Gasket for sealing electromagnetic waves filled with a conductive material

Abstract: An electromagnetic shielding gasket assembly includes a coil spring for blocking the propagation of electromagnetic waves therepast, with the coil spring means including a plurality of coils canted along a center line thereof. The plurality of coils are supported in an orientation for controlling the electromagnetic shielding effectiveness of the coil spring means, and a formable conductive material, disposed on said plurality of individual coils, is provided for being formed by the plurality of coils upon loading thereof. The formed conductive material fills an aperture of the coil spring to further block the propagation of electromagnetic waves.

Description: The present invention concerns an electromagnetic shielding spring assembly, including coils for blocking the propagation of electromagnetic waves therepast and having a conductive elastic material disposed around and between the coils and a coating for controlling the bonding therebetween. Heretofore, there has been no attempt to utilize an elastic or plastic in order to modify the force exerted by the spring assembly in response to deflection of the spring along a loading direction. Prior use of plastic material in combination with a canted-coil spring has been limited to the use of a canted-coil spring imbedded in an elastic material to provide a reinforcement therefor, totally sacrificing any load-deflection characteristics of the spring. It is well known that when an O-ring is loaded, the force developed thereby is generally directly proportional to the deflection; thus, in the case of U.S. Patent No. 3,183,010 to Bram, when the canted-coil spring is imbedded in the O-ring elastomer, it only provides in combination therewith a higher force which is directly proportional to the deflection. Thus, any advantage that a canted-coil spring may otherwise provide is sacrificed in this combination. It should be appreciated that cavities as hereinafter discussed may be linear or continuous with a desired shape, be it circular or otherwise. For as in circular cavities, the spring may have end coils welded forming a garter-type canted-coil spring. It is generally well known that electromagnetic transmission may be described by wave theory and, further, in terms of an electric and a magnetic field which travel in a 90° phase relationship with one another in free space. Wave theory describes electromagnetic energy in terms of wavelength and amplitude. It should be appreciated that electromagnetic energy is used here as a generic term including the full spectrum of electromagnetic energy frequencies. Generally speaking, electromagnetic interference (EMI) and radio frequency interference (RFI) are used to describe interference caused by unwanted electromagnetic energy or radio frequency energy entering or leaving a designated piece of equipment. The ability to shield a component from egress or ingress of electromagnetic energy is often called shield effectiveness. Microwave energy having frequencies from about 100 kHz to about 300 gHz is commonly referred to as microwaves. The microwave frequencies spectrum may be further divided into three general groups, which include commercial bands, 100 kHz to 10 gHz; military bands, 10 gHz to 50 gHz; and millimeter bands, 50 gHz to 300 gHz. Shield effectiveness of any particular frequency range is significantly affected by any gaps or openings therein which will allow electromagnetic fields to radiate therethrough unless current continuity can be preserved across the opening. The importance of the opening is, of course, dependent upon the frequency of the impinging electromagnetic energy. It should be understood that the shielding effectiveness is a measure of the efficiency of an EMI gasket measured in decibels (dB), the higher the shielding effectiveness, the more effective a gasket is. Such shielding effectiveness is affected by the opening between coils and an "aperture," as herein used, is defined as the maximum linear dimension (not area) of this opening. The greater the size of the aperture, the lower the shielding effectiveness. The aperture is the maximum shielding effectiveness. Two general types of spring gaskets disclosed in the present application include one having spacing between coils when loaded and another having coils butted at load. The butted coils provide the smallest aperture and thus the maximum shielding effectiveness. The length of the aperture in a spring gasket, as hereinafter disclosed, is affected by the wire diameter, the D/d ratio, and the spacing between coils. As a general rule, for effective shielding in commercial use, the opening size should be less than the wavelength of the electromagnetic energy divided by 20. For military bands, the opening size should be less than the wavelength divided by 50 and for the millimeter bands, the opening should be less than the wavelength divided by 200. Because of the dependence on the wavelength, the effectiveness of any gasket may be determined at a particular frequency the size of the openings or apertures therein. Heretofore available gaskets have been unable to provide dimensional stability throughout the life of equipment in order to prevent the formation of unwanted gaps therein which may be unacceptable in terms of shield effectiveness. This is especially true when the gaskets are utilized in situations where repeated disassembly and assembly are needed with the gasket relied on to prevent EMI or RFI between components requiring such assembly. EP 0 379 159 A1 describes an electromagnetic shielding gasket assembly, including coils for blocking the propagation of electromagnetic waves therepast and having a formable conductive member disposed adjacent to said coils for modifying a force exerted by said coils in response to deflection thereof, said formable conductive member being formed by said coils upon loading, said formable conductive member further blocking the propagation of electromagnetic waves upon loading of said coils. SUMMARY OF THE INVENTION In accordance with the invention a coating is disposed between said conductive member and said coils for controlling an adherence between said formable conductive member and said coils. With proper selection of the coating, which may be any conventional silicon release material or the like, depending on the elastomer, complete nonbonding of the elastomer or partial bonding of the elastomer to the coils may be achieved. When the individual coils are fabricated from a wire having a diameter, d, selected to achieve a desired shielding effectiveness, the shielding effectiveness increases with the increasing wire diameter. Additionally, when the individual coils are formed with the coil diameter, D, selected to achieve a desired shielding effectiveness, shielding effectiveness decreases with increasing D/d ratio. More particularly, the coil spring may have a turn angle greater than 0° and a means for supporting the plurality of individual coils includes groove means for increasing the turn angle of the circular coil spring to control the aperture of the circular coil spring through which electromagnetic waves may propagate. In one embodiment of the present invention, the means for supporting the plurality of coils includes means for causing the individual coils to abut upon one another. In this embodiment, when the individual coils are fabricated from wire having a diameter, d, selected to achieve a desired shielding effectiveness, the shielding effectiveness decreases with increasing wire diameter. Also, when the individual coils are formed with the coil diameter, D, selected to achieve a desired shielding effectiveness, the shielding effectiveness increases with the increasing D/d ratio. More particularly, the means for causing individual coils to abut may comprise a wedge which may be an integral part of the groove means or disposed within a groove and not fixed thereto. When sufficient formable means are provided, the electromagnetic shielding gasket assembly additionally functions as a low pressure seal. BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention will appear from the following description and considered in conjunction with the accompanying drawings in which: Figure 1 is a theoretical load vs. a deflection curve illustrating the various parameters of an electromagnetic shielding gasket; Figure 2 is an axial or radial load deflection curve for a typical axially resilient electromagnetic coil spring; Figures 3a and 3b are partially cutaway perspective views of a spring assembly in accordance with the present invention generally showing a plurality of coil means disposed within an elastic material; Figure 3c is a cross-sectional view of the spring assembly shown in Figure 40b with a fugative material therein which is later removed in accordance with the method of the present invention; Figure 3d is a round spring embodiment of the present invention filled with a conductive elastomer with a hollow core; Figure 4 shows an axial spring with an irregular shape in accordance with the present invention having a hollow interior DETAILED DESCRIPTION A resilient electromagnetic shielding gasket in accordance with the present invention may be described, in part, by its load-deflection characteristics. Depending on its application, a number of load-deflection characteristics may be utilized; however, of particular use is a gasket which exerts a generally constant force in response to deflection of the gasket. This feature enables the gasket, to accommodate variations in mating surfaces between which electromagnetic shielding is to be provided. Such variations may be due to surface irregularities, tolerances, or temperature and pressure effects. The shielding gasket provides a stable, long-lived gasket capable of shielding against electromagnetic radiation in a consistent manner over time, despite temperature and pressure effects, and/or continued assembly and disassembly of mating parts. For the purposes of describing a desirable load-deflection characteristic, an exemplary load-deflection curve 10 is shown in Figure 1. As can be seen when a load is applied to the gasket, it deflects in a generally linear fashion as represented by the line segment 12 until it reaches a minimum load point 14 which represents the point at which, after the initial deflection, the load remains relatively constant and relatively independent of gasket deflection. It should be appreciated that for an axially resilient gasket, hereinafter described, the load is applied axially and for a radially resilient gasket, hereinafter described, the load is applied radially. Between the minimum load point 14 and a maximum load point 16, the load-deflection curve may be constant or show a slight increase as shown in Figure 1. The area between the minimum load point 14 and the maximum working load point 16 is known as a working deflection range 18. The gasket is normally loaded for operation within this range, as indicated by the point 20, for a typical gasket for electromagnetic shielding purposes. In addition, the gasket also may provide sealing between the mating surfaces. Loading of the gasket beyond the maximum load point 16 results in an abrupt deflection response until it reaches a butt point 22, which results in a permanent set in the gasket as a result of overloading. Also indicated ind Figure 1 is the total deflection range 24, which is defined as the deflection between the unloaded gasket ad the deflection of the maximum working load point 16. A gasket, as hereinafter identified, may assume many forms. For example, it may be linear, have any irregular shape, or be interconnected to form a round, elliptical or other continuous shape, as hereinafter discussed. Figure 2 shows a specific load-deflection curve for a gasket. In this instance, the spring is made from wire having a circular cross-section with a diameter of 0.022 inches, a coil height of about 0.16 inches, a back angle of between about 13 and about 15°, a front angle between 29 and 31°, and coil spacing of between about 0.19 inches and 0.17 inches. Turning now to Figures 3a , 3b and 3c , there is shown a spring assembly 510a, 510b, in accordance with the present invention, which includes a plurality of coils 512a, 512b interconnected with one another in a spaced-apart relationship. The spring 514 may be assembled in order to exert a generally constant force in a loading direction, normal to a tangent to a center line 516, as hereinabove described. A conductive elastic material 518, 518b, such as that hereinabove described, may be disposed around and between the plurality of coils 512 and may include a hollow cross-section 520 which provides means for modifying the force exerted by the spring assembly 514 in response to deflection of the spring assembly 510b along a loading direction as hereinafter described in greater detail. As shown in Figure 3d, the spring assembly 514 may include circular coils, not canted to a center line thereof, or as hereinafter described, canted coil assemblies. The spring 510a is linear and may have ends attached to form the spring assembly 510b which may be circular as shown or have any other shape which may be useful in forming a gasket-type seal around a rectangular opening, for example. The conductive elastic, or elastomer, material 518a, 518b, may be of any suitable type plastic or the like, with resiliency suitable for the overall design objectives of the spring assembly 40a, 40b. Before application of the elastomer 518a, 518b, to the coils 512a, 512b, a coating 522 is applied to the coils 512a, 512b, in corder to control the bonding of the elastomer 518a, 518b thereto, as hereinabove noted. With proper selection of the coating, which may be any conventional silicon release material or the like, depending on the elastomer, complete nonbonding of the elastomer or partial bonding of the elastomer 518a, 518b, to the coils 512a, 512b may be achieved. However, any method for controlling the bonding of an elastomer to a metal, as may be well-known in the art, may be utilized. For example, many elastomers, such as Silastic 732 RTV adhesive sealant from Dow Corning of Midland, Michigan, requiring a metal surface to be primed with a suitable material to obtain adherence thereto. Consequently, if the coils 512 are not primed, or partially primed, some bonding or partial bonding will occur. While the spring assembly 510 shown in Figure 3b has a generally circular shape, it should be appreciated that any irregular shape such as the spring assembly 650 shown in Figure 4 as long as the cross-section thereof is hollow in cross-section. Alternatively, the spring 512 may not be joined, for example, as shown in Figure 3a . In this instance, the length of elastomer-coated, or filled, coils may be used in applications suitable for linear springs. The spring 514 may be filled by any manufacturing method suitable for the elastomer employed, such methods including extrusion, molding, spraying or any other suitable method for introducing elastomer 518 in and around the coils 512 of the spring 514, while leaving a hollow cross-section 520 along the center line 516. Such other methods may include the fusing of two tubular parts (not shown), one disposed on the inside of the spring 514 and one disposed on the outside of the spring 514 with the fusing of the parts filling of the spaces between the coils 512. Alternatively, the conductive elastomer 518 may be formed in and around the coils 512 while a rod 651 of fugative compound may be disposed within the coils 512 (see Figure 3c). Thereafter, the fugative compound may be dissipated or removed by heat or solution, as is well-known in the art. It is to be appreciated that when a canted-coil spring is filled with an elastomer, especially in an outer portion of the coil, the force applied thereon is transmitted more uniformly and the stress acting on the mating parts is substantially lower. In applications where the canted-coil spring assembly 510 is used in connection with the sealing device, it provides substantially better sealing ability and results in a substantial increase in the seal performance and seal life. The ability of the hollowed canted-coil spring filled with an elastomer having a hollow center to achieve a high degree of deflection and maintain a relatively constant load within a high deflection decreases the total amount of force that is needed to affect sealing and this offers a significant advantage, especially in dynamic situations. The ability of the hollowed elastomer-filled spring to stretch facilitates its assembly in a single groove construction, similar to that as can be found in O-ring applications for both mounting the assembly 510 in a piston (not shown) or in a housing (not shown). Another very important feature of the elastomer-filled spring 514 with hollow cross-section 520 is its use as a seal itself. Heretofore, elastomers have used the canted-coil spring as a loading means in order to maintain intimate contact between the sealing surfaces of the elastomer. In many prior art applications, the elastomer tends to age or relax due to gaining stiffness of the elastomer, or due to temperature cycling, and the spring is used to force the elastomer in contact with the sealing surfaces. In the present invention, by combining the elastomer 518 within the spring 514 and providing a hollow cross-section 520, both items can be accomplished in one, with a higher degree of reliability and sealing ability, and this can be done in reciprocating, rotating, oscillating, and static applications. Another important feature is to use the spring assembly 510 as a loading means to provide more uniform loading on a sealing surface (not shown) which results in substantially better performance because it enables the use of a smaller wire, thus uniformly distributing the load which results in more effective sealing ability and longer seal life. In all these applications, the elastomer 518 may be any type of elastomeric material of formable composition, such as silicone, Buna N, nitrile, fluorosilicones, PTFE, elastomers, etc. Also certain types of plastics that have the properties of elastomers can be used due to their high coefficient of friction, their operating temperature, etc. Conductive filters for the elastomer 518 have been hereinbefore described and include both metallic and non-metallic materials. Although there has been described hereinabove a specific arrangement of an electromagnetic shielding gasket assembly in accordance with the present invention, for the purposes of illustrating the manner in which the invention may be used to advantage, it should be appreciated that the invention is not limited thereto. Accordingly, any and all modifications, variations, or equivalent arrangements which may occur to those skilled in the art should be considered to be within the scope of the invention a defined in the appended claims.

Claims: A spring assembly (510a, 510b) for blocking the propagation of electromagnetic waves, including a plurality of coils (512a, 512b) interconnected with one another in a spaced-apart relationship and having a conductive elastic material (518a, 518b) disposed around and between the plurality of coils (512a, 512b) modifying a force exerted by the spring assembly (510a, 510b) in response to deflection thereof, characterized by a coating (522) applied to the plurality of coils (512a, 512b) for controlling bonding between the conductive elastic material (518a, 518b) and the plurality of coils (512a, 512b). The spring assembly of claim 1, characterized in that the coating (522) primes the plurality of coils (512a, 512b) to obtain adherence between the conductive elastic material (518a, 518b) and the plurality of coils (512a, 512b). The spring assembly of claim 1, characterized in that the conductive elastic material (518a, 518b) fills the plurality of coils (512a, 512b). The spring assembly of claim 1, characterized in that the conductive elastic material (518a, 518b) has a hollow cross-section (520).

Inventors: BALSELLS PETER J, BALSELLS, PETER J.
Applicants: BALSELLS JOAN C, BALSELLS PETER J, BALSELLS, JOAN C., BALSELLS, PETER J.
Application year: 1992
Main classification: H01R13
IPC codes: H05K9, F16F1