Seal for a joint or juncture – Seal between fixed parts or static contact against... – Contact seal for other than internal combustion engine – or...
Reexamination Certificate
2000-01-24
2002-03-19
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between fixed parts or static contact against...
Contact seal for other than internal combustion engine, or...
C277S651000, C277S652000, C277S904000, C277S919000
Reexamination Certificate
active
06357764
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a corrosion resistant gasket for aircraft, and more particularly to encapsulation of a conductive mesh in a pliable uncured, uncatalyzed fluorosilicone compound having a durometer of 40 or less that will not become bonded to mating surfaces and will also migrate upon compression during installation to the threads of attaching hardware utilized between the instrument, antenna, and aircraft structure, thereby reducing corrosion through the attaching hardware, by providing an outer bead around the periphery of the envelope to provide a hermetic seal of the attached device.
Present methods of providing coupling between mating surfaces in aircraft having aluminum structures were limited to the uses of cured elastomer gaskets, metallic gaskets using sealants, or a multiple use of corrosion inhibitors and plating. The cured elastomer gaskets would allow moisture between the mating surfaces and tended to become bonded/adhere or retain a memory under compression to the two surfaces after a period of time and temperature cycling. The metallic gaskets also had a permanent bonding problem due to the application of adhesives to reduce the moisture ingress between the two surfaces. Both elastomer and metallic gaskets tended to shift the frequency of antenna installations due to the gap they created between the two mating surfaces, causing a shift in the VSWR of the antenna. The use of the corrosion resistant compounds and sealants creates a time consuming process in application and removal and tend to crack during structure flexing, thereby allowing moisture to ingress between the mating surfaces and causing a breakdown of the inhibitors.
Also, most gaskets presently used have a base material so dissimilar to aluminum that they thereby cause galvanic corrosion, rather than prevent it, due to the fact that they cannot provide a hermetic seal by themselves and require the use of an outside sealant which when used in high vibration areas or under flexing conditions tends to crack and thereby introduces an electrolyte that creates a galvanic cell.
Other alloy meshes such as Monel, a nickel plated copper alloy manufactured by The Chomerics Co. of Woburn, Mass., embedded in silicone gels are dissimilar metals with respect to aircraft structure or aircraft antenna base, such that they are subject to corrosion themselves or cause additional corrosion through galvanic corrosion when exposed to certain unfavorable environments. Also, it has been proven, both by lab tests and in service applications, that the use of silicone on aircraft in areas where the silicone is either exposed to jet fuel or jet fuel vapors, the silicone deteriorates and consequently the corrosion protection is jeopardized in the use of aircraft applications.
SUMMARY OF THE INVENTION
The present invention provides a gasket which prevents the ingress of moisture and or other contaminates between the surfaces of aluminum causing corrosion or galvanic corrosion in this area. The gasket is constructed so that it eliminates present electrical bonding methods through the attaching screws and provides a positive bond through the structure to the base of the instrument, antenna, and/or aircraft skin lap joints, electrical receptacle outlets, waste outlets, lavatory installations and galley installations. Besides providing electrical bonding through surface to surface contact, eliminating all aspects of corrosion, the gasket is capable of reducing corrosion through the attaching hardware by migrating the insulating properties of the uncured unvulcanized fluorosilicone material of the gasket onto the threads of the attaching hardware and forms an outer bead of the fluorosilicone compound around the periphery of the installation upon compression. Although the gasket is shown in certain illustrative embodiments herein in aircraft applications, it is also useful in marine applications where salt water corrodes aluminum or steel installations and where maintaining electrically conductive properties between mating surfaces is a serious problem.
By using the present flexible gasket comprising silver-plated stranded aluminum when low electrical bonding resistance is required in certain aircraft installations requiring less than 0.02 milliohm encapsulated in an uncured unvulcanized fluorosilicone, application time is reduced, as well as removal and elimination of structural and component damage during removal. Since the fluorosilicone compound provides a hermetic seal under high vibration, flexing conditions, aerodynamic conditions of up to 0.8 Mach, and provides a seal for internal aircraft pressure of over 30 P.S.I., there is no fear of an introduction of either an outside, or inside introduction of an electrolyte that would create a galvanic cell. For the purposes of cost reductions in applications that do not require extremely low bonding resistance requirements, and only need to be less than 1 milliohm, then the aluminum woven metallic mesh or expanded aluminum screen can be of the same surface structure type currently used in the manufacture of aircraft structures. The expanded aluminum screen is favored over the metallic woven mesh for airplane applications. The reason for the preference of the expanded aluminum screen for airplane applications is the pliability of the expanded aluminum which is made from an expanded metal foil is one of several mesh (or open area) materials used in eliminating interference. It is made from solid foil gauge metals to precision tolerances. The process involves a shaped tool lancing and stretching a ductile metal in one motion. The resulting holes are diamond-shaped with a large variety of hole sizes. Dimensions range from a {fraction (1/32)}″ in the long way of the diamond (LWD) to approximately ½″ LWD. In shielding applications an LWD of ⅛″ or less is most common, although different shape holes can also be created. Material options include selvage edge, solid sections and annealing. Upon compression the screen will make contact with the two mating surfaces to provide electrical continuity, but will not cause any pits into the airplane structure as will the woven metallic mesh with the harder durometer. This has been observed on in-service airline field tests and discovered after a period of two years with periodic inspections using the woven metallic mesh.
By encapsulating the woven mesh or expanded aluminum screen with a fluorosilicone compound such as Dow Corning LS40 or GE FSE2120, or any other fluorosilicone compound that is not vulcanized and catalyzed the end product can be achieved. The fluorosilicone compound is best applied in a sheet form to a precut or die cut configuration of the metallic mesh or screen the configuration of the device to be applied to. The expanded aluminum screen should not exceed more than 20% of the thickness of the fluorosilicone compound. Application of the compound to the mesh can be accomplished by placing a layer of the compound on a warm pad and placing the aluminum screen or mesh on top. Then by the use of a vacuum bag, cover the heated gasket and drawing vacuum, the fluorosilicone compound will flow through the mesh, and also fill all the installation cut-outs for the attaching screws.
FIG. 8
shows a gasket that has an inner elastomer in the die cut for the attaching hardware that provides corrosion protection to the screws and nut plates or fasteners.
FIG. 8
shows the use of two layers of the screen or expanded aluminum mesh of FIG.
7
and die cut of the same to the desired configuration and the insertion of the uncured elastomer between the two conductive layers. This eliminates the need for special type release liners and can be packaged for application in any type of plastic or paper container. When applied under pressure the two aluminum mesh
32
meet and form the electrical contact needed between the two mating surfaces and the elastomer
31
squeezed between the openings and outer periphery of the exposed mesh to encapsulate the mesh and provide a hermetic seal. The desi
Gaines Patrick W.
Williamson Mickey A.
Beres John L.
Gardner Conrad O.
Knight Anthony
The Boeing Company
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