Abstract:
A ground electrode for a spark plug has a through hole located adjacent a firing end of the electrode, with a precious metal firing tip extending through the hole. The firing tip is compressed axially to define a bulging portion extending radially outwardly from its longitudinal axis to mechanically retain the firing tip within the through hole. The firing tip additionally has an enlarged head or otherwise expanded portion at each axial end of the tip to provide a second mechanical interlock of the tip to the ground electrode. The firing tip can then also be welded to further strengthen its connection to the ground electrode. A method of manufacturing the ground electrode and a spark plug containing the ground electrode is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 10/702,378 filed Nov. 5, 2003 now U.S. Pat. No. 7,011,560. The entire disclosure of the prior application Ser. No. 10/702,378, from which a copy of the oath or declaration is supplied, is considered as being part of the disclosure of the accompanying application and is hereby incorporated by reference therein. 
    
    
     BACKGROUND OF THE INVENTION 
     Related Art 
     Spark plugs for use in internal combustion engines typically have a center electrode and a ground electrode with a predefined gap therebetween. It is desirable to maintain the predefined gap distance so that a predictable and repeatable spark can arc between the two electrodes. To improve the useful life of a spark plug, it is known to incorporate precious metals, i.e. iridium-based alloys, platinum alloys, or other precious metals, on the electrodes to maintain the predetermined gap and to resist erosion in use. To ensure that the precious metal maintains the desired gap, it is beneficial to secure the precious metal to the electrode such that the precious metal does not become dislodged or move from its fixed position. To further maintain the desired gap, it is desirable to maximize the surface area of the precious metal exposed to the gap. As disclosed in U.S. Pat. No. 4,771,210 to K. Möhle et al., it is known to insert an electric discharge pad or firing tip in a through bore of a ground electrode and either laser or argon arc weld the firing tip to the electrode. Further, this patent discloses applying a radial load through opposite sides of the ground electrode perpendicular to an axis of the bore to plastically deform the ground electrode inwardly toward the firing tip in a pinched fashion to capture the firing tip. 
     SUMMARY OF THE INVENTION 
     A spark plug for an internal combustion engine has a ground electrode disposed adjacent a central electrode defining a spark gap therebetween. The ground electrode has a through hole extending axially toward the center electrode at the spark gap. A firing tip having a longitudinal axis is received at least in part in the through hole and the firing tip is compressed axially along its longitudinal axis to define a bulging portion extending radially outwardly from the longitudinal axis to mechanically retain the firing tip within the through hole. 
     In accordance with another aspect of the invention, there is provided a spark plug and a ground electrode therefore in which a firing tip is mechanically interlocked within a through hole in the ground electrode by engagement of an enlarged head or otherwise expanded portion of the firing tip with an outer surface of the ground electrode at each end of the firing tip. 
     Yet another aspect of the invention provides a method of constructing a ground electrode for a spark plug. The method includes providing a segment of metal wire and forming a through hole extending between generally opposite surfaces of the wire. A firing tip having a longitudinal axis is inserted within the through hole and then compressed along its longitudinal axis to mechanically secure the firing tip within the through hole. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred exemplary embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein: 
         FIG. 1  is a fragmentary cross-sectioned view of a spark plug constructed according to one embodiment of the invention; 
         FIG. 2A  is an enlarged fragmentary view of the spark plug of  FIG. 1  showing a firing tip partially assembled to a ground electrode of the spark plug of  FIG. 1 ; 
         FIG. 2B  is a view similar to  FIG. 2A  with the firing tip fully assembled to the ground electrode; and 
         FIG. 3  is a view similar to  FIG. 2B  showing an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A fragmentary view of a spark plug constructed according to one presently preferred embodiment of the invention is shown in  FIG. 1  generally at  10 . The spark plug  10  has a metal shell or housing  12  with a ground electrode  14  extending therefrom. The ground electrode  14  is generally L-shaped and extends from a first end that is welded to shell  12  to a second free end  16 . An electric discharge pad or firing tip  18  is received at least in part in a through hole  20  extending through the ground electrode  14  generally adjacent the free end  16 . The firing tip  18  is mechanically retained within the through hole  20  by compressing the firing tip  18  axially along a longitudinal axis  22  to deform it radially and establish an interference fit between the firing tip  18  and the bore  20 . To further secure the firing tip  18  to the ground electrode  14 , the firing tip  18  is preferably welded to the ground electrode  14 . 
     The spark plug  10  includes a number of other components that can be made and assembled in a conventional fashion. This includes a center electrode assembly  24  and insulator  36 . The center electrode assembly  24  has a center electrode  25  extending along a central axis  26  of the spark plug  10  and can include additional components (not shown) such as one or more conductive, non-conductive, or resistive glass seals, capsule suppressors and an associated compression spring, as well as a terminal attached to the top end of the insulator  36 . The center electrode  25  has a firing tip or electrical discharge member  28  extending from an end  30  of the center electrode  24  and terminating at a firing end  32 . The firing end  32  of the center electrode firing tip  28  and an upper surface  34  of the ground electrode firing tip  18  define a spark gap of a predetermined distance. It is desirable to maintain the predetermined gap throughout the life of the spark plug  10  so that its performance will not degrade significantly. Insulator  36  is secured within a central bore  37  of the housing  12 . The insulator  36  in turn includes a longitudinal bore in which center electrode assembly  24  is located. 
     As best shown in  FIG. 2A , the firing tip  18  is partially assembled within the through hole  20  of the ground electrode  14 . The ground electrode  14  is preferably fixed to the housing  12 , such as through a resistance weld joint, and is preferably straight, and not yet bent into the L-shaped configuration shown in  FIG. 1 . In addition, the casing  12  and ground electrode  14  are preferably coated, for example with nickel or a nickel-based alloy, prior to inserting the firing tip  18  into the through hole  20 . The ground electrode  14 , has an upper surface  38  and a lower surface  40  generally parallel to one another with the through hole  20  extending between the upper and lower surfaces  38 ,  40 . Preferably, a counterbore  42  is formed and extends from at least one of the upper and lower surfaces  38 ,  40 , shown here as the lower surface  40  of the ground electrode  14 , into the through hole  20  about 0.005–0.010″. The counterbore  42  is shown having a tapered surface that is oblique relative to the upper surface  38 , and preferably has a chamfer of about 15°–25° relative to axis  22 , though it should be recognized other configurations may be desirable, for example a generally stepped configuration. The ground electrode  14  is preferably constructed from a nickel-based material, for example and without limitation, an Inconel or 836 alloy, and can be made with or without a copper core. With the through hole  20  formed in the ground electrode  14 , the firing tip  18  is inserted within the through hole  20 . 
     The firing tip  18  has an end  46  generally opposite the end  34  wherein a first length, represented as (L 1 ), is defined between the ends  34 ,  46  prior to the firing tip  18  being compressed. Preferably, the end  34  has an enlarged head  48  for abutting the upper surface  38  upon inserting the firing tip  18  into the through hole  20 , which extends above upper surface  38 . As shown in  FIG. 2A , the end  46  of the firing tip  18  extends below the lower surface  40  of the ground electrode  14  preferably about 0.030″–0.040″ prior to compressing the firing tip  18  within the bore  20 . 
     Upon inserting the firing tip  18  at least in part within the through hole  20 , the head  48  is preferably maintained in contact with the upper surface  38 , while the end  46  is axially compressed along the longitudinal axis  22  to define a flared portion  50  of the firing tip  18  ( FIG. 2B ). Preferably, the head  48  is backed-up by a generally fixed surface while compressing the end  46  of the firing tip  18  generally toward the head  48  along the axis  22 . Generally, the axial force to compress the firing tip  18  is in a range of about 300 lbs.–380 lbs., and preferably within a range of 320 lbs.–360 lbs. This axial compression of the firing tip  18  expands the firing tip material at end  46  outwardly to thereby form the flared portion  50 . Upon completing the compression of the firing tip  18 , the firing tip  18  has a second length, wherein the second length, represented here as (L 2 ), is shorter than the first length (L 1 ) of the firing tip  18 . 
     Preferably, the end  46  is compressed to a degree such that it is generally flush with the lower surface  40 . The head  48  preferably presents an enlarged surface area having a diameter of approximately 0.120″–0.125″ to further enhance maintaining the gap and thus, extending the life of the spark plug  10 . 
     The enlarged head  48  and flared portion  50  form a first mechanical interlock. These features  48 ,  50  together retain the firing tip  18  in position by abutting opposing surfaces of the ground electrode. In addition to this first mechanical interlock, a bulging portion  51  is also formed during the compression operation. The bulging portion  51  is located generally between the head  48  and the flared portion  50  of the firing tip and is substantially centered longitudinally in this location (see  FIGS. 2B and 3 ) and bulges, or extends, radially outwardly about 0.005″–0.010″ on the radius. The bulging portion  51  further retains the firing tip  18  in position by creating additional interference (i.e., a second mechanical interlock) with the ground electrode  14  within the through hole  20 . Either this first mechanical interlock or the second mechanical interlock, or both, can be used without departure from the scope of the invention. 
     In the alternate embodiment shown in  FIG. 3 , similar features as the embodiment above are given similar reference numerals, but are offset by 100. A firing tip  118  is inserted within a generally straight through hole  120  and, upon being compressed, another head  52  is formed generally opposite a head  148  such that the head  52  defines a spaced or enlarged portion  150  to mechanically retain the firing tip  118  within the bore  120 . Otherwise, the embodiment shown in  FIG. 3  functions similarly as the embodiment of  FIG. 2B  and preferably includes a bulging portion  151  that extends radially into a widened center portion of through hole  120 . 
     Upon compressing the firing tip  18 ,  118  within the bore  20 , preferably the firing tip is welded to the ground electrode  14 ,  114  to provide yet another redundant interlocking of the firing tip  18  within the bore  20 . Preferably, a resistance weld is used to impart a weld joint between the ground electrode  14 ,  114  and the firing tip  18 ,  118  in both the area of the head  48 ,  148  and the compressed or coined end  46 ,  146 . 
     Other suitable welding processes may be used to impart the weld joint, for example, a laser welding process can be used to form a stitch around the head  48 ,  148 . 
     Once the firing tip  18 ,  118  is permanently attached to the through hole  20 ,  120  and the ground electrode  14 ,  114  is attached to the spark plug shell  12 , the gap can be established between the end  34 ,  134  of the firing tip  18 ,  118  and the firing end  32  of the electrical discharge member  28  by bending the ground electrode  14 ,  114  to the generally L-shape form. With the firing tip  18 ,  118  mechanically retained, the gap can be maintained and the life of the spark plug  10  can be extended in use. To further enhance the useful life of the spark plug  10 , it should be recognized that the firing tip  18 ,  118  is constructed from materials that resist erosion, for example iridium based materials, platinum based materials, and the like. 
     Although disclosed embodiment of firing tip is cylindrical, it will be understood that it can have other cross-sectioned shapes, including oval or other curved shapes or rectangular or other polygonal shapes, and that in such instances the term “radial” and its other forms do not require a cylindrical or curved shape but instead refer to a direction orthogonal to longitudinal axis of the tip. 
     Obviously, many 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 invention may be practiced otherwise than as specifically described. The invention is defined by the claims.