WO2016148433A1

WO2016148433A1 - Shear bolt

Info

Publication number: WO2016148433A1
Application number: PCT/KR2016/002335
Authority: WO
Inventors: 최진욱; 김정년; 오동윤; 이덕규; 이정진; 정근영
Original assignee: 엘에스전선 주식회사
Priority date: 2015-03-13
Filing date: 2016-03-09
Publication date: 2016-09-22
Also published as: CN107407314A; KR20160109823A; CN107407314B

Abstract

The present invention relates to a shear bolt. Specifically, the present invention relates to a shear bolt used for a bolt connector for the connection of a cable conductor, and capable of effectively solving resistance increases and heating problems of a connection part of the cable conductor by facilitating the adjustment of the insertion depth into the bolt connector.

Description

Shear bolt

The present invention relates to a shear bolt. Specifically, the present invention is a shear bolt used in the bolt connector for the conductor connection of the cable is easy to adjust the depth inserted into the bolt connector to effectively solve the problem of increased resistance and heat generation at the conductor connection portion of the cable To a shear bolt that can

Power cables usually use copper (Cu) conductors as conductors. On the other hand, although the conductivity is low at about 66% of copper (Cu), the use of lightweight (low-cost) aluminum (Al) conductors is increasing.

1 schematically shows various embodiments of the cross-sectional shape of an aluminum (Al) conductor. Specifically, FIG. 1A is a circular twisted-pair conductor in which a plurality of element wires having a circular cross section are circularly integrated as a whole, FIG. 1B is a circular compressed conductor compressing a plurality of element wires having a circular cross section as a whole, and FIG. A cross-sectional view of a divided compressed circular conductor, each of which consists of a plurality of element wires and a plurality of sections compressed in a fan shape, is generally circularly joined together.

As shown in FIG. 1, the aluminum (Al) conductor is configured in a form in which a plurality of element wires are associated with each other, and water penetrated from the outside by inserting a watertight paper tape or compound between the plurality of element wires in the longitudinal direction of the cable. You can prevent progress.

As a connection method of such an aluminum (Al) conductor, there exists a method using welding, a crimp sleeve, a bolt connector, etc. The method of connecting aluminum (Al) conductors by welding requires a long time because the plurality of wire strands constituting the aluminum (Al) conductor must be separated and the watertight paper tape or compound therebetween is removed for welding. There is a problem that is expensive and expensive.

In addition, the connection method of the aluminum (Al) conductor by the pressing sleeve is exposed to the external environment for a predetermined time for the connection between the conductors by the sleeve, whereby the oxide film is deposited on the surface of the aluminum (Al) conductor There is a problem that the resistance is increased at the connection of the cable, and as a result, the local heat generation may damage the cable or limit the rated current of the cable.

Meanwhile, FIG. 2 schematically illustrates a state in which aluminum (Al) conductors of adjacent cables are connected to each other by bolt connectors, and FIG. 3 schematically illustrates a cross-sectional view of A-A 'in FIG. 2.

As shown in FIG. 2, the bolt connector 300 is inserted into the conductor insertion groove 320 and the conductor insertion groove 320 into which the

conductors

210a and 210b of each of the

adjacent cables

200a and 200b are inserted. A plurality of bolt fastenings into which bolts 400 for pressing the

conductors

210a and 210b in one direction are inserted to fix the

conductors

210a and 210b and to contact the

conductors

210a and 210b with the inner wall of the connector 300. It may include a sphere 310.

That is, as shown in FIG. 3, the bolt 400 inserted through the bolt fastener 310 of the bolt connector 300 presses the conductor 210b inserted into the conductor insertion groove 320 in one direction. A lower portion of the conductor 210b is in contact with the lower inner wall of the bolt connector 300 to form a current path between the

conductors

210a and 210b and the bolt connector 300.

In this manner, if the depth of the bolt 400 inserted through the bolt fastener 310 of the bolt connector 300 is too shallow, the bolt 400 may not sufficiently press the conductor 210b so that the conductor 210b does not have sufficient depth. Since the contact area between the lower part of the lower part and the lower inner wall of the bolt connector 300 becomes smaller, there is a problem that resistance in the corresponding area increases and local heat generation occurs, whereas the depth into which the bolt 400 is inserted is too large. When the depth is deep inside the conductor insertion groove 320 of the bolt connector 300, the empty space for the subsequent insertion of the bolt to be inserted along the longitudinal direction or the circumferential direction of the bolt connector 300 is narrowed and thereby the subsequent As the other bolt is inserted into the shallow depth is inserted there is also a problem that the resistance in the corresponding area is increased and the local heating occurs.

Therefore, it is necessary to adjust the depth in which the bolt 400 is inserted, but according to the type of conductor 210b inserted into the conductor insertion groove 320 of the bolt connector 300, that is, the conductor 210b. 1A, the circular compressed conductor of FIG. 1B, the divided compressed circular conductor of FIG. 1C, etc., and the diameter and shape of the plurality of element wires constituting the conductor, Since the resistance of the conductor 210b to pressing is different, there is a problem that it is extremely difficult to consistently adjust the depth at which the bolt 400 is inserted while pressing the conductor 210b.

Therefore, there is an urgent need for a new method of conductor connection that can easily adjust the depth inserted into the bolt connector to effectively solve the problem of increased resistance and heat generation at the conductor connection portion of the cable.

The present invention is a shear bolt that is used in the bolt connector (bolt connector) for connecting the conductor of the cable is easy to adjust the depth inserted into the bolt connector can effectively solve the problem of increased resistance and heat generation at the conductor connection portion of the cable It is an object to provide a shear bolt.

In order to solve the above problems, the present invention,

A shear bolt for a bolt connector for conductor connection of a cable, comprising: a plurality of conductor fastening holes into which a conductor of a cable is inserted; and a plurality of bolt fasteners into which bolts for fixing a conductor inserted into the conductor insertion groove are inserted; A head capable of being fastened to a mechanism or equipment to allow shear bolts to be inserted into the bolt fasteners; A thread portion having a male thread corresponding to a female thread of the inner wall of the bolt fastener; A break portion disposed between the head and the thread portion and broken when a predetermined torque is applied to the shear bolt; And a stopper disposed between the threaded portion and the fracture portion and increasing the torque applied to the shear bolt by inhibiting the insertion of the shear bolt so that the increased torque reaches a predetermined torque at the fracture of the fracture portion. To provide a shear bolt, including.

Here, the stopper is provided with a shear bolt, characterized in that the flange or a non-threaded cylinder.

In addition, the outer diameter of the flange is greater than 3 mm or less than the outer diameter of the threaded portion, and the length of the flange or the thread unapplied cylinder is 2 to 5 mm, provides a shear bolt.

On the other hand, it is provided with a shear bolt, characterized in that it further comprises a protrusion which is disposed at the lower end of the threaded portion and can be inserted between the associated element wires constituting the conductor.

In the case where the conductor is a circular twisted pair conductor in which a plurality of element wires having a circular cross section are circular as a whole, or a circular compressed conductor in which a plurality of element wires having a circular cross section are compressed in a circular shape, the center of the circular stranded conductor or a circular compressed conductor The center of the section when the element wire is in contact with the distal end of the protruding part or when the conductor is a divided compressed circular conductor in which a plurality of sections having a circular cross section and a plurality of sections compressed in a fan shape are joined together in a circular shape as a whole When the element wire and the distal end of the projection contact, the stopper provides a shear bolt, characterized in that to increase the torque applied to the shear bolt.

And, when the break is broken, the stopper provides a shear bolt, characterized in that it does not protrude beyond the outer surface of the bolt connector.

In addition, the predetermined torque provides a shear bolt, characterized in that 50 to 150 N · m.

Further, it provides a shear bolt, characterized in that made of brass or aluminum alloy with a tensile strength of 250 MPa or more and the electrical conductivity 25% (@ 20 ℃) compared to pure copper.

In addition, the maximum outer diameter of the male screw constituting the threaded portion is Φ 12 to Φ 20, the pitch of the thread is 1 to 3 mm, the number of threads per unit length is 8 to 26, it provides a shear bolt.

And wherein the protrusion has an inclined shape which, in whole or in part, gradually decreases in cross-sectional area.

Here, the protrusion provides a shear bolt, characterized in that the cross-sectional area is a conical shape gradually decreasing and the end of the cone is a hemispherical shape with a radius of 2.5 to 8 mm.

In addition, the length of the protrusion provides a shear bolt, characterized in that 4 to 10 mm.

Furthermore, the break provides a shear bolt, characterized in that the diameter of the cone gradually decreases in diameter, the diameter is Φ8 to Φ14.

On the other hand, at least the threaded portion, the protrusion and the stopper provide a shear bolt, characterized in that plated with tin.

Shear bolt according to the present invention is a shear bolt used in the bolt connector (bolt connector) for connecting the conductor of the cable is easy to adjust the depth inserted into the bolt connector to effectively increase the resistance and heat generation problems at the conductor connection portion of the cable Excellent effect that can be solved.

1 schematically shows various embodiments of the cross-sectional shape of an aluminum (Al) conductor.

FIG. 2 schematically illustrates the connection of aluminum (Al) conductors of adjacent cables by bolt connectors.

FIG. 3 schematically illustrates a cross-sectional view of AA ′ in FIG. 2.

4 schematically shows an embodiment of a shear bolt according to the invention.

5 schematically shows another embodiment of a shear bolt according to the invention.

FIG. 6 schematically illustrates the use of the shear bolt shown in FIG. 5A in a bolt connector.

Hereinafter, preferred embodiments of the present invention will be described in detail. However, the invention is not limited to the embodiments described herein but may be embodied in other forms. Rather, the embodiments introduced herein are provided so that the disclosure may be made thorough and complete, and to fully convey the spirit of the present invention to those skilled in the art. Like numbers refer to like elements throughout.

The present invention relates to shear bolts for bolt connectors for conductor connection of cables. Here, the bolt connector has a cylindrical shape similar to the cable as a whole, and includes a plurality of conductor insertion grooves into which the conductors to be connected to both ends thereof are inserted, and a plurality of bolts for fixing the conductors inserted into the conductor insertion grooves. The bolt fastener of may be disposed around the cylindrical bolt connector. In addition, the bolt connector may have a resistance of 90% or less than that of the conductor of the cable.

4 schematically shows an embodiment of a shear bolt according to the invention.

As shown in Figure 4, the shear bolt 100 according to the present invention is the head 110, which can be fastened to a mechanism or equipment for inserting the shear bolt 100 into the bolt fastener of the bolt connector, the bolt When a screw thread 120 corresponding to the female screw of the inner wall of the fastener is applied, the head 110 is disposed between the head 110 and the screw thread 120, and a torque higher than a predetermined level is applied to the shear bolt 100. Breaking part 130 to be broken, and is disposed between the screw thread 120 and the breaking part 130 to suppress the insertion of the shear bolt 100 to increase the torque applied to the shear bolt 100 It may include a stopper 140.

The shear bolt 100 according to the present invention may be made of a metal material having a tensile strength of about 250 MPa or more and an electrical conductivity of about 25% (@ 20 ° C.) or more than that of pure copper. When the tensile strength of the shear bolt 100 is less than 250 MPa, it may be difficult to form a sufficient current path by breaking the conductor before sufficiently contacting the inner wall of the bolt connector, and also the electrical conductivity of the shear bolt 100 Is less than 25% (@ 20 ° C) compared to pure copper, the resistance at the conductor connection area of the cable may increase significantly. Here, the shear bolt 100 may be made of, for example, a metal such as brass or an aluminum alloy having higher electrical conductivity than brass.

The threaded portion 120 has a male screw corresponding to a female screw on an inner wall of the bolt fastener 310 of the bolt connector 300, and the shear bolt 100 is rotated by engaging the male screw with the female screw. Proceed toward the conductor 210b inserted into the conductor insertion groove 320 of the bolt connector 300, and as a result, the lower end of the shear bolt 100 pushes the conductor 210b to the bolt connector 300 By contacting the lower inner wall of the c) to form a current path between the conductor (210b) and the bolt connector (300).

In addition, the length of the threaded portion 120 and the size of the thread formed on the threaded portion 120, the shear bolt 100 is in contact with the inner wall of the bolt connector 300, the conductor 210b is sufficient to contact between them There is no particular limitation as long as a sufficient current path can be formed, for example, the maximum outer diameter of the male thread is Φ12 to Φ20, the pitch of the thread is 1-3 mm, and the number of threads per unit length (1 inch) is 8 To 26.

In addition, the threaded portion 120 has a length such that the stopper 140 disposed on the upper portion of the threaded portion 120 does not protrude from the surface of the bolt connector 300 when the broken portion 130 is to be described later. It can have When the stopper 140 protrudes from an outer surface of the bolt connector 300 when the breaker 130 is broken, it may be difficult to wrap the upper portion of the bolt connector 300 with a rubber tube made of a material such as silicon. Can be.

The breaker 130 is not particularly limited as long as it can perform a function of breaking when a predetermined torque is applied to the shear bolt 100. For example, the breaker 130 may have a conical shape that gradually decreases in diameter, and may have a different diameter depending on a material forming the breaker 130, but the breaker 130 may be brass. When made in diameter may be Φ8 to Φ14. Here, the predetermined torque applied to the shear bolt 100 at the time of breaking of the breaker 130 may be about 50 to 150 N · m.

As described above, the stopper 140 inserts the shear bolt 100 when the contact area between the conductor 210b and the inner wall of the bolt connector 300 increases to form a sufficient current path and decreases resistance. By suppressing this, the torque applied to the shear bolt 100 is increased to perform the function of breaking the breaker 130.

Shear bolt 100 according to the present invention is the bolt fastener 310 regardless of the type of conductor 210b that is inserted into the conductor insertion groove 320 of the bolt connector 300 by the stopper 140 By consistently adjusting the depth to be inserted through, by optimizing the contact area between the conductor (210b) and the inner wall of the bolt connector 300 to form a sufficient current passer and reduce the resistance to excellent solve the local heat problem effectively Effect.

The stopper 140 suppresses the insertion of the shear bolt 100 when the sufficient contact current between the conductor 210b and the inner wall of the bolt connector 300 is increased and the resistance is reduced, thereby preventing the insertion of the shear bolt 100. If it is possible to perform the function of breaking the fracture portion 130 by increasing the torque applied to the 100 is not particularly limited, for example, the flange shown in Figure 4a, the thread shown in Figure 4b Unapplied cylinders and the like.

As shown in FIG. 4A, when the stopper 140 is a flange, the shape of the flange is not particularly limited as long as it can increase the torque applied to the shear bolt 100 as a stopper. The outer diameter of the flange may be as large as about 3 mm or less than the outer diameter of the threaded portion 120, and the thickness may be about 2 to 5 mm.

On the other hand, as shown in Figure 4b, when the stopper 140 is a thread unapplied cylinder, the shape of the thread unapplied cylinder is not particularly limited as long as it can increase the torque applied to the shear bolt 100 as a stopper For example, the length of the thread unapplied cylinder may be about 2 to 5 mm.

Figure 5 schematically shows another embodiment of a shear bolt according to the present invention, Figure 6 schematically shows how the shear bolt of Figure 5a is used in a bolt connector.

As shown in FIG. 5, the shear bolt 100 according to the present invention is inserted between the wires constituting the conductor 210b of the cable disposed at the lower end of the threaded portion 120 and inserted into the bolt connector 300. It may further include a protrusion 150 having an inclined shape to reduce the cross-sectional area in whole or in part to be reduced.

As shown in FIG. 6, the shear bolt 100 according to the present invention has the protrusion 150 of the conductor of the bolt connector 300 when the bolt 150 is inserted into the bolt fastener 310 of the bolt connector 300. The wires may be inserted between the associated wires constituting the conductor 210b inserted into the insertion groove to push the wires in the left and right directions perpendicular to the traveling direction of the shear bolt 100.

As a result, the associated wires constituting the conductor 210b are in intimate contact with each other, and in particular, the wires disposed at the outermost of the wires are in close physical contact with the inner wall of the bolt connector 300. The contact area between the shear bolt 100 and the element wires is increased, thereby forming a current path connecting the conductor 210b, the bolt connector 300, and the shear bolt 100, thereby adding it. Furnace resistance increase can be avoided or minimized.

In addition, the protrusion 150 of the shear bolt 100 is inserted between the associated element wires constituting the conductor 210b to push the element wires, thereby between the protrusion 150 and the element wires or the element wires. It causes friction, thereby removing the oxide film on the surface of the aluminum wires to avoid or minimize the increase in resistance caused by the oxide film.

In addition, the shear bolt 100 according to the present invention forms a sufficient current path at the conductor connection by the bolt connector 300, and increases the resistance due to the oxide film on the surface of the element wires constituting the conductor 210b. By avoiding or minimizing, it is possible to effectively solve the problem of heat generation due to increased local resistance at the connection area of the cable conductor.

The protruding portion 150 is sheared by a male screw constituting the threaded portion 120 of the shear bolt 100 according to the present invention engaged with a female screw of an inner wall of the bolt fastener 310 of the bolt connector 300. If the bolt 100 can be inserted between the associated element wires constituting the conductor 210b as it travels in the direction of the conductor 210b inserted into the conductor insertion groove 320 of the bolt connector 300 in its shape. There is no special limitation.

For example, the protrusion 150 has a conical shape with a gradually decreasing cross-sectional area as a whole as shown in FIGS. 5A and 5B, and the ends of the cone are hemispherical or causal shapes having a radius of 2.5 to 8 mm, or in FIG. 5C. As shown, the cylinder may be generally cylindrical in shape and the ends of the cylinder may be conical in shape with gradually decreasing cross-sectional area.

In addition, the length of the protrusion 150 may be different according to the association method of the wires constituting the conductor 210b to be inserted into the conductor insertion groove 320 of the bolt connector 300.

For example, the protrusion 150 is the shear bolt 100 as shown in Figure 5a or 5c when the conductor 210b is a circular twisted-pair conductor shown in Figure 1a or a circular compression conductor shown in Figure 1b. End of the protrusion 150 may have a length such that the end of the protrusion 150 may contact the center element wire of the conductor, and when the conductor 210b is a split compressed circular conductor shown in FIG. 1C, as shown in FIG. 5B. The protruding portion 150 of the shear bolt 100 may have a length such that the distal end of the protrusion 150 may contact the center element wire of one section constituting the split compression circular conductor. For example, the length of the protrusion 150 may be 4 to 10 mm.

In addition, the protrusion 150 may have a length such that the stopper 140 disposed above the threaded portion 120 does not protrude from the surface of the bolt connector 300 at the time of breaking of the breaker 130 to be described later. Can have When the stopper 140 protrudes from an outer surface of the bolt connector 300 when the breaker 130 is broken, it may be difficult to wrap the upper portion of the bolt connector 300 with a rubber tube made of a material such as silicon. Can be.

When the distal end of the protrusion 150 of the shear bolt 100 contacts the center element wire of the circular twisted pair conductor shown in FIG. 1A or the circular compressed conductor shown in FIG. 1B, or the split compressed circular conductor shown in FIG. 1C. When reaching the center element wire of one section constituting the, the insertion of the shear bolt 100 by the stopper 140 is suppressed, thereby increasing the torque applied to the shear bolt 100, When the torque reaches a predetermined specific torque value, the breaking part 130 is broken, so that the breaking part 130 and the head 110 are separated. Here, the predetermined torque applied to the shear bolt 100 at the time of breaking of the breaker 130 may be about 50 to 150 N · m.

In particular, as shown in FIG. 6B, the conductor 210b inserted into the conductor insertion groove 320 of the bolt connector 300 is a split compression circular conductor, and the plurality of shear bolts 100 are split compression circular. When sequentially inserted toward the center element wire of each section of the conductor, the void space inside the conductor insertion groove 320 is gradually reduced as the number of shear bolts 100 inserted increases.

That is, the resistance of the conductor to the insertion of the shear bolt 100 to be inserted first is smaller than the resistance of the conductor to the insertion of the shear bolt 100 to be inserted later, so that the shear bolt 100 to be inserted first is later Compared to the shear bolt 100 to be inserted is inserted more easily is inserted more deeply.

Therefore, as shown in FIG. 6B, when a plurality of shear bolts 100 are inserted into the bolt connector 300, all of the shear bolts 100 to be inserted, in particular, protrusions of the shear bolts 100 inserted first. It is necessary to control this so that the end thereof is no longer inserted after contacting the central element wire of each section constituting the divided compression circular conductor, and the stopper 140 performs this control function.

That is, the stopper 140 is the shear in a state in which the end of the protrusion 150 of the shear bolt 100 is in contact with the central element wire of one or more sections constituting a circular twisted pair conductor, a circular compressed conductor, or a split compressed circular conductor By inhibiting the insertion of the bolt 100 may increase the torque applied to the shear bolt 100 to perform the function of breaking the breaking portion 130.

Thus, insertion of the shear bolt 100 when the distal end of the protrusion 150 of the shear bolt 100 contacts the central element wire of one section constituting the circular stranded conductor, the circular compressed conductor or the divided compressed circular conductor It is possible to achieve the formation of an optimum current path between the conductor 210b, the bolt connector 300 and the shear bolt 100, and the reduction of resistance.

In the shear bolt 100 according to the present invention, at least the threaded portion 120, the protrusion 150 and the stopper 140 may be plated with tin. When the shear bolt 100 and the bolt connector 300 or the conductor 210b are dissimilar metals, dissimilar metal contact corrosion, ie, galvanic corrosion, may occur at their contact surfaces, such galvanic corrosion. Tin is plated with a sacrificial metal to suppress the

The tin plating not only inhibits corrosion of the shear bolt 100, the conductor 210b, the bolt connector 300, etc., but also the male thread of the threaded portion 120 and the bolt of the bolt connector 300. The effect of forming a sufficient current path and further reducing the resistance by increasing the contact area between the shear bolt 100 and the bolt connector 300, such as filling a small void space between the internal threads of the fastener 310. Can be represented.

Although the present specification has been described with reference to preferred embodiments of the invention, those skilled in the art may variously modify and change the invention without departing from the spirit and scope of the invention as set forth in the claims set forth below. Could be done. Therefore, it should be seen that all modifications included in the technical scope of the present invention are basically included in the scope of the claims of the present invention.

Claims

A shear bolt for a bolt connector for conductor connection of a cable, comprising: a plurality of conductor fastening holes into which a conductor of a cable is inserted; and a plurality of bolt fasteners into which bolts for fixing a conductor inserted into the conductor insertion groove are inserted;

A head capable of being fastened to a mechanism or equipment to allow the shear bolt to be inserted into the bolt fastener;

A thread portion having a male thread corresponding to a female thread of the inner wall of the bolt fastener;

A break portion disposed between the head and the thread portion and broken when a predetermined torque is applied to the shear bolt; And

A stopper disposed between the threaded portion and the breaking portion and increasing the torque applied to the shear bolt by inhibiting the insertion of the shear bolt so that the torque reaches a predetermined torque at the time of breaking the breaking portion. Shearing bolts.
The method of claim 1,

And said stopper is a flange or a threadless cylinder.
The method of claim 2,

The outer diameter of the flange is greater than 3 mm or less than the outer diameter of the thread portion, the length of the flange or the thread unapplied cylinder is 2 to 5 mm, shear bolts.
The method according to any one of claims 1 to 3,

And a protrusion disposed at the bottom of the threaded portion and insertable between the associated strands of the conductor.
The method of claim 4, wherein

When the conductor is a circular stranded conductor in which a plurality of element wires having a circular cross section are circled as a whole or a circular compressed conductor in which a plurality of element wires having a circular cross section are compressed in a circular shape, the center element wire of the circular stranded conductor or a circular compressed conductor The center element wire of the section when the end of the protruding portion is in contact or when the conductor is a divided compressed circular conductor composed of a plurality of element wires having a circular cross section and a plurality of sections compressed in a fan shape together in a circular shape as a whole; Characterized in that the stopper increases the torque applied to the shear bolt when the distal end of the protrusion contacts.
The method according to any one of claims 1 to 3,

And when the break is broken, the stopper does not protrude beyond the outer surface of the bolt connector.
The method according to any one of claims 1 to 3,

And said predetermined torque is between 50 and 150 N · m.
The method according to any one of claims 1 to 3,

Shear bolt, characterized in that made of brass or aluminum alloy with a tensile strength of 250 MPa or more and the electrical conductivity 25% (@ 20 ℃) compared to pure copper.
The method according to any one of claims 1 to 3,

The maximum outer diameter of the male screw constituting the thread portion is Φ 12 to Φ 20, the pitch of the thread is 1 to 3 mm, the number of threads per unit length of 8 to 26, characterized in that the shear bolt.
The method of claim 4, wherein

Wherein the protrusion has an inclined shape which, in whole or in part, gradually decreases in cross-sectional area.
The method of claim 10,

Wherein the protrusion is conical in shape with gradually decreasing cross section and the end of the cone is in the shape of a hemisphere with a radius of 2.5 to 8 mm.
The method of claim 11,

Shear bolt, characterized in that the length of the protrusion is 4 to 10 mm.
The method according to any one of claims 1 to 3,

The fracture portion is a cone-shaped gradually decreasing in diameter, characterized in that the diameter is Φ8 to Φ14, shear bolts.
The method of claim 4, wherein

At least the threaded portion, the protrusion and the stopper are plated with tin.