US2400917A - Spark plug and the manufacture thereof - Google Patents

Description

1946- D. H. CORBIN SPARK PLUG AND THE MANUFACTURE THEREOF Filed Dec. 8, 1943 r g VQZ w N 502g 7 6W Bnoentor attorney- Patented May 28, 1946 SPARK PLUG AND THE MANUFACTURE THEREOF Douglas Haynes Corbin, signor to General Mot Mich., a corporation Application December 8, 1943 ,=Serial No.

Dunstable, England, asors Corporation, Detroit, v I Delaware In Great Britain December 12, 1942 Claims. (01. 123-w-169) This invention relates to spark plugs for internal combustion engines.

In the specification of my prior Letters Patent No. 2,162,062 granted June 13, 1939, there is described and claimed a method of making a spark plug by positioning in the insulator a spark electrode with the central bore, introducing ametal for constituting the core of the electrode into the bore and bringing it into contact with the said electrode tip whilst in a molten state and then letting the metal cool so as to retain the electrode in position.

In practice the spark electrode tip may be constituted of a platinum alloy and the core of silver so that there is a minimum of corrosion and erosion and good heat conductivity from the hot tip of the plug.

It has also been proposed in Patent No. 2,106,578 granted January 25, 1938, to Schwartzwalder and Kirk and in Patent No. 2,248,415 granted July 8, 1941, to Schwartzwalder and Rulka to seal an electrical conductor in a spark eiiected a gas-tight connection between the conductor and the insulator. According to the preferred embodiment of the invention disclosed in the second of the above patents, the glass seal consists of a mixtureof glass and copper. Nevertheless even with this latter seal it was found that there was a tendency for high resistances to develop during service and it was conjectured that this might be due to the production of a small deposit of silver oxide the silver core due to the fact that on cooling the silver column tended to shrink away from the seal leaving in some cases a very small area between the head of the silver core during service a small spark would be produced with the consequent formation of ozone which at the normal working temperature would rapidly produce a non-conductive silver oxide.

The present invention has for one of its objects to overcome the defect referredto.

According to the invention, an improved spark plug manufactured a hereinbefore described is tip so'as to be in communication between the seal and 7 diameter than the diameter trical connection between the core of the electrode and the seal. Another object of the invention consists in improving cast metal electrodes of the class described. It has been found that when the metal is cast into a smooth-walled bore there is some tendency for the metal to pull itself apart and to fracture at some phase of its setting, due to shrinkage. I have found that this tendency may be overcome by corrugating the bore, preferably spirally, whereby the accommodation of the silver core to the corrugations results in the shrinkage strains being spread evenly through the core instead of accumulating at any one place and causing fracture.

Figure 1 is a vertical insulator having a corrugated bore as hereinafter described.

Figure 2 is a view similar to Figure 1 but with the electrode in place in the bore.

Figure 3 is a front elevation of the electrode disclosed in sectionin Figure 2.

Figure 4 is a vertical section through a spark plug showing a modified form of electrode.

Referring first to the construction shown in Figures 1 to 3, the insulator 4 is provided with of three different diamences la, 4b, 4c.

The bore 4a which is of serves to accommodate the platinum alloy spark tip 5 which is provided with a head 5:: of greater of the bore in so that the said head So when resting on the shoulder 4d intermediate the bores la and ib correctly positions the said tip 5 relatively to the insulator 4.

The wall of the bore 4b is formed with corrugations arranged in a spiral form.

The tip 5 is secured in position by a silver core 6 which is introduced in the form of a length of silver rod which is rendered molten, then subthe smallest diameter jected to the influence of pressure and subsequently cooled so that the said core 6 is molded to the walls of the bores 4b and 4c and caused to secure the spark tip within the insulator as described in the said prior Patent No. 2,162,062.

It is found that as the molten silver in setting tlons results in the shrinkage strains being spread section of a spark plug rugation at any one place as was possible heretofore in the case of a core cast in a smooth bore. Furthermore, a secure fixing of the central electrode within the insulator I is insured by the accommodation of the core to the corrugations.

The corrugation of the section lb also assists in the manufacture of the insulator I since this may be molded around a central core which can subsequently be removed by unscrewing the core from the completed insulator.

In the modification shown in Figure 4 the corof the lower portion of the bore has been omitted to illustrate the prior smooth bore design previously referred to. This figure illustrates the second feature of my invention which consists in the provision of a spring about which the molten electrode metal is cast and the glass seal is fused to provide a good conducting path for the passage of current from the one to the other. This feature of my invention may obviously be employed whether the bore in the insulator is smooth or corrugated.

As in the embodiment of my invention, previously described, the headed spark tip 2 is inserted into the bore of the insulator 3 so that the head 2a rests on the shoulder 3a. A length of silver rod is then introduced into the bore followed by a small metallic coil spring 5. The assembly is first heated to render the silver molten and then is rotated rapidly and subsequently cooled so that the silver accommodates itself to the bore of,

the insulator 3 in the manner described in pending patent application Serial No. 508,549 filed November 1, 1943,

After cooling, the lower portion of the metallic coil spring 5' is found to be embedded n the adjacent portion of the silver core I.

A gas-tight sealing of the silver core is effected by inserting in the bore of the insulator 3 a pellet of glass and pulverized copper which is rendered plastic and subjected to pressure so that on cooling it constitutes a conductive seal 6' as described in the prior Patent No. 2,248,415.

The formation of the seal 6' as aforesaid results in the upper portion of the spring 5 becoming embedded in the seal Ii.

As illustrated, it will be seen that the spring 5' does not extend completely through the seal 6' and consequently it will be understood that the seal 6' serves as the conducting medium between the core I and the upper portion of the center electrode, not shown.

If desired, however, the spring 5' can be permitted to extend completely through the seal 6' so as to be capable of a metallic contact with the upper part of the electrode.

It is found in practice that by utilizing a coil spring 5' there is no tendency for high resistances to develop when a plug is in service due possibly to the formation of silver oxide in the minute gap which may obtain between the seal 5 and the core 1 owing to shrinkage on cooling, and it is considered that the efllcacy of the coil spring 5' is occasioned by the fact that the spring 5' not only bridges any gap which might be formed upon the shrinking of the silver core, but that in view of the very small angle at which the spring 5 enters the seal 6' any subsequent shrinkage movements would result in the pulling away of a very small piece of the seal 6' rather than the fracture of the spring 5'.

I claim:

1. The combination of a spark plug having a bore therein and an electrode in the bore comprising a section of metal cast in the bore, a conducting glass seal fused in the bore and overlying said cast metal section, and a spring embedded in the said section and seal and forming an electrically conductive connection therebetween,

2. In the combination as defined in claim 1, said spring being of spiral form and having its axis running lengthwise of the bore.

3. The combination of a spark plug having a bore therein, portions of said bore being of different diameter providing a shoulder therebetween, a spring resting on said shoulder, a metal electrode cast in the smaller portion of said bore and surrounding and embedding a portion of the spring, and a conducting glass seal fused in the upper portion of said bore, overlying said elec- .trode and surrounding and embedding at least a portion of said spring so that the spring forms an electrically conductive connection therebetween.

4. The combination of a spark plug having a bore therein and an electrode in the bore comprising a section of metal cast in the bore, a conducting glass seal fused in the bore and overlying said cast metal section, and a spring embedded in the said section and seal and formin an electrically conductive connection therebetween, a portion of the bore in which said metal section is cast being transversely corrugated to interlock with the cast metal.

5. The combination of a spark plug having a bore therein, portions of said bore being of different diameter providing a shoulder therebetween, a spring resting on said shoulder, a metal electrode cast in the smaller portion of said bore and surrounding and embedding a portion of the spring, and a conducting glass seal fused in the

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