US 3451110 A
Description (OCR text may contain errors)
June 24, 1969 5, v, BRAY 3,451,110
SPARK PLUG Filed Oct. 5, 1966 FIG. 1
STEWART M BRA) INVENTOR.
'BY ggzwazm ATTORNEYS United States Patent 3,451,110 SPARK PLUG Stewart V. Bray, Allen Park, Mich., assignor to Ford Motor Company, Dearborn, Mich., a corporation of Delaware Filed Oct. 3, 1966, Ser. No. 583,580 Int. Cl. H01t 13/16; F23g 3/01 US. Cl. 29-25.12 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a spark plug and to a method useful for the assembly of such a spark plug. This spark plug is conspicuous because of the economy with which it may be manufactured and its excellent performance.
The instant invention is best explained in terms of the drawings in which:
FIGURE 1 is a cross section through a spark plug at one stage of manufacture, and
FIGURE 2 is a cross section through the same spark plug after completion of the manufacturing process.
The drawings depict the central portion of a spark plug involving the invention. The drawings show the essential parts of the spark plug to be a central insulator, a complementary steel shell, a lower metallic seal and an upper seal which may be either metallic or a heat resisting plastic O-ring such as an O-ring fabricated from silicone rubber.
The central insulator and the complementary steel shell are dimensioned so as to effect a press fit between these two parts. The area of the press fit is indicated by the letters A and B and the double shading. Either the exterior of the central insulator, or the interior of the complementary steel shell, or both, are tapered to facilitate the assembly of the press fit. The assembly of the central insulator and the complementary steel shell is accomplished by forcing the central insulator axially into the interior of the complementary steel shell with a force of between four and five tons as indicated by the arrow. This type of assembly operation deforms the steel shell and establishes intimate contact between the central insulator and the complementary steel shell. This intimate contact serves to expedite the flow of heat from the central insulator to the steel shell. This deformation of the steel shell establishes a lasting radial sealing force be- 3,451,110 Patented June 24, 1969 tween the steel shell and the central insulator. This assembly force also tends to deform the lower metallic seal and so prevent leaks between the steel shell and the central insulator.
A typical automotive spark plug will have a shell diameter at the interference fit zone of 0.629 to 0.635 inch and an insulator diameter of 0.635 to 0.644 inch. The length of the interference zone is from 0.104 to 0.114 inch. The interference fit is typically 0.015 inch.
After the press fit has been made by axially thrusting the central insulator into the complementary steel shell the upper portion of the steel shell is rolled over the upper sealing ring to complete the assembly.
I claim as my invention:
1. A process for the manufacture of spark plugs comprising providing a central insulator and a complementary steel shell for the reception of the central insulator, the outside diameter of at least a portion of the central insulator being significantly greater than the mating interior diameter of the steel shell, and forcing the central insulator into place within the steel shell by means of an axially exerted force whereby a radial interference fit is established between the central insulator and the steel shell and permanent elastic stress is induced in the steel shell, said radial interference fit assisting in transferring heat from the insulator to the shell.
2. The process recited in claim 1 in which at least one of the central insulator and the steel shell is tapered to facilitate the insertion of the central insulator in the steel shell.
3. The process of claim 2 in which the force employed for establishing the interference fit also serves to establish a tight seal between the lower portion of the central insulator and the steel shell.
4. The process of claim 3 in which the seal is effected by the deformation of a metallic seal.
5. The process of claim 1 in which an additional upper seal is effected by crimping a heat resistant rubber O-ring seal in place between the upper end of the steel shell and the central insulator.
References Cited UNITED STATES PATENTS 1,353,785 9/1920 Platt 313-143 X 1,450,987 4/1923 Pethoud 313-143 X 2,283,164 5/1942 Brewster 313-118 2,309,236 1/1943 Burrell et al. 313-143 X JOHN F. CAMPBELL, Primary Examiner. RICHARD BERNARDLAZARUS, Assistant Examiner.
US. Cl. X.R. 313-118, 143