Abstract:
A sealing apparatus and a method for sealing, which make possible reduced installation space and assembly at room temperature, are proposed. The sealing apparatus encompasses a ceramic base element and a metallic housing. The ceramic base element comprises on an outer wall at least one circumferential flute in the region of which the housing is pressed in positively fitting fashion onto the ceramic base element.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is based on a sealing apparatus, and from a method for sealing.  
       BACKGROUND INFORMATION  
       [0002]     In order to ensure the functionality of, for example, a spark plug, engine gases must not escape between a spark plug housing and a ceramic insulator of the spark plug. The ceramic insulator must be installed sealedly into the spark plug housing in such a way that gas-tightness is guaranteed at up to 20 bar, at a maximum temperature of 220° C.  
         [0003]     Known for this purpose, for example, is hot assembly, in which the spark plug housing, after introduction of the ceramic insulator, is heated to approximately 950° C. in the region of a shrinkage zone. During this, the spark plug housing is pressed onto the ceramic insulator by applied forces. Upon cooling of the shrinkage zone, tensile stresses are produced in the spark plug housing with respect to the ceramic insulator. The applied forces are then removed. The ceramic insulator is thereby sealed in gas-tight fashion with respect to the spark plug housing.  
         [0004]     A further method for gas-tight sealing of the ceramic insulator with respect to the spark plug housing is achieved by way of a cold assembly method with powder sealing. Here the ceramic insulator, together with a fine ceramic powder, is pushed under load into the spark plug housing. The upper rim of the spark plug housing, through which the ceramic insulator was introduced into the spark plug housing, is then clinched over by axial forces in a crimping process, so that the spark plug housing abuts, at its upper rim as well, against the ceramic insulator and holds the latter in the spark plug housing in gas-tight and sealing fashion.  
       SUMMARY OF THE INVENTION  
       [0005]     The sealing apparatus and method for sealing according to the present invention having the features of the independent claims have, in contrast, the advantage that the ceramic base element comprises on an outer wall at least one circumferential flute in the region of which the housing is pressed in positively fitting fashion onto the ceramic base element. This makes possible assembly of the sealing apparatus at room temperature. As a result, all common corrosion protection coatings, for example zinc, transparent chromating, or corrosion protection lacquer, can be applied onto the metallic housing prior to assembly of the sealing apparatus. Furthermore, it is not necessary for the ceramic base element to have a shoulder in the region of the upper rim of the housing through which the ceramic base element is introduced into the housing, as is the case, for example, with spark plugs in order to receive a crimping of the upper rim of the housing. To the contrary, the gas-tight seal is ensured solely by the pressing of the housing onto the ceramic base element in the region of the at least one flute. Because the aforesaid shoulder is omitted, the ceramic base element can be implemented with a smaller cross-sectional area and thus with a smaller diameter. This is advantageous in particular for use of the sealing apparatus in a spark plug, a sheathed-element glow plug, or a lambda sensor, since space in the cylinder head or in the exhaust system is thus saved and is therefore available for other components, for example injection valves or cooling channels.  
         [0006]     It is advantageous if the ceramic base element is at least partially solder-joined to the housing. The gas-tightness of the sealing apparatus can be even further enhanced in this fashion.  
         [0007]     A particularly simple method for sealing the ceramic base element in the metallic housing results when the ceramic base element, in the context of a mechanical reshaping method, in a first step is introduced into the housing substantially coaxially with the housing, and when in the second step a reduction or drawing ring is placed on an outer boundary of the housing and is pushed in the radial direction into at least one flute, in order to press the housing sealingly onto the ceramic base element in the region of the at least one flute. This process requires little outlay in terms of assembly and tools.  
         [0008]     It is furthermore advantageous if the reduction or drawing ring is also pushed tangentially with respect to the at least one flute against the outer edge of the housing, while the ceramic base element is held in the housing against the tangential force. In this fashion the housing is pressed, in the region of the at least one flute, against a delimiting wall of the flute both radially and also tangentially with respect to the at least one flute, so that a greater gas-tightness of the resulting positive fit between housing and ceramic base element can be achieved.  
         [0009]     A further increase in gas-tightness can also be achieved by heating the housing, before the second step of pressing the housing in positively fitting fashion onto the ceramic base element in the region of the at least one flute, so that the housing elongates; and by cooling the housing after the second step so that it contracts and tensile stresses are produced in the housing with respect to the ceramic base element in the region of the at least one flute. This feature once again enhances the hot tightness of the seal that is formed between the ceramic base element and the metallic housing. “Hot tightness” is understood here as the tightness, in particular the gas-tightness, of the sealing apparatus upon heating.  
         [0010]     A further advantage lies in the fact that the housing is heated to approximately 300° C. As a result, all common corrosion protection coatings, for example zinc, transparent chromating, or corrosion protection lacquer, can be applied before assembly of the sealing apparatus and before sealing of the ceramic base element in the metallic housing, without causing those corrosion protection coatings to reach their melting point as a result of the heating.  
         [0011]     A further advantage lies in the fact that the ceramic base element is cooled during heating of the housing. This increases the temperature difference between the housing and the ceramic base element, thereby increasing, the tensile stresses produced in the housing, after cooling of the housing, with respect to the ceramic base element in the region of the at least one flute. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows the method step essential for the method for sealing according to the present invention.  
         [0013]      FIG. 2  shows the sealing apparatus according to the present invention constituted by such a method. 
     
    
     DETAILED DESCRIPTION  
       [0014]     In  FIG. 1, 1  designates a sealing apparatus that can be used, for example, for a spark plug, a sheathed-element glow plug, or a lambda sensor. In the case of the spark plug or sheathed-element glow plug, the sealing apparatus is used in an engine compartment, for example in a cylinder head; whereas in the case of a lambda sensor it is used in an exhaust duct. Sealing apparatus  1  encompasses a ceramic base element  5  that has, on an outer wall  15 , at least one circumferential flute  20 . In  FIG. 1 , sealing apparatus  1  that is to be constituted is shown in a longitudinal section, flutes  20  being implemented in the form of constrictions around the circumference of outer wall  15  that reduce the cross-sectional area and the diameter of the cross section of ceramic base element  5 . In a first method step upon assembly of sealing apparatus  1 , ceramic base element  5  is introduced or inserted into a metallic housing  10  along a longitudinal axis  45  of housing  10 . Ceramic base element  5  has a sealing seat  40  at which, upon introduction into housing  10 , it makes contact against a sealing ring  50  protruding in the interior of housing  10 .  
         [0015]     Ceramic base element  5  lies in housing  10  substantially coaxially with housing  10  with respect to longitudinal axis  45 , as shown in  FIG. 1 . In the region of a bottommost flute  55 , facing toward sealing seat  40 , of ceramic base element  5 , housing  10  has a circumferential outer edge  35 . In a second method step, a reduction or drawing ring  30  is placed on this outer edge  35 . The inside diameter of reduction or drawing ring  30  proceeds from a value that is less than the diameter of outer edge  35  to a value that is greater than the diameter of outer edge  35 . For this exemplary embodiment, it is to be assumed by way of example that ceramic base element  5  and housing  10  are disposed in substantially rotationally symmetrical fashion, and have a cross section of substantially circular or annular shape. When reduction or drawing ring  30  is then placed, with its inside diameter varying as described, on outer edge  35 , and is pushed by an application force against outer edge  35  oppositely to the insertion direction of ceramic base element  5 , in the arrow direction labeled with reference character  55 , radial and tangential forces thus act on ceramic base element  5  in the region of flutes  20 . The radial forces are directed toward longitudinal axis  45  and thus toward flutes  20 , and are thus perpendicular to arrow direction  55 . The tangential forces extend tangentially with respect to flutes  20  and thus in arrow direction  55 . In this operation, ceramic base element  5  is pushed into housing  10  in the insertion direction (which is identified in  FIG. 1  by reference character  60  and thus extends oppositely to arrow direction  55 ), and is held in housing  10  in the region of sealing ring  50  and sealing seat  40 . Outer edge  35  is part of an elevation  65  on an outer wall  70  of housing  10 . Elevation  65  of housing  10  extends substantially in the region in which ceramic base element  5 , inserted into housing  10 , has flutes  20 . Reduction or drawing ring  30  is displaced by corresponding pressure over elevation  65  in arrow direction  55  oppositely to insertion direction  60 , beginning at outer edge  35 , so that housing  10  is pressed in positively fitting fashion onto ceramic base element  5  in the region of elevation  65  and thus of flutes  20 . As a result of the variable inside diameter (as described) of reduction or drawing ring  30 , which diameter assumes smaller values even than the diameter of outer edge  35  and thus of elevation  65  as depicted in  FIG. 1 , elevation  65  is reduced to this smallest inside diameter of reduction or drawing ring  30 . This is depicted in  FIG. 2 , in which the positively fitting join thus formed between housing  10  and ceramic base element  5  after pressing is illustrated by way of reference character  75 . In this context, housing  10  conforms to a certain extent, in the region of flutes  20 , to the delimiting walls of flutes  20 . With suitable pressure from reduction or drawing ring  30  upon displacement over elevation  65  in arrow direction  55 , the join formed between housing  10  and ceramic base element  5  in the region of flutes  20  is also gas-tight, for example to  20  bar. The method described for pressing housing  10  onto ceramic base element  5  in the region of flutes  20  is a mechanical reshaping method.  
         [0016]     As an alternative to the mechanical reshaping method just described, provision can also be made to compress housing  10  at elevation  65  in the radial direction with respect to longitudinal axis  45  (and thus to flutes  20 ), for example by using round pliers, in order to press housing  10  onto ceramic base element  5  in the region of flutes  20 . A tangential force, as depicted by arrow direction  55  in  FIG. 1  for the first exemplified embodiment, is then not applied in this alternative embodiment. With appropriate radial pressure, however, a correspondingly gas-tight join can likewise be achieved between housing  10  and ceramic base element  5  in the region of flutes  20 , housing  10  once again, as depicted in  FIG. 2 , conforming to a portion of the delimiting walls of flutes  20 .  
         [0017]     The radial and/or tangential forces described can also, alternatively or additionally, be achieved by way of a magnetic reshaping method, in which a correspondingly strong magnetic field is created in a short period in the region of elevation  65  so that housing  10  is pressed onto ceramic base element  5  in the manner described.  
         [0018]     Provision can additionally be made for housing  10  to be heated, especially in the region of elevation  65 , before the second method step. As a result, housing  10  is elongated in the direction of longitudinal axis  45  in the region of elevation  65 . The heating of housing  10  can be accomplished before or after the introduction of ceramic base element  5  into housing  10 . When housing  10  is then cooled again after the second method step, it thus contracts in the region of elevation  65  so that tensile stresses are produced in housing  10 , with respect to ceramic base element  5 , in the region of flutes  20 . These tensile stresses enhance the gas-tightness achieved, by way of the magnetic and/or mechanical reshaping method described, in the join between housing  10  and ceramic base element  5  as shown in  FIG. 2 . This effect can also be intensified if ceramic base element  5  is cooled or kept cool during the heating of housing  10 . The temperature difference between ceramic base element  5  and housing  10  is thus increased, so that the tensile stresses produced after cooling of housing  10  are further increased. The tensile stresses brought about as a consequence of the heating of housing  10  also result in enhanced hot tightness of the sealing apparatus, i.e. an enhanced tightness when the sealing apparatus is operated at high temperatures, as is the case e.g. with spark plugs, sheathed-element glow plugs, or lambda sensors.  
         [0019]     Advantageously, in order to produce the desired tensile stresses, housing  10  is heated to a temperature that is below the melting temperature of common corrosion protection coatings, for example zinc, transparent chromating, or corrosion protection lacquer. The advantageous result is that metallic housing  10  can be equipped, before the assembly of sealing apparatus  1 , with such a corrosion protection coating, which then does not melt upon heating of housing  10  to produce the desired tensile stresses and is not thereby destroyed. Heating of the metallic housing  10  to approximately 300° C. satisfies the requirement that the desired tensile stresses be produced; this temperature also lies below the melting temperature of all common corrosion protection coatings.  
         [0020]     The heating operation just described will be referred to hereinafter as “semi-hot” assembly.  
         [0021]     Alternatively or in addition to semi-hot assembly, the gas-tightness of sealing apparatus  1  constituted by the above-described magnetic or mechanical reshaping operation can also be enhanced by the fact that in a third method step, ceramic base element  5  is at least partially soldered to housing  10 . This requires the use of a solder that bonds both to the metallic housing  10  and to ceramic base element  5 . This can be achieved, for example, with a silver solder. Gas-tightness is enhanced in particular, in this context, by the fact that ceramic base element  5  is soldered to housing  10  in the region of flutes  20  in which a seal between ceramic base element  5  and housing  10  has already been achieved, in the second method step, by way of the above-described magnetic and/or mechanical reshaping method and, optionally, by way of the above-described semi-hot assembly.  
         [0022]     The number of flutes mentioned in ceramic base element  5  can be selected to be equal to 1 or any number greater than 1.  
         [0023]     Ceramic base element  5  can be embodied as an insulator of a spark plug, and in that case is also referred to as a plug insulator. Metallic housing  10  is then, in this case, a plug housing of the spark plug.  
         [0024]     Alternatively, however, ceramic base element  5  can also be embodied as the heating element of a sheathed-element glow plug, the metallic housing  10  then being a plug housing of the sheathed-element glow plug.  
         [0025]     Alternatively, however, ceramic base element  5  can also be embodied as the base element of a lambda sensor, the metallic housing  10  then being a housing of the lambda sensor.