Battery stack and restraint hoop member

Abstract

To provide a battery stack capable of suppressing the concentration of stress of a restraint hoop member.SOLUTION: The battery stack includes: multiple single cells arranged in an array direction; and a constraint mechanism that constrains multiple single cells. The constraint mechanism includes: a pair of end plates arranged at both ends in the arrangement direction of multiple single cells; and an annular restraint hoop member 40. The multiple single cells and the pair of end plates are placed inside a ring of the restraint hoop member 40. The restraint hoop member 40 has an easily deformable part 50 where the restraint hoop member 40 partially deforms easily in the ring circumferential direction.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a battery stack and a restraint hoop member.

Background Art

[0002] Japanese Patent Application Laid-Open No. 2019-125444 (Patent Document 1) discloses that a restraint mechanism for restraining a plurality of single cells includes an annular restraint hoop portion. The restraint hoop portion applies a predetermined restraint pressure to the plurality of single cells in a direction of compression along the array direction of the single cells.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When restraining single cells by a restraint hoop portion, a gap may be formed between an end plate and the restraint hoop portion due to variations during component manufacturing. The inventors have found that when a gap is formed, stress concentrates at the end of the curvature of the restraint hoop portion.

[0005] In the present disclosure, a battery stack capable of suppressing stress concentration of a restraint hoop member and a restraint hoop member capable of suppressing stress concentration are proposed.

Means for Solving the Problems

[0006] According to an aspect of the present disclosure, a battery stack is proposed that includes a plurality of single cells arranged in an array direction and a restraint mechanism that restrains the plurality of single cells. The restraint mechanism includes a pair of end plates disposed at both ends in the array direction of the plurality of single cells and an annular restraint hoop member. The plurality of single cells and the pair of end plates are disposed inside the ring of the restraint hoop member. The restraint hoop member has a deformable portion that partially facilitates circumferential deformation of the ring of the restraint hoop member.

[0007] Since the restraint hoop member has a deformable portion, the restraint hoop member is made to be highly elastically deformable in the circumferential direction of the ring. By greatly elastically deforming the restraint hoop member during the manufacture of the battery stack, it is possible to suppress the occurrence of a gap between the end plate and the restraint hoop member in the assembled battery stack. By not creating a gap between the end plate and the restraint hoop member, it is possible to suppress the concentration of stress at the end of the curvature of the restraint hoop member.

[0008] In the above battery stack, the deformable portion may have a portion where the thickness of the restraint hoop member is smaller than that of other portions. By partially reducing the thickness of the restraint hoop member, the portion with the smaller thickness can be formed as the deformable portion.

[0009] In the above battery stack, a through hole penetrating the restraint hoop member in the thickness direction may be formed in the deformable portion. By forming a through hole in a part of the restraint hoop member, the periphery of the through hole can be formed as the deformable portion.

[0010] In the above battery stack, the deformable portion may have a portion where the width of the restraint hoop member is smaller than that of other portions. By partially reducing the width of the restraint hoop member, the portion with the smaller width can be formed as the deformable portion.

[0011] In the above-described battery stack, the pair of end plates has a first surface facing the single cell and a second surface opposite to the first surface, and the deformable portion may be in contact with the second surface. Thereby, it is possible to avoid stress concentration on the deformable portion.

[0012] In the above-described battery stack, the second surface of the end plate has chamfered portions with a curved surface at both circumferential edges of the ring of the restraining hoop member, has a flat portion between the pair of chamfered portions, and the deformable portion may be in contact with the flat portion. By arranging in such a manner, the restraining hoop member can continue to apply a high-load restraining force to the single cell.

[0013] In the above-described battery stack, the entire deformable portion in the circumferential direction of the ring of the restraining hoop member may be in contact with the flat portion. By arranging in such a manner, the restraining hoop member can continue to apply a high-load restraining force to the single cell.

[0014] According to an aspect of the present disclosure, there is proposed an annular restraining hoop member that restrains a plurality of single cells arranged in an array direction and a pair of end plates arranged at both ends in the array direction of the plurality of single cells in the array direction. The plurality of single cells and the pair of end plates are arranged inside the ring of the restraining hoop member. The restraining hoop member includes a deformable portion that partially facilitates deformation in the circumferential direction of the ring of the restraining hoop member.

[0015] Since the restraining hoop member has a deformable portion, the restraining hoop member is made to be largely elastically deformable in the circumferential direction of the ring. By largely elastically deforming the restraining hoop member during the manufacture of the battery stack, it is possible to suppress the occurrence of a gap between the end plate and the restraining hoop member in the assembled battery stack. By not creating a gap between the end plate and the restraining hoop member, it is possible to suppress stress concentration at the end of the curvature of the restraining hoop member.

Advantages of the Invention

[0016] According to the present disclosure, stress concentration of the restraining hoop member can be suppressed.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments will be described with reference to the drawings. In the following description, the same parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

[0019] FIG. 1 is a schematic diagram of a battery stack 1. The battery stack 1 includes a plurality of single cells 10 and a restraint mechanism. The plurality of single cells 10 are arranged in a predetermined arrangement direction (the left - right direction in the figure in FIG. 1). Hereinafter, the arrangement direction of the plurality of single cells 10 will also be simply referred to as the arrangement direction. The restraint mechanism is a mechanism for restraining the plurality of single cells 10 so that they can be used as a set of batteries.

[0020] The single cell 10 is typically a secondary battery capable of repeated charge and discharge, such as a lithium - ion secondary battery, a nickel - metal hydride battery, an electric double - layer capacitor, etc. The single cell 10 has a battery case 12 and an external terminal 14. The battery case 12 houses a power - generating element in which a solid electrolyte is arranged in layers between a positive - electrode active material layer and a negative - electrode active material layer inside. The external terminal 14 is disposed so as to protrude upward from the upper surface of the battery case 12. The external terminals 14 of adjacent single cells 10 are electrically connected by a bus bar (not shown) to form the battery stack 1.

[0021] The restraint mechanism includes a pair of end plates, namely a first end plate 20 and a second end plate 30. The first end plate 20 and the second end plate 30 are plate-like members for evenly applying a restraint pressure to the power generation elements of the single battery 10. The pair of end plates are arranged at both ends in the arrangement direction of the plurality of single batteries 10. The pair of end plates are arranged at both ends of the plurality of arranged single batteries 10 so as to sandwich the plurality of arranged single batteries 10 in the arrangement direction. The first end plate 20 is arranged at one end in the arrangement direction of the arranged single batteries 10. The second end plate 30 is arranged at the other end in the arrangement direction of the arranged single batteries 10.

[0022] The first end plate 20 and the second end plate 30 are configured to be slightly larger than the dimensions of the single battery 10. Assuming that the direction in which the external terminal 14 protrudes from the battery case 12 of the single battery 10 is the upward direction and the direction opposite to the upward direction is the downward direction, the upper end of the first end plate 20 is arranged above the tip of the external terminal 14. The upper end of the second end plate 30 is arranged above the tip of the external terminal 14. The lower end of the first end plate 20 is arranged below the lower surface (bottom surface) of the battery case 12. The lower end of the second end plate 30 is arranged below the lower surface of the battery case 12.

[0023] In the direction in which the external terminal 14 of the single battery 10 protrudes from the battery case 12, the dimensions of the first end plate 20 and the second end plate 30 are made larger than the dimensions of the single battery 10 in that direction. In the direction in which a line segment connecting the tip of the external terminal 14 and the bottom surface of the battery case 12 extends at the shortest distance, the dimensions of the first end plate 20 and the second end plate 30 in that direction are made larger than the dimensions of the single battery 10 in that direction.

[0024] The first end plate 20 has a first surface 21 facing the single battery 10 and a second surface 22 on the side opposite to the first surface 21. The first surface 21 has a planar shape. The first end plate 20 is arranged such that the first surface 21 is orthogonal to the arrangement direction of the single batteries 10.

[0025] The second surface 22 of the first end plate 20 has a curved chamfered portion 23 at the upper edge. The second surface 22 has a curved chamfered portion 24 at the lower edge. The second surface 22 has a planar portion 25 with a planar shape between the pair of chamfered portions 23, 24. The first end plate 20 has rounded and curved portions at the upper and lower corners when viewed from the left side of FIG. 1.

[0026] The second end plate 30 has a first surface 31 facing the single battery 10 and a second surface 32 on the side opposite to the first surface 31. The first surface 31 has a planar shape. The second end plate 30 is arranged such that the first surface 31 is orthogonal to the arrangement direction of the single batteries 10. The first end plate 20 and the second end plate 30 are arranged such that the first surface 21 of the first end plate 20 and the first surface 31 of the second end plate 30 are parallel to each other.

[0027] The second surface 32 of the second end plate 30 has a curved chamfered portion 33 at the upper edge. The second surface 32 has a curved chamfered portion 34 at the lower edge. The second surface 32 has a planar portion 35 with a planar shape between the pair of chamfered portions 33, 34. The second end plate 30 has rounded and curved portions at the upper and lower corners when viewed from the right side of FIG. 1.

[0028] The restraining mechanism includes a restraining hoop member 40. The restraining hoop member 40 has an annular shape. The restraining hoop member 40 has a flat belt-like shape. The restraining hoop member 40 has a first support portion 41 disposed along the second surface 22 of the first end plate 20. The restraining hoop member 40 has a second support portion 42 disposed along the second surface 32 of the second end plate 30. The restraining hoop member 40 has a connecting portion 43 that continuously connects the first support portion 41 and the second support portion 42 along the arrangement direction of the single batteries 10 so as to extend from the upper end of the first end plate 20 to the upper end of the second end plate 30. The restraining hoop member 40 has a connecting portion 44 that continuously connects the first support portion 41 and the second support portion 42 along the arrangement direction of the single batteries 10 so as to extend from the lower end of the first end plate 20 to the lower end of the second end plate 30.

[0029] The restraining hoop member 40 annularly surrounds a stack composed of the first end plate 20, the plurality of arranged single batteries 10, and the second end plate 30 along its outer periphery. The plurality of single batteries 10, the first end plate 20, and the second end plate 30 are disposed inside the ring of the restraining hoop member 40. The restraining hoop member 40 is in contact with the second surface 22 of the first end plate 20. The restraining hoop member 40 is in contact with the second surface 32 of the second end plate 30.

[0030] The restraining hoop member 40 restricts the distance between the first end plate 20 and the second end plate 30 by being closed in an annular shape. The dimension in the arrangement direction of the connecting portions 43, 44 of the restraining hoop member 40 is equal to the dimension in the arrangement direction of the plurality of single batteries 10 restrained in the arranged state. The dimension in the arrangement direction of the connecting portions 43, 44 of the restraining hoop member 40 is determined so that the restraining pressure applied to the single batteries 10 becomes a predetermined magnitude. The restraining hoop member 40 is configured such that the dimension in the arrangement direction of the connecting portions 43, 44 is smaller than the dimension in the arrangement direction of the plurality of single batteries 10 in the unrestrained state.

[0031] With the restraining hoop member 40 assembled around the first end plate 20, the plurality of single cells 10, and the second end plate 30, a circumferential tensile stress of the hoop of the restraining hoop member 40 acts on the restraining hoop member 40. The restraining hoop member 40 applies a force in a direction to reduce the distance between the first end plate 20 and the second end plate 30 to the first end plate 20 and the second end plate 30. Thereby, the restraining hoop member 40 generates a restraining pressure on the single cells 10 and restrains the plurality of single cells 10.

[0032] Since the dimensions of the first end plate 20 and the second end plate 30 are larger than the dimensions of the single cell 10 in the vertical direction of the single cell 10, as shown in FIG. 1, a gap is formed between the connecting portion 43 of the restraining hoop member 40 and the external terminal 14, and a gap is formed between the connecting portion 44 and the battery case 12. The restraining hoop member 40 assembled around the first end plate 20, the plurality of single cells 10, and the second end plate 30 is arranged away from the single cells 10, and interference between the restraining hoop member 40 and the single cells 10 is avoided. Thereby, the restraining hoop member 40 can firmly restrain the single cells 10 in the arrangement direction.

[0033] The forming material of the restraining hoop member 40 may be, for example, a metal material, a resin material, an inorganic material, or a composite material. The restraining hoop member 40 may have a seamless annular structure made of a single material. Or the restraining hoop member 40 may have a jointed annular structure, for example, formed by overlapping and joining the ends of two semi-circular members.

[0034] The restraining mechanism may include only one restraining hoop member 40. The restraining mechanism may include two or more restraining hoop members 40. When the restraining mechanism includes only one restraining hoop member 40, the restraining hoop member 40 may be formed wider than when the restraining mechanism includes two or more restraining hoop members 40.

[0035] The first end plate 20 and the second end plate 30 may have the entire upper and lower corners formed as curved surfaces. Alternatively, the first end plate 20 and the second end plate 30 may have a groove shape at the upper and lower corners, and the restraining hoop member 40 may be housed inside the groove shape, and the bottom surface of the groove shape may be formed as a curved surface to constitute a chamfered portion. In the latter case, displacement of the restraining hoop member 40 in the width direction of the battery stack 1 (perpendicular to the paper surface in FIG. 1) can be suppressed.

[0036] FIG. 2 is a schematic view of the restraining hoop member 40 seen from the front. FIG. 2 shows the restraining hoop member 40 viewed in the direction of arrow II in FIG. 1, and thus, the first support portion 41 of the restraining hoop member 40 is shown. As shown in FIG. 2, the restraining hoop member 40 has a deformable portion 50 that is more easily deformable in the circumferential direction of the ring than other portions in a part of the circumferential direction of the ring. The deformable portion 50 is a part of the restraining hoop member 40 that makes the deformation of the restraining hoop member 40 in the circumferential direction of the ring easier than other portions. The deformable portion 50 is configured to be more easily deformable in the circumferential direction of the ring of the restraining hoop member 40 compared to the portion of the restraining hoop member 40 that is not the deformable portion 50.

[0037] The boundary line 51 shown in FIG. 2 indicates the position corresponding to the boundary between the chamfered portion 23 and the flat portion 25 on the second surface 22 of the first end plate 20. The boundary line 52 shown in FIG. 2 indicates the position corresponding to the boundary between the chamfered portion 24 and the flat portion 25 on the second surface 22 of the first end plate 20. In the circumferential direction of the ring of the restraining hoop member 40, the deformable portion 50 is formed between the boundary line 51 and the boundary line 52.

[0038] As described with reference to FIG. 1, the restraining hoop member 40 is in contact with the second surface 22 of the first end plate 20. Therefore, the deformable portion 50 shown in FIG. 2 is in contact with the second surface 22 of the first end plate 20. The deformable portion 50 is in contact with the flat portion 25 of the second surface 22 of the first end plate 20. The entire deformable portion 50 in the circumferential direction of the ring of the restraining hoop member 40 is in contact with the flat portion 25 of the second surface 22 of the first end plate 20.

[0039] The deformable portion 50 has a width reduction portion 53. The width of the flat belt-shaped restraining hoop member 40 is reduced in the width reduction portion 53. The deformable portion 50 has a portion where the width of the restraining hoop member 40 is smaller than that of the restraining hoop member 40 other than the deformable portion 50. The width of the restraining hoop member 40 gradually decreases from the boundary line 51 shown in FIG. 2 toward the width reduction portion 53, and the width of the restraining hoop member 40 gradually decreases from the boundary line 52 toward the width reduction portion 53. The width reduction portion 53 is formed in the central portion of the deformable portion 50 in the circumferential direction of the ring of the restraining hoop member 40. In the width reduction portion 53, the width of the restraining hoop member 40 is the smallest.

[0040] By providing the width reduction portion 53, the cross-sectional area of the restraining hoop member 40 orthogonal to the circumferential direction of the ring is reduced in the deformable portion 50. Thereby, the rigidity of the deformable portion 50 is smaller than that of the restraining hoop member 40 other than the deformable portion 50. Since the deformable portion 50 has a low-rigidity structure, the circumferential deformation of the restraining hoop member 40 in the deformable portion 50 is facilitated more easily than other portions of the restraining hoop member 40.

[0041] The deformable portion 50 has a portion where the thickness of the restraint hoop member 40 is smaller than that of the restraint hoop member 40 other than the deformable portion 50. Typically, through holes 54 and 55 penetrating the restraint hoop member 40 in the thickness direction are formed in the deformable portion 50. That is, in the through holes 54 and 55, the thickness of the restraint hoop member 40 becomes zero. In the circumferential direction of the ring of the restraint hoop member 40, a through hole 54 is formed between the boundary line 51 and the width reduction portion 53, and a through hole 55 is formed between the boundary line 52 and the width reduction portion 53. Since the through holes 54 and 55 are formed in the restraint hoop member 40, narrow portions 56 with a small width are formed around the through holes 54 and 55.

[0042] By forming the through holes 54 and 55 that reduce the thickness of the restraint hoop member 40 in the deformable portion 50, typically making the thickness of the restraint hoop member 40 zero, the cross-sectional area orthogonal to the circumferential direction of the ring of the restraint hoop member 40 becomes smaller in the deformable portion 50. Thereby, the rigidity of the deformable portion 50 is smaller compared to the restraint hoop member 40 other than the deformable portion 50. Since the deformable portion 50 has a low-rigidity structure, the deformation of the restraint hoop member 40 in the circumferential direction of the ring in the deformable portion 50 is facilitated more easily than other portions of the restraint hoop member 40.

[0043] The restraint hoop member 40 of the embodiment described above has the deformable portion 50, so that the elastic deformation in the circumferential direction of the ring is facilitated compared to a conventional restraint hoop member without the deformable portion 50. When the restraint hoop member 40 of the embodiment is deformed to increase the circumferential dimension of the ring, the amount of elastic deformation until reaching the yield point is larger compared to a conventional restraint hoop member without the deformable portion 50.

[0044] When manufacturing the battery stack 1 using the restraining hoop member 40, first, an annular restraining hoop member 40 is arranged so as to surround the first end plate 20 and the second end plate 30. Subsequently, the first end plate 20 and the second end plate 30 are relatively moved to increase the distance between the first end plate 20 and the second end plate 30. At this time, the restraining hoop member 40 elastically deforms so as to increase the circumferential dimension of the ring. In this state, a plurality of single cells 10 are arranged between the first end plate 20 and the second end plate 30. Thereafter, the force applied for the relative movement of the first end plate 20 and the second end plate 30 is released.

[0045] The restraining hoop member 40 of the embodiment can elastically deform more greatly than the conventional restraining hoop member when manufacturing the battery stack 1. When the force applied to the first end plate 20 and / or the second end plate 30 for assembling the single cell 10 is released, the restraining hoop member 40 elastically deforms so as to decrease the circumferential dimension of the ring, and a restraining pressure is loaded from the restraining hoop member 40 to the single cell 10.

[0046] At this time, due to the large amount of elastic deformation of the restraining hoop member 40 that decreases the circumferential dimension of the ring, the restraining hoop member 40 is arranged along the curved surface of the chamfered portion 24 of the second surface 22 of the first end plate 20 as shown in FIG. 3. The restraining hoop member 40 is in surface contact with the chamfered portion 24, and no gap is generated between the restraining hoop member 40 and the second surface 22 of the first end plate 20. Note that FIG. 3 is a schematic diagram showing an enlarged view of region III in FIG. 1.

[0047] Even when the dimension of the first end plate 20 varies due to variations during manufacturing, the restraining hoop member 40 can favorably follow the actual shape of the first end plate 20, and the restraining hoop member 40 can be arranged without a gap between the restraining hoop member 40 and the first end plate 20. By not creating a gap between the first end plate 20 and the restraining hoop member 40, it is possible to suppress the concentration of stress at the end of the curvature of the restraining hoop member 40.

[0048] Referring to FIG. 3, the arrangement of the restraint hoop member 40 along the chamfered portion 24 at the lower end of the first end plate 20 has been typically described. Similarly, at the chamfered portion 23 at the upper end of the first end plate 20, the restraint hoop member 40 is arranged without a gap between the restraint hoop member 40 and the chamfered portion 23. A configuration in which no gap is formed between the first end plate 20 and the restraint hoop member 40 can be surely realized.

[0049] Although the deformable portion 50 formed in the first support portion 41 of the restraint hoop member 40 has been described, a similar deformable portion 50 can also be provided in the second support portion 42. By arranging the restraint hoop member 40 without a gap between the chamfered portions 33 and 34 of the second end plate 30, it is possible to suppress stress concentration at the end of the curvature of the restraint hoop member 40. By providing the deformable portions 50 at a plurality of locations in the circumferential direction of the ring of the restraint hoop member 40, the amount of elastic deformation of the restraint hoop member 40 can be made larger. By configuring the restraint hoop member 40 to include the deformable portions 50 at two equally spaced locations in the circumferential direction of the ring, the restraint hoop member 40 can be surely elastically deformed in the circumferential direction of the ring, and stress concentration of the restraint hoop member 40 can be suppressed.

[0050] The deformable portion 50 is formed in the first support portion 41 that contacts the second surface 22 of the first end plate 20 and the second support portion 42 that contacts the second surface 32 of the second end plate 30, and the deformable portion 50 is configured to contact the second surfaces 22 and 32. Thereby, it is possible to avoid the deformable portion 50 from becoming a stress concentration location, and the effect of not generating stress concentration in the restraint hoop member 40 can be more surely obtained.

[0051] The restraining hoop member 40 receives a reaction force by applying a restraining pressure to the single battery 10. Among the restraining hoop member 40, the portions that come into contact with the flat surface portion 25 of the first end plate 20 and the flat surface portion 35 of the second end plate 30 are the portions where the stress acting on the restraining hoop member 40 due to the reaction force is minimized. Therefore, the restraining hoop member 40 is arranged with respect to the first end plate 20 and the second end plate 30 such that the deformable portion 50 is in contact with the flat surface portions 25 and 35, and more typically, the entire deformable portion 50 is in contact with the flat surface portions 25 and 35. By arranging it in this way, it is possible to avoid the deformable portion 50 with low rigidity from degrading the function of the restraining hoop member 40, and the restraining hoop member 40 can continue to reliably apply a high-load restraining force to the single battery 10.

[0052] The embodiments disclosed this time should be considered illustrative in all respects and not restrictive. The scope of the present invention is indicated not by the above description but by the claims, and it is intended that all modifications within the meaning and scope equivalent to the claims be included.

Explanation of Reference Numerals

[0053] 1 Battery stack, 10 Single battery, 12 Battery case, 14 External terminal, 20 First end plate, 21, 31 First surface, 22, 32 Second surface, 23, 24, 33, 34 Chamfered portion, 25, 35 Flat surface portion, 30 Second end plate, 40 Restraining hoop member, 41 First support portion, 42 Second support portion, 43, 44 Connecting portion, 50 Deformable portion, 51, 52 Boundary line, 53 Width reduction portion, 54, 55 Through hole, 56 Detail.

Claims

1. A plurality of single cells arranged in an array direction, and a restraint mechanism for restraining the plurality of single cells, wherein the restraint mechanism includes a pair of end plates disposed at both ends of the plurality of single cells in the array direction and an annular restraint hoop member, the plurality of single cells and the pair of end plates are disposed inside the ring of the restraint hoop member, the restraint hoop member has a deformable portion that partially facilitates deformation in the circumferential direction of the ring of the restraint hoop member, the pair of end plates have a first surface facing the single cell and a second surface on the side opposite to the first surface, the deformable portion contacts the second surface, a battery stack.

2. The battery stack according to claim 1, wherein the deformable portion has a portion where the thickness of the restraint hoop member is smaller than that of other portions of the deformable portion.

3. The battery stack according to claim 2, wherein a through hole penetrating the restraint hoop member in the thickness direction is formed in the deformable portion.

4. The battery stack according to any one of claims 1 to 3, wherein the deformable portion has a portion where the width of the restraint hoop member is smaller than that of other portions of the deformable portion.

5. The second surface has chamfered portions with a curved surface shape at both edges in the circumferential direction of the ring of the restraint hoop member, and has a flat portion between the pair of chamfered portions, the battery stack according to any one of claims 1 to 4, wherein the deformable portion contacts the flat portion.

6. The battery stack according to claim 5, wherein the entire deformable portion in the circumferential direction of the ring of the restraint hoop member contacts the flat portion.

7. An annular restraint hoop member that restrains in the array direction a plurality of single cells arranged in the array direction and a pair of end plates disposed at both ends of the plurality of single cells in the array direction and having a first surface facing the single cell and a second surface on the side opposite to the first surface, the plurality of single cells and the pair of end plates are disposed inside the ring of the restraint hoop member, the restraint hoop member is provided with a deformable portion that partially facilitates deformation in the circumferential direction of the ring of the restraint hoop member, the deformable portion contacts the second surface, a restraint hoop member.

8. The restraint hoop member according to claim 7, wherein the deformable portion is provided at two locations in the circumferential direction at equal intervals.

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