An outer electrode for spark plug and a method of manufacturing thereof

Abstract: In an outer electrode, a rear end of which is securely connected to a metallic shell of a spark plug by means of welding to form a spark gap between a front end of the outer electrode and a firing tip of a center electrode which is concentrically placed within the metallic shell through an insulator,a middle core is provided which is made of copper, and the middle core is clad by a heat and spark-erosion resistant metal. A centermost core is clad by the middle core. The centermost core is made of a similar metal to the metallic shell, and rear end of the centermost core is welded to the metallic shell so as to reinforce a welding portion between the metallic shell and the outer electrode.

Description: This invention relates to an outer electrode for a spark plug (and a method of manufacture thereof) which includes a center electrode placed within a metallic shell through an insulator, with the outer electrode located to oppose the center electrode through a spark gap, and particularly concerns an outer electrode for a spark plug (and a method of manufacturing thereof)in which a rear end of the outer electrode is securely connected to a front end of the metallic shell by means of welding. In previously proposed spark plugs for use in internal combustion engines, the outer electrode has had one end securely welded to a metallic shell. The spark plug has included a center electrode placed within the metallic shell through an insulator. The outer electrode has been physically bent to oppose the center electrode through a spark gap. The outer electrode, thus welded to the metallic shell has generally been made of nickel-based alloy by adding Si Cr, Al and Mn to 95 % or more nickel which has a heat-dissipating (heat-conductive) property, in addition to a heat and spark-erosion resistant property. With recent high-output engines, however, it has been desired to introduce an outer electrode especially superior in heat-dissipating (heat-conductive) property. In an attempt to provide this, it has been suggested to make the outer electrode with a copper core to produce a good heat-dissipating (heat-conductive) property. Due to the relatively weak strength of the welding portion between the outer electrode and the metallic shell, cracks often occur on the welding portion when the outer electrode is physically bent to its final configuration. When the welding portion is cracked, the copper loses its good heat-dissipating property, resulting in a limited extension period of service life when used in an internal combustion engine. EP-A-0358319 discloses a spark plug, and a method of manufacture thereof, in accordance with the preambles of claims 1 and 9, respectively. According to a first aspect of the present invention there is provided a spark plug including an outer electrode, the rear end of which is securely connected to a metallic shell of the spark plug by means of welding, a spark gap being formed between the front end of the outer electrode and the firing tip of a center electrode which is concentrically placed within the metallic shell through an insulator, the outer electrode comprising: a middle core which is made of copper to provide heat-conduction, the middle core being clad by heat and spark-erosion resistant metal; characterised in that the outer electrode further comprises a centermost core clad by the middle core, the centermost core being made of a metal weldable to that of the metallic shell, and the rear end of the centermost core being welded to the metallic shell so as to reinforce a welding portion between the metallic shell and the outer electrode. There is thus provided an outer electrode for a spark plug which is capable of maintaining a good heat-dissipating property, and at the same time, obtaining a strong securement between an outer electrode and a metallic shell so as to ensure an extended period of service life. Preferably, the centermost core terminates its front end short of that of the middle core so as to maintain a heat-dissipating property. Further, preferably the centermost core has a thermal expansion characteristic substantially similar to that of the metal cladding the middle core and has an axial length similar to that of the middle core. This structure helps to alleviate thermal stresses among the centermost core, the middle core and the metal cladding the middle core and thus to maintain a predetermined spark gap between the electrodes. According to a second aspect of the present invention there is provided a method of providing a strengthened outer electrode of a spark plug comprising the step of providing a heat conductive middle core and cladding said conductive middle core (6) with a heat and spark-erosion resistant metal, characterised in that the method further comprises the step of providing a weldable centermost core extending substantially axially from the rear end of the outer electrode to reinforce a welding portion between the rear end of the outer electrode and a metallic shell of the spark plug. With this method, the outer electrode may be manufactured by the following steps of: forcing an elongated strip which serves as the centermost core into an axial bore provided in a metallic column made of copper to form a first body; forcing the first body into a cup-shaped metal piece to form a second body; forcing the second body into a circular hole provided in a die to extrude the second body so as to form an elongated bar; removing an end portion from the elongated bar; forcing the elongated bar into a rectangular hole provided in a rectangular die to extrude the elongated bar so as to form a rectangular bar; cutting a rear portion of the rectangular bar so that the rear end of the centermost core is flush with that of the outer metal and the middle core; and thermally annealing the rectangular bar. In order that the invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawings, in which:- Fig. 1 is a partially sectioned side elevation of part of a first embodiment of spark plug in accordance with the present invention; Fig. 2a is a cross sectional view along the line IIa-IIa of Fig. 1; Fig. 2b is a cross sectional view along the line 2b-2b of Fig. 1; Figs. 3 to 9 are schematic views of steps in a process of manufacturing the outer electrode of Fig. 1; Fig. 10 is an enlarged sectional view of the outer electrode of Fig. 1, prior to forming to its final shape; Fig. 11 is a graph showing a relationship between the axial length of the centermost core and the temperature of the firing tip of the center electrode of Fig. 1; Fig. 12 is a graph showing a relationship between the temperature of the firing tip of the center electrode and the cross sectional ratio of the middle core to the outer electrode of Fig. 1; Fig. 13 is a graph showing a relationship between the tensile strength and the cross sectional ratio of the middle core to the outer electrode of Fig. 1; Fig. 14 is a view similar to Fig. 1 illustrating a second embodiment of spark plug in accordance with the present invention; Fig. 15 is a cross sectional view taken along the line XV-XV of Fig. 14; Figs. 16 to 21 are views similar to Figs. 3 to 9, but showing a process for manufacturing the outer electrode of Fig. 14; Fig. 22 is a graph similar to Fig. 12, but relating to the spark plug of Fig. 14; Fig. 23 is a graph similar to Fig. 13, but relating to the spark plug of Fig. 14; and Figs. 24 and 25 are schematic views of steps in a process for manufacturing a modified version of the outer electrode of Fig. 14. Referring to Fig. 1 which shows a first embodiment of the invention, numeral 1 designates a spark plug which has a center electrode 2 concentrically placed within a cylindrical metallic shell 4 through a tubular insulator 3. The center electrode 2 is a composite metal consisting of a copper core clad by a nickel-based alloy, and connected to a high tension cable (not shown). The insulator 3 is made of ceramic material with alumina as the main component, and fixedly secures the center electrode 2 in position. The metallic shell 4 is made of ferro-based metal and its outer surface has a male thread suitable for mating the spark plug to the engine block of an internal combustion engine (not shown). The outer electrode 5 is elongate and has its rear end securely connected to a front end of the metallic shell 4 by means of welding. A front portion of the outer electrode 5 is physically bent into a generally L-shaped configuration so as to oppose a firing tip 2a of the center electrode 2 through a spark gap (1). The outer electrode 5 has a middle core 6 made of copper to provide a heat-dissipating (heat-conductive) property, and is clad by an outer metal 7 which is made of pure nickel (Ni), nickel-based alloy or Inconel (alloy of Ni and Cr and Fe) each welding intense to the metallic shell 4, and at the same time, providing good heat and erosion-resistance properties. A centermost core 8 is clad by the middle core 6 and is made of pure nickel, nickel-based alloy or pure iron, each welding intense to the metallic shell 4. The outer metal is, in use, generally exposed to a combustion chamber (Ch) and a spark discharge at once. The centermost core 8 terminates its rear end at the rear end of the middle core 6 and is flush with the end of the outer metal 7. The centermost core 8 terminates its front end considerably short of the front end of the middle core 6. By way of example, the axial length of the centermost core 8 may be 1/5 of that of the outer electrode 5 which may be approximately 15mm in length. In this example, the rear ends of the centermost core 8 and the outer metal 7 are directly welded to the front end of the metallic shell 4 when the rear end of the outer electrode 5 is securely connected to the metallic shell 4 by means of welding. The cross sectional area of the centermost core 8 is 60% or more of that of the middle core 6, as shown in Fig. 2a, while the cross sectional area of the middle core 6 is 40% or less of that of the outer electrode 5, as shown in Fig. 2b. Referring to Figs. 3 to 9, the outer electrode 5 is manufactured as follows: (i) As shown in Fig. 3, an elongated nickel strip 10, which serves as the centermost core 8, is prepared to have an axial length considerably shorter than that of a copper column 9. The column 9 is provided at its upper end with a flange portion 9a to act as a stopper. The elongation strip 10 is forced into an axial bore 9b provided within the copper column 9 to form a first body 11 as shown in Fig. 4. The column 9 includes a flange portion 9a at its upper end to act as a stopper. (ii) The first body 11 is forced into a hollow 12a of a cup-shaped nickel alloy piece 12 until the flange portion 9a reaches the upper end of the cup-shaped nickel alloy piece 12 to form a second body 13 as shown in Fig. 5. (iii) The second body 13 is forced into a circular hole provided in a circular die 14 to extrude the second body so as to form an elongated bar 15 circular in section as shown in Fig. 6. (iv) A flange head 16 which is provided with the elongated bar 15 is removed from the elongated bar 15 as shown in Fig. 7. (v) The elongated bar 15 is forced into a rectangular hole provided in a rectangular die 17 to extrude the elongated bar 15 so as to form a rectangular bar 18 rectangular in section as shown in Fig. 8. (vi) A rear portion 18a of the rectangular bar 18 is then cut so that the rear end of the centermost core 8 is flush with that of the outer metal 7 and the middle core 6 and the rectangular bar 18 is then thermally annealed to form the outer electrode 5 as shown in Fig. 9. Using the outer electrode 5 shown in Fig. 10, a first experiment was carried out to find how the axial length (a) of the centermost core 8 influences the heat-dissipating (heat-conductive) effect of the outer electrode 5 shown in Fig. 2a. The result obtained from the first experiment is shown in Fig. 11. This indicates that the heat-dissipating effect improves with a decrease in axial length (a) of the centermost core 8. A second experiment was carried out to find how the cross sectional ratio of the middle core 6 to the outer electrode 5 influences the heat-dissipating effect of the outer electrode 5 shown in Fig. 2b. The result obtained from the second experiment is shown in Fig. 12. This indicates that satisfactory heat-dissipation is obtained when the cross sectional ratio is 20% or more. With the cross section of the centermost core 8 constant, a third experiment was carried out to determine how the cross sectional ratio of the middle core 6 to the outer electrode 5 influences the tensile strength of the welding portion between the metallic shell 4 and the outer electrode 5. The result obtained from the third experiment is shown in Fig. 13. This indicates that a satisfactory tensile strength is obtained when the cross sectional ratio is 40% or less. As is apparent from these three experiments, when the centermost core 8 terminates short of the front end of the middle core 6, it is possible to obtain a good heat-dissipating effect of the outer electrode 5 and to strongly connect the outer electrode 5 to the metallic shell 4 by simultaneously welding the outer metal 7 and the centermost core 8 to the metallic shell 4. Furthermore, because the outer electrode 5 has manufacturing processes partly common to those of the center electrode 2, this contributes to reducing the manufacturing cost. Figs. 14 and 15 show a second embodiment of spark plug in accordance with the present invention. Like reference numerals in Figs. 14 and 15 denote similar parts to those of the spark plug of Figs. 1, 2a and 2b of the present invention. In the second embodiment of the invention, the centermost core 8 has a coefficient of thermal expansion similar to that of the outer metal 7, while the centermost core 8 has an axial length similar to that of the middle core 6. This structure helps to alleviate thermal stresses among the centermost core 8, the middle core 6 and the outer metal 7 and substantially prevents the outer electrode 5 from being unfavourably deformed, thus maintaining the spark gap (1) as the desired spacing. In this instance, the cross sectional area of the middle core 6 falls within a range from 20% to 50% of that of the outer electrode 5. The outer electrode 5, structured according to the second embodiment of the invention, is manufactured as follows:- (i) A nickel extension strip 106 which is to serve as the centermost core 8 is prepared. The axial length of this is substantially the same as that of a copper tube 104. The nickel extension strip 106 is forced into the copper tube 104, which has an outer flange portion 109a to serve as a stopper, to form a clad wire 109 as shown in Fig. 16. (ii) The clad wire 109 is forced into a hollow portion 110a of a cup-shaped nickel alloy piece 110 until the outer flange portion 109a reaches an upper end of the cup-shaped nickel alloy piece 110, to form a composite body 111 as shown in Fig. 17. (iii) The composite body 111 is forced into a circular hole provided in a circular die 112 to extrude the composite body 111 so as to form an elongated bar 113 circular in section as shown in Fig. 18. (iv) A flange head 114 formed integrally with the elongated bar 113 during the extrusion process is removed from the elongated bar 113 as shown in Fig. 19. (v) The elongated bar 113 is forced into a rectangular hole provided in a rectangular die 115 to extrude the elongated bar 113 so as to form a rectangular bar 116 rectangular in section as shown in Fig. 20. (vi) A rear portion 116a of the rectangular bar 116 is then cut so that a rear end of the rectangular bar 116 is flush with that of the outer metall 7, the middle core 6 and the centermost core 8 as shown in Fig. 21. The rectangular bar 116 is then thermally annealed to form the outer electrode 5. A first experiment was carried out to find how the cross sectional ratio of the middle core 6 to the outer electrode 5 influences the heat-dissipating effect of the outer electrode 5. The result obtained from the first experiment is shown in Fig. 22. This indicates that satisfactory heat-dissipation is obtained when the cross sectional ratio is 20% or more. A second experiment was carried out to find how the cross sectional ratio of the middle core 6 to the outer electrode 5 influences the tensile strength of the welding portion between the metallic shell 4 and the outer electrode 5. The result obtained from the second experiment is shown in Fig. 23. This indicates that a satisfactory tensile strength is obtained when the cross sectional ratio is 50% or less. In the two experiments, the middle core 6 was located between outer metal 7 and a centermost core 8 both having the same coefficient of thermal expansion. This structure protects the outer electrode 5 against unfavourable deformation when exposed to the combustion chamber (Ch) of the internal combustion engine. Further, the experiments indicate that a cross sectional ratio of the middle core 6 to the outer electrode 5 within the range from 20% to 50% provides a satisfactory heat-dissipation effect of the outer electrode 5, as well as a welded join of suitable strength between the metallic shell 4 and the outer electrode 5. Figs. 24 and 25 show a modification of the manufacturing method of the second embodiment of the invention. A copper tube 207, which serves as the middle core 6, has no flange portion as opposed to the second embodiment of the invention. A nickel extension strip 208 is forced into the copper tube 207 to provide a clad wire 217. The clad wire 217 is then forced into a hollow portion 210a of a nickel alloy cup 210 to form a composite body 211 as shown in Fig. 25. Processes subsequent to the formation of the composite body 211 are the same as those mentioned in relation to the second embodiment of the invention in conjunction with Figs. 18 to 21. In this instance, the cup 210 may be made of copper and the clad wire 217 of pure nickel. It is noted that a plurality of outer electrodes may be provided instead of a single outer electrode. It is further noted that the outer electrode may be bent into a C-shaped configuration instead of L-shaped configuration. It is appreciated that an outer electrode of straight type may be used, and the outer electrode may be slanted so as to oppose the periphery of the center electrode.

Claims: A spark plug including an outer electrode (5), the rear end of which is securely connected to a metallic shell (4) of the spark plug by means of welding, a spark gap (1) being formed between the front end of the outer electrode (5) and the firing tip of a center electrode (2) which is concentrically placed within the metallic shell (4) through an insulator (3), the outer electrode (5) comprising : a middle core (6) which is made of copper to provide heat-conduction, the middle core being clad by heat and spark-erosion resistant metal (7); characterised in that the outer electrode (5) further comprises a centermost core (8) clad by the middle core (6), the centermost core (8) being made of a metal weldable to that of the metallic shell (4), and the rear end of the centermost core (8) being welded to the metallic shell (4) so as to reinforce a welding portion between the metallic shell (4) and the outer electrode (5). A spark plug as claimed in claim 1, wherein the rear end of the centermost core (8) terminates flush with that of the middle core (6). A spark plug as claimed in claim 1 or claim 2, wherein the front end of the centermost core (8) terminates short of the front end of the middle core (6). A spark plug as claimed in claim 1 or claim 2, wherein the axial length of the centermost core (8) is substantially similar to that of the middle core (6). A spark plug as claimed in any of the preceding claims, wherein the centermost core (8) has a thermal expansion characteristic substantially the same as that of the metal (7) cladding the middle core (6). A spark plug as claimed in any of the preceding claims, wherein the cross sectional area of the middle core (6) is within a range of from 20% to 40% of the cross sectional area of the outer electrode (5). A spark plug as claimed in any of the preceding claims, wherein the centermost core (8) is made of one of pure nickel, nickel-based alloy or pure iron. A method of providing a strengthened outer electrode (5) of a spark plug comprising the step of providing a heat conductive middle core (6) and cladding said conductive middle core (6) with a heat and spark-erosion resistant metal (7), characterised in that the method further comprises the step of providing a weldable centermost core (8) extending substantially axially from the rear end of the outer electrode (5) to reinforce a welding portion between the rear end of the outer electrode (5) and a metallic shell (4) of the spark plug. A method as claimed in claim 8, wherein the outer electrode is manufactured by the steps of: forcing an elongated strip (10, 106, 208) which serves as the centermost core into an axial bore (9b) provided in a metallic column (9, 104, 207) made of copper to form a first body (11, 109); forcing the first body (11, 109) into a cup-shaped metal piece (12, 110, 210) to form a second body (13, 111, 211); forcing the second body (13, 111, 211) into a circular hole provided in a die (14, 112) to extrude the second body so as to form an elongated bar (15, 113); removing an end portion (16, 114) from the elongated bar (15, 113); forcing the elongated bar (15, 113) into a rectangular hole provided in a rectangular die (17, 115) to extrude the elongated bar so as to form a rectangular bar (18, 116); cutting a rear portion (18a, 116a) of the rectangular bar (18, 116) so that the rear end of the centermost core is flush with that of the outer metal (7) and the middle core (6); and thermally annealing the rectangular bar (18, 116). A method as claimed in claim 9, wherein a first end of the metallic column (9, 104) has a flange portion (9, 109a) to serve as a stopper and the first body (11, 109) is forced into the cup-shaped metal piece (12, 110) until the flange portion reaches the upper end of the cup-shaped metal piece and wherein subsequently the end portion (16, 114) including the flange portion (9a, 109a) is removed. A method as claimed in claim 9 or claim 10, wherein the axial bore (9b) has two diameters thereby forming an upper recess.

Inventors: ANDO MINORU C O NGK SPARK PLUG, OSHIMA TAKAFUMI C O NGK SPARK, ANDO, MINORU, C/O NGK SPARK PLUG CO.LTD., OSHIMA, TAKAFUMI, C/O NGK SPARK PLUG CO.LTD.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1991
Main classification: H01T21
IPC codes: H01T21, H01T13