Patent Publication Number: US-6701617-B2

Title: Spin-forming method for making catalytic converter

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to a method for manufacturing a catalytic converter by spin-forming a metal tube about a catalyst substrate to form a housing. More particularly, this invention relates to a spin-forming method wherein the catalyst substrate has a noncircular circumference and wherein the metal-forming tool is positioned during spin-forming to form a housing shaped similar to the catalyst substrate and sized greater than the catalyst substrate by a uniform distance. 
     BACKGROUND OF THE INVENTION 
     Automotive vehicles are equipped with a catalytic converter for treating exhaust gases to reduce noxious compounds prior to emission into the atmosphere. A typical catalytic converter comprises a catalyst substrate that is formed by extruding and firing a ceramic material and defines a plurality of passages that are coated with catalyst agents for treating exhaust gases caused to flow therethrough. The catalyst substrate is generally cylindrical and is enclosed in a metal housing. A thermally insulative material is interposed between the catalyst substrate and the metal housing to maintain the substrate at an elevated temperature effective for treatment and prevent overheating of the housing. 
     It has been proposed to manufacture a catalytic converter by spin-forming a metal tube about the catalyst substrate to form the housing. During spin-forming, the catalyst substrate is positioned in the metal tube, and the substrate and tube are rotated about a central axis. The metal forming tool is radially urged against the metal, while advancing axially, to reduce the circumference of the tube. Multiple passes are typically required in order to achieve the desired product size and shape. For each pass, the tool is radially advanced a small distance, so that the diameter is reduced incrementally. 
     In conventional spin-forming processes, the metal-forming tool is positioned a fixed distance from the axis, and produces a housing having a circular cross section. Thus the process is suited for producing a housing about a cylindrical substrate with a circular cross-section uniformly spaced from the housing. It is desired to produce a catalytic converter having other shapes, which would need to be based upon a substrate having a noncircular cross-section; for example, an oval circumference. Moreover, even for catalyst substrates that are designed to be cylindrical, the radial dimensions of the substrate tends to vary as a result of the extruding and firing of the ceramic material, so that the circumference of the catalyst substrate is not a true circle, but tends to have a radius that varies with direction, a condition referred to as out-of-round. During spin-forming, regions of the substrate having a greater radius than specified may experience higher pressure from the metal-forming tool, which may cause breakage of the fragile substrate. Furthermore, variations in the radius may result in a non-uniform thickness of insulation between the substrate and the housing. 
     Therefore, a need exists for a method of forming a catalytic converter by spin-forming that is suited for forming a metal housing about a catalyst substrate having a noncircular circumference, either by design or as a result of deviations that occur during processing of the ceramic. It is desired that the housing formed by spin-forming be spaced apart form the noncircular substrate by a uniform radial distance, such that a uniform layer of insulative material is disposed between the housing and substrate. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with this invention, a method is provided for forming a catalytic converter that includes a catalyst substrate having a noncircular circumference. The catalyst substrate is measured to determine the radial dimension of the noncircular circumference relative to an axis. The catalyst substrate is wrapped in a compressible mat and arranged in a metal tube. The arrangement is subjected to a spin-forming process that forms the metal tube about the catalyst substrate into a metal housing. The spin-forming process includes rotating the metal tube about the substrate axis and concurrently radially urging a metal-forming tool against the tube. In accordance with this invention, the metal-forming tool is programmed to follow a metal-forming path corresponding to the substrate circumference plus a predetermined radial distance. In this manner, a metal housing for the catalytic converter is produced having a noncircular circumference that corresponds in shape to the substrate and is spaced apart therefrom by an insulative layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be further illustrated with reference to the accompanying drawings wherein: 
     FIG. 1 is a cross-section of a catalyst substrate for forming a catalytic converter in accordance with a preferred embodiment of this invention; 
     FIG. 2 is a cross-sectional view showing an arrangement of components for spin-forming a catalytic converter in accordance with a preferred embodiment of this invention; 
     FIG. 3 is a cross-sectional view of the arrangement in FIG. 2 taken along lines  3 — 3  and looking in the direction of the arrows; 
     FIG. 4 is a cross-sectional view of the arrangement in FIG. 2 showing the components during spin-forming of a catalytic converter in accordance with this invention; 
     FIG. 5 is a cross-sectional view showing a catalytic converter spin-formed in accordance with this invention; 
     FIG. 6 is a cross-sectional view of the catalytic converter in FIG. 5 taken along the lines  6 — 6  in the direction of the arrows; 
     FIG. 7 is a cross sectional view of a catalyst substrate that is out-of-round for forming a catalytic converter in accordance with an alternate embodiment of this invention; and 
     FIG. 8 is a cross-sectional view of a catalytic converter comprising the catalyst substrate in FIG.  7  and spin-formed in accordance with an alternate embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with a preferred embodiment of this invention, referring briefly to FIGS. 5 and 6, a method is provided for manufacturing a catalytic converter  40  comprising a ceramic catalyst substrate  10  having an oval circumference and enclosed within a metal housing  42  spaced apart by a layer  43  of insulative material. The method uses a spin-forming process, carried out using a metal-forming tool  30  in FIG. 4, to form the metal housing having a similar oval shape to the substrate and sized greater than the substrate to provide a uniform layer of insulation therebetween. 
     Referring to FIG. 1, in this embodiment, substrate  10  has an axis  14  and an outer surface  18  characterized by an oval circumference. Substrate  10  comprises end faces in FIG.  2  and defines a plurality of axial passages between the ends, of which only a few are depicted. The substrate is formed by extruding and firing a ceramic material. During use, exhaust gas from an internal combustion engine flows through passages  12  and is treated by catalytic agents applied on the passage surfaces. 
     Prior to assembly, outer surface  18  of substrate  10  is mapped to determine the radial dimensions relative to axis  14 . For this purpose, the dimensions are measured between axis  14  and a series of points at the surface. The points are located in planes perpendicular to the axis and equidistantly spaced about the circumference. Thus, each point is readily identified by an angular displacement relative to a reference direction  16  and an axial distance relative to an end  15  of the substrate. Measurements may be made by any suitable technique that provides an accurate distance of a surface relative to a predetermined reference point, that is, axis  14 . In a preferred embodiment, a laser gauge is utilized that locates the surface without contact with the thin ceramic. Alternately, a mechanical instrument that contacts the surface may be employed. The measurements are correlated with the angle from reference direction  16  and the axial distance from an end  15  and stored in a computer memory. 
     Following measurement of the surface, substrate  10  is wrapped in a compressible mat  20  and inserted into a metal tube  22 , as shown in FIGS. 2 and 3. Mat  20  is formed of ceramic fibers and provides thermal insulation of the substrate in the product converter. Annular seals  24  are disposed about the substrate near the ends to restrict gas flow through the mat. 
     The resulting wrapped substrate is coaxially inserted into metal tube  22 . Tube  22  includes a midsection  27  about substrate  10  and end sections  28  that extend axially beyond midsection  27 . In the preferred embodiment, tube  22  has an oval cross-sectional shape similar to the substrate and is suitably sized to permit the wrapped pre-assembly to be readily inserted. 
     Referring to FIG. 4, the pre-assembly of substrate  10 , mat  20  and metal tube  22  is subjected to a spin-forming process to reduce the tube radius of midsection  27  to secure the substrate within the tube and form the catalytic converter. For this purpose, the pre-assembly is mounted onto a chuck  26  that spins tube  22  to rotate the tube about axis  14 . While the tube is rotated, a metal-forming tool  30  is radially urged against the outer surface of the metal tube along midsection  27 . In a preferred embodiment, tool  30  is a roller mounted on a yoke  31  to rotate about an axis  32  parallel to axis  14 . As roller  30  is radially urged against the metal tube, the roller is concurrently advanced axially to progressively reduce the tube diameter. As the diameter is reduced, compressible mat  20  is compressed about substrate  10 . In accordance with this invention, roller  30  is connected to an actuator  34 , such as a hydraulic actuator, that positions the roller relative to axis  14 , in response to a signal from a computer control module  36 . A suitable spin-forming machine is commercially available from M&amp;M Metal Forming Machinery, Inc., under the trade designation Spin Shrinking Machine Model SSM 350 TT. 
     In accordance with this invention, the computer control module determines the position of roller  30  based upon the radial dimensions of substrate  10  measured prior to assembly within the tube. As the tube spins about the axis, roller  30  traverses the metal tube in a plane perpendicular to the axis. The computer control module calculates desired radial dimensions for the tube circumference in the plane by adding a predetermined radial distance to the tube radial dimensions in the plane. Extrapolation is used to calculate dimensions of the substrate in planes other that those for which measured values are available. The computer control module then positions the metal-forming roller to follow a path corresponding to the desired housing dimensions. 
     In the final axial pass, tool  30  is positioned a radial distance equal to the total of the substrate dimensions, the desired thickness of insulation layer  43  and the thickness of housing  42 . In this manner, the method of this invention produces a housing having an outer surface corresponding in shape to the substrate and spaced apart by a uniform distance. Spin-forming may be carried out in a single axial pass of tool  30 . Alternately, multiple passes may be used to incrementally reduce the dimensions of the tube. In a process employing multiple passes, the distance added to the substrate dimensions is preferably chosen to reduce the dimensions of the tube a selected amount during each pass until the desired final size is achieved. 
     Following spin-forming of midsection  27  about the substrate  10 , end sections  28  are formed into the desired size and shape of the inlet and outlet for the catalytic converter. This is preferably accomplished by spin-forming in a manner similar to the process utilized for forming the midsection. The product catalytic converter  40  is shown in FIGS. 5 and 6. Converter  40  comprises a metal housing  42  that is formed by spin-forming in accordance with this invention. Housing  42  includes a midsection  44  about substrate  10 , with insulative layer  43  and seals  24  compressed therebetween. Housing  42  also includes end portions  46  that form the inlet and outlet to the catalytic converter. As can be seen in FIG. 6, regulation of the metal-forming tool during spin-forming in accordance with this invention produces a housing comprising a midsection  44  having a shape corresponding to substrate  10  and spaced apart by a substantially uniform distance. Moreover, compression of mat  20  between midsection  44  and substrate  10  produces layer  43  having a substantially uniform thickness. 
     Therefore, this invention provides a method for forming a midsection of a catalytic converter housing about a substrate having a noncircular circumference. The metal housing conforms in shape to the substrate and is uniformly sized about the substrate. Moreover, the insulative mat is uniformly compressed about the substrate to provide a uniform density within the housing midsection. 
     In the embodiment shown in FIGS. 1-6, a method of this invention was utilized in spin-forming a housing about a substrate having an oval circumference. The method may be applied to catalyst substrates having other suitable noncircular shapes, including a race track circumference or a nonsymmetrical shape. In an alternate embodiment, a housing is formed about a catalyst substrate that is designed to be cylindrical but has a circumference that is noncircular as a result of variations that occur during extruding and firing of the ceramic, commonly referred to as out-of-round. Referring to FIG. 7, there is shown an out-of-round catalyst substrate  50  having a circumference  53  that deviates from a circle  52 . In accordance with this invention, a housing may be spin-formed about substrate  50  to correspond in shape to the substrate despite the out-of-round deviations. This is accomplished by mapping circumference  53  of the substrate prior to spin-forming to determine the radial dimensions relative to an axis  54 , which corresponds to the central axis of the substrate as designed. The substrate is wrapped in a compressible mat and coaxially inserted within a metal tube. Thereafter, the substrate and metal tube are spun about a center axis  54  while forming the metal tube with a metal-forming roller. During forming, a control module adjusts the position of the roller to follow a path corresponding to the actual radial dimensions of the substrate plus a predetermined distance. The product catalytic converter  56  is shown in FIG.  8  and comprises substrate  50  surrounded by a layer  58  of insulative material and enclosed within a midsection of a metal housing  60 . The housing midsection is out-of-round to correspond in shape to the out-of-round dimensions of substrate  50 . In addition, the housing is sized greater than the substrate by predetermined distance to provide a uniform layer  50  of insulation therebetween. 
     While this invention has been described in terms of certain embodiments thereof, it is not intended to be limited to the described embodiments, but only to the extent set forth in the claims that follow.