Patent Publication Number: US-10320157-B1

Title: Spark plug and method for manufacturing the same

Description:
This application claims the benefit of Japanese Patent Application No. 2017-226469, filed Nov. 27, 2017, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a spark plug and a method for manufacturing the spark plug, and more particularly, to a spark plug having airtightness secured by a tapered portion and a method for manufacturing the spark plug. 
     BACKGROUND OF THE INVENTION 
     In a spark plug in which a ground electrode is connected to a metal shell having an external thread and a tapered portion formed on the outer peripheral surface thereof, a spark gap is formed between the ground electrode and a center electrode that is held by the metal shell in an insulated manner. In this type of spark plug, when the external thread of the metal shell is tightened into a threaded hole in a plug hole formed in an engine, a contact portion of the tapered portion is brought into contact with the inner surface of the plug hole. Thus, the ground electrode is positioned about an axis and in an axial direction. 
     Japanese Unexamined Publication No. 2001-121240 discloses a spark plug processed by cold forging over a range from an external thread of a metal shell to a tapered portion thereof. In the technology disclosed in Japanese Unexamined Publication No. 2001-121240, a contact portion subjected to work hardening by cold forging is difficult to deform. Thus, a ground electrode can easily be positioned by an appropriate tightening torque at a position about an axis where a flow of an air-fuel mixture is not hindered. 
     In the related art, however, the work hardening is caused by cold forging over the range from the external thread of the metal shell to the tapered portion thereof. Therefore, a portion between the external thread and the contact portion may be cracked and broken by the tightening torque of the external thread. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the problem described above and has an object to provide a spark plug and a method for manufacturing the spark plug, in which a portion between an external thread and a contact portion can be made difficult to crack while increasing the hardness of the contact portion. 
     In order to achieve this object, a spark plug according to a first aspect of the present invention includes a tubular metal shell having an external thread formed on a part of an outer peripheral surface of the metal shell, and a tapered portion that projects radially outward with respect to the external thread and is provided on a rear end side of the metal shell with respect to the external thread, a center electrode held in an insulated manner at a center of the metal shell on a front end side thereof, and a ground electrode connected to the metal shell to form a spark gap between the ground electrode and the center electrode. The tapered portion has a contact portion that contacts an inner surface of a plug hole formed in an engine when the external thread is tightened into a threaded hole in the plug hole. A Vickers hardness of the contact portion is higher than a Vickers hardness of at least a part of a portion between the contact portion and the external thread of the metal shell. 
     A method for manufacturing the spark plug according to a second aspect of the present invention includes a metal shell manufacturing step. The metal shell manufacturing step includes a softening step of reducing the Vickers hardness of at least the part of the portion between the contact portion and the external thread of the metal shell compared with the Vickers hardness of the contact portion of the metal shell. 
     A method for manufacturing the spark plug according to a third aspect of the present invention includes a metal shell manufacturing step. The metal shell manufacturing step includes a hardening step of increasing the Vickers hardness of the contact portion of the metal shell compared with the Vickers hardness of at least the part of the portion between the contact portion and the external thread of the metal shell. 
     According to the spark plug of the first aspect, the Vickers hardness of the contact portion is higher than the Vickers hardness of at least the part of the portion between the contact portion and the external thread of the metal shell. Therefore, at least the part of the portion between the contact portion and the external thread can be deformed easily. As a result, the portion between the external thread and the contact portion can be made difficult to crack while increasing the hardness of the contact portion. 
     According to the method for manufacturing the spark plug of the second aspect, in the softening step, the Vickers hardness of at least the part of the portion between the contact portion and the external thread of the metal shell is reduced compared with the Vickers hardness of the contact portion of the metal shell. Therefore, at least the part of the portion between the contact portion and the external thread can be deformed easily. As a result, the portion between the external thread and the contact portion can be made difficult to crack while increasing the hardness of the contact portion. 
     According to the method for manufacturing the spark plug of the third aspect, in the hardening step, the Vickers hardness of the contact portion of the metal shell is increased compared with the Vickers hardness of at least the part of the portion between the contact portion and the external thread of the metal shell. Therefore, at least the part of the portion between the contact portion and the external thread can be deformed easily. As a result, the portion between the external thread and the contact portion can be made difficult to crack while increasing the hardness of the contact portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein like designations denote like elements in the various views, and wherein: 
         FIG. 1  is a half sectional view of a spark plug according to one embodiment of the present invention. 
         FIG. 2  is a half sectional view of a part of the spark plug installed in an engine. 
         FIG. 3  is a half sectional view of a metal shell. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the present invention is described below with reference to the accompanying drawings.  FIG. 1  is a half sectional view of a spark plug  10  according to one embodiment of the present invention. In  FIG. 1 , the lower side of the drawing sheet is referred to as a front end side of the spark plug  10  and the upper side of the drawing sheet is referred to as a rear end side of the spark plug  10  (the same applies to  FIG. 2  and  FIG. 3 ). The spark plug  10  includes an insulator  11  and a metal shell  20 . 
     The insulator  11  is a member formed of, for example, alumina that is excellent in mechanical properties and insulation properties under high temperature. An axial hole  12  is formed through the insulator  11  along an axial line O. A center electrode  13  is arranged on the front end side of the axial hole  12 . 
     The center electrode  13  is a rod-shaped member extending along the axial line O. A core material that is copper or contains copper as a main component is covered with a base material that is nickel or a nickel-based alloy. The center electrode  13  is held by the insulator  11  and the front end is exposed from the axial hole  12 . 
     A metal terminal  14  is a rod-shaped member to which a high-voltage cable (not illustrated) is connected. The metal terminal  14  is formed of a conductive metal material (for example, low-carbon steel). The metal terminal  14  is fixed to the rear end of the insulator  11  in a state in which the front end side of the metal terminal  14  is inserted into the axial hole  12 . The metal terminal  14  is electrically connected to the center electrode  13  inside the axial hole  12 . 
     The metal shell  20  is fixed to the front end side of the outer periphery of the insulator  11  that is spaced away from the rear end of the insulator  11  in a direction of the axial line O by a predetermined distance while securing an insulation distance from the metal terminal  14 . The metal shell  20  is a substantially cylindrical member formed of a conductive metal material (for example, low-carbon steel). 
     The metal shell  20  has a front end portion  21 , a cylindrical portion  23 , a tapered portion  24 , and a seating portion  27  that are continuously connected to each other in this order from the front end side to the rear end side along the axial line O. An external thread  22  is formed on the outer peripheral surface of the front end portion  21 . The outside diameter of the cylindrical portion  23  is smaller than the minor diameter of the external thread  22 . The outside diameter of the seating portion  27  is larger than the major diameter of the external thread  22 . The tapered portion  24  is a conical portion that connects the seating portion  27  and the cylindrical portion  23  to each other. The outside diameter of the tapered portion  24  increases toward the rear end side. 
     In this embodiment, the tapered portion  24  has a first portion  25  continuously connected to the cylindrical portion  23 , and a second portion  26  continuously connected to the rear end side of the first portion  25  and adjacent to the seating portion  27 . The taper angle of the outer peripheral surface of the first portion  25  with respect to the axial line O is larger than the taper angle of the outer peripheral surface of the second portion  26  with respect to the axial line O. 
     The metal shell  20  has a bent portion  28 , a tool engagement portion  29 , and a rear end portion  30  that are continuously connected to the rear end side of the seating portion  27  in this order from the front end side along the axial line O. The rear end portion  30  is a portion that is bent inward at the time of assembling to restrict movement of the insulator  11  attached to the metal shell  20  toward the rear end side. The tool engagement portion  29  is a portion where a tool such as a wrench is engaged when the spark plug  10  is installed in a plug hole  41  of an engine  40  (described later). The bent portion  28  is a portion for fixing the metal shell  20  by crimping by being plastically deformed (bent) when the metal shell  20  is attached to the insulator  11 . 
     A ground electrode  31  is a member formed of a metal (for example, a nickel-based alloy) and joined to the metal shell  20 . In this embodiment, the ground electrode  31  is formed into a rod shape. The front end side of the ground electrode  31  is bent to face the center electrode  13 . The ground electrode  31  forms a spark gap between the ground electrode  31  and the center electrode  13 . 
     For example, the spark plug  10  is manufactured by the following method. First, the center electrode  13  is inserted into the axial hole  12  of the insulator  11  and is arranged so that the front end of the center electrode  13  is exposed to the outside from the axial hole  12 . The metal terminal  14  and the center electrode  13  are electrically connected to each other while inserting the metal terminal  14  into the axial hole  12 . Then, the insulator  11  is inserted into the metal shell  20  to which the ground electrode  31  is joined in advance. The bent portion  28  and the rear end portion  30  are plastically deformed while pressing the tapered portion  24  of the metal shell  20  against a tool (not illustrated), thereby attaching the metal shell  20  to the outer periphery of the insulator  11 . Subsequently, the ground electrode  31  is bent so as to face the center electrode  13 . Thus, the spark plug  10  is attained. 
       FIG. 2  is a half sectional view of a part of the spark plug  10  (near the tapered portion  24 ) installed in the engine  40 . As illustrated in  FIG. 2 , the plug hole  41  that communicates with a combustion chamber (not illustrated) is formed in the engine  40 . An inner surface  42  of the plug hole  41  is a conical surface having a diameter decreasing toward the combustion chamber of the engine  40 . A threaded hole  43  having an internal thread  44  is continuously connected to a part of the inner surface  42  of the plug hole  41  where the bore is smallest. 
     In this embodiment, when the external thread  22  formed on the metal shell  20  of the spark plug  10  is tightened into the threaded hole  43 , the second portion  26  (contact portion) of the tapered portion  24  is brought into contact with the inner surface  42  of the plug hole  41  and a space is formed between the inner surface  42  and the first portion  25  and between the threaded hole  43  and the first portion  25 . Through the contact of the second portion  26  with the inner surface  42  of the plug hole  41 , the ground electrode  31  is positioned in the direction of the axial line O and about the axial line O. 
     The metal shell  20  is manufactured so that the Vickers hardness of the second portion  26  becomes higher than the Vickers hardnesses of the first portion  25  and the cylindrical portion  23 . Therefore, the second portion  26  can be made more difficult to deform than the first portion  25  and the cylindrical portion  23 . Thus, the deformation amount of the second portion  26  in contact with the inner surface  42  of the plug hole  41  can be made difficult to vary by an appropriate tightening torque. Accordingly, the ground electrode  31  can easily be positioned at a position about the axial line O where a flow of an air-fuel mixture in the engine  40  is not hindered. 
     The first portion  25  and the cylindrical portion  23  can be deformed more easily than the second portion  26 . Therefore, the first portion  25  and the cylindrical portion  23  can be made difficult to crack by the tightening torque of the external thread  22 . As a result, the first portion  25  and the cylindrical portion  23  (in particular, the thin cylindrical portion  23 ) can be prevented from being broken. The Vickers hardness is a value measured based on JIS Z 2244: 2009. 
     The second portion  26  of the tapered portion  24  of the spark plug  10  is brought into contact with the inner surface  42  of the plug hole  41 . As a result, when the same axial force is applied to the external thread  22 , the load applied to the inner surface  42  of the plug hole  41  by the second portion  26  can be increased compared with a case in which the entire tapered portion  24  is brought into contact with the inner surface  42  of the plug hole  41 . As a result, the airtightness attained by the tapered portion  24  can be improved. 
     The entire second portion  26  need not essentially be brought into contact with the inner surface  42  of the plug hole  41 . The contour of the second portion  26  may be set so that a part of the second portion  26  near the boundary between the seating portion  27  and the second portion  26  or a part of the second portion  26  near the boundary between the first portion  25  and the second portion  26  (both are parts of the second portion  26 ) is brought into contact with the inner surface  42  of the plug hole  41 . Also when the part of the second portion  26  (contact portion) is brought into contact with the inner surface  42  of the plug hole  41 , the airtightness can be secured as long as the entire periphery of the second portion  26  is continuously brought into contact with the inner surface  42  of the plug hole  41 . The first portion  25  and the cylindrical portion  23  each having a Vickers hardness lower than that of the second portion  26  are present between the second portion  26  and the external thread  22 . Therefore, the first portion  25  and the cylindrical portion  23  can be made difficult to crack by the tightening torque of the external thread  22 . 
     In the manufacturing process of the spark plug  10 , when the metal shell  20  is attached to the outer periphery of the insulator  11 , the metal shell  20  is fixed by crimping by plastically deforming the bent portion  28  and the rear end portion  30  while pressing the second portion  26  against the tool (not illustrated). Since the hardness of the second portion  26  is high, sinking subsidence (deformation) of the second portion  26  can be prevented when the metal shell  20  is attached to the outer periphery of the insulator  11 . 
     Next, a method for manufacturing the metal shell  20  is described with reference to  FIG. 3 .  FIG. 3  is a half sectional view of the metal shell  20  before the metal shell  20  is attached to the insulator  11  and before the ground electrode  31  is joined to the metal shell  20 . In  FIG. 3 , illustration of ridges of the external thread  22  is omitted. 
     As illustrated in  FIG. 3 , the metal shell  20  has a first hole  32 , a second hole  33 , a third hole  34 , and a fourth hole  35  that are continuously connected to each other in this order from the front end side to the rear end side along the axial line O. The first hole  32  and the second hole  33  are located on the inner side of the front end portion  21 . The bore of the second hole  33  is smaller than the bore of the first hole  32 . The third hole  34  and the fourth hole  35  are located on the inner side of the metal shell  20  in a range from the rear end side of the front end portion  21  to the rear end portion  30 . The bore of the fourth hole  35  is larger than the bore of the third hole  34 . 
     The metal shell  20  is manufactured by processing an intermediate product (not illustrated). The intermediate product is a substantially columnar member formed by, for example, cold forging of a metal material such as low-carbon steel or stainless steel. In a metal shell manufacturing step of manufacturing the metal shell  20 , the tool engagement portion  29  is first formed on the outer periphery of the intermediate product (not illustrated) by cold forging. Subsequently, the front end portion  21 , the cylindrical portion  23 , the tapered portion  24 , the seating portion  27 , and the bent portion  28  are formed on the outer periphery of the intermediate product (not illustrated) by cutting work that uses a lathe or the like. Subsequently, the first hole  32 , the second hole  33 , the third hole  34 , and the fourth hole  35  are formed by using drills having different diameters (not illustrated). By forming those portions by cutting work, the accuracies of the contour, hole shapes, dimensions, and the like can be increased. 
     Next, in a hardening step, the surface layer of the second portion  26  of the intermediate product that rotates about the axial line O is plastically deformed by roller burnishing in which a hard roller (not illustrated) is pressed against the second portion  26 . Thus, the surface roughness of the second portion  26  is reduced and the surface of the second portion  26  is subjected to work hardening so that the Vickers hardness of the second portion  26  becomes higher than the Vickers hardnesses of the cylindrical portion  23  and the first portion  25 . 
     Subsequently, a thread is formed on the front end portion  21  by rolling and the external thread  22  is subjected to work hardening. 
     According to this method, the second portion  26  is hardened by roller burnishing. Therefore, the surface roughness of the second portion  26  can be reduced and the dimensional accuracy of the second portion  26  can be increased. By reducing the surface roughness of the second portion  26 , the airtightness can be improved. By the roller burnishing, the second portion  26  alone can be hardened within a short period of time. 
     The means for hardening the second portion  26  in the hardening step is not limited to the roller burnishing. Examples of other means for hardening the second portion  26  include quenching such as laser quenching, spark hardening, and shot peening. 
     The embodiment described above is directed to the case in which the second portion  26  of the metal shell  20  is hardened compared with the cylindrical portion  23 . The present invention is not limited to this case. Another method for manufacturing the metal shell  20  in which the second portion  26  is hardened compared with the cylindrical portion  23  (the cylindrical portion  23  is softened compared with the second portion  26 ) is described below with reference to  FIG. 3 . 
     In the metal shell manufacturing step of manufacturing the metal shell  20 , the contour of the metal shell  20  and the holes thereof are formed by cold forging such as extrusion or punching that uses a press. The second portion  26  of the metal shell  20  is subjected to work hardening by cold forging. 
     Next, in a softening step, the surface layers of the first portion  25  and the cylindrical portion  23  are removed by cutting. Thus, the Vickers hardness of the second portion  26  becomes higher than the Vickers hardnesses of the first portion  25  and the cylindrical portion  23 . By the cutting, the boundary between the first portion  25  and the second portion  26  is formed. Therefore, the dimensional accuracy of the boundary between the first portion  25  and the second portion  26  can be improved compared with the case in which the whole of the first portion  25  and the second portion  26  is formed by forging. 
     By increasing the Vickers hardness of the second portion  26  compared with the Vickers hardnesses of the first portion  25  and the cylindrical portion  23 , the second portion  26  can be made more difficult to deform than the first portion  25  and the cylindrical portion  23 . As a result, the deformation amount of the second portion  26  in contact with the inner surface  42  of the plug hole  41  can be made difficult to vary by an appropriate tightening torque. Further, the first portion  25  and the cylindrical portion  23  can be deformed more easily than the second portion  26 . Therefore, the first portion  25  and the cylindrical portion  23  can be made difficult to crack by the tightening torque of the external thread  22 . 
     The means for softening the first portion  25  and the cylindrical portion  23  in the softening step is not limited to the cutting work. Examples of other means for softening the first portion  25  and the cylindrical portion  23  include annealing. 
     The present invention has been described above based on the embodiment but is not limited to the embodiment described above. It can easily be understood that various modifications may be made without departing from the spirit of the present invention. 
     The embodiment is directed to the case in which the Vickers hardness of the second portion  26  is higher than the Vickers hardnesses of the cylindrical portion  23  and the first portion  25 . The present invention is not necessarily limited to this case. For example, as long as the Vickers hardness of the second portion  26  is higher than the Vickers hardness of the cylindrical portion  23  or the first portion  25 , the deformation amount of the second portion  26  in contact with the inner surface  42  of the plug hole  41  can be made difficult to vary by an appropriate tightening torque. Further, the first portion  25  or the cylindrical portion  23  can be deformed more easily than the second portion  26 . Therefore, a crack can be made difficult to occur between the external thread  22  and the second portion  26  (contact portion) by the tightening torque of the external thread  22 . 
     The embodiment is directed to the case in which the tapered portion  24  of the metal shell  20  has the first portion  25  and the second portion  26  having different taper angles. The present invention is not necessarily limited to this case. The cylindrical portion  23  and the seating portion  27  may be connected to each other by a conical surface as the tapered portion  24  without providing the first portion  25  and the second portion  26  having different taper angles. In this case, the contact portion is a part of the tapered portion  24  that is brought into contact with the inner surface  42  of the plug hole  41  (at least a part of the tapered portion  24 ). 
     The embodiment is directed to the case in which the outer peripheral surface of the tapered portion  24  is a conical surface. The present invention is not necessarily limited to this case. A tapered portion  24  whose outer peripheral surface has a spherical band shape may be provided. Also in this case, the contact portion is a part of the tapered portion  24  that is brought into contact with the inner surface  42  of the plug hole  41  (at least a part of the tapered portion  24 ). 
     The embodiment is directed to the case in which the outside diameter of the cylindrical portion  23  of the metal shell  20  is smaller than the minor diameter of the external thread  22  of the metal shell  20 . The present invention is not necessarily limited to this case. The outside diameter of the cylindrical portion  23  of the metal shell  20  and the minor diameter of the external thread  22  of the metal shell  20  may be set to equal dimensions. The embodiment is directed to the case in which the outside diameter of the bent portion  28  of the metal shell  20  is smaller than the outside diameter of the seating portion  27  of the metal shell  20 . The present invention is not necessarily limited to this case. The outside diameter of the bent portion  28  of the metal shell  20  and the outside diameter of the seating portion  27  of the metal shell  20  may be set to equal dimensions. Those cases are advantageous when the metal shell  20  is formed by cold forging. 
     Although description is omitted in the embodiment, a plating layer of zinc plating, nickel plating, or the like may be formed on the metal shell  20 . Further, the surface of the plating layer may be subjected to chromate treatment. 
     The embodiment is directed to the case in which the ground electrode  31  joined to the metal shell  20  is bent. The present invention is not necessarily limited to this case. A linear ground electrode may be used instead of using the bent ground electrode  31 . In this case, the linear ground electrode is caused to face the center electrode  13  by extending the front end side of the metal shell  20  in the direction of the axial line O and joining the ground electrode to the metal shell  20 .