Shaft body assembly structure and motor valve

Abstract

To provide a shaft body assembly structure that eliminates the need to remove shavings generated when a shaft body is press-fitted into a through hole, and a motor valve having the same.SOLUTION: A motor valve has a resin output gear 56 and a metal output shaft 57. The output gear 56 has a circular through hole 562a. A press-fit end part 573 of the output shaft 57 is press-fitted into the through hole 562a. The press-fit end part 573 has a cylindrical part 573a having a circular cross section, and a ridge part 573b in which a plurality of ridges 573c extending in an axial direction is arranged in a circumferential direction, which are arranged in order from its tip side. A diameter of the cylindrical part 573a is equal to or smaller than a diameter of the through hole 562a. A diameter of an imaginary circle connecting the tips of the plurality of ridges 573c is larger than the diameter of the through hole 562a. A portion of an outer peripheral surface of the cylindrical part 573a that is in contact with an inner peripheral surface of the through hole 562a is larger than a portion of the outer peripheral surface that is arranged outside the through hole 562a.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a shaft assembly structure, an electric valve having the same, and a shaft assembly method.

Background Art

[0002] Patent Document 1 discloses a conventional electric valve. The electric valve of Patent Document 1 has a reduction gear device. The reduction gear device has an output gear and an output shaft. The output gear is made of resin and has a bottomed cylindrical shape. A through hole is formed in the bottom wall portion of the output gear. The output shaft is made of metal and is press-fitted into the through hole of the output gear.

[0003] The output gear is manufactured by injection molding. The gate is arranged at a position corresponding to the inner peripheral surface of the through hole in the bottom wall portion of the mold for the output gear, and gate residue is generated on the inner peripheral surface of the through hole. The gate residue is scraped off from the inner peripheral surface and pushed out of the through hole by the output shaft press-fitted into the through hole.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the chips generated when the output shaft is press-fitted into the through hole may be held between the outer peripheral surface of the output shaft and the inner peripheral surface of the through hole in a state where they protrude from the through hole. Therefore, an operation for removing the chips is required.

[0006] Therefore, an object of the present invention is to provide a shaft assembly structure in which chips generated when a shaft body is press-fitted into a through hole do not need to be removed, an electric valve having the same, and a shaft assembly method.

Means for Solving the Problems

[0007] In order to achieve the above object, a shaft assembly structure according to one aspect of the present invention is a shaft assembly structure for assembling a resin gear and a metal shaft, wherein the gear has a circular through-hole, and an end portion of the shaft body press-fitted into the through-hole has, in order from the tip side of the end portion, a cylindrical portion having a circular cross-section, and a rib portion in which a plurality of axially extending ribs are arranged side by side in the circumferential direction, the diameter of the cylindrical portion is equal to or less than the diameter of the through-hole, and the diameter of a virtual circle connecting the tips of the plurality of ribs of the rib portion is larger than the diameter of the through-hole.

[0008] In the present invention, it is preferable that the axial length of the cylindrical portion is 1 / 10 or more of the axial length of the rib portion.

[0009] In the present invention, it is preferable that an inclined surface that is connected to the outer peripheral surface of the cylindrical portion and extends radially outward as it moves axially away from the cylindrical portion is formed at an end portion of the plurality of ribs on the cylindrical portion side.

[0010] In order to achieve the above object, an electric valve according to another aspect of the present invention is an electric valve having a valve body having a valve port, a valve element disposed opposite to the valve port, and a drive mechanism for driving the valve element, wherein the drive mechanism has a resin output gear and a metal output shaft, and the output gear and the output shaft are assembled by the shaft assembly structure.

[0011] In order to achieve the above object, a shaft body assembling method according to another aspect of the present invention is a shaft body assembling method for assembling a resin gear and a metal shaft body. The gear is manufactured by injection molding using a mold in which a gate is arranged at a position corresponding to the inner peripheral surface of the circular through hole of the gear. At the end of the shaft body, a cylindrical portion having a circular cross section arranged in order from the tip side of the end portion, and a plurality of ridges extending in the axial direction are arranged side by side in the circumferential direction. A ridge portion is formed. The cylindrical portion is inserted into the through hole while sliding the outer peripheral surface of the cylindrical portion on the inner peripheral surface of the through hole, and the ridge portion is press-fitted into the through hole while biting the plurality of ridges of the ridge portion into the inner peripheral surface of the through hole.

Advantages of the Invention

[0012] According to the present invention, the gear has a circular through hole. The end portion of the shaft body that is press-fitted into the through hole has, in order from the tip side thereof, a cylindrical portion having a circular cross section, and a plurality of ridges extending in the axial direction are arranged side by side in the circumferential direction. A ridge portion is provided. The diameter of the cylindrical portion is equal to or smaller than the diameter of the through hole. The diameter of the virtual circle connecting the tips of the plurality of ridges of the ridge portion is larger than the diameter of the through hole. As a result, the cylindrical portion is inserted into the through hole while the outer peripheral surface of the cylindrical portion is slid on the inner peripheral surface of the through hole. When there is gate residue on the inner peripheral surface of the through hole, the gate residue is held in a state of being crushed between the outer peripheral surface of the cylindrical portion and the inner peripheral surface of the through hole. Then, the ridge portion is press-fitted into the through hole while the plurality of ridges of the ridge portion bite into the inner peripheral surface of the through hole. When the inner peripheral surface of the through hole and the gate residue are scraped by the plurality of ridges of the ridge portion, the scrapings are held between the plurality of ridges and the inner peripheral surface of the through hole. Since the ridge portion is located at a position deep from the opening of the through hole, it is possible to suppress the scrapings from protruding from the through hole. Therefore, in the assembly of the shaft body, the operation of removing the scrapings can be made unnecessary.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiment for Carrying Out the Invention

[0014] Hereinafter, an electric valve according to an embodiment of the present invention will be described with reference to FIGS. 1 to 4. The electric valve 1 of this embodiment is used, for example, to adjust the refrigerant flow rate in a refrigeration cycle or the like.

[0015] FIG. 1 is a longitudinal sectional view of an electric valve according to an embodiment of the present invention. FIG. 2 is a sectional view of the output gear and the output shaft of the electric valve shown in FIG. 1. FIG. 3 is a diagram for explaining the output shaft of FIG. 2. FIG. 3A is a front view of the output shaft. FIG. 3B is a plan view of the output shaft. FIG. 4 is a diagram for explaining the assembling method of the output gear and the output shaft of FIG. 2. FIG. 4A is a sectional view showing the state before assembling the output gear and the output shaft. FIG. 4B is a sectional view showing the state where the cylindrical portion of the output shaft is inserted into the through hole of the output gear. FIG. 4C is a sectional view showing the state where the ribbed portion of the output shaft is press-fitted into the through hole of the output gear.

[0016] As shown in FIG. 1, the electric valve 1 according to this embodiment has a valve body 10, a holder 20, a valve body support member 25, a cam 30, a drive mechanism 40, a valve body 70, and a stator unit 80.

[0017] The valve body 10 has a rectangular parallelepiped shape. The valve body 10 has a valve chamber 13 and a valve port 14 connected to the valve chamber 13. The valve body 10 has a first passage 17 and a second passage 18. One end of the first passage 17 is connected to the valve chamber 13, and the other end of the first passage 17 opens to the left side surface 10a of the valve body 10. One end of the second passage 18 is connected to the valve chamber 13 via the valve port 14, and the other end of the second passage 18 opens to the right side surface 10b of the valve body 10. The valve body 10 has a mounting hole 19. The mounting hole 19 opens to the upper surface 10c of the valve body 10. A female thread is formed on the inner peripheral surface of the mounting hole 19. The valve chamber 13 opens to the bottom surface 19a of the mounting hole 19.

[0018] The holder 20 has a cylindrical shape. A male thread is formed on the lower part of the outer peripheral surface of the holder 20. The male thread of the holder 20 is screwed into the female thread of the mounting hole 19 of the valve body 10. The holder 20 is attached to the valve body 10 by a screw structure.

[0019] The valve body support member 25 has a cylindrical shape. The valve body support member 25 is disposed between the valve body 10 and the holder 20 inside the mounting hole 19. The lower part of the valve body support member 25 is press-fitted into the valve chamber 13 from the side of the mounting hole 19. An annular flat surface 25a facing downward is formed on the outer peripheral surface of the valve body support member 25. The annular flat surface 25a is in contact with the bottom surface 19a of the mounting hole 19. The valve body support member 25 supports the valve body 70 so as to be movable in the vertical direction.

[0020] The cam 30 has a cylindrical shape with its upper end closed and its lower end open. The lower end of the cam 30 is joined to the outer peripheral edge of a joint member 35 having an annular plate shape. The upper part of the holder 20 is disposed inside the joint member 35. The inner peripheral edge of the joint member 35 is joined to the holder 20.

[0021] The drive mechanism 40 moves the valve body 70 in the vertical direction. The drive mechanism 40 has a magnet rotor 41, a planetary gear mechanism 50, a guide member 60, a drive shaft 65, and a ball 68.

[0022] The magnet rotor 41 has a cylindrical shape. On the outer peripheral surface of the magnet rotor 41, N poles and S poles are alternately arranged in the circumferential direction. The outer diameter of the magnet rotor 41 is smaller than the inner diameter of the can 30. The magnet rotor 41 is rotatably arranged inside the can 30. A disk-shaped connecting member 42 is joined to the upper end portion of the magnet rotor 41. The connecting member 42 closes the upper end portion of the magnet rotor 41. The rotor shaft 43 passes through the center of the connecting member 42. The magnet rotor 41 is connected to the rotor shaft 43 via the connecting member 42.

[0023] The planetary gear mechanism 50 is arranged inside the magnet rotor 41. The planetary gear mechanism 50 includes a gear case 51, a fixed ring gear 52, a sun gear 53, a plurality of planetary gears 54, a carrier 55, an output gear 56, and an output shaft 57. The gear case 51 has a cylindrical shape. The gear case 51 is coaxially joined to the upper end portion of the holder 20. The fixed ring gear 52 is an internal gear. The fixed ring gear 52 is fixed to the upper end portion of the gear case 51. The sun gear 53 is arranged coaxially with the connecting member 42. The sun gear 53 is integrated with the connecting member 42. The rotor shaft 43 passes through the sun gear 53. The sun gear 53 rotates together with the magnet rotor 41 and the connecting member 42. The plurality of planetary gears 54 are arranged between the fixed ring gear 52 and the sun gear 53. The carrier 55 has a disk shape. The rotor shaft 43 passes through the center of the carrier 55. The carrier 55 is rotatable about the rotor shaft 43. The carrier 55 has a plurality of support shafts 55a. The plurality of support shafts 55a rotatably support the plurality of planetary gears 54. The output gear 56 has a bottomed cylindrical shape. The output gear 56 is an internal gear. The plurality of planetary gears 54 are arranged between the output gear 56 and the sun gear 53. The output shaft 57 is assembled with the output gear 56. The rotation of the sun gear 53 is decelerated by the fixed ring gear 52, the plurality of planetary gears 54, the carrier 55, and the output gear 56 and transmitted to the output shaft 57.

[0024] Here, the configuration of the output gear 56 and the output shaft 57 will be described with reference to FIGS. 2 and 3.

[0025] The output gear 56 is made of a synthetic resin such as polyphenylene sulfide (PPS). As shown in FIG. 2, the output gear 56 integrally has a peripheral wall portion 561 and a bottom wall portion 562. The peripheral wall portion 561 has a cylindrical shape. A plurality of teeth of an internal gear are formed on the inner peripheral surface of the peripheral wall portion 561. The bottom wall portion 562 has a disc shape. The bottom wall portion 562 is connected to the lower end portion of the peripheral wall portion 561. A circular through-hole 562a is formed at the center of the bottom wall portion 562.

[0026] The output shaft 57 is a shaft body. The output shaft 57 is made of a metal such as stainless steel. As shown in FIGS. 2 and 3, the output shaft 57 integrally has a connection portion 571, a stopper portion 572, and a press-fitting end portion 573. The connection portion 571 has a cylindrical shape. The stopper portion 572 has a disc shape. The diameter of the stopper portion 572 is larger than the diameter of the connection portion 571. The stopper portion 572 is coaxially connected to the upper end portion of the connection portion 571.

[0027] The press-fitting end portion 573 is press-fitted into the through-hole 562a of the output gear 56. The press-fitting end portion 573 has a cylindrical portion 573a and a rib portion 573b, which are arranged in order from the tip side (upper end side) of the press-fitting end portion 573. The cylindrical portion 573a has a circular cross-section (a cross-section perpendicular to the axial direction). The up-and-down direction in each drawing is the axial direction of the output shaft 57. The diameter of the cylindrical portion 573a is constant throughout the axial direction. The diameter of the cylindrical portion 573a is the same as the diameter of the through-hole 562a of the output gear 56 or slightly smaller than the diameter of the through-hole 562a. The diameter of the cylindrical portion 573a is less than or equal to the diameter of the through-hole 562a, and when the cylindrical portion 573a is inserted into the through-hole 562a, the outer peripheral surface of the cylindrical portion 573a slides on the inner peripheral surface of the through-hole 562a. The axial length of the cylindrical portion 573a is 1 / 10 or more of the axial length of the rib portion 573b. More preferably, the axial length of the cylindrical portion 573a is 1 / 5 or more of the axial length of the rib portion 573b. Also, preferably, the axial length of the cylindrical portion 573a is 1 / 2 or less of the axial length of the rib portion 573b. The rib portion 573b has a plurality of ribs 573c. The plurality of ribs 573c extend in the axial direction and are arranged side by side in the circumferential direction. Each rib 573c has a triangular cross-section. Each rib 573c has an inclined surface 573d at the end on the cylindrical portion 573a side. The inclined surface 573d is connected to the outer peripheral surface of the cylindrical portion 573a and is a surface that faces radially outward as it moves away from the cylindrical portion 573a in the axial direction. The diameter of the virtual circle VC connecting the tips (the portions most radially outward) of the plurality of ribs 573c is larger than the diameter of the output gear 56. Also, the diameter of the virtual circle VC is smaller than the diameter of the stopper portion 572. The rib portion 573b is formed, for example, by knurling.

[0028] A slit 57a extending upward is formed in the output shaft 57 from the lower end surface of the connecting portion 571. A support hole 57b extending downward is formed in the output shaft 57 from the upper end surface of the press-fitting end portion 573. The lower end portion of the rotor shaft 43 is disposed inside the support hole 57b. The support hole 57b rotatably supports the lower end portion of the rotor shaft 43.

[0029] A method for assembling a shaft body that assembles an output gear 56 and an output shaft 57 will be described with reference to FIG. 4.

[0030] The output gear 56 is manufactured by injection molding using a mold (not shown) in which a gate is arranged at a position corresponding to the inner peripheral surface of the through hole 562a. As shown in FIG. 4A, before assembling the output gear 56 and the output shaft 57, a resin piece X corresponding to the gate and runner of the mold remains inside the through hole 562a of the output gear 56. The resin piece X is connected to the inner peripheral surface of the through hole 562a.

[0031] Then, as shown in FIG. 4B, the cylindrical portion 573a of the press-fitting end portion 573 is inserted into the through hole 562a while sliding the outer peripheral surface of the cylindrical portion 573a on the inner peripheral surface of the through hole 562a. The resin piece X is pushed upward by the press-fitting end portion 573 and torn off from the inner peripheral surface of the through hole 562a. At this time, a gate residue (a connection mark between the resin piece X protruding from the inner peripheral surface and the inner peripheral surface) is formed on the inner peripheral surface of the through hole 562a, and the gate residue is held in a state of being crushed between the outer peripheral surface of the cylindrical portion 573a and the inner peripheral surface of the through hole 562a.

[0032] Then, as shown in FIG. 4C, the rib portion 573b of the press-fitting end portion 573 is press-fitted into the through hole 562a while biting the plurality of ribs 573c of the rib portion 573b into the inner peripheral surface of the through hole 562a. The press-fitting is advanced until the stopper portion 572 abuts against the bottom wall portion 562 of the output gear 56. The inner peripheral surface of the through hole 562a and the gate residue are scraped by the plurality of ribs 573c, and the scrapings are held between the plurality of ribs 573c and the inner peripheral surface of the through hole 562a.

[0033] The guide member 60 has a cylindrical shape. The guide member 60 is arranged inside the upper part of the holder 20. A female thread is formed at the lower part of the inner peripheral surface of the guide member 60. The output shaft 57 is arranged inside the guide member 60. The guide member 60 rotatably supports the output shaft 57.

[0034] The drive shaft 65 integrally has a cylindrical portion 66 and a flat plate portion 67. The flat plate portion 67 is connected to the upper end portion of the cylindrical portion 66. A male thread is formed on the outer peripheral surface of the cylindrical portion 66. The male thread of the cylindrical portion 66 is screwed with the female thread of the guide member 60. The flat plate portion 67 is disposed in the slit 57a of the output shaft 57 so as to be movable in the vertical direction. The drive shaft 65 is rotated by the output shaft 57 and moves in the vertical direction by a screw feed action. The ball 68 is disposed between the drive shaft 65 and the ball receiving portion 74 of the valve body 70.

[0035] The valve body 70 has a stem 71, a valve portion 72, a spring receiving portion 73, and a ball receiving portion 74. The stem 71 has a cylindrical shape. The stem 71 is disposed inside the valve body support member 25. The stem 71 is supported by the valve body support member 25 so as to be movable in the vertical direction. The valve portion 72 is disposed at the lower end portion of the stem 71. The valve portion 72 has an annular shape. The valve portion 72 projects radially outward from the outer peripheral surface of the stem 71. The valve portion 72 is disposed to face the valve port 14 in the vertical direction. The spring receiving portion 73 has a cylindrical shape. The spring receiving portion 73 is joined to the upper end portion of the stem 71. The spring receiving portion 73 has a flange portion 73a that projects radially outward. The ball receiving portion 74 has a circular flat plate portion and a convex portion connected to the lower surface of the flat plate portion. In the ball receiving portion 74, the flat plate portion is in contact with the ball 68, and the convex portion is fitted into a hole formed in the spring receiving portion 73. An opening spring 75 is disposed between the flange portion 73a of the spring receiving portion 73 and the valve body support member 25. The opening spring 75 is a compression coil spring. The opening spring 75 presses the valve body 70 (flange portion 73a) upward. The valve body 70 steplessly changes the opening area of the valve port 14 by advancing and retreating the valve portion 72 with respect to the valve port 14. The valve body 70 may close the valve port 14 (that is, the opening area may be set to 0).

[0036] The stator unit 80 has a stator 90 and a cover 95. The stator 90 has a cylindrical shape. The cam 30 is arranged inside the stator 90. The cover 95 houses the stator 90. The stator unit 80 constitutes a stepping motor together with the magnet rotor 41.

[0037] In the electric valve 1, the valve port 14, the holder 20, the valve body support member 25, the cam 30, the magnet rotor 41, the connecting member 42, the rotor shaft 43, the output gear 56, the output shaft 57, the guide member 60, the drive shaft 65, and the valve body 70 have their respective central axes aligned.

[0038] Next, the operation of the electric valve 1 will be described.

[0039] In the electric valve 1, a current is passed through the coil of the stator 90 to rotate the magnet rotor 41 in one direction. The rotation of the magnet rotor 41 is transmitted to the drive shaft 65 via the planetary gear mechanism 50. Due to the screw feed action between the drive shaft 65 and the guide member 60, the drive shaft 65 moves downward. The valve body 70 is pushed downward by the drive shaft 65, and the opening area of the valve port 14 becomes smaller.

[0040] In the electric valve 1, a current is passed through the coil of the stator 90 to rotate the magnet rotor 41 in the other direction. The rotation of the magnet rotor 41 is transmitted to the drive shaft 65 via the planetary gear mechanism 50. Due to the screw feed action between the drive shaft 65 and the guide member 60, the drive shaft 65 moves upward. The valve body 70 is pushed upward by the valve opening spring 75, and the opening area of the valve port 14 becomes larger.

[0041] The drive mechanism 40 of the electric valve 1 has a resin output gear 56 and a metal output shaft 57. The output gear 56 has a circular through-hole 562a. The press-fitting end portion 573 of the output shaft 57 has a cylindrical portion 573a having a circular cross-section, which are arranged in order from the tip side, and a rib portion 573b in which a plurality of axially extending ribs 573c are arranged side by side in the circumferential direction. The diameter of the cylindrical portion 573a is equal to or less than the diameter of the through-hole 562a. And the diameter of the virtual circle VC connecting the tips of the plurality of ribs 573c of the rib portion 573b is larger than the diameter of the through-hole 562a. Because of this, the cylindrical portion 573a is inserted into the through-hole 562a while the outer peripheral surface of the cylindrical portion 573a slides on the inner peripheral surface of the through-hole 562a. If there is gate residue on the inner peripheral surface of the through-hole 562a, the gate residue is held in a state of being crushed between the outer peripheral surface of the cylindrical portion 573a and the inner peripheral surface of the through-hole 562a. Then, the rib portion 573b is press-fitted into the through-hole 562a while the plurality of ribs 573c of the rib portion 573b bite into the inner peripheral surface of the through-hole 562a. When the inner peripheral surface of the through-hole 562a and the gate residue are scraped by the plurality of ribs 573c, the chips are held between the plurality of ribs 573c and the inner peripheral surface of the through-hole 562a. Since the rib portion 573b is located at a position deep from the upper opening of the through-hole 562a, it is possible to suppress the chips from protruding from the through-hole 562a. Therefore, in the assembly of the output gear 56 and the output shaft 57, the operation of removing chips can be made unnecessary.

[0042] Also, the axial length of the cylindrical portion 573a is 1 / 10 or more of the axial length of the rib portion 573b. By doing so, the rib portion 573b is surely arranged at a position deep from the upper opening of the through-hole 562a, and it is possible to more surely suppress the chips from protruding from the through-hole 562a.

[0043] Also, on the end portion of the plurality of ribs 573c on the side of the cylindrical portion 573a, an inclined surface 573d is formed which is connected to the outer peripheral surface of the cylindrical portion 573a and extends radially outward as it moves axially away from the cylindrical portion 573a. By doing so, since the diameter of the rib portion 573b gradually increases due to the inclined surface 573d, the press-fitting end portion 573 can be smoothly press-fitted into the through-hole 562a.

[0044] Although the embodiments of the present invention have been described above, the present invention is not limited to the configurations of the embodiments. For the above-described embodiments, those obtained by appropriately adding, deleting, or modifying components by those skilled in the art, or those obtained by appropriately combining the features of the embodiments, are included in the scope of the present invention as long as they do not depart from the spirit of the present invention.

Explanation of Reference Numerals

[0045] 1... Electric valve, 10... Valve body, 10a... Left side surface, 10b... Right side surface, 10c... Upper surface, 13... Valve chamber, 14... Valve port, 17... First passage, 18... Second passage, 19... Mounting hole, 19a... Bottom surface, 20... Holder, 25... Valve body support member, 25a... Annular plane, 30... Cam, 35... Joining member, 40... Driving mechanism, 41... Magnet rotor, 42... Connecting member, 43... Rotor shaft, 50... Planetary gear mechanism, 51... Gear case, 52... Fixed ring gear, 53... Sun gear, 54... Planetary gear, 55... Carrier, 55a... Support shaft, 56... Output gear, 561... Peripheral wall portion, 562... Bottom wall portion, 562a... Through hole, 57... Output shaft, 57a... Slit, 57b... Support hole, 571... Connection portion, 572... Stopper portion, 573... Press-fitting end portion, 573a... Cylindrical portion, 573b... Ridge portion, 573c... Ridge, 573d... Inclined surface, 60... Guide member, 65... Driving shaft, 66... Cylindrical portion, 67... Flat plate portion, 68... Ball, 70... Valve body, 71... Stem, 72... Valve portion, 73... Receiving portion, 73a... Flange portion, 74... Ball receiving portion, 75... Valve opening spring, 80... Stator unit, 90... Stator, 95... Cover, VC... Virtual circle, X... Resin piece

Claims

【Claim 1】 A method for assembling a shaft body by assembling a resin gear and a metal shaft body, using a mold in which a gate is arranged at a position corresponding to the inner peripheral surface of the circular through-hole of the gear, to produce by injection molding a resin piece arranged inside the gear and the through-hole and connected to the inner peripheral surface, forming, at an end of the shaft body, a cylindrical portion having a circular cross-section, arranged in order from the tip side of the end portion, and a rib portion in which a plurality of axially extending ribs are arranged side by side in the circumferential direction, inserting the cylindrical portion into the through-hole while sliding the outer peripheral surface of the cylindrical portion on the inner peripheral surface, and pressing the resin piece at the end portion to tear it off from the inner peripheral surface, characterized in that the rib portion is press-fitted into the through-hole while causing the plurality of ribs of the rib portion to bite into the inner peripheral surface.

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