Abstract:
A spark plug includes an electrode which during assembly is inserted into a vertical bore of a ceramic insulator from beneath. The electrode has an expanded firing end protruding from the insulator bottom. Tabs at an upper end of the electrode are expanded inside the bore to engage the insulator, and this is done using a removable expanding tool inserted into the insulator. A conductive glass in granular form is poured into the bore and envelops the expanded upper end of the electrode. A sacrificial conductive push pin inserts into the upper bore over the glass granules. The sub-assembly is heated to melt the glass and the push pin is pressed down into the melted glass. Upon cooling, the glass within the insulator fuses forming a seal which further strengthens the flared engagement of the electrode to the insulator.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to spark plugs, and more particularly, to the center electrode assemblies used in glass seal spark plugs.  
       BACKGROUND OF THE INVENTION  
       [0002]     The spark plugs of an engine are directly exposed to high temperature condition and pressure transients of a combustion chamber. For this reason the spark plug should have gas tight sealant qualities to prevent loss of combustion pressure and degradation of the electrical continuity of the spark plug. To complicate matters, the spark plug is situated within an environment that creates temperature fluctuations in excess of 600° F. One avenue of gas leakage through the spark plug is via the central bore through the longitudinal length of the insulator. It is common to use a non-expanding glass seal and in some instances, a conductive glass seal to prevent leakage through the bore. The conductive glass seal is in contact with not only the bore wall but also other conductive members of the center electrode assembly. For this reason, axial movement or thermal expansion of the electrode assembly can lead to breakage of the glass seal thereby losing the gas tight characteristic of the spark plug.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention provides a spark plug having an insulator which extends between top and bottom ends. An upper portion of the insulator carries the top end and extends axially to a bottom portion of the insulator at an intermediate location from which the bottom portion projects further to the bottom end. An electrode inserts into a lower bore of the lower portion through a bottom end of the lower portion. An enlarged firing end of the electrode protrudes from the insulator and engages the bottom end preventing upward movement of the electrode. An expanded upper end of the electrode engages a shoulder of the insulator at the intermediate location formed by the difference in diameters of an upper bore of the upper portion and the smaller diameter of the lower bore. The upper end is expanded to engage the shoulder and prevent downward movement of the electrode by an expanding tool which inserts into the upper bore of the upper portion.  
         [0004]     Preferably, the expanded upper end of the electrode is located in place using a conductive glass seal. The seal can be made by pouring conductive glass powder into the insulator through the upper bore enveloping the expanded upper end of the electrode. A sacrificial push pin inserts into the upper bore over the glass granules. Preferably, a removable push rod tool inserts over the push pin. The resultant electrode assembly is heated to melt the glass. During cooling, the glass is fused under the pressure of the push rod tool to thereby form a conductive seal. The push rod tool is removed and final assembly of the spark plug can then be completed, which may include adding one or more suppressive devices and a compression spring along with a terminal, as well as attaching a metal shell circumferentially about at least a portion of the insulator. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     Preferred exemplary embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein:  
         [0006]      FIG. 1  is a longitudinal cross-sectional view of a spark plug;  
         [0007]      FIG. 2  is a partial exploded cross-sectional view of the spark plug;  
         [0008]      FIG. 3  is an enlarged cross-sectional view of an electrode;  
         [0009]      FIG. 4  is an enlarged cross-sectional view of the spark plug taken along lines  4 - 4  viewing in the direction of the arrows found in  FIG. 1 , but prior to plastic deformation of a plurality of tabs of the electrode;  
         [0010]      FIG. 5  is the same cross-sectional view of  FIG. 4 , but after plastic deformation of the plurality of tabs of the electrode;  
         [0011]      FIG. 6  is a perspective view of an expanding tool; and  
         [0012]      FIG. 7  is a perspective view of a push rod tool.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Referring first to  FIG. 1 , there is shown a spark plug  10  having an electrode  14  which axially penetrates a bottom surface  13  of an elongated insulator  12  of the spark plug  10 . Sequentially stacked axially above the electrode  14 , and oriented concentrically within the insulator  12  is a seal  16 , a push pin  18 , a spring  20 , a suppressor  22 , and a terminal  24 . Use of the suppressor  22  is optional but preferred because it reduces radio frequency interference, and can take the form of a wire wound conductor or resistor. The spark plug  10  has an exterior metallic shell  25  for engaging an engine block or head (not shown). The metallic shell  25  circumscribes and engages a portion of the insulator  12  and has a threaded portion for engaging the engine head and a generally hexagonal portion for engagement of a tool (not shown) for rotational installation and removal of the spark plug  10  from the engine.  
         [0014]     Referring to  FIG. 2 , the insulator  12 , has an upper portion  26  defining an axially extending upper bore  28  and a substantially concentric lower portion  32  which defines an axially extending lower bore  34 . The upper and lower portions  26 ,  32  engage substantially concentrically at an axially intermediate location  30 . Because the upper bore  28  has a greater diameter than the lower bore  34  the insulator  12  carries a shoulder  36  substantially annular in shape, at the intermediate location  30 .  
         [0015]     The electrode  14  inserts into the lower portion  32  of the insulator  12 , through the bottom end  13  of the lower portion  32  which like the shoulder  36  is also substantially annular in shape. The upward insertion of the electrode  14  ceases when an enlarged head or substantially cylindrical firing end  40  of the electrode  14  engages the bottom end  13 . The mechanical engagement of firing end  40  with bottom end  13  is appreciable in order to resist the upward combustion forces exerted upon the electrode  14  and occurring within the combustion cylinder of an engine. Thus, a diametric length  42  of the firing end  40  measured laterally with respect to the elongated insulator  12 , is appreciably greater than a diameter  46  of the lower bore  34 , as best shown in  FIGS. 2 and 3 .  
         [0016]     When electrode  14  inserts fully into the insulator  12 , an upper end  48  of electrode  14  aligns axially to the shoulder  36 . Preventing any subsequent downward movement of the electrode  14  with respect to the insulator  12 , the upper end  48  expands radially outward to engage the shoulder  36  at the intermediate location  30 . This engagement is preferably achieved by plastic deformation and radial expansion of the upper end  48  though use of a removeable expanding tool  50 , as shown in  FIG. 6 . The expanding tool  50  inserts into the insulator  12  from above through the upper bore  28  where tool  50  then makes contact with and deforms the upper end  48  of the electrode  14 .  
         [0017]     The electrode  14  is made of a conductive metallic material, and the insulator  12  is generally made of a heat resistant ceramic material. When flaring the upper end  48  of electrode  14  radially outward, care should be taken so as not to produce excessive stresses which could cause insulator  12  to crack. To assure that the insulator  12  does not crack, the upper end  48  has a substantially reduced cross sectional area with respect to the remainder of the electrode and is composed of at least one axially extending tab  52  prior to bending. The cross section of tab  52  is thereby sized to permit easy bending or flaring out within the upper bore  28  to make contact with the shoulder  36  at the intermediate location  30 .  
         [0018]     Referring now to  FIGS. 4 and 5 , the upper end  48  comprises a plurality of annularly-spaced tabs  52  divided by a plurality of longitudinally extending slots  54 . The plurality of tabs  52  together define a central bore  56  and are equally spaced circumferentially about the central bore  56  of the electrode  14 . The symmetric placement of tabs  52  assist the expanding tool  50  in deforming the tabs  52  without risking breakage of insulator  12 . Tabs  52  restrict and guide expanding tool  50  in the axial direction only, thereby preventing lateral or radial movement of expanding tool  50  which could potentially crack insulator  12 . To guide expanding tool  50  into the central bore  56 , tool  50  has a beveled end  58  wherein the beveled radius shapes the deformation of tabs  52  against the shoulder  36  of the insulator  12 .  
         [0019]     Although the shoulder  36  at the intermediate location  30  may take a variety of forms, it preferably has a substantially annular surface  62  which directly contacts the flared tabs  52 . An inner perimeter  64  of annular surface  62  aligns radially to or congruently forms into the circumference of the lower bore  34 . Likewise, an outer perimeter  66  of annular surface  62  congruently forms into the circumference of the upper bore  28 . This radial alignment to the upper bore  28 , however, is not required but does simplify the manufacturing process. If manufactured as such, the diameter of the upper bore  28  is greater than the diameter of the lower bore  34 . Moreover, with use of the annular surface  62  of insulator  12 , the plurality of symmetrically spaced tabs  52  of electrode  14 , the annular bottom end  13  of insulator  12 , and the conical firing end  40  of electrode  14 , the electrode  14  need not be aligned circumferentially to the insulator  12  when inserted.  
         [0020]     After bending of tabs  52 , the electrode  14  is rigid and will not move axially, up or down, with respect to the insulator  12  regardless of forces applied to spark plug  10 . It is thus interlocked mechanically to the insulator  12  by the enlarged firing end  40  and the expanded tabs  52 . Prior to further assembly of spark plug  10 , the expanding tool  50  is removed from above insulator  12 .  
         [0021]     Referring to  FIGS. 1 and 2 , the seal  16  seals directly to the lower portion  32  of the insulator  12 , to the push pin  18 , and to the electrode  14 . Seal  16  also provides the conductive pathway between the push pin  18  and the electrode  14 . To further strengthen the vertical hold of upper end  48  of electrode  14  and enhance electrical conductivity, the seal  16  generally extends into the slots  54  between the bent tabs  52 . Seal  16  may have several distinctive layers stacked axially within the insulator  12  which may be preferable depending upon the electrical engagement characteristics of the materials forming electrode  14  and the push pin  18 , and/or the functional requirements of seal  16 . For instance, a center seal layer composed of glass and carbon may be used as a resistor to suppress high frequency interference. An upper and lower layer of seal  16  could then be used to reliably complete the necessary conductive path between the push pin  18 , and the electrode  14 . Where seal  16  includes the resistive characteristics of a carbon mixture, locating suppressor  22  between the terminal  24  and the spring  20  may not be necessary. However, the suppressor  22  may be preferred over the use of a resistive carbon glass seal in applications where the high temperatures from the engine combined with the heat created by the ignition energy might otherwise break down the resistive carbon glass.  
         [0022]     Preferably, the suppressor  22  is as shown in  FIG. 1 , and the seal  16  functions electrically as a conductor. This conductor may take the form of a wire extending through seal  16  in which case non-conductive glass can be used for the remaining portion of the seal  16 . However, and preferably, seal  16  is made of a conductive glass material having metallic traces, such as copper, nickel, or silver, running integral and throughout the composition of the seal  16 . Glass is a desirable seal for a spark plug  10  environment because of glass&#39; high temperature resistance and non-expanding characteristics. During assembly of spark plug  10 , a pre-determined amount of glass granules or powder, which ultimately become glass seal  16  after heating, is poured into the upper and lower bores  28 ,  32  from above, enveloping the tabs  52  at the intermediate location  30 .  
         [0023]     After pouring of the glass granules, the push pin  18  inserts from above, followed by a push rod tool  68 . This sub-assembly is then placed within a heat source. The glass granules have a melting point temperature which is lower than the electrode  14 , the push pin  18 , the insulator  12  and the push rod tool  68 . Upon melting of the glass granules, the sub-assembly is removed from the heat source. A press device then exerts a downward force upon the heated push rod tool  68 , pushing the push pin  18  into the now liquid glass and to a predetermined axial location within the upper bore  28 . Heating of push rod tool  68  along with the assembly minimizes thermal shock to the assembly during the pressing process.  
         [0024]     The firing end of push pin  18  has a series of ribs  60  extending circumferentially about the push pin  18  and spaced axially apart from one another with respect to the elongated insulator  12 . The liquid glass, envelops the ribs  60  with the downward exertion of push rod tool  68 . The envelopment provides superior adhesion, assisting in a reliable seal to the push pin  18  which expands and contracts with temperature. The push rod tool  68  further urges the liquid glass to flow between the tabs  52  into the slots  54  of the electrode  14  strengthening the tab  52  engagement with the shoulder  36  and enhancing the conductive pathway.  
         [0025]     Once the glass has hardened or fused to form the glass seal  16  between the electrode  14 , the insulator  12 , and the push pin  18 , the once heated push rod tool  68  is removed from above the insulator  12 . The spring  20  followed by the optional suppressor  22  is then inserted into the insulator  12  from above, both residing within the upper bore  28  and both being conductive.  
         [0026]     The terminal  24  has threads  70  which threadably mate to threads  72  of the insulator  12  to secure the remainder of the spark plug  10  axially together. During assembly or threading of the terminal  24  to the insulator  12  the terminal  24  moves axially downward making electrical contact with the suppressor  22  and compressing the spring  20 . The compression of spring  20  assures electrical continuity and accounts for vertical heat expansion from the terminal  24  through the push pin  18 . Rotation or threading of the terminal  24  to the insulator  12  within the upper bore  28  ceases when a radially projecting flange  74  of the terminal  24  engages a substantially annular top end  76  of the insulator  12 .  
         [0027]     Accordingly, it should thus be apparent that the present invention provides a spark plug assembly having superior sealing qualities which can better resist expansion and contraction caused by temperature fluctuations, and better resist external axial forces applied to the electrode in either axial direction. It will of course be understood that the foregoing description is of preferred exemplary embodiments and that the invention is not limited to the specific embodiments shown. Various changes and modifications will become apparent to those skilled in the art. For example, the upper end  48  of the electrode  14  can be pre-expanded, wherein the electrode inserts into the insulator  12  from above, not below. It may then be the firing end  40  which is flared radially outward to prevent axial movement of the electrode  14  with respect to the insulator  12 . All such changes and modifications are intended to come within the scope of the appended claims.  
         [0028]     As used in this specification and appended claims, the terms “for example,” “for instance,” and “such as,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.