Patent GB-712870-A (GB)
Title: Improvements in the construction and methods of making sparking plugs of the surfacedischarge type
IPC: H01T13/52
Filed: 1952-05-06
Abstract: 712,870. Sparking plugs. BENDIX AVIATION CORPORATION. May 6, 1952 [May 7, 1951], No. 11458/52. Class 7(4) In a method of making a low-tension sparking-plug electrode-assembly comprising making a metallic tube 15, Fig. 1, adapted to serve as an outer electrode and having a stepped bore, inserting into the bore a centre electrode 16 having one part closely fitting one section of the bore and another part adapted to be so housed in the other section of the bore that a space is present in the last mentioned bore section, filling this space with a dielectric material 17 and applying heat to render the latter plastic and then permitting the material to set, there is included a step which provides for cutting away that portion of the tube 15 which surrounds the electrode part that closely fits the first bore section; the centre electrode 16 may also comprise a shank portion 27 and an enlarged head portion 24, 25, which latter is spaced from the end of the tube by a thin layer of the dielectric material 17. Removably mounted within the steel or like casing 10 is the steel tube 15, the electrode 16 and the borosilicate or lime silica glass insulator 17, the tube 15 having shoulders 18 and 19, the former of which co-operates with a similar shoulder in the casing 10, a washer 20 of copper or similar soft metal being interposed therebetween; a tubular nut 21 acting through a sleeve 22 and a washer 23 secures the electrode assembly in position. The electrode 16 comprises the active portion 24 whilst the part 25 of reduced diameter incorporates one or more grooves 26 and the shank 27 to which the ignition lead, not shown, is attached. The sparks travel over the annular end surface of the insulator 17 between electrodes 15 and 16 which insulator surface is flush with the end surfaces of the electrodes; the insulator also acts as a seal to prevent gas leakage through the electrode 15. The centre electrode 16 is either forced into the powdered or solid insulator 17 after the latter has been made plastic by heating or the insulator is injected into position in a plastic form; the inner surface of the electrode 15 may be roughened or grooved to prevent loosening of the insulator. In a modification, the active portion of the centre electrode has the shape of the frustrum of a cone and in either embodiment the inner surface of the electrode tube 15 near the active end thereof may diverge from the longitudinal plug axis. In another modification, the active portion 24b, Fig. 4, of the centre electrode which has an external diameter equal to the external diameter of the electrode 15 is seated inwardly of the end surface of the tube so that the sparks travel vertically over the exposed annular surface 28a of the insulator. The upper and/or lower end surfaces of the portion 24b and tube 15 may diverge towards this longitudinal plug axis, so that the insulation between these members is disc shaped. In a further modification, the active portion 24c, Fig. 6, of the centre electrode has a diameter intermediate that of the inner and outer diameters of the tube electrode 15a, the sparks travelling vertically over the exposed annular surface of the insulator 17. The centre electrode 16 is made as previously described with the portion 24c flush with the end of the tube 15a and, after the insulator 17 has set, the lower end of the tube is recessed 33 to form the active sparking surfaces. In any of the above embodiments the portion of the insulator over which the sparks travel may be rendered semi-conductive by a metal powder dispersed therein, and the electrode tube arranged for use independently of the plug casing.