Bolt fastening structure

Abstract

To provide a bolt fastening structure capable of attaining reduction in weight while presenting a sufficient fastening force.SOLUTION: A bolt fastening structure 1 comprises: a bolt 10; a screw part 20 in which a female screw 21 is formed; a first interposing part 30 which is formed integrally with or separately from the screw part 20 and in which a first bolt hole 31 connected to the female screw 21 is formed; and a second interposing part 40 which is formed separately from the bolt 10 and the first interposing part 30 and disposed while being held between a head part 12 and the first interposing part 30 and in which a second bolt hole 41 connected to the first bolt hole 31 is formed. The first bolt hole 31 includes a first diameter expanded hole 31b which is formed so as to expand a hole diameter gradually from the side of the female screw 21 to the side of the head part 12, and the second bolt hole 41 includes a second diameter expanded hole 41b which is formed so as to expand a hole diameter gradually from the side of the head part 12 to the side of the female screw 21.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a bolt fastening structure. [Background technology]

[0002] In bolt fastening structures, the bolt hole diameter is specified in JIS B 1001-1985 (ISO 237-1979). The bolt hole diameter is divided into several classes and specified according to the nominal bolt diameter. The main part of the bolt hole, excluding chamfering and countersinking, is formed into a cylindrical inner circumferential shape.

[0003] Patent Document 1 describes a bolt fastening structure in which a number of connecting members are fastened using a bolt, in which a recess is formed in the portion through which the bolt passes on at least one of the connecting surfaces between a first connecting member having a female threaded portion that screws onto the bolt and a second connecting member that contacts the first connecting member.

[0004] Patent Document 2 describes the Letschel cone of influence. According to the concept of the Letschel cone of influence, the stress caused by fastening acts within a range within a cone that spreads at an angle of 45° from a point located on the outer edge of the part that abuts against the bolt head.

[0005] Also, Figure 6 of Patent Document 3 describes that the bolt hole is composed of a reference hole portion and a tapered hole portion whose hole diameter gradually increases from the reference hole portion toward the bolt head side. Figure 7 of Patent Document 3 describes that the bolt hole is composed of a reference hole portion and a tapered hole portion whose hole diameter gradually increases from the reference hole portion toward the female thread side. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2006-2828 A [Patent Document 2] JP 2020-23948 A [Patent Document 3] JP 2017-219052 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, there is a demand for reducing the weight of bolt fastening structures. To achieve this, the bolt diameter is sometimes reduced. However, if the bolt hole diameter is reduced in conjunction with the reduction in the bolt diameter, the mass of the mating member increases even if the bolt mass is reduced. Therefore, even if the bolt diameter is reduced, it is not possible to reduce the weight of the entire bolt fastening structure.

[0008] The bolt fastening structure shown in Figures 6 and 7 of Patent Document 3 is believed to be able to reduce weight, but there is a problem as to whether it can exert sufficient fastening force. Also, there is room for improvement to achieve further weight reduction.

[0009] The present invention has been made in view of the above background, and aims to provide a bolt fastening structure that can achieve weight reduction while exerting sufficient fastening force. [Means for solving the problem]

[0010] One aspect of the present invention is a bolt having a shaft portion on which a male thread is formed and a head portion having a diameter larger than the diameter of the shaft portion; A screw-engagement portion having a female thread that screws into the male thread of the bolt; A first intermediate portion formed integrally or separately from the screw-engagement portion and having a first bolt hole connected to the female thread; a second intermediate portion that is formed separately from the bolt and the first intermediate portion, that is sandwiched between the head portion and the first intermediate portion, and that has a second bolt hole that is connected to the first bolt hole; Equipped with The first bolt hole includes a first enlarged diameter hole formed so that the hole diameter gradually increases from the female thread side toward the head side, The second bolt hole is in a bolt fastening structure including a second enlarged diameter hole formed so that the hole diameter gradually increases from the head side toward the female thread side. Effect of the Invention

[0011] When the bolt is fastened, a compressive force in the axial direction is generated in the first and second intermediate portions. In particular, the compressive force originates from the portion where the male and female threads are screwed together, and from the portion of the bolt head that is in contact with the second intermediate portion. According to the concept of the Letschel cone of influence, in the first intermediate portion, the compressive stress due to fastening is thought to act in a range within a cone whose diameter is expanded at an angle of 45° from the portion where the male and female threads are screwed together. On the other hand, in the second intermediate portion, the compressive stress due to fastening is thought to act in a range within a cone whose diameter is expanded at an angle of 45° from the portion of the bolt head that is in contact with the second intermediate portion.

[0012] Therefore, as in the above-described embodiment, the first enlarged diameter hole of the first bolt hole in the first intermediate portion located on the female thread side is formed so that the hole diameter gradually increases from the female thread side toward the head side, and the second enlarged diameter hole of the second bolt hole in the second intermediate portion located on the head side of the bolt is formed so that the hole diameter gradually increases from the head side toward the female thread side.

[0013] In this way, by forming each of the first bolt hole and the second bolt hole in a shape that takes into account the Letschel influence cone, sufficient compressive stress due to fastening can be applied to the first intermediate portion and the second intermediate portion. As a result, sufficient fastening force can be exerted. Furthermore, by providing the first enlarged diameter hole in the first bolt hole and the second enlarged diameter hole in the second bolt hole, weight reduction can be achieved.

[0014] As described above, according to the above aspect, it is possible to provide a bolt fastening structure that can achieve a reduction in weight while exerting sufficient fastening force.

[0015] It should be noted that the reference characters in parentheses in the claims indicate the corresponding relationship with the specific means described in the embodiments described later, and are not intended to limit the technical scope of the present invention. [Brief description of the drawings]

[0016] [Figure 1] FIG. 2 is an axial cross-sectional view showing the bolt fastening structure in the first embodiment. [Diagram 2] 2A and 2B are diagrams showing the female-thread integral member (including a screw-fitting portion and a first intermediate portion) of FIG. 1, in which (a) is a plan view and (b) is a cross-sectional view taken along line IIb-IIb in (a). [Diagram 3] 2A and 2B are diagrams showing the second intermediate portion in FIG. 1, where (a) is a plan view and (b) is a cross-sectional view taken along line IIIb-IIIb in (a). [Figure 4] FIG. [Diagram 5] FIG. [Figure 6] FIG. 11 is an axial cross-sectional view showing a bolt fastening structure in a second embodiment. [Figure 7] FIG. 11 is an axial cross-sectional view showing a bolt fastening structure in a third embodiment. [Figure 8] FIG. 11 is an axial cross-sectional view showing a bolt fastening structure in a fourth embodiment. [Figure 9] FIG. 13 is an axial cross-sectional view showing a bolt fastening structure in a fifth embodiment. [Figure 10] FIG. 13 is an axial cross-sectional view showing a bolt fastening structure in a sixth embodiment. [Figure 11] Cross-sectional view taken along line XI-XI in FIG. [Figure 12] 11A and 11B are views showing the second intermediate portion in FIG. 10, where (a) is a plan view, (b) is a cross-sectional view taken along line XIIb-XIIb in (a), and (b) is a cross-sectional view taken along line XIIc-XIIc in (a). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The bolt fastening structure comprises a bolt, a threaded portion, a first intermediate portion, and a second intermediate portion. The first bolt hole comprises a first enlarged diameter hole formed so that the hole diameter gradually increases from the female thread side toward the head side, and the second bolt hole comprises a second enlarged diameter hole formed so that the hole diameter gradually increases from the head side toward the female thread side.

[0018] In the bolt fastening structure, the maximum hole diameter of the first enlarged hole and the maximum hole diameter of the second enlarged hole may be formed to be larger than the diameter of the outer edge of the portion of the head of the bolt that is in contact with the second intermediate portion, thereby making it possible to achieve further weight reduction.

[0019] The first expansion hole may be formed radially inward from a first imaginary line expanding from an end of the head side of the portion where the male thread and the female thread are screwed together at an angle of 45° toward the head side, and the second expansion hole may be formed radially inward from a second imaginary line expanding from an outer edge of a portion of the head that contacts the second intermediate portion side toward the female thread side at an angle of 45°. In other words, by positioning each of the first bolt hole and the second bolt hole radially inward from the Letschel influence cone, sufficient compressive stress due to fastening can be applied to the first intermediate portion and the second intermediate portion. As a result, sufficient fastening force can be exerted.

[0020] The first expansion hole and the second expansion hole may be formed in a tapered shape. The first expansion hole and the second expansion hole may be formed in the following manner, in addition to being tapered. First, the first expansion hole may be formed in a curved shape in the axial cross section so that the inclination angle with respect to the central axis gradually decreases from the female thread side toward the head side, and the second expansion hole may be formed in a curved shape in the axial cross section so that the inclination angle with respect to the central axis gradually decreases from the head side toward the female thread side.

[0021] Secondly, the first expansion hole may be formed in a curved shape in axial cross section so that the inclination angle with respect to the central axis gradually increases from the female thread side toward the head side, and the second expansion hole may be formed in a curved shape in axial cross section so that the inclination angle with respect to the central axis gradually increases from the head side toward the female thread side.

[0022] The head side surface of the first intermediate portion and the female thread side surface of the second intermediate portion may be in contact with each other, the opening having the maximum hole diameter of the first expanded diameter hole and the opening having the maximum hole diameter of the second expanded diameter hole may be located on the contact surface between the first intermediate portion and the second intermediate portion, the first expanded diameter hole may be formed such that the stress generated on the inner edge portion of the contact surface of the first intermediate portion with the second intermediate portion is less than the yield stress of the first intermediate portion, and the second expanded diameter hole may be formed such that the stress generated on the inner edge portion of the contact surface of the second intermediate portion with the first intermediate portion is less than the yield stress of the second intermediate portion. This ensures the durability of the bolt fastening structure.

[0023] The first enlarged diameter hole may have an axial length that is at least half that of the first bolt hole, and the second enlarged diameter hole may have an axial length that is at least half that of the second bolt hole, thereby ensuring weight reduction.

[0024] (Embodiment 1) 1. Configuration of bolt fastening structure 1 A bolt fastening structure 1 in the first embodiment will be described with reference to Figs. 1 to 3. The bolt fastening structure 1 is a structure in which a plurality of members are fastened together using a bolt 10. The thickness of the plurality of members to be fastened together is, for example, plate material having a thickness of 3 mm or more, preferably 5 mm or more. The thread diameter (nominal diameter) of the bolt 10 is, for example, 10 mm to 14 mm. However, the thickness of the plurality of members to be fastened together and the thread diameter of the bolt 10 are not limited to those described above.

[0025] 1, a bolt fastening structure 1 in this embodiment includes a bolt 10, a threaded portion 20, a first intermediate portion 30, and a second intermediate portion 40. The threaded portion 20 and the first intermediate portion 30 constitute an integrally formed female-threaded member 50. In other words, the bolt fastening structure 1 uses the bolt 10 to fasten the female-threaded member 50 constituted by the threaded portion 20 and the first intermediate portion 30, and a member constituted by the second intermediate portion 40.

[0026] 1, the bolt 10 has a shank 11 with a male thread 11a formed thereon, and a head 12 having a diameter larger than the diameter of the shank 11. The male thread 11a of the shank 11 may be formed only on the tip side of the shank 11 as shown in FIG. 1, or may be formed over the entire length of the shank 11, although not shown.

[0027] In FIG. 1, the bolt 10 is exemplified as a flanged bolt. In this case, the head 12 is a flanged head and constitutes a hexagonal head. That is, the head 12 includes a head body 12a and a flange portion 12b. However, the head 12 may be a head without a flange, may have a shape other than a hexagon, or may constitute a holed head. In this embodiment, the diameter of the flange portion 12b of the head 12 is larger than the diameter of the shank 11. Also, the width across faces of the head body 12a of the head 12 (the diameter of the inscribed circle of the head body 12a) is larger than the diameter of the shank 11.

[0028] The bolt 10 can also be configured with a stud bolt and a nut. In this case, the shaft portion 11 of the bolt 10 corresponds to the stud bolt portion, and the head portion 12 of the bolt 10 corresponds to the nut portion. In this case, the nut may be a flanged nut or a flangeless nut.

[0029] 1 and 2, the threaded portion 20 is formed with at least a female thread 21 that screws into the male thread 11a of the bolt 10. In this embodiment, the threaded portion 20 constitutes one of a plurality of members fastened by the bolt 10. In this embodiment, the female thread 21 formed in the threaded portion 20 constitutes a blind hole having a hole bottom.

[0030] In this embodiment, the screwing portion 20 has a bolt introduction hole 22 that is continuous with the opening of the female thread 21. However, the screwing portion 20 may not have the bolt introduction hole 22. The bolt introduction hole 22 starts from the opening of the female thread 21 and expands in diameter toward the head 12 side of the bolt 10. The bolt introduction hole 22 is formed radially inward of a first imaginary line L1 that expands in diameter at an angle of 45° toward the head 12 side from an end P1 on the head 12 side of the portion where the male thread 11a and the female thread 21 are screwed together.

[0031] 1 and 2, the first intermediate portion 30 is formed integrally with the screw-threaded portion 20. Thus, like the screw-threaded portion 20, the first intermediate portion 30 constitutes one of a plurality of members fastened by the bolt 10. In this embodiment, the boundary between the screw-threaded portion 20 and the first intermediate portion 30 is a surface that includes the contour line of the opening of the bolt introduction hole 22. However, the boundary between the screw-threaded portion 20 and the first intermediate portion 30 may also be a surface that includes the contour line of the opening of the female thread 21.

[0032] The first intermediate portion 30 is connected to the female thread 21 of the screw-fitting portion 20, and has a first bolt hole 31 through which the shaft portion 11 of the bolt 10 can be inserted. The first bolt hole 31 includes, in order from the screw-fitting portion 20 side, a cylindrical reference hole 31a and a first enlarged diameter hole 31b.

[0033] The cylindrical reference hole 31a is a portion formed continuously with the bolt introduction hole 22, and is formed in a cylindrical inner peripheral shape. That is, the cylindrical reference hole 31a constitutes the end portion of the first bolt hole 31 on the female thread 21 side. The inner diameter of the cylindrical reference hole 31a is formed to be larger than the inner diameter of the female thread 21 of the screwing portion 20. The inner diameter of the cylindrical reference hole 31a may be, for example, a bolt hole diameter according to the nominal diameter of the bolt 10 specified in JIS B 1001-1985 (ISO 237-1979). In addition, the inner diameter of the cylindrical reference hole 31a is formed to be smaller than the diameter of the portion of the head 12 of the bolt 10 that contacts the second intermediate portion 40 side, i.e., the flange portion 12b.

[0034] Here, the bolt introduction hole 22 is formed at an inclination angle (taper angle) suitable for inserting the shaft portion 11 of the bolt 10 into the female thread 21. In order to form the bolt introduction hole 22 at a desired inclination angle, it is effective to provide a cylindrical reference hole 31a. For example, when the thread diameter of the bolt 10 is 10 to 14 mm, the cylindrical reference hole 31a may be 1 to 2 mm. In particular, when the inclination angle of the bolt introduction hole 22 is different from the inclination angle θA of the first enlarged diameter hole 31b described later, it is preferable to form the cylindrical reference hole 31a. However, the cylindrical reference hole 31a does not have to be formed. In other words, the first intermediate portion 30 only needs to have at least the first enlarged diameter hole 31b, and may not have the cylindrical reference hole 31a.

[0035] The first enlarged diameter hole 31b is a portion formed continuous with the cylindrical reference hole 31a. The first enlarged diameter hole 31b is formed such that the hole diameter gradually increases from the female thread 21 side of the screw-engagement portion 20 toward the head 12 side of the bolt 10. Furthermore, the first enlarged diameter hole 31b is formed such that the hole diameter gradually increases over the entire circumferential direction.

[0036] In this embodiment, first enlarged diameter hole 31b constitutes the end of first bolt hole 31 on the opposite side to female thread 21. In other words, the opening of first bolt hole 31 on the side of head 12 of bolt 10 is constituted by an opening having a maximum hole diameter φA of first enlarged diameter hole 31b. However, although not shown, first bolt hole 31 may be formed so that, for example, a cylindrical hole is continuously formed from the portion of maximum hole diameter φA of first enlarged diameter hole 31b.

[0037] As described above, for example, when the thread diameter of the bolt 10 is 10 to 14 mm, the cylindrical reference hole 31a may be 1 to 2 mm. In this case, when the axial length of the first bolt hole 31 is 5 mm, the first enlarged hole 31b is in the range of 3 to 4 mm. In this way, the first enlarged hole 31b has an axial length that is more than half that of the first bolt hole 31. For example, when the axial length of the first bolt hole 31 is 5 mm, the axial length of the first enlarged hole 31b is 2.5 mm or more. This is similar for other thread diameters. Note that when the cylindrical reference hole 31a is not formed, the first enlarged hole 31b constitutes the entire first bolt hole 31.

[0038] Furthermore, as shown in FIG. 2, the first expanded hole 31b is tapered. In this embodiment, the first expanded hole 31b is tapered with a constant inclination angle θA relative to the central axis. That is, the cross-sectional shape of the first expanded hole 31b perpendicular to the central axis is a concentric circle. Therefore, the first expanded hole 31b is formed in an inverted cone shape. The inclination angle θA of the first expanded hole 31b is, for example, in the range of 5° to 40°, and preferably in the range of 10° to 30°. In particular, when the plate thickness of the first intermediate portion 30 is thin, when the strength of the bolt 10 is relatively high and the axial force is large, or when the strength of the first intermediate portion 30 is relatively low, the range of 10° to 20° is suitable.

[0039] Here, in this embodiment, since the first expanded diameter hole 31b is in an inverted cone shape, the line connecting the center points of the cross-sectional shape perpendicular to the central axis of the first expanded diameter hole 31b is linear. However, the line connecting the center points of the cross-sectional shape perpendicular to the central axis of the first expanded diameter hole 31b does not have to be linear. Also, the inclination angle θA of the first expanded diameter hole 31b may be different depending on the circumferential position. In this case, the cross-sectional shape perpendicular to the central axis of the first expanded diameter hole 31b is not circular.

[0040] Furthermore, the first expanded diameter hole 31b is formed radially inward from a first imaginary line L1 that expands at an angle of 45° toward the head 12 from an end P1 on the head 12 side of the portion where the male thread 11a and the female thread 21 are screwed together. In this embodiment, not only the first expanded diameter hole 31b but the entire first bolt hole 31 is formed radially inward from the first imaginary line L1.

[0041] 1, the maximum hole diameter φA of the first expanded diameter hole 31b is formed larger than the portion of the head 12 of the bolt 10 that is in contact with the second intermediate portion 40. In detail, the maximum hole diameter φA of the first expanded diameter hole 31b is formed larger than the inscribed circle diameter of the head body 12a of the head 12 of the bolt 10. Furthermore, the maximum hole diameter φA of the first expanded diameter hole 31b is formed larger than the diameter of the flange portion 12b of the head 12 of the bolt 10.

[0042] 1, the hole diameter of the first expanded diameter hole 31b is gradually enlarged continuously, but it may be gradually enlarged discontinuously. When the hole diameter of the first expanded diameter hole 31b is gradually enlarged discontinuously, the first expanded diameter hole 31b is formed in a stepped shape.

[0043] When the cylindrical reference hole 31a is not formed, the first enlarged diameter hole 31b is formed continuously with the bolt introduction hole 22. Alternatively, the cylindrical reference hole 31a and the bolt introduction hole 22 may not be formed, and the first enlarged diameter hole 31b may be formed from the opening of the female thread 21.

[0044] 1 and 3, the second intermediate portion 40 is formed separately from the bolt 10 and the first intermediate portion 30, and is disposed between the head 12 of the bolt 10 and the first intermediate portion 30. Thus, the second intermediate portion 40 constitutes another of the multiple members fastened by the bolt 10.

[0045] The second intermediate portion 40 is connected to the first bolt hole 31, and has a second bolt hole 41 through which the shaft portion 11 of the bolt 10 can be inserted. Therefore, one surface of the second intermediate portion 40 (the upper surface in FIG. 1) constitutes the surface with which the head 12 of the bolt 10 comes into contact. Meanwhile, the other surface of the second intermediate portion 40 (the lower surface in FIG. 1) constitutes the surface with which the first intermediate portion 30 comes into contact. Therefore, the surface on the head 12 side of the first intermediate portion 30 (the upper surface in FIG. 1) and the surface on the female thread 21 side of the second intermediate portion 40 (the lower surface in FIG. 1) come into contact with each other.

[0046] The second bolt hole 41 includes, in order from the head 12 side of the bolt 10, a cylindrical reference hole 41a and a second enlarged diameter hole 41b. The cylindrical reference hole 41a is formed into a cylindrical inner peripheral shape and constitutes the end of the second bolt hole 41 on the head 12 side. The inner diameter of the cylindrical reference hole 31a is formed smaller than the diameter of the portion of the head 12 of the bolt 10 that contacts the second intermediate portion 40 side, i.e., the flange portion 12b. The inner diameter of the cylindrical reference hole 41a may be a bolt hole diameter corresponding to the nominal diameter of the bolt 10 specified in, for example, JIS B 1001-1985 (ISO 237-1979).

[0047] Here, if the cylindrical reference hole 41a is not formed, the acute angle portion of the second enlarged diameter hole 41b constitutes the opening of the second bolt hole 41. By providing the cylindrical reference hole 41a, it is possible to prevent the opening of the second bolt hole 41 from being acute-angled. For example, when the thread diameter of the bolt 10 is 10 to 14 mm, the cylindrical reference hole 41a may be 1 to 2 mm. However, when the inclination angle θB (described later) of the second enlarged diameter hole 41b is small, the cylindrical reference hole 41a does not need to be formed. In other words, the second intermediate portion 40 only needs to have at least the second enlarged diameter hole 41b, and may not have the cylindrical reference hole 41a.

[0048] The second enlarged diameter hole 41b is a portion formed continuous with the cylindrical reference hole 41a. The second enlarged diameter hole 41b is formed such that the hole diameter gradually increases from the head 12 side of the bolt 10 toward the female thread 21 side of the screw-engagement portion 20. Furthermore, the second enlarged diameter hole 41b is formed such that the hole diameter gradually increases over the entire circumferential direction.

[0049] In this embodiment, the second enlarged diameter hole 41b constitutes the end of the second bolt hole 41 on the female thread 21 side. In other words, the opening of the second bolt hole 41 on the female thread 21 side is constituted by an opening having a maximum hole diameter φB of the second enlarged diameter hole 41b. However, although not shown, the second bolt hole 41 may be formed so that, for example, a cylindrical hole is continuously formed from the portion of the maximum hole diameter φB of the second enlarged diameter hole 41b.

[0050] As described above, for example, when the thread diameter of the bolt 10 is 10 to 14 mm, the cylindrical reference hole 31a may be 1 to 2 mm. In this case, when the axial length of the second bolt hole 41 is 5 mm, the second enlarged hole 41b is in the range of 3 to 4 mm. In this way, the second enlarged hole 41b has an axial length that is more than half of the second bolt hole 41. For example, when the axial length of the second bolt hole 41 is 5 mm, the axial length of the second enlarged hole 41b is 2.5 mm or more. This is the same for other thread diameters. In addition, when the cylindrical reference hole 41a is not formed, the second enlarged hole 41b constitutes the entire second bolt hole 41.

[0051] Furthermore, as shown in FIG. 3, the second expanded hole 41b is tapered. In this embodiment, the second expanded hole 41b is tapered with a constant inclination angle θB with respect to the central axis. That is, in the second expanded hole 41b, the cross-sectional shape perpendicular to the central axis is a concentric circle. Therefore, the second expanded hole 41b is formed in a conical shape. The inclination angle θB of the second expanded hole 41b is, for example, in the range of 5° to 40°, preferably in the range of 10° to 30°. In particular, when the plate thickness of the second intermediate portion 40 is thin, when the strength of the bolt 10 is relatively high and the axial force is large, or when the strength of the second intermediate portion 40 is relatively low, the range of 10° to 20° is suitable.

[0052] Here, in this embodiment, since the second expanded diameter hole 41b is conical, the line connecting the center points of the cross-sectional shape perpendicular to the central axis of the second expanded diameter hole 41b is linear. However, the line connecting the center points of the cross-sectional shape perpendicular to the central axis of the second expanded diameter hole 41b does not have to be linear. Also, the inclination angle θB of the second expanded diameter hole 41b may be different depending on the circumferential position. In this case, the cross-sectional shape perpendicular to the central axis of the second expanded diameter hole 41b is not circular.

[0053] Furthermore, the second expanded diameter hole 41b is formed radially inward from a second imaginary line L2 that expands at an angle of 45° from a point P2 that is the outer edge of a portion of the head 12 of the bolt 10 that contacts the second intermediate portion 40 toward the female thread 21. In this embodiment, not only the second expanded diameter hole 41b but the entire second bolt hole 41 is formed radially inward from the second imaginary line L2.

[0054] Furthermore, in this embodiment, the second expanded diameter hole 41b is formed radially inward of a third imaginary line L3 that expands at an angle of 45° toward the female thread 21 from point P3, which is the intersection point between an extension line of an inscribed circle portion (portion corresponding to the two face width) of the head body 12a of the head 12 of the bolt 10 and a portion of the head 12 of the bolt 10 that contacts the second intermediate portion 40. In this embodiment, not only the second expanded diameter hole 41b but the entire second bolt hole 41 is formed radially inward of the third imaginary line L3.

[0055] 1, the maximum hole diameter φB of the second expanded diameter hole 41b is formed larger than the portion of the head 12 of the bolt 10 that is in contact with the second intermediate portion 40. In detail, the maximum hole diameter φB of the second expanded diameter hole 41b is formed larger than the inscribed circle diameter of the head body 12a of the head 12 of the bolt 10. Furthermore, the maximum hole diameter φB of the second expanded diameter hole 41b is formed larger than the diameter of the flange portion 12b of the head 12 of the bolt 10.

[0056] In this embodiment, the second expanded diameter hole 41b has a maximum hole diameter φB, which is the same as the maximum hole diameter φA of the first expanded diameter hole 31b. However, the maximum hole diameter φA of the first expanded diameter hole 31b and the maximum hole diameter φB of the second expanded diameter hole 41b may be slightly different. In addition, in FIG. 1, the hole diameter of the second expanded diameter hole 41b is gradually enlarged continuously, but it may be gradually enlarged intermittently. When the hole diameter of the second expanded diameter hole 41b is gradually enlarged intermittently, the second expanded diameter hole 41b is formed in a stepped shape.

[0057] 1, the face of the first intermediate portion 30 on the head 12 side (the lower face in FIG. 1) and the face of the second intermediate portion 40 on the female thread 21 side (the lower face in FIG. 1) are in contact with each other. The opening having the maximum hole diameter φA of the first expanded diameter hole 31b and the opening having the maximum hole diameter φB of the second expanded diameter hole 41b are located on the contact surface between the first intermediate portion 30 and the second intermediate portion 40. When the first bolt hole 31 and the second bolt hole 41 are concentrically located, the opening having the maximum hole diameter φA of the first bolt hole 31 and the opening having the maximum hole diameter φB of the second bolt hole 41 coincide with each other.

[0058] Further, attention is paid to the stress generated on the contact surface between the first intermediate portion 30 and the second intermediate portion 40. First, attention is paid to the first intermediate portion 30. The first expanded diameter hole 31b is formed so that the average stress generated in the region inside the first virtual line L1 of the contact surface of the first intermediate portion 30 with the second intermediate portion 40 is less than the yield stress of the first intermediate portion 30. Here, the stress generated in the part located on the innermost side of the contact surface of the first intermediate portion 30 with the second intermediate portion 40, that is, the inner edge part of the contact surface, is the largest. Therefore, more preferably, the first expanded diameter hole 31b is formed so that the stress generated in the inner edge part (innermost part) of the contact surface of the first intermediate portion 30 with the second intermediate portion 40 is less than the yield stress of the first intermediate portion 30.

[0059] Next, attention is paid to the second intermediate portion 40. The second expanded diameter hole 41b is formed so that the average stress generated in the region inside the second virtual line L2 or the third virtual line L3 of the contact surface of the second intermediate portion 40 with the first intermediate portion 30 is less than the yield stress of the second intermediate portion 40. Here, the stress generated in the innermost part of the contact surface of the second intermediate portion 40 with the first intermediate portion 30, that is, the inner edge part of the contact surface, is the largest. Therefore, more preferably, the second expanded diameter hole 41b is formed so that the stress generated in the inner edge part (innermost part) of the contact surface of the second intermediate portion 40 with the first intermediate portion 30 is less than the yield stress of the second intermediate portion 40.

[0060] In this way, by making the average stress generated in the first intermediate portion 30 and the second intermediate portion 40 less than the yield stress, it is possible to ensure the durability of the bolt fastening structure 1. In particular, by making the maximum stress generated in the first intermediate portion 30 and the second intermediate portion 40 less than the yield stress, it is possible to more sufficiently ensure the durability of the bolt fastening structure 1.

[0061] 2. Analysis of bolt fastening structure 1 An analysis of the bolt fastening structure 1 will be described with reference to Fig. 4 and Fig. 5. The analysis was performed on an analysis model shown in Fig. 4(a) corresponding to this embodiment and an analysis model shown in Fig. 4(b) as a comparative example. Fig. 5(a) and (b) show the analysis results.

[0062] 4(a) corresponding to this embodiment includes the bolt 10 and the second intermediate portion 40, and is a planar model in an axial cross section of the bolt 10. However, since the axial cross section of the bolt 10 is line-symmetric with respect to the center line of the bolt 10, the target planar model is only one portion of the bolt 10 from the center line.

[0063] The analytical model shown in FIG. 4(a) corresponding to this embodiment assumes that the inclination angle θB of the second enlarged diameter hole 41b of the second intermediate portion 40 (a rectangular member in FIG. 4) is 15°. In addition, the lower surface of the second intermediate portion 40, i.e., the surface that contacts the first intermediate portion 30, is restrained. In other words, the second intermediate portion 40 is fixedly supported by the first intermediate portion 30. Furthermore, a contact condition between the bolt 10 and the second intermediate portion 40 is given. In this state, when a predetermined load is applied downward to a line that is the tip of the bolt 10 (the lower end of FIG. 4), the stress generated on the lower surface of the second intermediate portion 40 is obtained. The predetermined load applied to the bolt 10 is a value calculated based on the torque applied when the bolt 10 is fastened. In this analysis, the material of the second intermediate portion 40 is S45C, and the predetermined load applied to the bolt 10 is 8388N. In addition, the analytical model shown in FIG. 4(b) as a comparative example was performed in the same manner as the analytical model shown in FIG. 4(a) corresponding to this embodiment, except that the bolt hole of the member corresponding to the second intermediate portion 40 was cylindrical.

[0064] In the case of the analysis model of Fig. 4(a) corresponding to this embodiment, as shown in Fig. 5(a), the stress generated on the lower surface of the second intermediate portion 40 was in the range of -18.9 MPa to +1.3 MPa. Here, negative values ​​for stress indicate compressive stress and positive values ​​for stress indicate tensile stress. On the other hand, in the case of the analysis model shown in Fig. 5(b) as a comparative example, the stress generated on the lower surface of the member corresponding to the second intermediate portion 40 was in the range of -7.8 MPa to +0.8 MPa.

[0065] In both the present embodiment and the comparative example, the compressive stress occurring on the lower surface of the second intermediate portion 40 is the largest on the second expanded diameter hole 41b side. Furthermore, compared to the comparative example, the compressive stress occurring on the lower surface of the second intermediate portion 40 is larger in the present embodiment. Here, the yield stress of S45C is approximately 400 MPa. Therefore, in the present embodiment, the compressive stress occurring on the lower surface of the second intermediate portion 40 is sufficiently small compared to the yield stress. For this reason, in the present embodiment, the stress occurring in the second intermediate portion 40 is sufficiently small, and the bolt fastening structure 1 in the present embodiment can ensure sufficient durability.

[0066] 3. Effects According to the bolt fastening structure 1 of this embodiment, a compressive force in the axial direction is generated in the first intermediate portion 30 and the second intermediate portion 40 by fastening with the bolt 10. In particular, the portion where the male thread 11a and the female thread 21 are screwed together, and the portion of the head 12 of the bolt 10 that is in contact with the second intermediate portion 40 are the starting points of the compressive force. According to the concept of the Letschel influence cone, in the first intermediate portion 30, it is considered that a compressive stress due to fastening acts in a range within a cone whose diameter is expanded at an angle of 45° from the starting point of the portion (point P1) where the male thread 11a and the female thread 21 are screwed together. On the other hand, in the second intermediate portion 40, it is considered that a compressive stress due to fastening acts in a range within a cone whose diameter is expanded at an angle of 45° from the starting point of the portion (point P2 or point P3) of the head 12 of the bolt 10 that is in contact with the second intermediate portion 40.

[0067] In this embodiment, the first expanded diameter hole 31b of the first bolt hole 31 in the first intermediate portion 30 located on the female thread 21 side is formed so that the hole diameter gradually expands from the female thread 21 side toward the head 12 side, and the second expanded diameter hole 41b of the second bolt hole 41 in the second intermediate portion 40 located on the head 12 side of the bolt 10 is formed so that the hole diameter gradually expands from the head 12 side toward the female thread 21 side.

[0068] In this way, by forming each of the first bolt hole 31 and the second bolt hole 41 in a shape that takes into account the Letschel influence cone, it is possible to ensure that sufficient compressive stress due to fastening acts on the first intermediate portion 30 and the second intermediate portion 40. As a result, a sufficient fastening force can be exerted. Furthermore, the first bolt hole 31 has the first enlarged diameter hole 31b, and the second bolt hole 41 has the second enlarged diameter hole 41b, thereby enabling weight reduction. Therefore, according to the bolt fastening structure 1 in this embodiment, it is possible to achieve weight reduction while exerting a sufficient fastening force.

[0069] (Modification of the first embodiment) In the bolt fastening structure 1 in the first embodiment, the head 12 of the bolt 10 is in direct contact with the second intermediate portion 40. Alternatively, a washer may be interposed between the head 12 of the bolt 10 and the second intermediate portion 40. In this case, point P2, which is the starting point of the second imaginary line L2, is the outer edge of a portion of the head 12 of the bolt 10 that is in contact with the washer. Similarly, point P3, which is the starting point of the third imaginary line L3, is the intersection of an extension of an inscribed circle portion (a portion corresponding to the width across flats) of the head body 12a of the head 12 of the bolt 10 and the portion of the head 12 of the bolt 10 that is in contact with the washer.

[0070] (Embodiment 2) A bolt fastening structure 2 in the second embodiment will be described with reference to Fig. 6. Note that, among the reference symbols used in the second embodiment, those which are the same as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0071] 6, a bolt fastening structure 2 in this embodiment differs from the bolt fastening structure 1 in the first embodiment in a first enlarged diameter hole 131b of a first intermediate portion 30 and a second enlarged diameter hole 141b of a second intermediate portion 40. The other configurations are the same.

[0072] The first expanded hole 131b is formed in a curved shape in the axial cross section so that the inclination angle θA with respect to the central axis gradually decreases from the female thread 21 side toward the head 12 side. The second expanded hole 141b is formed in a curved shape in the axial cross section so that the inclination angle θB with respect to the central axis gradually decreases from the head 12 side toward the female thread 21 side.

[0073] In this embodiment, the maximum hole diameter φA of the first expanded diameter hole 131b (showing a portion corresponding to FIG. 2) and the maximum hole diameter φB of the second expanded diameter hole 141b (showing a portion corresponding to FIG. 3) are formed to be the same. However, the maximum hole diameter φA of the first expanded diameter hole 131b and the maximum hole diameter φB of the second expanded diameter hole 141b may be slightly different. When the first bolt hole 31 and the second bolt hole 41 are positioned concentrically, the opening with the maximum hole diameter φA of the first bolt hole 31 and the opening with the maximum hole diameter φB of the second bolt hole 41 are in a state of being aligned. Furthermore, the first expanded diameter hole 131b of the first bolt hole 31 and the second expanded diameter hole 141b of the second bolt hole 41 may be configured to form a continuous surface.

[0074] The bolt fastening structure 2 in this embodiment has the same effects as the bolt fastening structure 1 in the first embodiment.

[0075] (Embodiment 3) A bolt fastening structure 3 in the third embodiment will be described with reference to Fig. 7. Among the reference symbols used in the third embodiment, those which are the same as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0076] 7, a bolt fastening structure 3 in this embodiment differs from the bolt fastening structure 1 in the first embodiment in a first enlarged diameter hole 231b of a first intermediate portion 30 and a second enlarged diameter hole 241b of a second intermediate portion 40. The other configurations are similar.

[0077] The first expanded hole 231b is formed in a curved shape in the axial cross section so that the inclination angle θA with respect to the central axis gradually increases from the female thread 21 side toward the head 12 side. The second expanded hole 241b is formed in a curved shape in the axial cross section so that the inclination angle θB with respect to the central axis gradually increases from the head 12 side toward the female thread 21 side.

[0078] In this embodiment, the maximum hole diameter φA of first expanded diameter hole 231b (showing a portion corresponding to FIG. 2) and the maximum hole diameter φB of second expanded diameter hole 241b (showing a portion corresponding to FIG. 3) are formed to be the same. However, the maximum hole diameter φA of first expanded diameter hole 231b and the maximum hole diameter φB of second expanded diameter hole 241b may be slightly different. When first bolt hole 31 and second bolt hole 41 are positioned concentrically, the opening with the maximum hole diameter φA of first bolt hole 31 and the opening with the maximum hole diameter φB of second bolt hole 41 will be aligned.

[0079] The bolt fastening structure 3 in this embodiment has the same effects as the bolt fastening structure 1 in the first embodiment.

[0080] (Embodiment 4) A bolt fastening structure 4 in the fourth embodiment will be described with reference to Fig. 8. Note that, among the reference symbols used in the fourth embodiment, those which are the same as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0081] 8, the bolt fastening structure 4 in this embodiment differs from the bolt fastening structure 1 in the first embodiment in that the first intermediate portion 330 is formed separately from the threaded portion 320, and that the threaded portion 320 has a cylindrical hole 323 communicating with the bolt introduction hole 22. The other configurations are the same.

[0082] Since the first intermediate portion 330 and the screw-threaded portion 320 are formed separately, the first intermediate portion 330 and the second intermediate portion 40 are sandwiched in an overlapping state between the head 12 of the bolt 10 and the screw-threaded portion 320.

[0083] As in embodiment 1, the first expanded diameter hole 31b of the first intermediate portion 330 is formed radially inward from the end P1 on the head 12 side of the portion where the male thread 11a and the female thread 21 are screwed together, and is expanded at an angle of 45° toward the head 12 side from the first imaginary line L1.

[0084] Furthermore, the first expanded diameter hole 31b of the first intermediate portion 330 is formed radially inward from a fourth imaginary line L4 that expands in diameter at an angle of 45° toward the head 12 from an end P4 on the threaded portion 320 side of the first bolt hole 31 of the first intermediate portion 330. Note that in this embodiment, not only the first expanded diameter hole 31b but the entire first bolt hole 31 is formed radially inward from the fourth imaginary line L4.

[0085] Furthermore, the screw-engagement portion 320 only needs to have at least the female thread 21 , and may not have the bolt insertion hole 22 and the cylindrical hole 323 .

[0086] The bolt fastening structure 4 in this embodiment has the same effects as the bolt fastening structure 1 in the first embodiment.

[0087] (Embodiment 5) A bolt fastening structure 5 in the fifth embodiment will be described with reference to Fig. 9. Among the reference symbols used in the fifth embodiment, those which are the same as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0088] As shown in Fig. 9, the bolt fastening structure 5 in this embodiment includes a bolt 10, a threaded portion 420 which is a nut, a first intermediate portion 330, and a second intermediate portion 40. The threaded portion 420 which is a nut is exemplified by a flanged nut. The threaded portion 420 includes a threaded portion main body 420a and a flange portion 420b. The outer periphery of the threaded portion main body 420a is formed in a hexagonal shape. The threaded portion main body 420a includes a female thread 21 on its inner circumferential surface. The threaded portion 420 which is a nut may also be a flangeless nut which does not include the flange portion 420b.

[0089] As in embodiment 4, the first expanded diameter hole 31b of the first intermediate portion 330 is formed radially inward from the end P1 on the head 12 side of the portion where the male thread 11a and the female thread 21 are screwed together, and is expanded at an angle of 45° toward the head 12 side from the first imaginary line L1.

[0090] Furthermore, the first expanded diameter hole 31b of the first intermediate portion 330 is formed radially inward of a fifth imaginary line L5 that expands at an angle of 45° from a point P5 that is the outer edge of a portion of the threaded portion 420 that is a nut and that is in contact with the first intermediate portion 330 toward the head 12 of the bolt 10. In this embodiment, not only the first expanded diameter hole 31b but the entire first bolt hole 31 is formed radially inward of the fifth imaginary line L5.

[0091] Furthermore, the first expanded diameter hole 31b of the first intermediate portion 330 is formed radially inward from a sixth imaginary line L6 that expands at an angle of 45° toward the head 12 of the bolt 10 from a point P6, which is the intersection point between an extension line of an inscribed circle portion (a portion corresponding to the two-face width) of the screw-threaded portion main body 420a of the screw-threaded portion 420, which is a nut, and a portion of the screw-threaded portion 420 that is in contact with the first intermediate portion 330.

[0092] Also, the maximum hole diameter φA (showing the portion corresponding to FIG. 2) of the first expanded diameter hole 31b is formed larger than the portion of the screw-in portion 420, which is a nut, that is in contact with the first intermediate portion 330. In detail, the maximum hole diameter φA of the first expanded diameter hole 31b is formed larger than the inscribed circle diameter of the screw-in portion main body 420a of the screw-in portion 420, which is a nut. Furthermore, the maximum hole diameter φA of the first expanded diameter hole 31b is formed larger than the diameter of the flange portion 420b of the screw-in portion 420, which is a nut.

[0093] Also, the maximum hole diameter φB (showing the portion corresponding to FIG. 3) of the second expanded diameter hole 41b is formed larger than the portion of the screwing portion 420, which is a nut, that is in contact with the second intermediate portion 40. In detail, the maximum hole diameter φB of the second expanded diameter hole 41b is formed larger than the inscribed circle diameter of the screwing portion main body 420a of the screwing portion 420, which is a nut. Furthermore, the maximum hole diameter φB of the second expanded diameter hole 41b is formed larger than the diameter of the flange portion 420b of the screwing portion 420, which is a nut.

[0094] The bolt fastening structure 5 in this embodiment has the same effects as the bolt fastening structure 4 in the fourth embodiment.

[0095] (Modification of the fifth embodiment) In the bolt fastening structure 5 in the fifth embodiment, the screw portion 420, which is a nut, is in direct contact with the first intermediate portion 330. Alternatively, a washer may be interposed between the screw portion 420, which is a nut, and the first intermediate portion 330. In this case, the point P5, which is the starting point of the fifth imaginary line L5, is the outer edge of a portion of the screw portion 420, which is a nut, that is in contact with the washer side. Similarly, the point P6, which is the starting point of the sixth imaginary line L6, is the intersection of an extension of an inscribed circle portion (a portion corresponding to the two-face width) of the screw portion main body 420a of the screw portion 420, which is a nut, and a portion of the screw portion 420, which is a nut, that is in contact with the washer side.

[0096] (Embodiment 6) A bolt fastening structure 5 in the sixth embodiment will be described with reference to Fig. 10 to Fig. 12. Note that, among the reference symbols used in the sixth embodiment, those which are the same as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0097] 10 to 12, a bolt fastening structure 6 in this embodiment differs from the bolt fastening structure 1 in the first embodiment in a second intermediate portion 540. The other configurations are the same.

[0098] 10 to 12, the second bolt hole 541 of the second intermediate portion 540 includes, in order from the head 12 side of the bolt 10, an elliptical reference hole 541a and a second enlarged diameter hole 41b. The elliptical reference hole 541a is formed in an elliptical shape so that the relative position between the bolt 10 and the second intermediate portion 540 can be changed in a state in which the shaft portion 11 of the bolt 10 is inserted. The short width of the elliptical reference hole 541a is the same as the inner diameter of the cylindrical reference hole 41a in the first embodiment.

[0099] The bolt fastening structure 6 in this embodiment has the same effects as the bolt fastening structure 1 in the first embodiment. [Explanation of symbols]

[0100] 1,2,3,4,5,6: bolt fastening structure, 10: bolt, 11: shaft portion, 11a: male thread, 12: head portion, 12a: head body, 12b: flange portion, 20,320,420: screw-in portion, 21: female thread, 22: bolt guide hole, 30,330: first intermediate portion, 31: first bolt hole, 31a: cylindrical reference hole, 31b,131b,231b: first enlarged hole, 40,540: second intermediate portion, 41,541: second bolt hole, 41a: cylindrical reference hole, 41b,141b,241b: second enlarged hole, 50: female thread integral member, 323: cylindrical hole, 420a: screw-in portion body, 420b: flange portion, 541a: oval reference hole, L1: first virtual line, L2: second virtual line, L3: third virtual line, L4: fourth virtual line, L5: fifth virtual line, L6: sixth virtual line, P1: starting point of the first virtual line, P2: starting point of the second virtual line, P3: starting point of the third virtual line, P4: starting point of the fourth virtual line, P5: starting point of the fifth virtual line, P6: starting point of the sixth virtual line, θA: inclination angle of the first expansion hole, θB: inclination angle of the second expansion hole, φA: maximum hole diameter of the first expansion hole, φB: maximum hole diameter of the second expansion hole

Claims

1. A bolt (10) having a shaft portion (11) on which a male thread (11a) is formed and a head portion (12) having a diameter larger than the diameter of the shaft portion; A threaded portion (20, 320, 420) having a female thread formed thereon to be threadedly engaged with the male thread of the bolt; A first intermediate portion (30) formed integrally or separately with the screw-engagement portion and having a first bolt hole (31) connected to the female thread; a second intermediate portion (40, 540) that is formed separately from the bolt and the first intermediate portion, that is sandwiched between the head portion and the first intermediate portion, and that has a second bolt hole (41, 541) that is connected to the first bolt hole; Equipped with The first bolt hole includes a first expanded diameter hole (31b, 131b, 231b) formed so that the hole diameter gradually expands from the female thread side toward the head side, A bolt fastening structure (1, 2, 3, 4, 5, 6), wherein the second bolt hole is provided with a second enlarged diameter hole (41b, 141b, 241b) formed so that the hole diameter gradually increases from the head side toward the female thread side.

2. 2. A bolt fastening structure (1, 2, 3, 4, 5, 6) as described in claim 1, wherein a maximum hole diameter (φA) of the first expanded hole and a maximum hole diameter (φB) of the second expanded hole are formed larger than a diameter of an outer edge of a portion of the head of the bolt that is in contact with the second intermediate portion.

3. The first expansion hole is formed radially inward from a first imaginary line (L1) that expands at an angle of 45° toward the head side from the head side end (P1) of the portion where the male thread and the female thread are screwed together, 3. The bolt fastening structure (1, 2, 3, 4, 5, 6) according to claim 1 or 2, wherein the second expansion hole is formed radially inward from a second imaginary line (L2) that expands at an angle of 45° from an outer edge (P2) of a portion of the head that is in contact with the second intermediate portion toward the female thread side.

4. The bolt fastening structure (1, 4, 5, 6) according to claim 1 or 2, wherein the first enlarged diameter hole and the second enlarged diameter hole are formed in a tapered shape.

5. The first expansion hole is formed in a curved shape in the axial cross section so that the inclination angle (θA) with respect to the central axis line gradually decreases from the female thread side toward the head side, 3. The bolt fastening structure (2) according to claim 1 or 2, wherein the second expansion hole is formed in a curved shape in an axial cross section so that an inclination angle (θB) relative to a central axis gradually decreases from the head side toward the female thread side.

6. The first expansion hole is formed in a curved shape in the axial cross section so that the inclination angle (θA) with respect to the central axis gradually increases from the female thread side toward the head side, 3. The bolt fastening structure (3) according to claim 1 or 2, wherein the second expansion hole is formed in a curved shape in an axial cross section so that an inclination angle (θB) relative to a central axis gradually increases from the head side toward the female thread side.

7. The head side surface of the first intermediate portion and the female thread side surface of the second intermediate portion are in contact with each other, an opening having a maximum hole diameter (φA) of the first expanded diameter hole and an opening having a maximum hole diameter (φB) of the second expanded diameter hole are located on a contact surface between the first intermediate portion and the second intermediate portion, the first expanded diameter hole is formed so that a stress generated in an inner edge portion of a contact surface of the first intermediate portion with the second intermediate portion is less than a yield stress of the first intermediate portion, 3. The bolt fastening structure (1, 2, 3, 4, 5, 6) according to claim 1 or 2, wherein the second expansion hole is formed so that a stress generated in an inner edge portion of a contact surface of the second intermediate portion with the first intermediate portion is less than a yield stress of the second intermediate portion.

8. the first expansion hole has an axial length that is equal to or greater than half the axial length of the first bolt hole, The bolt fastening structure (1, 2, 3, 4, 5, 6) according to claim 1 or 2, wherein the second enlarged diameter hole has an axial length that is equal to or greater than half that of the second bolt hole.

Images