GB2184294A - An electromagnetic shielding device - Google Patents

Abstract

A transversely symmetrical electromagnetic shielding device comprises an electrically conductive contact strip (18) having a resilient, transversely bowed portion (36) and inwardly directed longitudinal edge portions (24,26), a track strip (20) overlapping the edge portions (24,26), which track strip (20) is for securing to a surface of a structure to be shielded. <IMAGE>

Description

SPECIFICATION An electromagnetic shielding device The present invention relates to electromagnetic shielding devices and more particularly to electromagnetic shielding devices adapted for members which are moved relative to each other.

Electromagnetic shielding is provided at joints and crevices in electronic equipment and testing facilities to dissipate electrical charges and fields. Optimally, the shielded item is isolated from the effects of external circuitry and other sources of electromagnetic disturbances. Electromagnetic shielding is often required for doors of electronic laboratory rooms, casings of radio transmitters, receivers and computers and othersimilarfacilities and equipment.

Commonly, such structures and equipment require shielding along gaps between surfaces of doors, access panels, drawers, cabinets orthe like, where one surface is slid on another more often than occasionally. For example, electrical components may be arranged individually in cases which are slid into and out of cabinets every time there arises a need to test, service or update the component. Accordingly, it is highly desirable to have electromagnetic shielding on the casing which is adapted to withstand the bi-directional shearing action induced upon it by each insertion and removal ofthecasing into and from the cabinet.Failing that, the electromagnetic shielding of the electrical component may become broken and the component as well as the entire electrical system to which it is connected be comes vulnerable to electromagnetic interference.

One prior shielding arrangement is disclosed in U.S. Patent No. 3,504,095 issued March 31, 1970 to Roberson et a I which discloses an electromagnetic gasket including a curved bearing portion and a longitudinal attaching portion connected to one side of the curved bearing portion. A plurality of notches divide the curved bearing portion into a row of integral fingers along the entire length of the gasket.

These fingers have free ends which are capable of being snagged and/or bent excessively. Also, the otherends ofthefingers are held rigidly in place re lativeto one ofthe shielded surfaces so that the gasket offers more resistance to shear in one trans verse direction than in the opposite one.

Another prior shielding arrangement is disclosed in U.S. Patent No. 3,277,230, issued on October 4, 1966 to Stickney et al, which discloses an electronic shielding gasketthat includes a linear metallic strip having a central portion with spaced holes for receiv ing a special fastening device and fingers extending outwardly in rows from opposite edges of the central portion of the gasket. The free ends of both rows of fingers are left exposed and unprotected from being snagged upon one of the surfaces to be shielded when the surfaces are slid transversely relative to each other and to the gasket. Mounting ofthe Stickney et al gasket also requires a relativelysub stantial amount of space due to the lateral extension ofthefingers in opposite directions.

Other types of electromagnetic shielding arrange mentswhich include longitudinal orcircularstrips arranged upon one ofthe opposed surfaces are disclosed in U.S. Patent Nos. 2,825,042 issued February 25, 1958 to Tollefson et al; 2,844,644 issued July 22, 1958to SouleJr.; and 3,904,810 issued September 9, 1975 to Kraus.

Another known joint seal for electromagnetic wave shielding includes spring strips of angular cross section providing an electromagnetic seal between a pair of movable members. The spring strips extend from one movable member at equal angles from beneath a clamp which is attached to the mov able member. An arrangementofthistypeisdis- closed in U.S. Patent No.3,962,550 issued June 8, 1976 to Kaiserswerth. Being asymmetrical, the Kaiserswerth device is notwell-suitedto effect electro magneticshielding between two memberswhich slide bi-directionally with respect to each other.

In IBM Technical Disclosure Bulletin Vol.20, No.1, page 281 (June 1977), a conductive seal for electromagnetic shielding is disclosed which is usable between movable panel sections. The seal comprises a transversely arched rubber strip whose upper sur- face is laminated with a metal foil. The flattened longitudinal edges of the laminated rubber strip are held in place by a metal strip 6 having longitudinal edge regions curled upwardly aboutthe longitudinal edges ofthe laminated rubberstrip.

In the IBM arrangement the longitudinal ends of the rubber strip are confined such that the compliant deflection ofthe assembly occurs almost entirely in the arched portion of the laminated rubber strip. Ac cordingly, it is desirableto have the laminated metal foil relativelythinandyetsuitablybondedtothe rubber layer so as to remain attached even under excessive and repeated deflection ofthe rubber strip.

It is a main object of the present invention to provide an electromagnetic shielding device suitable for use between opposing surfaces of two spaced apart members which slide by each other in two opposing directions such as in bi-directional sliding,which is effective to maintain good electrical contact between surfaces of two spaced apart members even after repeated bi-directional sliding motion between the members, which is useful on a widevarietyofdifferenttypes of surface textures and materials, and which is not prone to wear from the repeated movements of members.

Yet another object of the present invention is to provide an electromagnetic shielding device that is positively retained upon a surface, which is configured so as to permitplacementalongawidevarietyof surface contours, and which is relatively inexpensive to manufacture and easy to install such that nospecial tools are required to effect installation.

According to the invention there is provided an electromagnetic shielding device comprising an electrically conductive contact strip having a resilient, transversely bowed portion and longitudinal edge portions extending transversely inwardly, and means for retaining the longitudinal edge portions of the contact strip adjacent a surface of a member to be shielded, whereby upon deflection of the resilient, transversely bowed portion the longitudinal edge portions may displace transversely relative to the surface ofthe memberto be shielded.

The invention thus provides an electromagnetic shielding device having a contacting portion whose longitudinal edges are protected from being lifted or bent from the effects of sliding motion between the shielding surfaces, even when the sliding motion is reversed, and which is equally compliant in opposing transverse directions.

The electromagnetic shielding device of the invention may be repeatedly compressed at severe shear angles and with great shearforces, yet will still provide effective electromagnetic shielding, requires a minimum of space for its placement, and permits repetitive insertion and removal of electrical componentcasings orthe like into and from a cabinetorthe like without compromising the electromagnetic shielding of same.

Some embodiments ofthe present invention will now be described, by way of example, with reference to the accompanying drawings, wherein like members bear like reference numerals and wherein Figure 1 is a perspective view of an electromagnetic shielding device in accordance with a first embodimentofthe present invention, Figure2isa planarviewofametallicstripsuitable for being formed into a contact strip of the shielding device of Figure 1.

Figure 3 is a top view of the shielding device of Figure 1, Figure 4 is a bottom view ofthe shielding device of Figure 1, Figure 5is an enlarged end view of the shielding device of Figure 1 in an undeflected condition, Figure 6A is an end view of the shielding device of Figure 1 in partial compression between two movable members, Figure 6B is an end viewofthe shielding device of Figure 1 in substantial compression between two movable members, Figure 7 is an end view of the arrangement of Figure 6Awith the upper member undergoing sliding motion relative to the other member in a first transverse direction, Figure 8 is an end view of the arrangement of Figure 6Awith the upper member undergoing sliding motion relative to the other member in an opposite transverse direction from that of Figure 7, FigureS is an end view of an electromagnetic shielding device in accordance with anotherprefer- red embodiment of the present invention, and Figure 10 is a bottom view of the shielding device of Figure 9.

Referring to Figures 1 and 6A, an electromagnetic shielding device 1 is provided for effecting electrical contact across a gap 12 between two spaced apart members 14 and 16 so asto create an electromagnetic shielding effect across the gap 12. The el ectromagnetic device 1 includes a transversely bowed, electrically conductive contact strip 18 and a track strip 20 for securing the contact strip 18 in sliding contactwith either one of the members 14 and 16. In the first preferred embodiment, the track strip 20 is secured to the lower member 16 by a plurality of spaced apart rivets 22 that longitudinal edge port- ions 24 and 26 ofthe contact strip 28 are retained in sliding contact with the surface 28 ofthe lower member 16.The track strip 20 serves to hold down and protectthe ends 30 and 32 of the edge portions 24 and 26 so asto prevent or discourage a sliding motion between the members 14and 16from lifting up and irreversibly bending the ends 30 and 32.

The pair of longitudinal edge portions 24and 26 of the contact strip 18 are folded transversely inwardly toward each other with their ends 30 and 32 spaced apart and parallel to each other so as to leave a gap 34 between the longitudinal edge portions. In this fashion, the longitudinal edge portions 24 and 26 can move independently of each other as a result ofthe flexing of the transversely bowed portion 36 of the contact strip 18. The gap 34is also provided to ac commodatethe passage of the rivet 22 when the trackstrip 20 is slid into place within contact strip 18 during assembly ofthe shielding device 1.

The contact strip 18 includes a plurality of spaced apart, transversely directed slits 38, which in the pro ferred embodiment extend completely across the transversely bowed portion 36, around folded edges 40 and 42 of the strip 18and partially into each ofthe longitudinal edge portions 24 and 26. Bysucharran- gement, the transversely bowed portion 36 ofthe contact strip 18 is divided into a series of transverse segments 44 which can flex substantially independ- ently of one another.As a result, the contact strip 18 may deflect according to local loadings and is rende red moreflexible, with the resultthat a more complete electrical connection may be effected between the members 14 and 16 along the entire length ofthe contact strip 18.

The contact strip 18 is preferably made from a con ductivemetal alloywhich may be easilyshaped into the desired configuration without breakage. Referring to Figure 2, the entire contactstrip 18 is pre ferablyformedfromasinglepieceofanelectrically conductive metal while it is in a soft condition. After formation such as by die stamping andfolding,the metal is heat treated for stability. A beryllium copper alloy has been found to particularlyamenableto the formation process without risk of breakage. The thickness of the contact strip is selected such that a desired degree of resiliency is obtained in the transversely bowed portion 36 under the expected loads.

Other methods of construction and choice of materials would be readily apparent to one of ordinary skill in the pertinent art.

Referring now particularly to Figure 5, the track strip 20 has a width generally corresponding with the distance between the folded edges 40 and 42 ofthe contact strip 18 in an undeflected condition, althoughthetrackstrip 20 could be made somewhat narrower (but generallywiderthan the gap 34) without affecting the proper working ofthe electromagnetic shielding device 1. The track strip 20 inclu des a plurality of spaced projections 46 extending downwardly from the track strip 20 by a distance generally corresponding with the thickness of the contact strip 18 and in the preferred embodiment, slightly greaterthan that thickness so that when the shielding device 1 is mounted to a member 16 and in an undeflected condition, the edge portions 24and 26 fit loosely underneath the track strip 20. However, the track strip 20 may be provided with projections 46 configured to provide a snug fitforthe edge portions 24 and 26 or may be provided with no indentations at all.

In the first preferred embodiment, the projections 46 are provided integrally in the track and have an oblong shape with the long axis extending transversely with respect to the track strip 20. Recesses 48 and 50 are provided in the longitudinal edge portions 24 and 26, respectively, of the contact strip 18 to ac commodatethe projections 46. With the recesses, the longitudinal edge portions 24 and 26 may be made as wide as necessary to assure that they remain held down by the track strip 20 even under con- ditions offull compression as in Figure 6B. Referring now also to Figure 4, the fit between the recesses 48 and 50 and the oblong projections 46 in thetrack strip 20 also serve to key the contact strip 18 into place relative to the track strip 20 in the longitudinal direction.

The projections 46 are elongated transversely so as to also provide wide surface contact with the su rface 28 of the member 16. The projections 46 raise the track strip 20 slightly above the surface 28 so that the underside ofthetrackstrip 20 and the surface 28 form two parallel, longitudinally extending slots along both sides of the track strip 20, which slots receivethe longitudinal edge portions 24and 26 ofthe contact strip.

At each projection 46, a hole 52 is formed through which a rivet 22 is placed. Preferably, a head 54 of the rivet 22 extends beyond the periphery of the projection 46 so as to contact the planar portion ofthetrack member 20 surrounding each projection 46.

In the preferred embodiment, the rivets 22 area commercially available push type, having a region of maximum diameter 56 spaced a distance below the head 54 ofthe rivet so that the rivet may be popped into a hole provided in a sheet metal wall orthe like.

Referring also to Figure 5, the rivet 22 is constructed from a plastic or metallic material and has a hollowed shank 58 with a flattened side 60 so that when the rivet 22 is turned so asto presenttheflattened side 60 transversely, the rivet 22 has a width which fits within the gap 34 between the longitudinal edge portions 24 and 26. So oriented, the flattened side 60 allows the contact strip 18 to more easily slide past the rivet 22. However, assembly ofthe shielding device 1 may be effected without orienting the flattened sides 60, because of the flexibility ofthe contact strip 18. The shank 58 is given sufficient length so thatthe contact strip 18 prevents the rivet 22 from being pushed entirely out ofthe hole 52 in the track strip 20.

In orderto facilitate handling of the shielding device, the holes 52 are sized to fit closely with the rivets 22 so that the rivets 22 are held steady in thetrackstrip 20.

The track strip 20 of the preferred embodiment is constructed from brass or a bronze and is provided with a thickness greater than that of the contact strip 18 so as to have a degree of rigidity. However, the track strip 20 may be formed from other materials including non-conductive ones such as plastics.

In mounting the electromagnetic shielding device 1 to the member 16to be shielded, holes 62 are drilled into the member 16, which holes 62 are spaced to correspond with the spacing of the holes 52 of the shielding device 1. The electromagentic shielding device 1 is first assembled by placing rivets 22 in the holes 52 in thetrackstrip 20 and optionallyturn- ing them so that theirflattened sides 60 face trans- versely. The contact strip 18 is then slid onto the track strip 20 until the respective ends of the contact strip 18 and track strip 20 coincide and each pair of opposing recesses 48 and 50 of the longitudinal edges 24 and 26 coincide with a projection 46 of the track strip 20.The assembled shielding device 1 is then posi tioned overthesurface28ofthe memberl6withthe rivets 22 being initially placed in the holes 62 provided in the member 16. The userthen goes from rivet to rivet and presses each rivet 22 into a hole 62 by urging a transverse segment 44 of the contact strip 18 downwardly against the head 54 of each rivet 22 until each rivet 22 is fully engaged in one of holes 62.

Such action will fully compress the contact strip 18 in the region adjacent each rivet 22 in the manner shown in Figure 6B, however as earlier noted, the ends 30 and 32 ofthe longitudinal edge portions 24 and 26 remain beneath the track strip 20 despite this full deflection. It is important to note that recesses 48 and 50 ofthe longitudinal edge portions 24 and 26 are situated between pairs oftransverse slits 38so that a transverse segment 44 is positioned squarely over each hole 52 and rivet 22. Accordingly, a transverse segment44 is squarely compressed onto the rivet head 54 during the pressing operation, which arrangement minimizes local twisting loads upon thecontactstrip 18 during the pressing step.

Referring again to Figure 5, it is to be noted that in the uncompressed state of the contact strip 18,the longitudinal edge portions 24 and 26 preferably lie within a common horizontal plane. Accordingly, a substantial surface-to-surface, sliding contact is maintained between the surface 28 of the member 16 and the undersides ofthe longitudinal edge portions 24 and 26 when the contact strip 18 undergoes compression as in Figure 6, or Figure 6A even when shearing action is also present as in Figures 7 and 8.

Referring to Figure 5, in the first preferred embodi ment, the transversely bowed portion 36 ofthe contact strip 18 describes an arc which is symmetrical to a longitudinally extending, vertical plane bisecting the contact strip 18. Preferably, the arc of the transversely bowed portion 36 approximates a segment ofasemicirclesuchthatintheunflexedcondition, the transversely bowed portion 36 intersects a horizontal planethrough thefolded edges40 and 42 at equal but opposite acute angles. Preferably, the track 20 is also symmetrical to the aformentioned plane.

Referringto Figure6A,asanothermemberl4tobe shielded moves in a vertical direction as indicated by the arrow in Figure 6A, both longitudinal edge portions 24 and 26 may slide transversely outwardly re lative to the surface 28 of member 16 as the compression of the bowed portion 36 causes the folded edges 40 and 42 to displace outwardly. The folded edges 40 and 42 as well as the undersides of the edges 24 and 26 are placed in substantial contact with the surface 28 of the member 16. The track 20, being affixed to the member 16 by the rivets 22, covers the ends 30 and 32 ofthe longitudinal edge portions 24 and 26so as to protect them.Because the transversely bowed portion 36 is in substantial surface contact with the other member 14 being urged upon it,electriccon- tact and the desired electromagnetic shielding effect is achieved across the gap 12.

Referring particularly to Figures7and8,inthesitu- ation where the member 16 is initially or anytime thereafter caused to slide transversely to the other member 14,the electromagnetic shielding device 1 can deform in response to the resultant shearing action and maintain the electrical connection and shielding effect, irrespective ofwhetherthe member 14 is moved in afirsttransverse direction orion the opposite transverse direction.The shearing action imposed upon the electromagnetic shielding device 1 simply causes the transversely bowed portion 36 to deflect asymmetrically such thatthe longitudinal edge portion on onesideofthetrackstrip20istrans- lated transversely outwardly away from one side of thetrack strip 20 and the other is translated transversely inwardly toward the other side of track strip 20 Any shear loads transferred to the track strip 20 are resisted bytheconnection atthe rivets 22.Because of its resiliency, the transversely bowed por tion 36 ofthe contact strip 18 may return to its sym- metrical compressed condition as in Figure 6A if the friction between the transversely bowed portion 36 and the surface 64ofthe member l4which it contacts is insufficient to maintain it in the asymmetrical bowed condition. The contact strip 18 of the preferred embodiment is provided with sufficient resili encysuchthatwhen all loads arewithdrawn, the contactstrip 18will returnto its original bowedform.

Importantly, the electromagnetic shielding device of the present invention may be fully compressed at very severe shear angles approaching 900 from either direction with regard to the center line of the uncompressed height ofthe bowed portion and retain its capacity to effect electromagnetic shielding and its capacity to withstand oppositely directed shearing action parallel to the mounting surface.

Moreover, the shearing may be imposed time and time again, backwardlyand forwardly, as would occurwith the insertion and removal of electrical componentsfrom cabinets and the like where one wall is slid past another during the insertion and removal of the component for servicing orthe like.

Referring particularly to Figure7, during the imposition of compressive shearing loads upon the electromagneticshielding device 1,thefolded edge 40 tends to close upon itself and be drawn in contact with an adjacent edge 66 of the track strip 20 such that the adjacent edge 66 serves to limit the trans- verse movementofthe longitudinal edge portion 24 toward one side of the track strip 20. Because the track strip 20 preferably has a width approximately equal to that ofthe space between the folded edges 40 and 42 of the contact strip in an unflexed condition, the folded edge 40 comes into contact with the edge 66 ofthe track strip 20 relatively quickly.During the motion,the underside ofthetrackstrip 20 cooperates with the surface 28 to guide the end 30 ofthe edge portion 24so asto prevent its becoming bent or snagged.

It is to be noted thatwhateverthe loading imposed upon the electromagnetic shielding device 1, whether it includes shearing loads or not,the ends 30 and 32 ofthe longitudinal edge portions 24 and 26 are protected by the track strip 20 and prevented from being raised, snagged and/or bent. Accordingly, even if a shearing action in one direction is suddenly reversed to the opposite direction, the ends 30 and 32 of the contact strip 18 are not exposed and are protected from being damaged.

Referring to Figures 9 and 10, a second embodiment ofthe present invention is constructed similarlyto thefirst embodiment, exceptfor a con tinuouslyextending projection 46' formed in the track strip 20 with a layer of an adhesive agent 68 applied to the underside of the projection 46'forsec urging the track strip 20 to a selected surface such as the surface 28 of the member 16. For convenience, a removable protective paper70 is placed overthe adhesive layer68to ease handling priorto installation. It is to be noted that the projection 46' is made with a continuous width equal approximately to the elongated axis ofthe oblong projection 46 ofthefirst embodiment.Because the projection 46' ofthe second embodiment is continuous, the longitudinal edge portions 24' and 26' are shortened in the inwardly transverse direction. The adhesive layer 68 facilitates the attachment of the shielding device 1 to a surface 28 without having to drill holes orthe like.

The present invention provides a relatively simple yet highly effective device for electromagnetic shielding of two relatively movable surfaces. The device may be readily mounted to any type of surface and ensures proper electrical contact between two opposed surfaces, even during and after they are slid transversely past each other. Also, buckling, breakage and/or permanent deformation of the device is minimized because of the arcuate and transversely symmetrical shape of the contact strip 18 and the protection of its longitudinal edge portions 24 and 26 beneath thetrackstrip 20. In that the longitudinal edge portions 24 and 26 are folded inwardly, the shielding device 1 is given a compactness which minimizes space requirements for its installation.

Claims (17)

1. An electromagnetic shielding device comprising an electrically conductive contact strip having a resilient, transversely bowed portion and longitudinal edge portions extending transversely inwardly, and means for retaining the longitudinal edge portions of the contact strip adjacent a surface of a memberto be shielded, whereby upon deflection ofthe resilient, transversely bowed portion the longitudinal edge portions may displace transversely relative to the surface ofthe memberto be shielded.

2. An electromagnetic shielding device as claimed in Claim 1,whereinthecontactstripinclu- des a plurality of transverse slits.

3. An electromagnetic shielding device as claimed in Claim 2, wherein the contact strip is constructed from a beryllium copper alloy.

4. An electromagnetic shielding device as claimed in Claim 2 or Claim 3, wherein each ofthe plurality of transverse slits extends across the transversely bowed portion and into the longitudinal edge portions of the contact strip.

5. An electromagnetic shielding device as claimed in any one of Claims 1 to 4, wherein said longitudinal edge portions are substantially coplanar.

6. An electromagnetic shielding device as claimed in any one of Claims 1 to 5, wherein the means for retaining includes a track strip.

7. An electromagnetic shielding device as claimed in Claim 6, wherein the track strip is metallic, or nonmetallic.

8. An electromagnetic shielding device as claimed in Claim 6 or Claim 7, wherein the track strip includes a plurality of projections provided in a central region ofthetrackstrip, each of which projections protrudes downwardly between the longitudinal edge portions of the contact strip.

9. An electromagnetic shielding device as claimed in Claim 8, wherein each of the projections is elongated in a transverse direction of the track strip and the longitudinal edge portions include recesses adapted to receive the projections.

10. An electromagnetic shielding device as claimed in Claim 9, comprising a plurality of rivets, each of which rivets is provided at one of the projections of the track strip.

11. An electromagnetic shielding device as claimed in Claim 10, wherein each of the rivets is constructed from plastic or metallic material and has awidth approximately equal to the width ofthegap between the longitudinal edge portions of the contact strip in an uncompressed condition.

12. An electromagnetic shielding device as claimed in Claim 6 or Claim 7, wherein thetrackstrip has a projection provided in a central region ofthe track strip, which projection extends continuously along a longitudinal axis ofthetrackstrip.

13. An electromagnetic shielding device as claimed in Claim 1 2, wherein the track strip means includes a layer of an adhesive agent provided along a surface ofthe projection.

14. An electromagnetic shielding device as claimed in Claim 6 or Claim 7, wherein thetrackstrip is adapted to form a pair of parallel, longitudinal slots adjacent to a selected surface, which slots open transversely outwardly, and the longitudinal edge portions ofthe contact strip fit into those track slots, whereby upon deflection of the transversely bowed portion of the contact strip, the longitudinal edge portions may move transversely in the track slots.

15. An electromagnetic shielding device as claimed in any one of Claims 6 to 14, wherein the width ofthetrackstrip isgreaterthan the maximum distance between the longitudinal edge portions when the device is in a compressed state.

16. An electromagnetic shielding arrangement comprising first and second surfaces of a structure to be shielded and an electromagnetic shielding device as claimed in any one of Claims 6 to 15, with the transversely bowed portion of the contact strip adapted to be contacted by the second surface and the track strip secured to the first surface, which track strip retains the longitudinal edge portions ofthe contact strip in sliding contact with the first surface.

17. An electromagnetic shielding device sub- stantiallyas herein described with referencetoac- companying drawings.