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AIRCRAF T ENGINEERING May, 1933 Th e Design and Construction of a Compact, Adequately Insulated , Screening Cap Fully Described By A. M. Lodge S early as 1894, in a paper on the subject place with the production of ozone, which is of electrical radiation read a t the Royal fatal to rubber insulation. Enclosing the high Institution, Sir Oliver Lodge explained tension circuit in an "earthed " metal casing th e steps tha t it was necessary to take t o screen increases the electrical capacity by, say, four electrical radiation. He pointed out that lor or five times over tha t of an unscreened circuit effective screening every part of the oscillating and thereby considerably increases the brush circuit must be completely enclosed in metal discharge. Therefore, in a screened circuit all casing, and all the metal enclosures well con sharp bends in the high tension cable should be nected together. He also explained that while avoided. As previously mentioned, there must small round holes in the casing did not matter, be a right-angle bend or connection at the anything in th e nature of a slot or chink, or the sparking plug, and usually also a t the magneto, smallest "live " part of the circuit projecting bu t where this occurs some insulating material through the casing, would allow the radiation other tha n rubber should be used. t o pass. The ignition system of a petrol engine generates an oscillating discharge, sending out Suitable Insulating Materials electrical radiation, and to prevent this radia The best plan is for the screening cap itself tion interfering with a neighbouring radio t o be made in th e shape of a right-angle junction receiver it is necessary to enclose the whole ignition circuit—magneto, cables and sparking box or elbow, but this is where the manu plugs—in metal casing. facturing difficulty arises. There would not be much difficulty in making a cap using air in There is no difficulty in making a metal case sulation, bu t such a cap would be too bulky for t o enclose the sparking plug and to connect up use on most engines, and, therefore, some form this metal case t o the metal sheath round the of solid insulation must be used. The practical high tension cable provided the cable is attached materials that appear suitable are mica, one in line with the axis of the sparking plug as of th e hard rubber compounds (such as ebonite), shown in Fig. 1. With such an arrangement, synthetic resin (such as bakelitc), or one of the however, either the cable will project a con A screening cap for a sparking plug must be potter y materials. Mica would be an excellent siderable distance from the cylinder of the capable of being quickly attached or detached, material for th e purpose, bu t owing t o its nature engine, which is not practicable in the case of and this is provided for by an outer metal casing it cannot be built up in th e form of an insulating most aircraft engines, or the cable where it right-angle tube. The hard rubber compounds with a quick-acting catch locking on to the leaves the terminal of the sparking plug must could easily be moulded to the necessary shape gland nut of the plug, and a spring between the be bent sharply, which is bad electrically. and would be good electrically, but soften at outer casing an d the cap itself to give th e neces Wherever a sharp bend occurs in a high- sary pressure on to the sparking plug. Fig. 2 too low a temperatur e for the purpose. Bakelite tension cable, electrical brush discharge takes shows the construction of the cap (which is is about the best material from a moulding covered by patents), with the high tension cable poin t of view, on account of the accuracy with and sparking plug in position. It is importan t in which it can be finished, and would be quite man y engines tha t the screened plug should not suitable for exposure to moisture and oil. It project far out from the cylinder, and in the is also just good enough from the heat point of view, but, unfortunately, the synthetic resins design illustrated the addition of the screening are damaged by electrical brush discharge, cap only adds 5/16 in. to the projection of a normal sparking plug. The fitting of such a which gradually carbonises the surface and cap, besides its screening qualities, has the destroys the insulation. Probably, therefore, advantag e of keeping the sparking plug clean th e screening cap is best made in one of the and watertight. potter y materials, such as porcelain, as this will easily withstand the heat and is not affected I t might be though t tha t boxing-in th e sparking b y oil or moisture, nor by electrical brush plug by th e screening cap would cause i t t o over discharge. Porcelain can be moulded to the heat. No doubt the terminal end of the plug required shape, bu t moulded porcelain is usually does get slightly hotter, but the effect of attach not vitreous throughout and the moulded finish ing a large cooling-surface to th e metal body of is hardly accurate enough in dimensions t o be th e plug tends to cool that part better than worked satisfactorily with the necessary metal withou t the screening cap. So far as engine part s for fixing t o the sparking plug and cable. tests have been made at present, the resultant Porcelain should, therefore, be machined to the effect on a plug fitted with this cap appears to required shape, but this is rather an expensive show that the temperature of the plug at its process. sparking end is in practice not affected by the The Lodge company were fortunate in having boxing in. a suitable material ready t o hand in their "re The increased capacity of a screened ignition constructed " steatite used for sparking-plug circuit produces slightly different results. The insulators, which can be machined and fired to outpu t of the magneto should be sufficiently size with almost the accuracy of metal. In large to ensure tha t the extra capacity of the order to make the cap opaque to electrical radiation, the outside is coated all over with circuit will be charged up and discharge at metal to a sufficient thickness t o form a perfect th e plug gap with certainty at every break of radio screen. A 1/16-in. wall of steatite is really th e primary circuit. The larger capacity com sufficient to withstand the voltage of an ignition ponen t in th e spark gives i t less tendenc y t o leak spark, but in order to have the material under over surfaces coated with carbon or moisture, bu t the spark itself is more disruptive, and, very slight electrical strain and to provide a therefore, the sparking points of th e plug should large factor of safety for all conditions of have rather greater surface to make up for the service, the steatite caps are made with 3/16-in. more rapid wearing away than would other dielectric insulation; which also gives the caps ample mechanical strength. wise be the case.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: May 1, 1933
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