Seal structure and back-up material
Patent Information
- Application Number
- JP2023022987
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2023-02-17
- Publication Date
- 2025-09-10
AI Technical Summary
Conventional seal structures experience damage such as cracking in the O-ring due to high pressure, as the backup ring, being harder than the O-ring, causes the O-ring to deform and fill the gap between the backup ring and the seal groove bottom.
The seal structure incorporates a backup material with an inclined surface that follows the low-pressure side groove bottom, preventing deformation and gap formation between the backup material and the seal groove, thereby maintaining the seal even under high pressure.
This configuration effectively suppresses damage to the seal material by preventing deformation and gap formation, ensuring a stable seal under high-pressure conditions.
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Abstract
Description
[Technical field]
[0001] The present invention relates to a seal structure and a backup material. [Background technology]
[0002] As an example of a sealed structure, there are technologies disclosed in International Publication No. WO2004 / 061353 (Patent Document 1), Japanese Patent No. 4636281 (Patent Document 2), Japanese Patent No. 4949492 (Patent Document 3), and Japanese Patent No. 5126462 (Patent Document 4), which disclose sealing structures for gases (such as high-pressure hydrogen).
[0003] Here, a structure in which a sealing O-ring is arranged on the high pressure side of the seal groove and a backup ring is arranged on the low pressure side will be described as an example of a seal structure with reference to Figures 15 to 17. Figure 15 is an enlarged cross-sectional view showing a conventional seal structure (before high pressure operation), Figure 16 is an enlarged cross-sectional view showing the conventional seal structure (after high pressure operation), and Figure 17 is an enlarged cross-sectional view showing a problem with the conventional seal structure (after high pressure operation).
[0004] 15, a cylindrical shaft 1A is accommodated in a cylindrical groove 2A. The outer peripheral surface of the shaft 1A constitutes a first seal surface 1a. The inner peripheral surface of the cylindrical groove 2A constitutes a second seal surface 2a. Gas is sent into the cylindrical groove 2A from a through hole 1h of the shaft 1A, creating a high pressure state inside the cylindrical groove 2A.
[0005] An annular seal groove 1G is provided on the outer circumferential surface of the shaft 1A, recessed inward from the first seal surface 1a. In the seal groove 1G, a sealing O-ring 10X is disposed on the high pressure side, and a backup ring 40X supporting the O-ring 10X is disposed on the low pressure side. The cross-sectional view shown in Fig. 14 illustrates the O-ring 10X and backup ring 40X in an undeformed state.
[0006] 16 shows a state in which the O-ring 10X and the backup ring 40X are housed in the seal groove 1G. In this state, the O-ring 10X is sandwiched between the bottom 1Gs of the seal groove 1G and the second seal surface 2a and deformed into an elliptical shape. In this figure, gas pressure is not applied to the O-ring 10X from the high-pressure side (the lower side of the figure), so the O-ring 10X does not move toward the backup ring 40X.
[0007] 17 shows a state in which gas pressure is applied to the O-ring 10X from the high pressure side (lower side of the figure). When high pressure is applied to the O-ring 10X, as shown in the figure, the O-ring 10X moves toward the backup ring 40X. At this time, the backup ring 40X suppresses the movement of the O-ring 10X from the low pressure side (upper side of the figure), thereby maintaining the seal structure by the O-ring 10X that separates the high pressure side from the low pressure side. [Prior art documents] [Patent documents]
[0008] [Patent Document 1] International Publication No. WO2004 / 061353 [Patent Document 2] Patent No. 4636281 [Patent Document 3] Patent No. 4949492 [Patent Document 4] Patent No. 5126462 Summary of the Invention [Problem to be solved by the invention]
[0009] 17, in the conventional seal structure, when high pressure is applied to the O-ring 10X, the O-ring 10X is pressed against the backup ring 40X. Because the backup ring 40X is made of a harder material than the O-ring 10X, if pressure continues to be applied to the O-ring 10X, a deformation region 10y is formed in which part of the O-ring 10X enters the gap h generated between the backup ring 40X and the bottom 1Gs of the seal groove 1G.
[0010] If such a deformation region 10y is formed in the O-ring 10X, there is a risk that damage such as cracks will occur in the O-ring 10X, starting from the deformation region 10y.
[0011] The object of the present invention is to solve the above-mentioned problems, and to provide a sealing structure and back-up material that have a configuration that can suppress the occurrence of damage such as cracks in the sealing material, even when high pressure is applied to the sealing material, in a sealing structure composed of a sealing material and a back-up material. [Means for solving the problem]
[0012] [1]: The seal structure disclosed herein is a seal structure separating a high pressure side and a low pressure side, and when viewed in longitudinal section, comprises: one member having a first seal surface; a seal groove provided on the first seal surface; a seal material mounted in the seal groove and positioned on the high pressure side; a backup material mounted in the seal groove and positioned on the low pressure side; and a counterpart member having a second seal surface positioned opposite the one member and abutting against the seal material to form the seal structure, and has the following configuration:
[0013] The seal groove has, on the high pressure side, a high pressure groove bottom located at a fixed distance from the second seal surface, and, on the low pressure side, a low pressure groove bottom that is continuous with the high pressure groove bottom and approaches the second seal surface as it moves from the high pressure groove bottom to the low pressure side, the seal material is arranged between the high pressure groove bottom and the second seal surface, and the backup material is arranged between the low pressure groove bottom and the second seal surface, and the backup material has an abutment surface shaped to follow the low pressure groove bottom on the side facing the low pressure groove bottom.
[0014] [2]: The seal structure according to [1], wherein the backup material is an inclined surface on the side facing the second seal surface that inclines away from the second seal surface as it approaches the low pressure side.
[0015] [3]: A seal structure according to [1] or [2], wherein the low-pressure gutter bottom is formed by a portion of a circular arc.
[0016] [4]: A seal structure according to any one of [1] to [3], wherein the first seal surface is an outer peripheral surface of a cylindrical member, the second seal surface is a bottomed cylindrical inner peripheral surface provided in a housing, accommodating the cylindrical member, and in contact with the first seal surface, the seal groove is an annular groove provided in the outer peripheral surface of the cylindrical member, and the seal material and the backup material have an annular shape.
[0017] [5]: The backup material disclosed herein has an annular shape and is used together with a seal material in a seal groove provided in a seal structure that separates a high pressure side and a low pressure side, and the cross section of the backup material when viewed in a longitudinal section includes a first surface, a second surface rising from a radially outer end of the first surface, a third surface rising from a radially inner end of the first surface and facing the second surface, and a fourth surface connecting the second surface and the third surface and facing the first surface, and the third surface is inclined toward the fourth surface as it approaches the second surface.
[0018] [6]: In the backup material described in [5], the third surface includes a curved surface that bulges inward.
[0019] [7]: In the backup material described in [5] or [6], the second surface is inclined toward the fourth surface and therefore closer to the third surface. Effect of the Invention
[0020] This sealing structure and back-up material make it possible to provide a sealing structure and back-up material that has a configuration that can suppress the occurrence of damage such as cracks in the sealing material, even when high pressure is applied to the sealing material, in a sealing structure composed of a sealing material and a back-up material. [Brief description of the drawings]
[0021] [Figure 1] 1 is a cross-sectional view showing a sealed structure in the first embodiment. [Diagram 2] 2 is a partially enlarged cross-sectional view showing the seal structure in the first embodiment. FIG. [Diagram 3] 3 is a partially enlarged cross-sectional view showing a seal groove employed in the seal structure in the first embodiment. FIG. [Figure 4] FIG. 2 is a plan view of the backup ring in the first embodiment. [Diagram 5] 5 is a cross-sectional view taken along line VV in FIG. 4. [Figure 6] 3 is a partially enlarged cross-sectional view showing a sealed state of the seal structure in the first embodiment. FIG. [Figure 7] 5A to 5C are schematic diagrams showing changes in the sealing state of the seal structure in the first embodiment. [Figure 8] FIG. 11 is a plan view of a backup ring in the second embodiment. [Figure 9] 9 is a cross-sectional view taken along line IX-IX in FIG. 8. [Figure 10] FIG. 11 is a plan view of a backup ring according to a third embodiment. [Figure 11]11 is a cross-sectional view taken along line XI-XI in FIG. [Figure 12] FIG. 13 is a plan view of a backup ring in the fourth embodiment. [Figure 13] 13 is a cross-sectional view taken along line XIII-XIII in FIG. 10. [Figure 14] FIG. 13 is a cross-sectional view showing a sealed structure in a fifth embodiment. [Figure 15] FIG. 1 is an enlarged cross-sectional view showing a conventional seal structure (before high pressure operation). [Figure 16] FIG. 1 is an enlarged cross-sectional view showing a conventional seal structure (after high pressure operation). [Figure 17] FIG. 1 is an enlarged cross-sectional view showing a problem with a conventional seal structure (after high pressure operation). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] The seal structure in each embodiment will be described below with reference to the drawings. In the embodiments described below, when the number, amount, etc. are mentioned, the scope of the present invention is not necessarily limited to the number, amount, etc., unless otherwise specified. In addition, the same reference numbers are given to the same parts and corresponding parts, and duplicated descriptions may not be repeated. In the following description, for convenience of explanation, the positional relationship is clearly indicated using the words up and down, but this does not exclude a configuration in which the up and down arrangement is upside down or a configuration in which the parts are arranged left and right.
[0023] (Embodiment 1) The seal structure of the present embodiment will be described with reference to Fig. 1 to Fig. 5. Fig. 1 is a cross-sectional view showing a sealed structure 1000 in which a seal structure 100 is adopted, Fig. 2 is a partially enlarged cross-sectional view showing the seal structure 100, Fig. 3 is a partially enlarged cross-sectional view showing a seal groove adopted in the seal structure 100, Fig. 4 is a plan view of a backup ring, and Fig. 5 is a cross-sectional view taken along line VV in Fig. 4.
[0024] (Sealed structure 1000) 1, a sealed structure 1000 of this embodiment employs a seal structure for gas (high-pressure hydrogen, etc.), and includes a header 1 and a housing 2. A cylindrical shaft 1A extending toward the housing 2 is provided in the header 1. A through hole 1h through which gas is fed is provided in the center of the shaft 1A.
[0025] The housing 2 is provided with a cylindrical groove 2A with a bottom in which the shaft 1A is accommodated. The outer peripheral surface of the shaft 1A constitutes a first seal surface 1a provided on the one material side. The inner peripheral surface of the cylindrical groove 2A constitutes a second seal surface 2a provided on the other material side. When gas is sent into the cylindrical groove 2A from the through hole 1h of the shaft 1A, the inside of the cylindrical groove 2A becomes a high-pressure state.
[0026] The header 1 and the housing 2 are fastened by bolts 3 with an O-ring 30 interposed therebetween, thereby maintaining the airtightness of the seal structure 100 described below.
[0027] (Seal structure 100) The seal structure 100 will be described in detail with reference to Fig. 2. An annular seal groove 1G recessed inward from the first seal surface 1a is provided on the outer circumferential surface of the shaft 1A. A seal ring is disposed as a seal material on the high pressure side of the seal groove 1G, and in this embodiment, an O-ring 10 is disposed. A backup ring 20 is disposed as a backup material on the low pressure side of the seal groove 1G. The cross-sectional view shown in Fig. 2 illustrates the O-ring 10 and the backup ring 20 in an undeformed state.
[0028] The cross-sectional shape of the O-ring 10 is a typical circular shape, and a commercially available O-ring can be used. Note that, instead of a commercially available O-ring, a sealing material for fixed applications such as a U-packing may be used as the seal ring. The material of the backup ring 20 is preferably harder than the material of the O-ring 10 and softer than the material of the shaft 1A, such as PTFE (polytetrafluoroethylene), POM (polyoxymethylene), or PEEK (polyether ether ketone). Details of the cross-sectional shape of the backup ring 20 and the function of the seal structure 100 will be described later.
[0029] (Cross-sectional shape of seal groove 1G) The cross-sectional shape of the seal groove 1G will be described with reference to Fig. 3. The cross section of Fig. 3 shows a cross section of one of the grooves as viewed in a longitudinal section cut along a plane including the shaft, with the axial direction of the shaft 1A being the vertical direction.
[0030] The seal groove 1G has a high-pressure groove bottom 1Ga provided at a position spaced a certain distance from the second seal surface 2a on the high-pressure side, and a low-pressure groove bottom 1Gb that is continuous with the high-pressure groove bottom 1Ga on the low-pressure side and approaches the second seal surface 2a from the high-pressure groove bottom 1Ga toward the low-pressure side. The low-pressure side end of the low-pressure groove bottom 1Gb intersects with the first seal surface 1a.
[0031] The low-pressure gutter bottom 1Gb is formed by a part of a circular arc with a radius Ra. When the depth of the high-pressure gutter bottom 1Ga is W, it is preferable that the radius Ra satisfies Ra>W.
[0032] (Shape of backup ring 20) 4 and 5, the overall shape and cross-sectional shape of the backup ring 20 will be described. The backup ring 20 has an annular shape. The backup ring 20 is provided with cut surfaces 20k so as to be mounted in the seal groove 1G of the cylindrical shaft 1A.
[0033] The cross-sectional shape of the backup ring 20 is approximately rectangular, having a bottom surface 20a located on the high-pressure side (O-ring 10 side), a side surface 20b located on the second seal surface 2a side, an abutment surface 20c shaped to conform to the low-pressure groove bottom 1Gb on the side facing the low-pressure groove bottom 1Gb, and an upper surface 20d located on the bottom pressure side.
[0034] The shape of the contact surface 20c that conforms to the low-pressure gutter bottom 1Gb includes a case where a slight gap occurs between the contact surface 20c and the low-pressure gutter bottom 1Gb, as described later. When the radius Ra of the low-pressure gutter bottom 1Gb shown in FIG. 3 is R2.6 mm, the radius Rb of the contact surface 20c is a curved surface with R2.4 mm.
[0035] The bottom surface 20a corresponds to the first surface, and the side surface 20b corresponds to the second surface rising from the radially outer end of the bottom surface 20a. The abutment surface 20c corresponds to the third surface rising from the radially inner end of the bottom surface 20a and facing the side surface 20b. The top surface 20d connects the side surface 20b and the abutment surface 20c and corresponds to the fourth surface facing the bottom surface 20a. The abutment surface 20c is inclined toward the top surface 20d so as to approach the side surface 20b. Specifically, the abutment surface 20c includes a curved surface that bulges inward.
[0036] According to the backup ring 20 of this embodiment, the side surface 20b located on the second seal surface 2a side is inclined radially inward from the bottom surface 20a toward the top surface 20d. Specifically, the side surface 20b is inclined toward the top surface 20d side so as to approach the contact surface 20c.
[0037] In this embodiment, the angle (α) between the second seal surface 2a and the side surface 20b is approximately 85 degrees. As a result, the relationship between the diameter φA on the bottom surface 20a side of the backup ring 20 and the diameter φB on the top surface 20d side is diameter φA > diameter φB.
[0038] (Function of seal structure 100) Next, the function of the seal structure 100 will be described with reference to Fig. 6 and Fig. 7. Fig. 6 is a partially enlarged cross-sectional view showing the sealing state of the seal structure 100, and Fig. 7 is a schematic diagram showing the change in the sealing state of the seal structure 100.
[0039] 6 shows a state in which the O-ring 10 and the backup ring 20 are housed in the seal groove 1G. In this state, the O-ring 10 is sandwiched between the bottom 1Gs of the seal groove 1G and the second seal surface 2a and is deformed into an elliptical shape. In this figure, gas pressure is not applied to the O-ring 10 from the high-pressure side (the lower side of the figure), so the O-ring 10 does not move toward the backup ring 20.
[0040] 7, the states in which the gas pressure gradually increases on the O-ring 10X are shown as state I, state II, and state III. State I is the same as the state shown in Fig. 6. State III shows the state in which the pressurizing force on the backup ring 20 is at its highest.
[0041] In state I, no large pressure is applied to the backup ring 20 from the O-ring 10. Therefore, there is a clearance between the side surface 20b and the first seal surface 1a, and the line L1 on the bottom surface 20a does not move (rotate).
[0042] In state II, pressure begins to be applied to the backup ring 20 from the O-ring 10. Therefore, since the abutment surface 20c has a shape that follows the low-pressure groove bottom 1Gb, the backup ring 20 begins to move (rotate) along the low-pressure groove bottom 1Gb. As a result, the clearance between the side surface 20b and the first seal surface 1a becomes smaller, and the line L1 of the bottom surface 20a begins to move (clockwise) in the figure.
[0043] In state III, further pressure is applied to the backup ring 20 from the O-ring 10X. Therefore, the backup ring 20 moves further (clockwise) along the low-pressure groove bottom 1Gb. As a result, the clearance between the side surface 20b and the first seal surface 1a disappears, and the line L1 of the bottom surface 20a moves further (clockwise) in the figure.
[0044] In this way, when pressure is applied to the backup ring 20 from the O-ring 10, the backup ring 20 moves (rotates) along the low-pressure groove bottom 1Gb. As a result, no gap is generated between the backup ring 20 and the low-pressure groove bottom 1Gb as in the conventional structure, and no deformation region is generated in the backup ring 20. As a result, it is possible to suppress the occurrence of damage to the backup ring 20.
[0045] As described above, according to this embodiment, the low-pressure side bottom (low-pressure groove bottom 1Gb) of the seal groove 1G in which the backup ring 20 is housed is made round, and the abutment surface 20c of the backup ring 20 that faces this bottom is shaped to conform to the low-pressure groove bottom 1Gb, so that no gap is generated between the backup ring 20 and the seal groove 1G. As a result, even when high pressure is applied to the O-ring 10, deformation of the O-ring 10 can be suppressed.
[0046] In the following embodiments, other embodiments of the cross-sectional shape of the backup ring will be described.
[0047] (Embodiment 2: Backup ring 20A) The overall shape and cross-sectional shape of another embodiment of backup ring 20A will be described with reference to Figures 8 and 9. Figure 8 is a plan view of backup ring 20A, and Figure 9 is a cross-sectional view taken along line IX-IX in Figure 8. The basic configuration is the same as the backup ring 20 described above, so only the differences will be described below.
[0048] The above-mentioned backup ring 20, when viewed in cross section, has a generally rectangular shape having a bottom surface 20a, a side surface 20b, a contact surface 20c, and a top surface 20d. However, backup ring 20A does not have a top surface 20d, and has a generally triangular shape in which the curved contact surface 20c intersects with the side surface 20b.
[0049] With the backup ring 20A having this cross-sectional shape, the same effects as those of the backup ring 20 described above can be obtained.
[0050] (Embodiment 3: Backup ring 20B) The overall shape and cross-sectional shape of a backup ring 20B in still another embodiment will be described with reference to Figures 10 and 11. Figure 10 is a plan view of backup ring 20B, and Figure 11 is a cross-sectional view taken along line XI-XI in Figure 10. The basic configuration is the same as the backup ring 20A described above, so only the differences will be described below.
[0051] In the backup ring 20A, the curved contact surface 20c intersects with the side surface 20b, but in this embodiment, the contact surface 20c has a curved portion 20c1 on the high pressure side and a straight portion 20c2 on the low pressure side.
[0052] Even with this configuration, the curved surface portion 20c1 can move (rotate) along the low-pressure groove bottom portion 1Gb, so that the same effects as those of the backup ring 20A can be obtained.
[0053] (Embodiment 4: Backup ring 20C) The overall shape and cross-sectional shape of a backup ring 20C in still another embodiment will be described with reference to Figures 12 and 13. Figure 12 is a plan view of backup ring 20C, and Figure 13 is a cross-sectional view taken along line XIII-XIII in Figure 12. The basic configuration is the same as the backup ring 20A described above, so only the differences will be described below.
[0054] The contact surface 20c of the backup ring 20C of the present embodiment has a curved surface portion 20c1 (radius Ra) and a curved surface portion 20c3 (radius Rb) having different curvatures.
[0055] Even with this configuration, the contact surface 20c can move (rotate) along the low-pressure groove bottom portion 1Gb, so that the same effects as those of the backup ring 20A can be obtained.
[0056] In each of the above embodiments, a configuration has been described in which a seal groove 1G is provided in the shaft 1A and a seal material and a backup material are provided in the seal groove 1G. However, for example, when adopting a seal structure of the above configuration between a sliding rod and a housing, a seal groove may be provided on the housing side.
[0057] (Embodiment 5: Sealed structure 1000A) With reference to Fig. 14, a case where the above-mentioned seal structure 100 is applied to a sealing structure 1000A different from the above-mentioned sealing structure 1000 will be described. Fig. 14 is a cross-sectional view showing the sealing structure 1000A. The cross-sectional view shown in Fig. 14 illustrates the O-ring 10 and the backup ring 20 in an undeformed state.
[0058] In this sealed structure 1000A, an annular seal groove 1G is provided on the flat surface of an annular flange 200f provided on a housing 200 having a pipe 200h provided in the center. The flat surface of the annular flange 200f constitutes a first seal surface 200a, and the lower surface of a plate 300 constitutes a second seal surface 300a. The plate 300 is fixed to the annular flange 200f at a position not shown.
[0059] Since the pipeline 200h is the high pressure side, in the seal groove 1G, a high pressure side groove bottom 1Ga is provided on the pipeline 200h side, and a low pressure side groove bottom 1Gb is provided outside the high pressure side groove bottom 1Ga. An O-ring 10 is disposed on the pipeline 200h side, and a backup ring 20 is disposed outside the O-ring 10. The shape of the backup ring 20 may be the backup ring 20A, backup ring 20B, or backup ring 20C shown in the above-mentioned embodiments 2 to 4.
[0060] In the sealed structure 1000A in the present embodiment, it is possible to obtain the same effects as those of the sealed structure 1000 in the first embodiment.
[0061] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]
[0062] 1 header, 1A shaft, 1G seal groove, 1Ga high pressure side groove bottom, 1Gb low pressure side groove bottom, 1Gs, 20a bottom surface, 1a, 200a first seal surface, 1h through hole, 2,200 housing, 2A cylindrical groove, 2a, 300a second seal surface, 3 bolt, 10, 10X, 30 O-ring, 10y deformation area, 20, 20A, 20B, 20C, 40X backup ring, 20b side, 20c abutment surface, 20c1, 20c3 curved surface portion, 20c2 straight portion, 20d upper surface, 20k cut surface, 50, 300 plate, 200h pipe, 200f annular flange, 100 seal structure, 200h pipe, 1000, 1000A sealed structure.
Claims
1. A seal structure that separates a high pressure side from a low pressure side, When viewed from a vertical cross-sectional view, one member having a first sealing surface; a seal groove provided on the first seal surface; A seal material that is installed in the seal groove and disposed on a high pressure side; A backup material that is installed in the seal groove and disposed on the low pressure side; a counterpart member having a second seal surface disposed opposite the one member and abutting against the seal member to form the seal structure; Equipped with The seal groove is A high pressure side groove bottom portion provided at a position spaced a certain distance from the second seal surface on the high pressure side; a low-pressure groove bottom portion that is continuous with the high-pressure groove bottom portion on the low-pressure side and approaches the second seal surface side as it moves from the high-pressure groove bottom portion to the low-pressure side; having the sealant is disposed between the high pressure side groove bottom and the second seal surface; The backup material is disposed between the low-pressure gutter bottom and the second seal surface, The backup material has a contact surface on a side facing the low-pressure gutter bottom, the contact surface being shaped to fit along the low-pressure gutter bottom. Seal structure.
2. The backup material is an inclined surface that is inclined away from the second seal surface toward the low pressure side on the side facing the second seal surface. The seal structure according to claim 1 .
3. The low pressure gutter bottom is formed by a part of a circular arc. The seal structure according to claim 1 .
4. the first sealing surface is an outer circumferential surface of a cylindrical member, the second seal surface is a bottomed cylindrical inner circumferential surface provided in a housing, accommodating the cylindrical member, and in contact with the first seal surface; the seal groove is an annular groove provided in an outer circumferential surface of the cylindrical member, The sealing material and the back-up material have an annular shape. The seal structure according to claim 1 .
5. A backup material having an annular shape is used together with a seal material in a seal groove provided in a seal structure that separates a high pressure side from a low pressure side, When viewed from the longitudinal cross-sectional structure of the backup material, the cross section is as follows: The first page and a second surface rising from a radially outer end of the first surface; a third surface rising from a radially inner end of the first surface and facing the second surface; a fourth surface connecting the second surface and the third surface and facing the first surface, The third surface is inclined toward the fourth surface side and thus toward the second surface side. Backup material.
6. The third surface includes a curved surface that bulges inward. The back-up material according to claim 5 .
7. The second surface is inclined toward the fourth surface side and thus toward the third surface side. The back-up material according to claim 5 .