Abstract:
An electronic system having circuit cards insertable into a shelf unit, with shielding between adjacent circuit cards, is provided with a guide fin assembly having protruding guide fins between adjacent circuit card faceplates to limit cumulative tolerance build up between inserted circuit cards.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the provision of electromagnetic interference and electrostatic discharge shielding of an electronic system and, more particularly, to a guide assembly for circuit cards which are part of such an electronic system. 
     Modern electronic systems equipment, such as for telecommunications purposes, is often constructed as modular circuit cards inserted into guide slots of mechanical card cages, or shelf units, for engagement with a backplane mounted to the shelf unit at the inward ends of the guide slots. The outward edge of each circuit card is fastened to a faceplate assembly which holds a latch for releasably retaining the circuit card in the shelf unit. Although the circuit cards are relatively closely packed, there are still gaps between adjacent faceplate assemblies. There are also gaps in the faceplate assemblies to allow portions of the latches to pass therethrough. All of these gaps allow electromagnetic interference radiation to leak therethrough. In addition, the gaps allow electrostatic discharge to pass therethrough, possibly causing damage to circuit components mounted on the circuit cards. A need therefore exists to provide shielding against electromagnetic interference and electrostatic discharge for such electronic systems equipment. 
     It is known to provide such shielding by encasing the electronic components within a conductive housing. While this“closed door” approach is satisfactory, there exists a need to provide such shielding under“open door” conditions, for example where a door to the enclosure is open to allow access to the equipment for maintenance purposes. 
     These needs have been addressed, for example, by a shielding assembly disclosed in co-pending U.S. patent application Ser. No. 09/309,930, filed May 11, 1999, and assigned to the assignee of this invention. As disclosed in the referenced application, each faceplate assembly secured to a circuit card includes an elongated conductive U-shape bracket member extending at least the length of the circuit card edge with the base wall of the U-shape being orthogonal to the circuit card. Secured to one side wall of each of the bracket members is a respective resilient compressible conductive gasket which extends the length of its respective bracket member. Each bracket member has its respective gasket secured to the same corresponding side wall as all the other bracket members so that between each adjacent pair of bracket members there is a single respective gasket. This seals all the gaps between adjacent faceplate assemblies. 
     While the aforedescribed assembly is effective, it has been found that the circuit card faceplate width can have dimensional variation that is considered acceptable in the manufacture process. However, this accumulation of variation (tolerance build-up) along with the springiness of the gaskets between adjacent faceplates can decrease the available opening for a circuit card to plug in between previously inserted cards. The tolerance build up is such that the first few circuit cards can be plugged in, but as there are more circuit cards in an adjacent group, the faceplates and gaskets start flaring out like an“accordion”. The build up can be such that the last few circuit cards are difficult or impossible to install. Accordingly, a need exists to overcome this problem without at the same time diminishing the shielding effect of the aforedescribed assembly. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a plurality of guide fins are mount ed to the front of the shelf unit. Each of the guide fins is parallel to the guide slots and the guide fins are located and spaced so that each guide fin is between a respective pair of adjacent faceplate assembly bracket member side walls. 
     In accordance with an aspect of this invention, an elongated bracket is secured to the front of the shelf unit and extends across the plurality of guide slots. The plurality of guide fins are formed unitarily with the bracket. 
     In accordance with another aspect of this invention, an elongated angle bracket is secured to the front of the shelf unit and extends across a plurality of guide slots. The angle bracket has first and second orthogonally related sides and a plurality of aligned pairs of openings. One of the openings in each pair is in the bracket first side and the other opening in each pair is in the bracket second side. Each fin includes a pair of tabs each adapted for insertion in a respective opening of a respective pair of openings. 
     In accordance with a further aspect of this invention, each fin includes a third tab bent orthogonally to the fin and adapted to be spot welded to one of the angle bracket sides. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing will be more readily apparent upon reading the following description in conjunction with the drawings in which like elements in different figures thereof are identified by the same reference numeral and wherein: 
     FIG. 1 is a perspective view showing a first embodiment of the present invention; 
     FIG. 2 is an enlarged and exploded perspective view of a portion of the embodiment shown in FIG. 1; 
     FIG. 3 is a perspective view, partially broken away, showing a second embodiment of the present invention; 
     FIG. 4 is an enlarged and exploded perspective view of a portion of the embodiment shown in FIG. 3; and 
     FIG. 5 is a perspective view showing a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a shelf unit  10  for an electronic system. The shelf unit  10  has a plurality of parallel spaced guide slots  12  for holding a plurality of circuit cards  14  in a parallel spaced array. Secured to an edge of each of the circuit cards is a faceplate assembly  16 . Each of the faceplate assemblies  16  is accessible from outside the shelf unit  10  so that the respective circuit card  14  can be selectively installed and removed from the shelf unit  10 . Illustratively, each faceplate assembly  16  includes an elongated conductive U-shaped bracket member  18  extending at least the length of the circuit card  14  edge. The base wall of the bracket member  18  is orthogonal to the circuit card  14  and the two side walls of the bracket member  18  extend parallel to and away from the circuit card  14 . 
     Secured to the same corresponding side wall of each of the bracket members  18  is a respective resilient compressible conductive gasket  20  (FIG.  5 ). The gasket  20  extends substantially the entire length of its respective bracket member  18  and in its uncompressed state extends away from its respective bracket member  18  in a direction orthogonal to the respective circuit card  14  by a distance greater than the spacing between facing bracket member side walls of adjacent circuit card faceplate assemblies. The foregoing is fully disclosed in the afore referenced co-pending U.S. patent application. 
     It is the build up caused by the accumulation of the gaskets  20  which causes the circuit cards  14  to flex forwardly of the guide slots  12  so as to decrease the space between faceplate assemblies  16 . Briefly, according to the present invention, the faceplate assemblies  16  are maintained in position by providing a plurality of guide fins mounted to the front of the shelf unit  10 . Each of the guide fins is parallel to the guide slots  12  and the guide fins are located and spaced so that each guide fin is between a respective pair of adjacent faceplate assembly bracket member side walls. 
     Thus, according to a first embodiment of this invention, as shown in FIGS. 1 and 2, an elongated angle bracket  22  is secured to the front of the shelf unit  10 , as by adhesive, welding, or any other suitable means, and extends across a plurality of the guide slots  12 . As best seen in FIG. 2, the angle bracket  22  has first and second orthogonally related sides  24 ,  26 . Aligned pairs of openings  28 ,  30  are provided each in a respective one of the sides  24 ,  26 . As shown, the openings  28 ,  30  are elongated and rectangular in shape. Each guide fin  32  is formed of sheet material so that it is generally planar with a pair of tabs  34 ,  36 . The tabs  34 ,  36  are adapted each for insertion in a respective opening  28 ,  30 . In addition, each guide fin  32  is formed with a third tab  38  which is bent orthogonally to the major plane of the guide fin  32  and is adapted to be spot welded to the side  26  of the angle bracket  22  after the tabs  34 ,  36  are inserted in the openings  28 ,  30 . The guide fin  32  is preferably chamfered at its leading edge  40  and the gasket  20  (FIG. 5) is similarly chamfered at its end. This chamfering of the guide fin  32  eases the insertion of the faceplate assembly  16  between adjacent guide fins  32  and the removal of gasket material insures that the faceplate assembly bracket member  18  will fit between adjacent guide fins  32 . To maintain the shielding effect, gaskets  42  may be secured to the side  24  of the angle bracket  22  between adjacent guide fins  32 , as shown in FIG.  1 . While the guide fin structure has been shown only along a lower edge of the shelf unit  10 , it is understood that a similar, but inverted, guide fin structure is provided along an upper edge of the shelf unit  10  as well. 
     FIGS. 3 and 4 disclose a second embodiment according to the present invention wherein the angle bracket  44  has three orthogonally related sides  46 ,  48 ,  50  to form a channel for holding the guide fins. The sides  46 ,  48  have the aligned pairs of openings  28 ,  30 . The guide fin  52  is formed the same as the guide fin  34  with the tabs  54 ,  56 ,  58  as before and with an additional tab  60  which is adapted to be spot welded to the side  50  of the angle bracket  44 . 
     FIG. 5 shows a third embodiment of this invention wherein the bracket  62  and the plurality of guide fins  64  are formed unitarily as a single cast piece. As shown, the bracket  62  is secured to the front of the shelf unit  10  by double faced adhesive tape  66 . 
     Accordingly, there has been disclosed an improved guide assembly for circuit cards which are part of an electronic system which is shielded from electromagnetic interference and electrostatic discharge. While several embodiments of the present invention have been disclosed herein, it is understood that various adaptations and modifications to the disclosed embodiments are possible and it is intended that this invention be limited only by the scope of the appended claims. Thus, while the faceplate assembly has been shown as having a U-shaped bracket, the present invention can be utilized with other types of faceplate assemblies, such as those having solid blocks or box-like brackets, for example.