1. Field of Invention
The invention relates to a spin-forming method, a spin-forming apparatus, and a catalytic converter. More particularly, the invention relates to a spin-forming method and a spin-forming apparatus for modifying a material pipe into a shape with a predetermined angle, and a catalytic converter wherein a catalyst support is disposed within a catalyst container.
2. Description of Related Art
In a typical combustion engine, for example, a vehicular internal combustion engine and the like, the engine is connected to an exhaust system for conducting and discharging exhaust gas. The exhaust system is equipped with a catalytic converter for performing emission control by causing chemical reactions, such as oxidation, reduction and the like, of exhaust gas discharged from the combustion engine via an exhaust manifold.
The catalytic converter is formed by containing a catalyst support within a catalyst container. As shown in FIG. 20, a catalyst container 1″ generally has a relatively large-diameter catalyst installation portion 1a that is disposed substantially at a middle portion of the catalyst container 1″, in a lengthwise direction for containing a catalyst support 2. Relatively small-diameter joint portions 1b are disposed at opposite ends and are connected to the exhaust pipe of the internal combustion engine and to an outlet pipe, respectively. Funnel-shaped cone portions 1c are formed between opposite end portions of the catalyst installation portion 1a and the joint portions 1b, so that the diameter of each cone portion 1c gradually reduces from the end portion of the catalyst installation portion 1a toward the joint portion 1b. 
In the production of a catalytic converter as described above, an ordinary spin-forming apparatus shown in FIGS. 1 and 2 is used to reduce the diameter of a hollow material pipe 1 so as to form a catalyst installation portion 1a, cone portions 1c, and joint portions 1b as one unit.
The spin-forming apparatus shown in FIGS. 1 and 2 has a clamp portion 11 for retaining a material pipe 1, a diameter-direction moving portion 12 for moving the material pipe 1 in a direction of a diameter of the material pipe 1, an axis-direction moving portion 13 for moving the material pipe 1 in the direction of an axis of the material pipe 1, a revolutional drive portion 14 for revolving a roller 10, that is, a forming tool, with respect to the material pipe 1, and a revolution radius changing portion (not shown) for changing the diameter of revolution of the roller 10.
In the production of a catalytic converter using a spin-forming apparatus as described above, a hollow material pipe 1 having substantially the same diameter as the catalyst installation portion 1a is held by the clamp portion 11. Then, the revolutional drive portion 14 revolves the roller 10, and the axis-direction moving portion 13 moves the material pipe 1 so that the roller 10 is positioned relative to the material pipe 1 so as to face a spin-forming initiation position on the material pipe 1, that is, a boundary between the catalyst installation portion 1a and a cone portion 1c. While the roller 10 is pressed against the material pipe 1 by the revolution radius changing portion (not shown), the material pipe 1 is moved by the axis-direction moving portion 13 so that the roller 10 moves relatively to the material pipe 1 in the direction of the axis thereof from the spin-forming initiation point toward an end of the material pipe 1. The catalyst support 2 wound on a mat 3 (see FIG. 20) is then inserted into the material pipe 1 from the other end portion thereof, which has not yet been spin-formed. Subsequently, the other end portion of the material pipe 1 is reduced in diameter by spin-forming.
The spin-forming apparatus shown in FIGS. 1 and 2 is also able to form a joint portion 1b eccentrically to the catalyst installation portion 1a. That is, it is formed in such a manner that a center axis C2 of the joint portion 1b lies apart from and parallel to a center axis C1 of the catalyst installation portion 1a, by relatively shifting the material pipe 1 in directions of the diameter during the spin-forming process.
Some catalyst containers 1″, due to demands concerning arrangement of an exhaust system or the like, are formed in such a deflective fashion that the center axis C2 of a cone portion 1c and the adjacent joint portion 1b finally form a predetermined angle θ5 with respect to the center axis C1 of the catalyst installation portion 1a as indicated in FIG. 20. Some other catalyst containers 1″ as shown in FIGS. 17 and 18 are formed in a deflective fashion such that a center axis C2 of the cone portion 1c and the joint portion 1b on one end side and a center axis C2 of the cone portion 1c and the joint portion 1b on the other end side form a predetermined angle therebetween about the center axis C1 of the catalyst installation portion 1a in a view as shown in FIG. 18, and thus are in different phases.
A related-art technology for processing a material pipe by a spin-forming process so as to, achieve a predetermined deflective angle with respect to the center axis of the material pipe is disclosed in Japanese Patent Application Laid-Open No. 11-151535. There exists a pipe-end forming method wherein a workpiece (material pipe) is subjected to a spinning process while the axis of the material pipe and the axis of revolution of a roll (forming tool) are tilted relatively to each other. There also exists a pipe-end forming method and apparatus wherein the axis of a workpiece pipe and the axis of revolution of a roll are tilted relatively to each other. The aforementioned laid-open patent application also describes a technology in which the roll is moved in radial directions with respect to the axis of revolution of the roll, while the roll is revolved around the axis of revolution, and the workpiece pipe is held so that the workpiece does not turn around its own axis.
The aforementioned patent application also indicates that in order to tilt the axis of the workpiece pipe and the axis of revolution of the roll with respect to each other, a clamp device for holding a workpiece is equipped with a workpiece tilter portion, such as an electric motor or the like, and the clamp device is turned by the tilter portion.
In the aforementioned related-art technologies, however, there is a need to equip a workpiece clamping device of a spin-forming apparatus with a workpiece tilter portion, such as a motor or the like, in order to tilt the axis of a workpiece (material pipe) and the axis of revolution of a roll (forming tool) relative to each other when a spinning process is performed to deflect the center axis of a cone portion and an adjacent joint portion with respect to the center axis of a catalyst installation portion as indicated in FIG. 20. Therefore, it is difficult or impossible to reduce the size or cost of the spin-forming apparatus.
If spin-forming is performed while the clamp portion is turned by the tilter portion so that the axis of a workpiece pipe and the axis of revolution of the roll are tilted relatively to each other, a boundary plane K between a cone portion 1c and the catalyst installation portion 1a of a catalyst container 1″ is not substantially perpendicular to, but is oblique to the center axis C1 of the catalyst installation portion 1a as indicated in FIG. 20. The catalyst support 2 contained in the catalyst installation portion 1a typically has a generally cylindrical shape in which a center axis is substantially perpendicular to end surfaces. Therefore, the boundary plane K between the catalyst installation portion 1a and the cone portion 1c, and an end surface of the catalyst support 2 partially define a wedge-shaped space S in which the distance between the boundary plane K and the end surface of the catalyst support 2 varies in accordance with position. If such a spin-formed product is used as a catalyst container 1″ of a catalytic converter as described above, exhaust gas is led from the cone portion 1c to the end surface of the catalyst support 2 via a space within the wedge-shaped space S, in which distance varies depending on position. Therefore, exhaust gas cannot be uniformly introduced into the catalyst support 2 via the entire end surface thereof, and exhaust gas cleaning efficiency cannot be improved.
Furthermore, in the catalyst container 1″ formed so that the boundary plane K between the catalyst installation portion 1a and the cone portion 1c is oblique to the center axis C1 of the catalyst installation portion 1a as shown in FIG. 20, the presence of the wedge-like space S formed between the boundary plane and the end surface of the catalyst support increases the entire length L″ of the catalyst container 1″. As such, a large installation space is needed for the catalytic converter, and a large amount of material is needed to form a catalyst container, etc.