Gas sensor

Abstract

The invention provides a gas sensor which suppresses breakage of a terminal fitting on the rear end side of a separator. A gas sensor is provided with: a plate-shaped sensor element having a plurality of electrode pads on a main surface on the rear end side; a plurality of plate-shaped terminal fittings electrically connected to the electrode pads; a separator having a housing part for housing the rear end side of the sensor element and the terminal fitting; an outer cylinder; and a backing ring made of an elastic member disposed on the rear end side of the separator, in which the inner surface of the housing portion has an expanded diameter portion, the backing ring has a backing ring hole through which the rear end portion of the terminal fitting and the lead wire are inserted, and a diameter reducing member for reducing the diameter of the backing ring in a direction perpendicular to the main surface is fitted to the outer periphery of the front end side of the backing ring. The diameter-reducing member is disposed in a region that overlaps the terminal fitting in the backing ring in the axial direction and that is closer to the distal end side than a fastening section that is formed on the outer cylinder and fixes the backing ring.

Description

Technical Field

[0001] The present invention relates to a gas sensor suitable for detecting the gas concentration of a specific gas contained in combustion gas or exhaust gas of, for example, a burner or an internal combustion engine. Background Art

[0002] As a detector for detecting oxygen and NO in exhaust gas of automobiles, etc. x As a gas sensor for measuring the concentration of a gas, a gas sensor having a plate-shaped sensor element using a solid electrolyte is known.

[0003] As such a gas sensor, a gas sensor is used in which a plurality of electrode pads are provided on the rear ends of opposing main surfaces of a plate-shaped sensor element, and terminal fittings are electrically contacted with the electrode pads to extract a sensor output signal from the sensor element to the outside (Patent Document 1).

[0004] In the gas sensor of Patent Document 1, each terminal fitting is held in a two-part insulating connector (separator). The connector houses each terminal fitting in a box-shaped housing of the same shape, and the housings are coupled and fixed with a metal clamp. When the rear end side of the sensor element is inserted into the insertion hole of the connector, the terminal fitting inside the connector is in electrical contact with the electrode pad.

[0005] On the other hand, at the rear end side of the connector, the insertion hole is oriented toward the rear end surface and expands radially outward in a direction perpendicular to the main surface of the sensor element, thereby preventing the terminal fittings led to the rear end side of the connector from interfering with the inner surface of the connector.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Publication No. 2014-209104 Summary of the invention

[0009] Problems to be solved by the invention

[0010] However, it is known that when the rear end side of the connector is enlarged in diameter, the distance between the terminal fitting and the connector increases, and the terminal fitting is easily vibrated and broken in the connector due to vibration during use of the gas sensor.

[0011] Therefore, an object of the present invention is to provide a gas sensor in which breakage of a terminal fitting on the rear end side of a separator is suppressed.

[0012] Means for solving problems

[0013] In order to solve the above-mentioned problems, the gas sensor of the present invention comprises: a plate-shaped sensor element extending in the axial direction and having a plurality of electrode pads on the main surface on the rear end side; a plurality of plate-shaped terminal fittings electrically connected to the electrode pads respectively; a separator having a receiving portion penetrating in the axial direction and receiving the rear end side of the sensor element and the terminal fitting, and holding the terminal fitting; an outer cylinder receiving the sensor element and the separator; and a bushing made of an elastic component, arranged on the rear end side of the separator, closing the opening on the rear end side of the outer cylinder, wherein the gas sensor is characterized in that the inner surface of the receiving portion has an expanded diameter portion, and the expanded diameter portion is expanded as the The bushing expands in diameter in a direction perpendicular to the main surface along the surface toward the rear end of the separator, the bushing has a bushing hole, the rear end of the terminal fitting and the lead connected to the rear end are inserted through the bushing hole, the multiple electrode pads and the multiple terminal fittings are arranged separately in the width direction of the sensor element, and a reducing component is embedded in the outer periphery of the front end side of the bushing, the reducing component reduces the diameter of the bushing at least in the direction perpendicular to the main surface, and the reducing component is arranged in an area that is closer to the front end side than the fastening portion formed on the outer cylinder and fixing the bushing, and overlaps with the terminal fitting in the bushing in the axial direction.

[0014] The terminal fitting is plate-shaped, with the plate surface facing the main surface of the sensor element. The plate-shaped terminal fitting vibrates in a direction perpendicular to the plate surface, so the terminal fitting is easily vibrated radially inward (toward the center of the sensor element) and broken due to vibrations during use of the gas sensor.

[0015] Therefore, according to the gas sensor, the bushing hole around the terminal fitting is pressed by the reducing member to shrink radially inward, and the inner wall of the bushing hole contacts or approaches the terminal fitting to hold the terminal fitting. As a result, the vibration of the terminal fitting radially inward can be suppressed or reduced, and the breakage of the terminal fitting can be suppressed.

[0016] In the gas sensor of the present invention, the diameter-reducing member may surround the entire circumference of the backing ring.

[0017] According to this gas sensor, the diameter of the backing ring can be reliably reduced.

[0018] In the gas sensor of the present invention, a connection portion to be connected to the lead wire may be provided at a rear end portion of the terminal fitting, and the diameter-reducing member may be arranged closer to the front end side than the connection portion.

[0019] In order to hold the lead wire, the connection portion is thicker than other parts of the terminal fitting. Therefore, the diameter of the backing ring can be reliably reduced by reducing the diameter of the backing ring at a position avoiding the connection portion through the diameter reducing member.

[0020] In the gas sensor of the present invention, the diameter-reducing member may extend toward the front end side of the backing ring.

[0021] When the front end of the reducing member is arranged on the rear side of the front end of the backing ring, the front end of the backing ring may not be sufficiently reduced in diameter. Therefore, by extending the reducing member toward the front end side than the backing ring, the front end of the backing ring can also be reliably reduced in diameter.

[0022] In the gas sensor of the present invention, the reducing part may have a tapered portion on its inner surface that expands toward the rear end edge and a straight portion that is closer to the front end than the tapered portion, and the inner diameter D1 of the straight portion is smaller than the outer diameter D2 of the bushing when not engaged with the reducing part.

[0023] According to this gas sensor, the diameter reducing member can be easily fitted into the backing ring, and the straight portion having a smaller diameter than the backing ring can be pressed in by fitting, so that the backing ring hole around the backing ring and the terminal fitting can be reliably shrunk, thereby improving airtightness.

[0024] Effects of the Invention

[0025] According to the present invention, it is possible to obtain a gas sensor in which breakage of the terminal fitting on the rear end side of the separator is suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a cross-sectional view of a gas sensor according to an embodiment of the present invention.

[0027] Figure 2 It is a three-dimensional diagram of a partition.

[0028] Figure 3 It is along Figure 2 Cross-sectional view along line AA.

[0029] Figure 4 This is a perspective view of the connection terminal.

[0030] Figure 5 It is a three-dimensional diagram of the reduced diameter component.

[0031] Figure 6 This is a diagram showing a state where the diameter-reducing member is fitted from the front end side of the bushing ring. DETAILED DESCRIPTION

[0032] The following, with the attached Figure 1 Embodiments of the present invention will now be described.

[0033] First, refer to Figure 1 to Figure 6 , a gas sensor 10 according to an embodiment of the present invention will be described.

[0034] Figure 12 is a cross-sectional view of a gas sensor (sensor) 10 according to an embodiment of the present invention, taken along the axis O direction. Figure 2 is a perspective view of the partition 50, Figure 3 It is along Figure 2 The cross-sectional view of line AA, Figure 4 is a perspective view of the connection terminal 71, Figure 5 is a perspective view of the diameter-reducing component 80, Figure 6 This is a diagram showing a state in which the diameter-reducing member 80 is fitted from the front end side of the backing ring 47 .

[0035] in addition, Figure 2 , Figure 3 It shows the state before the backing ring 47 is arranged.

[0036] like Figure 1 As shown, the gas sensor 10 includes: a sensor element 20 for measuring a specific gas component in the measured gas; a protector 30 for protecting the front end of the sensor element 20; and a sensor assembly 40 including a partition 50 for accommodating the rear end side of the sensor element 20. The gas sensor 10 is installed in the exhaust pipe of a vehicle, for example, to measure gas components such as NOx and O2 contained in the measured gas (exhaust gas).

[0037] The sensor element 20 is a long and thin plate-like element extending in the direction of the axis O, and is formed by laminating a ceramic substrate composed of an oxygen ion conductive solid electrolyte layer such as zirconium oxide (ZrO2). In addition, the end of the sensor element 20 on the protector 30 side is marked as the front end, and the end on the separator 50 side is marked as the rear end.

[0038] On the main surfaces 20m1 and 20m2 ( Figure 2 , Figure 3 ) is formed with a plurality of electrode pads 21a, 21b ( Figure 3 ).

[0039] Specifically, if Figure 3 As shown, on one main surface 20m1 of the sensor element 20, a plurality of electrode pads 21a are arranged separately in the width direction of the sensor element 20. Similarly, on the other main surface 20m2 of the sensor element 20, a plurality of electrode pads 21b are arranged separately in the width direction of the sensor element 20.

[0040] Furthermore, the plurality of electrode pads may be arranged only on one main surface 20 m 1 or 20 m 2 of the sensor element 20 .

[0041] The electrode pads 21 a and 21 b are used to apply a voltage to the sensor element 20 , or to output a detection signal of the sensor element 20 , or to energize the heater when the sensor element 20 has a heater.

[0042] like Figure 1 As shown in FIG. 1 , the protector 30 is arranged so as to surround the front end of the sensor element 20. The protector 30 includes an inner protector 31 covering the front end of the sensor element 20 and an outer protector 32 covering the inner protector 31. The inner protector 31 is formed in a cylindrical shape and includes an inner gas introduction hole 31a for introducing the measured gas at the front end of the sensor element 20. The outer protector 32 is formed in a bottomed cylindrical shape and includes an outer gas introduction hole 32a for introducing the measured gas at the side. The inner protector 31 and the outer protector 32 are made of metal such as SUS, for example.

[0043] The sensor assembly 40 includes a metallic shell 41 , a cylindrical inner cylinder 42 and an outer cylinder 46 fixed to the metallic shell 41 by welding, and a spacer 50 connected to the rear end of the sensor element 20 .

[0044] The main body fitting 41 is mounted on, for example, an exhaust pipe of a vehicle through the external threaded portion 41a. A plurality of ceramic sleeves 43a to 43c and powder filling layers 44a and 44b such as talc filled between the ceramic sleeves 43a and 43b and between the ceramic sleeves 43b and 43c are sealed inside the inner tube 42, and they are clamped and sealed by the metal ring 45 and the inner wall of the main body fitting 41.

[0045] The outer cylinder 46 covers the inner cylinder 42, the sensor element 20, and the separator 50. In addition, the rubber bushing 47 closes the opening on the rear end side of the outer cylinder 46. Figure 4 ) A lead wire 48 is connected by crimping or the like, and the lead wire 48 passes through a bushing hole 47a that penetrates the bushing 47 in the direction of the axis O and is led out from the rear end of the bushing 47 to the outside.

[0046] Furthermore, the connection portion 71 g of the terminal fitting 71 and the lead wire 48 are inserted through the backing ring hole 47 a .

[0047] Next, the separator 50 will be described. Figure 2 As shown, the separator 50 includes a first member 51 a and a second member 51 b made of ceramic such as alumina sintered body, and a metal clamp 90 for sandwiching and fixing the first member 51 a and the second member 51 b.

[0048] Furthermore, the terminal fittings 71 are held by the first member 51 a or the second member 51 b , and are arranged so as to face and contact the electrode pads 21 a and 21 b of the sensor element 20 in a one-to-one manner.

[0049] The first component 51a and the second component 51b respectively arrange and hold four terminal fittings 71 in a width direction perpendicular to the length direction (=axis O direction) of the terminal fitting 71. The first component 51a and the second component 51b have the same box-shaped shape, so the same components are marked with the same reference numerals for description. In addition, the first component 51a and the second component 51b are collectively described as the housing 51.

[0050] The housing 51 comprises: a housing portion 50h extending in the direction of the axis O; four locking grooves 52 for locking the front end side of the terminal fitting 71; four insertion holes 53 for inserting the upright portion 71d in the central portion of the terminal fitting 71; and a locking portion 54 formed in each insertion hole 53 for locking the terminal fitting 71.

[0051] The housing 51 includes a convex portion 55 on one side in the width direction across the sensor element 20, and includes limiting members 56 and 57 on the other side for limiting the distance between the first member 51a and the second member 51b in the thickness direction (see FIG. Figure 2 ). The protrusion 55 is inserted into the recess between the limiting member 56 and the limiting member 57 of the housing 51 , thereby limiting the relative position of the first member 51 a and the second member 51 b in the thickness direction.

[0052] like Figure 4 As shown, the terminal fitting 71 is a metal component held by the housing 51 in a one-to-one relative position to the electrode pads 21a, 21b of the sensor element 20, and comprises: a front end portion 71a, which is locked in the locking groove 52 by utilizing a bent shape; a protruding portion 71b, which protrudes in a bent manner toward the sensor element 20; a contact portion 71c, which contacts the electrode pads 21a, 21b by protruding in a bent manner toward the sensor element 20; an upright portion 71d, which is inserted into the insertion hole 53; a bent portion 71f, which is led out to the outside of the partition 50; and a connecting portion 71g, which is crimped on the outside of the partition 50 and holds the multiple core wires 48a of the lead 48.

[0053] The protrusion 71b and the contact portion 71c are arranged along the length direction of the terminal fitting 71, and the contact portion 71c is arranged closer to the connection portion 71g than the protrusion 71b. The protrusion 71b and the contact portion 71c are both formed to be elastically deformable. The upright portion 71d has a locking portion 71e that is locked to the locking portion 54 by a bent shape.

[0054] Furthermore, the contact portion 71c of the terminal fitting 71 held by the first component 51a is opposed to and in contact with the electrode pad 21a of the sensor element 20 in a one-to-one manner, and the contact portion 71c of the terminal fitting 71 held by the second component 51b is opposed to and in contact with the electrode pad 21b of the sensor element 20 in a one-to-one manner ( Figure 3 ).

[0055] On the other hand, Figure 3 As shown, the electrode pads 21a, 21b are formed from the rear end of the sensor element 20 to a position between the contact portion 71c and the protrusion 71b.

[0056] like Figure 2 As shown, the clamp 90 is formed by bending a plate-shaped metal and has an elastic force capable of clamping the first component 51a and the second component 51b and pressing them toward each other. When the first component 51a and the second component 51b are clamped by the elastic force, the limiting components 56 and 57 of the first component 51a contact the second component 51b, and the limiting components 56 and 57 of the second component 51b contact the first component 51a. As a result, the distance between the first component 51a and the second component 51b is fixed.

[0057] In addition, when the clamping member 90 clamps the first component 51a and the second component 51b in a state where the sensor element 20 and the terminal fitting 71 are clamped by the first component 51a and the second component 51b in a manner such that the contact portion 71c of the terminal fitting 71 is opposite to the electrode pads 21a and 21b of the sensor element 20, the sensor element 20 is clamped and fixed by the elastic deformation of the protrusion 71b and the contact portion 71c due to the pressing force from the clamping member 90.

[0058] At this time, the protrusion 71b and the contact portion 71c are elastically deformed and pressed, thereby reliably clamping and fixing the sensor element 20. In addition, since the contact portion 71c is elastically deformed, the electrical contact between the contact portion 71c and the electrode pads 21a and 21b can be more reliably maintained.

[0059] Next, the characteristic features of the present invention will be described.

[0060] First, if Figure 3 As shown, the separator 50 has a housing portion 50 h penetrating in the direction of the axis O, and the housing portion 50 h houses the rear end side of the sensor element 20 and the terminal fitting 71 .

[0061] More specifically, the housing portion 50h opens in a rectangular shape slightly larger than the outer shape of the sensor element 20 at the front end surface of the housing 51 and communicates with the insertion holes 53 at its rear end. The housing portion 50h may be an insertion hole opened only at the front and rear ends of the housing 51.

[0062] In this example, the housing portion 50h is formed by recessing a portion of the facing surfaces of the first member 51a and the second member 51b.

[0063] The inner surface of the housing portion 50h has an expanded diameter portion 50s which expands in a direction perpendicular to the two main surfaces 20m1 and 20m2 of the sensor element 20 as it goes toward the surface of the separator 50 toward the rear end ( Figure 3 The diameter is expanded in the up and down directions.

[0064] It should be noted that “in a direction perpendicular to the main surfaces 20m1, 20m2” means that as long as there is a directional component perpendicular to the main surfaces 20m1, 20m2, it also includes the case where the degree of expansion of the expanded diameter portion 50s varies depending on the location. For example, it also includes the case where the expanded diameter portion 50s is not parallel to the main surfaces 20m1, 20m2, the case where the expanded diameter portion 50s is a curved surface, the case where the expanded diameter portion 50s itself is parallel to the main surfaces 20m1, 20m2 but the front edge and rear edge of the expanded diameter portion 50s are not parallel to the main surfaces 20m1, 20m2 (the case where the length and position of the line segment when the expanded diameter portion 50s is cut along the main surfaces 20m1, 20m2 are different).

[0065] On the other hand, the terminal fitting 71 is in a plate shape, and the plate surface faces the main surfaces 20m1 and 20m2 of the sensor element 20. The plate-shaped terminal fitting 71 vibrates in a direction perpendicular to the plate surface, so the terminal fitting 71 is easily moved radially inward (the center side of the sensor element 20, the ... Figure 3 IN direction) and breaks due to vibration.

[0066] Therefore, if Figure 1 As shown, a diameter reducing member 80 for reducing the diameter of the backing ring 47 at least in a direction F perpendicular to the main surfaces 20 m 1 and 20 m 2 of the sensor element 20 is fitted to the outer periphery of the front end side of the backing ring 47 .

[0067] The diameter reducing member 80 is disposed on the front end side of the fastening portion 46 c formed in the outer cylinder 46 and fixing the bushing 47 , and in a region overlapping with the terminal fitting 71 in the bushing 47 in the axis O direction.

[0068] As a result, the bushing hole 47a around the terminal fitting 71 is pressed by the reducing member 80 and shrinks radially inward, and the inner wall of the bushing hole 47a contacts or approaches the terminal fitting 71 to hold the terminal fitting 71. As a result, the terminal fitting 71 can be suppressed or reduced from moving radially inward ( Figure 3 IN direction) vibration, which can suppress the breakage of the terminal fitting 71.

[0069] Here, in Figure 5 In the example shown, the diameter reducing member 80 is made of a metal ring such as SUS304, and has a tapered portion 80t formed on the inner surface at the rear end side, which expands toward the rear end edge. In addition, the inner surface of the diameter reducing member 80 closer to the front end than the tapered portion 80t is a straight portion 80s.

[0070] Then, if Figure 6 As shown, in the semi-finished sensor assembly (an assembly in which the bushing 47, the separator 50, the terminal fitting 71 and the lead wire 48 are assembled and the outer cylinder 46 is not installed), the reduced diameter component 80 is inserted into the bushing 47 from the front end side ( Figure 6 (a)).

[0071] Here, the inner diameter D1 of the straight portion 80 s of the diameter reducing member 80 is smaller than the outer diameter D2 of the backing ring 47 in the non-compressed state (the state in which the diameter reducing member 80 is not fitted).

[0072] The reducing member 80 is fitted into the front end portion 47s of the bushing 47 from the tapered portion 80t side and then inserted toward the rear end, thereby pressing in the straight portion 80s with a smaller diameter than the bushing 47, so that the bushing 47 and the bushing hole 47a around the terminal fitting 71 are contracted radially inward ( Figure 6 (b)).

[0073] Then, the assembled body is housed in the outer tube 46, and the outer tube 46 is fastened to the outside of the bushing 47 to form a fastening portion 46c.

[0074] In addition, as to whether the bushing ring 47 is reduced in diameter by the reducing component 80, after removing the outer tube 46 from the bushing ring 47, the reducing component 80 is removed. If the outer diameter D2 of the bushing ring 47 near the reducing component 80 is larger than the minimum inner diameter D1 of the reducing component 80 in contact with the bushing ring, it is determined that the bushing ring 47 is reduced in diameter.

[0075] The diameter-reducing member 80 can be formed of a metal such as SUS304. In addition, the hardness of the diameter-reducing member 80 is preferably 200 Hv or more.

[0076] The diameter reducing member 80 is not limited to an annular shape, and may be a C-shape, a clip shape, or the like. However, an annular shape surrounding the entire circumference of the backing ring 47 is preferred because the backing ring 47 can be reliably reduced in diameter.

[0077] In addition, if Figure 6 As shown in (b) of FIG. 1 , a connection portion 71 g connected to the lead wire 48 may be provided at the rear end of the terminal fitting 71 , and the diameter-reducing member 80 may be arranged closer to the front end side than the connection portion 71 g .

[0078] The connection portion 71g is thicker than other portions of the terminal fitting 71 in order to hold the lead wire 48. Therefore, the diameter of the backing ring 47 is reduced by the diameter reducing member 80 at a position avoiding the connection portion 71g, so that the diameter of the backing ring 47 can be reduced reliably.

[0079] In addition, if Figure 6 As shown in (b) of FIG. 8 , the diameter-reducing member 80 may extend toward the front end side of the backing ring 47 .

[0080] When the front end of the reducing member 80 is arranged on the rear side of the front end of the bushing 47, the front end of the bushing 47 may not be sufficiently reduced in diameter. Therefore, by extending the reducing member 80 toward the front end of the sleeve 47, the front end of the sleeve 47 can also be reliably reduced in diameter.

[0081] The present invention is not limited to the above-described embodiments, but may include various modifications and equivalents within the concept and scope of the present invention.

[0082] The diameter reducing member is not limited to the member having the tapered portion described above, and a ring having a larger diameter than the backing ring may be fitted into the backing ring and then the ring may be reduced in diameter.

[0083] The separator is not limited to a box-like shape divided into two, but may be in a cylindrical shape such as a cylinder.

[0084] The diameter reducing member only needs to reduce the diameter of the backing ring at least in a direction perpendicular to the main surface of the sensor element. However, for example, by making the diameter reducing member annular, the backing ring may be reduced in a direction other than the main surface.

[0085] Description of Reference Numerals

[0086] 10 Gas Sensor

[0087] 20 Sensor element

[0088] 20m1, 20m2 Main surface of sensor element

[0089] 21a, 21b Electrode pads

[0090] 46 outer cylinder

[0091] 46c Fastening part

[0092] 47 Bushing

[0093] 47a Bushing hole

[0094] 48 Lead

[0095] 50 Dividers

[0096] 50h Containment

[0097] 50s Expanded diameter

[0098] 71 Terminal accessories

[0099] 71g Connection

[0100] 80 Reduced diameter parts

[0101] 80t cone

[0102] 80s Straight Line

[0103] O Axis

[0104] F Direction perpendicular to the main surface

Claims

1. A gas sensor comprising: A plate-shaped sensor element extending in the axial direction and having a plurality of electrode pads on a main surface on the rear end side; A plurality of plate-shaped terminal fittings, electrically connected to the electrode pads respectively; a separator having a receiving portion penetrating in the axial direction to receive the rear end side of the sensor element and the terminal fitting, and holding the terminal fitting; an outer cylinder for accommodating the sensor element and the separator; and A bushing ring made of an elastic member is arranged at the rear end side of the partition to close the opening at the rear end side of the outer tube. The gas sensor is characterized in that The inner surface of the housing portion has an expanded diameter portion, and the expanded diameter portion expands in a direction perpendicular to the main surface as it goes toward the rear end surface of the partition. The bushing has a bushing hole, through which the rear end of the terminal fitting and a lead wire connected to the rear end are inserted. The plurality of electrode pads and the plurality of terminal fittings are arranged separately in a width direction of the sensor element, Furthermore, a diameter reducing member is fitted on the outer periphery of the front end side of the bushing ring, and the diameter reducing member reduces the diameter of the bushing ring at least in a direction perpendicular to the main surface. The diameter-reducing member is disposed on a region that is located on the front end side of a fastening portion that is formed on the outer cylinder and that fixes the backing ring, and that overlaps with the terminal fitting in the backing ring in the axial direction.

2. The gas sensor according to claim 1, characterized in that The diameter-reducing member surrounds the entire circumference of the backing ring.

3. The gas sensor according to claim 1 or 2, characterized in that: A connection portion connected to the lead wire is provided at the rear end of the terminal fitting. The diameter-reducing member is disposed on the front end side of the connecting portion.

4. The gas sensor according to claim 1 or 2, characterized in that: The diameter-reducing member extends toward the front end side relative to the backing ring.

5. The gas sensor according to claim 1 or 2, characterized in that: The diameter-reducing member has a tapered portion on the inner surface thereof that expands in diameter toward the rear end edge and a straight portion that is closer to the front end side than the tapered portion. An inner diameter D1 of the straight portion is smaller than an outer diameter D2 of the bushing ring in a state where the bushing ring is not fitted with the diameter-reducing member.

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