JP2012160351A - Spark plug and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drastically improve fracture resistance of a ground electrode.SOLUTION: A spark plug 1 comprises: an electric insulator 2 having a shaft hole 4 penetrating in a direction of an axis line CL; a center electrode 5 inserted in a tip of the shaft hole 4; a main body metal fitting 3 provided on an outer periphery of the electric insulator 2; and a ground electrode 27 arranged at the tip of the main body metal fitting 3. The ground electrode 27 is bent toward the center electrode 5 at its own bending part 27C. A recess 29 extending in a longitudinal direction of the ground electrode 27 is formed at least at one part of a back face 27b and both side faces 27s1, 27s2 of the ground electrode 27 at the bending part 27C. On a cross section orthogonal to a center shaft of the ground electrode 27, hardness of a bottom part 29b of the recess 29 is made larger than hardness of a center portion of a face of the ground electrode 27 positioned on the center electrode 5 side.

Description

  The present invention relates to a spark plug used in a combustion apparatus such as an internal combustion engine and a method for manufacturing the spark plug.

  The spark plug is attached to, for example, an internal combustion engine (engine) and is used to ignite an air-fuel mixture in a combustion chamber. In general, a spark plug is composed of an insulator having a shaft hole, a center electrode inserted into the tip end side of the shaft hole, a metal shell provided on the outer periphery of the insulator, and a ground connected to the tip of the metal shell. An electrode. The ground electrode is bent back so that the tip of the ground electrode is opposed to the center electrode at a bent portion provided at a substantially intermediate portion of the ground electrode, and a spark discharge is generated between the tip of the ground electrode and the tip of the center electrode. A gap is formed. When a high voltage is applied to the center electrode, a spark discharge is generated in the spark discharge gap, and the mixture is ignited (see, for example, Patent Document 1).

JP 2008-108478 A

  By the way, in recent years, there has been a demand for downsizing and reducing the diameter of the spark plug. To make it possible to join to a metal shell with a reduced diameter, the ground electrode is made thinner, or the curvature radius of the bent part is made smaller. Is required to do. Furthermore, in recent high-power engines, the stress applied to the ground electrode (particularly the bent portion) due to vibration or the like is larger. That is, there are cases where the ground electrode, which has low strength or stress is more likely to concentrate on the bent portion, must be used in a harsh environment where breakage is more likely to occur. Therefore, it is desired to further improve the breakage resistance of the ground electrode.

  This invention is made | formed in view of the said situation, The objective is to provide the spark plug which can improve the breakage resistance of a ground electrode dramatically, and its manufacturing method.

  Hereinafter, each configuration suitable for solving the above-described object will be described in terms of items. In addition, the effect specific to the corresponding structure is added as needed.

Configuration 1. The spark plug of this configuration includes a cylindrical insulator having an axial hole penetrating in the axial direction;
A center electrode inserted on the tip side of the shaft hole;
A cylindrical metal shell provided on the outer periphery of the insulator;
A ground electrode disposed at the tip of the metal shell,
The ground electrode is a spark plug bent to the center electrode side at its bent portion,
A recess extending in the longitudinal direction of the ground electrode is formed in at least a part of the back surface of the ground electrode located on the opposite side of the center electrode from the bent portion and both side surfaces adjacent to the back surface,
In the cross section orthogonal to the central axis of the ground electrode, the hardness of the bottom of the concave portion of the ground electrode is made larger than the hardness of the central portion of the surface of the ground electrode located on the center electrode side. Features.

  According to the configuration 1, the concave portion extending along the longitudinal direction of the ground electrode is provided on at least a part of the back surface or the side surface of the ground electrode in the bent portion, and the hardness of the bottom of the concave portion is It is larger than the central portion of the surface located on the side of the central electrode (that is, the portion where the change in hardness due to the formation of the recess hardly occurs). Accordingly, the strength of the bent portion where stress is particularly likely to concentrate can be improved extremely effectively, and the breakage resistance of the ground electrode can be greatly improved.

  Although it is conceivable to provide a concave portion on the surface on the center electrode side of the ground electrode, if a concave portion is provided on the surface on the central electrode side, cracks based on the concave portion are likely to occur. Therefore, it is preferable to provide a recess on the back surface or side surface of the ground electrode from the viewpoint of more reliably improving the breakage resistance.

  Further, it is conceivable to increase the hardness of the entire area of the side surface of the ground electrode by pressing the entire area such as the side surface of the ground electrode without providing a recess, but in this case, the manufacturing cost is increased. There is a fear. Therefore, it is preferable to increase the hardness of at least the bottom of the recess while providing the recess in terms of effectively improving the breakage resistance while suppressing an increase in manufacturing cost.

  Configuration 2. The spark plug of this configuration is characterized in that, in the above configuration 1, the concave portion is formed in the entire longitudinal direction of the bent portion.

  According to the configuration 2, the strength can be increased in the entire bent portion, and the breakage resistance of the ground electrode can be further improved.

Configuration 3. In the configuration 1 or 2, the spark plug of this configuration is in a cross section orthogonal to the longitudinal direction of the ground electrode,
The depth of the recess is D (mm), the thickness of the ground electrode along the depth direction of the recess is TD (mm), the width of the recess is W (mm), and the width direction of the recess When the thickness of the ground electrode along TW is TW (mm), the following expressions (1) and (2) are satisfied.

0 <D ≦ TD / 2 (1)
TW / 20 ≦ W <TW (2)
When the concave portions are formed on both side surfaces of the ground electrode, “the depth of the concave portion” means the sum of the depths of the concave portions.

  According to the configuration 3, the hardness of the bottom can be further reliably improved while sufficiently maintaining the thickness of the ground electrode. That is, the strength of the bent portion can be further increased while more reliably preventing the strength of the ground electrode from being lowered. As a result, the breakage resistance can be further improved.

  Configuration 4. The spark plug of this configuration satisfies the following expressions (3) and (4) in the above configuration 3.

TD / 6 ≦ D ≦ TD / 3 (3)
TW / 6 ≦ W ≦ TW / 3 (4)
According to the said structure 4, the hardness of a bottom part can be improved more reliably and the intensity | strength of a bending part can be improved further. As a result, even better breakage resistance can be achieved.

  Configuration 5. The spark plug of this configuration is characterized in that, in any one of the above configurations 1 to 4, the recess is formed on each side surface of the ground electrode.

  According to the configuration 5, since the concave portions are formed on both side surfaces of the ground electrode, the ground electrode is sandwiched between the both concave portions, and as a result, the hardness of the bottom portion of the concave portion can be further increased. As a result, the breakage resistance can be further improved.

Configuration 6. The spark plug manufacturing method of this configuration is the spark plug manufacturing method according to any one of the above configurations 1 to 5,
A bending step of forming the bent portion in the ground electrode;
Including a recess forming step of forming a recess in the ground electrode,
The concave portion forming step is performed after the bending step.

  If the concave portion is formed before the ground electrode is bent, the hardness of the bottom portion may decrease as the ground electrode is bent. In this regard, according to the configuration 6, the recess is formed after the ground electrode is bent. Therefore, a decrease in hardness can be prevented more reliably, and the breakage resistance can be further improved.

It is a partially broken front view which shows the structure of a spark plug. It is a partial expanded front view which shows the structure of the front-end | tip part of a spark plug. It is a partial expanded sectional view which shows the cross-sectional shape of a recessed part. (A), (b) is a partial expanded sectional view which shows another example of the surface which forms a recessed part. (A)-(c) is a partial enlarged front view which shows another example of the formation range of a recessed part. (A)-(c) is a partial expanded sectional view which shows another example of the cross-sectional shape of a recessed part. It is a figure which shows the jig | tool etc. which are used at a recessed part formation process, Comprising: (a) is an enlarged plan view, (b) is an enlarged front view.

  Hereinafter, an embodiment will be described with reference to the drawings. FIG. 1 is a partially cutaway front view showing a spark plug 1. In FIG. 1, the direction of the axis CL <b> 1 of the spark plug 1 is the vertical direction in the drawing, the lower side is the front end side of the spark plug 1, and the upper side is the rear end side.

  The spark plug 1 includes an insulator 2 as an insulator and a cylindrical metal shell 3 that holds the insulator 2.

  As is well known, the insulator 2 is formed by firing alumina or the like, and in its outer shape, a cylindrical rear end side body portion 10 formed on the rear end side, and the rear end side body portion 10. A large-diameter portion 11 that protrudes outward in the radial direction on the distal end side, a middle trunk portion 12 that is formed to have a smaller diameter on the distal end side than the large-diameter portion 11, and the middle trunk portion 12 Furthermore, it is provided with the leg length part 13 formed more narrowly than this at the front end side. In addition, of the insulator 2, the large diameter portion 11, the middle trunk portion 12, and most of the leg long portions 13 are accommodated inside the metal shell 3. In addition, a tapered step portion 14 is formed at a connecting portion between the middle body portion 12 and the long leg portion 13, and the insulator 2 is locked to the metal shell 3 at the step portion 14.

  Further, the insulator 2 is formed with a shaft hole 4 penetrating along the axis CL1, and a center electrode 5 is inserted and fixed to the tip end side of the shaft hole 4. The center electrode 5 is made of a nickel (Ni) alloy and has a rod shape (columnar shape) as a whole. In addition, the tip surface of the center electrode 5 is formed flat and protrudes from the tip of the insulator 2. Note that an inner layer made of copper or copper alloy having excellent thermal conductivity may be provided inside the center electrode 5. In this case, the heat extraction of the center electrode 5 is improved, and the wear resistance can be improved.

  A terminal electrode 6 is inserted and fixed on the rear end side of the shaft hole 4 in a state of protruding from the rear end of the insulator 2.

  Further, a cylindrical resistor 7 is disposed between the center electrode 5 and the terminal electrode 6 of the shaft hole 4. Both ends of the resistor 7 are electrically connected to the center electrode 5 and the terminal electrode 6 through conductive glass seal layers 8 and 9, respectively.

  In addition, the metal shell 3 is formed in a cylindrical shape from a metal such as low carbon steel, and a screw for attaching the spark plug 1 to a combustion device such as an internal combustion engine or a fuel cell reformer on the outer peripheral surface thereof. A portion (male screw portion) 15 (for example, the screw diameter is M14) is formed. Further, a flange-like seat portion 16 that protrudes radially outward is formed on the outer peripheral surface on the rear end side of the screw portion 15, and a ring-shaped gasket 18 is formed on the screw neck 17 at the rear end of the screw portion 15. It is inserted. Furthermore, a tool engaging portion 19 having a hexagonal cross section for engaging a tool such as a wrench when the spark plug 1 is attached to the combustion device is provided on the rear end side of the metal shell 3. Further, a caulking portion 20 that is bent toward the inner side in the radial direction is provided on the rear end side of the tool engaging portion 19, and the insulator 2 is held by the caulking portion 20.

  In addition, a tapered step portion 21 for locking the insulator 2 is provided on the inner peripheral surface of the metal shell 3. The insulator 2 is inserted from the rear end side to the front end side of the metal shell 3, and the rear end of the metal shell 3 is engaged with the step portion 14 of the metal shell 3. It is fixed to the metal shell 3 by caulking the opening on the side inward in the radial direction, that is, by forming the caulking portion 20. An annular plate packing 22 is interposed between the two step portions 14 and 21. Thereby, the airtightness in the combustion chamber is maintained, and the fuel gas entering the gap between the leg long portion 13 of the insulator 2 exposed to the combustion chamber and the inner peripheral surface of the metal shell 3 is prevented from leaking outside.

  Furthermore, in order to make the sealing by caulking more complete, ring members 23 and 24 are interposed between the metal shell 3 and the insulator 2 on the rear end side of the metal shell 3, and the ring member Between 23 and 24, talc 25 is filled. That is, the metal shell 3 holds the insulator 2 via the plate packing 22, the ring members 23 and 24, and the talc 25.

Further, a ground electrode 27 formed of a Ni alloy and having substantially the same width and thickness is joined to the front end portion 26 of the metal shell 3 along the longitudinal direction of the metal shell 3. The ground electrode 27 is bent back toward the center electrode 5 by a curved (non-linear) bent portion 27 </ b> C formed substantially in the middle of the ground electrode 27. Opposite. A spark discharge gap 28 is formed between the electrodes 5 and 27, and a spark discharge is generated in the spark discharge gap 28 substantially along the direction of the axis CL1. In the present embodiment, the area of the tip surface of the metal shell 3 is relatively small. Therefore, the cross-sectional area of the base end portion of the ground electrode 27 joined to the tip end portion 26 of the metal shell ^{3 is} also relatively small (for example, 4.48 mm ² or less).

  In addition, as shown in FIGS. 2 and 3, the ground electrode 27 includes a back surface 27b located on the opposite side of the center electrode 5, and both side surfaces 27s1 and 27s2 adjacent to the back surface 27b. In the embodiment, a concave concave portion 29 extending along the longitudinal direction of the ground electrode 27 is formed from the proximal end side to the distal end of both side surfaces 27S1 and 27s2. That is, the concave portion 29 is formed over the entire longitudinal direction of the bent portion 27C.

  In addition, the recess 29 has a V-shaped cross section in a cross section orthogonal to the longitudinal direction of the ground electrode 27. In addition, since the recess 29 in the present embodiment is formed by performing plastic working on the ground electrode 27 (the manufacturing method will be described later), the bottom 29b of the recess 29 of the ground electrode 27 (that is, plastic processing is performed). Of the ground electrode 27 is greater than the hardness of the central portion of the surface located on the side of the center electrode 5 (that is, the portion not specially processed). The bottom portion 29b is a portion of the concave portion 29 that is farthest along the depth direction of the concave portion 29 from the surface of the ground electrode 27 where the concave portion 29 is formed (side surfaces 27s1, 27s2 in this example).

  Further, the recess 29 is configured to satisfy both of the following expressions. That is, the depth of the recess 29 is D (mm), the thickness of the ground electrode 27 along the depth direction of the recess 29 is TD (mm), the width of the recess 29 is W (mm), When the thickness of the ground electrode 27 along the width direction is TW (mm), 0 <D ≦ TD / 2 (more preferably, TD / 6 ≦ D ≦ TD / 3) and TW / 20 ≦ W <TW (more preferably, TW / 6 ≦ W ≦ TW / 3) is satisfied. Note that “the depth D of the concave portion 29” means the sum of the depths of the concave portions 29 formed on both side surfaces 27s1 and 27s2.

  The position where the recess is provided is not limited to the both side surfaces 27s1, 27s2, and it is sufficient that the recess is provided on at least one of the back surface 27b and the both side surfaces 27s1, 27s2. Therefore, as shown in FIG. 4A, the concave portion 31 may be provided on one of the side surfaces 27s1 and 27s2, and the concave portion 32 is provided on the back surface 27b as shown in FIG. 4B. It is good as well. Furthermore, a recess may be provided from the back surface 27b to the side surface 27s1 (side surface 27s2).

  Further, as shown in FIG. 5A, the concave portion 33 is provided corresponding to only the formation range of the bent portion 27C of the ground electrode 27 without providing the concave portion from the base end side to the distal end of the ground electrode 27. Alternatively, as shown in FIG. 5B, a recess 34 may be provided corresponding to only a part of the bent portion 27C. Furthermore, it is good also as providing the recessed part 35 intermittently along the longitudinal direction of the ground electrode 27, as shown in FIG.5 (c), without providing a recessed part continuously.

  In addition, the cross-sectional shape of the concave portion is not limited to a V shape, and the concave portion 36 may be formed in a rectangular cross section as shown in FIG. As shown, it is good also as forming the recessed part 37 so that cross-sectional curve shape (for example, cross-sectional semicircle shape) may be made | formed. Moreover, as shown in FIG.6 (c), it is good also as forming the recessed part 38 so that cross-sectional trapezoid shape may be made.

  Next, the manufacturing method of the spark plug 1 comprised as mentioned above is demonstrated.

  First, the metal shell 3 is processed in advance. That is, a through-hole is formed by subjecting a cylindrical metal material (for example, an iron-based material such as S17C or S25C or a stainless steel material) to a cold forging process, and a rough shape is manufactured. Thereafter, the outer shape is adjusted by cutting to obtain a metal shell intermediate.

  Subsequently, a straight bar-shaped ground electrode 27 made of an Ni alloy is resistance-welded to the front end surface of the metal shell intermediate. When the welding is performed, so-called “sag” is generated. After the “sag” is removed, the threaded portion 15 is formed by rolling at a predetermined portion of the metal shell intermediate body. Thereby, the metal shell 3 to which the ground electrode 27 is welded is obtained. The metal shell 3 to which the ground electrode 27 is welded is galvanized or nickel plated. In order to improve the corrosion resistance, the surface may be further subjected to chromate treatment.

  On the other hand, the insulator 2 is formed separately from the metal shell 3. For example, a raw material powder containing alumina as a main component and containing a binder or the like is used to prepare a green granulated material for molding, and a rubber molded product is used to obtain a cylindrical molded body. Then, the insulator 2 is obtained by subjecting the obtained molded body to grinding and shaping the outer shape, followed by firing.

  Further, the center electrode 5 is manufactured by forging the Ni alloy.

  Then, the insulator 2 and the center electrode 5, the resistor 7, and the terminal electrode 6 obtained as described above are sealed and fixed by the glass seal layers 8 and 9. The glass seal layers 8 and 9 are generally prepared by mixing borosilicate glass and metal powder, and the prepared material is injected into the shaft hole 4 of the insulator 2 with the resistor 7 interposed therebetween. Then, the terminal electrode 6 is pressed from behind, and then baked in a baking furnace. At this time, the glaze layer may be fired simultaneously on the surface of the rear end body portion 10 of the insulator 2 or the glaze layer may be formed in advance.

  Thereafter, the insulator 2 provided with the center electrode 5 and the terminal electrode 6 and the metal shell 3 provided with the ground electrode 27 are fixed. More specifically, after the insulator 2 is inserted into the metal shell 3, the opening on the rear end side of the metal shell 3 formed relatively thin is caulked radially inward, that is, the caulking portion 20 is By forming, the insulator 2 and the metal shell 3 are fixed.

  Next, in a bending step, the ground electrode 27 is bent back toward the center electrode 5, whereby a bent portion 27 </ b> C is formed in the ground electrode 27. Thereafter, in the recess forming step, as shown in FIGS. 7A and 7B, the outer peripheral surfaces of the roller-shaped jigs JG1 and JG2 held so as to be freely rotatable are respectively connected to the side surfaces 27s1 and 27s2 of the ground electrode 27. After pressing, both jigs JG1 and JG2 are synchronously moved along the longitudinal direction of the ground electrode 27 while the ground electrode 27 is sandwiched between the two jigs JG1 and JG2. As a result, the ground electrode 27 is plastically deformed, and the recesses 29 are formed on both side surfaces 27s1 and 27s2. Thereafter, the spark plug 1 described above is obtained by adjusting the size of the spark discharge gap 28 formed between the center electrode 5 and the ground electrode 27.

  In addition, when forming a recessed part intermittently along the longitudinal direction of the ground electrode 27, it is good also as forming a recessed part by carrying out the press deformation | transformation of the ground electrode 27 with a predetermined press apparatus (not shown). Further, a recess may be formed in the ground electrode 27 before the bonding step of the ground electrode 27 to the metal shell 3 (that is, a recess forming step may be performed before the bonding step). Therefore, the ground electrode 27 may be joined to the metal shell 3 after the concave portion is formed in the straight rod-shaped ground electrode 27. Alternatively, the concave portion may be formed after the ground electrode 27 is bent, and then the ground electrode 27 may be joined to the metal shell 3.

  As described above in detail, according to the present embodiment, the concave portion 29 extending along the longitudinal direction of the ground electrode 27 is provided on at least a part of the back surface 27b and the side surfaces 27s1 and 27s2 of the ground electrode 27 in the bent portion 27C. Thus, the hardness of the bottom 29b of the recess 29 is set to be greater than that of the central portion of the ground electrode 27 (part where the change in hardness due to the formation of the recess 29 hardly occurs). Therefore, the strength of the bent portion 29C where stress is particularly likely to concentrate can be improved extremely effectively, and the breakage resistance of the ground electrode 27 can be dramatically improved. It should be noted that the present invention is particularly significant when the ground electrode 27 is used, which is relatively thin and is feared to be deteriorated in breakage resistance, as in this embodiment.

  Moreover, since the recessed part 29 is formed in the longitudinal direction whole region of the bending part 27C, intensity | strength can be raised in the bending part 27C whole region, and breakage resistance can be improved more.

  In addition, the depth D (mm) of the recess 29, the thickness TD (mm) of the ground electrode 27 along the depth direction of the recess 29, the width W (mm) of the recess 29, and the width direction of the recess 29 The thickness TW (mm) of the ground electrode 27 along the line is configured to satisfy 0 <D ≦ TD / 2 and TW / 20 ≦ W <TW. Therefore, the hardness of the bottom 29b can be more reliably improved while sufficiently maintaining the thickness of the ground electrode 27. As a result, the breakage resistance can be further improved.

  In addition, since the concave portions 29 are formed on both side surfaces 27s1 and 27s2 of the ground electrode 27, the ground electrode 27 is sandwiched between both concave portions 29, and the hardness of the bottom portion 29b of the concave portion 29 can be further increased.

  If the concave portion 29 is formed before the ground electrode 27 is bent, the hardness of the bottom portion 29b may be lowered when the ground electrode 27 is bent. In the present embodiment, the ground electrode 27 is bent. The concave portion 29 is formed after being formed. Therefore, a decrease in hardness can be prevented more reliably, and the breakage resistance can be further improved.

  If the recess 29 is formed before joining the ground electrode 27, the recess 29 can be formed more easily. Further, if the cross-sectional shape of the recess is curved (see FIG. 6B), the wear of the jig used when forming the recess can be suppressed, and productivity can be improved.

  Next, in order to confirm the effect achieved by the above embodiment, a sample of the spark plug (sample A) in which the outer surface of the ground electrode was formed flat (that is, no recess was provided), and the ground electrode A sample of a spark plug in which recesses are provided by cutting on both side surfaces (that is, the hardness of the bottom of the recess is set to the same degree as the hardness of the center portion of the ground electrode on the side of the center electrode) ( Sample B; both samples A and B correspond to comparative examples), and a recess is formed by plastic processing on the back and side surfaces of the ground electrode, and the hardness of the bottom of the recess is greater than the hardness of the central portion of the ground electrode Spark plug samples (corresponding to Examples) were prepared, and each sample was subjected to a fracture resistance evaluation test. The outline of the fracture resistance evaluation test is as follows. That is, each sample was assembled in a motorcycle single cylinder engine (displacement of 125 cc) with relatively large vibrations, and then the engine was driven. Then, the ground electrode is observed every predetermined time, and the time until the crack occurs in the ground electrode (endurance time) is measured, and the ratio of the endurance time of each sample to the endurance time of sample A corresponding to the comparative example ( (Durability improvement rate) was calculated. In addition, for the sample provided with the recess, the hardness of the bottom of the recess (an arbitrary recess when the recess is provided on both sides of the ground electrode) is measured with a predetermined Vickers hardness tester, and the ground electrode The ratio of the hardness of the bottom portion to the hardness of the center portion (hardness increase rate) was calculated.

  Table 1 shows the durability improvement rate and the hardness increase rate in each sample. For the samples corresponding to the examples, the surface of the ground electrode where the concave portion is formed (the concave portion forming surface), the range of the concave portion along the longitudinal direction of the ground electrode (the concave portion is provided over substantially the entire longitudinal direction of the ground electrode). Or provided corresponding to only the bent part or only corresponding to a part of the bent part), the formation position of the concave part along the width direction of the concave part on the concave part forming surface (the concave part is defined as the width of the concave part forming surface). Provided in the center of the direction or shifted from the center), the shape of the recess (cross-sectional shape and shape along the longitudinal direction (continuous or intermittent)), and the depth D and width W of the recess, respectively. changed. In Table 1, in each sample, the concave formation surface, the concave formation range in the longitudinal direction (concave formation range), the concave formation position in the width direction (concave formation position), the concave shape, and the concave depth D, The width W is also shown. In each sample, the thickness TD of the ground electrode along the depth direction of the recess was 1.3 mm, and the thickness TW of the ground electrode along the width direction of the recess was 2.7 mm. In addition, with respect to sample 23, a concave portion is provided by performing plastic working on the straight electrode (before bending) of the ground electrode, and for other samples, plastic processing is performed after forming a bent portion on the ground electrode. A recess was provided.

  As shown in Table 1, the samples (samples 1 to 29) in which the recesses are provided on the side surface and the back surface of the ground electrode and the hardness of the bottom portion of the recess is larger than the hardness of the central portion of the ground electrode are excellent in resistance to resistance. It was found to have breakability. This is considered to be because the strength of the bent portion was improved by increasing the hardness of the bottom portion.

  In addition, the depth (D) and width (W) of the recess satisfy the following conditions with respect to the thicknesses TD and TW of the ground electrode: 0 <D ≦ TD / 2 and TW / 20 ≦ W <TW (samples 2 to 10, 12-29) were confirmed to have even better breakage resistance. Further, among these samples, samples satisfying TD / 6 ≦ D ≦ TD / 3 and TW / 6 ≦ W ≦ TW / 3 (samples 4 to 7) have a durability improvement rate of 2.5, It was revealed that it was extremely excellent in breakage resistance. This is considered to be because the hardness of the bottom portion is further increased by setting the depth D and the width W of the recess within the above numerical range.

  Furthermore, comparing Samples 3 and 12, Samples 15 and 16, etc., it was confirmed that the sample with the recesses on both sides of the ground electrode is superior to the sample with the recesses on one side of the ground electrode. It was done.

  In addition, comparing Samples 3 and 23 that differ only in the timing of forming the recesses, Sample 3 in which the recesses are provided after bending the ground electrode has increased bottom hardness and superior breakage resistance. I understood that. This is because if the concave portion is provided before the ground electrode is bent, the hardness of the bottom portion decreases with the bending of the ground electrode, and since the concave portion is formed after the ground electrode is bent, the hardness is prevented from lowering. It is thought that.

  In addition, it was confirmed that if at least a part of the bent portion has a concave portion, the breakage resistance can be sufficiently improved. In particular, by providing the concave portion over the entire longitudinal direction of the bent portion, It has become clear that even better performance can be realized. In addition, it was found that the breakage resistance can be sufficiently improved even if the shape along the longitudinal direction of the recess (continuous or intermittent) and the cross-sectional shape of the recess are different.

  From the above test results, in order to improve the breakage resistance of the ground electrode, a concave portion extending in the longitudinal direction of the ground electrode is formed on at least a part of the back surface and both side surfaces of the ground electrode, and the bottom portion of the concave portion. It can be said that it is preferable to make this hardness larger than the hardness of the central portion of the ground electrode.

  Further, in terms of further improving the breakage resistance, a recess is formed so as to satisfy 0 <D ≦ TD / 2 and TW / 20 ≦ W <TW, or a recess is formed on both side surfaces of the ground electrode. It is more preferable to form the concave portion after forming the bent portion, or to form the concave portion so as to satisfy TD / 6 ≦ D ≦ TD / 3 and TW / 6 ≦ W ≦ TW / 3. Even more preferable.

  In addition, it is not limited to the description content of the said embodiment, For example, you may implement as follows. Of course, other application examples and modification examples not illustrated below are also possible.

  (A) In the above embodiment, the spark discharge gap 28 is formed between the center electrode 5 and the ground electrode 27. However, at least one of the electrodes 5 and 27 is made of a noble metal alloy (for example, platinum alloy or iridium alloy). A noble metal tip may be provided, and a spark discharge gap may be formed between the noble metal tip provided on one electrode and the other electrode, or between both noble metal tips provided on both electrodes.

  (B) In the above embodiment, the case where the ground electrode 27 is joined to the distal end portion 26 of the metal shell 3 is embodied. However, a part of the metal shell (or the tip metal fitting previously welded to the metal shell is used. The present invention is also applicable to the case where the ground electrode is formed by cutting out a part of the ground (for example, Japanese Patent Application Laid-Open No. 2006-236906).

  (C) In the above embodiment, the tool engaging portion 19 has a hexagonal cross section, but the shape of the tool engaging portion 19 is not limited to such a shape. For example, it may be a Bi-HEX (deformed 12-angle) shape [ISO 22777: 2005 (E)].

1 ... Spark plug 2 ... Insulator (insulator)
3 ... metal shell 4 ... shaft hole 5 ... center electrode 27 ... ground electrode 27b ... back surface (of ground electrode) 27C ... bent portion 27s1, 27s2 ... side surface (of ground electrode) 29 ... concave portion 29b ... bottom CL1 ... axis

Claims (6)

A cylindrical insulator having an axial hole penetrating in the axial direction;
A center electrode inserted on the tip side of the shaft hole;
A cylindrical metal shell provided on the outer periphery of the insulator;
A ground electrode disposed at the tip of the metal shell,
The ground electrode is a spark plug bent to the center electrode side at its bent portion,
A recess extending in the longitudinal direction of the ground electrode is formed in at least a part of the back surface of the ground electrode located on the opposite side of the center electrode from the bent portion and both side surfaces adjacent to the back surface,
In the cross section orthogonal to the central axis of the ground electrode, the hardness of the bottom of the concave portion of the ground electrode is made larger than the hardness of the central portion of the surface of the ground electrode located on the center electrode side. Features a spark plug.   The spark plug according to claim 1, wherein the concave portion is formed in the entire longitudinal direction of the bent portion. In a cross section orthogonal to the longitudinal direction of the ground electrode,
The depth of the recess is D (mm), the thickness of the ground electrode along the depth direction of the recess is TD (mm), the width of the recess is W (mm), and the width direction of the recess 3. The spark plug according to claim 1, wherein the following formulas (1) and (2) are satisfied when the thickness of the ground electrode along the line is TW (mm):
0 <D ≦ TD / 2 (1)
TW / 20 ≦ W <TW (2) The spark plug according to claim 3, wherein the following expressions (3) and (4) are satisfied.
TD / 6 ≦ D ≦ TD / 3 (3)
TW / 6 ≦ W ≦ TW / 3 (4)   The spark plug according to any one of claims 1 to 4, wherein the recesses are respectively formed on both side surfaces of the ground electrode. A spark plug manufacturing method according to any one of claims 1 to 5,
A bending step of forming the bent portion in the ground electrode;
Including a recess forming step of forming a recess in the ground electrode,
The method for manufacturing a spark plug, wherein the recess forming step is performed after the bending step.

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