US2863080A - Spark plug and method for making same - Google Patents

Description

Dec. 2, 1958 w. A. BYCHINSKY' ET AL 6 SPARK PLUG AND METHOD FOR MAKING SAME Filed April 15, 1955 2 Sheets-Sheet 2 inventors United States Patent 9 hub SPARK PLUG AND METHOD FOR MAKING SAME Wilfred A. Bychinslry, Ann Arbor, and Nlichael Skunda,

Davison, Mich, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application April 15, 1955, Serial No. 501,633

8 Claims. (Cl. 313-137) This invention relates to spark plugs and to a process for making same. It has particular reference to spark plugs of the type having an unusually long tubular insulator, either straight or angular, enclosed in a concentric metal shell, and designed primarily for use as igniters in gas turbine engines.

One of the major problems in the production of extra long metal shielded spark plugs such as are often required for gas turbine installations is that of insulator warpage. Such warpage results in frequent insulator breakage and poor sealing properties. There is, of course, almost always a small amount of warping which occurs in the firing of any insulator regardless of size; however, in the case of an exceptionally long insulator, even slight warpage will result in an insulator having extremities which are so far out of line as to render it defective and perhaps useless. There is no commercially feasible production technique presently known for completely eliminating firing warpage. Thus, it has heretofore been the practice to merely scrap the rather large portion of insulators which are too badly warped. Obviously, this raises production costs considerably. Also, it results in a final product which may be inferior since even those insulators which are used may be slightly warped.

It is an object of the present invention to provide a spark plug and a process for making same which serve as solutions to the above-outlined problem. In other words, it is an object of the invention to provide a metal shielded spark plug of exceptional length which has increased accuracy of dimensions, better sealing characteristics and greater durability. Another object is the provision of a process for manufacturing exceptionally long spark plugs and more particularly the insulators for such spark plugs which results in a more accurate and therefore a better product and which also results inlower production costs due to a substantial reduction in the amount of scrap.

It has been found that slight insulator warpage has no ill efiect provided that such warpage is near the axial center portion such that it does not cause the end portions to be too far out of substantial alignment. The chief difficulties which arise because of inaccurately shaped or warped insulators are excessive insulator breakage and poor sealing. When an insulator is warped beyond a certain extent its upper extremity may, for example, contact the metal shield of the spark plug and thus the least amount of shock will cause breakage. Such insulator warpage may also result in a poor seating of the gaskets between the insulator and the shell thereby resulting in a defective gas seal.

As stated previously, the warpage which occurs in firing a relatively short insulator is small in comparison to that which occurs in the manufacture of a long insulator. Also, the same amount of warpage in a short insulator does not cause the end portions thereof to be as far out of alignment as in the case of a long one.

The present invention contemplates an extra long metal shielded spark plug having an insulator which is made in two axial sections joined together by a suitable bonding material. The two sections may be joined in axial alignment for use in a straight spark plug or may be joined at an angle to each other for use in an elbowshaped plug such as is required for certain types of installations. The invention also contemplates an improved method for the production of such spark plugs and more particularly for the production of the insulator elements. Various other objects and advantages to be gained by the present invention will appear in the following description and in the accompanying drawings in which:

Figure 1 is a sectional view of a straight spark plug embodying the invention;

Figure 2 is a side view of the mating portions of the two insulator sections shown in Fig. 1 prior to joining during the process of manufacture;

Figure 3 is an enlarged view of the insulator joint shown in Fig. 1 after the bonding operation is completed during the process of manufacture;

Figure 4 is a sectional view of an elbow-shaped spark plug embodying the invention; and

Figure 5 is a side View in partial section of the insulator joint shown in Fig. 4.

Referring now to the drawings and more particularly to Fig. 1, there is shown a metal shielded igniter plug of the type to which this invention relates, namely, one having exceptional length and adapted particularly for use in gas turbine or jet engine installations. Its chief component parts are a cylindrical shaped tubular metal shell 4, a tubular insulator 6 enclosed by the shell, and a center electrode assembly 8 within the centerbore 9 of the insulator.

In the embodiment shown, the metal shell 4 consists of three tubular metal sections ill, 12 and 14 which may be secured together by any conventional means such as a flange and shoulder arrangement as shown at 16 or by threaded engagement as shown at 18 or by welding or brazing. Upper section 14 is for the purpose of providing radio shielding to prevent interference with radio reception and is thus often referred to as the shield. Screw threads are provided at 29 for securing the ignition harness to the igniter in the conventional manner. In the particular embodiment shown, the lower end of the shell is bent inwardly to form a flange 22 the inner periphery of which serves as the ground electrode and the upper annular surface of which serves as a seating surface for insulator 6.

insulator 6, which consists of two elongated sections.

24 and 2s bonded together end-to-end at 44 as hereinafter more specifically described, is secured substantially concentrically within the shell 4 by means of the external annular shoulder 25 which is seated and clamped between the lower edge of the shield 14 and the upper edge of a metal spacer ring 27 the bottom of which engages the internal shoulder in the shell 4 as shown at 16. Metal gaskets 28 and 39 are provided to assure a good seating and to form a gas-tight seal.

in the particular embodiment shown, the center electrode assembly comprises a metal spindle 32, the upper end of which is threadedly engaged with a nut member 33 and embedded in a conductive glass seal 34, and a metal contact plug 36 having a lower portion which is also embedded in the conductive glass seal 34, thereby establishing electrical contact with the spindle 32. Above the contact plug 36, the insulator centerbore is of enlarged diameter to accommodate the terminal at the end of the ignition lead. The center electrode spindle 32 extends to the exterior of the flat end surface of the insulator and terminates in a head 38 which is in spaced Y prevent undermining and 5 excessive electrode wear.

By reference to Fig. 1and theab'ove descriptionit will be obvious that the insulator element 6 must be formed relatively accurately. For example, if there is more than ju'st'a slight amount-of Warpage, the thin-walled upper end of the insulator will contact the metal shield 14 thereby'. subjecting said insulator to mechanical shock and breakage; Also, suchcontact between the insulator and the shield 14' may cause a misalignment of the insulator shoulder 25 with the cooperating metal seating portions or ami'salignment of the base of the insulator with the seat ing surface formed by flange 22 thereby causing a defective gas seal.

To assure the desirable accuracy of alignment and thereby prevent the above-mentioned difficulties, the relativelylong insulator 6 is, in accordance with the present invention; constructed of two short sections 24 and 26 permanently joinedtogether' by a suitable bonding material at 44. For the purpose of assuring a good bonded joint, -the sections are provided with mating sur-' faces. Thus, in the embodiment shown in Fig. 1, the lower end of upper insulator section 24 isprovided With a cylindrical projection 46 which fits into and is bonded to a mating cylindrical recess in the upper end of lower insulator section 26. It will be noted that sections 24 and 26 are of substantially equal length and that the joint 44 is thus located substantially in the middle of the insulator. In this manner each of the upper and lower sections is maintained as short as possible thereby reducing warpage difiiculties. Also, as pointed out earlier, a slight amount of inaccuracy near the axial center of the insulator is of no particular disadvantage. Thus, it will be obvious that by locating the joint near the middle of the insulator a substantially perfect alignment of the ends of the insulator can be assured, any inaccuracy due to warpage of the small insulator sections manifesting itself adjacent the joint where it will do no harm.

A more complete understanding of the structure and of the advantages to be derived therefrom will be had from the following description of our process for making same.

To manufacture the insulator member 6, upper and lower sections 24 and 26 are first formed by any one of the well-known techniques such, for example, as by injection or compression molding wherein an organic resin is used as a binder and subsequently burned out during the firing operation, by the rubber molding technique utilizing paraflin as a binder which is burned out during firing, or by grinding or turning to final shape from a cylindrical blank. Because of its superior qualifies and desirable characteristics, we prefer to use as the insulator material a sintered high alumina composition such, for example, as is covered by U. S. Patent 2,272,618 Fessler et al., assigned to the assignee of the present invention.

Next, at least those portions of the insulator sections 24 and 26 to be joined are coated with a thin layer of ceramic bonding material and fired until said material is fully matured. Then, after cooling, at least the mating portion of one of the sections 24 or 26 is coated with a second layer of the ceramic bonding material after which the two sections are fitted together as shown in Fig. 3, and fired, Whilepressure is applied, to mature the bonding material and'form the joint 44.

It is to be particularly noted that joint 44 is preferably attained by two distinct coating operations. That is, the mating portions of the two insulator sections are first individually coated and fired and then a second application of bonding material is made to at least one of the two sections, after which the sections are fitted together and again fired to complete the joint. We have found that this technique of using two distinct glazing operations results in'a' joint which is far superior in strength and other desirable--characteristicsto that which may be obtained by means of a single application of bonding material to the parts.

A variety of ceramic bonding materials can be used, ranging from those which mature at relatively low temperatures of 1600 F. to relatively high temperatures of 2500 F. p

The exactbonding material which can best be used will, of course, be-determined by the composition of the-insulators. Preferred materials for joining high-alumina type insulators of the type referred to above are thelow temperature maturing glazes. Such glazes are well known to those skilled in" the art andthe following example is given by way of illustration and not by way of limitation:

Percent Ferro frit #3285 1 63.21 Ferro frit #3505 V 3005 Ball Clay -e.-- 6.45 Epsom salts .09 Sodium boro phosphate r .18 Fuchsin dye .02

Ferro frit #3285 Percent SiO 51.52 B 0 25.97 :A1203 3.8 FeO sa 1.31 'Li O 2.26 Na O 6.06 K 0 2.51 CaO 4.10

F erro frit #3505 Percent SiO 33.3 PbO 39.4 B 0 19.8 A1 0 4.6 Li O 2.9

The glaze joint should be as thin as possible'rather than heavy in order to obtain maximum strength. The bonding material should wet the surfaces of the ceramic and should not craze or'crawl and the joint should be free from blebs-and bubbles, this being accomplished by'the application of a slight amount of pressure during the firing of the bonding material.

' If the joint is to be in a position where it may undergo heat and electrical stress at the same time, the bond should be as free of alkali as possible. If the joint is in such a position as to be in a'reducing atmosphere, it is desirable to use lead free bonding materials.

The' followingis a'more detailed descriptionof the" procedure which may be used in forming the joint in accordance with the invention, the bonding material used being the above-listed glaze.

The mating portions of the insulator sections 24 and 26- are coated with about from .003 to .004 inch thickness of the low temperature glaze and fired at 1600 'F. on a 15 minute schedule to allow the glaze to mature. After cooling, the mating portion of one of the sections 24 or 26 is again sprayed or otherwise coated with a second layer of the low temperature glaze, the thickness of this layer being sutficient to provide a snug fit between the two insulator sections when fittedtogether as shown in- Fig. 3. The two sections 24 and 26'are then fitted together, projection 46 extending into the mating well provided at the'top. of insulator section 26. Center electrode spi-ndle'32 (together with gasket 4%) is then secured into place by means of the threaded nut 33 which fitsintothe enlarged diameter counterboreStl of the top insulatorsection 24 as shown in Fig. 1., threaded member 33 being'left rather loose so as to allow'a small amount of movement of the two insulator sections with respect to each other. This assembly is then placed in a jig indicated at 52 in Fig. 3 so as to obtain a perfect alignment of at least the extremities of the overall insulator. In this last regard a small amount of clearance should be provided between the mating portions of the two insulator sections so that correct dimensions and alignment can always be attained at this stage of the process. it is also to be noted that the center electrode spindle 32 assists in aligning the insulators and in some instances it may not be essential to use an aligning jig.

With insulator sections held properly aligned in the jig 52, the entire assembly including the jig is heated to about 1600 F. on a 15 minute schedule while the insulator sections are being pressed together at the joint 44 with a total pressure of about 3 to 4 pounds. After cooling, the threaded nut 33 is tightened on the center wire spindle, thus completing the insulator assembly.

It is highly advantageous to use a bonding material which matures at a relatively low temperature since this allows the use of jigs constructed from material such as metal having relatively low melting temperatures. In those instances where a high-fired glaze is used, it is necessary that the jig be of ceramic or a similar material having a very high resistance such that it is not destroyed during firing of the bonding material.

As the next step in the process after the insulator is completed, the conductive glass seal material, consisting of a mixture of glass and metal powder or similar conducting material, is tamped into the enlarged diameter counterbore 50 of the insulator, contact plug 36 inserted and the entire assembly fired so as to soften the glass and form the seal. Examples of such conductive glass seals and processes for forming same are described in U. S. Patents 2,106,578 and 2,248,415, both assigned to the assignee of the present invention.

To complete the spark plug assembly, gasket 42 is positioned in the spark plug shell 4 together with the insulatorcenter electrode assembly, spacer ring 27, gaskets 28 and 30, after which shield 14 is screwed tightly into place as shown. The final step is a Cico weld operation whereby the thin shell section 54 is softened by passage of electric current while pressure is applied on the ends of the shell so as to tightly clamp the insulator shoulder 25 into place between the mating metal portions and firmly seat the base of the insulator against the flange 22.

Figure 4 shows an angular type spark plug embodying the invention and comprises an elongated cylindricalshaped metal shell 60, a cylindrical-shaped insulator 62 concentrically enclosed by the shell and formed of two elongated sections 64 and 66 bonded together end to end at 68, and a center electrode assembly 70. The shell 60 is formed of two angularly disposed portions 72 and 74, the latter of which concentrically encloses the thin-walled upper insulator section 64 and constitutes a radio interference shield. Center electrode assembly 70 is quite similar to that shown in Fig. 1 and includes a spindle 76, seals '78 and 80 and contact plug 82. Electrode spindle 76 terminates in a head portion 84 which extends to the exterior of the insulator in spaced relation to the lower end of the shell 60 which serves as a ground electrode.

The insulator 62 is seated and sealed within the shell by means of gasket 86 which contacts the surfaces of the shell and insulator shoulder 88 as shown, and by the gasket 96 which is pressed between the top surface of the insulator and the shell.

As can best be seen in Fig. 5, the mating surfaces of the insulator sections 64 and 66 consist of the cylindrical surface of section 64 which fits into and is bonded to the surface of a cylindrical recess 92 extending transversely across the top of section 66. The bonding material for joining the two sections may be the same as that described with reference to the embodiment shown in Fig. 1. Also, the method for making the spark plug and, in particular, the insulator is preferably the same as that prea viously described, in this instance the jig used being suitable for attaining the proper angularity between the insulator sections.

The present invention is particularly important and advantageous with respect to spark plugs such as shown in Fig. 4 since accurate forming of an angular one-piece insulator is especially diflicult.

By means of the present invention, accurate alignment of at least the end portions of the insulator with the end portions of the shell is assured. Thus, substantially, concentricity is attained, particularly at the extremities of the insulator where it is most important. Also, because of this accurate alignment, firm and continuous contact of the insulator shoulder and base against the mating shell seating surfaces is assured, thereby providing a good seal.

It is to be understood that, although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

l. A spark plug comprising an elongated tubular metal shell provided with a ground electrode at one end thereof, an elongated tubular ceramic insulator secured within and enclosed along its entire length by said shell, said insulator comprising two elongated sections of substantially the same ceramic composition, One of said sections having a recess at one end thereof defined by a generally cylindrical wall and the other of said sections having a cylindrical mating surface extending into said recess and permanently bonded to said wall to thereby form a joint between said sections, said joint providing substantial alignment of the extremities of said insulator with the extremities of said shell, and a centerwire fixedly secured within said insulator sections having an end portion in spaced relation to said ground electrode to form a spark gap therewith.

2. A spark plug comprising an elongated tubular metal shell having at least two spacially separated internal seating surfaces and provided with a ground electrode at one end thereof, an elongated tubular ceramic insulator enclosed along its entire length by said shell and provided with at least two external surfaces adapted to cooperate with said seating surfaces to secure said insulator within said shell and form a seal therewith, said insulator comprising two elongated sections of substantially the same ceramic composition having adjacent end portions permanently bonded together to provide substantial alignment of the extremities of said insulator with the extremities of said shell and uniform and continuous contact between said external surfaces of the insulator and the seating surfaces of said shell, and a centerwire fixedly secured within said insulator sections having an end portion in spaced relation to said ground electrode to form a spark gap therewith.

3. A spark plug comprising an angular elongated tubular metal shell having at least two spacially separated internal seating surfaces and provided with a ground electrode at one end thereof, an elongated tubular ceramic insulator concentrically enclosed along its entire length by said shell and provided with at least two external surfaces adapted to cooperate with said seating surfaces to secure said insulator within said shell and form a seal therewith, said insulator comprising two elongated sections of substantially the same ceramic composition permanently bonded together at an angle with respect to each other to provide substantial alignment of the extremities of said insulator with the extremities of said shell and uniform and continuous contact between said surfaces of the insulator and said seating surfaces of the shell, and a centerwire fixedly secured within said insulator sections having an end portion in spaced relation to said ground electrode to form a spark gap therewith.

4. A spark plug comprising an elongated straight cylindrical shaped tubular metal shell having at least two spacially separated internal seating; surfaces and provided with a ground electrode at one end thereof, an elongated cylindrical shaped tubular .ceramic insulator concentrically enclosed along its entire length by said shell and provided with at least two external surfaces adapted to cooperate with said seating surfaces to secure said insulator within said shell and form a seal therewith, said insulator having a lower elongated section with a cylindrical recess at the upper end thereof and an upper section with a bore of enlarged diameter toward the upper end thereof and a cylindrical surface at the lower end thereof extending into and bonded within said recess to form a straight joint between said sections, said joint providing substantial alignment of the extremities of said insulator with the extremities of said shell and uniform and continuous contact between the said external surfaces of the insulator and the seating surfaces of said shell, and a centerwire fixedly secured within said insulator, said centerwire comprising a conductive glass seal within said bore and a metal rod having a lower end in spaced relation to said ground electrode to form a spark gap therewith and an upper end in threaded engagement with a nut member positioned in said bore and in electrical contact with said conductive glass seal.

5. A method for making an elongated tubular spark plug insulator comprising the steps of forming two separate elongated ceramic insulator sections, applying to one end of each of said insulator sections a coating of ceramic bonding material, firing said insulator sections to mature said ceramic bonding material, applying a second coating of ceramic bonding material over said first coating to the end of at least one of said insulator sections, pressing the coated ends of said insulator sections together and firing said sections to mature said second coating of bonding material and form a bonded joint between said insulator sections.

6. A process for manufacturing a metal-shielded spark plug of the type comprising an elongated metal shell having an elongated tubular ceramic insulator enclosed along its entire length therein, said process including the steps of forming an upper insulator section with a cylindrical surface at the base thereof, forming a lower insulator section with a cylindrical recess at the upper end thereof, applying ceramic bonding material to said cylindrical surface and the walls of said recess, pressing said cylindrical surface into said recess, inserting a centerwire through said insulator sections to form an insulatorcenterwire assembly, firing said assembly to mature said bonding material and form a joint, the extremities of said sections being maintained in position during said firing step for substantial alignment with the extremities of the metal shell and then securing said assembly concentrically within the metal shell.

7. A method for manufacturing a metal-shielded spark plug of the type comprising an elongated cylindrical shaped metal shellhaving an elongated cylindrical shaped tubular insulator enclosed along its entire length therein, said process: including the steps of forming two separate cylindrical insulator sections, applying to one end of each of said insulator sections a coating of ceramic bonding material, firing said insulator sections to mature said ceramic bonding material, applying a second coating of ceramic bonding material over said first coating to the end of at least one of said insulator sections, pressing the coated ends of said insulator sections together with a pressure of about 3 to 4 pounds while firing said sections to mature said second coating of bonding material and form one elongated bonded insulator, the extremities of said sections being maintained in position during said second-mentioned firing step for substantial alignment with the extremities of the metal shell, and securing said elongated insulator'within said shell so that said insulator is concentrically enclosed along its entire length thereby.

8. A method for manufacturing a metal-shielded spark plug of the type comprising an elongated metal shell having an elongated tubular ceramic insulator enclosed along its entire length therein, said process including the steps of forming two separate cylindrical insulator sections, applying'to one end of each of said insulator sections a coating of ceramic bonding material, pressing the coated ends of said insulator sections together while firing said sections to mature said bonding material and form one elongated bonded insulator, the extremities of said sections being maintained in position during said firing step for substantial alignment with the extremities of the metal shell, and securing said elongated insulator within said shell so that said insulator is concentrically enclosed along its entire length thereby.

References Cited in the file of this patent UNITED STATES PATENTS Johnson Mar. 18, 1924

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