Abstract:
A flap valve for a motor vehicle exhaust system includes a valve housing in which a valve flap is pivotally mounted about a shaft. A bearing housing is disposed in a gas-tight manner outside the valve housing. The shaft is extended through a first orifice in the valve housing and emerges from the bearing housing through a second orifice. Inside the bearing housing, the shaft is surrounded by a bearing ring having a supporting surface pointing towards the valve flap. A spring element, which is mounted in the bearing housing, surrounds the shaft and is supported at the supporting surface and directly or indirectly at the shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a continuing application, under 35 U.S.C. § 120, of copending International Application No. PCT/EP2005/007332, filed Jul. 7, 2005, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German Patent Application DE 10 2004 032 856.0, filed Jul. 7, 2004; the prior applications are herewith incorporated by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a flap valve for a motor vehicle exhaust system. 
   Such a valve which is known, for example, from European Patent EP 0 835 998 B1, is used generally as a shut-off member in branched exhaust systems. It has a valve housing in which a flap is mounted rotatably about a shaft running transversely relative to a central longitudinal axis of the valve housing or transversely relative to a flow duct delimited by the latter. That valve flap is pivotable between a closing position covering the inner cross-sectional area of the valve housing and an opening position uncovering or releasing the inner cross-sectional area. A bearing housing is mounted, for example welded, gas-tightly, on the outside of the valve housing. The shaft projects through a first orifice in the valve housing into the bearing housing and projects through a second orifice in the bearing housing out of the latter again. Within the bearing housing, the shaft is surrounded by a bearing ring which has a spring-loaded supporting surface pointing toward the flap and oriented transversely with respect to the shaft and a sliding surface pointing away from the flap and cooperating with an inner wall region of the bearing housing to provide a slide pairing. The inner wall region delimits the second orifice. The sliding surface is pressed against the inner wall region by a spring element which surrounds the shaft and which is supported, on one hand, on the supporting surface and, on the other hand, indirectly or directly on the shaft. In such a flap valve, the material of the bearing ring is selected in such a way that, when the shaft is actuated rotationally, frictional forces as low as possible have to be overcome, but sufficient sealing off is nevertheless ensured. In other words, a situation is prevented in which exhaust gas may pass outward through a separating gap between the bearing ring and the inner wall region of the bearing housing. Materials which fulfill those requirements, for example ceramic materials, have, as a rule, a lower thermal expansion than the material of the shaft, for example stainless steel. During operation, temperatures of 600° C. and above are reached in the bearing housing. In order to allow a radial expansion of the shaft in the event of such heating, without the bearing ring being destroyed in that case, a correspondingly large play must be provided between those parts. As a result of that play, however, exhaust gases may pass outward between the shaft and the bearing ring. In the flap valve known from European Patent EP 0 835 998 B1, such a leakage of exhaust gas is prevented by using a spring washer stack as a spring element. The individual spring washers lie in each case with their inner and outer edges one on the other in a more or less gastight manner and thus form a cylinder with a closed cylinder wall. The two outer spring washers sealingly bear respectively flat against the supporting surface of the bearing ring and against a countersurface on the shaft. That region of the shaft which is disposed between the bearing ring and the countersurface is thus sealed off, substantially gas-tightly, so that no exhaust gas can pass outward through the annular gap present between the bearing ring and shaft due to the bearing play. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a flap valve for a motor vehicle exhaust system, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which manages without costly spring washers, while an escape of exhaust gas through an annular gap between a bearing ring and a shaft is nevertheless prevented or at least reduced. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a flap valve for a motor vehicle exhaust system. The flap valve comprises a valve housing having a central longitudinal axis, an inner cross-sectional area, an outside and a first orifice. A bearing housing which is mounted gas-tightly on the outside of the valve housing has an inner wall region and a second orifice delimited by the inner wall region. A shaft extended transversely relative to the central longitudinal axis of the valve housing projects through the first orifice in the valve housing into the bearing housing and emerges from the bearing housing again through the second orifice in the bearing housing. The shaft has a central longitudinal axis and is formed of a material having a thermal expansion. A valve flap is pivotable in the valve housing about the shaft between a closing position covering the inner cross-sectional area and an opening position uncovering the inner cross-sectional area. A bearing ring surrounding the shaft within the bearing housing has a supporting surface pointing toward the valve flap and oriented transversely relative to the shaft and a sliding surface pointing away from the valve flap and cooperating with the inner wall region of the bearing housing to provide a slide pairing. The bearing ring has at least one region, including the sliding surface, being formed of a material with a lower thermal expansion than the material of the shaft. At least one mineral fiber mat is compressed in direction of the central longitudinal axis of the shaft and is provided as a spring element surrounding the shaft in the bearing housing. The at least one mineral fiber mat is supported on the supporting surface of the bearing ring and indirectly or directly on the shaft. 
   In accordance with another feature of the invention, the at least one mineral fiber mat is a mat used for mounting monoliths in exhaust systems of motor vehicles. Such mats, particularly when they are mats which are used for the mounting of monoliths in exhaust systems of motor vehicles, generate return forces in the compressed state which equate to those of a helical spring or of a spring washer stack and are sufficient to press the bearing ring against the bearing housing with sufficiently high force and to ensure the required leak-tightness. In the compressed state, the fibers of these mats are pressed together, leak-tightly, in such a way that a passage of exhaust gas can be observed, at most, to an insignificant extent. 
   In accordance with a further feature of the invention, mats with polycrystalline fibers are preferably used. These mats are resistant to high temperature and can be employed at temperatures of more than 1000° C. Polycrystalline fibers are fibers with an aluminum oxide content of &gt;63% by weight and are produced by the sol-gel method from aqueous spinning solutions. Preferred mats contain 80 to 99% by weight of polycrystalline fibers. In order to make handling and mounting easier, polycrystalline as well as other mineral fiber mats are used which contain an organic binder, preferably a binder on an acrylic base. Such binders, as well as other organic binders which are contained in a fraction of 1 to 20% by weight, burn virtually without any residue, at the prevailing operating temperatures. 
   In accordance with an added feature of the invention, the material for the bearing ring, which has both good sliding properties and sealing properties in cooperation with a metallic surface, that is to say the above-mentioned inner wall region of the bearing housing, is a ceramic material. This material, of course, has a substantially lower thermal expansion than the metallic shaft which is formed, for example, of stainless steel. A correspondingly large play is therefore required between the shaft and the bearing ring in order to ensure an unimpeded radial expansion of the shaft at high temperatures. The bearing ring may be formed entirely of ceramic material. It is also conceivable, however, that only a region carrying the sliding surface is formed of ceramic. 
   In accordance with a concomitant feature of the invention, the spring element is supported directly on the shaft. For this purpose, an outwardly extending radial shoulder is present on the latter. Preferably, however, between the spring element and the radial shoulder, there is a bushing which surrounds the shaft and which with its circumferential surface cooperates with the inner surface of the bearing housing providing a slide pairing. Since the bushing is a separate part, it can be manufactured from a material which has lower coefficients of friction, as compared with the metallic bearing housing, than would be the case in a metal-to-metal friction pairing. A bushing formed of ceramic material is preferably employed. 
   Other features which are considered as characteristic for the invention are set forth in the appended claims. 
   Although the invention is illustrated and described herein as embodied in a flap valve for a motor vehicle exhaust system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic, cross-sectional view of a flap valve; 
       FIG. 2  is a fragmentary, cross-sectional view orthogonal to a portion of  FIG. 1 ; and 
       FIGS. 3A to 3C  are fragmentary, cross-sectional views, corresponding to a portion of  FIG. 2 , illustrating different slide pairings between a bearing ring and a bearing housing. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there is seen a flap valve which includes a valve housing  1  constructed as a tubular segment, a valve flap  3  disposed in the latter and mounted rotatably with the aid of a shaft  2 , and a bearing housing  4 . The shaft  2  extends transversely with respect to a central longitudinal axis  11  of the valve housing  1  or of a flow duct  21  surrounded by the latter. A wall  5  of the valve housing  1 , having a circular inner cross section, is pierced by a first orifice  6 . The shaft  2  extends through this orifice into the bearing housing  4 . An outer end  8  of the shaft, facing away from the valve flap  3 , passes through a second orifice  7  which is present in the bearing housing  4 . An inner end  9  of the shaft  2  is widened radially, and a transition between narrower and wider shaft parts is constructed as a radial shoulder  12  extending at right angles with respect to a central longitudinal axis  10  of the shaft  2 . The inner end  9  of the shaft  2  carries the valve flap  3 . For this purpose, the end  9  has a non-illustrated axial slot, in which the valve flap  3  is inserted at an edge region and fixed, for example through the use of a weld. At a location lying diametrically opposite the inner end  9  and pierced by the central longitudinal axis  10 , the valve flap  3  has fixed to it a bearing journal  13  which engages in a cup-shaped protuberance  14  in the wall  5  of the valve housing  1 . 
   The approximately sleeve-shaped bearing housing  4  is inserted, with its end  15  facing an inner space of the valve housing  1 , into the first orifice  6  of the valve housing and is welded to the wall  5 . That region of the bearing housing  4  which, for example, adjoins the second orifice  7 , is constructed in the form of a cone  16  opening toward the flap  3 . A bearing ring  17 , being formed of a ceramic material, is disposed in this region of the bearing housing  4 . As is seen in  FIG. 2 , the bearing ring  17  surrounds the shaft  2  so as to leave an annular gap  18  free. In this case, the annular gap  18  is dimensioned in such a way that, in the event of heating to temperatures of &gt;600° C. while the vehicle is in operation, the shaft can expand radially, without expanding the bearing ring  17  radially in this case, which would result in the destruction of the latter due to the brittle ceramic material. The bearing ring  17  has a planar supporting surface  19  facing the valve flap  3  and running transversely with respect to the central longitudinal axis  10 . Furthermore, a sliding surface  20 , which is present on the bearing ring  17 , points away from the valve flap  3  and cooperates with an inner wall region  22  of the cone  16  or of the bearing housing  4  with the effect of a slide pairing. The sliding surface  20  is curved and is part of a spherical or toroidal surface. By virtue of this configuration, only a narrow, virtually linear contact region  23  is present between the inner wall region  22  and the sliding surface  20 . 
   A mineral fiber mat  25  is disposed in an annular space  24  which is present between the shaft  2  and the inner wall of the bearing housing  4 . This mat  25  is a mat having polycrystalline fibers. Such a mat, which is also used for the mounting of monoliths, is obtainable under the trademark MAFTEC® of the company Thermal Ceramics, in the United States. This is a mat with 80 to 99% by weight of polycrystalline fibers which have a fraction of more than 63% by weight of aluminum oxide. The fibers are connected to one another through the use of 1 to 20% by weight of acrylate binder. The mineral fiber mat  25  is disposed in the annular space  24  in such a way that its fibers run substantially transversely with respect to the shaft  2  or with respect to the central longitudinal axis  10  of the latter. In the compressed state, the mineral fiber mat  25  is supported, on one hand, on the supporting surface  19  of the bearing ring  17  and, on the other hand, on an end face  26 , facing away from the valve flap  3 , of a bushing  29  surrounding the shaft  2 . The bushing  29 , in turn, bears flat, with its other end face  27 , against the radial shoulder  12 . In an actual exemplary embodiment, as is indicated in  FIG. 2 , the annular space  24  has a height  28  of 8 mm, an outside diameter  36  (corresponding to the inside diameter of the bearing housing  4  in the region of the mineral fiber mat  25 ) of 14 mm and an inside diameter  37  (corresponding to the diameter of the shaft  2 ) of 6 mm. Two mat rings  25   a ,  25   b  punched out of a mat blank and having an initial thickness in each case of 8 to 9 mm and a weight per unit area of 1200 g/m 2  are pressed into the annular space  24 . The outside diameter of the uncompressed mat rings  25   a ,  25   b  is 14 mm and their inside diameter is 5.5 mm. By virtue of these dimensions, the mat rings  25   a ,  25   b  can be introduced easily into the annular space  24 , but fill the latter completely. The initial density of the mat rings  25   a ,  25   b  is approximately 160 kg/m 3 , whereas, after axial compression, the mat has a nominal density of approximately 320 kg/m 3 . In the compressed state of the mineral fiber mat  25  being formed of two parts, a surface pressure of approximately 12 N/cm 2  and a spring force of approximately 18N are achieved. This spring force is sufficient to press the bearing ring  17  with its sliding surface  20  against the inner wall region  22  of the bearing housing  4  in such a way that sufficient leak-tightness is ensured. The mineral fiber mat  25  performs its sealing and spring functions even at temperatures of more than 1000° C. Leakages of less than 2 l/min at 300 mbar are achieved in this case. 
   The bushing  29  likewise surrounds the shaft  2  so as to leave an annular gap  18   b  free, in order to allow the above-mentioned radial expansion of the shaft  2 . The outside diameter of the bushing  29  and the inside diameter of an inner wall region  32  of the bearing housing  4  which surrounds the bushing, are dimensioned in such a way that, even in the cold state, an annular gap  33  is present between those parts. In the event that the bushing  29  comes into contact with the inner wall region in spite of this annular gap, the rotational actuation forces for the shaft  2  are only insignificantly increased due to the low friction between the ceramic material of the bushing  29  and the metallic bearing housing  4 . 
     FIGS. 3A to 3C  show a bearing housing  4   a  having an inner wall region  22   a  which cooperates with a bearing ring  17   a  and does not extend obliquely, but rather at right angles, with respect to the central longitudinal axis  10  of the shaft  2 . A sliding surface  20   a  of the bearing ring  17   a  extends parallel to the inner wall region  22   a . However, the inner wall region  22   a  and the sliding surface  20   a  do not bear one against the other over their entire area, but only over a smaller contact region  23   a . This is implemented in such a way that either the sliding surface  20   a  or the inner wall region  22   a  has an annular projection  34  (see  FIGS. 3A and 3B ) or  35  (see  FIG. 3C ) projecting from it, which cooperates with the other surface in each case, with the effect of a slide pairing. It is noted that the location of the annular projection  34  is different in  FIGS. 3A and 3B .