Spark plug and manufacturing method

Abstract

To provide a spark plug capable of securing airtightness even if such a flaw as to exceed an entire tapered surface is formed or a depth of the flaw increases to some extent.SOLUTION: A spark plug 10 comprises: an insulator 12 in which a center electrode 13 is accommodated; a housing 11 in which the insulator 12 is accommodated; and a ground electrode 14 which forms a predetermined clearance with respect to the center electrode 13. In the housing 11, there are provided a screw part 114 for screwing into a mounting hole of a combustion device and a tapered sheet part 112 including an inclined plane, of which the outer diameter becomes gradually small toward a distal end side, and being positioned at a proximal end side. When a flaw depth in the tapered sheet part 112 is defined as d [μm] and a flaw width is defined as W [μm], d>5 μm and d≤-0.25×W+68 μm are satisfied, and a deposition amount of a rust preventive oil applied to the tapered sheet part 112 after vaporization is 1.8 mg or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a spark plug and a method for manufacturing a spark plug. [Background technology]

[0002] The spark plug described in Patent Document 1 below includes a cylindrical insulator extending in the axial direction and a cylindrical metallic shell attached to the outer periphery of the insulator. The metallic shell has a threaded portion for threaded engagement with a mounting hole of a combustion device, and a flange-shaped seat portion located rearward of the threaded portion, with a tapered surface whose outer diameter gradually decreases toward the tip. When the threaded portion of this spark plug is threaded into the mounting hole of the combustion device, the tapered surface contacts the seat surface of the combustion device. A ring-shaped or spiral-shaped protrusion having a length of at least one revolution is formed on the tapered surface within a range from at least the outermost periphery to a portion whose outer diameter is 95% of the outer diameter of the outermost periphery, and extends circumferentially of the metallic shell. In a cross section including the axis of the spark plug, the arithmetic mean roughness of the tapered surface within this range is 1 μm or more and 5 μm or less. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-258007 Summary of the Invention [Problem to be solved by the invention]

[0004] The relatively long scratches formed in the wire drawing process of the material that will become the metallic shell are formed along the longitudinal direction of the spark plug, and therefore may cross the protrusions formed on the tapered surface and extend beyond the entire tapered surface, which may compromise the airtightness provided by the protrusions described in Patent Document 1. Furthermore, the process of producing the metallic shell from the material that will become the metallic shell includes processes in which large quantities are processed in a barrel, such as a plating process, and therefore contact between the materials cannot be avoided, making it difficult to achieve an arithmetic mean roughness of 5 μm or less.

[0005] An object of the present disclosure is to provide a spark plug and a manufacturing method thereof that can ensure airtightness even if scratches that extend beyond the entire tapered surface are formed or the scratches are relatively deep. [Means for solving the problem]

[0006] The present disclosure provides a spark plug comprising an insulator (12) that houses a center electrode (13), a housing (11) that houses the insulator, and a ground electrode (14) provided in the housing so as to form a predetermined gap with respect to the center electrode. The housing is provided with a threaded portion (114) for threadedly engaging with a mounting hole of a combustion device, and a tapered seat portion (112) that has an inclined surface whose outer diameter gradually decreases toward its tip and is located proximal to the threaded portion. When the scratch depth in the tapered seat portion is d [μm] and the scratch width is W [μm], the relationships d > 5 μm and d ≦ −0.25 × W + 68 μm are satisfied, and the amount of rust preventive oil applied to the tapered seat portion after evaporation is 1.8 mg or more.

[0007] The present disclosure relates to a method for manufacturing a spark plug comprising an insulator (12) that houses a center electrode (13), a housing (11) that houses the insulator, and a ground electrode (14) that is provided in the housing so as to form a predetermined gap with respect to the center electrode, wherein in the process of manufacturing the housing, the vertical drop distance of the workpiece that will become the housing is 100 mm or less. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to provide a spark plug and a manufacturing method thereof that can ensure airtightness even if a scratch is formed that extends beyond the entire tapered surface or the scratch is relatively deep. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a partial cross-sectional view of a spark plug according to this embodiment. [Figure 2] 2 is a cross-sectional view showing a half section of the housing shown in FIG. 1 taken along line II-II. [Figure 3] FIG. 3 is a diagram showing the relationship between the scratches shown in FIG. 2, the amount of anti-rust oil attached, and the amount of gas leakage. [Figure 4] FIG. 4 is a graph of the relationship shown in FIG. [Figure 5] FIG. 5 is a diagram showing the relationship between the amount of adhered rust preventive oil and the amount of gas leakage. [Figure 6] FIG. 6 is a graph showing the relationship shown in FIG. [Figure 7] FIG. 7 is a diagram showing the relationship between housing hardness and scratch depth. [Figure 8] FIG. 8 is a diagram showing the relationship between housing hardness and scratch depth. [Figure 9] FIG. 9 is a diagram showing the relationship between the vertical drop distance and the scratch depth in the manufacturing process of the housing. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, the present embodiment will be described with reference to the accompanying drawings. To facilitate understanding of the description, the same components in the drawings will be denoted by the same reference numerals as much as possible, and duplicated descriptions will be omitted.

[0011] The schematic configuration of a spark plug 10 according to an embodiment will be described with reference to FIG. 1. The spark plug 10 is attached to, for example, an engine head block. The spark plug 10 ignites an air-fuel mixture in an engine cylinder by generating a spark discharge based on the application of voltage. The spark plug 10 includes a housing 11, an insulator 12, a center electrode 13, and a ground electrode 14.

[0012] The housing 11 is cylindrically formed around a plug central axis m10, which is the central axis of the spark plug 10. The housing 11 is formed of a metal material such as carbon steel. The lower end of the insulator 12 is inserted into the housing 11. A threaded portion 114 is provided on the outer peripheral surface of the lower portion of the housing 11. The spark plug 10 can be fastened and fixed to the engine head block by screwing the threaded portion 114 of the housing 11 into a threaded hole, which is an attachment hole formed in the engine head block. Hereinafter, the direction along the plug central axis m10 will also be referred to as the "plug axial direction Da." The circumferential direction about the plug central axis m10 will also be referred to as the "plug circumferential direction Dc." The side facing the ground electrode 14 will also be referred to as the tip side, and the opposite side will also be referred to as the base side.

[0013] A tapered seat portion 112 is provided on the housing 11 closer to the base end than the threaded portion 114. The tapered seat portion 112 has an inclined surface whose outer diameter gradually decreases toward the tip end. When the threaded portion 114 is screwed into a mounting hole formed in the engine head block, the inclined surface of the tapered seat portion 112 comes into contact with the seat surface of the engine head block, ensuring airtightness.

[0014] The insulator 12 is formed in a cylindrical shape centered on the plug central axis m10. The insulator 12 is made of an insulating material such as alumina. A housing 11 is integrally assembled to the outer periphery of the insulator 12. An axial hole 120 is formed inside the insulator 12. The axial hole 120 is formed so as to penetrate the insulator 12 from the tip end to the base end along the plug central axis m10. A center electrode 13, a first seal body 15, a resistor 16, a second seal body 17, and a terminal fitting 18 are inserted into the axial hole 120 in this order from the tip end. The center electrode 13 is inserted into the insulator 12 so as to be exposed from the tip end of the insulator 12.

[0015] The center electrode 13 has a center electrode base material 30 and a center electrode tip 40. The center electrode base material 30 is formed in a cylindrical shape centered on the plug central axis m10. The center electrode base material 30 is made of a nickel (Ni) alloy or the like, which has excellent heat resistance. The center electrode tip 40 is joined to the front end of the center electrode base material 30. The center electrode tip 40 is formed in a cylindrical shape centered on the plug central axis m10. The center electrode tip 40 is made of an iridium alloy or the like. A first seal body 15, a resistor 16, and a second seal body 17 are sandwiched between the base end side of the center electrode 13 and the front end side of the terminal fitting 18.

[0016] The terminal fitting 18 is formed in a substantially cylindrical shape centered on the plug central axis m10. The terminal fitting 18 is made of steel or the like. A terminal portion 180 is provided on the base end side of the terminal fitting 18. The terminal portion 180 is exposed to the outside from the base end side of the insulator 12.

[0017] The ground electrode 14 has a ground electrode base material 50 and a ground electrode tip 60. The ground electrode base material 50 is made of a nickel alloy or the like. The ground electrode base material 50 is joined to a front end surface 110 of the housing 11. The ground electrode base material 50 is formed so as to extend from the front end surface 110 of the housing 11 to a position facing the center electrode tip 40. The ground electrode tip 60 is joined to the front end of the ground electrode base material 50. The ground electrode tip 60 is made of a precious metal alloy such as an iridium alloy or a platinum alloy. The ground electrode tip 60 is positioned so as to face the center electrode tip 40. The gap formed between the center electrode tip 40 and the ground electrode tip 60 is called a spark gap 19.

[0018] In the spark plug 10, an external circuit capable of applying a high voltage is connected to the terminal portion 180 of the terminal fitting 18. When a high voltage is applied to the terminal portion 180 from the external circuit, a spark discharge is formed between the center electrode tip 40 of the center electrode 13 and the ground electrode tip 60 of the ground electrode 14. This spark discharge ignites the air-fuel mixture in the cylinder of the engine, forming a flame, and the air-fuel mixture burns.

[0019] As shown in Fig. 2, scratches 112a are formed on the tapered seat portion 112. Although one scratch 112a is shown in Fig. 2 for the sake of simplicity, multiple scratches 112a may be formed. In this embodiment, rust-preventive oil 80 is applied to the inclined surface of the tapered seat portion 112. The rust-preventive oil 80 also serves to fill the scratches 112a.

[0020] 2 is a cross section taken along line II-II of the housing 11 shown in FIG. 1, and is therefore a cross section taken along the plug central axis m10. The depth d and width W of the scratch 112a are not limited to those in this cross section. For example, the width W may be determined by visually observing the scratch 112a and finding the width of the widest visible portion. Because the direction in which the scratch 112a is generated is not necessarily limited to along the plug central axis m10, the depth d and width W are measured as appropriate depths and widths representative of the scratch 112a.

[0021] As shown in Figure 3, the depth d and width W of the scratch 112a were varied, and the amount of anti-rust oil 80 applied to the inclined surface of the tapered seat portion 112 was varied to measure the amount of gas leakage. The amount of anti-rust oil shown in "Anti-rust oil application amount" was applied to the inclined surface of the tapered seat portion 112, and the sample was left at room temperature for 10 hours. "Anti-rust oil application amount (after evaporation)" is the amount of anti-rust oil applied after the sample was left at room temperature for 10 hours.

[0022] The prerequisites for measuring the leakage rate were as follows. The spark plug 10 was attached to an engine jig. The engine jig had a mounting hole for attaching the spark plug 10. The leakage rate measurement was based on the airtightness test of ISO 11565, and is summarized as follows. A sample spark plug 10 was attached to the engine jig, a test stand simulating an internal combustion engine, with a tightening torque of 14 N·m, and the seating surface of the engine jig was heated to 200°C. In this state, an air pressure of 2±0.2 MPa was continuously applied to the tip of the sample, and the amount of air leakage per minute (cc / min) from between the tapered seat portion 112 of the spark plug 10 and the seating surface of the engine jig was measured. The specific measurement conditions were as follows: Tightening torque of the screw: 14Nm Engine jig material: S45C Taper angle (housing side): 63° Taper angle (engine fixture side): 60.5°

[0023] The measurement results shown in Fig. 3 are plotted on the horizontal axis as the width W of the scratch 112a and on the vertical axis as the depth d of the scratch 112a in the graph shown in Fig. 4. In the graph shown in Fig. 4, "◯" indicates that the airtightness meets the standard, and "×" indicates that the airtightness does not meet the standard.

[0024] From the graph shown in FIG. 4, when the depth of the scratch 112a in the tapered sheet portion 112 is d [μm] and the width of the scratch is W [μm], d>5μm and d≦―0.25×W+68μm If the above requirements are met, the application of anti-rust oil can ensure airtightness.

[0025] As shown in FIG. 5, the amount of leakage was measured for spark plug A and spark plug B, which have different depths d and widths W of the scratches 112a, while varying the amount of adhered rust preventive oil.

[0026] The prerequisites for measuring the leakage rate were as follows. Spark plugs A and B were attached to an engine jig. The engine jig had mounting holes for attaching spark plugs A and B. The leakage rate measurement was based on the ISO 11565 airtightness test, and is summarized as follows. A sample spark plug 10 was attached to the engine jig, a test stand simulating an internal combustion engine, with a tightening torque of 14 N·m, and the seating surface of the engine jig was heated to 200°C. In this state, an air pressure of 2±0.2 MPa was continuously applied to the tip of the sample, and the amount of air leakage per minute (cc / min) from between the tapered seat portion 112 of the spark plug 10 and the seating surface of the engine jig was measured. The specific measurement conditions were as follows: Tightening torque of the screw: 14Nm Engine jig material: S45C Taper angle (housing side): 63° Taper angle (engine fixture): 60.5° Anti-rust oil viscosity: 4.0mm 2 / s (measured based on JIS K2283)

[0027] The measurement results shown in Figure 5 are plotted on a graph in Figure 6, with the horizontal axis representing the amount of anti-rust oil adhered (after evaporation) and the vertical axis representing the amount of leakage. From the graph in Figure 6, it can be seen that as long as the amount of anti-rust oil applied to the tapered seat portion 112 after evaporation is 1.8 mg or more, the application of anti-rust oil can ensure airtightness. Note that the same effect can be achieved with anti-rust oil specified in JIS K2246, Type 3.

[0028] As shown in Figure 7, the crimping load during the process of assembling the housing and insulator was varied to measure the depth of scratches that occurred for two spark plugs with different housing materials, S35C and S17C. In the process of assembling the housing and insulator, the housing 11 is held by inserting the front end of the housing 11 into a standard receiving die with the insulator 12 inserted in the housing 11. The receiving die has an insertion hole into which the threaded portion 114 can be inserted and an annular receiving surface connected to the opening of the insertion hole. The receiving surface is inclined at an angle so as to fit along the tapered seat portion 112 of the housing 11. Therefore, the tapered seat portion 112 is pressed against the receiving surface by the crimping load. At a crimping load of 30 kN, the scratch depth in a portion of the S17C housing material was 28 μm or more, which is the NG condition.

[0029] As shown in Figure 8, the housing hardness [Hv] of two spark plugs with different housing materials, S35C and S17C, was compared using Vickers hardness. As shown in Figure 8, when the Vickers hardness was less than 180 Hv, the scratch depth met the NG condition. Therefore, it is preferable that the Vickers hardness of the tapered seat portion 112 be 180 Hv or more.

[0030] Next, adjustment of the size of the scratches 112a in the manufacturing method of the spark plug 10 will be described with reference to Fig. 9. Fig. 9 is a diagram showing the relationship between the vertical drop distance and the size of the scratches in the manufacturing process of the housing 11. Fig. 9(A) shows the pre-plating cleaning process. Fig. 9(B) shows the masking process. Fig. 9(C) shows the plating process. Fig. 9(D) shows the packaging process.

[0031] In either case, the allowable size of the scratch 112a is a depth d of 29 μm or less and a width W of 152 μm or less. From the graph shown in Figure 9, it is clear that the scratch 112a can be kept within the allowable size as long as the vertical drop distance of the workpiece to be made into the housing is 100 mm or less. The reason for studying the above manufacturing process for the housing 11 is that it was selected as a representative process because, in the process of manufacturing the spark plug 10, many workpieces are sometimes transferred into a container at once.

[0032] As described above, the spark plug 10 according to this embodiment includes the insulator 12 that houses the center electrode 13, the housing 11 that houses the insulator 12, and the ground electrode 14 that is provided in the housing 11 so as to form a predetermined gap with respect to the center electrode 13. The housing 11 is provided with a threaded portion 114 for threadedly engaging with a mounting hole of a combustion device, and a tapered seat portion 112 that has an inclined surface whose outer diameter gradually decreases toward the tip end and is located closer to the base end than the threaded portion 114. When the scratch depth in the tapered seat portion 112 is d [μm] and the scratch width is W [μm], d>5μm and d≦―0.25×W+68μm and the amount of anti-rust oil applied to the tapered seat portion 112 after evaporation is 1.8 mg or more.

[0033] By limiting the depth and width of the scratches 112a in the tapered seat portion 112 to the above ranges and ensuring that the amount of anti-rust oil applied to the tapered seat portion 112 adheres to the above amount after evaporation, the anti-rust oil remains not only in the scratch-free parts of the tapered seat portion 112 but also inside the scratches 112a, ensuring airtightness.

[0034] In addition, in the spark plug 10 according to this embodiment, the Vickers hardness of the tapered seat portion 112 can be set to 180 Hv or more.

[0035] By ensuring the hardness of the tapered seat portion 112, it is possible to prevent scratches from being too deep during the crimping step in the manufacturing process.

[0036] The manufacturing method according to this embodiment is a method for manufacturing a spark plug 10 including an insulator 12 that houses a center electrode 13, a housing 11 that houses the insulator 12, and a ground electrode 14 that is provided in the housing 11 so as to form a predetermined gap with respect to the center electrode 13, and in the process of manufacturing the housing 11, the vertical drop distance of the workpiece that will become the housing 11 is 100 mm or less.

[0037] By setting the vertical drop distance to 100 mm or less in the manufacturing process of the housing 11, the width and depth of the scratches 112a can be kept within a predetermined range.

[0038] The present embodiment has been described above with reference to specific examples. However, the present disclosure is not limited to these specific examples. Design modifications to these specific examples made by a person skilled in the art as appropriate are also included within the scope of the present disclosure as long as they comprise the features of the present disclosure. The elements of the above-described specific examples, as well as their arrangement, conditions, shape, etc., are not limited to those exemplified and can be modified as appropriate. The elements of the above-described specific examples can be combined in various ways as appropriate, as long as no technical contradictions arise. [Explanation of symbols]

[0039] 10: Spark plug 11: Housing 12: Insulator 13: Center electrode 14: Ground electrode 112: Tapered seat part 114:Threaded part

Claims

1. an insulator (12) that houses a center electrode (13); a housing (11) for accommodating the insulator; a ground electrode (14) provided in the housing so as to form a predetermined gap with respect to the center electrode, The housing includes: a threaded portion (114) for threadably engaging with a mounting hole of a combustion device; a tapered seat portion (112) having an inclined surface whose outer diameter gradually decreases toward the tip side and located on the base end side of the threaded portion, When the scratch depth in the tapered sheet portion is d [μm] and the scratch width is W [μm], d>5μm and d≦-0.25×W+68μm and the amount of rust preventive oil applied to the tapered seat portion after evaporation is 1.8 mg or more.

2. 2. The spark plug according to claim 1, wherein the tapered seat portion has a Vickers hardness of 180 Hv or more.

3. an insulator (12) that houses a center electrode (13); a housing (11) for accommodating the insulator; a ground electrode (14) provided in the housing so as to form a predetermined gap with respect to the center electrode, In the step of manufacturing the housing, the workpiece to be formed into the housing is dropped vertically a distance of 100 mm or less.

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