Patent Publication Number: US-2013248508-A1

Title: Glow plug

Description:
TECHNICAL FIELD 
     The present invention relates to a glow plug used for assisting in start-up of a diesel engine. 
     BACKGROUND ART 
     A glow plug used for assisting in start-up of a diesel engine is such that a heater which has a heat-generating resistor at its forward end portion is held directly or indirectly at a forward end portion of a tubular metallic shell having an axial bore. A rodlike center shaft is inserted into the axial bore of the metallic shell and disposed in a condition electrically insulated from the metallic shell. One end portion of the center shaft is connected to a rear end portion of the heater, and the other end portion projects from the rear end of the metallic shell. Two electrodes led from the heater are electrically connected to the metallic shell and the center shaft, respectively. 
     In the thus-configured glow plug, in order to ensure gastightness through the axial bore of the metallic shell, an O-ring is disposed between the center shaft and the inner circumferential surface (wall surface) of the axial bore at the rear end side of the axial bore. Furthermore, an insulation member for ensuring electrical insulation between the metallic shell and the center shaft is disposed, on the rear end side of the O-ring, between the center shaft and the inner circumferential surface of the axial bore. A taper surface is provided in a region of the inner circumferential surface of the axial bore and/or a region of the center shaft where the O-ring is disposed. The end surface of the insulation member presses the O-ring toward the taper surface, thereby ensuring close contact of the O-ring with three contact surfaces of the inner circumferential surface of the axial bore, the center shaft, and the insulation member (refer to, for example Patent Document 1). 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2007-292444 
       
    
     SUMMARY OF THE INVENTION 
     However, in the configuration in which the three contact surfaces of the inner circumferential surface, the center shaft, and the insulation member surround the O-ring, difficulty is encountered in providing a space for accepting deformation of a compressed O-ring. Thus, the positions of disposition of the contact surfaces must be determined accurately in relation to the O-ring. For example, in the case where the insulation member is strongly pressed into a space between the center shaft and the inner circumferential surface of the axial bore, the disposed O-ring assumes a small size. Accordingly, the O-ring is deformed in such a manner as to enter a narrow gap between the contact surfaces; thus, internal stress in a deformed portion of the O-ring increases, potentially resulting in breakage of the O-ring. By contrast, in the case where the insulation member is loosely pressed into the space, the disposed O-ring assumes a large size. Accordingly, the O-ring fails to maintain sufficient close contact with the contact surfaces; thus, a clearance is formed therebetween, potentially resulting in a failure to ensure gastightness. 
     Also, conventional glow plugs employ O-rings having a circular or elliptic cross section. In the process of assembling a glow plug, when the O-ring is to be disposed in a narrow gap between the center shaft and the inner circumferential surface of the axial bore, there has been involved the risk of twist of the O-ring along the circumferential direction. Also, the O-ring has a circular cross section, or the O-ring having an elliptic cross section assumes a substantially circular cross section when disposed in a space surrounded by the three contact surfaces. Thus, since a portion of the O-ring which intervenes between the center shaft and the inner circumferential surface of the axial bore assumes a short axial length, when external vibration is imposed on the center shaft, the effect of restraining oscillation of the center shaft is unlikely to be exhibited. Joint use of an axially long tube-like vibration insulating rubber or the like encounters difficulty in disposition between the center shaft and the inner circumferential surface of the axial bore and increases cost. 
     The present invention has been conceived to solve the above problems, and an object of the invention is to provide a glow plug having a contact member which enhances close contact with the center shaft and with the inner circumferential surface of the axial bore of the metallic shell, ensures vibration insulation for the center shaft, and facilitates attachment thereof. 
     According to an embodiment of the present invention, there is provided a glow plug comprising a heater having, in a forward end portion, a heat-generating resistor which generates heat through energization; a metallic shell assuming the form of a tube having an axial bore extending along a first axis which is the axis of the metallic shell, and holding the heater directly or indirectly at a forward end portion; a center shaft assuming a rodlike form, disposed in the axial bore of the metallic shell with a gap formed between the same and an inner circumferential surface of the axial bore, having one end portion connected to a rear end portion of the heater, and having the other end portion projecting from a rear end of the metallic shell; and a contact member formed from an electrically insulative elastic member, assuming an annular form, inserted into the axial bore at a rear end portion of the axial bore, and disposed in such a manner as to be in contact with the inner circumferential surface of the axial bore and with the center shaft; the glow plug being characterized in that a contour of one of two cross sections resulting from cutting the contact member in a state prior to attachment to the glow plug by a plane which contains a second axis which is the axis of the contact member has a first contour segment assuming the form of a curve extending along the second axis and swelling radially outward with a radius R 1  of curvature, and a second contour segment assuming the form of a straight line extending along the second axis or the form of a curve extending along the second axis and swelling radially inward with a radius R 2  of curvature which satisfies a relational expression R 1 &lt;R 2 . 
     In the present embodiment, the second contour segment having the radius R 2  of curvature extends along the second axis with a radius of curvature greater than that of the first contour segment having the radius R 1  of curvature. Thus, when the contact member is pressed in along the second axis, the second contour segment can function as a core which supports the entire contact member and restrains the contact member from bending and being dragged inward. Therefore, the contact member is restrained from bending or wrinkling at the second contour segment. 
     Also, when the contact member is disposed between the center shaft and the inner circumferential surface of the axial bore, the radial thickness of the contact member perpendicular to the second axis is compressed. Since the first contour segment assumes the form of a curve swelling outward in a radial direction of the contact member, compressive deformation can be performed smoothly. Also, since, in the course of deformation, deformation can be such that material moves from a thick region to a thin region, a deformed region having high internal stress does not arise locally; thus, even when the glow plug is subjected to external vibration or the like, the contact member is unlikely to be broken. Also, the contact member is in contact with two members (two surfaces); namely, the center shaft and the inner circumferential surface of the axial bore. Therefore, in view of establishment of gastightness of the axial bore by the contact member, there is no need to form a complicated seal surface on the metallic shell and the center shaft, which are counter members of contact with the contact member, so that machining is facilitated, leading to a reduction in cost. 
     In the present embodiment, the contact member may be such that, as viewed on the one cross section, a length along an extending direction of the second axis is longer than a length along a direction orthogonal to the second axis. Through employment of such configuration, when the contact member is disposed between the center shaft and the inner circumferential surface of the axial bore, there can be increased an axial intervening length of the contact member intervening between the center shaft and the inner circumferential surface of the axial bore. Therefore, the center shaft can be more reliably held in the axial bore; thus, there can be more reliably restrained oscillation of the center shaft stemming from imposition of external vibration on the center shaft. 
     In the present embodiment, the center shaft may further comprise a rear trunk portion disposed at such a position with respect to the extending direction of the first axis of the metallic shell as to face the rear end portion of the inner circumferential surface of the axial bore, and having a diameter greater than that of the other end portion, and a shoulder portion connecting the rear trunk portion and the other end portion in a tapered manner. The metallic shell may further comprise a taper portion expanding in a tapered manner at the rear end portion of the axial bore from a position located forward of a forward end of the shoulder portion toward the rear end located rearward of the position with respect to the extending direction of the first axis. The contact member may be disposed such that the second contour segment is in contact with the center shaft and such that the first contour segment is in contact with an inner circumferential surface of the axial bore located forward of the taper portion. 
     For example, even in the case where the first axis of the metallic shell and the axis of the center shaft fail to coincide with each other, when the contact member is disposed between the center shaft and the inner circumferential surface of the axial bore, the taper portion can guide the contact member toward the center of the axial bore. Therefore, misalignment between the first axis and the axis of the center shaft can be corrected via the contact member. By specifying that the outside diameter of the rear trunk portion be greater than the inside diameter of the contact member, in attachment of the contact member to the glow plug, first, the contact member can be brought into contact with the axial bore in a condition that the inside diameter of the contact member is expanded by the shoulder portion. Then, in a state in which the contact member is in contact with the center shaft (i.e., in a state in which a big gap is not formed between the contact member and the center shaft), the outside diameter of the contact member is narrowed along the taper portion; thus, the contact member is free from inward dragging and, thus, free from twist along the circumferential direction and wrinkling. Therefore, the contact member can be reliably in contact with the inner circumferential surface of the axial bore and with the outer circumferential surface of the center shaft, whereby gastightness through the axial bore can be reliably established. 
     In the present embodiment, the contour of the contact member may further comprise a third contour segment which is connected, at one end, to the second contour segment; which extends radially outward from the one end toward the other end thereof while extending along the second axis; which is connected, at the other end, to the first contour segment; and whose point of connection to the first contour segment is an end of the contact member with respect to the extending direction of the second axis. In attachment of the contact member to the glow plug, the contact member first comes into contact with the shoulder portion. At the time of this contact, first, the third contour segment, which extends radially outward while extending along the second axis, comes into contact with the shoulder portion. Thus, friction of the contact member against the shoulder portion is small, so that the contact member can be prevented from being caught by the shoulder portion and dragged inward. 
     In the present embodiment, the contour may form a mirror image with respect to a center position in the extending direction of the second axis. That is, since the contact member is symmetrical along the extending direction of the second axis, in attachment of the contact member to the glow plug, the contact member can be fitted, in either orientation with respect to the direction of the axis P, to a rear end portion of the center shaft. Thus, there can be omitted labor for checking and correcting orientation in attachment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [ FIG. 1 ] Longitudinal sectional view of a glow plug  1 . 
       [ FIG. 2 ] Sectional view on an enlarged scale of a rear end portion of the glow plug  1 . 
       [ FIG. 3 ] View showing, perspectively and in section, a contact member  7  in a state prior to attachment to the glow plug  1 . 
       [ FIG. 4 ] Sectional view on an enlarged scale of a rear end portion of the glow plug  1  in the process of attaching the contact member  7  to the glow plug  1 . 
       [ FIG. 5 ] Sectional view on an enlarged scale of the rear end portion of the glow plug  1  in the process of attaching the contact member  7  to the glow plug  1 . 
       [ FIG. 6 ] View showing, perspectively and in section, a contact member  107  in a state prior to attachment to the glow plug  1 . 
       [ FIG. 7 ] View showing, perspectively and in section, a contact member  207  in a state prior to attachment to the glow plug  1 . 
       [ FIG. 8 ] View showing, perspectively and in section, a contact member  307  in a state prior to attachment to the glow plug  1 . 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     A glow plug according to an embodiment of the present invention will next be described with reference to the drawings. The entire structure of a glow plug  1  is described, by way of example, with reference to  FIGS. 1 and 2 . The drawings referred to herein are used for explaining technical features which the present invention can employ, and the configuration, etc., of the glow plug appearing in the drawings are given by way of illustration and not of limitation. In the following description, the axis of a metallic shell  4  is referred to as the axis O, and the axis O serves as reference in describing the positional relationship, orientations, and directions of those component members of the glow plug  1  which are attached to the metallic shell  4 . With respect to the extending direction of the axis O (hereinafter, may be referred to as “the direction of the axis O”), a side on which a ceramic heater  2  is disposed (the lower side in  FIG. 1 ) is referred to as the forward side of the glow plug  1 . The axis O corresponds to the “first axis” in the present invention. An axis P, which will be described later, indicates the axis of a contact member  7  in a state prior to attachment to the glow plug  1 , and, in the process of assembly and after assembly, the axis O is used as reference for description. The axis P corresponds to the “second axis” in the present invention. 
     The glow plug  1  shown in  FIG. 1  is attached to, for example, a combustion chamber of a direct-injection-type diesel engine (not shown), and is used as a heat source for assisting in ignition at start-up of an engine. The glow plug  1  includes the metallic shell  4 , a holding member  8 , the ceramic heater  2 , a center shaft  3 , a connection terminal  5 , an insulation member  6 , the contact member  7 , and a connection ring  85 . 
     First, the ceramic heater  2  is described. The ceramic heater  2  assumes the form of a round bar and has a substrate  21  which is formed from an electrically insulating ceramic and whose forward end portion  22  is formed into a hemispherical shape. A heat-generating element  24  formed from an electrically conductive ceramic and having a substantially U-shaped section is embedded in the substrate  21 . The heat-generating element  24  includes a heat-generating resistor  27  and leads  28  and  29 . The heat-generating resistor  27  is disposed in the forward end portion  22  of the ceramic heater  2  and is curved and bent at opposite ends in a shape resembling the letter U according to the curved surface of the forward end portion  22 . The leads  28  and  29  are connected to opposite ends, respectively, of the heat-generating resistor  27  and extend substantially in parallel with each other toward a rear end portion  23  of the ceramic heater  2 . The cross-sectional area of the heat-generating resistor  27  is smaller than that of each of the leads  28  and  29 , and, upon energization, heat is generated mainly by the heat-generating resistor  27 . Electrode lead portions  25  and  26  project radially from the leads  28  and  29 , respectively, at respective positions located rearward of the center of the ceramic heater  2 . The electrode lead portions  25  and  26  are exposed at the outer circumferential surface of the ceramic heater  2  at positions deviated from each other in the direction of the axis O. 
     Next, the holding member  8  is described. The holding member  8  is a cylindrical metal member extending in the direction of the axis O and radially holds a trunk portion of the ceramic heater  2  within its tubular hole  84 . The forward end portion  22  and the rear end portion  23  of the ceramic heater  2  project from the opposite ends of the holding member  8 . A trunk portion  81  of the holding member  8  has a thick-walled flange portion  82  formed on a side toward the rear end thereof. The holding member  8  has a stepped metal-shell engagement portion  83  which is located rearward of the flange portion  82  and is engaged with a forward end portion  41  of the metallic shell  4 , which will be described later. Of the electrode lead portions  25  and  26  of the ceramic heater  2 , the electrode lead portion  25  located on a side toward the forward end is in contact with the inner circumferential surface of the tubular hole  84  of the holding member  8 , whereby the electrode lead portion  25  and the holding member  8  are electrically connected to each other. 
     Also, the tubular connection ring  85  of metal is press-fitted to the rear end portion  23  of the ceramic heater  2  projecting rearward from the metallic-shell engagement portion  83  of the holding member  8 . The electrode lead portion  26  of the ceramic heater  2  is in contact with the inner circumferential surface of the connection ring  85 , whereby the electrode lead portion  26  and the connection ring  85  are electrically connected to each other. As a result of the forward end portion  41  of the metallic shell  4 , which will be described later, being joined to the metallic-shell engagement portion  83  of the holding member  8 , the electrode lead portion  25  is electrically connected to the metallic shell  4 . While the connection ring  85  connected to the electrode lead portion  26  is disposed within the metallic shell  4 , the ceramic heater  2  and the metallic shell  4  are positioned by the holding member  8  such that the connection ring  85  and the metallic shell  4  are held mutually in an electrically insulated condition. 
     Next, the metallic shell  4  is described. The metallic shell  4  is a slender tubular metal member having an axial bore  43  extending therethrough in the direction of the axis O. The inner circumference of the forward end portion  41  of the metallic shell  4  is engaged with the outer circumference of the metallic-shell engagement portion  83  of the above-mentioned holding member  8 , whereby the metallic shell  4  is electrically connected, via the holding member  8 , to the electrode lead portion  25  of the ceramic heater  2 . The overlap region of the forward end portion  41  and the metallic-shell engagement portion  83  is subjected to laser welding, whereby the metallic shell  4  and the holding member  8  are joined together. The metallic shell  4  has an intermediate trunk portion  44  formed between the forward end portion  41  and the rear end portion  45  and extending long in the direction of the axis O, and the intermediate trunk portion  44  has a mounting portion  42  formed on the outer circumferential surface of a portion located on a side toward the rear end, the mounting portion  42  having threads for mounting the glow plug  1  to an engine head of an internal combustion engine (not shown). The intermediate trunk portion  44  also has a tool engagement portion  46  located rearward of the mounting portion  42 , having a hexagonal cross section, and adapted to allow a tool to be engaged therewith in mounting the glow plug  1  to the engine head. As shown in  FIG. 2 , the rear end portion  45  of the metallic shell  4  has a taper portion  47  formed on the inner circumferential surface of the axial bore  43  and expanding in a tapered form from the axial bore  43  to the opening of a rear end  48 . 
     Next, the center shaft  3  is described. As shown in  FIG. 1 , the center shaft  3  is a rodlike metal member extending in the direction of the axis O and is inserted into the axial bore  43  of the metallic shell  4 . An intermediate trunk portion  33  located between a forward end portion  31  and a rear end portion  32  of the center shaft  3  is smaller in outside diameter than the forward end portion  31  and the rear end portion  32 . The forward end portion  31  has a small-diameter ring engagement portion  34  formed at its forward end so as to be engaged with the inner circumference of the connection ring  85 . As a result of the ring engagement portion  34  being engaged with the connection ring  85 , the ceramic heater  2  and the center shaft  3  are unitarily connected together along the axis O via the connection ring  85 . Although unillustrated, the overlap region of the forward end portion  31  and the connection ring  85  is subjected to laser welding, whereby the forward end portion  31  and the connection ring  85  are joined together. Through this joining, the center shaft  3  is electrically connected to the electrode lead portion  26  of the ceramic heater  2  via the connection ring  85 . As mentioned above, since the ceramic heater  2  and the metallic shell  4  are positioned by the holding member  8 , the center shaft  3  and the metallic shell  4  are held mutually in an electrically insulated condition in the axial bore  43 . 
     As shown in  FIG. 2 , the rear end portion  32  of the center shaft  3  has a connection end portion  36  projecting from the rear end  48  of the metallic shell  4 , and a connection base portion  37  which connects the connection end portion  36  and the intermediate trunk portion  33 . The connection end portion  36  has a lock portion  39  formed by knurling its outer circumferential surface. The connection end portion  36 , including the lock portion  39 , is smaller in outside diameter than the connection base portion  37 . A shoulder portion  38  is formed between the connection end portion  36  and the connection base portion  37  for connecting, in a tapered manner, the connection end portion  36  and the connection base portion  37 . 
     The contact member  7  and the insulation member  6  are disposed on the rear end portion  32  of the center shaft  3 . The contact member  7 , which will be described later, is disposed between the inner circumferential surface of the axial bore  43  of the metallic shell  4  and the connection base portion  37  of the center shaft  3 , holds the center shaft  3  in the axial bore  43  to thereby restrain oscillation of the center shaft  3 , and maintains gastightness of the axial bore  43 . 
     The insulation member  6  is a tubular member formed from a heat-resistant, electrically insulative material; for example, nylon (registered trademark), for preventing short circuit which could otherwise result from contact between the metallic shell  4  and the center shaft  3  or the connection terminal  5  (which will be described later). The insulation member  6  is fitted to the center shaft  3  such that the rear end portion  32  of the center shaft  3  is inserted through the insulation member  6 ; is positioned such that a taper portion  63  provided on its outer circumference is in contact with the taper portion  47  of the metallic shell  4 ; and maintains an electrically insulating condition between the metallic shell  4  and the center shaft  3 . In this condition, a rear end  65  of the insulation member  6  projects rearward from the rear end  48  of the metallic shell  4 , and a flange portion  51  (which will be described later) of the connection terminal  5  is in contact with the rear end  65 , whereby the connection terminal  5  and the metallic shell  4  are held mutually in an electrically insulated condition. 
     The connection terminal  5  is fixedly attached to the connection end portion  36  of the center shaft  3 . The connection terminal  5  has a cap-like trunk portion  52  which is fitted externally to the connection end portion  36 , and a pin-like protrusion  53  protruding rearward from the trunk portion  52 . The trunk portion  52  has a flange portion  51  provided at its forward open end in such a manner as to radially project along the entire circumference. When the connection terminal  5  is fitted externally to the connection end portion  36  of the center shaft  3 , the flange portion  51  comes into contact with the rear end  65  of the insulation member  6 . Also, in a state in which the connection terminal  5  is pressed forward with respect to the direction of the axis O, the trunk portion  52  is crimped radially inward, whereby the inner circumferential surface of the trunk portion  52  is firmly locked to the lock portion  39  of the connection end portion  36 . Since the lock portion  39  is knurled, the force of fixation is enhanced for the trunk portion  52  which is crimped to the lock portion  39 , whereby the connection terminal  5  and the center shaft  3  are unitarily fixed and electrically connected to each other. 
     In mounting the glow plug  1  to the engine head (not shown), a plug cap (not shown) is fitted to the protrusion  53  of the connection terminal  5 . The heat-generating element  24  (see  FIG. 1 ) of the ceramic heater  2  generates heat through application of electricity between one end of the heat-generating resistor  27  which is grounded to the engine via the holding member  8  and the metallic shell  4 , and the other end of the heat-generating resistor  27  which is connected to the plug cap via the connection terminal  5  and the center shaft  3 . 
     Next, the contact member  7  is described. As mentioned above, the contact member  7  is a member disposed between the inner circumferential surface of the axial bore  43  of the metallic shell  4  and the connection base portion  37  of the center shaft  3 , holds the center shaft  3  in the axial bore  43  to thereby restrain oscillation of the center shaft  3 , and maintains gastightness of the axial bore  43 . As shown in  FIG. 3 , the contact member  7  is formed into a cylindrical shape from a heat-resistant, electrically insulative material; for example, fluororubber, acrylic rubber, or silicone rubber. Preferably, the contact member  7  has a Knoop hardness of 60 to 80. 
     Specifically, the contact member  7  has such a cylindrical shape as to have a tubular hole  76  extending in the extending direction of its axis P (hereinafter, may be referred to as “the direction of the axis P”), and is formed in such a manner as to have a substantially D-shaped cross section in a state prior to attachment to the glow plug  1 . More specifically, when the cylindrical contact member  7  is cut (split) into two pieces by a plane which contains the axis P, each of the pieces has two cross sections. In the present embodiment, when attention is focused on one  75  of two cross sections of the piece, a contour  70  of the cross section  75  assumes the following form. 
     The contour  70  has three kinds of contour segments (line segments which constitute the contour); namely, a second contour segment  71 , a first contour segment  72 , and third contour segments  73 . The second contour segment  71  extends in the form of a straight line along the axis P. The first contour segment  72  assumes the form of a curve extending along the axis P and swelling radially outward in a direction orthogonal to the axis P. The third contour segments  73  connect the second contour segment  71  and the first contour segment  72  at their ends located on the same side with respect to the axis P, and each of the third contour segments  73  assumes the form of a curve extending toward the outside of the cross section  75  while swelling. Also, connections between the first contour segment  72  and the third contour segments  73  are upper and lower (forward and rear) ends of the contact member  7  with respect to the direction of the axis P. 
     Also, the first contour segment  72  is disposed radially outward of the second contour segment  71  and is longer in length along the direction of the axis P than the second contour segment  71 . Therefore, each of the third contour segments  73  extends radially outward from the end of the second contour segment  71  and is connected to the end of the first contour segment  72 . Furthermore, the contour  70  forms a mirror image with respect to the center position (represented by the dash-dot-dot line A-A in  FIG. 3 ) in the direction of the axis P. That is, the contact member  7  has a symmetrical shape along the direction of the axis P. 
     The following provisions are made for the contact member  7  having the above-mentioned cross-sectional shape, with respect to the shapes of the second contour segment  71  and the first contour segment  72 . The first contour segment  72  assumes, as mentioned above, the form of a curve swelling outward in a radial direction perpendicular to the axis P and has a radius R 1  of curvature. The second contour segment  71  assumes the form of a straight line; however, assuming that the second contour segment  71  assumes the form of a curve having a radius R 2  of curvature, the second contour segment  71  can be considered as a curve having infinite R 2 . Therefore, the present embodiment specifies that the radius R 2  of curvature of the second contour segment  71  and the radius R 1  of curvature of the first contour segment  72  satisfy the relational expression R 1 &lt;R 2 . In other words, the first contour segment  72  swelling radially outward is greater in radial swelling than the second contour segment  71  in the form of a straight line (according to the above assumption, swelling radially inward). Also, the present embodiment specifies that the cross-sectional shape of the contact member  7  is such that a length L 1  along the extending direction of the axis P is longer than a length L 2  along a direction orthogonal to the extending direction of the axis P (i.e., along a radial direction) (i.e., the cross-sectional shape satisfies the relational expression L 1 &gt;L 2 ). 
     The contact member  7  having such a shape can be manufactured by an ordinary method for manufacturing an O-ring except that an employed die or mold differs from that employed in the ordinary method. For example, the contact member  7  can be manufactured by compression forming; specifically, upper and lower dies having shapes corresponding to the shape of the contact member  7  are pressed, from above and from underneath, against a sheet of fluororubber. The manufacturing method is not limited thereto. Injection molding can be utilized; specifically, a material, such as fluororubber, is injected into a split-type mold whose cavity has the shape of the contact member  7 . Alternatively, machining can be utilized; specifically, an annular member (ring) of fluororubber or a like material is machined into the shape of the contact member  7 . 
     The glow plug  1  having such a structure is assembled as outlined below. A material composed of an electrically conductive ceramic powder, binder, etc., is injection-molded into an element green-body which is to become the heat-generating element  24  of the ceramic heater  2 . Also, an electrically insulating ceramic powder is die-pressed into substrate green-body halves which are collectively to become the substrate  21  of the ceramic heater  2 . An assembly of the substrate green-body halves with the element green-body accommodated therein in a sandwiched condition is subjected to press compression. The compressed assembly is subjected to a debindering process, a firing process, such as hot pressing, and then a surface polishing process, thereby yielding the rodlike ceramic heater  2  having a hemispherical forward end. The method of manufacturing the ceramic heater  2  may be modified as appropriate. For example, The substrate green-body may be manufactured as follows: a previously formed substrate green-body half is placed in a die; the element green-body is placed on the substrate green-body half; the electrically insulating ceramic powder is charged into the die; and press compression is performed. 
     The ceramic heater  2  is press-fitted into the connection ring  85  formed by forming a steel material, such as stainless steel, into the shape of pipe, thereby establishing electrical connection between the connection ring  85  and the electrode lead portion  26 . Similarly, the ceramic heater  2  is press-fitted into the holding member  8  formed into a predetermined shape, thereby establishing electrical connection between the holding member  8  and the electrode lead portion  25 . Meanwhile, the center shaft  3  is formed as follows: a rodlike member formed by cutting an iron-based material (e.g., Fe—Cr—Mo steel) into a predetermined dimension is subjected plastic working, cutting, etc. In a state in which the ring engagement portion  34  of the center shaft  3  is engaged with the connection ring  85  fitted to the ceramic heater  2 , the overlap region is subjected to laser welding, thereby uniting the center shaft  3  and the ceramic heater  2 . 
     The tubular metallic shell  4  is formed from an iron-based material, such as S45C, and threads are formed on the mounting portion  42  by rolling. Furthermore, the taper portion  47  is formed, by cutting or the like, on the inner circumferential surface of the axial bore  43  at the rear end portion  45  of the metallic shell  4  in such a manner as to expand in a tapered form from the axial bore  43  to the opening of the rear end  48 . The center shaft  3  united to the ceramic heater  2 , etc., is inserted through the axial bore  43  of the metallic shell  4 . The overlap region of the metallic shell  4  and the holding member  8  is subjected to laser welding, whereby the metallic shell  4  and the holding member  8  are joined together. 
     Next, the contact member  7  is fitted to the rear end portion  32  of the center shaft  3  projecting from the rear end  48  of the metallic shell  4 . As mentioned above, since the contact member  7  has a symmetrical shape (mirror image) along the direction of the axis P, the contact member  7  can be fitted in either orientation with respect to the direction of the axis P. Also, as shown in  FIG. 4 , an inside diameter C 1  of the tubular hole  76  of the contact member  7  is greater than an outside diameter C 2  of the connection end portion  36  of the center shaft  3 . Thus, when the connection end portion  36  is inserted through the contact member  7 , there can be prevented damage to the tubular hole  76  (the inner circumferential surface of the contact member  7 ) which could otherwise result from rubbing between the inner circumferential surface of the tubular hole  76  and the lock portion  39  of the connection end portion  36 . 
     With respect to the direction of the axis O, a forward end position B 1  (corresponding to the boundary between the shoulder portion  38  and the connection base portion  37 ) of the shoulder portion  38  of the center shaft  3  is located rearward of a forward end position B 2  (corresponding to the starting position of expansion of the taper portion  47  of the axial bore  43 ) of the taper portion  47  of the metallic shell  4 . Therefore, after the connection end portion  36  is inserted through the contact member  7 , the contact member  7  reaches the shoulder portion  38  of the center shaft  3  before coming into contact with the taper portion  47  of the metallic shell  4 . 
     Also, the inside diameter C 1  of the tubular hole  76  of the contact member  7  is smaller than the outside diameter C 3  of the connection base portion  37  of the center shaft  3 . Therefore, when the contact member  7  which has reached the shoulder portion  38  is further pressed in forward along the direction of the axis O, the tubular hole  76  is expanded along the taper of the shoulder portion  38 . At this time, the third contour segment  73  of the cross section  75  of the contact member  7  (see  FIG. 3 ) is pressed against the tapered shoulder portion  38 . Since the third contour segment  73  assumes the form of a curve swelling toward the outside of the cross section  75 , the inside diameter of the contact member  7  can be smoothly expanded along the taper of the shoulder portion  38 . 
     When the contact member  7  whose tubular hole  76  is expanded by the shoulder portion  38  is pressed in forward along the direction of the axis O in a state in which the inner circumferential surface of the tubular hole  76  is in contact with the outer circumferential surface of the connection base portion  37 , next, the outer circumferential surface of the contact member  7  comes into contact with the taper portion  47  of the metallic shell  4 . When the contact member  7  is further pressed in, as shown in  FIG. 5 , a portion of the contact member  7  on a side toward the outer circumferential surface is elastically deformed along the taper portion  47 , and the contact member  7  is inserted forward beyond the forward end position B 2  of the taper portion  47 . The first contour segment  72  assumes the form of a curve swelling outward in a radial direction of the contact member  7  with the radius R 1  of curvature. Therefore, as compared with the second contour segment  71  assuming the form of a straight line (assuming that the second contour segment  71  assumes the form of a curve, the radius R 2  of curvature is infinite), the first contour segment  72  can be more smoothly deformed such that the radial thickness of the contact member  7  is compressed. The contact member  7  is first expanded in inside diameter by the shoulder portion  38  of the center shaft  3  and then reaches a state in which the inner circumferential surface of the tubular hole  76  is in contact with the outer circumferential surface of the connection base portion  37 . While the contact member  7  is in this state (i.e., in a state in which a big gap is not formed between the contact member  7  and the center shaft  3 ), the outside diameter of the contact member  7  is narrowed by the taper portion  47  of the metallic shell  4 ; thus, the contact member  7  is free from inward dragging and, thus, free from twist along the circumferential direction and wrinkling. Meanwhile, a jig may be used, or, as shown in  FIG. 5 , the insulation member  6  may be utilized for pressing the contact member  7  into a space between the connection base portion  37  and the inner circumferential surface of the axial bore  43 . 
     As mentioned above, the second contour segment  71  of the contact member  7  assumes the form of a straight line along the axis P. Thus, when the contact member  7  is pressed in along the axis O such that the pressing force acts mainly along its axis P, the second contour segment  71  can function as a core which supports the entire contact member and restrains the contact member from bending and being dragged inward. Therefore, the contact member  7  is restrained from bending or wrinkling at the second contour segment  71 . 
     Also, when the contact member  7  is disposed between the connection base portion  37  and the inner circumferential surface of the axial bore  43 , the contact member  7  is guided toward the center of the axial bore  43  by the taper portion  47  of the metallic shell  4 . Thus, for example, even when misalignment arises between the axis O of the metallic shell  4  and the axis of the center shaft  3 , the center shaft  3  is guided toward the center of the axial bore  43  via the contact member  7 ; therefore, misalignment between the axis O and the axis of the center shaft  3  can be corrected. 
     In this manner, since the cross section of the contact member  7  has the first contour segment  72  which swells radially outward with the radius R 1  of curvature, the contact member  7  can be smoothly elastically deformed. Furthermore, since the cross section has the second contour segment  71  in the form of a straight line, the contact member  7  is reliably disposed between the connection base portion  37  and the inner circumferential surface of the axial bore  43  without involvement of bending and wrinkling. Also, the contact member  7  whose cross section before attachment has the D-shaped contour  70  is deformed such that its radial thickness is compressed to thereby impart, to the contact member  7 , a flat profile along the axis P. Thus, deformation is such that material in a thickest region (located near the center with respect to the direction of the axis P) moves toward thin regions (opposite end regions with respect to the direction of the axis P). Therefore, in the contact member  7 , a deformed region having high internal stress does not arise locally; thus, even when the glow plug  1  is subjected to external vibration or the like, the contact member  7  is unlikely to be broken. As viewed after attachment, the contact member  7  does not maintain the form of a mirror image along the direction of the axis P, and such deformation reliably establishes close contact of the contact member  7  with the inner circumferential surface of the axial bore  43  and with the connection base portion  37 . Therefore, sufficient reaction is generated against the outer circumferential surface of the connection base portion  37  and against the inner circumferential surface of the axial bore  43 . Thus, the contact member  7  can reliably hold the center shaft  3  in the axial bore  43  and thus can restrain oscillation of the center shaft  3  when the glow plug  1  receives external vibration or the like. 
     Also, in a space between the outer circumferential surface, encircling the axis O, of the connection base portion  37  of the center shaft  3  and the inner circumferential surface, encircling the axis O, of the axial bore  43  of the metallic shell  4 , the contact member  7  has two radially separated contact surfaces for contact with the connection base portion  37  and for contact with the inner circumferential surface of the axial bore  43 . For example, the position of the disposed contact member  7  may shift due to vibration generated as a result of operation of the glow plug  1 . However, for the contact member  7  having the two radially separated contact surfaces for contact with the connection base portion  37  and for connection with the inner circumferential surface of the axial bore  43 , such a positional shift emerges merely as shifts of positions of contact with the two members, and the sizes of the contact surfaces can be maintained intact. Additionally, since the contact member  7  can maintain, through elastic deformation, a state of contact with the inner circumferential surface of the axial bore  43  and with the connection base portion  37 , gastightness through the axial bore  43  can be reliably ensured. Also, the contact member  7  is in contact with two surfaces; namely, the inner circumferential surface of the axial bore  43  and the outer circumferential surface of the connection base portion  37 . Therefore, in view of establishment of gastightness of the axial bore  43  by the contact member  7 , there is no need to form a complicated seal surface on the metallic shell  4  and the center shaft  3 , which are counter members of contact with the contact member  7 , so that machining is facilitated, leading to a reduction in cost. 
     Since the contact member  7  satisfies the relational expression L 1 &gt;L 2 , when the contact member  7  is disposed between the center shaft  3  and the inner circumferential surface of the axial bore  43 , there can be increased the intervening length, along the direction of the axis O, of the contact member  7  which intervenes between the center shaft  3  and the inner circumferential surface of the axial bore  43 . Therefore, the center shaft  3  can be more reliably held in the axial bore  43 ; thus, there can be more reliably restrained oscillation of the center shaft  3  stemming from imposition of external vibration on the center shaft  3 . 
     After the contact member  7  is disposed between the connection base portion  37  and the inner circumferential surface of the axial bore  43 , as shown in  FIG. 2 , the insulation member  6  is fitted to the rear end portion  32  of the center shaft  3 . In a state in which the insulation member  6  is positioned such that the taper portion  63  of the insulation member  6  is in contact with the taper portion  47  of the metallic shell  4 , the insulation member  6  is fitted to the rear end portion  32  of the center shaft  3 . The trunk portion  52  of the connection terminal  5  is crimped, whereby the connection terminal  5  is fixed to the connection end portion  36  of the center shaft  3 , and the glow plug  1  is completed. 
     The present invention can be modified in various forms. In the embodiment described above, the connection member  7  is formed such that connections between the first contour section  72  and the third contour sections  73  are its upper and lower (forward and rear) ends. However, the present invention is not limited thereto. For example, the proportion of the third contour sections  73  to the contour  70  may be increased such that the third contour segments are the upper and lower ends with respect to the direction of the axis P. 
     Also, for example, the contact member may be formed as in the case of a contact member  107  shown in  FIG. 6 ; specifically, a contour  170  of a cross section  175  does not have the third contour segments and has a shape resembling the letter D such that a second contour segment  171  and a first contour segment  172  are directly connected. Also, the contact member may be formed as in the case of a contact member  207  shown in  FIG. 7 ; specifically, a contour  270  of a cross section  275  does not assume the form of a mirror image with respect to the center position in the direction of the axis P; i.e., the contour  270  is asymmetric along the direction of the axis P. 
     Also, for example, the contact member may be formed as in the case of a contact member  307  shown in  FIG. 8 ; specifically, a contour  370  of a cross section  375  has a second contour segment  371  assuming the form of a curve swelling radially inward with the radius R 2  of curvature. In this case, similar to the present embodiment, it suffices for the radius R 1  of curvature of a first contour section  372  swelling radially outward and the radius R 2  of curvature of the second contour section  371  to satisfy the relational expression R 1 &lt;R 2 . Through employment of this relationship, when the contact member  307  is disposed between the center shaft  3  and the inner circumferential surface of the axial bore  43 , the first contour segment  372  larger in the degree of swelling than the second contour segment  371  can be smoothly elastically deformed. The second contour segment  371  smaller in the degree of swelling and closer in shape to a straight line than the first contour segment  372  can function as a core which supports the entire contact member and restrains the contact member from bending and being dragged inward. 
     The contact members  107 ,  207 , and  307  shown in  FIGS. 6 ,  7 , and  8 , respectively, are similar in preferred features to the contact member  7  of the present embodiment shown in  FIG. 3 . Specifically, preferably, the cross section is such that the length L 1  along the extending direction of the axis P is longer than the length L 2  along a direction orthogonal to the extending direction of the axis P (along a radial direction); i.e., the cross section has a short radial length so as to have a flat profile (L 1 &gt;L 2 ). More preferably, the cross section is such that the radial length L 2  is half the length L 1  along the direction of the axis P or less (L 1 / 2 ≧L 2 ). However, depending on material and detailed configurational features, this relationship (half of the length along the axis P&gt;the radial length (L 1 / 2 ≧L 2 )) may not be satisfied so long as the relationship L 1 &gt;L 2  is satisfied). 
     The glow plug  1  has the ceramic heater  2 . However, the present invention is not limited thereto. The glow plug  1  may have a sheath heater configured such that a coil-like heat-generating resistor and a controlling resistor are disposed within a metallic sheath tube whose tip is hemispherically closed. Also, the glow plug may be such that the forward end portion  41  of the metallic shell  4  directly holds the ceramic heater  2  or the sheath heater without use of the holding member  8 . The ceramic heater may be of a so-called surface heat-generation type in which the heat-generating element  24  is externally disposed around the substrate  21 . 
     The connection base portion  37  of the center shaft  3  may have, at a position located toward its forward end, a stopper in the form of a flange or a protrusion for forming some level difference. Through provision of such a stopper, even when the contact member  7  is positionally shifted as a result of reception of vibration or the like, the stopper prevents movement of the contact member  7  to the intermediate trunk portion  33  of the center shaft  3 . Also, the contact member  7  may be disposed between the connection base portion  37  and the inner circumferential surface of the axial bore  43  in a noncontacting manner in relation to the insulation member  6 . 
     The contact member  7  of the present embodiment is expected primarily to ensure gastightness of the axial bore  43  and is also expected to hold the center shaft  3  to thereby restrain radial oscillation of the center shaft  3 . Therefore, preferably, the contact member  7  is in close contact with the inner circumferential surface of the axial bore  43  and with the outer circumferential surface of the center shaft  3 , along the full circumference around the axis O, since vibration isolation and gastightness can be ensured. 
     In the present embodiment, the forward end portion  31  corresponds to “one end portion,” and the connection end portion  36  corresponds to “the other end portion.” The connection base portion  37  corresponds to “a rear trunk portion.”