Method for producing a bellows with a cross section of elliptical, egg-shaped, shaped as two equal semicircles connected by two parallel straight lines, non circular ring or polygonal roundes form.

Abstract: After a metallic cylindrical tube (1) is bulged to form a bellows with a circular cross section, this bellows is ovalized to form a bellows with an oval cross section. For this reason, an inexpensive round tube (cylindrical tube) can be used as the material for a bellows with an oval cross section and it is unneces­ sary to use an expensive oval tube. Accordingly, a bellows with an oval cross section can be produced at low cost.

Description: This invention relates to a method for producing a metallic bellows. More specifically, this invention relates to a method for producing a metallic belows with a choice of final cross section. This bellows is used not only as a flexible joint and the like in fluid piping but, especially in a small size, as a cooling water passage for cooling a large-scale IC chip used in a large-scale computer and the like, which serves also as an elastic support for such chip. In this specification, the final cross section to be produced includes not only a "mathematically or geometrically elliptical" shape but an "egg-shaped" configuration, and further includes also a profile at least one part of which, for example, the portion which is almost parallel to the major axis of an oval (hereinafter referred to as the "long side"), is linear. In this specification, noncircular ring forms are included in the term "final cross section" with the exception of polygonal shapes the vertexes or apexes of which are angular. In other words, a polygonal with all vertexes or apexes are rounded is included in the term "final cross section". Also, in this specification, the term "bellows" means a tube whose side wall alternately provides ridges or large-diameter portions (bulged portions) and grooves or small-diameter portions (non-bulged portions). In a bellows, the cross-sectional size of all large-diameter portions may be the same or may not be the same, and that of all small-diameter portions also may be the same or may not be the same. Furthermore, the intervals of large-diameter portions may be the same or may vary in the longitudinal direction of the tube or bellows and those of small-diameter portions also may be the same or may vary in the longitudinal direction. Moreover, in a bellows, the cross-sectional shape of bulged portions may be or may not be substantially similar to that of nonbulged portions. The above-mentioned configurational conditions for a bellows can be selected conveniently depending on the elasticity, mechanical strength, etc. as in tension, compression and bending in various directions that the bellows is required to provide according to its application. When a bellows is used as a flexible joint in fluid piping and the like, the major or minor axis of the oval in the cross section of the bellows may be several millimeters to several centimeters or more in size. When a bellows is used as a cooling water passage or elastic support for a large-scale IC chip, the major or minor axis of the oval in the cross section of the bellows may be several millimeters or less in size. Also, in this specification, the term "circular bellows" means a bellows with a circular cross-sectional shape. Description of the Prior Art Hitherto, bellows with an oval cross section have been produced, for example, as described in JP-A-95927/86, from stock tubes with an oval cross section by means of a forming die that has protuberances formed in part of the semi-oval so that the portions of the ridges of the oval bellows to be formed which are virtually parallel to the major axis may be generally linear. In this method of the prior art, however, expensive oval stock tubes are required and this may result in high production costs of oval bellows. Moreover, in the method disclosed in JP-A-95927/86, the above-mentioned protuberances provided in the forming die cause a decrease in the cross section of the fluid passage at parts of the formed article including the inwardly protruded portions corresponding to the above-mentioned protuberances and no attention has so far been paid to this point. The inwardly protruded portions may also affect the fluid flow in the passage of the bellows to be disturbed. Furthermore, when folding is conducted to form a bellows by axially applying compressive loads to the bulged part, the circumferentially non-uniform strain or deformation is caused to results in the circumferentially non-uniform stress remained in the bellows due to the inwardly protruded portions circumferentially locally formed in the grooves of the bulged part corresponding to the above-mentioned protuberances (which are formed in portions almost parallel to the major axis of the oval in terms of the cross section of the bulged part). Therefore, there is fear that the bellows thus produced is likely to be broken after repeated deformation in use. When a stock tube with an oval cross section is bulged to form bulged portions as in this prior art and when the forming die is not provided with the protuberances disclosed in JP-A-95927/86, portions of small curvature (i.e., large radius of curvature) that are virtually parallel to the major axis of the oval are radially bulged more than portions of large curvature that are virtually parallel to the minor axis upon bulging because of the oval shape of the cross section of the tube. The higher the ratio of the major axis to the minor axis of an oval, the greater the difference in the degree of this bulging. When a bellows is to be formed by axially applying compressive forces to a tube with bulged portions whose degree of bulging thus differs radially and by folding the bulged and nonbulged portions, buckling may occur in the portions of low degree of bulging (a case where the bulged amount is too small in the above-mentioned portions of large curvature) or excessive tensile deformation may occur in the portions of small degree of bulging (a case where the bulged amount becomes too large when an attempt is made to increase the bulged amount in the above-mentioned portions of large curvature). Therefore, necking or ductile fracture may occur in these portions. GB-A-2 068 810 and GB-A-1 325 759 disclose a method for forming a circular bellows from a circular pipe by application of hydraulic pressure to bulge or expand parts of circular walls between adjacent and discrete discs or external guides. JP-A-61 86 030, which is considered to represent the nearest prior art, discloses to ovalize a circular bellows by means of upsetting, but without applying hydraulic pressure from the inside of the bellows during said upsetting. US-A-3 996 783 discloses to apply, from the inside of a tubular member, tensile forces for deforming said member to rectangular or flat/oval cross section. SUMMARY OF THE INVENTION This invention was made in view of the above-mentioned various problems, and an object of the invention is to provide a method for producing a bellows with a final cross section as defined above directly from an inexpensive cylindrical tube by means of a single apparatus. Said object is achieved, according to the present invention, by the method claimed in claim 1. Advantageous further features of the invention are claimed in the dependent claims 2 to 7. In a method according to a preferred embodiment of the present invention, the above-mentioned step of ovalizing comprises upsetting of the bellows with the circular cross section using a pair of dies for pressure application by applying upsetting forces to the circular bellows in the radial direction of its circle. In this case, hydraulic pressure is applied to the inside of the circular bellows beforehand for the upsetting. In the method of this embodiment, the pair of dies for pressure application installed on the hydraulic forming device are caused to operate to radially upset the circular bellows hydraulically bulged by sectional dies after the separation of these sectional dies. In this way, a circular bellows can be formed into a bellows with an oval cross section by means of the same apparatus as the hydraulic forming device and in a small number of processing steps or short man-hours for forming. Accordingly, a bellows with an oval cross section can be produced in a simple manner and at low cost, with the result that the production cost can be reduced. The objects of the present invention and effects obtained therefrom will become clear from the following description with reference to the accompanying drawings, wherein: Fig. 1 is a plan sectional view of a common parts to apparatuses for carrying out the method for producing bellows in first and second preferred embodiments of the present invention in its state of bulging by hydraulic pressure; Fig. 2 is a front sectional view of the apparatus of Fig. 1, which illustrates the hydraulic pressure forming condition for the upsetting by an apparatus for carrying out the method for producing the final bellows in the first preferred embodiment of the present invention; Fig. 3 is a sectional side view along line III-III of Fig. 2; Fig. 4 is a flowchart illustrating the forming process in the first embodiment of the present invention using the apparatus shown in Figs. 1 to 3; Fig. 5 is a plan view of an example of bellows produced in accordance with the present invention; Fig. 6 is a front view of the bellows shown in Fig. 5; Fig. 7 is a front sectional view of the apparatus of Fig. 1, illustrating an apparatus for carrying out the method for producing the final bellows in the second preferred embodiment of the present invention in its state of stretching; Fig. 8 is a sectional side view along line VIII-VIII of Fig. 7; and Fig. 9 is a flowchart illustrating the forming process in the second embodiment of the present invention using the apparatus shown in Figs. 1, 7 and 8. DESCRIPTION OF THE PREFERRED EMBODIMENTS The first embodiment of the present invention will be described hereinafter with reference to Figs. 1 to 6. Referring to Figs. 1 to 3, the reference numeral 1 is a stock tube for producing a final bellows 26 (shown in Figs. 5 and 6). The stock tube 1 is made of a metal, such as copper and its alloys, and initially has a circular cross section, as represented by the imaginary lines in Fig. 1. A forming die 28 used for forming this stock tube 1 into a circular bellows 1d (refer to Fig. 3) has the first to sixth sectional dies 11-13 and 16-18. These six sectional dies comprise two sets of sectional dies (one set 11-13 and another set 16-18), or three pairs of sectional half-die, i.e., a pair of the first and fourth sectional dies 11 and 16, a pair of the second and fifth sectional dies 12 and 17, and a pair of the third and sixth sectional dies 13 and 18. When integrated, sectional dies of each pair provide a forming part 28a which has an inside diameter almost equal to the outside diameter of the stock tube 1 and the same shape of inside surface as that of one of the grooves of the circular bellows, and a base 28b integral with the forming part 28a and thicker than the forming part 28a. When these sectional dies are moved axially, i.e., in the directions indicated by the arrows A and B and are superposed on each other, the thickness center distance of the forming parts 28a becomes equal to the pitch of the circular bellows 1c. The first to third sectional half-dies 11, 12, 13 on one side can be moved in the axial direction of the stock tube, i.e., in the directions A and B guided by a rod 14 attached to a sectional-die supporting member or plate 10. Similarly, the fourth to sixth sectional dies 16, 17, 18 on the other side can be moved in the axial direction of the stock tube, i.e., in the directions A and B guided by a rod 19 attached to a sectional-die supporting member or plate 15. The reference numerals 2 and 3 are the first cylinder and the second cylinder fixed to a mounting plate 2b and a mounting plate 3b, respectively, 2a and 2b represent rods of these cylinders 2 and 3, reference numeral 4 represents a seal holder which is attached to the end of the rod 2a and seals and supports one end of the stock tube 1 in conjunction with an inserted O-ring 6, and reference numeral 5 represents a seal holder which is attached to the end of the rod 3a and seals and supports the other end of the stock tube 1 in conjunction with an inserted O-ring 7. A pipe 26 for introducing hydraulic pressure into the inside of the stock tube 1 is attached to the seal holder 5. The seal holders 4 and 5 can compress the stock tube 1 axially, i.e., in the directions A and B when driven by the cylinders 2 and 3 while hydraulic pressure is applied to the inside of the tube 1. The numerals 8 and 9 denote the third cylinder and the fourth cylinder attached to a mounting plate 8b and a mounting plate 9b, respectively, 8a and 9a are rods of these cylinders 8 and 9. The sectional-die supporting plates 10 and 15 are fixed to the ends of the rods 8a and 9a respectively. When the cylinders 8 and 9 are actuated, the sectional dies 11 to 13 and 16 to 18 can be moved in the directions C and D in the course from the retreat position (Fig. 3) to the integration position (Fig. 1). In the integration position, closing loads can be applied to the sectional dies. In Figs. 2 and 3, an upper die for pressure application 23 and a lower die for pressure application 24 are adapted to radially upset the circular bellows 1d, which has been hydraulically formed by means of the first to sixth sectional dies 11-13 and 16-18, under the action of the fifth and sixth cylinders 21 and 22 to produce a final bellows. The ridges 1a are inserted in the grooves 23a, 24a of upper and lower upsetting dies 23, 24, to prevent the deformation of the ridges 1a. The upper and lower dies for pressure application 23 and 24 are fixed to the ends of rods 21a and 21b, respectively, of the fifth and sixth cylinders 21 and 22 arranged in perpendicular to the extending directions of the third and fourth cylinders 23 and 24. The fifth and sixth cylinders 21 and 22 are fixed to mounting plates 21b and 22b, respectively. A method for producing a bellows in this embodiment by the operation of the apparatus 27 for producing the bellows 26 thus constructed will be described in the following. Referring to Fig. 1, all of the six sectional dies are moved to be arranged around the stock tube 1 in the following manner. Both ends of the stock tube 1 (1c) are first hydraulically sealed by means of the seal holders 4 and 5 through the advance of the rods 2a and 3a in the directions A and B, respectively, which is accomplished by the actuation of the first and second cylinders 2 and 3, respectively. After that, the first to third sectional dies 11-13 and the fourth to sixth sectional dies 16-18, which are spaced from each other by a specified distance, for example, a distance corresponding to the outer diameter of the stock tube 1 multiplied by about 0.4 to 0.6, are moved in the directions C and D, respectively, through the advance of the rods 8a and 9a in the directions C and D by the actuation of the third and fourth cylinders 8 and 9, respectively, whereby three pairs of sectional dies 11 and 16; 12 and 17; and 13 and 18 are closed. Next, by applying hydraulic pressure 20 to the inside of the stock tube 1 through the pipe 29, free bulging is accomplished between the seal holders 4 and 5 and the sectional dies 11-13 and 16-18 and the ridges 1a are formed (the step 30 in the flowchart of Fig. 4). With the hydraulic pressure 20 kept at a constant level, the sectional dies 11-13 and 16-18 are moved in the direction A or B to deform the ridges 1a through the advance of the rods 2a, 3a of the first and second cylinders 2, 3 so that the ridges 1a (and grooves) of the bulged stock tube are folded when the axial compressive loads are applied to the stock tube in the directions A and B (the step 31 in the flowchart). On this occasion, this folding or compression step is completed when the sectional dies 11-13 and 16-18 are integrated into one piece (the step 32 in the flowchart). Then the oil in the circular bellows or tube 1 is recovered after the relief of hydraulic pressure 20. With the seal holders 4 and 5 stopped, the rods 8a and 9a of the third and fourth cylinders 8 and 9, respectively, are caused to retreat in the directions D and C, respectively, and the first to sixth sectional dies 11-13 and 16-18 are separated from the formed article 1d (the step 33 in the flowchart). As shown in Fig. 2, the ridges 1a of the circular bellows are then inserted in the grooves 23a and 24a of the upper and lower dies for pressure application 23 and 24, respectively, by advancing, the rods 21a and 22a of the fifth and sixth cylinders 21 and 22, respectively, arranged in the directions E and F respectively, orthogonal or perpendicular to the extending direction of the third and fourth cylinders 8 and 9, and then the advancement of the rods 21a and 22a are stopped. The rods 2a and 3a of the first and second cylinders 2 and 3, respectively, are then caused to retreat in the directions B and A, respectively, and the seal holders 4 and 5 are extracted from both ends of the stock tube 1. After that, by further advancing the rods 21a and 22a of the fifth and sixth cylinders 21 and 22, respectively, in the directions E and F, respectively, the hydraulically formed circular bellows 1d is radially upset and is caused to undergo plastic deformation, whereby the circular bellows 1 is deformed or converted into a final bellows 26. On this occasion, a bellows 26 whose portions extending almost in parallel to the major axis of an oval are linear can be produced by means of the upper and lower dies 23 and 24 whose surfaces for pressure application are flat, and a bellows in which the portions extending almost parallel to the major axis of the oval have a large curvature can be produced by means of dies whose surfaces 23, 24 are curved (the steps 34 and 35 in the flowchart). The appearance of a bellows 26 with linear long sides produced by the method for producing the bellows in this embodiment is shown in Figs. 5 and 6. Now the first embodiment of the present invention will be described in connection with the following examples: Example 1 An oval bellows in which the major and the minor axes of an oval at the outside the ridge were about 70 mm and about 45 mm, respectively, and which had four ridges at a pitch of 5 mm, were produced from cylindrical stock tubes of oxygen-free copper 38 mm in outside diameter, 0.4 mm in wall thickness and 80 mm in length. The hydraulic pressure for bulging was 6,37 MPa (65 kgf/cm²), the axial load was about 7840 N (800 kgf), the load for closing the sectional dies was about 19600 N (2,000 kgf), and the radial load for upsetting was 3920 N (400 kgf). When forming was carried out under the above-mentioned conditions by the production method and apparatus illustrated in Figs. 1 to 4, good bellows with linear long sides as shown in Figs. 5 and 6 could be produced. According to the embodiment described above, the present invention provides the following effects: (1) Since upsetting dies for pressure application were incorporated in the hydraulic device for producing circular bellows, final bellows can be produced by a single apparatus. (2) Oval bellows and other bellows as defined above can be produced by a simple method and at low cost. According to one aspect of the present invention described above, inexpensive oval and other bellows can be produced because circular bellows can be radially upset by a simple method and by a part of the apparatus including the hydraulic device for forming circular bellows. Example 2 In accordance with the foregoing first embodiment, oval bellows in which the major and the minor axes of the oval at the outside of ridges were 17.5 mm and 8 mm, respectively, and the major and the minor axis at the outside of grooves (small diameter portion) were 13.5 mm and 4 mm, respectively, and had a wall thickness between about 0.08 mm and 0.1 mm and five ridges at a pitch of 1 mm were produced from circular copper tubes of 10 mm in outside diameter, 0.1 mm in wall thickness and 30 mm in length under the conditions: hydraulic pressure for bulging of 3,92 MPa (40 kgf/cm²), load for closing sectional dies during bulging of 588 N (60 kgf), axial load for folding for producing circular bellows of 294 N (30 kgf), and radial upsetting load for ovalizing of 784 N (80 kgf). Two oval bellows thus produced were mounted perpendicular to a substrate for mounting and cooling a LSI chip and parallel to each other. Then, cooling water was introduced into a passage in the cooling substrate through one of the two oval bellows and the water which had passed through the water passage in the cooling substrate was caused to flow out of the other oval bellows. At the same time, the IC chip was elastically or resiliently supported by the two oval bellows through the mounting substrate. As a result, in spite of the amount of heat generated from the IC chip, deformation due to the thermal expansion resulting from heat generation, error in the chip mounting space, etc., the oval bellows could stably support the IC chip for a long time and could hold a temperature increase in the IC chip within a specified range. Instead of conducting upsetting as mentioned above to transform a bellows with a circular cross section, stretching may be conducted as will be described hereinafter referring to Figs. 7 to 9 in addition to Fig. 1. In Figs. 7 to 9, members and elements similar to those in Figs. 2 to 4 are designated by the same reference numerals or characters as used in Figs. 2 and 4. In Figs. 7 and 8 illustrating an apparatus for producing a final bellows according to the second embodiment of the present invention, an upper stretching die 43 and a lower stretching die 44 having the same configuration with each other are used for producing a bellows with a cross section as defined above, from a circular bellows hydraulically formed by means of the first to sixth sectional dies 11-13 and 16-18, by performing stretch forming. The stretch forming or stretching is accomplished with tensile loads applied in the radial directions F and E by moving the upper and lower stretching dies in the directions F and E, respectively, thereby causing the two to be spaced from each other, through the actuation of the fifth and sixth cylinders 21, 22. Incidentally, the deformation of a ridges 1a of the circular bellows can be prevented by fitting into the ridges 1a and groove of the circular bellows 1d, corrugations 43a and 44a of the dies 43, 44 having the same outside configuration as the inside configuration of a final bellows at ends of the major axis of the oval. The upper and lower stretching dies 43 and 44 are detachably fixed, through supporting members 43b and 44b, respectively, to the rods 21a and 22a of the fifth and sixth cylinders 21 and 22, respectively, arranged in perpendicular to the third and fourth cylinders 8, 9. The fifth and sixth cylinders 21 and 22 are fixed to mounting plates 21b and 22b, respectively. The operation of an apparatus thus constructed for carrying out the method for producing bellows in the second embodiment of the present invention is explained hereinafter. The circular bellows is first formed in the same manner as in the first embodiment with reference to Figs. 1 to 4. In the present case, however, after the completion of the forming of a circular bellows (after the step 32) the hydraulic pressure 20 is first relieved and the seal holders 4 and 5 are then caused to retreat (the step 33a in Fig. 9), unlike the first embodiment, to recover the oil. As shown in Figs. 7 and 8, the upper and lower stretching dies 43 and 44 are then introduced into the circular bellows and the convex parts 43a and 44a of corrugations of the upper and lower stretching dies 43 and 44 are placed in the inside of the ridges 1a of this circular bellows. The rods 21a and 22a are then advanced in the directions E and F, respectively, by actuating the fifth and sixth cylinders 21 and 22, and the upper and lower stretching dies 43 and 44 are fixed to the supporting members 43b and 44b at the ends of the rods 21a and 22a respectively (the step 51 in Fig. 9). Next, the sectional dies 11-13 and 16-18 are separated from the circular bellows by causing the rods 8a and 9a to retreat in the directions D and C (the step 52 in Fig. 9). After that, the rods 21a and 22a of the cylinders 21 and 22 are caused to retreat by a specified distance in the directions F and E, respectively, and stretch forming is conducted by axially applying tensile loads to the circular bellows in order to cause the circular bellows to undergo plastic deformation, whereby the desired final bellows 26 as shown in Figs. 5 and 6 can be produced (the steps 53 and 54). The manufacturing process of bellows in the second embodiment is schematically shown in Fig. 9. Now an example of the second embodiment is described. Example 3 Oval bellows in which the major axis of an oval at the outside of ridges was about 70 mm (47 mm at the outside of grooves) and the minor axis of the oval at the outside of ridges is about 45 mm (22 mm at the outside of grooves) and which have four ridges at a pitch of 5 mm, were produced from cylindrical stock tubes of phosphor bronze 38 mm in outside diameter, 0.4 mm in wall thickness and 80 mm in length by means of the apparatus of the second embodiment shown in Figs. 1, 7 and 8 according to the procedure shown in Fig. 9. The hydraulic pressure for bulging was set at 8,33 MPa (85 kgf/cm²), the axial loads in the directions A and B at about 24500 N (2,500 kgf), the closing loads for sectional dies in the directions C and D at about 29400 N (3,000 kgf), the radial stretching load of about 19600 N (2,000 kgf), and the retreat distance of upper and lower stretching dies in the directions F and E at 4.5 mm. Under these conditions, a circular bellows of 38 mm in diameter at the grooves and 61 mm in diameter at the ridges and with a pitch of 5 mm was first formed and the circular bellows was then formed to produce an oval bellows. As a result, an oval bellows 26 of the desired size and good shape could be produced. Incidentally, the retreat distances of the upper and lower stretching dies may be different from each other, or one of the upper and lower stretching dies may be retreated by a desired retreat distance. According to the second embodiment, the present invention provides the following effects: (1) Costs can be reduced because oval and similar bellows 26 can be produced from inexpensive circular (cylindrical) stock tubes 1. (2) Oval and similar bellows can be produced by means of the same apparatus as used for the forming of circular bellows by installing the upper and lower stretching dies 43 and 44 on the hydraulic forming device. (3) Buckling, wrinkling, etc., do not occur during forming because the deformation by stretching is the principal step of this forming method. (4) Because the passage area in the interior of a formed article (oval and similar bellows) does not decrease. (5) The ratio of the length of the major axis of the oval to the length of the minor axis thereof can be changed by changing the retreat distance of the upper and lower stretching dies. According to another aspect of the present invention, cost reductions can be accomplished because oval and similar bellows can be produced from inexpensive circular tubes as the stock. In addition, said bellows can be produced by means of the same apparatus as the hydraulic device for forming the circular bellows. Moreover, irregular deformation, such as buckling and wrinkling, is not apt to occur and few defective parts are produced. There is an additional effect that the passage area does not decrease.

Claims: A method for producing a bellows (26) with a cross section of elliptical, egg-shaped, shaped as two equal semi-circles connected by two parallel straight lines, non-circular ring or polygonal form with all vertexes and apexes being rounded, comprising the steps of bulging a metallic cylindrical tube (1) to form a bellows (1d) with a circular cross section and transforming the bellows (1d) with the circular cross section to form the final bellows (26) with the cross section of one of said forms, said transforming step comprising an upsetting of the bellows (1d) with the circular cross section by using a pair of dies (23, 24) for applying upsetting forces to the circular bellows (1d) in a radial direction (E,F) of the bellows (1d), and applying hydraulic pressure (20) inside of the circular bellows (1d) during said upsetting. A method as claimed in claim 1, wherein the bulging is conducted by applying fluid pressure to an inside of the cylindrical tube (1). A method as claimed in claim 2, wherein the fluid for pressure application is a liquid. A method as claimed in claim 1, wherein said step of forming the circular bellows (1d) includes a step of axially deforming a bulged metallic tube under pressing pressure so that bulged portions of the tube are folded. A method as claimed in claim 1, wherein said step of forming the bellows (1d) with the circular cross section comprises placing two sets of sectional half-dies (11-13, 16-18), which cooperatively define a circular shape by inside surfaces thereof, each set consisting of halves of sectional dies, to an outside of the metallic cylindrical tube (1) in a manner that the sectional dies in each set are spaced by a specified distance from each other in the axial direction, of the tube (1), bulging the metallic cylindrical tube (1) by applying hydraulic pressure (20) to an inside of said metallic tube (1), and axially pressing the metallic tube (1) to form the bellows (1d) with the circular cross section. A method as claimed in claim 5, wherein the circular bellows (1d) is formed by means of a bulge forming die (28) which comprises the two sets of sectional dies (11-13, 16-18), each of the sectional dies comprising a forming part (28a) which has an inside diameter almost equal to an outside diameter of the cylindrical tube (1) and a same shape of inside surface as one of the grooves of the circular bellows (1d) to be formed, and a base (28b) integrated with the forming part (28a) and of larger thickness than the forming part, each set of sectional half-die being superposed on each other so that the thickness center distance of the forming parts becomes equal to the pitch of the bellows (1d) with the circular cross section. A method as claimed in claim 1, wherein transforming is conducted by means of stretching dies (23, 24) which have a same external form as an internal form at ends on the major axis of the final bellows (26) to be formed and provide corrugations capable of being fitted to ridges and grooves of the bellows (1d) with the circular cross section.

Inventors: ASAO HIROSHI, NAKAMURA KEIICHI, OKADA KENICHI, ASAO, HIROSHI, NAKAMURA, KEIICHI, OKADA, KENICHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1987
Main classification: B21D15
IPC codes: B21D15, B21D26