Patent GB-734252-A (GB)
Abstract: 734,252. Sparking plugs. BENDIX AVIATION CORPORATION. Sept. 24, 1953 [Sept. 29, 1952], No. 26350/53. Class 7(4). In a centre-electrode assembly for a surfacedischarge sparking plug comprising a tubular insulator or semi-conductor element 30, Fig. 1, having a stepped bore the smallest section of which is threaded at, 36, and a centre electrode 31 having a threaded stem 37 screwed into the threaded bore, the electrode embodies an enlarged head 38 engaging the end surface of the element 30 and another enlarged part, such as a flange 40, engaging a shoulder in the bore. Terminal end construction. The shell is made of stainless steel or a nickel alloy and is formed by two parts 11 and 12 welded together, the upper part having threads 13 by which the plug is mounted in an engine, and a screwed portion 14 for the attachment of a shielded supply cable (not shown). Within shell part 11 is located the ceramic or aluminium oxide insulator 18 having upper and lower conical surfaces abutting conical soft-metal gaskets 19, 21, the former seating on a shoulder 17 and the latter being contacted by a threaded ring 20 which secures the insulator in position. A nickel-alloy tubular-connector 22 which is supported in a stepped part of the insulator bore and has secured thereto a nickel-alloy wire 23, is connected by a molybdenum wire 27 to the terminal 24, the remaining space within the insulator being filled with a heat-resistant glassseal 28. The wire 27 may be dispensed with if the seal 28 is made conductive. Sparking end construction. The lower shell part 12 which also acts as the earthed electrode has a reduced bore 32 at its inner end thus forming a square or (as shown) a tapered shoulder 33 by which the insulator 30 is guided during assembly, the reduced part 34 of the insulator being a tight fit in the bore 32 whilst the larger insulator part is a loose or sliding fit in the intermediate bore of the shell. The shell is inverted and made red hot whilst the insulator rests loosely therein after which the shell is pressed into position. A sleeve 35 which is threaded or force fitted into the outer end of shell part 12 also assists in maintaining the insulator 30 in position. The head 38 of the nickel-chrome alloy electrode is of smaller diameter than the insulator portion 34 thereby forming an annular spark-gap 39 ; the wire 23 is welded or otherwise fixed 41 to the electrode head 38. The space between the insulators 18 and 30 is vented through openings 42 in shell part 11. In a modification, the member 30 may be wholly semi-conductive or the outer face only made semi-conductive by coating it with a graphite or silver nitrate, for example. When member 30 is an insulator the plug is pre-sparked at a voltage higher than the normal operating voltage so depositing carbon on the active insulator surface. The shell part 12 may extend slightly below the insulator portion 34 so that the spark gap will be in the form of an annular groove. In another embodiment, further shoulders 45, Fig. 3, are provided on the insulator or semi-conductor 30<SP>1</SP> and the shell part 121 whilst the reduced part of the member 301 projects beyond the latter, The centre electrode comprises a bolt 46 with a nut 47 threaded and welded or otherwise fixed thereto, the nut having a diameter larger than that of the adjacent part of the insulator so that an annular and vertical spark gap is formed which is bridged by the insulator 301. The nut 47 may be substantially the same diameter as the adjacent end of member 301.