A SPARK PLUG

Abstract: A spark plug comprising: a cylindrical metallic shell 3; a tubular insulator 5 inserted into said metallic shell 3; a centre electrode 6 positioned inside said insulator, said electrode 6 having its rear end projecting from the rear end of said insulator; a sleeve terminal 7 located between the outer surface of the said electrode and the inner surface of said insulator 5, the terminal 7 being connected at its rear end to said centre electrode 6; a connector 2 having an insulator sleeve 9 fitted inside a metallic sleeve 8; and said connector 2 being securely capped to said metallic shell 3 at the time of assembly with the rear end of said tubular insulator 5 fitting inside said insulator sleeve 9, and with said metallic shell 3 fitting said metallic sleeve 8.

Description: This invention relates to spark plugs particularly surface gap types for use in various types of internal combustion engines, gas burners and the like, and particularly concerns the connecting structure between the connector and the metallic shell. Figure 3 shows a known type of low voltage surface gap type spark plug. The igniter plug (B) has a plug body 100, a metallic barrel 200, a connector 300 and a sleeve terminal 330. The plug body 100 has a cylindrical metallic shell 110 inside which an annular semi-conductor tip 120 and a tubular insulator 130 are placed end to end. A centre electrode 140 is placed inside tip 120 and the insulator 130. The centre electrode 140 consists of an insert axis 141 to one end of which a discharge tip 146 is welded. The other, rear, end of the insert axis 141 projects from the insulator 130, and is enclosed by an insulator tube 142. A sealant glass 150 is provided in the space between the centre electrode 140 and the insulator 130 around the position of the weld between the tip 146 and the insert axis 141. The metallic barrel 200 is securely connected to the rear end of the metallic shell 110 by a weld 160. The connector 300 has an insulator sleeve 310 fitted to the inside of a metallic sleeve 320, the front end of which is brazed to the rear end of the metallic barrel 200 as seen at numeral 250. The insulator sleeve 310 has a portion of smaller diameter at its front end which extends into the rear end of the metallic barrel 200 and encloses the insulator tube 142. The sleeve terminal 330 is fitted between the insulator sleeve 310 and the rear portion of the insert axis 141. The rear ends of the sleeve terminal 330 and the insert axis 141 being flush with each other, and secured by means of brazing as seen at numeral 350. The spark plug (B) is assembled as follows: (1) The plug body 100 is welded or brazed at 160 to the metallic barrel 200 after assembly of the plug body 100. (2) The insulator sleeve 310 is inserted into the metallic sleeve 320, and brazed to the metallic barrel 200 as indicated by 250. (3) The sleeve terminal 330 is brazed at 350 to the insert axis 141 while the plug is still hot from the brazing operation 250. The above spark plug, however, has the following disadvantages: Firstly, if a defect occurs during process (3) the results of the first two processes are wasted leading to increased costs. Second, process (3) is done in a small space in the metallic sleeve 320 which requires a well-trained and skilful technique. Therefore, it is an aim of this invention to provide a spark plug which can be easily assembled with fewer defects and in which it is possible to easily weld the sleeve terminal to the centre electrode and at the same time, ensure positive and sufficient contact between the terminal and the electrode. US-A-4315298 discloses a spark plug comprising a cylindrical metallic shell, a tubular insulator inserted into said metallic shell, a centre electrode positioned inside said insulator, said electrode having its rear end projecting from a rear end of said insulator, a sleeve terminal connected to said centre electrode and a connector having an insulator sleeve fitted inside a metallic sleeve. In common with the spark plug described above with reference to Fig. 3 the construction of the spark plug disclosed in US-A-4315298 adversely affects its assembly and/or performances. For example, the upper insulator is separate from the upper connector sleeve, requiring the upper connector to be incorporated into the rest of the plug in two stages, by first adding the upper insulator to the plug and then adding the upper connector sleeve. Secondly, the tubular insulator of the metallic shell does not fit inside the upper insulator and thus cannot be supported thereby. According to the present invention, there is provided a spark plug comprising a cylindrical metallic shell, a tubular insulator inserted into said metallic shell, a centre electrode positioned inside said insulator, said electrode having its rear end projecting from a rear end of said insulator, a sleeve terminal connected to said centre electrode and a connector having an insulator sleeve fitted inside a metallic sleeve, characterised in that the sleeve terminal is located between the outer surface of said centre electrode and the inner surface of said insulator and is connected at its rear end to said centre electrode, that the insulator sleeve is fitted inside said metallic sleeve by means of a stop ring and that said connector is securely capped to said metallic shell by means of brazing or welding at the time of assembly with the rear end of said tubular insulator fitting inside the insulator sleeve, and with said metallic shell fitting said metallic sleeve. In many embodiments, to allow the rear end of the tubular insulator to fit inside the insulator sleeve, the rear end of the tubular insulator projects from the end of the metallic shell and the insulator sleeve ends short of the front end of the metallic sleeve. The invention will be further described with reference to a preferred exemplary embodiment and the drawings, in which: Figure 1 is an exploded cross-sectional view of a surface gap type igniter plug according to an embodiment of this invention; Figure 2 is a longitudinal cross-sectional view of a surface gap type igniter plug according to an embodiment of this invention; and Figure 3 is a view similar to Figure 2 according to the prior art. With reference to Figures 1 and 2 of the drawings a low voltage surface gap type spark plug (A) has a plug body 1 and a connector 2. The plug body 1 includes a cylindrical metallic shell 3, an annular semi-conductor tip 4 and a tubular insulator 5 connector end to end and each located inside the metallic shell 3. Inside the bore 51 of the tubular insulator 5, an elongated centre electrode 6 is located, to the rear end of which a sleeve terminal is attached. The metallic shell 3 has a housing portion 3B, the front end of which is connected to the rear end of an annular firing tip 3A. The housing portion 3B is made of stainless steel, while the firing tip 3A is made from heat-and arc-resistant material such as SUS 304, Inconel or a tungsten-based alloy. The metallic shell 3 has a tapered inner surface 31 at the firing tip 3A, a male thread 32 on the outer surface of the housing portion 3B, and a flange 33 at its rear end portion. The rear end 34 of the metallic shell 3 is plugged to engage with a step portion 5a of the insulator 5 to axially push the insulator 5. The semi-conductor tip 4 is made of a sintered compact with silicon carbide (SiC), alumina (Al₂O₃) and magnesium oxide (MgO) as main components. This tip 4 has an outer beveled portion 42 to engage with the tapered surface 31, and is positioned to provide a surface discharge gap (Gp) between the tip 4 and the metallic shell 3. The tubular insulator 5 is made of a ceramic material, having alumina as its main component, and has a portion of reduced thickness 52 in the front region, so that the inner diameter of the tubular insulator 5 is greater than the outer diameter of the centre electrode 6, and the outer diameter of the tubular insulator 5 is smaller than the inner diameter of the metallic shell 3. The front end 53 of the portion of reduced thickness 52 extends to and contacts the rear end of the semi-conductor tip 4. The rear end portion 54 of the insulator 5 extends beyond the rear end 34 of the metallic shell 3. The centre electrode 6 has a discharge tip 61 made of tungsten-based alloy and an insert axis 62 made of nickel-based alloy connected end to end inside the portion of reduced thickness 52 by a weld 63. The discharge tip 61 extends slightly beyond the lowest end 43 of the semi-conductor tip 4 and is flush with the rear end of the firing tip 3A. The rear end of the insert axis 62 extends beyond the rear end portion 54 of the insulator 5. A glass sealant 64 fills the annular space between the thickness-reduced portion 52 and the centre electrode 6, and the space between the thickness-reduced portion 52 and the firing tip 3A. A sleeve terminal 7 has sleeve body 71 and diameter-increased head portion 72. The diameter-increased head portion 72 extends beyond the rear end portion 54 of the insulator 5 and engages with it, while the sleeve body 71 is inserted between the insert axis 62 and the rear end portion 54. In this situation, the rear end of the insert axis 62 extends through a bore of the sleeve terminal 7 and is flush with the rear end of the terminal 7. The axis 62 and the terminal 7 are brazed at the flush surface as indicated by numeral 73. A connector 2 has an insulator sleeve 9 fitted to the inner wall of a metallic sleeve 8. The front end of the insulator sleeve 9 ends somewhat short of that of the metallic sleeve 8. The metallic sleeve 8 further has a connector thread 81 on an outer rear surface thereof, and at the same time, has a stop rim 82 on an inner rear surface of the sleeve 8 which engages the rear end of the insulator sleeve 9 via a washer 21. A hexagonal portion 83 is integrally formed in the metallic sleeve 8 for convenience when moulding the plug. The insulator sleeve 9 is made of the same material as the tubular insulator 5, and constricted at a front portion 91 through a step area 92 to match the rear end portion 54 of the insulator 5. In this instance, a stop ring 22 is placed on the inside of the metallic sleeve 8 in a position remote from the connector thread 81, and around the outside of the constricted portion 91 of the insulator sleeve 9. The ring 22 is welded or brazed to the inner wall of the metallic sleeve 8 as indicated by a black dot 23. The insulator sleeve 9 is sandwiched between the stop rim 82 and the stop ring 22 so as to be securely positioned. The connector 2 is securely capped to the metallic shell 3 in a manner that the rear end portion 54 of the insulator 5 fits inside the constricted front portion 91, while the rear end 34 of the metallic shell 3 fits inside the front end of the metallic sleeve 8. During this insertion process, the sleeve end engages with flange 33, and the two are welded together as indicated by numeral 35 in Figure 2. In this embodiment, the igniter plug (A) is assembled as follows: (1) The connector 2 is assembled after the plug body 1 is completed. (2) The connector 2 is capped to the metallic shell 3, and welded at the engagement portion 35 between the flange 33 and the front end of the metallic sleeve 8. As may be seen from the above description, the following advantages are obtained according to the invention. (i) The brazing operation between the insert axis 62 and the sleeve terminal 7 can be carried out with the connector 2 unassembled, as seen in Figure 1. This makes the brazing operation easy. In addition, the condition of the finished braze may be easily visually examined, thus reducing braze defects to a considerable degree. (ii) The assembly is completed by the comparatively easy weld operation between the metallic sleeve 8 and the metallic shell 3, thus avoiding defects in the earlier assembly procedures and reducing the number of defective products. (iii) The welding or brazing operation between the stop ring 22 and the metallic sleeve 8 can be conducted remote from the position of the connector thread 81, thus avoiding the thread 81 from being thermally deformed. In addition, this eliminates the need for removing fluxes which otherwise would penetrate into the thread 81 at the time of the welding operation. (iv) Compared to the prior art of Figure 3, those components equivalent to the barrel 200 and the insulator tube 142 can be omitted, thus reducing the number of component parts and reducing the number of assembly procedures It is noted that this invention may be applied to high voltage type plugs. Further, it is appreciated that instead of welding 35, the sleeve 8 may be secured to the flange 33 by thermal fusion such as brazing. In addition, the sleeve terminal 7 may be incorporated into the rear end of the insert axis 62 to constitute an integral part of the centre electrode 6. Furthermore, the metallic sleeve 8 may have a thread at the inner wall, while the rear end 34 of the metallic shell 3 may have a complementary thread at the outer wall. The sleeve 8 may be secured to the metallic shell 3 by screwing the male thread into the female thread.

Claims: A spark plug comprising a cylindrical metallic shell (3), a tubular insulator (5) inserted into said metallic shell (3), a centre electrode (6) positioned inside said insulator (5), said electrode (6) having its rear end projecting from a rear end of said insulator (5), a sleeve terminal (7) connected to said centre electrode (6), and a connector (2) having an insulator sleeve (9) fitted inside a metallic sleeve (8) characterised in that the sleeve terminal (7) is located between the outer surface of said centre electrode (6) and the inner surface of said insulator (5) and is connected at its rear end to said centre electrode, that the insulator sleeve (9) is fitted inside said metallic sleeve (8) by means of a stop ring (22) and that said connector (2) is securely capped to said metallic shell (3) by means of brazing or welding at the time of assembly with the rear end of said tubular insulator (5) fitting inside the insulator sleeve (9) and with said metallic shell (3) fitting said metallic sleeve (8). A spark plug according to claim 1, in which said sleeve terminal (7) has an upper head portion (72) of increased diameter integrally formed with said sleeve terminal, a lower surface of which overhangs and engages with the rear end of said tubular insulator (5). A spark plug according to claim 1 or 2, in which the rear end of said centre electrode (6) is flush with that of said sleeve terminal (7). A spark plug according to claim 1, 2 or 3, in which said connector (2) has a connector thread (81) at a rear surface of said metallic sleeve (8) and said stop ring (22) is secured to the inside of said metallic sleeve (8) to secure said insulator sleeve (9), said stop ring (22) being located remote from said connector thread (81). A spark plug according to any preceding claim, wherein said metallic sleeve (8) has a hexagonal portion (83) on its outer surface. A spark plug according to any preceding claim, wherein said tubular insulator (5) and/or insulator sleeve (9) is made of a ceramic material with alumina as a main component. A spark plug according to any preceding claim, wherein said centre electrode (6) consists of discharge tip (61) and an insert axis (62). A spark plug according to claim 7, wherein said discharge tip (61) is made of a tungsten-based alloy and said insert axis (62) is made of a nickel-based alloy, ends of said discharge tip (61) and said insert axis (62) being welded together near the front portion of said tubular insulator (5). A spark plug according to any preceding claim, wherein said metallic sleeve (8) has female thread on its inside while said metallic shell (3) has a male thread on its outside, and the male thread of said metallic shell (3) is screwed into the female thread of said metallic sleeve (8). A spark plug according to any preceding claim, wherein the rear end (54) of said tubular insulator (5) projects from the end of said metallic shell (3) and said insulator sleeve (9) ends short of the front end of said metallic sleeve (8).

Inventors: MATSUMURA, SHUZO, SUZUKI, TAKAHIRO
Applicants: NGK SPARK PLUG CO., LTD
Application year: 1989
Main classification: H01T13
IPC codes: H01T13, F23Q3, H01T21