Spark plug and method of manufacturing the same

Abstract: A spark plug is disclosed in which a hexagonal portion of a metallic shell (5) has a width across flat W of not greater than 14 mm. Talc is filled into a space defined by the metallic shell (5) and an insulator (1) to thereby form a cushion-material charged portion (9). The cushion-material charged portion (9) has an axial length L of from 0.5 mm to 10.0 mm inclusive and a thickness M of from 0.5 mm to 1.3 mm inclusive.

Description: The present invention relates to a spark plug used as a source of ignition in an internal combustion engine, and more particularly, to a spark plug having a small-sized metallic shell for installation in a narrow space. Some conventional spark plugs employ a cushion material formed from talc powder and filled into a cylindrical space defined by the outer circumferential surface of an insulator and the inner circumferential surface of a metallic shell so as to improve impact resistance, and others do not employ such a cushion material (talc) but are configured such that the insulator is secured directly by means of the metallic shell through thermal caulking. These conventional spark plugs have a screw diameter of 14 mm (M14) or 12 mm (M12). Also, a hexagonal tightening portion with which a plug wrench is engaged has a distance of 20.8 mm or 16 mm between two parallel, diagonally opposed faces thereof (width across flat). With recent improvement in engine control technology and a tendency toward employment of a multi-valve type combustion chamber, the number of components mounted on and around an engine has been increasing. Particularly, in the case of a direct-injection-type engine, which is becoming popular, a volume allotted to a spark plug on a cylinder head is small. Accordingly, the width across flat of a tightening portion of a metallic shell has been required to be decreased from the conventionally employed width of 16 mm to not greater than 14 mm. When the width across flat is reduced to not greater than 14 mm, the wall thickness of a metallic shell decreases accordingly. As a result, the volume of the metallic shell decreases with a resultant decrease in strength. A spark plug having a width across flat not greater than 14 mm and not employing a cushion material (talc) suffers impairment in impact resistance; i.e., a considerable reduction in airtightness as measured after exposure to impact. Also, because the wall thickness of a tightening portion is decreased, a load imposed on the tightening portion during caulking causes swelling of the tightening portion. As a result, the width across flat may fail to fall within a predetermined tolerance, potentially causing a failure to establish engagement between the tightening portion and a plug wrench. The above-mentioned engagement problem will next be described specifically with reference to FIGS. 2 and 3. An insulator 1 is fixedly attached to a metallic shell 5 through caulking in the following manner. A caulking die is applied from underneath to a seat portion 5F of the metallic shell 5, while another caulking die is applied from above to a tightening portion 5A and a caulking portion 5C. The upper caulking die exerts a downward force so as to buckle a curved portion 5D by about 0.5 mm to 0.8 mm, whereby the insulator 1 is strongly pressed against an inner stepped portion 5E of the metallic shell 5 via a packing member 6. In this manner, the insulator 1 is fixedly attached to the metallic shell 5 through caulking. During such caulking, a strong force exerted by the upper caulking die causes plastic deformation of not only the curved portion 5D but also the tightening portion 5A. As a result, the tightening portion 5A swells slightly. This swelling has not raised a problem with respect to a conventional spark plug having a width across flat W of not less than 16 mm, since a wall thickness P of the tightening portion 5A is sufficient so that the tightening portion 5A exhibits a sufficient strength. However, a spark plug having a width across flat of not greater than 14 mm encounters a difficulty in bringing the width across flat W within a predetermined tolerance, since the wall thickness P of the tightening portion 5A is thin, with a resultant significant swelling of the tightening portion 5A. Unless the width across flat W falls within a predetermined tolerance, a plug wrench cannot be engaged with the tightening portion 5A. By contrast, when, in order to reduce the swelling of the tightening portion 5A, the wall thickness of the curved portion 5D is reduced so that a forced required to buckle the curved portion 5D can be reduced, the strength of the curved portion 5D of a spark plug becomes insufficient for enduring a tightening torque exerted when the spark plug is mounted on an engine. Alternatively, when a thickness M of talc 9 serving as a cushion material is reduced to accordingly increase the wall thickness P of the tightening portion 5A, the effect of the talc 9 as a cushion material is diminished, resulting in an impairment in impact resistance. An object of the present invention is to provide a spark plug capable of exhibiting high impact resistance even when the width across flat of a tightening portion of a metallic shell is small, and capable of maintaining airtightness even after subjection to strong impact. Another object of the present invention is to provide a spark plug having further improved impact resistance and capable of bringing the width across flat of a tightening portion into a predetermined tolerance through suppression of swelling of the tightening portion. A further object of the present invention is to provide a method of manufacturing a spark plug as mentioned above. To achieve the above objects, the present invention provides a spark plug comprising an insulator having a center through-hole formed therein; a center electrode held in the center through-hole; a metallic shell holding the insulator through caulking; and a ground electrode electrically connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode. The metallic shell has a male-threaded portion and a tightening portion. The male-threaded portion is formed on the outer circumferential surface of a front end portion of the metallic shell, and the tightening portion is formed on the outer circumferential surface of the metallic shell and is located at the rear side with respect to the male-threaded portion. In the specification, the term "front" refers to a spark discharge gap side with respect to an axial direction of the center electrode, and the term "rear" refers to a side opposite the front side. The tightening portion is used to screw the male-threaded portion into a female-threaded hole formed in an internal combustion engine. The distance between two opposed parallel faces of the tightening portion (hereinafter referred to as a width across flat W) is not greater than 14 mm (W ≤ 14.0 mm). A cushion material is charged into a cylindrical space defined by an outer surface of the insulator and an inner surface of the metallic shell to thereby form a cushion-material charged portion. The cushion-material charged portion has an axial length L of from 0.5 mm to 10.0 mm inclusive (0.5 mm ≤ L ≤ 10.0 mm) and a thickness M of from 0.5 mm to 1.3 mm inclusive (0.5 mm ≤ M ≤ 1.3 mm). Talc, for example, may be employed as the cushion material. The cylindrically filled cushion material eases impact exerted on the metallic shell, thereby preventing loosening of caulking between the metallic shell and the insulator even when the width across flat is not greater than 14 mm. Even when caulking between the metallic shell and the insulator loosens to some extent and thus the pressure produced at the packing potion between the metallic shell and the insulator decreases with a resultant leakage of combustion gas through the packing portion, the cushion-material charged portion serves as a second packing to prevent leakage of the combustion gas from the spark plug. When the axial length L of the cushion-material charged portion is less than 0.5 mm, the cushion-material charged portion fails to effect cushioning as expected. When the axial length L of the cushion-material charged portion is in excess of 10 mm, the cushion material cannot be sufficiently filled into the cylindrical space. The resultant cushion-material charged portion has a low cushion material density and thus fails to effect cushioning as expected. When the thickness M of the cushion-material charged portion is less than 0.5 mm, the cushion-material charged portion fails to effect cushioning as expected. When the thickness M of the cushion-material charged portion is in excess of 1.3 mm, the wall thickness of the tightening portion of the metallic shell decreases accordingly, resulting in an impairment in the strength of the metallic shell. Accordingly, even when the width across flat is not greater than 14 mm, the spark plug endures use at high temperature and exhibits excellent impact resistance. Preferably, the metallic shell has a seat portion which is located between the male-threaded portion and the tightening portion and has a diameter greater than that of the male-threaded portion, and a curved portion which extends between the tightening portion and the seat portion; and the curved portion is buckled through axial caulking while being heated, so as to integrate the metallic shell and the insulator into a single unit. Through employment of the above-mentioned caulking combined with heating; i.e., hot caulking, a load required for caulking; i.e., a load required for buckling of the curved portion, becomes smaller than that required for cold caulking. Therefore, the load imposed on the tightening portion during caulking is decreased accordingly. Thus, even in the case of the tightening portion having a thin wall thickness, swelling of the tightening portion becomes sufficiently small so as to bring the width across flat within a predetermined tolerance. Also, when the heated curved portion cools after caulking, the curved portion shrinks axially, so that the pressure produced at the packing portion through caulking further increases to thereby improve airtightness of the spark plug. Notably, whether a spark plug has been formed through employment of hot caulking or cold caulking can be easily determined through analysis of a halved piece of the spark plug. In a spark plug formed through employment of hot caulking, a buckled curved portion exhibits swelling in radially inward and outward directions; i.e., the curved portion is deformed such that the thickness thereof is increased. By contrast, in a spark plug formed through employment of cold caulking, the buckled curved portion is deformed in either a radially inward direction or a radially outward direction. The present invention further provides a method of manufacturing a spark plug comprising an insulator having a center through-hole formed therein; a center electrode held in the center through-hole; a metallic shell holding the insulator through caulking; a ground electrode electrically connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode; and a ground electrode electrically connected to the metallic shell and defining a spark discharge gap in cooperation with the center electrode. The metallic shell has a male-threaded portion and a tightening portion. The male-threaded portion is formed on the outer circumferential surface of a front end portion of the metallic shell, and the tightening portion is formed on the outer circumferential surface of the metallic shell and is located at the rear side with respect to the male-threaded portion. The tightening portion is used to screw the male-threaded portion into a female-threaded hole formed in an internal combustion engine. The method is characterized by comprising the steps of: forming the metallic shell such that the distance between two opposed parallel faces of the tightening portion (hereinafter referred to as a width across flat W) is not greater than 14 mm (W ≤ 14.0 mm) and that the metallic shell has a seat portion which is located between the male-threaded portion and the tightening portion and has a diameter greater than that of the male-threaded portion, and a curved portion which extends between the tightening portion and the seat portion; charging a cushion material into a cylindrical space defined by an outer surface of the insulator and an inner surface of the metallic shell to thereby form a cushion-material charged portion having an axial length L of from 0.5 mm to 10.0 mm inclusive (0.5 mm ≤ L ≤ 10.0 mm) and a thickness M of from 0.5 mm to 1.3 mm inclusive (0.5 mm ≤ M ≤ 1.3 mm); and pressing the tightening portion and the seat portion toward each other while applying current thereto so as to heat the curved portion, to thereby buckle the curved portion. Through employment of the above-mentioned steps, a load required for caulking can be decreased. Thus, a spark plug manufactured by the above method yields the effects mentioned previously. Also, swelling of the tightening portion can be reduced to a practically acceptable extent. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is a partially sectional view of a spark plug according to the present invention; FIG. 2 is an enlarged, partially sectional view of a portion of a metallic shell subjected to caulking; FIG. 3 is a sectional view taken along line A-A of FIG. 2; FIG. 4 is a partially sectional view showing a step of caulking a spark plug which does not have a cushion-material charged portion; FIG. 5A is a schematic plan view of a hexagonal portion of a metallic shell; and FIG. 5B is a schematic plan view of the hexagonal portion of FIG. 5A formed into a 12-point nut profile. FIG. 1 shows a spark plug 20 according to the present invention. As well known, an insulator 1 made of, for example, alumina has corrugations 1A formed at an upper portion in FIG. 1 and adapted to increase creeping distance, and has a leg portion 1B formed at a lower portion in FIG. 1 and exposed to the interior of the combustion chamber of an internal combustion engine. A center through-hole 1C is formed axially in the insulator 1. A center electrode 2 made of a nickel alloy, such as inconel, is held in the center through-hole 1C in such a manner as to be projected from the lower end (in FIG. 1) of the insulator 1. The center electrode 2 is not simply made of inconel, but includes a copper core extending axially within an inconel body in order to improve thermal conductivity. FIG. 1 does not show the copper core to avoid complication of the drawing. The center electrode 2 is electrically connected to a terminal 4 located at the top of the spark plug 20 in FIG. 1, via conductive glass seal layers 12 and 13 and a resistor 3 provided within the center through-hole 1C. An unillustrated high-tension cable is connected to the terminal 4 for application of high voltage to the terminal 4. The insulator 1 rests in a metallic shell 5. The metallic shell 5 is made of low-carbon steel and includes a hexagonal portion 5A serving as the tightening portion of the present invention and adapted to engage a spark plug wrench; a male-threaded portion 5B to be screwed into a cylinder head; and a seat portion 5F. As shown in FIG. 5A, the circumferential surface of the hexagonal portion 5A assumes a hexagonal profile of a hexagonal nut. The metallic shell 5 is caulked to the insulator 1 by means of a caulking portion 5C, whereby the metallic shell 5 and the insulator 1 are integrated into a single unit. A curved portion 5D extending between the hexagonal portion 5A and the seat portion 5F is adapted to absorb an axial deformation of the metallic shell 5 that accompanies caulking. In order to complement sealing effected by caulking, a sheetlike packing member 6 is disposed between an inner circumferential stepped portion 5E of the metallic shell 5 and the insulator 1 so as to seal the leg portion I B exposed to the interior of the combustion chamber against an upper portion of the insulator 1. Wire-like sealing members 7 and 8 are disposed between the caulking portion 5C and the insulator 1. Talc powder 9 serving as a cushion material is charged between the sealing members 7 and 8 so as to establish elastic sealing, thereby fixedly and completely engaging the metallic shell 5 and the insulator 1 together. A gasket 10 is disposed at an upper end of the male-threaded portion 5B. A ground electrode 11 of nickel alloy is welded to the lower end of the metallic shell 5. The ground electrode 11 is bent at a right angle such that a flat surface of an end portion thereof faces a tip end of the center electrode 2. Referring to FIGS. 2 and 3, the outer circumferential surface of the insulator 1, the inner circumferential surface of the hexagonal portion 5A, and the upper and lower sealing members 7 and 8 define a cylindrical space into which the talc powder is charged, to thereby form a cushion-material charged portion 9. As shown in FIG. 4, a lower caulking die 42 is brought into contact with the lower face of the seat portion 5F of the metallic shell 5, and an upper caulking die 41 is brought into contact with the caulking portion 5C and the upper face of the hexagonal portion 5A. The upper and lower dies 41 and 42 are pressed toward each other at a load of several tons so as to press the metallic shell 5. Through application of the above load, as shown in FIG. 2, the caulking portion 5C is deformed along the surface of the upper die 41, and the thin-walled, curved portion 5D is plastically deformed, or buckled, in an amount of about 0.8 mm in the axial direction. This axial buckling causes the caulking portion 5C to strongly press downward in FIG. 2 an outer circumferential stepped portion 1D of the insulator 1 via the sealing member 8, the talc powder 9, and the sealing member 7. As a result, the insulator 1 is strongly pressed against an inner circumferential stepped portion 5E of the metallic shell 5 via the packing member 6, thereby sealing the leg portion 1B exposed to the interior of the combustion chamber against an upper portion of the insulator 1. A strong force exerted on the talc powder 9 causes the hexagonal portion 5A of the metallic shell 5 to slightly, elastically swell in the radial direction. This elastic swelling of the hexagonal portion 5A induces a radially inward force similar to a spring force, which presses downward the outer circumferential stepped portion 1D of the insulator 1 via the talc powder 9. This downward force elastically presses the insulator 1 against the inner circumferential stepped portion 5E of the metallic shell 1 via the packing member 6. Thus, sealing effected by the packing member 6 becomes more elastic, thereby imparting excellent impact resistance on the spark plug 20. FIG. 4 illustrates a step of caulking a spark plug which does not have a cushion-material charged portion (talc) 9. An outer circumferential stepped portion 1'D of an insulator 1' is elongated axially such that the caulking portion 5C of the metallic shell 5 abuts the upper end of the outer circumferential stepped portion 1'D either directly or via a sealing material. The lower caulking die 42 is brought into contact with the lower face of the seat portion 5F of the metallic shell 5, and the upper caulking die 41 is brought into contact with the caulking portion 5C and the upper face of the hexagonal portion 5A. The upper and lower dies 41 and 42 are pressed toward each other at a load of several tons so as to press the metallic shell 5. In this state, a current of about 100 A is applied between the upper and lower dies 41 and 42 for 0.5 sec. to 1 sec. The current flows from the upper die 41 to the lower die 42 through the metallic shell 5; specifically, through the hexagonal portion 5A, the curved portion 5D, and the seat portion 5F. Since the curved portion 5D has the thinnest thickness and thus has the highest resistance, only the curved portion 5D is intensively heated and is thus red-heated. Thus, the curved portion 5D is softened, so that a load required to buckle the curved portion 5D is decreased. A load required for caulking is decreased accordingly. As the heated, curved portion 5D cools after completion of hot caulking, the curved portion 5D shrinks in the axial direction, thereby further increasing the packing pressure of the packing member 6 produced through caulking and thus improving airtightness of the spark plug. Hot caulking of the spark plug not having the cushion-material charged portion 9 has been described with reference to FIG. 4. However, a spark plug having the cushion-material charged portion 9 as shown in FIG. 2 may undergo hot caulking while current is applied to the metallic shell 5 through the caulking dies 41 and 42. Through employment of hot caulking, a load required to buckle the curved portion 5D decreases 30% or more, thereby reducing swelling of the hexagonal portion 5A associated with caulking to a practically acceptable extent. As the heated, curved portion 5D cools after completion of hot caulking, the curved portion 5D shrinks, thereby improving airtightness of the spark plug. In order to test the effect of hot caulking, a number of spark plugs were prepared. The spark plugs were divided into three groups-plugs A, B, and C. Plug A is a spark plug which has the cushion-material charged portion 9 and which has undergone cold caulking; plug B is a spark plug which has the cushion-material charged portion 9 and which has undergone hot caulking; and plug C is a spark plug which does not have the cushion-material charged portion 9 and which has undergone hot caulking. The spark plugs had the following dimensions. The male-threaded portion 5B of the metallic shell 5 had a diameter of 12 mm, or M12. The width across flat W of the hexagonal portion 5A was 14 mm with a tolerance of +0.0 mm and -0.27 mm. The wall thickness P of the hexagonal portion 5A was 1.0 mm. The cushion-material charged portion 9 had an axial length L of 7.0 mm and a thickness M of 1.0 mm. The spark plugs underwent an impact test and a heating test and were then tested for hot airtightness. The impact test was conducted according to Section 6.4 "Impact Test" of JIS B 8031. A spark plug was attached to a block having a mass of 2.3 kg. The block was hit against an anvil 400 times per minute while being biased by a spring, thereby exerting impact on the spark plug. According to JIS regulations, impact is to be exerted for 10 minutes. However, in this test, impact was exerted for 30 minutes. The heating test was conducted simultaneously with the impact test. By use of a burner, a spark portion of the spark plug was heated to about 800°C, and the seat temperature was increased to about 300°C. The spark plug which had undergone the impact and heating tests was subjected to a hot airtightness test, which was carried out in the following manner. After the spark plug was allowed to stand at a predetermined ambient temperature for 30 minutes, an air pressure of 15 kgf/cm 2 was applied to the spark portion. The amount of air leakage from the interior of the spark plug was measured at various ambient temperatures. The results are shown in Table 1. Hot Airtightness as Measured after Heating and Impact Tests Type Room temp. 50°C 100°C 150°C 200°C 250°C 300°C Plug A With talc Cold caulked AA AA AA BB CC CC CC AA AA BB BB CC CC CC AA AA AA BB BB CC CC AA AA AA BB CC CC CC AA AA BB CC CC CC CC Plug B With talc Hot caulked AA AA AA AA BB BB CC AA AA AA AA BB BB CC AA AA AA AA BB CC CC AA AA AA AA BB CC CC AA AA AA AA BB CC CC Plug C Without talc Hot caulked CC CC CC CC CC CC CC CC CC CC CC CC CC CC BB BB CC CC CC CC CC CC CC CC CC CC CC CC BB BB CC CC CC CC CC In Table 1, AA denotes a leakage of 0 cc per minute; BB denotes a leakage of from 0 cc to 10 cc per minute; and CC denotes a leakage of greater than 10 cc per minute. 5 spark plugs belonging to each of plugs A, B, and C were tested. As seen from Table 1, leakage increases with ambient temperature. This is conceivably because, as ambient temperature increases, the metallic shell 5 thermally expands in the axial direction; consequently, the packing pressure exerted on the packing member 6 decreases. As seen from comparison of the test results between plugs A and C in Table 1, spark plugs belonging to plug A show markedly better impact resistance as compared to those belonging to plug C. Because of absence of the cushion-material charged portion 9, the spark plugs belonging to plug C show a significant impairment in airtightness as measured after the impact test. Even at room temperature, more than half of the spark plugs belonging to plug C are marked with CC with respect to airtightness. By contrast, the spark plugs belonging to plug A, which have the cushion-material charged portion 9, are all marked with AA at an ambient temperature of up to 50°C. Even at an ambient temperature of 100°C, more than half of the spark plugs belonging to plug A are marked with AA, indicating that those belonging to plug A are sufficiently applicable to practical use. As seen from comparison of the test results between plugs A and B in Table 1, spark plugs belonging to plug B, which are hot-caulked, show better impact resistance as compared to those belonging to plug A, which are cold-caulked. The spark plugs belonging to plug A are all marked with AA at an ambient temperature of up to 50°C, whereas the spark plugs belonging to plug B are all marked with AA at an ambient temperature of up to 150°C. Moreover, those belonging to plug B are all marked with BB at an ambient temperature of up to 200°C, indicating that those belonging to plug B exhibit excellent impact resistance. Next, swelling of the hexagonal portion 5A associated with caulking will be verified. Accurate measurement of the width across flat W was carried out with respect to two kinds of spark plugs which had been manufactured through use the caulking dies 41 and 42 such that the amount of buckling of the curved portion 5D becomes 0.8 mm. One kind of spark plugs, which are categorized as plug A, had the cushion-material charged portion 9 and were subjected to cold caulking. The other kind of spark plugs, which are categorized as plug B, had the cushion-material charged portion 9 and were subjected to hot caulking. The width across flat W is 14 mm nominally and 13.70 mm as measured before caulking. 10 spark plugs belonging to each of plugs A and B were measured for the width across flat W in mm. The results are shown in Table 2. Width across flat W of Hexagonal Portion at 0.8 mm Buckling of Curved Portion Plug Plug A With talc Cold caulked Plug B With talc Hot caulked No. 1 13.924 13.790 No. 2 13.957 13.775 No. 3 13.980 13.792 No. 4 13.923 13.795 No. 5 13.928 13.796 No. 6 13.962 13.795 No. 7 13.988 13.788 No. 8 14.001 13.783 N9.9 13.968 13.791 No. 10 13.991 13.789 Average 13.962 13.789 As seen from Table 2, spark plugs belonging to plug type A have an average increase in width across flat W of 0.262 mm and show considerable variations in the width across flat W. The width across flat W of spark plug No. 8 belonging to plug type A is 0.001 mm beyond the tolerance. By contrast, in the case of spark plugs belonging to plug type B, swelling of the width across flat W is as small as an average of 0.089 mm, and variations in the width across flat W are slight. Accordingly, even when the width across flat W as measured before caulking is increased by 0.1 mm, the width W as measured after caulking may sufficiently fall within the tolerance. Through buckling of the curved portion 5D effected while the curved portion 5D is heated and softened through application of current thereto, swelling of the hexagonal portion 5A can be reduced to a practically acceptable extent. The spark plugs belonging to plugs A, B, and C mentioned above were examined for hot airtightness as measured after they were tightened at an excessive torque. Conceivably, when a spark plug is tightened at an excessive torque, the male-threaded portion 5B of the metallic shell 5 is stretched axially; as a result, the packing pressure exerted on the packing member 6 held between the inner circumferential stepped portion 5E and the insulator 1 decreases with a resultant impairment in airtightness. A rated torque for a spark plug having the male-threaded portion 5B of M12 and a width across flat W of 14 mm is 25 N-m (newton-meter). The rated torque is defined as a torque required to tighten the male-threaded portion 5B which is not coated with anything. However, in this test, in order to establish severer conditions, an anti-seizing agent, or a lubricant, which contains molybdenum was applied to the male-threaded portion 5B, and each spark plug was tightened. The tightening torque was varied from 25 N-m to 65 N-m. The hot airtight test was conducted in the following manner. The seat temperature was increased to 200°C, and an air pressure of 15 kgf/cm 2 was applied to the spark portion. Air leakage from the interior of each spark plug was measured. Specifically, air leakage along the packing member 6 and air leakage through the clearance between the metallic shell 5 and the insulator 1 of each spark plug were measured. The results are shown in Tables 3 and 4. Table 3 shows air leakage along the packing member 6, and Table 4 shows air leakage through the clearance between the metallic shell 5 and the insulator 1. Hot Airtightness as Measured after Tightening at Excessive Torque Air Leakage along Packing Member Tightening torque N-m 25 30 35 40 45 50 55 60 65 plug A With talc Cold caulking AA AA AA AA AA AA BB BB BB AA AA AA AA AA BB BB BB CC AA AA AA AA AA AA BB BB CC Plug B With talc Hot caulking AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA Plug C Without talc Hot caulking AA AA BB BB CC CC CC CC CC AA BB BB CC CC CC CC CC CC AA BB BB CC CC CC CC CC CC Hot Airtightness as Measured after Tightening at Excessive Torque Air Leakage to Exterior of Spark Plug Tightening torque N-m 25 30 35 40 45 50 55 60 65 plug A With talc Cold caulking AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA Plug B With talc Hot caulking AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA Plug C Without talc Hot caulking AA AA BB BB CC CC CC CC CC AA BB BB CC CC CC CC CC CC AA AA BB BB CC CC CC CC CC Tables 3 and 4 show the test results with respect to 3 spark plugs belonging to each of plugs A, B, and C. Symbols AA, BB, and CC hold the same meaning as in the case of Table 1. Specifically, AA denotes a leakage of cc per minute; BB denotes a leakage of from 0 cc to 10 cc per minute; and CC denotes a leakage of grater than 10 cc per minute. As seen from Table 3, spark plugs belonging to plug A, which have the cushion-material charged portion 9, show markedly better hot airtightness as compared to those belonging to plug C, which do not have the cushion-material charged portion 9. As mentioned previously, a spring force induced by a radially outward, elastic deformation of the hexagonal portion 5A of the metallic shell 5 is converted to a pressure of the talc powder 9. This pressure presses elastically the outer circumferential stepped portion 1D of the insulator 1 in the downward direction in FIG. 2. Thus, conceivably, even when the male-threaded portion 5B is stretched to some extent due to tightening at an excessive torque, the insulator 1 moves downward following the stretch, thereby maintaining airtightness in the position of the packing member 6. As seen from comparison of the test results between plugs A and B in Table 3, the spark plugs belonging to plug B, which are hot-caulked, show better hot airtightness. Since a load required for hot caulking is 30% or more lower than that required for cold caulking, as mentioned previously with reference to Table 2, the spark plugs belonging to plug B exhibit a smaller amount of plastic deformation with respect to the hexagonal portion 5A. Thus, the spark plugs belonging to plug B conceivably exhibit a larger amount of elastic deformation with respect to the hexagonal portion 5A. As seen from comparison between Table 3 and Table 4, the spark plugs belonging to plug C, which do not have the cushion-material charged portion 9, show little change in hot airtightness. By contrast, in the case of the spark plugs belonging to plugs A and B, which have the cushion-material charged portion 9, hot airtightness shown in Table 4 exhibits apparent improvement from that shown in Table 4. In the case of plug C as shown in FIG. 4, because of absence of the cushion-material charged portion 9, air which has leaked along the packing member 6 leaks through the clearance between the metallic shell 5 and the insulator 1. By contrast, in the case of plugs A and B as shown in FIG. 2, the cushion-material charged portion 9 serves as a second packing and prevents air which has leaked along the packing member 6, from leaking through the clearance between the metallic shell 5 and the insulator 1. The above embodiment is described while mentioning as the tightening portion of the present invention the hexagonal portion 5A having an hexagon-nut profile as shown in FIG. 5A. However, the present invention is not limited thereto. The tightening portion may assume a 12-point (bi-hexagon) nut profile as shown in FIG. 5B. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.

Claims: A spark plug comprising: an insulator (1) having a center through-hole formed therein; a center electrode (2) held in said center through-hole; a metallic shell (5) holding the insulator (1) through caulking and having a male-threaded portion (5B) formed on an outer circumferential surface of a front end portion of the metallic shell (5) and a tightening portion (5A) formed on an outer circumferential surface of the metallic shell (5) and located at a rear side with respect to the male-threaded portion (5B), the tightening portion (5A) being used to screw the male-threaded portion (5B) into a female-threaded hole formed in an internal combustion engine; and a ground electrode (11) electrically connected to the metallic shell (5) and defining a spark discharge gap in cooperation with the center electrode (2), characterized in that the distance (W) between two opposed parallel faces of the tightening portion (5A) is not greater than 14 mm; a cushion material is charged into a cylindrical space defined by an outer surface of the insulator (1) and an inner surface of the metallic shell (5) to thereby form a cushion-material charged portion (9); and the cushion-material charged portion (9) has an axial length L of from 0.5 mm to 10.0 mm inclusive (0.5 mm ≤ L ≤ 10.0 mm) and a thickness M of from 0.5 mm to 1.3 mm inclusive (0.5 mm ≤ M ≤ 1.3 mm). A spark plug according to claim 1, characterized in that the metallic shell (5) has a seat portion (5F) which is located between the male-threaded portion (5B) and the tightening portion (5A) and has a diameter greater than that of the male-threaded portion (5B), and a curved portion (5D) which extends between the tightening portion (5A) and the seat portion (5F); and wherein the curved portion (5D) has been buckled through axial caulking while being heated, so as to integrate the metallic shell (5) and the insulator (1) into a single unit. A method of manufacturing a spark plug comprising an insulator (1) having a center through-hole formed therein; a center electrode (2) held in said center through-hole; a metallic shell (5) holding the insulator (1) through caulking and having a male-threaded portion (5B) formed on an outer circumferential surface of a front end portion of the metallic shell (5) and a tightening portion (5A) formed on an outer circumferential surface of the metallic shell (5) and located at a rear side with respect to the male-threaded portion (5B), the tightening portion (5A) being used to screw the male-threaded portion (5B) into a female-threaded hole formed in an internal combustion engine; and a ground electrode (11) electrically connected to the metallic shell (5) and defining a spark discharge gap in cooperation with the center electrode (2); characterized by comprising the steps of: forming the metallic shell (5) such that the distance (W) between two opposed parallel faces of the tightening portion (5A) is not greater than 14 mm and that the metallic shell (5) has a seat portion (5F) located between the male-threaded portion (5B) and the tightening portion (5A) and having a diameter greater than that of the male-threaded portion (5B), and a curved portion (5D) extending between the tightening portion (5A) and the seat portion (5F); charging a cushion material into a cylindrical space defined by an outer surface of the insulator (1) and an inner surface of the metallic shell (5) to thereby form a cushion-material charged portion (9) having an axial length L of from 0.5 mm to 10.0 mm inclusive (0.5 mm ≤ L ≤ 10.0 mm) and a thickness M of from 0.5 mm to 1.3 mm inclusive (0.5 mm ≤ M ≤ 1.3 mm); and pressing the tightening portion (5A) and the seat portion (5F) toward each other while applying current thereto so as to heat the curved portion (5D), to thereby buckle the curved portion (5D).

Inventors: SUZUKI AKIRA, SUZUKI, AKIRA, Suzuki, Akira, NGK Spark Plug Co., Ltd.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD.
Application year: 2000
Main classification: H01T13
IPC codes: H01T13