10-500-11 Conductive Sealing Gaskets
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10-500-11 Conductive Sealing Gaskets An introduction to the problems of Radio Frequency bonding and the prevention of corrosion in bonded joints Techtest Limited Introduction To achieve efficient operation from antennas mounted on aircraft, it is essential to have the base of the antenna bonded to the airframe, which, in turn, must be adequately bonded to provide an effective ground plane. It is not sufficient for the resistance between the antenna base and airframe to meet some arbitrary limit. This could probably be achieved by a single point of contact which would not however suppress RF circulating currents. To ensure suppression of these circulating currents, it is important that the base should be in contact over a substantial amount of its surface area. A similar restriction applies to the bonding of aircraft skins, where, as with antennas, a high degree of contact is required, thereby removing the possibility of the formation of resonant slots at the operating frequencies being used. It is not only with antennas that effective bonding is required. In the construction of racks to hold avionic or electronic equipment, bonding of the structural members is of great importance in the control of electromagnetic interference. In fact, wherever current is being carried across a metal to metal joint, bonding integrity must be effectively established. When two metal surfaces must be bonded together and either brazing or welding cannot be used, then consideration must be give to the use of some form of conductive gasket to ensure adequate contact area and prevent the ingress of moisture. The HR Smith range of Conductive Sealing Gaskets have been specifically designed to combat the problems of both inadequate contact surface area and corrosion as described. Contact Surface Area and Bearing Pressure To produce an effective bond between two metal surfaces, the faces must be clean, untreated and flat. The removal of any protective treatment and cleaning of the faces can be readily accomplished, but the provision of flat surfaces cannot be assured. PLASTIC RANGE ELASTIC RANGE If we consider two perfectly flat surfaces brought together in a vacuum, contact is made over the total area with zero load. When the surfaces are not perfectly flat, then with zero pressure contact will only be made between those points which are necessary for stability. This will be three points of micro area, plus any other points which happen, coincidently, to just touch when the three primary points are in contact. The probability of any of these secondary points occurring under zero pressure is small. An increase load is applied to bring the surfaces together, the primary contact points crush to form a finite contact area which can be calculated from the load, and the elastic and plastic distortion characteristics of the material. A normal stress/strain curve is show Figure 1 and indicates the characteristics of the material used. As the primary contact points crush, the two surfaces move into closer proximity and more points make contact and crush according to their own particular position on the material stress/ strain curve. Therefore, under load the contact area will consist of multiple discrete areas, each with a different pressure according to the amount of distortion present, and the load will be reacted by the summation of the product of pressure and area for each contact surface i.e. HR Smith Group of Companies

Figure 2 - Uniform and distributed loading In perfectly elastic material all the contact areas can be determined by consideration of the modulus of elasticity; while in a plastic material, the plastic characteristics must be taken into account. The latter also applies when a material is worked beyond its elastic limit and compressed until the discrete points merge into a total contact surface. Regardless of the number of discrete points, it follows that within the constraints of elastic/plastic deformation; a particular load requires a particular bearing surface area to react it. In...

Gasket Types Application of the Type 10-500-11 Gaskets Various approaches to solving the problem of non-flat surfaces and corrosion inhibition have been made and these were evaluated before the current range of H.R. Smith Conductive Sealing Gaskets were designed. The different types of gasket were: Various greases had been devised, which were loaded with conductive particles of graphite or various metals. This approach was considered to be of doubtful value for use with aircraft antennas, because cabin pressure could bleed through the grease when gaps due to airframe curvature had to be...