.

8. The laminated spring according to claim 1, characterized in that the first surface modified portion has a formation range in the longitudinal direction of the second spring member that is larger than the diameter of the hole formed in the second spring member.

9. The laminated spring according to claim 1, characterized in that the area in which the first surface modified portion is formed forms a ring shape concentric with the hole formed in the second spring member.

10. A laminated spring as described in any one of claims 1 to 7, characterized in that the first surface modified portion passes through the longitudinal center of the second spring member, and the central axis extending in the width direction passes through the center of the hole formed in the second spring member.