Abstract:
A method and apparatus for the economical and reliable assembly of buss plates and components thereon, having a bushing positioned on one or more surfaces or therebetween. A resilient retainer is employed to retain bushings in proximity to buss plate through holes to facilitate easy assembly. The use of the resilient retainer eliminates the requirement for costly and potentially adverse pre-assembly soldering of the bushing into position.

Description:
[0001]    This application is a continuation in part of, and claims priority from, pending U.S. application Ser. No. 11/670,828 for “BUSS PLATE BUSHING RETAINER AND ASSEMBLY THEREOF,” filed Feb. 2, 2007 by S. Bader et al., and further from U.S. Application 60/894,300, filed Mar. 12, 2007, both of which are hereby incorporated by reference in their entirety. 
     
    
       [0002]    The present invention is directed to a mechanical and/or electrical interconnection device, in which a power distribution assembly (PDA), or circuit backplane, is formed having a plurality of conductive buss plates whereby the components and buss plates are fastened together having bushings there-between, forming a laminated multilayer buss board assembly for conveying electric power or signals to, from and between various electronic components within a power distribution assembly. In one embodiment, a resilient split cylinder retainer or ferrule is inserted within a bushing and a buss plate hole or aperture so as to retain the bushing in alignment during the buss board and component assembly processes. 
       BACKGROUND AND SUMMARY OF THE INVENTION 
       [0003]    In order to reduce the size, as well as increase efficiency of a power distribution assembly, conductive buss plates are employed having lateral direct interconnections to high current switching devices, thereby mitigating the traditional use of hard wiring and associated bulky cable harnesses. The buss plate is designed to be mounted to, and within, an enclosure whereby the components are then attached to the plate in such a manner as to complete the power supply circuit. Accordingly, buss plates lower the manufacturing costs by decreasing assembly time, as well as material costs. Furthermore, the flexibility of buss plates provide for a variety of form factors to accommodate obstacles inherent within the assembly, such as transformers, heat sinks, circuit breakers and the like. 
         [0004]    Additionally, there are significant technical and functional advantages to the use of buss plates over hard wires. For example, a determining factor in mitigating electrical noise is to reduce circuit inductance while increasing the capacitance. Accordingly, the use of relatively thin parallel conductive buss plates, having a dielectric laminate as a substrate, has a tendency to minimize the effect of inductance by increasing the capacitance between electrical circuit planes. Laminated buss plates are important as well for the reduction of power circuit inductance to reduce transient voltages and to control parasitic oscillations when using high current insulated gate bi-polar transistor (IGBT) modules. 
         [0005]    More specifically, it has been found that a PDA consisting of conductive buss plates, made from fabricated copper adhered onto a thin dielectric material and then sandwiched together to form a power buss circuit board, provides for both the mechanical mounting and electrical connectivity of components such as filter capacitors and semiconductor switching devices, for example IGBT&#39;s. The mechanical/electrical connection points or vias, are interposed between the conductive surfaces within the insulated buss plates. The components are secured with a fastener and a bushing or embossed conductive surface, the fastener passing through and contacting one or more of the plates and subsequently threading directly into a component. Notably, each connection through hole within the buss plate requires a copper bushing that is generally soldered into place, or alternatively an embossed surface, so as to be in direct alignment with the through hole. The copper bushing may be in the form of a flat-sided, washer-like component, and in one embodiment may also include a star-shaped locking washer to prevent problems with the backing-out or reversing of the fastener. 
         [0006]    As mentioned, it is generally necessary to incorporate and retain a large bushing or washer around each connection through hole in the buss plate prior to assembly. However, the present practice requires the bushing to be pre-assembled to the buss plate by soldering, or welding the bushing to the buss plate about a hole in order to retain the bushing in position during the assembly operation. This requires a solder reflow, or similar process, to ensure that a plurality of connection points, have bushings therein, are simultaneously aligned for subsequent assembly. This soldering process is complex and in some cases has proven to be counterproductive and detrimental to providing a solid and reliable connection due to; (i) assembly alignment issues, (ii) thermal distortion introduced in the assembled components, (iii) compromised co-planarity, and (iv) increased softness of the bushing material. The soldering process also often results in corrosion due to the use of a fluxing agent, which interferes with a “hard” bushing to buss plate connection. 
         [0007]    In order to solve the above-described problems, the present invention is directed to the assembly of a buss plate board connecting system comprising an electrically conductive buss plate having a plurality of through holes, a bushing disposed inline to a through hole having a hole therein coinciding with a hole in the buss plate and a compressible cylindrical retainer inserted within the hole of the bushing and the buss plate through hole so as to retain the bushing in general alignment with the buss plate through hole. 
         [0008]    One object of the present invention is to provide an electro-mechanical inner connective means between buss plates and components that will overcome the above-described problems associated with pre-soldering the bushing in place by perpetually eliminating the bushing to buss plate soldering process with the use of a cylindrical spring bushing or retainer. 
         [0009]    In accordance with a further aspect of the present invention, there is provided a compressible cylindrical retainer that mechanically retains the bushing in order to facilitate ease of assembly, whereby the bushing is permitted nominal movement to compensate for hole tolerances and offsets. 
         [0010]    The assembly of the buss board structure, according to the present invention, can be greatly simplified by eliminating the necessity to align and solder the bushing onto the board prior to assembly. In effect, the bushing in the completed assembly is fundamentally secured to the buss board by the fastener and not the solder. Under the present process soldering of the bushing serves primarily as an interim means for the positioning of the bushing to assist in the assembly process, although in low-voltage applications the solder may assist with electrical conductivity. Once assembled there is little residual benefit and, in fact, soldering is all too often counterproductive and detrimental to a reliable connection due to the potential for; (i) misalignment, (ii) thermal distortion, (iii) corrosion and (iv) a weakened connection due to the metal fatigue from heating. Breaking or fracture of the soldered joints is even observed during the buss plane assembly process. 
         [0011]    Additionally, by virtue of accumulated tolerances within the soldering process, the mechanical alignment of the bushing within the aperture of the first buss plate may not conform directly to the vertical axis of the mating buss plate or component because a bushing solder in place, unlike a cylindrical retainer, is incapable of yielding to compensate for coplanar alignment errors. As previously discussed the bushing must be in total contact in order to effectively conduct the high currents because contact surface area or diameter of the bushing is a function of the peak current capacity of the connection, expressed in circular mills. Consequently, one amp requires an area of approximately 400 circular mills of the bushing where a circular mill is defined as the square of the diameter of an equivalent round conductor expressed in units of 10 −3  inches. Accordingly a skewed bushing will significantly decrease the contact area and therefore increase the connection resistance resulting in loss of power due to the generated heat. Therefore an objective in designing a high current power supply buss is to maintain low contact resistance by maximizing the contact surface area so as to provide only a nominal voltage drop and minimize the associated resistive heating effect in watts (W) at the connection point according to Ohm&#39;s Law where R=V 2 /W and W=I 2 ×R. 
         [0012]    The embodiment described and disclosed herein details aspects of the present invention in accordance with an interlocking cylindrical retainer/bushing assembly providing for an interim means to reliably locate the bushing onto the buss plate until a positive connection is established with a fastener passing through the buss plate and into an electrical component. The present invention therefore provides for significantly improved ease of assembly, improved reliability and reduced cost of manufacture. It is further contemplated that aspects of the disclosed embodiments permit the use of various and alternative materials, where the bushing is constructed of a material different from the buss plate and which may be plated or formed from a highly-conductive material. 
         [0013]    In accordance with an embodiment disclosed herein, there is provided a buss plate connecting system comprising: an electrically conductive buss plate having a plurality of through holes; a bushing, disposed inline to a through hole within the buss plate having a hole therein said bushing coinciding with the through hole in the buss plate; and a circumferentially compressible cylindrical retainer inserted within the hole of the bushing and the through hole of the buss plate, to retain said bushing in general alignment with said buss plate through hole. 
         [0014]    In accordance with another embodiment disclosed herein there is provided a buss plate connecting system having a first and second bushing, comprising: an electrically conductive buss plate having a plurality of through holes; a first and second bushing disposed inline to a through hole within the buss plate and having a hole therein said bushings coinciding with the hole in the buss plate; and a circumferentially compressible cylindrical retainer inserted within the hole of the first bushing, the buss plate through hole and the hole of the second bushing, to retain said bushings in general alignment with said buss plate through hole. 
         [0015]    In accordance with a further embodiment disclosed herein there is provided a method of interconnecting a buss plate with a component comprising: placing at least one bushing in proximity to a through hole within a buss plate; compressing a cylindrical retainer, having a longitudinal slot, to reduce its diameter; inserting said retainer through a hole in the bushing and the buss plate through hole; and releasing the retainer thereby urging the bushing in alignment with said through hole in said buss plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Other objects and features of the invention will become apparent to those skilled in the art as the disclosure is made in the following detailed description of a preferred embodiment of the invention as illustrated in the accompanying sheets of drawing, which are not necessarily drawn to scale, and in which: 
           [0017]      FIG. 1  is an exploded view of multiple buss plates within a buss board assembly; 
           [0018]      FIG. 2  is an exploded view of a power distribution assembly (PDA); 
           [0019]      FIG. 3  is cut-away view of an exemplary buss assembly; 
           [0020]      FIG. 4  is a planar cross-section view of the bushing assembly; 
           [0021]      FIG. 5  is a isometric exploded view of the bushing and spring retainer; 
           [0022]      FIG. 6  is a fragmentary cross sectional view of a bushing buss connection; 
           [0023]      FIGS. 7 and 8  are illustrative cross-sections of various embodiments of known bushings and the current invention, respectively; and 
           [0024]      FIG. 9  is an exemplary view of an alternative retainer configuration. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    For a general understanding of the present invention, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements. 
         [0026]    The disclosed embodiment(s) provides for the reliable union of a plurality of buss plates and components without the requirement for soldering the bushing in place. The bushing assembly is arranged in such a manner so as to secure one or more bushings in place with the insertion of a circumferentially compressed split cylinder retainer or ferrule or similar device having an interference fit within and between the inside diameter of the bushing and the hole within the buss plate. The split cylinder or ferrule is made from either a conductive or dielectric material containing resilient properties and having a longitudinal opening, or slit, to enable the pin to flex (e.g., compress) and therefore allow for the reduction of the inherent diameter for ease of insertion. In alternative embodiments, the split cylinder may be made from heat-treated stainless steel and copper alloys such as phos-bronze, beryllium copper, etc. In an alternative configuration it is contemplated that the split cylinder may be made from a wire-form embodied within plastic molded pieces. 
         [0027]    Although generally depicted in a cylindrical shape, it is further contemplated that the retainer  320 , described in detail below, may be non-cylindrical in its cross-sectional shape. For example, the retainer may have an elliptical or polygonal (e.g., hex-shaped) cross-section so as to prevent or reduce the rotation of the bushings relative to the plates. In other words, the retainer may also decrease the likelihood that the bushing may move—in any direction, including rotation. 
         [0028]    In a further alternative embodiment, as depicted in  FIG. 9 , the cylinder or ferrule may not be split but may be made from a material that provides resilient properties that enable the cylinder wall, or at least the outwardly-projecting portions thereof, to flex inwardly (e.g., compress) during insertion and therefore effectively reduce the outer diameter for ease of insertion. In such an embodiment, the cylinder may again be made from heat-treated stainless steel and copper alloys such as phos-bronze, beryllium copper, or a wire-form embodied within plastic molded pieces. It is further contemplated that the cylinder, in such an embodiment may be placed in an interference fit with the buss plates and other components once inserted. An example of such a retainer  320  is found, for example, in  FIG. 9 . At the ends of the cylinder flared rims  322 , where each of the flared rims may be placed in interfering contact with the adjacent busing or similar component (not shown), when inserted therein. It is also contemplated that while retainer  322  is formed of a continuous cylinder, the rims themselves may be split or segmented, thereby increasing the flexibility of the rims when they are being inserted through buss plates, into bushings, etc. Such splitting of the rims may improve the ease of inserting the retainer. 
         [0029]    Although generally depicted in a cylindrical shape, it is further contemplated that the retainer  320  depicted in  FIG. 9  may be non-cylindrical in its cross-sectional shape. For example, the retainer of  FIG. 9  may also have an elliptical or polygonal (e.g., hex-shaped) cross-section so as to prevent or reduce the rotation of the bushings relative to the plates. In other words, the retainer&#39;s shape may further decrease the likelihood that the bushing may move—in any direction, including rotation. 
         [0030]    The split cylinder further comprises a profile, including a flared rim, flange or formed ridge at each of the ends, whereby the first flared end interacts with a complementary chamfer, annular ring or groove fashioned around the open end of the hole on the opposite side of the buss plate. The second flared end of the split cylinder retainer engages, for example, an annular groove or recess about at least a portion of the internal diameter of the bushing hole. In one embodiment, the annular grove may be replaced by a coined rim, which may produce a continuous or regularly-spaced projection of the inner diameter for the bushing—thereby providing a feature on the bushing that positively engages with the flared end of the retainer. 
         [0031]    This engaging feature of the spring cylinder, such as the flared end or formed ridge, “locks” the bushing in position and maintains alignment between the bushing and the buss plate hole. Once the first end flare of the retainer is seated in the buss plate hole chamfer and the second end engaged within the groove of the bushing hole, the bushing/spring assembly is in order to accept a fastener, which is then screwed into a component connection. Accordingly, the construction of the power distribution assembly is now accomplished by simply fastening the plates to the components and thereby providing a well aligned electrical and mechanical interconnect without the requirement for pre-soldering a bushing in place. Furthermore, because the alignment of the contact bushing is accomplished using the retainer, the need for embossing or other pre-working of the conductive plates is reduced—leading to less distortion and fewer processing steps in the assembly process. 
         [0032]    Referring to  FIGS. 1 and 2 , an exemplary buss board assembly  100  constitutes a first electric power input circuit buss plate  105  and a second electric power input circuit buss plate  106  that connects a plurality of devices  125  (e.g., IGBTs) and capacitors  120  therebetween and first electric power output circuit buss  107  and second electric power output circuit buss  108  also connecting a plurality of devices  125  and capacitors  120 , as seen in  FIG. 2 . Located in direct proximity of each connection hole  420  within the buss plates is surface bearing bushing  110  that provides a high current electrical connection between buss board assembly  100  and the components, such as capacitor  120  and switch  125 . Conductive bushing  110  is an essential element in providing a sufficiently large connection surface bearing area that has the ability to physically contact a mating surface to provide a solid electrical connection with a minimum voltage drop. The conductive bushing may also be in the form of a flat-sided, washer-like component, and in one embodiment may also include a star-shaped locking washer, or similar features, to prevent problems with the backing-out or reversing of the fastener. 
         [0033]    One aspect of the present invention deals with the basic problem of reliably locating bushing  110  in proximity to hole  420  without necessitating the step of first soldering bushing  110  into position as a pre-assembly requirement. In order to maintain bushing  110  in position for assembly, cylindrical retainer  320  is compressed and inserted through bushing  110  and subsequently pressed into hole  420  within buss plate  105 , as depicted in  FIG. 3 . Once cylindrical retainer  320  is decompressed, buss plate hole  420  and bushing hole  430  provide a reactive force to the tension exerted from retainer  320 , thereby creating a contact or friction fit, as well as a positive interlock developed between the incorporation of anterior flare  425  within groove  122  (also see  FIG. 5 ). The retainer  320  thereby assures alignment of the conductive bushing  110  with the buss plate  105 . Furthermore, retainer  320  assures that a connector  310  can be easily and reliably inserted therethrough in order to complete the electrical connection between the buss plate  105 , bushing  110 , and a device or component  125  (e.g., a threaded hole  350  or nut in the device). 
         [0034]    Now referring to  FIG. 5 , cylindrical retainer  320  may be constructed from a conductive material having a high spring constant or resiliency, such as tempered high carbon steel, stainless steel and possibly alternative materials such as copper-berrylium, phos. bronze or other materials that exhibit spring-like resiliency. Non-conductive materials may also be used for retainer  320  such as polymers and reinforced carbon or fiberglass having the required elasticity without demonstrating evidence of a permanent deformation. Moreover, some materials may be composites that exhibit high resistance to heat, provide some level of conductivity, etc. 
         [0035]    The tubular profile of cylindrical retainer  320  contains a number of distinctive features, one of which is a flare located about each of the open ends of the cylindrical retainer. Posterior flare  415  rests within bevel  435  of buss plate hole  420  so as to limit retainer  320  from pulling directly through hole  420 . On the other hand anterior flare  425  engages groove or annular recess  122  within the annular diameter of hole  430  in bushing  110 . Additionally, installation apertures  325  allow for the engagement of a compression tool (not shown) to facilitate the insertion of retainer  320  within buss plate  105 . And lastly, a longitudinal slit  410 , or opening, allows for clearance during the reduction in diameter when retainer  320  is compressed. 
         [0036]    Also depicted in  FIG. 5  is an illustrative representation of a detent or notch  416  or similar removal means for permitting a tool to interact with the lead edge of the cylindricval retainer  320 , whereby the retainer  320  may be unseated or removed from a hole  430  or similar orifice into which it had been inserted. Removal means may be included at one or more locations around the periphery of the retainer  320 , and may be at or near the longitudinal slit  410  to facilitate disengagement or the removal of the anterior flare from the annular recess  122 . 
         [0037]    Referring next to  FIG. 6 , there is shown an embodiment of retainer  320  used in conjunction with two or more bushings  110  and at least one buss plate  105 . In this configuration anterior flare  425  snaps into the first bushing  110  while posterior flare  415  snaps into second bushing  110  having one or more buss plates therebetween. In this configuration, the bushings  110  are each held in place and permit the subsequent insertion of threaded members or other fastening means, such as bolt  310 , therethrough to enable the make-up and joining of components using the buss plates. 
         [0038]    As will be appreciated, particularly from the illustrations in  FIGS. 3-6 , one embodiment contemplates a method of interconnecting a buss plate with a component. The method includes placing at least one bushing  110  in proximity to a through hole  420  within a buss plate  105 , compressing the cylindrically-shaped retainer  320 , to reduce its diameter, inserting the retainer through a hole  430  in the bushing and the buss plate through hole  420 , and then releasing the retainer  320 . The release of the spring-like retainer allows it to return to a nominal state where it urges the bushing to remain in alignment with the through hole in the buss plate. 
         [0039]    As a further illustration of several advantages of the embodiments disclosed herein, reference is made to  FIGS. 7 and 8 . In  FIG. 7 , there are depicted conventional connections  710 ,  720  and  730  relative to the various buss plates  105 ,  106  and  107 . As illustrated, the connections  710 ,  720  and  730  include a shoulder  750 , and a wall thickness suitable to produce the shoulder and to provide a portion  752  that extends through an aperture  420  in the buss plate. Compared with  FIG. 8 , where the connector assemblies  810 ,  820  and  830  include upper and lower connection bushings joined by the compressible retainer  320 , it is apparent that the embodiments of  FIG. 8  provide greater contact with the buss plate, and result in greater ease of assembly of the buss plate, where there is improved ability to align the resulting connectors. 
         [0040]    In recapitulation, the present invention is a method and apparatus for the expeditious and reliable assembly of buss plates and components thereon, by eliminating the operation of soldering a bushing in place. A unique cylindrical retainer has been discovered that serves to retain the bushing in a desired position in order to facilitate ease of assembly. While this invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.