A New Plug's Aero "Up-bringing"
Page 31
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MUCH information about the new " aircraft-insulated" A-C Sphinx sparking plug was imparted by Mr. D. Bennion Browne, M.I.A.E., managing director of the company, at the Dunstable works last week.
High-light of the story of the new plug is the sintered-alumina insulator. In 1939, mica insulation was found inadequate in aero-engines and something new had to be devised. On to the scene came Mr. D. H. Corbin,. Minst.B.E., works director and keen research worker, who had already been experimenting with aluminium oxide as an insulator base.
The result was quantity production of silver-cored, alumina-insulated aircraft plugs which stood up to fantastic tests. The same insulator is now used in the latest A-C plug as sold to the trade and the public.
Although the process of conversion from war to peace is only just being completed at Dunstable, the new motor plug is being produced in quantity. The manufacturing process is most impressive and its sequence is roughly as follows:—
Alumina powder is mixed with bakelite powder and moulded one-fifth larger than the insulator, to allow for shrinking in subsequent pro cesses. It is then sintered in hightemperature kilns, producing a 100 per cent. pure alumina insulator nearly as hard as a diamond. (We signed a sheet of plate glass with a pointed piece of insulator.) The " nailhead " electrode is fitted, followed by Pyrex glass and copper sealing compound, and tike steel upper half. After passing through kilns at 900 degrees C., the plugs enter presses which force the upper half of the electrode on to the lower half, the fused vitreous seal flowing into every internal space. The remaining processes include the fitting of the insulator and electrode to the body by means of special seals, and exhaustive high-voltage tests at every stage. Increased production is being obtained with a special multi-moulder designed by Mr. Corbin, which cuts out the separate weighing and " pelleting" of the insulator materials by the use of 24 presses rotating on a drum. The machine can turn out 10,000 moulded insulators, ready• for sintering, in 24 hours. One girl can operate it.
The manufacturer, A-C Sphinx Sparking Plug Co., Ltd., Dunstable, Beds, claims that the plug is three times stronger, three times better insulated, and three times more heat-conductive than earlier types. We tested its strength by tugging on the insulator with an iron tithe. and broke the material only with the greatest difficulty and effort.