Abstract:
An annular elastomeric EMI shielding gasket includes a metallic mesh embedded in an elastomeric sheet for sealing and protecting aircraft fuel tanks and panels. In one embodiment, the gasket has a central region with a protruding portion to prevent leakage of fluid under the dynamic loading of the wing assembly experienced during flight. In another embodiment, the gasket has a rubberized tip portion providing the same advantages in an alternative configuration. In further embodiments, the annular inner section of the gasket includes a grommet bordering the gasket opening and encapsulating the inner most section of the gasket. In still further embodiments, the elastomeric sheet is formed from a fluorosilicate reinforced with Kevlar fibers, and the metallic mesh is formed from triple strand wire.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a gasket assembly for use in sealing and EMI shielding applications, particularly for aircraft related applications, and more particularly for sealing and protecting aircraft fuel tank access doors and panels. The gasket assembly of the invention includes the following components: (1) an improved seal design; (2) a molded grommet design; (3) an improved reinforced elastomeric sealing element; and (4) a conductive triple stranded knitted wire mesh. These various components are each novel and include inventive features which may have applicability beyond the specific applications illustrated herein. 
         [0002]    Specifically, and without intending to limit its scope or reach, the present invention relates to an annular elastomeric gasket having a conductive mesh integrated into the gasket to provide electrical conductivity between adjacent aircraft surfaces to be sealed. The gaskets of the invention are designed to prevent failure under the dynamic load conditions experienced by the aircraft wing assembly during flight. This design provides a void space for the volumetric expansion of the sealing member under the compressive loading experienced during flight, thereby preventing leakage from the seal. 
         [0003]    In addition to the foregoing, the gaskets of the invention may contain additional features designed to improve the sealing effectiveness, survivability, and overall performance of the gasket as explained in more detail herein. 
         [0004]    Gaskets are typically used in EMI shielding applications where flexibility in addition to shielding effectiveness is required due to the particular application. Composite gaskets generally comprising a metal core material enclosed or encapsulated within a resilient polymeric material are known in the art. Such gaskets have sufficient structural integrity to be useful in sealing components in corrosive and high performance environments, such as for aircraft. Examples of such gaskets are disclosed in U.S. Pat. No. 2,477,267 and U.S. Pat. No. 3,126,440. 
         [0005]    U.S. Pat. No. 4,900,877 discloses a gasket for EMI shielding and environmental sealing. The gasket of this patent has a metallic electrically conductive deformable element adapted to be positioned in a gap between adjacent conductive surfaces. The gap is filled with a gel for sealing the space between the surfaces and for encapsulating the metallic structure to provide environmental protection against moisture and corrosion. 
         [0006]    The gaskets described in the aforementioned patents may not be acceptable for high performance applications, typically aircraft applications, where a variety of performance characteristics may be required in harsh working environments. For example, in addition to EMI shielding and sealing, electrical bonding of components and protection against corrosive environments may be a necessity. 
         [0007]    Additional problems are created when gaskets are used in external aircraft applications, such as for sealing aircraft fuel tanks. For example, during assembly of the fuel tank access panel, tightening of the bolts used in the panel can rupture the gasket, allowing fluids to leak from the sealing area and adjacent metal surfaces to come into direct contact with each other. Failure can also occur as a result of the dynamic loads placed on the aircraft wing surfaces during flight. This can also cause the gasket to be compressed beyond its normal tolerances, resulting in the separation of the reinforcing mesh from the elastomer. 
         [0008]    Various attempts have been made to solve problems associated with aircraft fuel access ports and panels. For instance, U.S. Pat. No. 4,579,248 describes an improved gasket for sealing aircraft fuel tanks. The gaskets of the reference incorporate outwardly extending seal members fabricated from a fluorosilicate rubber to provide the gasket with improved corrosion resistance and fluid retention. As shown in the reference however, the outwardly extending gasket end portion does not contact either of the opposed aircraft fuel tank surfaces upon compression of the gasket. U.S. Pat. No. 7,900,412 is directed to polysulfide sealing materials containing fiberglass reinforcing materials which can be applied to aircraft fuel tank access ports as caulking materials. 
         [0009]    While the above references provide solutions to some of the problems associated with fuel tank panel sealing, the use of caulking materials such as polysulfide, in particular, usually requires significant downtime due to the curing time required for the polysulfide. This is a result of the time required for the application and curing of the polysulfide caulk. Additionally, when a gasket is replaced, the old caulk must first be removed, and the removal procedure can result in damage to the equipment. Furthermore, most caulking compounds have a limited shelf life which can create inventory obsolescence and increase associated costs. 
         [0010]    It will be readily appreciated that it would be advantageous to develop an improved gasket for sealing aircraft fuel tank access panels and doors which overcomes the problems associated with the techniques and materials currently in use, which would be able to provide environmental sealing between mating surfaces, and which contains such additional features as may be useful to optimize the performance of the gasket. 
         [0011]    The disclosure of each of the patents set forth above is incorporated by reference herein in their entirety. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention is directed generally to improved gaskets for sealing and protecting aircraft fuel tank access doors and panels. The improvement of the invention results in more effective sealing and protection of aircraft wing-mounted fuel tanks, access doors and panels. The improvement comprises the use of a gasket having a flexible design for withstanding the dynamic stresses and forces associated with aircraft wings during flight, and results in greater sealing and environmental protection of the fuel tank and aircraft. 
         [0013]    The gasket of the invention comprises an EMI shielding gasket for aircraft fuel tank access doors and panels. The gasket, which can be circular, oval, rectangular, square, or any other shape designed to fit the access door configuration, has an annular configuration with a hollow core section, such as a ring gasket. The gasket further comprises an elastomeric sheet having embedded therein an electrically conductive mesh for establishing electrical conductivity between adjacent metallic surfaces, and particularly adjacent aircraft wing mounted fuel tank and access door surfaces. The aircraft surfaces are typically aluminum or titanium surfaces. 
         [0014]    In one aspect, the electrically conductive mesh is a metallic mesh formed from metal wire or fiber. Suitable metals include, for example, copper, nickel, silver, aluminum, bronze, steel, tin, or an alloy or combination thereof. Electrically conductive fibers can also be non-conductive fibers having an electrically-conductive coating, metal wires, carbon fibers, graphite fibers, inherently-conductive polymer fibers, or a combination thereof. Preferably, the mesh is a metallic mesh formed from triple strand wire. 
         [0015]    In another aspect, a grommet is affixed to the inner annular region of the gasket, forming a border between the gasket and the hollow annular space. The grommet is designed to encapsulate the innermost end section of the gasket, effectively forming a border between the inner annular region of the gasket and the void of the annular space. The grommet can be formed from a metal or a molded plastic. 
         [0016]    The elastomeric material can, in general, be any of a variety of natural or synthetic rubbers, such as EPDM, nitrile rubber, silicone, urethane, polysulfide, polytetrafluoroethylene, butadiene, neoprene, and the like. Preferably, the elastomeric material is a fluorosilicone polymer reinforced with Kevlar fibers. 
         [0017]    In one embodiment, the gasket is divided into three zones or regions extending outwardly from the assumed center of the gasket to the end portion of the gasket: an inner region, a central region, and an outer region, all of said regions being contiguous and part of a unitary gasket structure. The inner and outer regions can preferably have approximately the same thickness. The central region has an intermediate zone positioned between two peripheral zones. The peripheral zones are generally thinner than the inner and outer regions of the gasket, while the intermediate zone, which is positioned between the peripheral zones, can be at least approximately the same thickness as the inner and outer regions. Preferably, the intermediate zone is tubular in configuration. The gasket can optionally include a rubberized tip portion extending outwardly from the edge or rim of the outer region. 
         [0018]    In another embodiment, the gasket comprises, as a single unitary structure, an inner region and an outer region, wherein the inner region and outer region are contiguous, and further wherein the inner region is thicker than the outer region. The outer region has affixed to its outwardly extending end portion a rubberized tip portion. Preferably, the rubberized tip portion has a bulb or tubular shape, and is thicker in dimension than the inner region of the gasket. Upon compression of the gasket as a result of the installation and securing of the aircraft fuel tank access door, the rubberized tip portion, which extends beyond the lateral surface of the access door, is compressed, thereby sealing the interior section of the gasket. 
         [0019]    A further embodiment of the invention comprises an assembly including a gasket embodiment as described above, the aircraft fuel tank outer surface access port, and the aircraft fuel tank sealing panel or door. The aircraft surfaces are generally fabricated from aluminum or titanium. The gasket is inserted into the space between the access port and sealing panel, a series of bolts is inserted between the panel, the gasket and the access port, and the bolts are tightened to compress the gasket and seal the port. The shape of the gasket is designed to conform to the shape of the access port, and will typically comprise an annular shape with a series of radial holes for accommodating the sealing bolts. 
         [0020]    The present invention, accordingly, comprises the construction, combination of elements and components, and/or the arrangement of parts and steps which are exemplified in the following detailed disclosure. The foregoing aspects and embodiments of the invention are intended to be illustrative only, and are not meant to restrict the spirit and scope of the claimed invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The foregoing and other advantages and features of the invention will become apparent upon reading the following detailed description with reference to the accompanying drawings in which: 
           [0022]      FIG. 1  is a cross-sectional view of one embodiment of the gasket design of the invention. 
           [0023]      FIG. 2  is a cross-sectional view of another embodiment of the gasket design of the invention. 
           [0024]      FIG. 3  is a perspective view of the gasket of  FIG. 1 . 
           [0025]      FIG. 4  is a perspective view of the gasket of the invention depicting a circular grommet positioned on the innermost surface of the gasket, encapsulating the end section thereof. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    The gasket of the present invention is primarily intended to be used for sealing and EMI shielding of aircraft fuel tank access doors and panels to the fuel tank access port which receives the fuel for the aircraft. These access doors and panels are typically mounted on the wing of the aircraft which also contains the external fuel tank. The gasket design of the invention includes a feature which provides effective sealing upon initial compression of the gasket during installation of the panel, and during flight when the external wing surfaces are subjected to a variety of dynamic loading forces due to variations in air pressure. As described in more detail herein, this feature permits the expansion of the gasket without rupturing or damaging the gasket, and thereby results in increased service life for the gasket. Other features useful for the practice of the invention are also described herein. 
         [0027]    As used herein the term “aircraft” is intended to designate both commercial and military aircraft, jet and prop aircraft, including both large commercial and smaller private aircraft. 
         [0028]    Also as used herein, the term “horizontal”, as used with reference to the dimensions of the gasket, is intended to denote the plane in the x-axis and y-axis (commonly the length and width) of the gasket. The “thickness” of the gasket denotes the part of the gasket extending upwardly along the z-axis, which extends vertically from the plane of the x-axis and y-axis. 
         [0029]    The term “grommet” is intended to denote a ring designed as a collar fitted on the innermost section of the annular gasket of the invention. The grommet can be a molded plastic or metal part. The function of the grommet is to effectively terminate the end section of the mesh facing the inner annular void, reinforcing this section and preventing premature failure of the gasket. 
         [0030]    In particular, the gasket of the invention is intended to be installed between adjacent aircraft surfaces, and in particular between the fuel tank opening and the fuel tank access panel. The gasket is intended for sealing, to prevent leakage of the fuel from the fuel tank, to prevent the entry of external fluid into the gasket, and to protect the fuel tank from static electrical charges. 
         [0031]    Typically, the gasket is shaped or prepared as an annular sheet with an open core section. The annular sheet can be cut from a larger sheet, for instance, or prepared as an annular component. Preferably, the gasket includes a grommet affixed to the innermost section of the gasket, i.e. the section of the gasket bordering on the inner void space. The grommet, which can be formed from a metal or plastic, encapsulates the inner section of the gasket, thereby terminating the gasket at the annular space. The grommet replaces small pieces of secondary wire mesh which had previously been mechanically inserted at the gasket interface. The secondary wire mesh pieces can create a mechanical standoff or stress riser which could lead to premature gasket failure. However, the use of a grommet to encapsulate the end section of the elastomer permits a more uniform compression around the circumference of the gasket. 
         [0032]    The overall shape of the gasket is dictated by the shape of the fuel tank opening and access panel, and will typically be oval, round, square or rectangular. The gasket will also typically include holes for the attachment bolts or screws which secure the access panel to the fuel tank opening. When the gasket is tightened in place, the conductive mesh embedded in the elastomer contacts the adjacent aircraft surfaces thereby establishing an electrical bridge between the surfaces, thereby protecting the surfaces from EMI radiation and electrical discharges. 
         [0033]    The gasket of the invention includes a zone or region of diminished thickness which allows for expansion of the gasket as a result of dynamic loading on the wing in flight due to changes in air pressure. In addition, adjacent to the zone of diminished thickness, the gasket also includes a zone of increased thickness to provide sealing for the interior sections of the gasket. This prevents the contamination and failure of the gasket as a result of dynamic forces acting on the gasket during flight. Contamination can result from moisture and other environmental fluids entering the seal area when the gasket is under stress. 
         [0034]    The elastomeric material which can be used to fabricate the gasket of the invention can include, in general, both natural and synthetic rubbers, such as, for example, polyethylene, polypropylene, polypropylene-EPDM blends, butadiene, styrene-butadiene, nitrile rubber, chlorosulfonate, neoprene, urethane, polytetrafluoroethylene, polysulfide, silicone, or a copolymer, blend or combination of any of the foregoing polymers. Preferably, the elastomeric material is a fluorosilicone polymer reinforced with fibers, and particularly Kevlar fibers. The fibers provide increased toughness, tensile strength, and a high modulus of elasticity to the elastomeric sheet. 
         [0035]    A conductive mesh is embedded in the gasket during the gasket fabrication process. This can be accomplished using known techniques, for instance, by incorporating the mesh in a mold cavity prior to injection of the elastomeric resin in liquid form, and subsequently curing the elastomer to form the gasket material. This material can be in the form of a sheet, which can then be cut into the appropriate annular shape required for the particular gasket. Alternatively, the gasket can be formed in a mold cavity having the desired annular shape. 
         [0036]    The conductive mesh material of the gasket can be, for example, an expanded metal mesh or a metal wire screen or a metal-plated fabric sheet. Typically, the mesh sheet may be formed from metal or metal alloy wires or fibers, graphite or carbon fibers, or metallized or metal-coated or metal plated non-conductive woven or non-woven fabric, such as nylon fabric or nylon fibers. In general, the surface resistivity of the mesh sheet is less than about 0.1 Ω/sq. 
         [0037]    As used herein, the term “mesh” includes fabrics, cloths, webs, mats, screens, meshes and the like, which may be open, such as in the case of a screen, or closed, such as in the case of a fabric. 
         [0038]    The mesh can be inherently conductive if formed from a metal or metal alloy, graphite, carbon, etc., as wires, monofilaments, yarns, bundles, or other fibers or materials which are inherently conductive. Alternatively, the mesh can be non-conductive and rendered electrically-conductive by means of an applied coating, plating, sputtering, or other treatment of the electrically conductive material. Representative of the inherently electrically conductive materials include metals, such as copper, nickel, silver, aluminum, steel, tin and bronze, alloys thereof, such as Monel nickel-copper alloys, non-metals, such as carbon, graphite, and inherently conductive polymers, and plated or clad wires or other fibers such as one or more of copper, nickel, silver, aluminum, steel, tin, bronze, or an alloy thereof, e.g. silver-plated copper, nickel-clad copper, Ferrex® (Parker Chomerics, Woburn, Mass.), tin-plated copper-clad steel, tin-clad copper, and tin-plated phosphor bronze. Representative non-conductive fibers include cotton, wool, silk, cellulose, polyester, polyamide, nylon, and polyimide monofilaments or yarns which are plated, clad or otherwise coated with an electrically-conductive material which may be a metal mesh such as copper, nickel, silver, aluminum, tin, or an alloy or combination thereof, or a non-metal such as carbon, graphite, or a conductive polymer. The plating, cladding or other coating may be applied to individual fiber strands or to the surface of the fabric after weaving, knitting or other fabrication. Combinations of one or more of the foregoing conductive fibers and/or one or more of the foregoing coated non-conductive fibers may also be employed. 
         [0039]    Preferably, the conductive mesh of the invention is formed from a metal wire, such as aluminum wire, and most preferably from triple stranded metal wire, i.e. metal wire in which three strands of wire are combined into a single strand. It has been found that a metallic mesh formed from triple stranded wire is superior to single stranded wire mesh which may not be robust enough to withstand high stress environments, and can therefore fail prematurely. The triple stranded wire creates a stronger poly-filament wire mesh. In addition, triple stranded wire reduces the relative movement between the strands typically induced by vibration, thereby reducing gasket abrasion and prolonging gasket life. 
         [0040]    The various embodiments of the invention will now be more clearly described by reference to the drawings, which are illustrative only and not intended to be limiting in any way. 
         [0041]      FIG. 1  illustrates one embodiment of the EMI shielding gasket of the invention. As shown in  FIG. 1 , gasket  1  is positioned generally between aircraft fuel tank access port  2  and fuel tank access door  4 . Gasket  1 , shown in the uncompressed state, is divided into several regions or zones. These include inner region  6 , outer region  8 , and a central region positioned there between. The central region includes two peripheral regions, outer peripheral region  10  and inner peripheral region  12 , both adjoining intermediate region  14 . As shown, peripheral regions  10  and  12  are thinner than adjacent regions  8  and  6 . However, intermediate region  14  is thicker than the adjoining peripheral regions  12  and  14 . This configuration allows the gasket to be compressed without compromising its structural integrity. The intermediate region  14  is shown in the shape of a bulb or as a 3-dimensional annular rod having a cross-section as shown. Also depicted in  FIG. 1  is an optional rubberized tip element formed at the end of outer region  8 . The peripheral regions  10  and  12  are thinner than the remainder of the gasket, and this allows expansion of the gasket into this space, thereby preventing gasket failure. 
         [0042]      FIG. 2  illustrates another embodiment of the EMI shielding gasket of the invention. Gasket  18 , positioned between aircraft fuel tank access port  2  and fuel tank access door  4 , is shown in the uncompressed state. Gasket  18  is divided into two regions or zones. These include inner region  20  and outer region  22 . Outer region  22  has a thickness which is less than the thickness of inner region  20 . Affixed to the end portion of outer region  22  is a tip element  24  having a greater thickness than either inner region  20  or outer region  22 . Tip element  24  is fabricated from a rubberized material, and can be in the shape of a bulb or aqs a cross-section of a 3 dimensional annular rod. As gasket  18  is compressed, rubberized tip element  24  compresses and seals the gasket from outside fluids, and also prevents leakage of fuel from the fuel tank. 
         [0043]      FIG. 3  is a perspective view of gasket  1  depicted in  FIG. 1 .  FIG. 3  illustrates the different regions or zones of gasket  1 , including inner region  6 , outer region  8 , outer peripheral region  10 , inner peripheral region  12 , and intermediate region  14 . Intermediate region  14  is depicted in the shape of an annular rod, but one skilled in the art will readily appreciate that a variety of shapes are possible. 
         [0044]      FIG. 4  is a perspective view of the inner region or inner end portion of annular gasket  1  depicting a grommet  32  sealing the gasket around opening  30 . The grommet is preferably a molded plastic element positioned to seal or encapsulate the end section of the gasket, i.e. the section of the gasket surrounding interior opening  30 . 
         [0045]    The gaskets of the invention can be used in a variety of applications and under a variety of conditions. Preferably, the gaskets of the invention are used for sealing fuel tank access doors and panels to fuel tank access ports commonly used in wing-mounted fuel tanks for military and civilian aircraft. 
         [0046]    As it is anticipated that certain changes may be made in the present invention without departing from the precepts herein involved, it is intended that all matter contained in the foregoing description shall be interpreted as illustrative and not in a limiting sense. All references cited herein are expressly incorporated herein by reference thereto in their entirety.