Method for manufacturing a double pipe

A method for manufacturing a double pipe includes: drawing a plate (1), which has a predetermined shape corresponding to the desired final shape of a curved outer pipe (4), into a curved, substantially U-shape in cross-section to produce a half pipe (2). Then, an inner pipe (3), which has a curved shape similar to the desired final curved shape of the outer pipe, is inserted into and positioned within the interior space of the half pipe. Subsequently, the inner pipe is welded to the half pipe at one longitudinal end thereof. Thereafter, longitudinally-extending edges of the half pipe are curled towards each other until they abut and a substantially circular cross-section is formed that surrounds the exterior of the inner pipe. The abutting longitudinally-extending edges (22) of the half pipe are then welded together to produce the curved outer pipe (4).

This application claims priority to Japanese patent application no. 2015-237034 filed on Dec. 4, 2015, the contents of which are incorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to a method for manufacturing a double pipe, such as a method for manufacturing a curved double pipe.

Description of the Related Art

Catalytic converters are utilized, e.g., in vehicles, to convert toxic pollutants, such as carbon monoxide, unburned hydrocarbons and nitrogen oxides, into less toxic or non-toxic substances, such as carbon dioxide, water and nitrogen gas. To perform this purification, catalytic converters utilize a “redox” (i.e. reduction and oxidation) catalyst (purification catalyst) that typically functions most optimally or efficiently at a predetermined temperature or higher. Therefore, to avoid a decrease in the temperature of an exhaust gas flowing from the internal combustion engine (or a similar fuel burning device) into the catalytic converter, an exhaust pipe between the internal combustion engine, etc. and the catalytic converter may include an inner-outer double pipe, in which an air space or clearance is provided between the inner and outer pipes. This air space or clearance serves as an insulator to minimize cooling of the exhaust gas as it flows through the inner pipe to the catalytic converter.

Exhaust pipes typically have a curved shape, because it is necessary to pass the exhaust pipe through narrow spaces underneath the vehicle so as to avoid interference with other parts of the engine, transmission, vehicle chassis, etc. However, if the curved exhaust pipe is formed simply by bending a straight double pipe, the curved portion becomes flattened, which means that a prescribed air space (clearance) can not be reliably and readily formed between the inner pipe and the outer pipe.

As one example for overcoming this flattening problem, Japanese Patent Laid-Open No. 2000-79417 discloses a method for forming a curved double pipe that includes filling a straight double pipe with water and then immersing it in pressurized liquid nitrogen to form ice inside the double pipe. In this frozen state, the entire double pipe is bent by performing a draw-bending process or a similar metal working technique. After the double pipe has been bent, the ice is melted and the water is drained.

Because it is necessary to quickly freeze the water to form the ice, the above-described known manufacturing method incurs additional equipment and material costs due to the use of pressurized liquid nitrogen. Moreover, it involves additional labor costs for the managing (procuring) and handling the liquid nitrogen.

SUMMARY

In view of the above circumstances, it is one object of the present teachings to disclose a relatively simple and cost-effective method for manufacturing a double pipe, e.g., a curved double pipe.

A method according to a first non-limiting aspect of the present teachings includes: drawing a plate (1), which has a predetermined planar shape that at least substantially corresponds to a desired final shape of a curved outer pipe (4), into a curved, substantially U-shape in cross-section, thereby producing a half pipe (curved U-shaped intermediate part) (2); inserting and positioning an inner pipe (3), which has a curved shape similar (corresponding or complementary) to the final curve shape of the outer pipe (4), into an interior space of the half pipe (curved U-shaped intermediate part) (2) having the substantially U-shaped cross-section; welding together the inner pipe (3) and the half pipe (curved U-shaped intermediate part) (2) at one longitudinal end thereof; then bending (curving or curling) longitudinally-extending edges of the half pipe (curved U-shaped intermediate part) (2) so as to cause the longitudinal edges to abut (touch), whereby a substantially circular cross-section is formed that surrounds or encloses the inner pipe; and welding together the abutted longitudinally-extending edges (22), thereby producing the curved outer pipe (4) that seals or encloses (e.g., hermetically seals) the air space (clearance) located around the exterior surface of curved inner pipe (3).

In the first aspect of the present teachings, the inner pipe, which has a curved shape similar (corresponding or complementary) to the curved shaped of the half pipe, is inserted into the interior space of the curved half pipe which was formed into a substantially U-shape in cross-section in the preceding step. Then, the longitudinally-extending edges of the half pipe are bent, curved or curled around the longitudinal centerline of the half pipe so that the longitudinally-extending edges are brought into abutment and a substantially circular shape is formed in a transverse cross-section. As a result, the half pipe surrounds or encloses the exterior of the inner pipe and the curved outer pipe is thus formed. According to such a method, it is possible to easily and cost-effectively manufacture a curved double pipe, e.g., without using ice and liquid nitrogen. Furthermore, because it is not necessary to bend a straight double pipe, flattening or other undesirable deformations of the outer pipe, in particular, can be avoided, thereby ensuring that the prescribed annular gap (air space or clearance) between the inner and outer pipes of the final double pipe can be reliably achieved along the entire length of the double pipe.

In a second aspect of the present teachings, prior to the drawing step, the plate (1) is curved or bent into a predetermined mountain-like (peak) shape along the direction that the double pipe is intended to curve. By performing such a bending step, the subsequent drawing step(s) become(s) easier to perform.

In a third aspect of the present teachings, after the outer pipe (4) has been welded along its longitudinal direction, the diameter of the outer pipe (4) at one longitudinal end (41) thereof is reduced, whereby the inner circumferential surface of the outer pipe (4) is brought into contact with the outer circumferential surface of the inner pipe (3). The longitudinal ends of the inner pipe (3) and outer pipe (4) may then be affixed together, e.g., by welding, so that the inner pipe (3) is fixedly supported or suspended within the outer pipe (4).

In the third aspect of the present teachings, by reducing the diameter end portion (swaged part), the longitudinal end of the double pipe can be easily welded and coupled to another pipe or structure, such as the engine or catalytic converter. Furthermore, by affixing the longitudinal ends of the inner pipe to the corresponding longitudinal ends of the outer pipe, the inner pipe can be suspended and held within the outer pipe in a simple and efficient manner such that the prescribed annular clearance between the inner pipe and the outer pipe is provided. In such an embodiment, the inner pipe contacts the outer pipe only at the longitudinal ends thereof, thereby improving the insulating capabilities of the double pipe.

The reference numbers in parentheses above provide representative, non-limiting examples of structures that may be utilized or formed according to the present methods, as will be further described below in the context of the preferred embodiment. These reference numbers should not be interpreted as limiting the scope of the invention to the preferred embodiment.

By eliminating the need for water and liquid nitrogen in the double pipe manufacturing process, equipment, material and labor costs can be kept down during the manufacture of a curved double pipe.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The manufacturing method that will be described below is merely a representative, non-limiting example of the present teachings, and various design improvements or modifications, which can made by those skilled in the art without departing from the gist of the present invention, are also intended to be included in the scope of the present invention.

In a representative manufacturing method of the present teachings, a stainless-steel or titanium (or titanium alloy) metal plate1is formed into a predetermined shape (blank) by blanking, e.g., cutting or punching, as shown inFIG. 1. The optimal blank shape may be determined empirically such that an outer pipe having a predetermined curved shape can be obtained after the subsequent metal working (shaping) steps. The shape of the blank thus need only be determined once prior to mass production.

Next, as shown inFIG. 2, the plate (blank)1optionally may be bent or deformed (plastically deformed), e.g., by stamping, into a predetermined mountain-like (peak) shape so as to conform to the shape of the drawing (forming) die that will be utilized in the subsequent drawing process. For example, the plate1may be bent along the direction (i.e. the direction of the arrow inFIG. 1) of the final bend of the outer pipe after the subsequent shaping steps have been performed. Thus, the plate1may be bent or deformed along a straight and/or curved line, depending upon the intended final shape of the outer pipe4(seeFIG. 8). Thereafter, the planar or bent plate1is subjected to a drawing process, in which the plate1is stretched and/or compressed into a die by a punch to create a three-dimensional part (i.e. a sheet metal blank1is radially drawn into a forming die by the mechanical action of a punch). In case the depth of the drawing is relatively large, the drawing may be performed in multiple steps (e.g., two steps in this embodiment) as shown inFIG. 3andFIG. 4. Depending upon the dimensions of the plate1and the resulting half pipe2, the drawing process may also be characterized as a “deep drawing” process. As a result of this drawing or deep drawing process, the plate1is formed into an inverted U-shape in cross-section, as shown inFIG. 4. It is noted that it is also possible to bend the plate1concurrently with drawing process.

As shown inFIG. 5, after the drawing process, flanges11remaining on the two longitudinally-extending side edges of the drawn plate1may be removed by trimming to produce a half pipe (curved U-shaped intermediate part)2which is curved along its longitudinal direction (longitudinal centerline) and has an inverted U-shaped cross-section. Thereafter, the half pipe2is turned (rotated) upside down, and a smaller-diameter inner pipe3, which is curved in the same direction and to the same extent as the half pipe2, is inserted into and positioned within the space of (the interior space defined by) the half pipe2having the U-shaped cross-section, as shown inFIG. 6. Then, the inner pipe3and the half pipe2are spot-welded together (reference number21) at one longitudinal end thereof to affix the two pipes2,3to each other.

Then, as shown inFIG. 7, the longitudinally-extending edges of the half pipe2are bent (curved or curled) around (along) the longitudinal centerline of the half pipe2by pressing so that the left and right longitudinally-extending edges of the half pipe2come into abutment against each other (reference number22inFIG. 7), thereby closing the opening (enclosing the interior space) of the half pipe2along the longitudinal direction. In this step, the U-shaped cross-section of the half pipe2is transformed into a circular cross-section so as to form the U-shaped half pipe2into an enclosed pipe (tubular) shape. Because the inner pipe3and the half pipe2were spot-welded together at one longitudinal end prior to this bending (curling) step, shifting of the inner pipe3relative to the half pipe2(outer pipe4) during the bending step is prevented, and the half pipe2is curved so that a predetermined (prescribed) annular (ring-shaped) clearance is provided around the exterior of the inner pipe3along the entire longitudinal length of the double pipe. Next, the abutting longitudinally-extending edges22of the half pipe2are welded together along their length (dashed line inFIG. 8) to produce the (e.g., hermetically sealed) outer pipe4. The welding may be performed, e.g., using butt welding techniques or seam welding techniques. This welding step completes the curved inner-outer double pipe.

Thereafter, one or both longitudinal ends41of the outer pipe4optionally may be swaged (reduced in diameter) by pressing, e.g., in a die, whereby the inner circumference (inner circumferential surface) of the outer pipe4is brought into contact with the outer circumference (outer circumferential surface) of the inner pipe3at this swaged part, as shown inFIG. 9. This process may also be referred to as “necking”. The swaged part makes it easier to couple the double pipe to another pipe or another structure by performing three-plate full-circumferential welding or the like. If necessary, the overall shape of the double pipe can be adjusted or modified by restriking to form the final, desired shape of the outer pipe4.

While the representative embodiment of the present teachings has been described in the context of a double pipe for automobiles, it should be understood that the present teachings are not limited to automobiles and can be advantageously utilized in a variety of fields, including but not limited to trucks, boats, planes, or any other field in which a curved double pipe may be utilized.

Although the term “double pipe” was utilized herein, the structures produced according to the present teachings also may be characterized as “double-walled pipes” that have an annulus (interstitial space) between the diameters of the inner pipe (e.g., primary or carrier pipe) and the outer pipe (e.g. secondary or containment pipe). Furthermore, although stainless steel and titanium were specifically mentioned as materials for the inner and outer pipes, the present teachings are, of course, applicable to any kind of plastically deformable material, including other types of metals and metal alloys, as well as polymers that may be softened for bending, e.g., by raising the temperature thereof. The materials of the inner pipe3and the outer pipe4may be the same or different.

As was described above, the plate or blank1preferably has a predetermined planar shape that at least substantially corresponds to the desired final shape of the curved outer pipe4. “At least substantially corresponds” is intended to mean that the shape of the plate or blank1may be selected such that the curved outer pipe4can be formed with some excess material that may be trimmed either before or after the welding step. Naturally, it is preferable, but not mandatory, to minimize the amount of material that must be trimmed from an efficiency standpoint.

Although the inner pipe3and outer pipe4have smooth inner and outer circumferential surfaces in the preferred embodiment, naturally it is possible to provide projections or other structures on any of the inner or outer surfaces of the inner pipe3and/or the outer pipe4.

For example, if the double pipe is intended to be utilized, e.g., as a heat exchanger or a jacketed pipe, in which heat exchange between fluids (e.g., liquids or gases) in the inner and outer pipes is desirable, the inner pipe and/or the outer pipe may contain projections (e.g., heat conductive projections) extending within the annular clearance between the inner and outer pipes that help to facilitate heat exchange. In such an embodiment, the longitudinal ends are preferably not swaged or affixed to each other, so that a fluid can be introduced into the annular space between the inner and outer pipes. Therefore, the inner pipe may be supported within the outer pipe via one or more projections extending from the outer circumferential surface of the inner pipe to the inner circumferential surface of the outer pipe.

In addition or in the alternative, e.g., in case the double pipe is intended to be used as a double-walled pipe, wherein a dangerous or toxic fluid is transported through the inner pipe and the outer pipe is provided for the purpose of preventing leakages to the surrounding environment in the event that a crack forms in the inner tube, the longitudinal ends may be swaged and/or projections may be provided between the outer circumferential surface of the inner pipe and the inner circumferential surface of the outer pipe, in order to support the inner pipe within the outer pipe.

Although the inner pipe3and outer pipe4have circular cross-sections in the preferred embodiment, one or both of the inner pipe and outer pipe may have a different cross-section, such as, e.g., oval or polygonal.