US1950629A - Spark plug - Google Patents

Description

I March 13, 1934.

H. RABEZZANA S PARK PLUG Filed Feb. 16, 1931 gmentot Patented Mar. 13, 1934 UNITED STATES.

SPARK PLUG Hector Rabezzana, Flint, Mich., assignor to A Spark Plug Company, Flint, Mich., a company of Michigan Application February 16, 1931, Serial No. 515,968 4 Claims. (01. 123-169) This invention relates to ignition devices for firing an explosive mixture in the combustion chamber of an internal combustion engine, and

particularlyto a spark plug of the type wherein u an insulator core of porcelain or other similar material surrounds a central electrode or wire and is secured between axially spaced portions of an outer metallic shell carrying a grounded electrode or side wire arranged in spaced relation 19 to the central electrode projecting through the core to afford a gap across which electric current jumps at predetermined times to fire the combustible charge.

Among other requirements, a spark plug must be leak proof togive satisfactory and eiiicient service, since the escape of compressed gases through the assembly results in power losses. More importance attaches to this necessity as the degree of engine compression increases and the present trend in the industry is toward hotter running and higher compression engine designs. Therefore, the spark plug manufacturer is now called upon to give more attention than ever before to the matter of producing a product that will stand up under these more severe conditions. 7

One of the outstanding difllculties lies in the fact that material most suitable for the insulator core has a coefiicient of expansion difierent from that of the attachment shell, and therefore, an assembly which may be leak proof at one temperature may not be so under difierent conditions. Utmost care and precision has had to be exercised in the manufacture and assembly of spark plugs to fit the parts for satisfactory service throughout the range of temperatures in which they operate and so that neither looseness occurs at one temperature nor such extreme tightness as would impose damaging stresses on the parts and cause fracture thereof at another temperature.

To provide a spark plug which will better meet these new conditions, and in addition eliminate the need for extreme accuracy and thereby facilitate economical manufacture, is one of the primary objects of this invention. To this end there is contemplated, in a specific embodiment, the slitting of the wall of the shell between its core abutment portions in a manner to weaken the wall and so render it sufllciently resilient to compensate for the differences in expansion while maintaining a tight seal at all times.

A better understanding of the invention will be had from an inspection of the. accompanying drawing wherein Figure 1 is a side elevation, partly in section, of a spark plug made in accordance herewith. I Figure 2 is a vertical section of the outer metallic shell and is taken on line 2--2 of Figure 1.

Figure 3 is an elevation showing a slightly modified plug.

Figure 4 is a vertical section of the shell taken on line 4-4 of Figure 3, and

Figure 5 is a portion of a shell embodying an alternative arrangement.

Referring to the drawing, the outer shell, which is preferably of steel or other similar material, includes a tubular member 1 having at its upper end a hexagonal head or tool engaging part 2 and atits lower end a skirt 3 which may carry screw threads for engagement within the customary spark plug opening of an engine and through which the side wire or electrode 4, carried thereby, is grounded. The bore of the tu- 7 bular member 1 is enlarged at its upper end and provides a tapered shoulder or abutment 5, the outer end of the bore being screw threaded to adjustably receive the gland or nut 6 whose lower end provides a tapered shoulder 7 which cooperates with the shoulder 5 in engaging corresponding shoulders on an enlargement 8 intermediate the ends of the insulator core 9 which may be formed of porcelain or other suitable di-electric material, encloses .a central wire or electrode 10 whose lower end is positioned in spaced relation with the grounded electrode 4, while its upper end carries screw threads on which is adjustably engaged a terminal nut 11 for securing thereto the conventional ignition wiring.

- Since the porcelain core 9 has a relatively lower coefiicient of expansion than the steel shell, it is desirable that provision be made to compensate for this difierence in expansion. As one means of accomplishing the purpose the shell may be provided, as shown in the drawing, with a number of saw kerfs or slots 12-12 extending through the shell wall between the axially spaced abutment shoulders 5 and '7. These kerfs are preferably grouped in two sets of two pairs each with the slots of each pair cut inwardly through the wall from diametrically opposite points and the sets arranged in staggered relation preferably apart. By the provision of these saw kerfs the wall of. the shell intermediate the abutment portions is sufliciently weakened that the material of the wall may give or be distended axially. Upon assembly of the parts the gland 6 is drawn up sufficiently to spring the weakened portion of the shell. As a result a leak proof joint is provided between the shoulder 5 and the portion of the core with which it engages, not only when the parts are cold but also upon their expansion under heat since with the relief of stress due to the difference in the rate of core and shell expansion the spring portion tends to contract and thereby eliminates looseness. Thus the resilient action of the shell wall serves to automatically compensate for dimensional variations and prevents fracture of the parts, particularly the core, maintaining practically constant pressure on the sealing gaskets throughout the range of temperature in which a spark plug normally operates.

While the above description refers to a two part metal shell it will be readily understood that the invention may be applied to the socalled one-piece shell, such as is illustrated in Figures 3 and 4 and wherein, in place of an adjustable gland retainer, the shell 13 is initially provided with an upstanding flange 14 which may be bent down and turned inwardly over the central core enlargement, as shown in Figure 3 and as illustrated by dotted lines in Figure 4. Figures 3 and 4 are also used to illustrate a modified arrangement of the wall slitting, there being shown two slots l515, each in the form of a helix and each extending throughout. approximately 270 and being arranged so that adjacent ends of the two slits are in overlapping relation with their intermediate portions on opposite sides of the wall. The function and purpose of the helical grooves is, of course, the same as the' group of saw kerfs shown in the Figure 1 embodiment.

Instead of providing grouped slits, as heretofore referred to, it will be obvious that there may be employed a single spiral slit extend ing preferably throughout more than 380, as is illustrated in Figure 5, wherein 16 indicates -a helical cut to give to the shell portion 1? the desired elasticity.

Various alternative arrangements of the structure described may be readily made as come within the scope of appended claims.

I claim:

1'. In a spark plug, an insulator core having an enlargement intermediate its ends to provide, spaced apart shoulders thereon, and a shell surrounding the core and having spaced shoulders corresponding to and engaging with the shoulders of the core to secure the core in the shell and also having in the wall portion thereof between the shoulders a series of horizontal slits grouped in pairs-with the slits of each pair diametrically arranged and with the pairs angularly related to impart resiliency thereto, said resilient wall portion being under tension to maintain tight engagement of the core and shell shoulders regardless of relative expansion.

2. In a spark plug, a core having a relatively low rate of expansion and a shell having a relatively high rate of expansion, said shell having a pair of spaced seats for engagement with the core and between said seats a, resilient portion under tension to compensate for the expansion thereof.

. 3..A. spark plug in which the shell has spaced seats to engage the core, characterized by a resilient shell portion between the seats under tensile strain so as automatically to take up such clearance between the seats and core as might tend to occur due to expansion of said portion under heat.

4. In a spark plug, an insulator core and a 1 shell having spaced seats for engagement with the core to position the parts, said core and 110 shell having diflerent rates of expansion under heat, said shell having circumferential slits in the region between said seats so constructed and arranged as to impart axial resiliency thereto and said resilient region being under tensile 115 strain.

HECTOR RABEZZANA.

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