Glow plug

Abstract

Glow plug, comprising: a tubular metal sleeve (40) having an axial opening (43) extending through the metal sleeve (40) along the direction of an axis (O); a rod-shaped central shaft (30) extending along the direction of the axis (O) and disposed in the axial opening (43) of the metal sleeve (40) with a clearance formed between the central shaft (30) and a wall surface of the axial opening (43), an end part of the central shaft (30) protruding from a rear end surface of the metal sleeve (40); an O-ring (70) disposed at a rear end of the axial opening (43) between the wall surface of the axial opening (43) and the center shaft (30), the O-ring (70) being in close contact with the wall surface of the axial opening (43) and an outer peripheral surface of the center shaft (30); and an annular pressing member (60) having an insertion opening (62) into which the central shaft (30) is inserted, the pressing member (60) being at least partially disposed between the wall surface of the axial opening (43) and the central shaft (30), and having an end surface for pressing the O-ring (70) from the rear side thereof, a part of the wall surface of the axial opening (43) with which part the O-ring (70) comes into close contact and / or a part of the outer peripheral surface of the central shaft (30) with which part the O-ring (70) comes into close contact is / is formed as a chamfer surface (47) which increases the clearance between the wall surface of the axial opening (43) and the central shaft (30) in the direction of the axis (O) towards the rear end side; and wherein the O-ring (70) assumes an annular shape which extends around the axis (O, P) and is configured such that, in one of the two cross sections of the O-ring (70) perpendicular to its circumferential direction, a distance (V) between two tangential lines which are parallel to a radial direction of the O-ring and are tangents to a contour line of the one cross section is greater than a distance (H) between two tangential lines which are perpendicular to the radial direction and are tangents to the contour line before the O-ring (70) is installed in the glow plug (100), characterized in that the central shaft (30) has a terminal connecting part (36) which is provided at a rear end of the central shaft (30) with respect to the direction of the axis (O) and to which a connection terminal of an external circuit is directly or indirectly connected; and a relationship D1 > d1 is satisfied before the O-ring (70) is installed in the glow plug (100), wherein D1 represents the minimum inner diameter of the O-ring (70) and d1 represents the maximum diameter of the terminal connecting part (36) of the center shaft (30), further wherein a relationship d1 < d2 is satisfied, wherein d2 represents the diameter of the center shaft (30) at a part which is arranged in the direction of the axis (O) in front of the terminal connecting part (36), and a relationship D1 < d2 is satisfied.

Description

Technical area

[0001] The present invention relates to a glow plug which is used to assist in starting a diesel engine. Background technology

[0002] Conventionally, a glow plug that assists starting a diesel engine includes a tubular metal shell and a heater mounted at the front end of an axial opening of the shell, and is configured such that a front end portion of the heater protrudes into the engine. Furthermore, a rod-shaped center shaft made of metal is inserted into the axial opening of the metal shell while being insulated from the metal shell, such that an end portion of the center shaft protrudes from the rear end of the metal shell. Two electrodes extending from the heater to supply electricity to the heater are electrically connected to the metal shell and the center shaft, respectively.

[0003] In a glow plug with such a structure, to maintain the airtightness of the axial opening of the metal shell, an O-ring is interposed between the wall surface of the axial opening and the center shaft at the rear end of the axial opening. Furthermore, an insulating member is interposed between the wall surface of the axial opening and the center shaft, and the O-ring is pressed from the rear end side by an end surface of the insulating member. Thus, the O-ring is brought into close contact with the end surface of the insulating member, the wall surface of the axial opening, and the outer peripheral surface of the center shaft, thereby sealing the interior of the axial opening (see, for example, Patent Document 1, Japanese Patent Application Laid-Open (kokai) No. 2005-315474). Disclosure of the invention Problems to be solved by the invention

[0004] However, glow plugs have been reduced in diameter to meet the recent demand for downsized diesel engines, and the distance between the wall surface of the axial hole and the center shaft of the glow plug has been reduced. Therefore, when assembling a glow plug, it becomes difficult to arrange an O-ring in the clearance. Since a cross section of an O-ring perpendicular to its circumferential direction is circular, when such an O-ring is arranged in the narrowed clearance between the wall surface of the axial hole and the center shaft, it has a portion that suffers large local deformation because the O-ring is sandwiched between the wall surface of the axial hole and the outer peripheral surface of the center shaft and pressed by the insulating member.When the internal stress of the O-ring increases due to this deformation, an elastic force of the O-ring decreases, resulting in a reduction in the degree of close contact against the wall surface of the axial opening and the outer peripheral surface of the center shaft, and resulting in difficulty in maintaining the airtightness of the axial opening.

[0005] The present invention has been achieved to solve the above problems, and an object of the invention is to provide a glow plug configured such that an O-ring is easily disposed between the wall surface of an axial opening of a metal shell and a center shaft in the course of manufacturing, and the disposed O-ring reliably maintains the airtightness of the axial opening. Means to solve the problems

[0006] To achieve the above object, a glow plug of an invention according to claim 1 comprises a tubular metal shell having an axial opening extending through the metal shell along the direction of an axis; a rod-shaped central shaft extending along the direction of the axis and disposed in the axial opening of the metal shell with a clearance formed between the central shaft and a wall surface of the axial opening, an end portion of the central shaft protruding from a rear end surface of the metal shell; an O-ring disposed at a rear end of the axial opening between the wall surface of the axial opening and the central shaft, the O-ring being in close contact with the wall surface of the axial opening and an outer peripheral surface of the central shaft; and an annular pressing member having an insertion hole into which the central shaft is inserted.wherein the pressing element is at least partially disposed between the wall surface of the axial opening and the central shaft and has an end surface for pressing the O-ring from the rear side thereof. A part of the wall surface of the axial opening, with which part the O-ring comes into close contact, and / or a part of the outer peripheral surface of the central shaft, with which part the O-ring comes into close contact, is / is formed as a chamfered surface that increases the clearance between the wall surface of the axial opening and the central shaft along the direction of the axis toward the rear end side. In this application, "chamfered surface" is understood in particular to mean a conical surface, and "chamfered" has a particular meaning of conical. The O-ring assumes an annular shape that extends circumferentially around the axis and is configured such thatthat in one of two cross-sections of the O-ring perpendicular to its circumferential direction, a distance between two tangential lines parallel to a radial direction of the O-ring and tangential to a contour line of one cross-section is greater than a distance between two tangential lines perpendicular to the radial direction and tangential to the contour line before the O-ring is assembled into the glow plug. The O-ring has this shape, in particular, when it is newly manufactured and unused.

[0007] In addition, having the structure of the invention described in claim 1, a glow plug of an invention according to claim 2 is characterized in that a relationship of 1.2 ≤ V / H ≤ 2.0 is satisfied, where V represents a distance between two tangential lines which are parallel to a radial direction of the O-ring and are tangential to a contour line of the one cross section, and H represents a distance between two tangential lines which are perpendicular to the radial direction and are tangents to the contour line in the one of the two cross sections of the O-ring perpendicular to the circumferential direction thereof before the O-ring is assembled to the glow plug.

[0008] In addition to having the structure of the invention described in claim 1 or 2, a glow plug of an invention according to claim 3 is characterized in that the center shaft has a terminal connecting part which is provided at a rear end thereof in the direction of the axis and to which a connecting terminal of an external circuit is directly or indirectly connected; and a relationship D1 > d1 is satisfied before the O-ring is assembled to the glow plug, where D1 represents the smallest inner diameter of the O-ring and d1 represents the largest diameter of the terminal connecting part of the center shaft.

[0009] In addition, having the structure of the invention described in claim 3, a glow plug of an invention according to claim 4 is characterized in that a relationship d1 < d2 is satisfied, where d2 represents the diameter of the center shaft at a part located forward of the terminal connecting part in the direction of the axis, and a relationship D1 < d2 is satisfied.

[0010] In addition to having the structure of the invention described in any one of claims 1 to 3, a glow plug of an invention according to claim 5 is characterized in that the chamfer surface is formed at a part of the outer peripheral surface of the center shaft with which part the O-ring comes into close contact; and a relation d2 < D1 < d3 is satisfied, where d2 represents the diameter of the center shaft at a part between the terminal connecting part and the chamfer surface, and d3 represents the diameter of the center shaft at a part located forward of the chamfer surface.

[0011] In addition to having the structure of the invention described in any one of claims 1 to 5, a glow plug of an invention according to claim 6 is characterized in that a relationship D2 < d4 is satisfied before the O-ring is assembled to the glow plug, where D2 represents the maximum outer diameter of the O-ring, and d4 represents a diameter of an edge line formed between the rear end surface of the metal shell and the wall surface of the axial opening.

[0012] In addition to having the structure of the invention described in any one of claims 1 to 6, a glow plug of an invention according to claim 7 is characterized in that the metal shell further comprises: a fastening threaded portion for fastening in a threaded opening of an internal combustion engine, a tool engagement portion for engaging a tool used when the fastening threaded portion is screwed into the threaded opening, wherein the metal shell has a corner width of 8.8 mm or less at the tool engagement portion, and the metal shell has a nominal outer diameter of 8 mm or less at the fastening threaded portion. Effects of the invention

[0013] In the glow plug of the invention according to claim 1, the O-ring disposed between the wall surface of the axial opening of the metal shell and the outer peripheral surface of the center shaft is configured such that its cross section perpendicular to the circumferential direction of the O-ring has a length in a direction perpendicular to the radial direction of the O-ring that is longer than that in the radial direction. That is, the cross section of the O-ring taken along the radial direction assumes an oval shape, an elliptical shape, or a similar shape whose longitudinal direction coincides with a direction perpendicular to the circumferential direction; that is, the axial direction (including a shape of an incomplete oval or ellipse; for example, an elliptical shape in which one of the semicircles of an ellipse differs from the other in a radius).When the O-ring is pressed from the rear end side by the pressing member, and the O-ring experiences a reaction force in the pressing direction from the chamfered surface formed on the wall surface of the axial opening and / or the outer peripheral surface of the center shaft, and is elastically deformed, the O-ring comes into contact with the two surfaces over a larger area due to such a shape compared to the case of a conventional O-ring having a circular cross-section. Therefore, a glow plug assembled with the O-ring can more reliably maintain the airtightness of the axial opening.

[0014] Specifically, a part of the wall surface of the axial opening with which the O-ring comes into close contact and / or a part of the outer peripheral surface of the center shaft with which the O-ring comes into close contact is formed as a chamfered surface. Therefore, the O-ring can be engaged with the chamfered surface without failure and can generate a tensile force (resistance force) against the chamfered surface. Even in the case where one of the two surfaces is not a chamfered surface, the other or the remaining surface is a chamfered surface. Therefore, by adjusting the direction in which the pressing member presses the O-ring, a constraining force is generated such that a tensile force can be generated between the non-chamfered surface and the O-ring, whereby the degree of close contact between the non-chamfered surface and the O-ring can be sufficiently increased.

[0015] As described above, the O-ring has a cross-section elongated in the direction perpendicular to the direction of the clearance between the wall surface of the axial opening of the metal sleeve and the outer peripheral surface of the center shaft. When such an O-ring is disposed between the wall surface of the axial opening of the metal sleeve and the outer peripheral surface of the center shaft, the amount of deformation of the O-ring required to ensure a large contact area with the two surfaces is small, and an increase in the internal stress of the O-ring is small. Therefore, the O-ring can easily deform to conform to the surface shapes of the two surfaces, and thus the contact areas can be further increased.Accordingly, the degree of close contact between the O-ring and the wall surface of the axial opening and between the O-ring and the outer peripheral surface of the center shaft can be further increased, whereby the airtightness of the axial opening can be more reliably maintained. Furthermore, since local concentration of the internal stress of the O-ring can be avoided, deterioration of the O-ring material or breakage of the O-ring, which would otherwise occur due to expansion and contraction of the O-ring with changes in ambient temperature, vibration, or other reasons, hardly occurs. Thus, a contact state can be sufficiently ensured to maintain the airtightness of the axial opening. In particular, the O-ring preferably has the shape described above before the O-ring is assembled into the glow plug.A shape of the O-ring before the O-ring is assembled to the glow plug can be confirmed by disassembling an O-ring from a glow plug.

[0016] Preferably, the O-ring satisfies a relationship of 1.2 ≤ V / H between an axial length (V) and a radial length (H) before the O-ring is assembled to the glow plug. Such an O-ring can ensure a tighter connection within an axial opening and maintain better airtightness compared to a conventional O-ring having a circular shape in cross section. Furthermore, an O-ring satisfying a relationship of V / H ≤ 2.0 before the O-ring is assembled to the glow plug can reduce contact friction generated when the O-ring is inserted between an inner wall surface of the axial opening and an outer surface of the center shaft, and can facilitate the O-ring assembly step.Furthermore, distortion of the O-ring caused by a difference between the axial length and the radial length in the cross section can be suppressed and an appropriate constraining structure can be realized.

[0017] Meanwhile, the O-ring is typically assembled into a glow plug by an operation of fitting the O-ring onto one end of the center shaft disposed in the axial opening of the metal shell, and moving the O-ring to a position between the wall surface of the axial opening and the outer peripheral surface of the center shaft. A rear end portion of the center shaft is used as a terminal connecting portion to which a connecting terminal for connection to an external circuit is connected. In some cases, an intermediate member (corresponding to a terminal metal piece in the embodiment) for connection with the connecting terminal is attached to the terminal connecting portion. In some cases, surface processing such as knurling is performed on the outer peripheral surface of the terminal connecting portion to thereby more reliably attach the intermediate member.Furthermore, in the case where the connecting terminal is directly connected without using the intermediate member, in some cases, a male thread is formed for connection to the connecting terminal. In such a case, the outer peripheral surface of the terminal connecting part becomes non-uniform, and the inner peripheral surface of the O-ring rubs against the machined part and is damaged when the O-ring is moved, which may deteriorate the performance of the O-ring. To solve this problem, it is preferable to make the minimum inner diameter D1 of the O-ring larger than the maximum diameter d1 of the terminal connecting part, as in the invention according to claim 2. Specifically, examples of the external circuit include an external power source circuit including a battery, etc.and a signal processing circuit which is used when the glow plug includes a pressure sensor and is used to output a signal from the pressure sensor.

[0018] When the diameter d2 of the center shaft at a portion located forward of the terminal connecting portion in the axial direction is made larger than the minimum inner diameter D1 of the O-ring, as in the invention according to claim 3, the O-ring having the minimum inner diameter D1 is expanded by the portion of the center axis having the diameter d2. Therefore, the O-ring generates a tensile force in the radial direction against the outer peripheral surface of the center shaft, achieving a high degree of close contact. This structure is more effective in the case where a portion of the outer peripheral surface of the center shaft, with which the O-ring comes into close contact, is not chamfered.In this case, by adjusting the direction in which the pressing member presses the O-ring, a tensile force can be generated between the O-ring and the outer peripheral surface of the center shaft, thereby increasing the degree of close contact of the O-ring against the outer peripheral surface of the center shaft. However, in the case where a portion of the outer peripheral surface of the center shaft, with which the O-ring comes into close contact, is chamfered, the direction of the tensile force generated between the O-ring and the chamfered surface does not coincide with the radial direction of the O-ring.When the diameter d2 of the center shaft at a portion between the terminal connecting portion and the chamfer surface is made smaller than the minimum inner diameter D1 of the O-ring, as in the invention according to claim 4, the above-described tensile force exerted on the outer peripheral surface of the center shaft due to the deformation of the O-ring is not generated. However, since a tensile force can be generated between the O-ring and the chamfer surface when the O-ring is pressed by the pressing member, the airtightness of the axial opening can be sufficiently maintained. Since the minimum inner diameter D1 of the O-ring is larger than the diameter d2 of the center shaft and the maximum diameter d1 of the terminal connecting portion, the O-ring fitted on the center shaft from one end portion thereof can be easily moved until it comes into close contact with the chamfer surface.At this time, if the diameter d3 of the center shaft at a part located in front of the chamfer surface is larger than the minimum inner diameter D1 of the O-ring, the O-ring can be reliably brought into contact and engagement with the chamfer surface of the center shaft. Therefore, when the O-ring is pressed by the pressing element, the O-ring can easily generate a tensile force against the chamfer surface.

[0019] As described above, the O-ring is fitted onto the center shaft from an end portion thereof protruding from the rear end surface of the metal shell, is moved axially, and is accommodated within the axial opening of the metal shell. When the maximum outer diameter D2 of the O-ring is made smaller than the diameter d4 of the edge line formed between the rear end surface of the metal shell and the wall surface of the axial opening, as in the invention according to claim 5, the O-ring has no effect on the rear end surface. Accordingly, the O-ring can be easily guided into the axial opening and moved to a seating position between the wall surface of the axial opening and the outer peripheral surface of the center shaft.

[0020] The O-ring of the present invention is suitably applicable to what we call a narrow diameter glow plug in which the metal shell at the tool engaging portion has a corner width of 8.8 mm or less, and the metal shell at the fastening thread portion has a nominal outside diameter of 8 mm or less. Best mode of carrying out the invention

[0021] An embodiment of a glow plug embodying the present invention will next be described with reference to the drawings. First, with reference to Fig. 1 to 3 the general structure of an example glow plug 100 will be described. Fig. 1 is a vertical sectional view of the glow plug 100. Fig. 2 is an enlarged sectional view of a rear end and its vicinity of the glow plug 100. Fig. 3 is a perspective view of a center shaft 30. In the following description, one end of the glow plug 100 in which a ceramic heater 20 is arranged (a lower end in Fig. 1) with respect to the direction of an axis O may be referred to as the front end of the glow plug 100.

[0022] The glow plug 100, which is Fig. 1, is attached to a combustion chamber (not shown) of, for example, a direct-injection diesel engine, and is used as a heating source to assist ignition at the time of engine start-up. The glow plug 100 is mainly composed of the center shaft 30; the ceramic heater 20 having a heat-generating body 27; a tubular member 80 radially supporting the ceramic heater 20; and a metal shell 40 having an axial opening 43 through which the center shaft 30 is inserted, and a front end portion 41 connected to the tubular member 80.

[0023] First, the ceramic heater 20 will be described. The ceramic heater is configured in such a manner that a heat generating element 24, which is formed of a conductive ceramic and has a generally U-shaped cross section, is embedded in a round-rod-shaped substrate 21, which is formed of an insulating ceramic and has a front end portion 22 formed in a hemispherical shape. The heat generating element 24 is composed of a heat generating body 27, which is disposed in the front end portion 22 of the ceramic heater 20 and has opposite end portions folded back in a generally U-like shape along the curved surface of the front end portion 22, and lead portions 28 and 29, which are connected to the opposite ends of the heat generating body 27 and extend generally in parallel along the axis O to a rear end portion 23 of the ceramic heater 20.The heat generating body 27 is formed to be smaller in cross-sectional area than the conduction parts 28 and 29. Therefore, when electricity is supplied to the heat generating element 24, heat is mainly generated at the heat generating body 27. Furthermore, on an outer peripheral surface of the ceramic heater 20, which is located on the rear end side with respect to the center thereof, electrode output parts 25 and 26, which respectively protrude from the conduction parts 28 and 29, are exposed at different positions with respect to the direction of the axis O.

[0024] Next, the tubular member 80 will be described. The tubular member 80 is formed of a cylindrical tubular metallic member extending along the direction of the axis O. The tubular member 80 radially supports a body portion of the ceramic heater 20 within a cylindrical opening 84 thereof such that the front end portion 22 and the rear end portion 23 are exposed from the opposite ends of the cylindrical opening 84. The tubular member 80 has a thick flange portion 82 at the rear end side of a body portion 81 thereof. The tubular member also has a stepped sleeve engagement portion 83 formed at the rear end thereof. The sleeve engagement portion 83 engages the inner periphery of the front end portion 41 of the metallic sleeve 40, which will be described later, so as to be connected to the metallic sleeve 40.Of the electrode output portions 25 and 26 of the ceramic heater 20, the electrode output portion 25 formed in front of the electrode output portion 26 is in contact with the wall surface of the cylindrical opening 84 of the tubular member 80, thereby electrically connecting the electrode output portion 25 to the tubular member 80. Furthermore, a connecting ring 75 formed of metal and having a tubular shape is fitted to the rear end portion 23 of the ceramic heater 20, which protrudes rearward from the sleeve engaging portion 83 of the tubular member 80. The electrode output portion 26 of the ceramic heater 20 is in contact with the inner peripheral surface of the connecting ring 75, thereby electrically connecting the electrode output portion 26 to the connecting ring 75.The front end portion 41 of the metal sleeve 40 is connected to the sleeve engagement portion 83 of the tubular member 80, thereby electrically connecting the metal sleeve 40 and the tubular member 80. Although the rear end portion 23 of the ceramic heater 20 and the connecting ring 75 are disposed within the metal sleeve 40, the ceramic heater 20 and the metal sleeve 40, respectively, are positioned with respect to the tubular member 80, and the metal sleeve 40 and the connecting ring 75 are left in a non-contact state, thereby electrically insulating these two members from each other.

[0025] Next, the metal sleeve 40 will be described. The metal sleeve 40 is an elongated tubular metallic member having the above-mentioned axial opening 43 penetrating the sleeve in the direction of the axis O. A male threaded portion 42 for attaching the glow plug 100 to an engine head (not shown) of an internal combustion engine is formed on the rear end side of an intermediate body portion 44 of the metal sleeve 40. As shown in Fig. 2, a tool engaging part 46, with which a tool used for attaching the glow plug to the engine head is engaged, is further formed at the rear end of the intermediate body part 44 (see Fig. 1). In the present embodiment, the tool engagement portion 46 has a hexagonal cross section, and the axial opening 43 is enlarged in diameter within the tool engagement portion 46. This portion is called a diameter-enlarged portion 45. Before reaching the diameter-enlarged portion 45, the axial opening 43 is formed to have a chamfered surface 47 whose diameter gradually increases toward the rear end of the axial opening. The axial opening 43 is opened to a rear end surface 48 of the metal sleeve 40 at the diameter-enlarged portion 45, and the opening portion is chamfered. As shown in Fig. 1, the inner periphery of the front end part 41 of the metal sleeve 40 is engaged with the outer periphery of the sleeve engaging part 83 of the tubular member 80, and the connecting part between these members is laser welded from the outside, whereby the metal sleeve 40 and the tubular member 80 are integrally connected.

[0026] Next, the center shaft 30 will be described. As shown in the Fig. 1 and Fig. 3, the center shaft 30 is a metal rod extending along the axis O direction and inserted into the axial opening 43 of the metal sleeve 40. A small-diameter portion 35 having a reduced diameter is formed at the front end side of an intermediate body portion 33 of the center shaft 30. Further, a small-diameter ring engaging portion 34 for engaging with the inner periphery of the connecting ring 75 is formed at the distal end of a front end portion 31 located at the front end side with respect to the small-diameter portion 35. By engaging this ring engaging portion 34 with the connecting ring 75, the ceramic heater 20 and the center shaft 30 are integrally connected along the axis O via the connecting ring 75.Specifically, the front end portion 31 of the center shaft 30 and the connecting ring 75 are integrally joined by laser welding, which is performed externally for the connection portion between these members. Thus, the center shaft 30 is electrically connected to the electrode output portion 26 of the ceramic heater 20 via the connecting ring 75. Since the ceramic heater 20 and the metal sleeve 40 are positioned with respect to the tubular member 80 as described above, the center shaft 30 is left in a non-contact state within the axial opening of the metal sleeve 40 and is electrically insulated therefrom.

[0027] As continued in Fig. 2 and Fig. 3, the center shaft 30 has a rear end portion 32 including a small-diameter connecting portion 36 projecting from the rear end surface 48 of the metal sleeve 40 and a sealing portion 37 that comes into contact with an O-ring 70 (to be described later) arranged to maintain the airtightness of the axial opening 43 of the metal sleeve 40. In the present embodiment, the sealing portion 37 has the same diameter as the intermediate body portion 33 and is continuous with the intermediate body portion 33. As shown in the Fig. 3, the outer peripheral surface of the terminal connecting part 36 is surface-treated by knurling so as to form an engaging part 39.

[0028] The O-ring 70 and an insulating pressing member 60 are provided at the rear end portion 32 of the center shaft 30. The pressing member 60 takes a cylindrical tubular shape and has an insertion opening 62. The O-ring 70 is formed into an annular shape from an insulating elastic material such as fluororubber, acrylic rubber, or silicone rubber. A body portion 65 of the pressing member 60 is disposed between the wall surface of the diameter-enlarged portion 45 of the axial opening 43 and the sealing portion 37 of the center shaft 30, so that the center shaft 30, which is inserted into the insertion opening 62 of the pressing member 60, is positioned within the diameter-enlarged portion 45, thus providing insulation between the center shaft 30 and the metal shell 40.

[0029] The O-ring 70, which is disposed between the outer peripheral surface of the seal part 37 and the tapered surface 47 of the axial opening 43, is pressed toward the front end side by a front end side end surface 63 of the pressing member 60, so that the O-ring 70 is in close contact with the tapered surface 47 of the axial opening 43 and the outer peripheral surface of the seal part 37 of the center shaft 30. The end surface 63 of the pressing member 60 is tapered so that the apex of an imaginary conical surface passing through the end surface 63 is located on the axis O. This structure reduces the difference between the tensile force between the pressed O-ring 70 and the sealing part 37 and the tensile force between the O-ring 70 and the chamfer surface 47 of the axial opening 43, thereby reducing the imbalance of the degree of close contact of the O-ring 70.Furthermore, a flange portion 61 is provided at the rear end of the pressing member 60. This flange portion 61 is engaged with the rear end surface 48 of the metal shell 40 to be interposed between the metal shell 40 and a terminal metal piece 50 (which will be described later) and engaged with the terminal connecting portion 36, thereby maintaining insulation between the terminal metal piece 50 and the metal shell 40.

[0030] The terminal metal piece 50 is fitted to the terminal connecting part 36, which protrudes from the rear end surface 48 of the metal shell 40. The terminal metal piece 50 includes a cap-shaped body part 52 that fits onto and covers the terminal connecting part 36, a pin-shaped protrusion part 53 that protrudes rearward from the body part 52, and a flange part 51 that protrudes radially from the front end of the body part 52. The flange part 51 of the terminal metal piece 50, which is fitted to the terminal connecting part 36, is brought into contact with the flange part 61 of the pressing member 60 so as to press the pressing member 60 forward along the axial direction, and the outer periphery of the body part 52 is crimped, whereby the inner peripheral surface of the body part 52 is firmly engaged with the engaging part 39 of the terminal connecting part 36.In this way, the terminal metal piece 50 and the center shaft 30 are integrally fixed and electrically connected to each other. Since the engaging portion 39 is knurled, the attachment force of the crimped terminal metal piece 50 to the engaging portion 39 can be increased. When the glow plug 100 is mounted on the engine head (not shown), a plug cap (not illustrated) is fitted onto the protrusion 53, and electrical power is supplied to the glow plug 100 from an external circuit.

[0031] To increase the airtightness of the axial opening 43 in the glow plug 100 having the above-described structure, it is necessary to increase the respective contact areas between the O-ring 70 and the outer peripheral surface of the sealing part 37 of the center shaft 30 and between the O-ring 70 and the chamfered surface 47 of the axial opening 43 without affecting the stresses (surface pressure) exerted on the respective contact surfaces. As described above, when the diameter of the glow plug 100 is reduced, the clearance between the above-mentioned two surfaces decreases. It is theoretically possible to increase the contact areas between the O-ring 70 and the two surfaces by greatly deforming the O-ring 70 to equalize the clearance.Since the O-ring 70 has a significantly reduced cross-sectional area due to the downsizing of the glow plug 100, the shape of the O-ring 70 changes significantly in response to a slight difference in the pressing amount by the pressing member 60, and thus, stable production of glow plugs 100 whose O-rings have the same shape becomes difficult. If the pressing amount is excessive, the O-ring 70 is likely to harden and deteriorate, and the elastic force cannot be maintained at a sufficient level. As a result, it becomes difficult to maintain airtightness for a long period of time.

[0032] To solve the above-mentioned problem, in the present embodiment, in order to reduce an increase in internal stress due to deformation, the shape of the cross section of the O-ring 70 perpendicular to the circumferential direction is defined such that contact areas of the O-ring with the above-mentioned two surfaces are increased while preventing the O-ring 70 from deforming significantly, thereby more reliably maintaining the airtightness of the axial opening 43. Furthermore, to facilitate the work of disposing the O-ring 70 between the outer peripheral surface of the sealing part 37 of the center shaft 30 and the chamfer surface 47 of the axial opening 43 in the course of manufacturing the glow plug 100, the relationship between the size of the O-ring 70 and the diameters of the axial opening 43 and the center shaft 30 is also defined. The following describes the details of the O-ring 70 with reference to Fig. 4 and Fig. 5 are described. Fig. 4 is a perspective view of an O-ring 70, and Fig. 5 is a sectional view showing a state before the O-ring 70 is arranged between the center shaft 30 and the metal sleeve 40.

[0033] As in Fig. 4, the O-ring of the present embodiment assumes an annular shape extending circumferentially around an axis P. The axis P coincides with the axis O when the O-ring 70 is assembled as a part of the glow plug 100. A cross section of the O-ring 70 perpendicular to the circumferential direction thereof (cross section generated by cutting the O-ring 70 by an imaginary plane including the axis P) has an elliptical contour line S elongated along the direction of the axis P. Specifically, the contour line S of the cross section is such that on an imaginary plane containing the cross section (for example, the paper plane of the Fig. 4), the axial (vertical) distance V between tangential lines T1 and T2, which are parallel to the radial direction of the O-ring 70 (a direction perpendicular to the direction of the axis P) and tangential to the contour line S, is greater than the radial (horizontal) distance H between tangential lines T3 and T4, which are perpendicular to the radial direction of the O-ring 70 and tangential to the contour line S. More specifically, a relationship is satisfied between the axial distance V and the radial distance H: 1.2 ≤ V / H ≤ 2.0. When V / H is 1.2 or greater, a wider contact area can be obtained between the outer peripheral surface of the O-ring 70 and the inner wall surface of the axial opening 43 and the outer surface of the center shaft 30.When V / H is 2.0 or smaller, contact friction between the O-ring 70 and the inner wall surface of the axial opening 43 and the outer surface of the center shaft can be prevented, and the step of assembling the O-ring can be facilitated. Furthermore, when V / H is 2.0 or smaller, deformation of the O-ring caused by the difference between the axial distance and the radial distance when the ring is pressed by the pressing member 60 is suppressed. Specifically, since the O-ring 70 assumes an annular shape whose center is located on the axis P when the O-ring 70 is cut by the above-mentioned imaginary plane, two symmetrical cross sections are formed on the respective sides of the axis P. However, the shape of such a cross section is described here.

[0034] Incidentally, the wall surface of the axial opening 43 and the outer peripheral surface of the center shaft 30 face each other in a direction perpendicular to the axis O. When the O-ring 70, which has an elongated cross-sectional shape in the direction of the axis O at the time of assembly, is disposed between the chamfered surface 47 and the outer peripheral surface of the seal part 37, the chamfered surface 47 and the outer peripheral surface of the O-ring 70 come into mutual contact by an area larger than that in the case where a conventional O-ring with a circular cross section is used. Similarly, the seal part 37 and the inner peripheral surface of the O-ring 70 come into mutual contact by an area larger than that in the case where a conventional O-ring with a circular cross section is used.In this state, the O-ring 70 is pressed by the end surface 63 of the pressing member 60 so that the tensile forces are generated against the above-mentioned two surfaces. In this case, the areas of contact between the O-ring 70 and the chamfer surface 47 and between the O-ring 70 and the outer peripheral surface 40 of the seal member 37 extend in the direction of the axis O, and the contact areas between them can be increased while increasing the surface pressures. Furthermore, since the shape of the contour line S of the cross section of the O-ring 70 is initially elliptical, the amount of deformation at the time of insertion between the above-mentioned two surfaces is small, and an increase in internal stress is small, whereby deterioration or breakage of the O-ring 70 can be suppressed.Accordingly, it is possible to promote the degree of close contact between the O-ring 70 and the chamfer surface 47 of the axial opening 43 and between the O-ring 70 and the outer peripheral surface of the sealing part 37 of the center shaft 30, and to maintain the airtightness of the axial opening 43 more reliably.

[0035] Next, the definition regarding the size relationship between the metal sleeve 40, the center shaft 30, and the O-ring 70 will be described in a state before the O-ring 70 is arranged between the chamfer surface 47 of the axial opening 43 and the outer peripheral surface of the sealing part 37 of the center shaft 30 (state before assembling the glow plug 100). As shown in Fig. 5, first, the minimum inner diameter D1 of the O-ring 70 is set to be larger than the maximum outer diameter d1 of the center shaft 30 at the terminal connecting part 36. As shown in Fig. 4, tangential lines T3, T4, T5, and T6 are considered here, which are parallel to the axis P (perpendicular to the radial direction) and which are tangential to two cross sections of the O-ring 70 obtained by cutting the O-ring 70 by an imaginary plane passing through the axis P. The minimum inner diameter D1 of the O-ring 70 refers to the distance between the tangential lines T4 and T5, which are closer to the axis P than the other tangential lines. As described above, the engaging part 39 of the terminal connecting part 36 of the center shaft 30 is further subjected to knurling, which is a type of surface treatment. Depending on the type of surface treatment, parts protruding from the surface (before machining) of the engaging part 39 can be produced.Therefore, in the present embodiment, the maximum outer diameter d1 refers to the outer diameter of a part whose outer diameter is the largest among parts of the terminal connecting part 36, including the protruding parts generated as a result of surface processing. Since the minimum inner diameter D1 of the O-ring 70 is made larger than the maximum outer diameter d1 of the terminal connecting part 36, when the O-ring 70 is fitted onto the center shaft 30 from the rear end part 32 (an end part in the present invention), the O-ring 70 can easily pass through the terminal connecting part 36. In a case where the minimum inner diameter D1 of the O-ring 70 is not larger than the maximum outer diameter d1 of the terminal connecting part 36, the inner periphery of the O-ring 70 rubs against the knurled engaging part 39 when the O-ring 70 passes over the terminal connecting part 36.If the surface of the O-ring 70 is damaged, the degree of close contact with the sealing part 37 decreases and sealing of the axial opening 43 becomes insufficient.

[0036] As in Fig. Next, as shown in FIG. 5, the minimum inner diameter D1 of the O-ring 70 is set to be smaller than the outer diameter d2 of the seal portion 37 of the center shaft 30. As described above, the seal portion 37 opposes the diameter-enlarged portion 45 and the chamfered surface 47 of the axial opening 43 of the metal shell 40 at the rear end portion 32 of the center shaft 30. That is, the seal portion 37 is located where the O-ring 70 is present after assembly of the glow plug 100. In the case of the center shaft 30 of the present embodiment, because the seal portion 37 has a cylindrical peripheral surface extending along the direction of the axis O, a tensile force is generated between the O-ring 70 and the seal portion 37 in a direction perpendicular to the axis O. As described above, the tensile force that the O-ring 70 receives from the end surface 63 of the pressing member 60 includes a component perpendicular to the axis O.Therefore, the O-ring 70 generates a tensile force against the outer peripheral surface of the seal member 37, thereby achieving close contact. When the minimum inner diameter D1 of the O-ring 70 is smaller than the outer diameter d2 of the seal member 37, in addition to the above-mentioned tensile force, a tensile force may be generated between the O-ring 70 and the seal member 37, which is caused by deformation of the O-ring 70 itself. Therefore, the degree of close contact between the O-ring 70 and the seal member 37 can be increased.

[0037] As also in Fig. 5, the maximum outer diameter D2 of the O-ring is set to be smaller than the diameter d4 of the edge line between the rear end surface 48 of the metal shell 40 and the wall surface of the axial opening 43. As described above, the axial opening 43 is opened to the rear end surface 48 of the metal shell 40 at the diameter-enlarged part 45, and its opening part is chamfered. In the present embodiment, this chamfered part is regarded as a part of the wall of the axial opening 43, and the opening diameter (the diameter of the opening) of the rear end surface 48 is regarded as the diameter d4 of the edge line between the rear end surface and the wall surface of the axial opening 43. Furthermore, as in the case of the above-mentioned minimum inner diameter D1 of the tangential lines T3, T4, T5, and T6 shown in Fig. 4 which are parallel to the axis P (perpendicular to the radial direction) and which are tangential to two cross sections of the O-ring 70 obtained by cutting the O-ring 70 by an imaginary plane passing through the axis P, the distance between the tangential lines T3 and T6, which are further separated from the axis P than the remaining tangential lines, is considered to be the maximum outer diameter D2 of the O-ring 70. When the O-ring 70 is fitted onto the center shaft 30 from the rear end portion 32 and moved to the inside of the axial opening 43 along the center shaft 30, the O-ring 70 does not come into contact with the rear end surface 48 and the O-ring 70 can be easily guided into the axial opening 43 if the maximum outer diameter D2 of the O-ring 70 is smaller than the diameter d4, which is the opening diameter of the rear end surface 48.

[0038] The O-ring 70 having a size and a shape as defined and described above is arranged between the metal sleeve 40 and the center shaft 30 in a final step of assembly as shown in Fig. 6, which is a step of a process of manufacturing the glow plug 100, thereby completing the glow plug 100. Before describing the final step of assembly, the process of manufacturing the glow plug 100 will be generally described with reference to Fig. 1, Fig. 2 and Fig. 6. Particularly Fig. 6 is a view schematically showing the final step of assembling a process of manufacturing the glow plug 100.

[0039] In the process of manufacturing the glow plug 100, which is Fig. 1, first, an element green body (prototype) of a heat-generating element 24 of the ceramic heater 20 is formed by injection molding a material including a conductive ceramic powder, a binder, etc. Meanwhile, a substrate green body (prototype) of the substrate 21 of the ceramic heater 20, composed of two half-green bodies, is formed by compression molding from an insulating ceramic powder such that the half-green bodies have a recess for receiving the element green body at their partial surfaces. The substrate green body then undergoes pressure compression, with the element green body received and held in the recesses of the substrate green body, followed by a coking process and a firing process such as hot pressing.Subsequently, the resulting product is formed into the shape of a rod having a hemispherical end by milling the outer peripheral surface thereof, thereby forming the ceramic heater 20.

[0040] Next, the connecting ring 75, which is made of a steel material such as stainless steel and formed into a tube-like shape, is pressure-fitted onto the ceramic heater 20 to establish electrical continuity between the connecting ring 75 and the electrode output part 26. Similarly, the tubular member 80, which is formed into a predetermined shape, is pressure-fitted onto the ceramic heater 20 to establish electrical continuity between the tubular member 80 and the electrode output part 25. Specifically, it is preferable to coat the connecting ring 75 and the tubular member 80 with Au, Cu, or the like to stabilize the electrical continuity.

[0041] Meanwhile, the center shaft 30 is formed by performing plastic machining, cutting, etc. on a rod-shaped member obtained by cutting an iron-based material (e.g., Fe-Cr-Mo steel) to a predetermined length. The outer periphery of the engaging part 34 of the center shaft 30 is engaged with the inner periphery of the connecting ring 75, which is fitted to the ceramic heater 20, and laser welding is performed on the connecting part between these members, thereby integrally connecting the center shaft 30 and the ceramic heater.

[0042] Next, the tubular metal sleeve 40, which has the tool engagement part 46, etc., is formed from an iron-based material such as S45C, and a screw thread is formed at the male threaded part (fastening threaded part) 42. The center shaft 30, which is integrated with the ceramic heater 20, etc., is inserted into the axial opening 43 of the metal sleeve 40 from the rear end 32 thereof. Subsequently, the connecting part between the metal sleeve 40 and the tubular member 80 is laser-welded, thereby integrally joining these members. Specifically, to prevent rusting of the metal sleeve 40 formed from an iron-based material, the metal sleeve 40 may be pre-coated before being joined to the tubular member 80, or a rust-preventing process such as plating or painting may be performed after the metal sleeve 40 and the tubular member are joined together.

[0043] The following is the final step of assembling in Fig. 6. The O-ring 70 is fitted onto the terminal connecting portion 36 of the rear end portion 32 of the center shaft 30, which protrudes from the rear end surface 48 of the metal sleeve 40. As described above, the minimum inner diameter D1 of the O-ring 70 is larger than the maximum outer diameter d1 of the terminal connecting portion 36 of the center shaft 30. Therefore, the O-ring 70 can easily pass over the terminal connecting portion 36 and reach the sealing portion 37. Furthermore, since the maximum outer diameter D2 of the O-ring 70 is smaller than the diameter d4, which is the opening diameter of the axial hole 43 opened to the rear end surface 48 of the metal sleeve 40, the O-ring 70 is easily accommodated within the diameter-enlarged portion 45.Meanwhile, since the outer diameter d2 of the sealing part 37 is larger than the minimum diameter D1 of the O-ring 70, the O-ring 70 is moved to the front end side while sliding on the outer peripheral surface of the sealing part 37 and reaches the chamfer surface 47 of the axial opening 43 of the metal sleeve 40 (see . Fig. 2). The pressing member 60 is fitted onto the rear end portion 32 of the center shaft 30 in this state, and the body portion 65 thereof is interposed between the wall surface of the diameter-enlarged portion 45 of the axial opening 43 of the metal sleeve 40 and the sealing portion 37 of the center shaft 30. Furthermore, the terminal metal piece 50 is fitted onto the terminal connecting portion 36 of the rear end portion 32 of the center shaft 30, the terminal connecting portion 36 having the knurled engaging portion 39, and the pressing member 60 is pressed toward the front end side by the flange portion 51 of the terminal metal piece 50. As a result, the O-ring 70 is pressed toward the front end side by the end surface 63 of the pressing member 60.The O-ring 70, which is pressed by the end surface 63 of the pressing member 60, comes into close contact with the chamfered surface 47 of the axial opening 43 of the metal sleeve 40 and the outer peripheral surface of the sealing part 37 of the center shaft 30 without excessive deformation, thereby achieving robust sealing of the axial opening 43. The outer periphery of the body part 52 of the terminal metal piece 50 is crimped to secure the terminal metal piece 50 to the center shaft 30, thus completing the glow plug 100. Example 1

[0044] In order to confirm the effects achieved by defining the size and shape of the O-ring 70 incorporated in the glow plug 100 manufactured in the manner described above, an evaluation test was conducted as shown below. In this evaluation test, three O-rings 70 were manufactured to have the shape described in the section on embodiments such that in the contour line of a cross section perpendicular to the circumferential direction of the O-ring, the distance (the distance V shown in Fig. 4) between two tangential lines parallel to the radial direction is 1.65 mm, the distance (the distance H in Fig. 4) between two tangential lines perpendicular to the circumferential direction becomes 1.2 mm, and the minimum inner diameter D1 becomes 3.9 mm (the maximum outer diameter D2 becomes 6.3 mm). Three glow plug samples having these O-rings assembled therein were manufactured (sample group 2). Furthermore, three O-rings 70 were manufactured such that the contour line of a cross section perpendicular to the radial direction of the O-ring assumes a completely circular shape having a diameter of 1.1 mm, and the minimum inner diameter D1 becomes 4.1 mm (the maximum outer diameter D2 is 6.3 mm). Three glow plug samples having these O-rings assembled therein were manufactured as comparative examples (sample group 1).

[0045] These glow plug samples were manufactured by using a center shaft formed such that the maximum diameter of the terminal connecting part is 3.75 mm and the diameter of the sealing part is 3.95 mm; and a metal shell formed such that the diameter of the axial opening is 5.0 mm, the diameter of the diameter-enlarged part is 6.3 mm, the opening diameter of the axial opening at the rear end surface is 6.7 mm, and the chamfer angle of the chamfer surface extending from the diameter-enlarged part (the angle between the axis O and the chamfer surface in a cross section including the axis O) is 15 degrees.

[0046] The prepared glow plug samples were subjected to a determination test to determine airtightness. Specifically, a shock of 2500 G was applied to each sample 10,000 times. An opening communicating with the axial opening was formed at the front end portion of the metal sleeve of each sample. Air was supplied to the axial opening through the formed opening at a pressure subsequently set to three values: 0.6 MPa, 1.5 MPa, and 4.0 MPa. At each air pressure, it was confirmed whether air leaked from the clearance between the rear end surface of the metal sleeve and the pressing member through the O-ring. In the test, each sample that exhibited air leakage was designated as "×," and each group that did not exhibit air leakage was designated as "◯." Table 1 shows the results of this determination test. Table 1 Probengruppe Luftdichtigkeit 0,6 MPa 1,5 MPa 4,0 MPa 1 ◯ x x x x x x x x 2 ◯ ◯ ◯ ◯ ◯ ◯

[0047] In the case of the glow plugs of Sample Group 1, which includes the O-ring formed such that the contour line of a cross section perpendicular to the radial direction assumes a completely circular shape, as shown in Table 1, one of the three samples was able to maintain airtightness when the air pressure was 0.6 MPa, but all of the samples caused air leakage when the air pressure was set to 1.5 MPa and when the air pressure was set to 4.0 MPa. In contrast, in the case of the glow plugs in Sample Group 2, which include the O-ring formed such that the contour line of a cross section perpendicular to the radial direction assumes an elliptical shape, the samples did not cause air leakage at any air pressure.The results of this determination test confirm that when the O-ring is formed such that the contour line of a cross section perpendicular to the radial direction assumes an elliptical shape, the contact area of ​​the O-ring with the outer peripheral surface of the sealing part of the center shaft and the chamfer surface of the axial opening of the metal sleeve can be increased so as to increase the degree of close contact with these two surfaces, whereby the airtightness of the axial opening can be reliably maintained.

[0048] The present invention can be modified in various ways. For example, a center shaft 130, which is Fig. 7. A rear end portion 132 of the center shaft 130 includes a rear body portion 137 which is disposed forward of the terminal connecting portion 136 and within the diameter-enlarged portion 45 of the axial opening 43 of the metal shell 40 and which has a diameter d2 different from a diameter d3 of an intermediate body portion 130 of the center shaft 130. A chamfered surface 138 is provided between the intermediate body portion 133 and the rear body portion 137. The O-ring 70 is brought into contact with this chamfered surface 138, and the O-ring 70 is pressed forward by the pressing member so as to generate a tensile force between the O-ring 70 and the chamfered surface 138, thereby increasing the degree of close contact therebetween. In this case, the airtightness of the axial opening 43 can be maintained more reliably, as in the case of the embodiment described above.In order to make the tensile force generated between the tapered surface 138 of the central shaft 130 and the O-ring 70 equal to the tensile force generated between the tapered surface 47 of the axial opening 43 and the O-ring 70, it is particularly preferable that the end surface 63 of the pressing member 60 of the above-described embodiment be formed by a flat surface extending perpendicular to the axis O. In the present modification, the direction of the tensile force generated as a result of contact between the O-ring 70 and the tapered surface 138 of the central shaft 30 intersects the direction of the axis O.Therefore, in the case where the minimum diameter D1 of the O-ring 70 is made smaller than the diameter d2 of the rear body part 137 of the central shaft 130, when the O-ring 70 is fitted on the central shaft 130 from the rear end part 132, the O-ring 70 passes through the terminal connecting part 136 whose maximum diameter d1 is smaller than the diameter d2 of the rear body part 137, then passes over the inner body part 137 and easily reaches the chamfer surface 138. Meanwhile, in the case where the central shaft 130 is configured such that the diameter d3 of the intermediate body part 133, which is arranged in front of the rear body part 137, is larger than the minimum inner diameter D1 of the O-ring 70 when the O-ring 70 is fitted on the central shaft 130 from the rear end part 132, the O-ring 70 can be easily brought into contact with the chamfer surface 138.That is, when the O-ring 70 is pressed from the rear end side by the pressing member 60 (see . Fig. 2), a tensile force can be easily generated between the O-ring 70 and the chamfered surface 138, and they can be brought into close contact. Furthermore, in the case where the maximum outer diameter D2 of the O-ring 70 is made smaller than the diameter d4, which is the opening diameter of the rear end surface 48 of the metal shell 40, the O-ring 70 can be easily inserted into the axial opening 43 without interfering with the rear end surface 48, as in the above-described embodiment.

[0049] When a center shaft 130 is used in which a bevel 138 is provided, as in Fig. Further, as shown in Fig. 8, a metal shell 140 configured such that a chamfered surface or a diameter-enlarged portion is not formed on an axial opening 143 thereof can be used, and the airtightness of the axial opening 143 can be reliably maintained, as in the above-described embodiment. Also in this case, it is preferable to make the minimum inner diameter D1 of the O-ring 70 larger than the diameter d2 of the rear body portion 137 and smaller than the diameter d3 of the intermediate body portion 133. Further, as in the above-described case, it is preferable to make the maximum outer diameter D2 of the O-ring 70 smaller than the diameter d4, which is the opening diameter of the rear end surface 148 of the metal shell 140.

[0050] In the above-described embodiment, the cross section of the O-ring 70 perpendicular to the circumferential direction thereof assumes an elliptical shape. The cross section may have a general shape and may have any shape as long as the contour line S of the cross section forms a shape whose length in the direction of the axis P is greater than that in the direction perpendicular to the direction of the axis P, as described in Fig. 4 described.

[0051] In the above-described embodiment, the engaging portion 39 formed at the terminal connecting portion 36 of the center shaft 30 is knurled. However, the engaging portion 39 may be formed in the form of bellows or protrusions and is preferably configured such that the terminal metal piece 50 can be engaged with the terminal connecting portion 36, and these elements can be firmly connected together by crimping. As in a glow plug 200 in Fig. 9, a nut may of course be used instead of the terminal metal piece. Specifically, a male thread is formed on the outer peripheral surface of the terminal connecting part 236 of the center shaft 230, and a nut 250 is screwed onto the male thread so as to press the pressing member 60 forward along the axis. A connection terminal (not shown) of an external circuit is screwed onto the male thread of the terminal connecting part 236, which is exposed rearward from the nut 250, so as to establish an electrical connection. In particular, the connection between the center shaft 230 and the connection terminal of the external circuit via the terminal connecting part 236 shown in the present embodiment is an example of the case where the center shaft and the connection terminal are "directly" connected in the present invention.Meanwhile, the case where the terminal metal piece 50 is provided at the rear end of the center shaft 30 and the connection terminal (not shown) of the external circuit is connected to the terminal metal piece 50 is an example of the case where the center shaft and the connection terminal are "indirectly" connected in the present invention. The connection terminals of some external circuits take the shape of a disk. Such a connection terminal can be connected to the center shaft directly or indirectly. When the connection terminal takes the shape of a disk having a large inner diameter, for example,The connecting terminal is engaged with the center shaft 230, as in the modification described above, and a second nut is screwed onto the center shaft 230 to thereby hold the connecting terminal between the nut 250 and the second nut, thereby "indirectly" connecting the connecting terminal to the center shaft 230 via the nut. When the connecting terminal takes the form of a disc having a small inner diameter, the connecting terminal is "directly" connected to the center shaft 230 as a result of the connecting terminal being held by the nut in a state in which the inner periphery of the connecting terminal is in contact with the male thread of the center shaft 230 and an electrical continuity is established therebetween.

[0052] The glow plug of the above-described embodiment includes the ceramic heater 20 in which the heat-generating element 24 formed of a conductive ceramic is embedded in the substrate 21 formed of an insulating ceramic. However, the heater is not limited to this, and the glow plug may include a sleeve heater configured such that a coil-shaped heat-generating resistor and a control resistor are arranged within a metallic sleeve tube whose distal end portion is closed to form a hemispherical shape. Industrial applicability

[0053] The present invention can be applied not only to glow plugs having only a heat generating function, but also to glow plugs including a temperature sensor, a pressure sensor or the like.

[0054] In summary, a glow plug is provided which is configured such that an O-ring can be easily interposed between the wall surface of an axial opening of a metal sleeve and a center shaft during manufacturing, and such that the interposed O-ring reliably maintains the airtightness of the axial opening. An O-ring, whose cross section perpendicular to the circumferential direction has an elliptical contour line elongated in a direction perpendicular to the radial direction, is interposed between a chamfer surface of an axial opening of a metal sleeve and a sealing part of a center shaft. The O-ring is pressed from the rear end side by an end face of a pressing member.Since surfaces of a longitudinal side of the O-ring, viewed in cross section, come into contact with the chamfer surface and the outer peripheral surface of the sealing part, the contact areas between the O-ring and the chamfer surface and the outer peripheral surface of the sealing part 37 can be increased. In addition, because the amount of deformation from the cross-sectional shape is small, the internal stress is prevented from increasing and the degree of close contact is prevented from increasing, whereby the airtightness of the axial opening can be reliably maintained. Short description of the drawings Fig. 1 is a vertical sectional view of a glow plug 100; Fig. 2 is a perspective view of a center shaft 30; Fig. 3 is an enlarged sectional view of a rear end and its vicinity of the glow plug 100; Fig. 4 is a perspective view of an O-ring 70; Fig. 5 is a sectional view showing a state before the O-ring 70 is disposed between a center shaft 30 and a metal shell 40; Fig. 6 is a view showing a final step of assembling a process of manufacturing the glow plug 100; Fig. 7 is a sectional view showing a state before the O-ring 70 is disposed between a center shaft 130 and a metal shell 40 according to a modification; Fig. 8 is a sectional view showing a state before the O-ring 70 is disposed between a center shaft 130 and a metal shell 140 according to another modification; And Fig. 9 is a sectional view showing a rear end and its vicinity of a glow plug 200 according to still another modification. Description of reference numbers 30 middle shaft 32 Rear end part 33 Interbody part 36 Connection part 37 Sealing part 39 engaging part 40 metal sleeve 43 axial opening 45 diameter enlarged part 47 Bevel surface 48 Rear end face 50 connecting metal pieces 60 push element 62 insertion opening 63 End face 70 O-ring 100 glow plugs

Claims

[1] Glow plug, comprising: a tubular metal sleeve (40) having an axial opening (43) extending through the metal sleeve (40) along the direction of an axis (O); a rod-shaped central shaft (30) extending along the direction of the axis (O) and disposed in the axial opening (43) of the metal sleeve (40) with a clearance formed between the central shaft (30) and a wall surface of the axial opening (43), an end part of the central shaft (30) protruding from a rear end surface of the metal sleeve (40); an O-ring (70) disposed at a rear end of the axial opening (43) between the wall surface of the axial opening (43) and the center shaft (30), the O-ring (70) being in close contact with the wall surface of the axial opening (43) and an outer peripheral surface of the center shaft (30); and an annular pressing member (60) having an insertion opening (62) into which the central shaft (30) is inserted, the pressing member (60) being at least partially disposed between the wall surface of the axial opening (43) and the central shaft (30), and having an end surface for pressing the O-ring (70) from the rear side thereof, a part of the wall surface of the axial opening (43) with which part the O-ring (70) comes into close contact and / or a part of the outer peripheral surface of the central shaft (30) with which part the O-ring (70) comes into close contact is / is formed as a chamfer surface (47) which increases the clearance between the wall surface of the axial opening (43) and the central shaft (30) in the direction of the axis (O) towards the rear end side; and wherein the O-ring (70) assumes an annular shape which extends around the axis (O, P) and is configured such that, in one of the two cross sections of the O-ring (70) perpendicular to its circumferential direction, a distance (V) between two tangential lines which are parallel to a radial direction of the O-ring and are tangents to a contour line of the one cross section is greater than a distance (H) between two tangential lines which are perpendicular to the radial direction and are tangents to the contour line before the O-ring (70) is installed in the glow plug (100), characterized by , that the central shaft (30) has a terminal connecting part (36) which is provided at a rear end of the central shaft (30) with respect to the direction of the axis (O) and to which a connection terminal of an external circuit is directly or indirectly connected; and a relationship D1 > d1 is satisfied before the O-ring (70) is installed in the glow plug (100), wherein D1 represents the minimum inner diameter of the O-ring (70) and d1 represents the maximum diameter of the terminal connecting part (36) of the center shaft (30), further wherein a relationship d1 < d2 is satisfied, wherein d2 represents the diameter of the center shaft (30) at a part which is arranged in the direction of the axis (O) in front of the terminal connecting part (36), and a relationship D1 < d2 is satisfied. [2] Glow plug, comprising: a tubular metal sleeve (40) having an axial opening (43) extending through the metal sleeve (40) along the direction of an axis (O); a rod-shaped central shaft (30) extending along the direction of the axis (O) and disposed in the axial opening (43) of the metal sleeve (40) with a clearance formed between the central shaft (30) and a wall surface of the axial opening (43), an end part of the central shaft (30) protruding from a rear end surface of the metal sleeve (40); an O-ring (70) disposed at a rear end of the axial opening (43) between the wall surface of the axial opening (43) and the center shaft (30), the O-ring (70) being in close contact with the wall surface of the axial opening (43) and an outer peripheral surface of the center shaft (30); and an annular pressing member (60) having an insertion opening (62) into which the central shaft (30) is inserted, the pressing member (60) being at least partially disposed between the wall surface of the axial opening (43) and the central shaft (30), and having an end surface for pressing the O-ring (70) from the rear side thereof, a part of the wall surface of the axial opening (43) with which part the O-ring (70) comes into close contact and / or a part of the outer peripheral surface of the central shaft (30) with which part the O-ring (70) comes into close contact is / is formed as a chamfer surface (47) which defines the clearance between the wall surface of the axial opening (43) and the central shaft (30) in the direction of the axis (O) to the rear enlarged towards the end; and where the O-ring (70) assumes an annular shape which extends around the axis (O, P) and is configured such that, in one of the two cross sections of the O-ring (70) perpendicular to its circumferential direction, a distance (V) between two tangential lines which are parallel to a radial direction of the O-ring and are tangents to a contour line of the one cross section is greater than a distance (H) between two tangential lines which are perpendicular to the radial direction and are tangents to the contour line before the O-ring (70) is installed in the glow plug (100), characterized by , that the chamfer surface (47) at a part of the outer peripheral surface of the central shaft (30), with which part the O-ring (70) comes into close contact; and a relationship d2 < D1 < d3 is satisfied before the O-ring (70) is installed in the glow plug (100), where D1 represents the minimum inner diameter of the O-ring (70), d2 represents the diameter of the center shaft (30) at a part between the terminal connecting part (36) and the chamfer surface (47), and d3 represents the diameter of the center shaft (30) at a part which is arranged in front of the chamfer surface (47). [3] The glow plug according to claim 2, wherein the center shaft (30) has a terminal connecting part (36) which is provided at a rear end of the center shaft (30) with respect to the direction of the axis (O) and to which a connection terminal of an external circuit is directly or indirectly connected; and a relationship D1 > d1 is satisfied before the O-ring (70) is installed in the glow plug (100), where d1 represents the maximum diameter of the terminal connecting part (36) of the center shaft (30). [4] The glow plug according to claim 1, 2 or 3, wherein a relationship of 1.2 ≤ V / H ≤ 2.0 is satisfied before the O-ring (70) is installed in the glow plug (100), where V represents a distance between two tangential lines which are parallel to a radial direction of the O-ring (70) and are tangents to a contour line of the one cross section, and where H represents a distance between two tangential lines which are perpendicular to the radial direction and are tangents to the contour line in the one of the two cross sections of the O-ring (70) perpendicular to the circumferential direction thereof. [5] Glow plug according to one of claims 1 to 4, wherein a relationship D2 < d4 is satisfied before the O-ring (70) is installed in the glow plug (100), where D2 represents the maximum outer diameter of the O-ring and d4 represents a diameter of an edge line formed between the rear end surface of the metal shell (40) and the wall surface of the axial opening (43). [6] Glow plug according to one of claims 1 to 5, wherein the metal sleeve (40) further comprises: a fastening threaded part for fastening in a threaded opening of an internal combustion engine, a tool engaging portion for engaging a tool used when the fastening threaded portion is screwed into the threaded opening, wherein the metal housing at the tool engaging portion has a corner width of 8.8 mm or less, and wherein the metal housing has a nominal outer diameter of 8 mm or less at the fastening threaded portion.

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