Fastening tool and nut

Abstract

To provide a fastening tool that enhances corrosion resistance of a joining part of a steel structure and enhances tensile strength.SOLUTION: A bolt 2 of a fastening tool 1 includes a shaft part 21 and a bolt countersunk head part 22. The shaft part 21 has a columnar shape and includes a male screw part 211. The bolt countersunk head part 22 is connected to the shaft part 21, is disposed coaxially with the shaft part 21 and includes a truncated cone part 221 of which diameter increases as separating from the shaft part 21 in the axial direction of the bolt 2. The nut 3 includes a cylindrical part 31, a nut countersunk head part 32, a through hole 33 and a female screw part 34. The cylindrical part 31 has a cylindrical shape. The nut countersunk head part 32 is connected to the cylindrical part 31, is disposed coaxially with the cylindrical part 31 and includes a truncated cone part 321 of which diameter increases as separating from the cylindrical part 31 in the axial direction of the nut 3. The through hole 33 is disposed coaxially with the cylindrical part 31 and the nut countersunk head part 32 and penetrates through the cylindrical part 31 and the nut countersunk head part 32. The female screw part 34 is formed on an inner surface of the through hole 33.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to fasteners and nuts, and more particularly to fasteners comprising a bolt and a nut, and to nuts. [Background technology]

[0002] Steel structures, such as steel bridges, buildings, steel towers, and chemical plants, are constructed on-site by joining multiple steel members together using fasteners. Fasteners include bolts and nuts. FIG. 1 is a schematic diagram of a joint in a steel structure. Referring to FIG. 1, the joint in a steel structure includes, for example, a pair of mother plates 100 and a pair of connecting plates 110. Through holes 140 are formed in the mother plates 100 and the connecting plates 110. Bolts 130 of fasteners 120 are inserted into the through holes 140. The mother plate 100 is sandwiched between the pair of connecting plates 110 and joined together by multiple fasteners 120. Specifically, the mother plate 100 is joined to the connecting plates 110 by friction welding when the bolts 130 and nuts 150 of the fasteners 120 are tightened. Joints such as those shown in FIG. 1 are formed at multiple locations on the steel structure, and the steel structure is constructed. The fastener 120 used in the joint where the mother plate 100 and the connecting plate 110 are joined by friction welding is required to have high tensile resistance (axial force) that can realize friction welding.

[0003] Furthermore, the above-mentioned steel structures are installed outdoors. Therefore, corrosion resistance is required for the joints of steel structures. Typically, paint is applied to the joints of steel structures to improve corrosion resistance, forming a coating film. However, as shown in Figure 1, the joints of steel structures have unevenness formed by the heads of bolts 130 and nuts 150. It is difficult to paint the surface of such an uneven joint, and it is difficult to form a uniform coating film. As a result, thin areas of the coating film are formed in the joints. Thin areas of the coating film have poor corrosion resistance. Furthermore, rainwater is likely to remain in the uneven surface of the joint for a long period of time. As a result, after a certain period of time has passed since the joint was formed, the uneven areas of the joint (near the nut 150 in Figure 2) may corrode, as shown in Figure 2.

[0004] To prevent corrosion over time at the joints of such steel structures, Japanese Patent Laid-Open Publication No. 2000-160700 (Patent Document 1) proposes a fastener that includes a so-called flat head bolt and nut. This document prevents the flat head of the flat head bolt from protruding from the connecting plate when the fastener joins the mother plate and connecting plate of the joint. As a result, the formation of irregularities at the joint can be prevented to some extent (see Figure 2 of Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-160700 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when the fastener disclosed in Patent Document 1 is used to join the mother plate and connecting plate of the joint, unevenness still remains on the surface of the connecting plate on the nut side, making it difficult to form a uniform coating film and sometimes resulting in insufficient corrosion resistance.

[0007] An object of the present disclosure is to provide a fastener that can improve the corrosion resistance of a joint formed by joining steel members that constitute a steel structure, and can improve the tensile strength after fastening. [Means for solving the problem]

[0008] The fastener of the present disclosure comprises a bolt and a nut. The bolt includes a shank and a countersunk head. The shank has a cylindrical shape and includes a male thread. The countersunk head is connected to the shank and is arranged coaxially with the shank, and includes a truncated cone portion whose diameter increases with increasing distance from the shank in the axial direction of the bolt. The nut includes a cylindrical portion, a nut countersunk head, a through hole, and a female thread portion. The cylindrical portion has a cylindrical shape. The nut countersunk head is connected to the cylindrical portion, is arranged coaxially with the cylindrical portion, and has a truncated cone portion whose diameter increases with increasing distance from the cylindrical portion in the axial direction of the nut. The through hole is arranged coaxially with the cylindrical portion and the nut countersunk head, and passes through the cylindrical portion and the nut countersunk head. The female thread portion is formed on the inner surface of the through hole.

[0009] The nut of the present disclosure includes a cylindrical portion, a nut countersunk head, a through hole, and a female thread portion. The cylindrical portion has a cylindrical shape. The nut countersunk head is connected to the cylindrical portion and arranged coaxially with the cylindrical portion, and has a truncated cone portion whose diameter increases with increasing distance from the cylindrical portion in the axial direction of the nut. The through hole is arranged coaxially with the cylindrical portion and the nut countersunk head, and passes through the cylindrical portion and the nut countersunk head. The female thread portion is formed on the inner surface of the through hole. [Effects of the Invention]

[0010] The fastener of the present disclosure can improve the corrosion resistance of a joint formed by joining steel members that constitute a steel structure, and can improve the tensile strength after fastening. The nut of the present disclosure is used in the above-mentioned fastener. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of a joint in a conventional steel structure. [Figure 2] FIG. 2 is a photographic image showing the corrosion of the joint shown in FIG. [Figure 3] FIG. 3 is a side view of the fastener of this embodiment. [Figure 4] 4 is a side view and a plan view of the bolt in FIG. [Figure 5] 5 is a cross-sectional view parallel to the axial direction of the nut in FIG. 3 and a plan view. [Figure 6] FIG. 6 is a schematic diagram showing a state in which joints are joined using the fastener of this embodiment. [Figure 7] FIG. 7 is a schematic diagram showing a state where joints are joined using the fastener of this embodiment, which is different from FIG. [Figure 8] FIG. 8 is a schematic diagram showing a state where joints are joined by the fastener of this embodiment, which is different from FIG. 6 and FIG. [Figure 9] FIG. 9 is a schematic diagram showing a state where joints are joined by the fastener of this embodiment, which is different from FIGS. [Figure 10] FIG. 10 is a schematic diagram showing an axisymmetric analysis model of a nut among the axisymmetric analysis models of a fastener used to investigate the relationship between the nut shape and the load bearing ratio LSR of the female thread of the nut. [Figure 11] FIG. 11 is a schematic diagram showing a state in which the axisymmetric analytical model of the nut shown in FIG. 10 is deformed after fastening. [Figure 12] FIG. 12 is a diagram showing the relationship between the female threads of each number and the load bearing ratio LSR of the female threads, obtained from the axisymmetric analysis model of the nut of FIG. [Figure 13] FIG. 13 is a side view of the fastener of the preferred embodiment. [Figure 14] FIG. 14 is a diagram showing the relationship between the rotational torque T (N·m) and the axial force N (kN) when fastening the fastener. [Figure 15] FIG. 15 is a cross-sectional view and a plan view of a nut of the fastener of the second embodiment. [Figure 16] FIG. 16 is a plan view of the upper surface of a nut having a blind hole with a different shape from that of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] The fastener of the first configuration of this embodiment includes a bolt and a nut. The bolt includes a shank and a countersunk head. The shank has a cylindrical shape and includes a male thread. The countersunk head is connected to the shank and is arranged coaxially with the shank, and includes a truncated cone portion whose diameter increases with increasing distance from the shank in the axial direction of the bolt. The nut includes a cylindrical portion, a nut countersunk head, a through hole, and a female thread portion. The cylindrical portion has a cylindrical shape. The nut countersunk head is connected to the cylindrical portion, is arranged coaxially with the cylindrical portion, and has a truncated cone portion whose diameter increases with increasing distance from the cylindrical portion in the axial direction of the nut. The through hole is arranged coaxially with the cylindrical portion and the nut countersunk head, and passes through the cylindrical portion and the nut countersunk head. The female thread portion is formed on the inner surface of the through hole.

[0013] In the fastener of the first configuration, the bolt includes a tapered countersunk head bolt, and the nut includes a tapered countersunk head nut. Therefore, after fastening, the countersunk head bolt and the countersunk head nut are housed in the countersunk groove formed in the steel members. As a result, at the joints of steel structures, the fastener can be prevented from protruding from the steel members, and the surface of the joint can be made smooth. This makes it easier to apply paint evenly to the surface of the joint, allowing for the formation of a uniform coating film. As a result, peeling of the coating film can be prevented, and corrosion resistance can be improved.

[0014] Furthermore, in the fastener of the first configuration, the nut includes a cylindrical portion, which ensures a sufficient threading length. This increases the shear resistance of the threaded portions (male and female threaded portions) and increases the tensile strength (axial force). As a result, the fastener of the first configuration can withstand friction welding and can be used in friction-welded joints. Furthermore, by forming a through hole in the cylindrical portion of the nut and the countersunk head of the nut, it is possible to prevent stress from concentrating locally in the thin portion of the female thread on the inner surface of the through hole, thereby improving delayed fracture resistance.

[0015] The fastener of the second configuration is the fastener according to the first configuration, wherein the cylindrical portion includes a tapered portion at the tip of the inner surface of the cylindrical portion, the inner diameter of which increases toward the opening of the cylindrical portion.

[0016] In the fastener of the second configuration, the tapered portion acts as a guide for the bolt shank. This makes it easier for the tip of the bolt shank to be inserted into the cylindrical portion of the nut during fastening. The tapered portion also reduces stress concentration on the first thread of the male thread of the bolt during fastening.

[0017] A fastener of the third configuration is a fastener according to the first or second configuration, in which the taper angle of the countersunk head of the bolt is larger than the taper angle of the countersunk head of the nut.

[0018] In the fastener of the third configuration, when fastening, the nut head and the countersunk head are in surface contact, and the bolt head and the countersunk head are in line contact. In this case, local stress concentration occurs on the bolt head due to line contact. Therefore, when fastening, the bolt is fixed and does not easily rotate around its axis. As a result, co-rotation is suppressed, making the fastening operation easier.

[0019] A fastener of the fourth configuration is a fastener of any one of the first to third configurations, wherein the nut further has a plurality of blind holes formed around the opening of the through hole on the top surface of the nut countersunk head.

[0020] In the fastener of the fourth configuration, the nut can be easily fastened to the bolt by rotating the nut around its axis using a fastening jig that fits into the blind hole.

[0021] A fastener according to a fifth configuration is the fastener of the fourth configuration, and when the upper surface of the nut is viewed from above, the blind hole has a rectangular shape.

[0022] In the fastener of the fifth configuration, the blind hole is rectangular, which makes it easier to transmit the external force of the fastening jig around the axis of the nut, making it even easier to fasten the nut to the bolt.

[0023] The nut of this embodiment constitutes the above-mentioned fastener.

[0024] Hereinafter, the fastener of this embodiment will be described with reference to the drawings. In each drawing, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0025] First Embodiment [Overall configuration of fastener 1] 3 is a side view of the fastener of this embodiment. Referring to FIG. 3, fastener 1 includes bolt 2 and nut 3.

[0026] [Bolt 2 Configuration] Fig. 4 is a side view and a plan view of the bolt 2 in Fig. 3. Referring to Fig. 4, the bolt 2 includes a shank 21 and a countersunk head 22. The shaft portion 21 has a cylindrical shape. The shaft portion 21 includes an externally threaded portion 211. The externally threaded portion 211 has a plurality of external threads. The plurality of external threads are arranged in the axial direction of the bolt 2.

[0027] The shank 21 may further include a chamfered portion 212 and a tip portion 213. The chamfered portion 212 is disposed between the male threaded portion 211 and the countersunk bolt head portion 22. An incomplete male thread is formed in the chamfered portion 212, or no male thread is formed. The tip portion 213 is disposed at the tip of the shank 21. An incomplete male thread is formed in the tip portion 213, or no male thread is formed. The tip of the tip portion 213 is chamfered.

[0028] The countersunk bolt head 22 is connected to the shank 21 and is arranged coaxially with the shank 21. The countersunk bolt head 22 includes a truncated cone portion 221. The truncated cone portion 221 has a truncated cone shape whose diameter increases with increasing distance from the shank 21 in the axial direction of the bolt 2. The surface of the truncated cone portion 221 comes into contact with the countersunk portion of the connecting plate after the fastener 1 is tightened. This stabilizes the tensile strength (axial force) and crimping force of the fastener 1.

[0029] The countersunk head bolt 22 may further include a seat portion 222 and a surface layer portion 223. The seat portion 222 is connected to the shank portion 21. The seat portion 222 is disk-shaped and has a constant outer diameter. The diameter of the seat portion 222 is preferably the same as the outer diameter of the cylindrical portion 31 of the nut 3, which will be described later. The seat portion 222 and the cylindrical portion 31 resist shear when the fastener 1 tightened at the joint is subjected to shear in a direction perpendicular to the axial direction. To increase resistance to shear, the outer diameters of the seat portion 222 and the cylindrical portion 31 are preferably smaller than the inner diameter of the through hole formed in the connecting plate and are the maximum diameters that allow the seat portion 222 and the cylindrical portion 31 to be inserted into the through hole.

[0030] Of the two bottom surfaces of the truncated cone portion 221, the surface layer 223 is disposed on the bottom surface with the larger diameter. The surface layer 223 has a taper angle that is smaller than the taper angle A1 of the truncated cone portion 221. The diameter of the surface layer 223 may be constant. In other words, the surface layer 223 may be disk-shaped. The surface layer 223 prevents the shape of the surface of the truncated cone portion 221 from changing due to dents or the like when the bolt 2 is transported.

[0031] [Nut 3 configuration] Fig. 5 is a cross-sectional view parallel to the axial direction of the nut 3 in Fig. 3 and a plan view. Referring to Fig. 3, the nut 3 includes a cylindrical portion 31 and a nut countersunk head portion 32. The cylindrical portion 31 has a cylindrical shape. The nut countersunk head portion 32 is connected to the cylindrical portion 31 and is arranged coaxially with the cylindrical portion 31. The nut countersunk head portion 32 has a truncated cone portion 321. The truncated cone portion 321 has a truncated cone shape whose diameter increases as it moves away from the cylindrical portion 31 in the axial direction of the nut 3. Of the two bottom surfaces of the truncated cone portion 321, the diameter of the bottom surface closest to the cylindrical portion 31 is the same as the outer diameter of the cylindrical portion 31.

[0032] The nut countersunk head 32 may further include a surface layer 322. The surface layer 322 is disposed on the bottom surface with the larger diameter of the two bottom surfaces of the truncated cone portion 321. The surface layer 322 has a taper angle smaller than the taper angle A2 of the truncated cone portion 321. The diameter of the surface layer 322 may be constant. In other words, the surface layer 322 may be disk-shaped. The surface layer 322 prevents the shape of the surface of the truncated cone portion 321 from being changed due to dents or the like when the nut 3 is transported.

[0033] The nut 3 further has a through hole 33. The through hole 33 is arranged coaxially with the cylindrical portion 31 and the countersunk nut head 32, and passes through the cylindrical portion 31 and the countersunk nut head 32. The nut 3 includes a female thread portion 34 on the inner surface of the through hole 33. The female thread portion 34 is formed continuously with the inner surface of the cylindrical portion 31 and the inner surface of the countersunk nut head 32, and has multiple female threads. The female thread portion 34 is formed in the axial direction of the through hole 33. This ensures a sufficient screw-in length. As a result, the shear resistance of the male thread portion 211 and the female thread portion 34 can be increased, and the tensile resistance (axial force) of the fastener 1 can be increased.

[0034] Preferably, the nut 3 further includes a tapered portion 35 at the tip of the inner surface of the through-hole 33 of the cylindrical portion 31. The tapered portion 35 has a tapered shape, and the inner diameter increases toward the opening 36 of the cylindrical portion 31. If the cylindrical portion 31 has the tapered portion 35, the tapered portion 35 serves as a guide for the shank 21 of the bolt 2. Therefore, when fastening, the tip of the shank 21 of the bolt 2 can be easily inserted into the cylindrical portion 31 of the nut 3. The tapered portion 35 further relieves stress concentration on the first thread of the male thread portion 211 of the bolt 2 when fastening.

[0035] [Function of fastener 1] [Function of the bolt head 22 and the nut head 32] In the fastener 1 having the above configuration, the bolt 2 includes a tapered countersunk head bolt 22, and the nut 3 also includes a tapered countersunk head nut 32. As shown in FIG. 6 , after fastening, the countersunk head bolt 22 and the countersunk head nut 32 are housed in a countersunk groove 111 formed on a steel member, such as a connecting plate 110. As a result, the fastener 1 is prevented from protruding from the joint in the steel structure. Specifically, the countersunk head bolt 22 and the countersunk head nut 32 are prevented from protruding from the surface of the connecting plate 110. In this way, by joining steel members (the mother plate 100 and the connecting plate 110) using the fastener 1, the surface of the joint can be made smooth. This makes it easy to apply paint evenly to the surface of the joint, facilitating the formation of a uniform coating. As a result, peeling of the coating can be prevented, and corrosion resistance can be improved.

[0036] [Effect of Providing Cylindrical Portion 31] In the fastener 1 of this embodiment, the nut 3 further has a cylindrical portion 31. A female thread portion 34 is formed on the inner surface of the cylindrical portion 31 and the inner surface of the nut countersunk head 32. Therefore, the male thread portion 211 of the bolt 2 and the female thread portion 34 are fastened together over a long range in the axial direction. In other words, in this case, the screw-in length is long. This increases the tensile resistance (axial force) of the fastener 1. As a result, the fastener 1 can adequately withstand frictional joining. In particular, when a tensile load or repeated load acts on the fastened fastener 1, the fatigue strength of the fastener 1 is increased. Furthermore, the long screw-in length reduces the occurrence of loosening.

[0037] Furthermore, fastener 1 can increase shear force when the joint changes from a friction joint to a bearing state. As described above, joints in steel structures are friction-joined by fastener 1. However, when the load transmission due to friction exceeds its limit, the joint changes from a friction joint to a bearing state. In the bearing state, shear force is applied to fastener 1. Here, the shear resistance is affected by the diameter of fastener 1. In fastener 1, the shank 21 of bolt 2 is inserted into and fitted into the cylindrical portion 31 of nut 3. Therefore, the diameter of cylindrical portion 31 corresponds to the diameter of fastener 1. The diameter of cylindrical portion 31 is larger than the diameter of shank 21. Therefore, the cylindrical portion 31 can increase the shear resistance of fastener 1.

[0038] Furthermore, fastener 1 can accommodate variations in the thickness of the plates being fastened by providing cylindrical portion 31. For example, as shown in Figures 7 to 9, even if thickness T2 of mother plate 100 is greater than thickness T1 of mother plate 100 in Figure 6, fastener 1 can still perform fastening because cylindrical portion 31 ensures the required screw-in length.

[0039] Specifically, compared to the case of Figure 6, it is sufficient to elongate a portion of the bolt 2 or a portion of the nut 3. For example, it is sufficient to elongate one or more of the seat portion 222, shank 21 of the bolt 2, and cylindrical portion 31 of the nut 3. In Figure 7, the seat portion 222 of the bolt 2 is elongated. In Figure 8, the shank 21 of the bolt 2 is elongated. In Figure 9, the cylindrical portion 31 of the nut 3 is elongated. In the fastener 1, a sufficient screw-in length can be ensured by the cylindrical portion 31. Therefore, even if one or more of the seat portion 222, shank 21, and cylindrical portion 31 are elongated, a sufficient screw-in length can be ensured. Therefore, the fastener 1 can ensure sufficient tensile resistance (axial force).

[0040] [Effect of the nut 3 having the through hole 33] Furthermore, in the fastener 1 of this embodiment, the nut 3 has a through hole 33, and a female thread portion 34 is formed not only on the inner surface of the cylindrical portion 31 but also on the inner surface of the nut countersunk head 32. This makes it possible to prevent local stress concentration from occurring in a part of the female thread portion 34 formed on the cylindrical portion 31. This makes it possible to improve the delayed fracture resistance and tensile strength of the fastener 1. As a result, the fastener 1 can be used as a fastener for friction-bonded joints. This point will be explained below.

[0041] Assuming the analysis cases of the two nuts 3 shown in FIGS. 10(A) and 10(B), an axisymmetric analysis was carried out to simulate the tightening of the fastener 1.

[0042] The analysis solver used was Abaqus / Standard, a product name of Dassault Systèmes. The elements and boundary conditions used were as follows: In the analysis model, the base plate 100 was an 8-node quadrilateral reduced integration second-order element with a minimum element side length of 1 mm. The bolt 2, nut 3, and connecting plate 110 were 6-node triangular second-order elements with a minimum element side length of 0.05 mm.

[0043] Coulomb's law of friction was applied to the engagement of the screw, the contact between the countersunk head 22 of the bolt and the countersunk head 111 of the connecting plate 110, and the contact between the countersunk head 32 of the nut and the countersunk head 111 of the connecting plate 110, and the friction coefficient of the contact surfaces was set to 0.1. The stress-strain relationship of the fastener 1, the mother plate 100, and the connecting plate 110 was calculated by setting the elastic modulus E to 2.0×10 5 N / mm 2 The Poisson's ratio ν was set to 0.3, and the work hardening coefficient H was set to E / 100, which is a bilinear type. The yield point σy of fastener 1 was set to 1260 N / mm 2 , tensile strength σt is 1400N / mm 2 The yield point σy of the mother plate 100 and the connecting plate 110 was set to 355 N / mm 2 , tensile strength σt is 490N / mm 2 It was decided.

[0044] The nuts in Fig. 10(A) and (B) were both axisymmetric analysis models. In Fig. 10(A), the nut did not have a through hole, but had a blind hole and a lid portion. On the other hand, in Fig. 10(B), the fastener 1 of this embodiment was assumed, and the nut had a through hole. Here, the female threads were numbered in order from the opening of the cylindrical portion of the nut in Fig. 10(A) and Fig. 10(B). Specifically, the nut in Fig. 10(A) has a lid portion, so there are 16 female threads, P1 to P16. On the other hand, in Fig. 10(B), the nut has a through hole, and female threads are formed along the entire inner surface, so there are 19 female threads, P1 to P19.

[0045] An axisymmetric analysis was performed under the above conditions to determine the load sharing ratio (LSR) (%) of the female thread for each number. Fig. 11 is a schematic diagram showing an axisymmetric analysis model of the nut shown in Fig. 10 after the axisymmetric analysis. Fig. 12 is a diagram showing the relationship between the female thread for each number and the load sharing ratio LSR of the female thread, obtained from the axisymmetric analysis model of the nut in Fig. 10.

[0046] Graph G10A in Figure 12 shows the results of the analytical model for the nut in Figure 10(A). Graph G10B shows the results of the analytical model for the nut in Figure 10(B). Referring to Figure 12, for the nut in Figure 10(A), the load sharing ratio (LSR) of the female threads in the cylindrical portion of the nut is high for the female thread P15 near the nut countersunk head. The thickness of the cylindrical portion is thinner than the thickness of the nut countersunk head. Therefore, if high stress is applied locally to this area, delayed fracture may occur.

[0047] On the other hand, in the nut of FIG. 10(B), which corresponds to the fastener 1 of this embodiment, the load sharing ratio (LSR) is kept low for the P15 female thread near the nut countersunk head of the female thread of the cylindrical portion. This prevents high stress from being locally applied to this thin portion. The P17 female thread inside the nut countersunk head has a high load sharing ratio (LSR). However, the portion where the P17 female thread is formed is inside the nut countersunk head, so it has a sufficient thickness. Therefore, even if the load sharing ratio of the P17 female thread is high, delayed fracture is unlikely to occur.

[0048] As shown in Figure 12, in the fastener 1, the nut 3 includes a cylindrical portion 31, and furthermore, a through hole 33 is formed, thereby improving delayed fracture resistance. In addition, the corrosion resistance of the joints of the steel structure can be further improved.

[0049] As described above, in the fastener 1 of this embodiment, both the bolt 2 and the nut 3 have countersunk heads (the countersunk head bolt 22 and the countersunk head nut 32). This prevents a portion of the fastener from protruding from the surface of a joint in a steel structure. As a result, a uniform coating film is more easily formed on the surface of the joint, improving corrosion resistance. In the fastener 1, the nut 3 further includes a cylindrical portion 31. This allows for a longer screw-in length. This increases the shear resistance of the threaded portions (the male threaded portion and the female threaded portion) and increases the tensile strength (axial force). As a result, the fastener of the first configuration can adequately withstand frictional joining. It also increases the strength of the fastener 1 and prevents loosening. Furthermore, the nut 3 has a through hole that penetrates the cylindrical portion 31 and the countersunk head nut 32, and the female threaded portion 34 is formed on the inner surface of the through hole. This prevents high stress from being locally applied to the thinner portions of the female threaded portion 34, improving delayed fracture resistance and tensile strength.

[0050] [Preferred form of fastener 1] Preferably, as shown in FIG. 13 , the taper angle A1 of the countersunk bolt head 22 of the bolt 2 of the fastener 1 is larger than the taper angle A2 of the countersunk nut head 32 of the nut 3. In this case, during the fastening operation, the countersunk nut head 32 makes surface contact with the countersunk groove 111 formed on a steel member such as the connecting plate 110, while the countersunk bolt head 22 makes line contact with the countersunk groove 111 at the large-diameter portion of the truncated cone portion 221. This causes local stress concentration in the truncated cone portion 221. As a result, during the fastening operation, the rotational resistance of the bolt 2 is greater than the rotational resistance of the nut 3. As a result, the bolt 2 is fixed and less likely to rotate when fastened by the rotation of the nut 3. This suppresses co-rotation and makes the fastening operation easier.

[0051] There are no particular limitations on the angle as long as the taper angle A1 is larger than the taper angle A2. Preferably, the taper angle A2 is 90°, and the taper angle A1 is 1 to 3° larger than the taper angle A2.

[0052] Figure 14 shows the relationship between rotational torque T (N m) and axial force N (kN) when fastening a fastener. Figure 14(A) corresponds to the case where fastener 1 shown in Figure 13 is used for fastening, and taper angle A1 is 92° and taper angle A2 is 90°. Figure 14(B) shows the relationship between rotational torque T (N m) and axial force N (kN) when taper angle A1 and taper angle A2 are the same. In Figure 14(B), both taper angles A1 and A2 are 92°.

[0053] 14(A), in which taper angle A1 is larger than taper angle A2, the tensile resistance (axial force) per unit rotational torque is increased compared to FIG. 14(B), in which taper angle A1 is the same as taper angle A2. Therefore, by making taper angle A1 larger than taper angle A2 in fastener 1, the occurrence of co-rotation is suppressed, and further, the tensile resistance (axial force) per unit rotational torque can also be increased.

[0054] <Second embodiment> FIG. 15 is a cross-sectional view and a plan view of the nut 3 of the fastener 1 of the second embodiment. Referring to Figure 15, the nut 3 has a plurality of blind holes 38 on the nut top surface 37 of the nut countersunk head 32. The blind holes 38 are formed around the opening 39 of the through hole 33. Preferably, the blind holes 38 are arranged at equal intervals around the opening 39. The blind holes 38 are blind holes with a bottom. The nut 3 can be easily fastened to the bolt 2 by rotating the nut 3 around its axis using a fastening jig that fits into the blind holes 38.

[0055] When the nut upper surface 37 is viewed from above, the blind hole 38 is preferably rectangular. Because the blind hole 38 is rectangular, the external force of the fastening jig can be easily transmitted around the axis of the nut 3. This makes it even easier to fasten the nut 3 to the bolt 2.

[0056] As shown in FIG. 16, when the nut upper surface 37 is viewed from above, the blind hole 38 may have a circular shape.

[0057] [Other configurations of fastener 1] The male thread portion 211 of the bolt 2 of the fastener 1 may include a play thread portion having one or more male threads that are not inserted into the cylindrical portion 31 when the bolt 2 is tightened with the nut 3 and the tip of the shank portion 21 reaches the nut top surface 37 of the nut 3. The play thread portion homogenizes the stress applied to each male thread of the male thread portion. Preferably, the play thread portion includes two or more male threads. In this case, the stress applied to each male thread can be further homogenized.

[0058] [Regarding the shape of the raised portion 212] Preferably, the raised portion 212 of the shank 21 of the bolt 2 may be concavely curved. In other words, a fillet R may be formed in the raised portion 212. In this case, it is possible to alleviate local stresses that are applied to the male and female threads after fastening, and to homogenize the stresses.

[0059] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure. [Explanation of symbols]

[0060] 1 Fastener 2 bolts 3 nuts 21 Shaft 22 Bolt countersunk head 31 Cylindrical part 32 Countersunk head nuts 33 Through hole 34 Female thread 211 Male thread 221, 321 truncated cone

Claims

1. Bolt and a nut; The bolt is a cylindrical shaft portion including a male thread portion; a countersunk head bolt portion connected to the shank portion, arranged coaxially with the shank portion, and having a truncated cone portion whose diameter increases as it moves away from the shank portion in the axial direction of the bolt, The nut is a cylindrical portion having a cylindrical shape; a nut head portion having a truncated cone portion that is connected to the cylindrical portion, is arranged coaxially with the cylindrical portion, and has a diameter that increases as it moves away from the cylindrical portion in the axial direction of the nut; a through hole that is disposed coaxially with the cylindrical portion and the nut countersunk head and penetrates the cylindrical portion and the nut countersunk head; A female thread portion formed on the inner surface of the through hole, Fasteners.

2. 2. The fastener of claim 1, the cylindrical portion includes a tapered portion at a tip end of an inner surface of the cylindrical portion, The tapered portion has an inner diameter that increases toward the opening of the cylindrical portion. Fasteners.

3. 2. The fastener of claim 1, The taper angle of the countersunk head of the bolt is larger than the taper angle of the countersunk head of the nut. Fasteners.

4. 2. The fastener of claim 1, The nut further comprises: A plurality of blind holes are formed around the opening of the through hole on the nut top surface of the nut countersunk head. Fasteners.

5. 5. The fastener of claim 4, When the upper surface of the nut is viewed in plan, the bottomed hole is rectangular. Fasteners.

6. A nut, a cylindrical portion having a cylindrical shape; a nut head portion having a truncated cone portion that is connected to the cylindrical portion, is arranged coaxially with the cylindrical portion, and has a diameter that increases as it moves away from the cylindrical portion in the axial direction of the nut; a through hole that is disposed coaxially with the cylindrical portion and the nut countersunk head and penetrates the cylindrical portion and the nut countersunk head; A female thread portion formed on the inner surface of the through hole, nut.

7. The nut according to claim 6, further comprising: A plurality of blind holes are formed around the opening of the through hole on the nut top surface of the nut countersunk head. nut.

8. 8. The nut according to claim 7, When the upper surface of the nut is viewed in plan, the bottomed hole is rectangular. nut.

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