PRE-CAST SEALANT DOME AND METHOD

Abstract: A pre-cast dome-like shell (20) is loaded with a predetermined amount of curable sealant (31). The sealant and dome are similar in composition. The dome (20) is pressed over the exposed end o fa fastener (10). An elastic portion (24) of the shell can expand, thereby increasing the internal volume of the shell (20) to accommodate small variations in sealant (31) amount. A close fit (31) between the shell and fastener, combined with an adequate shell wall, insure proper shell orientation and barrier thickness.

Description: This invention pertains to the protective sealing of fasteners, and more particularly to using an adhesive sealant with a pre-cast dome which is made from the same sealant material, thereby electrically insulating and otherwise protecting the fasteners. Fasteners used in the manufacture of aircraft are exposed to stress, corrosive fluids and, potentially, to electrical current. Owing to these factors, polymeric sealants have been applied to aircraft fasteners. One rather elaborate method is disclosed in U.S. Patent No. 2,710,113, issued to P.J. Prichard on June 7, 1955. The method in this patent involves the deposition of a first rubber-like sealant layer over the fasteners followed by the application of a dome having a hole in it over the first layer of sealant. Excess sealant and air is said to exit the hole, whereupon another layer of sealant is applied. In actuality, unwanted air was known to have been retained by this type of method. Some methods involve the injection of sealant into a mold or into the space formed between a dome and a fastener. Other methods require precise metering of sealant either around fasteners prior to dome placement, or into a dome before placement over fasteners. Such known prior art methods are either costly, impractical or inconsistent in the quality of the result. The present invention is addressed to overcoming the deficiencies of the prior art fastener sealing methods and products. Furthermore, US-A-4 519 974 discloses a method and apparatus for applying a uniform sealant coating over a fastener. This document further discloses to apply a shell and a sealant over an exposed portion of the fastener and the retaining shell as brought into contact with the panel such that the uncured sealant is introduced between the shell and the exposed portion of the fastener and the panel. The sealant is allowed to cure and thereafter the shell is removed leaving a uniform sealant coating over the fastener's exposed end. SUMMARY OF THE INVENTION The present invention is directed to a protective and insulating seal for a fastener which protrudes from a workpiece, according to claim 1. Furthermore, the present invention is directed to a method for installing a pre-cast protective cover to a fastener on a workpiece. The seal includes a pre-cast continuous and uninterrupted shell having a base portion defining a mouth opening into a central cavity. The shell or dome may be filled with a curable sealant which is made from the same material as the shell. The shell is dimensioned so that it may be placed over the exposed portion of a fastener extending from a workpiece with the final thickness of the sealant and shell meeting the specifications required for thickness of sealant over an exposed fastener. The shell is provided with a top portion having a reduced thickness which may expend slightly upon application of pressure on the shell as it is seated over the fastener. This expansion results in a slight increase in volume of the cavity within the shell thereby permitting excess sealant to be accommodated within the shell. The present invention provides a protective and insulating seal for fasteners which is economical and practical and which requires less employee training than prior art methods. Similarly, the present invention provides a faster installation which does not require injection equipment or complex tools. The invention provides a structurally and chemically stable protection to a fastener which does not retain air and an insulation which automatically conforms to protective thickness specifications. The sealant dome of the present invention provides a professional appearance to the finished product while minimizing squeeze out of excess sealant. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective view showing the pre-cast dome of the present invention prior to installation on a fastener, and FIGURE 2 is a partial section view showing the fastener with the protective pre-cast dome in place. DESCRIPTION OF THE PREFERRED EMBODIMENT FIGURE 1 illustrates an aircraft fastener 10, used to attach several sheets 30 together, with a pre-cast shell-like elastomeric dome 20 positioned prior to installation. The fastener includes a threaded bolt 11, a nut 12 and in the illustrated example, two washers 13, 14. Bolt 11 has a head 15. FIGURE 2 illustrates the pre-cast dome or shell in its installed position over the end of fastener 10 which extends to one side of one of the sheets 30. Dome 20 surrounds the threaded end of the fastener and seats against sheet 30. The pre-cast dome is an oven cured or accelerated polymer. This results in improved toughness and abrasion resistance. A bevel 21 is formed on the inside edge of the mouth of dome 20 which defines an interior cavity 25. In the final installed position, a predetermined gap 23 is provided between the inside surface of the dome and the fastener. The sidewall thickness of the dome is also predetermined so that even in the event that the dome is placed in the position of maximum eccentricity with respect to the fastener, the total minimum protective barrier thickness still meets the required specification. For nearly al fastener sizes, a .025 inch (.635 mm) clearance between the fastener and the inside surface of the dome cavity is used. The final wall thickness of the assembly is generally 0.25 inches (6.35 mm) measured from the outermost dimension of the washers, and 0.12 inches (3.05 mm) measured from the top of the fastener. The dome is provided with a top portion 24 of reduced thickness, the purpose of which will be explained below. For a #8 fastener, the base diameter is 0.85 inches (21.59 mm), the height of the dome is 0.75 inches (19.05 mm) and the reduced portion 24 is 0.05 inches (1.27 mm). An air-free sealant layer 31 occupies the space between the dome and the fastener. A 1058 type 2B2 sealant conforming to MIL SPEC 8802 is known to be a suitable sealant. Test trials have shown that sealants having a designated B2 viscosity provide optimum adherence without being too runny or promoting air entrapment. The structural integrity of the final assembly is enhanced by casting the dome from the same sealant. A pre-cast cap 41 covers the head 15 of the fastener, and a sealant layer 42 occupies the space between head 15 ad cap 41. As a result of the method of installation, a small bead of sealant 43, 44 may be formed around both the dome and cap. The preferred method for applying the pre-cast dome of the present invention begins with the cleansing of the fastener with an abrasive pad and MEK solvent. Once clean, an adhesion promoter such as PR148 is applied to and around the fastener. A brush coat of 1057 class A1/2 sealant is then applied to the fastener and allowed to dry for about 30 minutes. A fine adhesive pad is then used to roughen up the inside of the dome to be applied. The dome 20 is then cleaned with MEK solvent, inside and out. Type PR148 adhesion promoter is applied to the inside and base of the dome and allowed to dry for 15 minutes. Sealant (1048 type 2B2) is injected into the dome cavity. The sealant has a viscosity which promotes an air-free layer between the fastener and dome while not being runny. The amount of sealant is predetermined, in accordance with the squeeze-out that is desired. In general, the sealant amount is equal to or slightly more than the void volume between the seated dome and the fastener. A small amount of squeeze-out can usually be tolerated. For a #8 fastener dome weighing 3.3 grams, 2.5 grams of salant is adequate. The dome is then pushed over the fastener and held in place for five seconds, while turning alternately clockwise and counterclockwise. Turning promotes distribution of sealant evenly, particularly around the base of the dome. After 2 to 4 minutes, the dome is again pressed and rotated. As is seen in FIGURE 2, dome 20 has a reduced thickness top section 24 which is thin relative to the remaining dome side wall thickness. In the preferred embodiment shown, this reduced thickness permits the dome to expand slightly upon application of pressure on the dome as it is seated over the fastener. This expansion results in a slight increase in volume of the cavity thereby permitting excess sealant to be accommodated within the dome. In the preferred embodiment shown, the reduced thickness section is disposed about the axis of symmetry of the dome. The method of applying cap 41 is substantially the same as that used to apply dome 20. Although cap 41 is not shown with a reduced thickness portion, it will be understood that it may be so designed to provide the benefit described above with respect to dome 20. Thus, the present invention provides a protective and insulating seal for a fastener used in aircraft and other manufacturing where a sealant must be applied over the fastener. In the present invention, the seal includes a continuous and uninterrupted dome or shell having a mouth at the base thereof which defines an opening to a central cavity within the shell. The shell has a sidewall with a portion thereof having a reduced thickness when compared to the remainder of the shell. In the preferred embodiment, the portion of the reduced thickness is disposed about the axis symmetry of the fastener. This reduced thickness permits the dome to expand slightly upon application of pressure on the dome as it is seated over the fastener. Thus, the internal volume of the dome changes from a first or resting volume, to a second or expanded volume. This increase in volume of the interior cavity thereby permits excess sealant to be accommodated within the dome and thereby reduces the amount of material which is squeezed out during use of the present invention.

Claims: A protective and insulating seal for a fastener (10) which protrudes from a workpiece (30), the seal comprising: a continuous and uninterrupted shell (20) having a base portion surrounding an opening into a central cavity (25) characterized in that, said continuous and uninterrupted shell (20) having a portion of reduced wall thickness (24); a curable sealant (31) for adhering the shell (20) over the fastener; and wherein the portion of reduced wall thickness (24) is integral with the shell (20) and is deformable from a first position to a second position, whereby the volume of the cavity (25) changes from a first volume to a second volume. The seal according to Claim 1, characterized in that the portion of reduced wall thickness is disposed about the axis of symmetry of the fastener. The seal according to Claim 1, characterized in that the shell is an accelerated polymer. The seal according to Claim 1, characterized in that the curable sealant (31) and the shell (20) substantially are of the same material composition. The seal according to Claim 1, characterized in that the shell (20) further comprises a bevel (21) leading into the central cavity (25). The seal according to Claim 1, characterized in that the curable sealant (31) has a viscosity to promote an air-free layer between the fastener and the shell (20) without being runny. The seal according to Claim 1, characterized in that the second volume of the cavity (25) is greater than the first volume. The seal according to Claim 1, characterized in that the interior dimensions of the cavity (25) closely conform to the dimensions of the fastener covered by the shell (20) and when the base of the shell (20) is flush with the workpiece, a minimum predetermined protective thickness is provided by the shell, irrespective of the misalignment of the shell with respect to the fastener. The seal according to Claim 1, characterized in that the shell (20) is pre-cast. A method for installing a pre-cast protective cover to a fastener (10) on a workpiece (30) characterized by establishing a minimum protective thickness required around the fastener; selecting a continuous and uninterrupted shell (20) having a flat base, lateral walls, an elastically resilient end portion (24) of reduced wall thickness and a central cavity (25), the lateral walls providing substantially all of the required protective thickness, the cavity dimensions closely conforming to the dimensions of the fastener and the end portion (24) being deformable from a first position to a second position; metering an amount of curable sealant (31) into the cavity (25) for adhering the shell (20) over the fastener, the amount of curable sealant (31) being substantially equal to or slightly more than the void volume in the cavity (25) when the shell (20) covers the fastener and the flat base rests flatly against the workpiece; and pressing the shell (20) containing the sealant over the fastener thus deforming the end portion (24) from the first position to the second position. The method according to Claim 10, characterized by further comprising the step of applying an adhesion promoter around the fastener prior to pressing on the shell (20). The method according to Claim 10, characterized by further comprising the step of applying an adhesion promoter to the cavity (25) and base prior to metering the sealant (31) into the cavity (25). The method according to Claim 10, characterized in that the step of pressing the shell (20) over the fastener includes rotating the shell (20) with respect to the fastener. The method according to Claim 10, characterized by further comprising the step of allowing the sealant to cure while the shell (20) is in the deformed state to adhere the shell (20) over the fastener. The method according to Claim 10, characterized by further comprising the step of curing the shell (20) prior to use by passing the shell (20) through an accelerated cure.

Inventors: HENRY, WINSTON L.
Applicants: VOUGHT AIRCRAFT COMPANY
Application year: 1989
Main classification: F16B33
IPC codes: B64D45, F16B33