Coiled spring electromagnetic shielding gasket

Abstract: An electromagnetic shielding gasket (210) is provided which includes a coil spring (212) for blocking the propagation of electromagnetic waves therepast. The coil spring (212) includes a plurality of individual coils (214) which provide shielding of electromagnetic waves independent of compression of the coil spring (212) within a range of deflection of the individual coils (214).

Description: The present invention generally relates to shielding gaskets for interfacing electronic/electromagnetic components, such as filters, attenuators, connectors, circuit boards and other electronic or non-electronic equipment which requires shielding or grounding of such components in order to prevent interference or damage by electromagnetic radiation. More particularly, the invention relates to electrically conductive gaskets for enabling the establishment of component interface without the transmittal or leakage of electromagnetic energy therepast independent of loading forces on the gasket. It is well known that modern electronic equipment is typically produced in modular form where discreet components or units of electronic equipment are inter-connected by cables or the like and where such components themselves are comprised of discreet sub-systems which may be interchanged or replaced for servicing. It is important to realize that in many, if not most instances, all of the modular units as well as the sub-system components are preferably isolated from electromagnetic emanations from either other sub-components within the equipment or from exterior sources. Inefficient coupling of components results in both the loss of operation efficiency, due to the energy loss, and also the interference with effective operation of other associated electronic components. The problems associated with unwanted electromagnetic energy leakage and interference has been known for some time. The problems is particularly acute for components which must be repeatedly interconnected and disconnected, either to vary the operational characteristics of the equipment, provide for mobility of the equipment, or for maintenance. In these situations, the mechanical and electrical coupling of components many times includes a gasket or the like surrounding critical electromagnetic transmission lines or equipment which is comprised of a conductive elastomer or a combination of elastomeric materials in any number of configurations. A common type of gasket now being used in such applications is a conductive elastomer, which consists of an elastomeric material, silverplated aluminum composites or similar materials that have a high degree of conductivity, for the purpose of providing effective shielding. Other types of gaskets now in use consist of metallic or metallic combinations in the form of mesh strips in round or rectangulasr shapes; braided wire in a round or rectangular tubular shape; expanded metals; circular, round strips, etc. These various materials are made from metallic materials or filled with elastomeric materials to provide a higher degree of resiliency. WO-A-8 701 901 discloses a device for achieving a seal between the edge portions of two metal plates which are used as screens against electromagnetic fields. The device includes a metallic sealing strip which is intended to be compressed along these edge portions. The sealing strip comprises a helically wound metal element with a cornered cross section, and the edge portion of one metal plate is intended, by engaging against a support, to be pressed against the sealing strip both in the cross-sectional and in the longitudinal direction of the strip, so that the turns of the helically wound metal elements are twisted in the longitudinal direction of the strip in relation to the original positions. Parts of the cornered sides of the metal element will thus be pressed against said edge portion, which gives good metallic contact. US-A-4 441 780 discloses a plug and receptacle connector having a hollow cylindrical shell of such internal diameter as to permit its receipt onto and over a cylindrical receptacle shell during mating. A groove formed circumferentially about the inner wall of the plug includes a pair of intertwined continuous helically wound springs secured within the groove which wipes along the external wall of the receptable as it is fitted within the plug. At crossover points the springs, they are bonded one to another. US-A-3 505 463 discloses a configuration of electrical conducting, flexible wires or members arranged to have a height, width and length. The barrier material being inserted between two conducting surfaces forming portions of an RF energy barrier enclosure. While many, if not all, of the hereinabove identified gasket configurations provide electromagnetic shielding to a certain extent, each of them have the disadvantage of permanent deformation thereof upon loading. That is, they take a "set" upon loading and thereafter upon a change in such loading create a gap, or separation, which allows the transmission of electromagnetic energy therepast. The hereinabove identified disadvantage described in general terms may be illustrated by a specific example of a screw-type microwave connector which includes an elastomeric gasket which is deformed upon coupling the connector to a component by mechanical threads or the like. Upon either repeated, or intermittent use, the elastomeric member takes a "set" and therefore does not provide reliable electromagnetic shielding upon repeated coupling and uncoupling of the connectors. This problem becomes more acute when the gasket is subjected to thermocycling. Repeated heating and cooling of the gasket may cause significant relaxation, thereby leaving openings between normally coupled components through which electromagnetic energy can pass. through, even though the gasket may not be subject to loading changes due to repeated coupling and uncoupling. With reference to shielding gaskets, including elastomeric components has been made, it is also well known that other metallic materials such as braided wire, woven gaskets, circular strips or expanded metal, take a set upon loading, are typically very rigid and do not accommodate well to small diameters or irregularities, therefore, enabling the leakage of electromagnetic energy therepast. The irregularities hereinbefore identified include mating components which have surface deformations or surface finishes which do not intimately engage one another, the latter relying on a gasket for providing electrical continuity to a degree necessary to provide electromagnetic shielding for a particular electromagnetic energy frequency. Regarding the frequency of electromagnetic energy, in particular, microwave bands for commercial service applications generally range from about 100 MHz to about 1 GHz, while military service applications may range from 1 GHz to 300 GHz. Electromagnetic energy as used herein is a generic term including the full spectrum of electromagnetic energy frequencies and specifically as used hereinafter, electromagnetic interference (EMI) and radio frequency interference (RFI) may be used interchangeably, with both referring to interference caused by unwanted electromagnetic energy or radio frequency energy, entering a designated piece of equipment. In general, the ability to shield a component from egress or ingress of electromagnetic energy is often called shield effectiveness. A factor of prime importance in electromagnetic shielding is the frequency of the electromagnetic energy or the wave length thereof. It is known that all electromagnetic waves consist of two essential components, a magnetic field and an electric field. These two fields are perpendicular to each other, and the direction of wave propagation is at right angles to the plane containing these two components. The relative magnitude between the magnet (H) field and the electric (E) field depends on how far away the wave is from its source, and on the nature of the generating source itself. The ratio of E to H is called the wave impedance, Z w . Thus, it can be seen that shield effectiveness for a particular gasket may differ, depending on whether the electromagnetic energy is generated in associated equipment or in equipment remote from the gasket. If the source contains a large current flow compared to its potential, such as may be generated by a loop, a transformer, or power lines, it is called a current, magnetic, or low impedance source, having a small E to H value. On the other hand, if the source operates at a high voltage, and only a small amount of current flows, the source impedance is said to be high, and the wave is commonly referred to as an electric field. It is important to realize that at very large distances from a source, the ratio of E to H to equal for either wave, regardless of its orientation. When this occurs, the wave is said to be a plane wave, and the wave impedance is equal to 377 ohms, which is the intrinsic impedance of free space. It is known that metals have an intrinsic impedance that approaches zero as conductivity approaches infinity. Because of the great difference between the intrinsic impedance of metals and the intrinsic impedance of free space, waves generated from sources remote from the metal receptor will have most of the energy reflected therefrom and very little will be transmitted therepast. This, of course, is not true for magnetic or low impedance fields, in which less energy is reflected, and more is absorbed thereby making it more difficult to shield against magnetic fields. Because magnetic shields have less than infinite conductivity, part of the field is transmitted across the boundary which is dependent upon the thickness of metallic shield. A far greater factor in shield effectiveness is the presence of openings or gaps in the shield. A gap or opening in the shield will allow electromagnetic fields to radiate through the shield unless the current continuity can be preserved across the gaps. Therefore, the function of an EMI gasket is to preserve the continuity of current flow in the shield. The importance of the gap is, of course, dependent upon the frequency of the impinging electromagnetic energy. For example, electromagnetic energy having frequency of one GHz has a wavelength of approximately 29,46 cm (11.6 inches), while electromagnetic energy of 100 GHz has a wavelength approaching 3 mm (0.12 inches). 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 and for avionics use the opening size should be less than 1/50th of the wavelength of the electromagnetic energy. Other factors bearing directly on the gap size and therefore shield effectiveness is the surface finish of mating parts to be sealed, the ability of the shielding material to withstand environmental changes with no or little changes in conductivity, due to corrosion galvanic action, etc. The ability of the gasket to provide dimensional stability with a constant load throughout the life of the gasket between the mating parts is important in order to prevent changes in the continuity of the gasket and to prevent the opening of gaps therein, which may be unacceptable in terms of shield effectiveness. The gasket of the present invention provides an effective electromagnetic shield through the use of a canted-coil spring with closely spaced coils that deflect upon loading, for providing a nearly constant force between mating points, and/or surfaces, in order to assure a high degree of conductivity, and therefore a high shield effectiveness within useful temperature and cycling requirements. In addition, the gasket of the present invention provides sufficient flexibility to take up for variations that may occur due to torquing, eccentricities, irregularities and other variables, and still maintain the required loading and open low area for effective shieldings from very low frequencies to extremely high frequencies. SUMMARY OF THE INVENTION In accordance with the present invention, an electromagnetic shielding gasket includes coil spring means for blocking the propagation of electromagnetic waves therepast with the coil springs means including a plurality of individual coil means for causing the coil spring means to block the propagation of electromagnetic waves therepast independent of compression of the coil spring means within a range of deflection of the individual coil means. The individual coil means are interconnected with one another and disposed in a canted relationship with the centerline of the coil spring means. Because the blockage of electromagnetic wave propagation is independent of the compression of the coil spring means, the gasket enables the mating of irregular surfaces and is independent of spacing variations between the mating surfaces due to temperature effects. More particularly, the electromagnetic sealing gasket, in accordance with the present invention, takes the form of a continuous coil spring when the individual coil means are joined together. Additionally, inner coil spring means may be provided which are disposed within the coil spring means for blocking the propagation of electromagnetic waves there-past. The inner coil spring means includes a plurality of individual inner coil means for causing the inter-coil spring means to block the propagation of electromagnetic waves therepast independent of compression of the inner coil spring means within a range of deflection of the individual inter coil means. The individual inner coil means may be interconnected with one another and disposed in a canted relationship with a centerline of the canted coil spring means and further may be canted in an opposite direction as the coil means or in the same direction as the coil means. More particularly, an electromagnetic shielding gasket in accordance with the present invention may include garter-type axially resilient coil spring means for blocking the propagation of electromagnetic waves therepast with the said coil spring means comprising a plurality of individual coil means for canted along a centerline thereof and comprising back angle means for both defining the disposition of a trailing portion of each coil means with respect to a line normal to the centerline and for determining the force-deflection characteristics of the coil spring means. Front angle means may be provided for defining the disposition of a leading portion of each coil means with respect to the normal line and said front angle means is greater than said back angle means. The coil means may be interconnected in the manner forming a garter-type axially resilient coil spring with the trailing portion along an outside diameter of the garter-type axially resilient coil spring and the leading portion along an inside diameter of the garter-type axially resilient coil spring. In another embodiment, the electromagnetic shielding gasket in accordance with the present invention may include radially resilient canted coil spring means for blocking the propagation of electromagnetic waves therepast which comprise a plurality of individual coil means canted along the centerline thereof and including back angle means for defining the disposition of a trailing portion of each coil means with respect to a line normal to the centerline and front angle means for defining the disposition of a leading portion of each coil means with respect to the normal line. Means are provided orienting the plurality of coil means at a turn angle for defining the load-deflection characteristics of the radially resilient canted coil spring means with said turn angle being greater than 0 degrees and less than 90 degrees. BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention will appear from the following description when considered in conjunction with the accompanying drawings in which: Figure 1 is a theoretical load versus deflection curve illustrating the various parameters of a resilient coil spring in accordance with the present invention; Figures 2a and b are plan and side views, respectively, of a circular welded clockwise wound spring with a back angle defining a trailing portion along the outside diameter of the spring and a front angle defining a leading portion along the inside diameter of the spring, in accordance with the present invention. Figure 3a and b are plan and side views, respectively, of a circular counter-clockwise wound spring having the same physical dimensions as the spring shown in figure 2a, 2b, also having a back angle defining a trailing portion along the outside diameter of the spring and a front angle defining a leading portion along an inside diameter of the spring; Figure 4 is a load versus deflection curve for the springs shown in Figures 2 and 3; Figure 5 shows load deflection curves for a variety of axially loaded counterclockwise springs with the trailing portion along the outside diameter thereof corresponding to the spring dimensions set forth in table 1; Figure 6 shows load deflection curves for axially springs having different back angles; Figure 7a and b are plan and side views, respectively, of a circular welded clockwise spring, in accordance with the present invention, with a front angle on the outside of the spring and a back angle on the inside of the spring; Figure 8 is a perspective view of an electromagnetic shielding gasket generally including an annular axially resilient coil, in accordance with the present invention, generally showing a plurality of coils interconnected in a manner forming a garter-type axially resilient coil spring, with the spring being disposed in an annular seal with a preselected turn angle orientation for controlling a resilient characteristics of the annular axially resilient coil spring, non-invasive support of the spring within the annular seal enabling independent working of the spring therein, thereby providing preselected force concentration on the sealing portions of the seal; Figure 9 shows load deflection curves for springs made in accordance with the present invention. Figure 10 is a schematic of an axially canted coil spring with turn angle ϑ shown for the purpose of illustrating how the turn angle ϑ may be calculated; Figures 11a, b, c and d and e illustrate axial springs having various turn angles; Figure 12 is a plurality of force-deflection curves, each corresponding to an annular axially resilient coil spring having different turn angle orientation; Figure 13 is a perspective view of an electromagnetic shielding gasket including a radially loaded coiled spring seal, in accordance with the present invention, generally showing a plurality of coils interconnected in a manner forming a radially resilient canted coiled spring, with the spring being disposed in an annular seal with a preselected turn angle orientation for controlling the load-deflection characteristics of the radially resilient canted coiled spring, non-invasive support of the spring within the annular seal enabling independent working of the spring therein, thereby providing pre-selected force concentration on the sealing portions of the seal; Figure 14 shows test jig apparatus for the determination of the load deflection characteristics; Figure 15 is a cross-section view of an electromagnetic gasket of the present invention disposed in an RF connector; Figure 16 is an enlarged cross-section view of the gasket shown in Figure 13 in an unloaded state; Figure 17 is an enlarged cross-section view of the gasket shown in Figure 13 in a load state; Figure 18 is a cross-section view of another electromagnetic shield gasket application showing the mounting of a card in a frame; Figure 19 is a diagram showing the effect of spring deflection on open areas of a canted coil spring; Figure 20a and b are side views of a coil spring showing the dependence of open area size on the coil back angle; Figure 21a and b are views of an electromagnetic shield gasket having two canted coil springs in an unloaded state; Figure 22a and b are views of an electromagnetic shield gasket having two canted coil spring in a loaded state; Figure 23a and b are views of an electromagnetic shield gasket having two coil springs with interstitial areas filled with an elastomer; and Figure 24a and b are views of an electromagnetic shield gasket having a coil spring and a central rod or tube. DETAILED DESCRIPTION Turning now to Figure 1 there is shown an exemplary load-deflection curve 10 for the purpose of illustrating the characteristics of canted coil resilient coiled springs suitable for electromagnetic shields gasket in accordance with the present invention. As shown in Figure 1 when a load is applied to the spring, the spring deflects in a generally linear fashion as shown by the line segment 12 until it reaches a minimum of load point 14 which represents the point at which, after the initial deflection, the load begins to remain relatively constant. It is to be appreciated that for an axially resilient garter-type spring, hereinafter described, the load is applied axially and for a radially resilient garter-type spring, hereinafter described, the load is radially applied. 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 maximum load point 16 is known as the working deflection range 18. The spring is normally loaded for operation within this range, as indicted by point 20,for a typical spring utilized in conjunction with a seal, gasket, or the like, for both sealing and electromagnetic shielding purposes. Loading of the spring 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 spring as a result of overloading. Also indicated in Figure 1, is the total deflection range 24, which is defined as the deflection between the unloaded spring and the deflection at the maximum load point 16. Figures 2a and 2b show a circular welded spring 30, in accordance with the present invention, generally showing a plurality of coils 32 wound in a clockwise fashion (see arrow 34) which are canted in a counterclockwise direction along a centerline 36 thereof. As more clearly shown in figure 2b, each coil includes a trailing portion 40 and a leading portion 42, with the trailing portion having a back angle 48 which provides for means for both defining the disposition of the trailing portion 40 of each coil 32 with respect to a normal line 50 and for determining the working resilient range of the spring 30 as hereinafter described in greater detail. In addition, a front angle 54 provides means for defining the disposition of the leading portion 42 of the coil 32 with respect to the normal line. The spring 30 is formed by interconnecting the coils 32 in a manner forming a garter-type axially resilient coil spring with the trailing portion 40 along an outside diameter 58 (see Figure 2a) of the spring 30 and the leading portion 42 along an inside diameter 60 of the spring 30. As can be seen most clearly in Figure 2b, the spring 30, in accordance with the present invention, always has a leading portion 40 disposed at a front angle 54, which is greater than the back angle 48 defining the trailing portion 40. That is, as the coil is traced in the circular-like manner about the centerline 36, each revolution includes a trailing portion and a leading portion, with the leading portion advancing movement along the centerline 36 more than the advancement along the centerline 36 when following the trailing portion 40 of the coil 32. Figures 3a and 3b show a circular welded spring 68, in accordance with the present invention, having the same physical dimensions and wire sizes as spring 30, shown in Figures 2a and 2b, with the spring 68 wound in a counter-clockwise fashion (see arrow 70). In this instance, the spring 68 includes a plurality of coils 72, each having a trailing portion 74 and a leading portion 76 which are defined, respectively by a back angle 80 and a front angle 82, as shown in Figure 3b. Similar to spring 30, the coils 72 of spring 68 are interconnected in a manner forming a garter-type axially resilient coiled spring 68 in which the trailing portion falls along an outside diameter 86 and the leading portion falls along an inside diameter 84 of the spring 68. Curve A of Figure 4 represents the performance of the spring 30 or 68, while spring B represents the performance of a prior art spring identical in physical specification, but with the trailing portion along the outside of the spring. Although the two springs have almost identical force-deflection characteristics in their working deflection range, the maximum load points have a variation of about 40 percent. It has been found that, in accordance with the present invention, the back angle can be varied from as little as one degree to 35 degrees as long as the front angle 54 is greater than the back angle and is larger than 20 degrees and less than 55 degrees. Variation in the back angle of the spring significantly affects the resilient characteristics of the spring independent of the front angle. This is shown in Figure 5 which is a force-deflection curve for springs C and D having spring parameters set forth in Table 1. It should be appreciated that the spring parameters set forth herein are provided only to illustrate the effect of spring back angle and disposition of the trailing portion. Actual spring parameters depend upon the desired spring size, loading and application. Springs C and D are identical springs having the same wire diameter, spring inside diameter coil height and approximately the same front angle, but the back angle and correspondingly the coil spacing is varied. As can be seen in Figure 5, the working deflection of spring D is approximately 50 percent whereas the working deflection of spring C is 45 percent. This is independent of the front angle. Hence, springs can be designed with varying resilient characteristics, such as the force required to deflect the spring, utilizing a spring having the same wire diameter inside diameter and coil height than was heretofore possible with only varying the front angle of the spring. As hereinabove described, increased force-deflection characteristics can be used to advantage in conjunction with a sealant, or gasket materials, wherein the spring cavity is predetermined, the latter dictating spring I.D. and coil height. When the coil spacing is held constant, the back angle along with the front angle may be varied to custom design a spring to tailor the resilient characteristics. For example, the smaller the back angle, the higher the force necessary to deflect the spring, as shown in Figure 6, for Springs E, F and G defined in Table 2. This enables springs to be made with smaller wire and close coil spacing. Conversely, as the back angle is increased, the working deflection is increased when the coil spacing is held constant. This characteristic is important and enables the coiled spring in accordance with the present invention to be effective as electroinagnetic shielding gasket, either by themselves or in conjunction with conductive elastomers as hereinafter described. Turning now to Figure 7a and 7b, there is shown a circular welded clockwise spring 100 in accordance with the present invention, generally showing a plurality of coils 102, which are canted in a clock-wise direction along a centerline 104 thereof. As more clearly shown in 7b, each coil 102 includes a trailing portion 108 and a leading portion 110, with the trailing portion having a back angle 112 which provides for means for both defining the orientation of the trailing portion 108 of each coil 102 with respect to a normal line 114 and for determining the working resilient range of the spring 100 as hereinafter described in greater detail. In addition, a front angle 116 provides the means for defining the orientation of the leading portion 110 of the coil 102 with respect to a normal line 114. The spring 100 is formed by interconnecting the coils 102 in a manner forming a garter-type axially resilient coil spring with the trailing portion 108 along an inside diameter 120 (see Figure 7a of the spring 100 and a leading portion 110 along an outside diameter 122 of the spring 100. As can be seen most clearly in Figure 7b, the spring 100, in accordance with the present invention, always has a leading portion 110 disposed at a front angle 116, which is greater than the back angle 112, defining the trailing portion 108. That is, as the coil is traced in the circular-like manner about the centerline 104, each revolution includes a trailing portion 108 and a leading portion 110, with the leading portion advancing movement along the centerline 104 more than the advancement along the centerline 104 when following the trailing portion 108 of the coil 102. It should be appreciated that the inside back angle canted coil spring 100, while having the same general load/deflection characteristics as the outside back angle canted coil spring 30 hereinbefore described, the specific load/deflection characteristics of each spring are different. For example, an outside back angle canted coil spring 30 having the same wire size and dimension as an inside back angle canted coil spring 100 will generally have a higher maximum load point (See Figure 1) than the inside back angle canted coil spring. Turning now to Figure 8, there is shown an electromagnetic shielding gasket 210 in accordance with the present invention, generally showing a garter-type axial spring 212 with a plurality of coils 214 in an annular seal 216 which provides means for non-invasively supporting the garter-type axially resilient coil spring 212, in a preselected orientation for controlling the resilient characteristics thereof, as hereinafter described in greater detail. In Figure 9, there is shown a load-deflection curve A, representative of the hereinbefore discussed spring 30 and 100 for comparison purposes. Also shown is a load-deflection curve B for a spring made in accordance with the present invention having a turn angle for the purpose of illustrating the characteristics thereof. The load-deflection curve B shows the characteristics of the spring 212 made in accordance with the present invention, showing a linear load-deflection segment 236 until a peak load point is reached. After the peak point 238, the load decreases with deflection in segment 240. This results in a saddle-type deflection range between the peak point 238 and the butt point 242. This type of load-deflection characteristic may have specific advantage for electromagnetic spring seals which are locked in position, such as a groove, with the tension thereof being caused by the spring. In this instance, while the spring produces a relatively constant load over a given working deflection range 244, changes beyond the working range limits at points 246, 248, causes an abrupt increase in load. This causes the spring seal to be self-centering within a groove, or the like. Figure 10 shows a schematic form a cross-section of a canted coil spring, in accordance with the present invention, with a turn angle of ϑ a measured coil width of CW, a measured coil height of CH and a measured spring height H of the spring 212, as shown in Figures 11a, b, c and d. The turn angle may be clockwise (bold lines) or counterclockwise. (Dashed lines). As shown in Figure 11c, an axially flat spring 212 may be turned up counterclockwise, for example, 30 degrees, as shown in Figure 11b, or turned clock-wise, as for example, shown in Figures 11d and 11e, with 30 degrees and 60 degrees turn angles, respectively. It should be appreciated that while the springs shown are shown with a circular shape, other shapes are possible, such as elliptical or rectangular, depending upon the configuration of the mating parts between which the spring 212 and/or seal 216 are to be placed. As shown in the Figures, the turn angle ϑ is defined as an angle formed by a generally circular spring forming a cone or an inverted cone, depending on the position of the spring and measuring the angle ϑ from the horizontal to the intersection through the centerline of each cone, or inverted cone. By varying the turn angle ϑ, different loads can be obtained and the degree of loading depends upon the turn angle ϑ. That is, the higher the turn angle ϑ, the higher the force developed, as will be hereinafter demonstrated. It should be noted that the force developed upon loading is independent upon whether the spring is in the cone shape as shown in figure 11b, or an inverted cone as shown in 11d. That is the springs in 11b and 11d will perform in identical fashion. Curves A, B, C and D, in Figure 12 show the force-deflection characteristics of a series of springs, with ϑ varied from zero degrees to 90 degrees, with a specification of these springs being set forth in Table 3. Each of the springs A, B, D, D, are identical in all respects, except for the turn angle ϑ. Curve A in Figure 12 represents a spring 212 with a turn angle of zero and is representative of the springs 30 or 100. Curve B represents the spring 212 having a turn angle of 15 degrees and manifests a threshold rise 268 characteristic of springs made in accordance with the present invention. This gradual rise develops into the peak load characteristics more clearly shown by curves C, D and E, corresponding to springs C, D and E of Table 3. As shown in Figure 12, as the turn angle ϑ increases, the load increases at a maximum about 90 degrees. Importantly, after the peak load, as shown at 270, 272, 274, respectively, is achieved, the force decreases rapidly to approximately the forces shown by springs A and B. Thus, these springs have working regions 276, 278 and 280, approximately the same as unturned spring A; however, as can be seen in Figure 12, these working areas are bounded by steep load-deflection characteristics. Springs, in accordance with the present invention, have advantages in a variety of applications, as hereinabove described. As hereinbefore mentioned, although the spring shown generally are circular in shape, they could be used in other applications in irregular shapes. That is, the spring will easily fit into other shapes than round. As shown in Table 3, the peak load is substantially greater than the base load and in fact reaches to 1725 percent when the turn angle is 90 degrees. Hence, by using a turn angle, higher load can be provided. Consequently, as hereinbefore mentioned, a smaller wire can be used which will permit the use of more coils per inch thus decreasing the stress that will occur on the seal when loaded and increase the shielding effectiveness of the gasket 210. Also, as hereinbefore pointed out, springs, in accordance with the present invention, exhibiting the force-deflection curves as shown buy curve C, D and E of Figure 12, can be used in self-locking and self-centering applications not heretofore possible by springs exhibiting force-deflection curves as shown by curve A in Figure 9. Turning now to Figure 13, there is shown an electromagnetic shielding gasket 310 including a radially resilient coiled spring 312, in accordance with the present invention, generally showing the radially resilient spring 312 with a plurality of coils 314 in an annular seal 316 which provides means for non-invasively supporting the garter-type radially resilient coil spring 312, in a preselected orientation for controlling the resilient characteristics thereof, as hereinafter described in greater detail. The load-deflection curve for the spring 312 is illustrated in Figure 1. The load-deflection characteristics may be determined with a test jig 330 as shown in Figure 14. A radially resilient spring 332 is held within a housing 334 by a fixture 336, thereby entrapping the spring 332 in a cavity 338. A circumferential spacer 340 is used to load the outside diameter of the spring 332 and the force required to pass a plug 342 through the inside diameter of the spring 332 is measured. A radially resilient spring 312 may be turned up counterclockwise, as shown in Figures 11a, b, c, d and e, for example, 30 degrees, as shown in Figure 11b, or turned clockwise, as for example, shown in Figures 11d and 11e, with 30 degrees and 60 degrees turn angles, respectively. It should be appreciated that while the springs shown are shown with a circular shape other shapes are possible, such as elliptical or rectangular, depending upon the configuration of the mating parts between which the spring 312 and/or seal 316 are to be placed. As shown in the Figures, the turn angle ϑ is defined as an angle formed by a generally circular spring forming a cone or an inverted cone, depending on the position of the spring and measuring the angle ϑ from the horizontal to the intersection through the centerline of each cone, or inverted cone. By varying the turn angle ϑ, different loads can be obtained and the degree of loading depends upon the turn angle ϑ. That is, the higher the turn angle ϑ, the higher the force developed, as will be hereinafter demonstrated. It should be noted that the force developed upon loading is independent upon whether the spring is in the cone shape as shown in Figure 11b, or an inverted cone as shown in lid. That is, the springs in 11b and 11d will perform in identical fashion. The spring 312 may also have a trailing portion defined by a back angle and a leading portion defined by a front angle as hereinbefore described. When loading the spring 312 radially, the load is greater when the turn angle is 90 degrees than when the turn angle is 0 degrees and such load increases progressively from 0 degrees to 90 degrees. In addition, a spring 312 with a back angle, or trailing portion along the inside I.D. of the spring will develop substantially higher force than a spring having a back angle or trailing portion along the outside O.D. of the spring with both springs having the same turn angle. This enables greater tailoring ability. That is, a greater range of wire size and coil spacing can be used while still exerting the same or greater force in response to deflection. This has significant advantage when the spring is used in conjunction with a seal as hereinbefore described for electromagnetic shielding. With the hereinabove description of various canted coil springs suitable for use as electromagnetic shielding gaskets, the reader's attention is now directed to Figure 15 which shows a typical application of a gasket shield spring 400 in a coaxial connector 410. In general, the connector 410 is screw 412 mounted to a bulkhead 414 compressing the spring 400 between a connector housing 416 and the bulkhead 414. The connector includes a pin 420 for the transmission of electromagnetic energy. Temporary connection between the connector 410 and a coaxial cable 424 is made through a conventional screw connector 426 with threads 428 disposed on the connector 410. Figures 16 and 17 are enlarged views of the housing 416 which includes a groove 430 therein for supporting the spring 400. As shown in Figure 16, the groove 430 has a depth, d, less than the height of the spring 400, thereby enabling compression of the spring 400 between the housing 416 and bulkhead 414. (Figure 17). The size and configuration of the spring 400 is made in accordance with the hereinbefore set forth principles to achieve a nearly constant load versus deflection curve for the spring 400. Thus, the spring 400 provides a constant axial force between the housing and the bulkhead 414, despite any variations in the amount of spring compression due to surface irregularities in the groove 430 and the bulkhead 414. Importantly, such force is maintained, despite thermal cycling of the connector which may otherwise deform or assisted permanently setting prior art elastomer seals (not shown) used in a similar configuration. As a result, the spring 400, which may be formed from any suitable conducting material, maintains electrical continuity within the groove 430 and bulkhead 414 surrounding the conductor 420, thereby blocking the propagation of electromagnetic waves therepast. Turning to Figure 18, there is shown another application of a gasket 450 made in accordance with the present invention according to the principles hereinbefore discussed. In this instance, a spring 450 may be of general circular shape or any other configuration such as rectangular and is supported in a shelf 452 attached to a frame 454 in a manner enabling the spring 450 to support a sliding card 456 which must be electromagnetically shielded and grounded to the frame 454 and locked into position. As hereinbefore discussed, open areas in an electromagnetic shield can provide a source for leakage. With this in mind, coils and springs made in accordance with the present invention are selected in order that open areas between coils upon deflection is within that permissible in a particular application. It should be appreciated that this spacing is, of course, dependent upon the wave length of electromagnetic energy to be shielded. With that in mind, Figure 19 shows a variation in open areas between coils with a change in deflection of a shield gasket spring 460. Typical dimensions are provided on the Figure for the spring 460 which includes a back angle B of 17 degrees, a wire diameter of 0,089 mm (0.0035 inches). A spring height of 0,635 mm (0.025 inches) and a spring with 0,737 mm (0.029 inches). Figure 19 shows that at 0, deflection, D, the open area is 0,089 mm (0.0035 inches), while at a deflection of 24 percent, the open area is 0,157 mm (0.0062 inches) and with a deflection of 37-1/2 percent deflection is 0,226 mm (0.0089 inches). It should also be appreciated that the back angle of the spring has an effect on the open area of the spring. This is illustrated in Figures 20a and b, wherein a spring having a height of 4,114 mm (0.162 inches) and a wired diameter of 0,559 mm (0.022 inches) has an open area A₁ of 0,0402 mm² (0.0000623 sq. inches) with a back angle of 16.5 degrees and an open area A₂ of 0,0577 mm² (0.0000893 sq. inches) with a back angle of 30 degrees. The situations where additional electromagnetic shielding is desired, an electromagnetic shielding gasket 500 as shown in Figures 21a and b may include an outer coil spring 502 with an inner coil spring 504 disposed therein, both coil springs 502 and 504 may be made into axially resilient or radially resilient springs as hereinbefore discussed and have their coils canted along a same direction along a centerline 506 or in opposite directions (see Figure 22a and b). Figures 22a and 22b show the gasket 500 as it may be loaded compressing the outer spring 502 to a position where it contacts the inner coil 504. Alternative embodiments of the present invention are shown in Figures 23a and b and 24a and b, wherein the electromagnetic shield 510 includes one or more coiled springs 512, 514 which are filled with a conductive elastomer in order to provide further conductivity of the gasket 510. The gasket 520 shown in Figures 24a, 24b, includes a coil spring 522 as hereinbefore described with a center rod or tube 524 therein. The rod or tube 524 may be conductive to an electromagnetic shielding and also may be solid with sufficient strength to act as a stop for the spring 522 in order to prevent permanent damage thereto. Although there has been described hereinabove a specific electromagnetic shielding gaskets in accordance with the present invention for the purpose 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 as defined in the appended claims.

Claims: An electromagnetic shielding gasket characterized by a resilient coil spring (30, 68, 100, 212, 312, 332, 400, 450, 460, 502, 504, 512, 514, 522) for blocking the propagation of electromagnetic waves therepast, said coil spring comprises a plurality of closely spaced individual coils (32, 72, 102, 214, 314) for causing the coil spring to block the propagation of electromagnetic waves therepast independent of the compression of the coil spring within a range of deflection of the individual coils, said individual coils are interconnected with one another, thereby forming said coil spring and said coils are disposed in a canted relationship with a centerline (36, 104, 506) passing the centers of the individual coils. The electromagnetic shielding gasket according to claim 1, characterized in that two ends of the individual coils (32, 72, 102, 214, 314) are joined to form a continuous coil spring (30, 68, 100, 212, 312, 332, 400, 450, 460, 502, 504, 512, 514, 522). The electromagnetic shielding gasket according to claim 1 or 2, further characterized by an inner coil spring (504, 514), disposed within an outer coil spring (30, 68, 100, 212, 312, 332, 400, 450, 460, 502, 512, 522) separately therefrom, for blocking the propagation of electromagnetic waves therepast. The electromagnetic shielding gasket according to claim 3, characterized in that the inner coil spring (504, 514) comprises a plurality of individual inner coils for causing the inner coil spring (504, 514) to block the propagation of electromagnetic waves therepast independent of compression of the inner coil spring within a range of deflection of the individual inner coils, said individual inner coils being interconnected with one another and disposed in a canted relationship with the centerline (506) of the canted coil spring (30, 68, 100, 212, 312, 332, 400, 450, 460, 502, 504, 512, 514, 522). The electromagnetic shielding gasket according to claim 4, characterized in that the outer coils (502) and the inner coils (504) are canted in the same direction with respect to the centerline (506). The electromagnetic shielding gasket according to claim 4, characterized in that the outer coils (512) and the inner coils (514) are canted in opposite directions with respect to the centerline. The electromagnetic shielding gasket according to any one of claims 1 through 6, characterized in that each of said individual coils (32, 72, 102) have a trailing portion (40, 74, 108) and a leading portion (42, 76, 110), the leading portion advancing movement along the centerline is more than the advancement of the trailing portion along the centerline, said trailing portion (40, 74, 108) being disposed at a back angle (48, 80, 112) with respect to a line (50), normal to the centerline (36) and said leading portion (42, 76, 110) being disposed at a front angle (54, 82, 116) with respect to said line (50), said front angle (54, 82, 116) being greater than said back angle (48, 80, 112). The electromagnetic shielding gasket according to claim 7, characterized in that said individual coils (32, 72, 102) abut one another. The electromagnetic shielding gasket according to claim 7 or 8, characterized in that said back angle (48, 80, 112) is selected in order that the coil spring (30, 68, 100) exerts a force in response to deflection of the individual coils (32, 72, 102) and said force is generally constant over said range of deflection of the individual coils (32, 72, 102). The electromagnetic shielding gasket according to claim 9, characterized in that said back angle (48, 80, 112) is less than about 25 degrees. The electromagnetic shielding gasket according to any one of claims 2 through 10, characterized in that the individual coils (32, 72, 102, 214) are disposed in a manner forming an axially resilient electromagnetic shielding gasket. The electromagnetic shielding gasket according to claim 11, characterized in that said trailing portion (40, 74) of each individual coil (32, 72) is disposed along an outside diameter (58, 86) of the axially resilient electromagnetic shielding gasket and the leading portion (42, 76) of each individual coil (32, 72) is disposed along an inside diameter (60, 84) of the axially resilient electromagnetic shielding gasket. The electromagnetic shielding gasket according to claim 11, characterized in that said trailing portion (108) of each individual coil (102) is disposed along an inside diameter (120) of the axially resilient electromagnetic shielding gasket and the leading portion (110) of each individual coil (102) is disposed along an outside diameter (122) of the axially resilient electromagnetic shielding gasket. The electromagnetic shielding gasket according to any one of claims 11 through 13, further characterized by means orienting said individual coils (32, 72, 102, 214) at a turn angle for defining the load-deflection characteristics of the axially resilient electromagnetic shielding gasket, said turn angle being greater than zero degrees and less than 90 degrees. The electromagnetic shielding gasket according to any one of claims 2 through 10, characterized in that the individual coils (32, 72, 102, 312) are disposed in a manner forming a radially resilient electromagnetic shielding gasket. The electromagnetic shielding gasket according to claim 15 further characterized by means orienting said individual coils (32, 72, 102, 312) at a turn angle for defining the load-deflection characteristics of the radially resilient electromagnetic shielding gasket, said turn angle being greater than zero degrees and less than 90 degrees. The electromagnetic shielding gasket according to any one of claims 7 through 16, characterized in that said back (48, 80, 112) and front angles (54, 82, 116) are selected to enable the canted coil spring to exert a constant force over the range of deflection. The electromagnetic shielding gasket according to any one of claims 1 through 17, characterized by: a plurality of individual outer coils for causing the outer coil spring to block the propagation of electromagnetic waves therepast independent of compression of the coil spring within a range of deflection of the individual outer coils, said individual outer coils being interconnected with one another and disposed in a canted relationship with a centerline (506) of the coil spring, each said individual outer coils has a back portion disposed at an outer coil back angle with said centerline and a forward portion disposed at an outer coil front angle with said centerline, said outer coil front angle being greater than said outer coil back angle; and, a plurality of individual inner coils, disposed within said individual outer coils, for causing the inner coil spring to block the propagation of electromagnetic waves therepast independent of compression of the coil springs within a range of deflection of the individual inner coils, said individual inner coils being interconnected with one another and disposed in a canted relationship with the centerline (506) of the coil spring, each said inner coil has a back portion disposed at an inner coil back angle with said centerline and a forward portion disposed at an inner coil front angle with said centerline, said inner coil front angle being greater than said inner coil back angle. The electromagnetic shielding gasket according to claim 18, characterized in that said inner and outer coils are canted in the same direction in relationship with the centerline (506). The electromagnetic shielding gasket according to claim 18, characterized in that said inner and outer coils are canted in opposite directions in relationship with the centerline. The electromagnetic shielding gasket according to any one of claims 18 through 20, characterized in that two ends of the outer coils are interconnected and two ends of the inner coils are interconnected to form a continuous coil spring arrangement having continuous inner coils disposed within continuous outer coils. The electromagnetic shielding gasket according to any one of claims 18 through 21, characterized in that interstitial areas within the inner and outer coils are filled with a conductive elastomer. The electromagnetic shielding gasket according to any one of claims 7 through 22, characterized by a garter-type axially resilient coil spring (30, 68, 100, 212) for blocking the propagation of electromagnetic waves therepast, said coil spring comprises a plurality of individual coils (32, 72, 102, 214), comprising: a back angle for both defining the disposition of a trailing portion of each coil with respect to a line normal to the centerline (36) and for determining the force-deflection characteristics of the coil spring; a front angle for defining the disposition of a leading portion of each coil with respect to the normal line, said front angle being greater than said back angle; said coils are interconnected in a manner forming a garter-type axially resilient coiled spring with the trailing portion along an outside diameter of the garter-type axially resilient coiled spring and the leading portion along an inside diameter of the garter-type axially resilient coiled spring. The electromagnetic shielding gasket according to claim 23, characterized in that the back angle is greater than about one degree and less than about 40 degrees and the front angle is greater than about 15 degrees and less than about 55 degrees. The electromagnetic shielding gasket according to claim 23 or 24, characterized in that the back angle defines a working deflection in which the garter-type axially resilient electromagnetic shielding spring gasket exerts a generally constant force in an axial direction in response to deflection of the spring gasket in the axially direction, said working deflection being between about 5 percent and about 50 percent deflection of the spring gasket. The electromagnetic shielding gasket according to any one of claims 7 through 22, characterized by: a garter-type axially resilient coil spring (30, 68, 100, 212) for blocking the propagation of electromagnetic waves therepast, said coil spring comprises a plurality of individual coils, comprising: a back angle for both defining the disposition of a trailing portion of each coil with respect to a line normal to the centerline (36) and for determining the working resilient range of the coil spring; a front angle for defining the disposition of a leading portion of each coil with respect to the normal line, said front angle being greater than said back angle; said coils are interconnected in a manner forming a garter-type axially resilient coiled spring with the trailing portion along an inside diameter of the garter-type axially resilient coiled spring and the leading portion along an outside diameter of the garter-type axially resilient coiled spring. The electromagnetic shielding gasket according to claim 26, characterized in that the front angle is less than 35 degrees. The electromagnetic shielding gasket according to claim 26 or 27, characterized in that the plurality of coils are canted in a clockwise direction. The electromagnetic shielding gasket according to any one of claims 26 through 28, characterized in that the back angle is greater than one degree and less than 35 degrees. The electromagnetic shielding gasket according to any one of claims 26 through 29, characterized in that the back angle defines a working deflection in which the spring gasket exerts a generally constant force in an axial direction in response to deflection of the spring gasket in the axial direction, said working deflection being between about 5 percent and about 50 percent deflection of the spring gasket. The electromagnetic shielding gasket according to any one of claims 26 through 30, characterized in that the back angle is less than about 11 degrees. The electromagnetic shielding gasket according to any one of claims 7 through 17, characterized by an annular axially resilient coil spring (30, 68, 100, 212) for blocking the propagation of electromagnetic waves therepast, said coil spring comprises a plurality of individual coils, having a back angle for both defining the disposition of a trailing portion of each coil with respect to a line normal to the centerline (36) and for determining the load-deflection characteristics of the coiled spring; front angle for defining the disposition of a leading portion of each coil with respect to the normal line, said front angle being greater than said back angle; said coils being interconnected in a manner forming a garter-type axially resilient coiled spring; and an annular seal for non-invasively supporting the garter-type axially resilient coiled spring in a preselected orientation for controlling the resilient characteristics thereof. The electromagnetic shielding gasket according to claim 32, characterized in that said annular seal includes means defining a cavity for supporting and orienting said coiled spring with a turn angle of greater than zero degrees and less than 90 degrees. The electromagnetic shielding gasket to claim 33, characterized in that the turn angle is selected to provide a load-deflection characteristics having a saddle shape. The electromagnetic shielding gasket seal according to claim 34, characterized in that the turn angle is greater than about 15 degrees. The electromagnetic shielding gasket seal according to claim 34, characterized in that the turn angle is greater than about 60 degrees. The electromagnetic shielding gasket seal according to any one of claims 32 through 36, characterized in that the trailing portion is disposed along an outside diameter of the garter-type axially resilient coiled spring and the leading portion is disposed along an inside diameter of the garter-type axially resilient coiled spring. The electromagnetic shielding gasket according to any one of claims 32 through 36, characterized in that the trailing portion is disposed along an inside diameter of the garter-type axially resilient coiled spring and the leading portion is disposed along an outside diameter of the garter-type axially resilient coiled spring. The electromagnetic shielding gasket according to any one of claims 7 through 22, characterized by a radially resilient canted coil spring (30, 68, 100, 312) for blocking the propagation of electromagnetic waves therepast, said coil spring comprising a plurality of individual coils, having a back angle for defining the disposition of a trailing portion of each coil with respect to a line normal to the centerline (36); front angle for defining the disposition of a leading portion of each coil with respect to the normal line, said front angle being greater than said back angle; and means orienting said plurality of coils at a turn angle for defining the load-deflection characteristics of the radially resilient canted coiled spring, said turn angle being greater than zero degrees and less than 90 degrees. The electromagnetic shielding gasket according to claim 39, characterized in that the trailing portion is disposed along an outside diameter of the radially loaded canted coiled spring and the leading portion is disposed along an inside diameter of the radially resilient canted coiled spring. The radially resilient canted coiled spring according to claim 39, characterized in that the trailing portion is disposed along an inside diameter of the radially loaded canted coiled spring and the leading portion is disposed along an outside diameter of the radially resilient canted coiled spring.

Inventors: BALSELLS PETER J, BALSELLS, PETER J.
Applicants: BALSELLS PETER J, BALSELLS, PETER J.
Application year: 1990
Main classification: H05K9
IPC codes: F16F1, H05K9, H01R13