Patent Publication Number: US-2011056916-A1

Title: Vaccum Switch Chamber with a Protective Sleeve that is Applied by Heat Shrinking

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 11/630,804, filed Dec. 27, 2006, (to issue as U.S. Pat. No. 7,832,079), which is a national phase application under 35 U.S.C. §371 of International Application No.: PCT/EP2005/006885, which has an international filing date of Jun. 27, 2005 and claims priority from German patent application, No.: DE102004031089.0, having a filing date of Jun. 28, 2004, all of the foregoing previously-filed patent applications being hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention concerns a vacuum interrupter chamber and a method for producing the same. 
     The invention relates to what are known as resin-encapsulated pole pieces with a vacuum interrupter chamber, the pole pieces being used in low-, medium- and high-voltage technology. The vacuum interrupter chamber comprises a metal body which is evacuated inside and in which the switching contacts are located. Vacuum interrupter chambers of this type are usually embedded in epoxy resin or casting resin, or are enclosed by the same. It must be ensured here that the coefficients of expansion and other thermal parameters of the casting resin are different from those of the metal used for the vacuum interrupter chambers. 
     In operation, but also already in production, mechanical stresses may therefore occur during the encapsulation of such vacuum interrupter chambers as a result of different coefficients of thermal expansion, with the effect that intimate contact between the metal surface of the vacuum interrupter chamber and the encapsulation is not ensured. In order to avoid this, cushioning coatings of the vacuum interrupter chambers which consist of an elastic material and surround the vacuum interrupter chambers in an intimate manner are known in the prior art. In this case, the vacuum interrupter chamber with the coating or enclosure of elastic material already surrounding it is then cast in epoxy resin. 
     It is important here that the applied elastic material rests on the vacuum interrupter chamber in an intimate manner, that is to say without air bubbles, before it is encapsulated. Further requirements here are also that the system of layers remains stable during the casting. 
     To satisfy these requirements, it is known in the prior art to use tubes of elastic material which are made to expand by mechanical aids to the extent that they can be placed over the vacuum interrupter chamber. After removing the mechanical expanding means, the tube then comes to lie against the surface of the vacuum interrupter chamber. This generally means that the material comes to lie against the surface of the vacuum interrupter chamber with intimate contact, the elastic material in most cases remaining expanded even after it has come to lie against the surface of the vacuum interrupter chamber. 
     This means that, if the interrupter chamber were not placed there and the mechanical expanding means were removed, the diameter of the elastic tube that has relaxed as a result would be smaller than the outside diameter of the vacuum chamber. On the basis of these known parameters, already advantageously used in the prior art, the intimate contact with the surface that is described above occurs. 
     In the case of a vacuum interrupter chamber of the generic type described and in particular in the case of the production of such a described vacuum interrupter chamber for later encapsulation, the known parameters and measures of the method that are in themselves used with advantageous effect are expedient, but the production method is complex for such a vacuum interrupter chamber to be prepared for later encapsulation. 
     However, it is desired to continue with the arrangement of a corresponding tube, which on the one hand protects the interrupter chamber from damage and on the other hand brings about an increase in the insulating clearance and, furthermore, compensates for the different coefficients of expansion of the materials mentioned. Therefore, in the prior art, such a tube or protective sheath of the type described is applied at ambient temperature. However, it must be ensured here that the vacuum chamber together with the tube applied to it in this way is preheated to a temperature before it is encapsulated in casting resin. However, the use of such an elastic material that is cold when it is applied means that the heating during the further production process causes the permissible material temperature of the elastic material to be exceeded, thereby initiating premature aging. This kind of application increasingly causes the tube to be damaged before and during casting. This impairs the mechanical stability of the casting, and also the electrical parameters. 
     Based on the foregoing, it would be desirable to provide a resin-cast vacuum interrupter and a method for producing the same that has the advantages described above but avoids the disadvantages described above. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a resin-encapsulated pole piece is provided and includes a vacuum interrupter chamber having an outer surface. A protective sheath covers the outer surface of the vacuum interrupter chamber and is composed of a heat-shrinkable material. The protective sheath has been pulled over the vacuum interrupter chamber and then heat shrunk so as to be in firm contact with the outer surface of the vacuum interrupter. An outer casing encapsulates the vacuum chamber covered by the protective sheath. The outer casing is composed of a cast resin and is in contact with the protective sheath. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  is a schematic drawing of a tube composed of heat shrinkable material disposed over a vacuum interrupter chamber, wherein the tube has not yet been shrunk; and 
         FIG. 2  is a schematic drawing of the tube disposed over the vacuum interrupter chamber after the tube has been shrunk with the application of heat. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     It should be noted that in the detailed description that follows, identical components have the same reference numerals, regardless of whether they are shown in different embodiments of the present invention. It should also be noted that in order to clearly and concisely disclose the present invention, the drawings may not necessarily be to scale and certain features of the invention may be shown in somewhat schematic form. 
     The essence of the device-related invention concerning the vacuum interrupter chamber is that, in preparation for later casting, the vacuum interrupter chamber is still provided on the outer surface with a protective sheath consisting of an elastic, or elastomeric or plastomeric material, which is applied to the surface of the vacuum interrupter chamber without further mechanical aids by heat shrinkage. This achieves the advantage of a damping layer for the later casting of the vacuum interrupter chamber, that is to say this covering also ensures compensation for the different coefficients of thermal expansion and also satisfies the electrical and insulating requirements. 
     If appropriate, before this jacket or sheath is pushed over the vacuum interrupter chamber, the sheath is in this case also made to expand by mechanical means. When it is being pulled over the vacuum chamber, however, these means are no longer necessary, because in the cold state the heat shrinkage material initially remains in this stretched condition. However, it is important here that the sheath does not then relax immediately, as in the prior art, but initially remains in a stretched state until the heat shrinkage makes the tube come to lie against the surface of the vacuum interrupter chamber with intimate and firm contact. 
     In a further advantageous refinement, it is specified that the dimensioning of the tube or its final dimensioning once heat shrinkage has taken place is designed such that it comes to lie around the cylindrical outer surfaces with intimate contact, and also at the ends likewise comes to lie at least partially against the surface of the vacuum interrupter chamber around the edges. 
     With respect to a method for producing the same, a tube consisting of heat shrinkage material which has a greater circumference in its state before it is pulled onto the vacuum interrupter chamber than that of the interrupter chamber is pulled over the same and positioned there, and is subsequently shrink-fitted onto the surface of the vacuum interrupter chamber by a heat shrinkage process. Therefore, further mechanical means are no longer necessary, as customary in the prior art, for applying the tube to the vacuum interrupter chamber. In addition, the tube can consequently be easily pulled over the vacuum interrupter chamber and then, by means of a simple method step, be brought quickly and very efficiently into similarly close and intimate contact with the surface of the vacuum interrupter chamber. 
     This is followed by casting in epoxy resin. It is of particular advantage here that the heat shrinkage material is a material which is stable in its consistency at different temperatures as compared to the materials previously used in the prior art, which consisted of rubber or silicone rubber or the like. The subsequent casting in epoxy resin likewise takes place at a temperature of at least 130 degrees. By contrast with materials from the prior art, however, the heat shrinkage tube according to the invention remains stable from technical aspects of its material and in its consistency and the encapsulation as such does not cause aging of the material. 
     The invention is schematically represented in the drawing only to the extent necessary and is described in more detail below. 
       FIG. 1 : For shrink-fitting a heat shrinkage tube  2  onto a vacuum chamber  1 , the expanded tube is pulled over the chamber in the way described. However, the expanding of the tube may take place beforehand and, as a result, mechanical means that are otherwise necessary when pulling the heat shrinkage tube onto the vacuum chamber are no longer necessary. 
     This is followed by thermal treatment at 130 degrees Celsius, so that the tube shrinks onto the vacuum interrupter chamber. The tube is of such a nature that it assumes the outer contour of the chamber and therefore smooths the surface. A dielectrically sealed joint with an increase in the insulating clearance is produced. This helps to optimize the form of construction of vacuum interrupter chambers and to minimize the production costs. Suitable characteristic material values of the tube avoid impermissible thermal loading before and during casting. 
     The form of the expanded tube is conducive to automation of the production process to a much greater extent than was possible with known methods. 
       FIG. 2  shows how, after the heat treatment, the shrinkage tube comes to lie around the vacuum interrupter chamber. 
     The vacuum interrupter chamber created in this way and firmly encased in this way by the shrink-fitted heat shrinkage tube is then surrounded by an epoxy resin casting, or embedded in such, which is not represented any further here. The casting in this case lies directly against the outer skin of the heat shrinkage tube. 
     It is to be understood that the description of the foregoing exemplary embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.