Patent Publication Number: US-10763646-B2

Title: Spark plug

Description:
FIELD OF THE INVENTION 
     The present invention relates to a spark plug, and, in particular, to a spark plug in which an insulator is retained by a metal shell. 
     BACKGROUND OF THE INVENTION 
     International Publication No. 2010/035717 discloses, in a spark plug in which an insulator is retained by a metal shell, a technology of making a portion between the metal shell and the insulator airtight by using a metallic packing. When a load that the metal shell and the insulator apply to the packing is increased, the airtightness is increased, whereas, when the excessively deformed packing strongly compresses the insulator, the insulator breaks. In the technology of International Publication No. 2010/035717, the shape of a gap between the metal shell and the insulator is adjusted to suppress excessive deformation of the packing, so that airtightness is ensured while suppressing occurrence of cracking of the insulator. 
     However, in the above-described existing technology, there is a demand for increasing the airtightness between the metal shell and the insulator without excessively increasing the load when retaining the insulator by the metal shell. 
     SUMMARY OF THE INVENTION 
     The present invention is made to meet this demand. An advantage of the present invention is a spark plug that is capable of ensuring airtightness between a metal shell and an insulator while suppressing occurrence of cracking of the insulator. 
     In accordance with a first aspect of the present invention, there is provided a spark plug that includes an insulator that extends along an axial line from a front end side to a rear end side, and a cylindrical metal shell that is disposed on an outer peripheral side of the insulator, the metal shell including a stepped portion at an inner periphery of the metal shell, the stepped portion protruding inward in a radial direction and including a rear-end-side facing surface that retains the insulator either directly or via another member. The stepped portion includes a first convex portion that includes the rear-end-side facing surface, a second convex portion that is disposed on a front end side of the first convex portion and that is adjacent to the first convex portion, and a connection portion that connects the first convex portion and the second convex portion to each other; and when a cross section including the axial line is viewed, in a direction perpendicular to the axial line, the connection portion exists in a range where a portion of the rear-end-side facing surface that contacts the insulator or the other member is positioned. 
     According to the spark plug of the first aspect, in a direction perpendicular to the axial line, the connection portion that connects the first convex portion and the second convex portion of the stepped portion to each other exists within the range where the portion of the rear-end-side facing surface of the first convex portion that contacts the insulator or the other member is positioned. Therefore, when, in retaining the insulator by the metal shell, the first convex portion is subjected to a force acting towards the front end side in an axial-line direction from the insulator, a tensile stress is produced at the first convex portion along the rear-end-side facing surface, and a compression stress is produced at the first convex portion along a connection-portion-side surface adjacent to the second convex portion. As a result, it is possible to closely contact the rear-end-side facing surface with the insulator either directly or via the other member by an opposing force that is produced by elastic deformation of the first convex portion. Therefore, it is possible to ensure airtightness between the stepped portion of the metal shell and the insulator. 
     When the insulator is retained by the rear-end-side facing surface via the other member, since excessive deformation of the other member is suppressed by the elastic deformation of the first convex portion, it is possible to suppress occurrence of cracking of the insulator caused by the other member. When the insulator contacts the rear-end-side facing surface, since the other member does not exist, it is possible to suppress occurrence of cracking of the insulator caused by the other member. 
     According to a second aspect of the present invention, there is provided a spark plug as described above, wherein, in the cross section including the axial line, a length of the first convex portion on an imaginary straight line is less than a length of the second convex portion on the imaginary straight line, the imaginary straight line passing through the connection portion and extending along the axial line. Therefore, the first convex portion subjected to a force acting towards the front end side in the axial-line direction can be easily elastically deformed. As a result, since it is possible to ensure an opposing force that is produced by the elastic deformation of the first convex portion, it is possible to increase airtightness, in addition to providing the effects of the first aspect. 
     According to a third aspect of the present invention, there is provided a spark plug as described above, wherein, in the cross section including the axial line, a distance from the imaginary straight line, which passes through the connection portion and which extends along the axial line, to an innermost position of the second convex portion in the radial direction is greater than a distance from the imaginary straight line to an innermost position of the first convex portion in the radial direction. Therefore, a load that is applied to the connection portion by the first convex portion subjected to a force acting towards the front end side in the axial-line direction can be easily dispersed by the second convex portion. As a result, it is possible to make it less likely for the first convex portion to buckle, in addition to providing the effects of the first aspect or the second aspect. 
     According to a fourth aspect of the present invention, there is provide a spark plug as described above, wherein the insulator is directly retained by the rear-end-side facing surface. Since it is possible not to use the other member that is interposed between the stepped portion and the insulator, it is possible to reduce the number of components and to prevent occurrence of cracking of the insulator caused by excessive deformation of the other member, in addition to providing the effects of any one of the first to third aspects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of one side of a spark plug according to a first embodiment. 
         FIG. 2  is a sectional view of a part of the spark plug of  FIG. 1  that is enlarged. 
         FIG. 3  is a sectional view of a spark plug according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention are described below with reference to the attached drawings.  FIG. 1  is a sectional view of one side of a spark plug  10  according to a first embodiment of the present invention, with an axial line O as a boundary. In  FIG. 1 , a lower side in a sheet plane is called “front end side” of the spark plug  10 , and an upper side in the sheet plane is called “rear end side” of the spark plug  10  (this also applies in  FIGS. 2 and 3 ). As shown in  FIG. 1 , the spark plug  10  includes an insulator  11  and a metal shell  30 . 
     The insulator  11  is a substantially cylindrical member made of, for example, alumina having excellent insulation property and mechanical property under high temperatures. An axial hole extends through the insulator  11  along the axial line O. An inclined surface  13  whose diameter decreases towards the front end side while facing the rear end side is formed on a front end side of an inner peripheral surface  12  of the insulator  11  that defines the axial hole. In the insulator  11 , a rear end portion  14 , a large-diameter portion  15 , a small-diameter portion  16 , and a front end portion  17  are formed consecutively in order from the rear end side to the front end side. The large-diameter portion  15  is a part having the largest outside diameter in the insulator  11 . The small-diameter portion  16  is a part having an outside diameter than is smaller than the outside diameter of the large-diameter portion  15 . The front end portion  17  having an outside diameter than is smaller than the outside diameter of the small-diameter portion  16  is adjacent to a front-end side of the small-diameter portion  16  with a retaining portion  18  interposed therebetween. The diameter of the retaining portion  18  decreases towards the front end side. 
     A center electrode  20  is a rod-shaped electrode that is inserted into a front end side of the axial hole and that is held by the insulator  11  along the axial line O. In the center electrode  20 , a head portion  22  that protrudes axially at right angles to a shaft portion  21  extending in directions of the axial line O is formed consecutively with the shaft portion  21 . The head portion  22  is retained by the inclined surface  13 . In the center electrode  20 , a core material having excellent thermal conductivity is embedded in a base material. The base material is formed from a metallic material containing an alloy whose main component is Ni or Ni, and the core material is formed from copper or an alloy containing copper as the main component. The core material need not be used. 
     A metal terminal  23  is a rod-shaped member to which a high-pressure cable (not shown) is connected, and is made of a metallic material (such as low carbon steel) having conductivity. A front end side of the metal terminal  23  is inserted into the axial hole of the insulator  11 . The metal terminal  23  is electrically connected to the head portion  22  of the center electrode  20  by, for example, a conductor containing glass. 
     The metal shell  30  is a substantially cylindrical member made of a metallic material (such as low-carbon steel) having conductivity. The metal shell  30  includes a trunk portion  31  that surrounds a portion of the insulator  11  from the front end portion  17  to the small-diameter portion  16 , a seating portion  32  that is formed consecutively with a rear end side of the trunk portion  31 , a connecting portion  33  that is formed consecutively with a rear end side of the seating portion  32 , a tool engaging portion  34  that is formed consecutively with a rear end side of the connecting portion  33 , and a rear end portion  35  that is formed consecutively with a rear end side of the tool engaging portion  34 . An external thread  36  that is screwed into a threaded hole of an engine (not shown) is formed on an outer periphery of the trunk portion  31 . A stepped portion  37  that protrudes inward in a radial direction is formed along an entire inner periphery of the trunk portion  31 . 
     The seating portion  32  is a part for covering a gap between the threaded hole of the engine (not shown) and the external thread  36 , and has an outside diameter that is larger than the outside diameter of the trunk portion  31 . The connecting portion  33  is a part plastically deformed into a curved shape when the metal shell  30  is mounted on the insulator  11 . The tool engaging portion  34  is a part that is caused to engage with a tool, such as a wrench, when the external thread  36  is tightened in the threaded hole of the engine. The rear end portion  35  is a part bent inward in the radial direction, and is positioned on the rear end side of the large-diameter portion  15  of the insulator  11 . A seal portion  38  filled with, for example, talc powder is provided between the large-diameter portion  15  and the rear end portion  35  over the entire outer periphery of the rear end portion  14  of the insulator  11 . 
     The stepped portion  37  of the metal shell  30  is positioned on a front end side of the retaining portion  18  of the insulator  11 . When the metal shell  30  is mounted on the insulator  11 , a portion of the metal shell  30  from the stepped portion  37  to the rear end portion  35  of the metal shell  30  applies a compression load in a direction of the axial line O to a portion of the insulator  11  from the small-diameter portion  16  to the large-diameter portion  15  via the seal portion  38 . As a result, the metal shell  30  holds the insulator  11 . A ground electrode  39  is a rod-shaped metallic member (made of, for example, a nickel-based alloy) and that is joined to the trunk portion  31  of the metal shell  30 . A spark gap is formed between the ground electrode  39  and the center electrode  20 . 
       FIG. 2  is a sectional view of a part of the spark plug  10  of  FIG. 1  (vicinity of the stepped portion  37 ) that is enlarged, with the axial line O (see  FIG. 1 ) being included. The stepped portion  37  includes a first convex portion  41  that protrudes inward (towards the right in  FIG. 2 ) in the radial direction from the trunk portion  31  of the metal shell  30  and a second convex portion  42  that protrudes inward in the radial direction from the trunk portion  31 . The second convex portion  42  is adjacent to the first convex portion  41  on a front end side (lower side in  FIG. 2 ) of the first convex portion  41 . A connection portion  43  connects the first convex portion  41  and the second convex portion  42  to each other. 
     The first convex portion  41  includes a rear-end-side facing surface  44  and a front-end-side facing surface  45 . The rear-end-side facing surface  44  faces the retaining portion  18  of the insulator  11 . The rear-end-side facing surface  44  is a surface that retains the insulator  11 , and has a diameter that decreases towards the front end side in a direction of the axial line O (up-down direction in  FIG. 2 ). In the present embodiment, the rear-end-side facing surface  44  is in contact with the retaining portion  18  of the insulator  11 . The front-end-side facing surface  45  is a surface formed consecutively with the connection portion  43  and has a diameter that increases towards the front end side. 
     At the second convex portion  42 , in order from the rear end side to the front end side, a first surface  46 , a second surface  47 , and a third surface  48  are formed consecutively. The first surface  46  is a surface that faces the rear end side, and has a diameter that decreases towards the front end side. The second surface  47  is a surface that faces a direction perpendicular to the axial line O (towards the side of the front end portion  17  of the insulator  11 ). The third surface  48  is a surface that faces the front end side and has a diameter that increases towards the front end side. 
     The connection portion  43  is a surface that corresponds to a valley bottom that connects the front-end-side facing surface  45  of the first convex portion  41  and the first surface  46  of the second convex portion  42  to each other. In a direction perpendicular to the axial line O (left-right direction in  FIG. 2 ), the connection portion  43  exists in a range  49  where a portion of the rear-end-side facing surface  44  that contacts the insulator  11  is positioned). In retaining the insulator  11  by the metal shell  30  and mounting the metal shell  30  on the insulator  11 , when the first convex portion  41  is subjected to a force acting towards the front end side (lower side in  FIG. 2 ) in the direction of the axial line O from the insulator  11 , a tensile stress is produced at the first convex portion  41  along the rear-end-side facing surface  44 , and a compression stress is produced at the first convex portion  41  along the front-end-side facing surface  45 . As a result, it is possible to closely contact the rear-end-side facing surface  44  with the insulator  11  by an opposing force that acts towards the rear end side (upper side in  FIG. 2 ) and that is produced at the first convex portion  41 . Therefore, even if a load that the insulator  11  applies to the metal shell  30  is not made excessively large, it is possible to ensure airtightness between the stepped portion  37  and the insulator  11 . 
     Since the rear-end-side facing surface  44  is made to contact the insulator  11 , it is possible not to use a packing that is interposed between the stepped portion  37  and the insulator  11 . It is possible to reduce the number of components in proportion to a packing that is not used and to prevent occurrence of cracking of the small-diameter portion  16  and the front end portion  17  of the insulator  11  that is caused by excessive deformation of the packing. 
     In the present embodiment, in a cross section including the axial line O (see  FIG. 2 ), an angle θ 1  (acute-angle side) formed by an imaginary straight line  50 , which passes through the connection portion  43  and which is parallel to the axial line O, and the rear-end-side facing surface  44  is greater than an angle θ 2  (acute-angle side) formed by the imaginary straight line  50  and the front-end-side facing surface  45  (θ 1 &gt;θ 2 ). Therefore, compared to when θ 1 ≤θ 2 , it is possible to make it less likely for the first convex portion  41  that is subjected to a force from the insulator  11  to buckle and to increase the opposing force that acts towards the rear end side and that is produced at the first convex portion  41 . Consequently, it is possible to increase airtightness. 
     In the cross section including the axial line O, a length L 1  of the first convex portion  41  on the imaginary straight line  50  is less than a length L 2  of the second convex portion  42  on the imaginary straight line  50  (L 1 &lt;L 2 ). Therefore, compared to when L 1 ≥L 2 , the first convex portion  41  subjected to a force acting towards the front end side in the axial-line direction can be easily elastically deformed, and it is possible to ensure the opposing force that is produced by the elastic deformation of the first convex portion  41 . Consequently, it is possible to increase airtightness between the first convex portion  41  and the insulator  11 . 
     The length L 1  is a length of a line segment from a point of intersection of the rear-end-side facing surface  44  and the imaginary straight line  50  to the connection portion  43 . The length L 2  is a length of a line segment from a point of intersection of a perpendicular line and the imaginary straight line  50  to the connection portion  43 , the perpendicular line passing through a front end  51  of the third surface  48  and being perpendicular to the imaginary straight line  50 . Since the connection portion  43  is in contact with the imaginary straight line  50  at one point, the length of the connection portion  43  on the imaginary straight line  50  is 0. 
     In the cross section including the axial line O, a distance D 2  from the imaginary straight line  50  to an innermost position of the second convex portion  42  in the radial direction is greater than a distance D 1  from the imaginary straight line  50  to an innermost position of the first convex portion  41  in the radial direction. Therefore, a load that is applied to the connection portion  43  by the first convex portion  41  subjected to a force acting towards the front end side in the axial-line direction can be easily dispersed by the second convex portion  42 . As a result, it is possible to make it less likely for the first convex portion  41  to buckle. Further, since the connection portion  43  is rounded, compared to when the connection portion  43  is angular, it is possible to more easily disperse the load. 
     Since the second convex portion  42  includes the third surface  48  whose diameter increases towards the front end side, compared to when the second surface  47  is continuously formed up to a front end of the metal shell  30  without the third surface  48  existing, it is possible to ensure a gap between the trunk portion  31  and the front end portion  17 . Therefore, it is possible to suppress staining of the front end portion  17  by carbon produced by, for example, incomplete combustion of an air-fuel mixture and to suppress leaks. 
     Since the second convex portion  42  is surrounded by the first surface  46 , the second surface  47 , and the third surface  48 , compared to when the second surface  47  that faces inward in the radial direction does not exist (the third surface  48  is connected to the first surface  46 ), it is possible to increase cross-sectional second moment of the second convex portion  42 . As a result, since a buckling load of the second convex portion  42  can be increased, the second convex portion  42  can be subjected to a load that the first convex portion  41  applies to the connection portion  43 . Therefore, it is possible to make it less likely for the first convex portion  41  to buckle. 
     A second embodiment is described with reference to  FIG. 3 . In the first embodiment, the spark plug  10  in which the insulator  11  is directly retained by the metal shell  30  is described. In contrast, in the second embodiment, a case in which an insulator  11  is retained by a metal shell  61  via a packing  62  (different member) is described. Corresponding portions to those described in the first embodiment are given the same reference numerals and are not described below.  FIG. 3  is a sectional view of a spark plug  60  according to the second embodiment, with an axial line O (see  FIG. 1 ) being included.  FIG. 3  shows a portion that is similar to the portion shown in  FIG. 2 . 
     The spark plug  60  includes the insulator  11  and the metal shell  61 . The metal shell  61  is a substantially cylindrical member made of a metallic material (such as low-carbon steel) having conductivity. A stepped portion  70  that protrudes inward (towards the right in  FIG. 3 ) in a radial direction is formed along an entire inner periphery of a trunk portion  31  of the metal shell  61 . The stepped portion  70  is positioned on a front end side of a retaining portion  18  of the insulator  11 . The packing  62  is interposed between the retaining portion  18  and the stepped portion  70 . The packing  62  is a circular-ring-shaped plate member made of a metallic material, such as a soft steel plate, that is softer than the metallic material of the metal shell  61 . 
     When the metal shell  61  is mounted on the insulator  11 , a portion of the metal shell  61  from the stepped portion  70  to a rear end portion  35  (see  FIG. 1 ) of the metal shell  61  applies a compression load in a direction of the axial line O (up-down direction in  FIG. 3 ) to a portion of the insulator  11  from a small-diameter portion  16  to a large-diameter portion  15  (see  FIG. 1 ) via a seal portion  38  and the packing  62 . As a result, the metal shell  61  holds the insulator  11 . The packing  62  is deformed and compressed in the direction of the axial line O by the compression load. 
     The stepped portion  70  includes a first convex portion  71  that protrudes inward in the radial direction from the trunk portion  31  and a second convex portion  72  that protrudes inward in the radial direction from the trunk portion  31 . The second convex portion  72  is adjacent to the first convex portion  71  on a front end side (lower side in  FIG. 3 ) of the first convex portion  71 . A connection portion  73  connects the first convex portion  71  and the second convex portion  72  to each other. 
     The first convex portion  71  includes a rear-end-side facing surface  74  and a front-end-side facing surface  75 . The rear-end-side facing surface  74  faces the retaining portion  18  of the insulator  11 . The rear-end-side facing surface  74  is a surface that retains the insulator  11 , and has a diameter that decreases towards the front end side in a direction of the axial line O (up-down direction in  FIG. 3 ). In the present embodiment, the rear-end-side facing surface  74  is in contact with the packing  62 . The front-end-side facing surface  75  is a surface formed consecutively with the connection portion  73  and has a diameter that increases towards the front end side. 
     At the second convex portion  72 , in order from the rear end side to the front end side, a first surface  76 , a second surface  77 , and a third surface  78  are formed consecutively. The first surface  76  is a surface that faces the rear end side, and has a diameter that decreases towards the front end side. The second surface  77  is a surface that faces a direction perpendicular to the axial line O (towards the side of the front end portion  17  of the insulator  11 ). The third surface  78  is a surface that faces the front end side and has a diameter that increases towards the front end side. 
     The connection portion  73  is a surface that corresponds to a valley bottom that connects the front-end-side facing surface  75  of the first convex portion  71  and the first surface  76  of the second convex portion  72  to each other. In a direction perpendicular to the axial line O (left-right direction in  FIG. 3 ), the connection portion  73  exists in a range  79  where a portion of the rear-end-side facing surface  74  that contacts the packing  62  is positioned. 
     Therefore, in retaining the insulator  11  by the metal shell  61  and mounting the metal shell  61  on the insulator  11 , when the first convex portion  71  is subjected to a force acting towards the front end side (lower side in  FIG. 3 ) in a direction of the axial line O from the insulator  11 , a tensile stress is produced at the first convex portion  71  along the rear-end-side facing surface  74 , and a compression stress is produced at the first convex portion  71  along the front-end-side facing surface  75 . As a result, it is possible to closely contact the rear-end-side facing surface  74  with the retaining portion  18  of the insulator  11  via the packing  62  by an opposing force that acts towards the rear end side (upper side in  FIG. 3 ) and that is produced at the first convex portion  71 . Therefore, even if a load that the insulator  11  applies to the metal shell  61  is not made excessively large, it is possible to ensure airtightness between the stepped portion  70  of the metal shell  61  and the insulator  11 . Further, since the first convex portion  71  is elastically deformed and, thus, excessive deformation of the packing  62  is suppressed, it is possible to suppress occurrence of cracking of the small-diameter portion  16  and the front end portion  17  of the insulator  11  that is caused by the packing  62 . 
     In the present embodiment, in a cross section including the axial line O (see  FIG. 3 ), an angle θ 1  (acute-angle side) formed by an imaginary straight line  80 , which passes through the connection portion  73  and which is parallel to the axial line O, and the rear-end-side facing surface  74  is less than or equal to an angle θ 2  (acute-angle side) formed by the imaginary straight line  80  and the front-end-side facing surface  75  (θ 1 ≤θ 2 ). Therefore, compared to when θ 1 &gt;θ 2 , it is possible to suppress an opposing force of the first convex portion  71  that is subjected to a force from the insulator  11 , and to make it easier to suppress excessive deformation of the packing  62 . 
     In the cross section including the axial line O, a length L 1  of the first convex portion  71  on the imaginary straight line  80  is less than a length L 2  of the second convex portion  72  on the imaginary straight line  80  (L 1 &lt;L 2 ). Therefore, compared to when L 1 ≥L 2 , the first convex portion  71  subjected to a force acting towards the front end side in the axial-line direction can be easily elastically deformed, and it is possible to ensure an opposing force that is produced by the elastic deformation of the first convex portion  71 . Consequently, it is possible to increase airtightness between the first convex portion  71  and the insulator  11  via the packing  62 . 
     The length L 1  is a length of a line segment from a point of intersection of the rear-end-side facing surface  74  and the imaginary straight line  80  to a rear end of the connection portion  73 . The length L 2  is a length of a line segment from a point of intersection of the third surface  78  and the imaginary straight line  80  to a front end of the connection portion  73 . The connection portion  73  is in line-contact with the imaginary straight line  80 . A length L 3  of the connection portion  73 , which is a length of contact of the imaginary straight line  80  with the connection portion  73 , is less than or equal to 0.1 mm. Since the length L 3  of the connection portion  73  is less than or equal to 0.1 mm, the load that is applied to the connection portion  73  by the first convex portion  71  subjected to a force acting towards the front end side in the axial-line direction can be easily dispersed by the second convex portion  72 . Therefore, it is possible to suppress buckling of the first convex portion  71 . 
     In the cross section including the axial line O, a distance D 2  from the imaginary straight line  80  to an innermost position of the second convex portion  72  in the radial direction is less than a distance D 1  from the imaginary straight line  80  to an innermost position of the first convex portion  71  in the radial direction (D 1 &gt;D 2 ). Therefore, compared to when D 1 ≤D 2 , since a spatial distance between the second surface  77  of the second convex portion  72  and the front end portion  17  of the insulator  11  can be made long, it is possible to suppress, for example, accumulation of carbon produced by, for example, incomplete combustion of an air-fuel mixture and to make it easier to produce a predetermined spark discharge between a center electrode  20  (see  FIG. 1 ) and a ground electrode  39 . 
     Although the present invention has been described on the basis of the embodiments, it can be easily inferred that various improvements and modifications are possible within a scope that does not depart from the spirit of the present invention. For example, the shapes and dimensions (the distances D 1  and D 2  and the lengths L 1 , L 2 , and l 3 ) of the first convex portions  41  and  71  and the second convex portions  42  and  72  are examples and are settable as appropriate. 
     Although, in the embodiments, the front-end-side facing surface  45  of the first convex portion  41  and the front-end-side facing surface  75  of the first convex portion  71  each have been described as having a conical shape whose diameter increases towards the front end side (circular conical surface), the front-end-side facing surfaces  45  and  75  are not necessarily limited thereto. The front-end-side facing surfaces  45  and  75  may obviously be surfaces perpendicular to the axial line O. 
     Although, in the embodiments, the first surface  46  of the second convex portion  42  and the first surface  76  of the second convex portion  72  each have been described as having a conical shape whose diameter decreases towards the front end side (circular conical surface), the first surfaces  46  and  76  are not necessarily limited thereto. The first surfaces  46  and  76  may obviously be surfaces perpendicular to the axial line O. 
     Although, in the embodiments, the second convex portions  42  and  72  have been described as including the respective second surfaces  47  and  77  (cylindrical surfaces) facing inward in the radial direction, the second convex portions  42  and  72  are not necessarily limited thereto. The third surfaces  48  and  78  may obviously be connected to the respective first surfaces  46  and  76  without using the respective second surfaces  47  and  77 . 
     Although, in the embodiments, the third surface  48  of the second convex portion  42  and the third surface  78  of the second convex portion  72  each have been described as having a conical shape whose diameter increases towards the front end side (circular conical surface), the third surfaces  48  and  78  are not necessarily limited thereto. The third surfaces  48  and  78  may obviously be surfaces perpendicular to the axial line O. 
     Although, in the embodiments, the second convex portions  42  and  72  have been described as including the respective third surfaces  48  and  78 , the second convex portions  42  and  72  are not necessarily limited thereto. The second surfaces  47  and  77  may obviously be continuously formed up to the front end of the metal shell  30  without using the respective third surfaces  48  and  78 . 
     Although, in the first embodiment, a case in which the first convex portion  41  directly retains the insulator  11  has been described, the present invention is not necessarily limited thereto. As in the second embodiment, a packing  62  (another member) may obviously be interposed between the first convex portion  41  and the insulator  11 . Similarly, in the second embodiment, the first convex portion  71  may obviously directly retain the insulator  11  without using the packing  62 . 
     Although, in the embodiments, a case in which one ground electrode  39  is joined to the metal shell  30  has been described, the present invention is not necessarily limited thereto. A plurality of ground electrodes may obviously be joined to the metal shell  30 .