Abstract:
A double pipe connection structure includes a joint member and a double pipe having an inner pipe and an outer pipe coaxially disposed therein. Further, the double pipe connection structure includes a supporting rib formed integrally therewith in an axial direction, wherein the supporting rib connects the inner pipe to the outer pipe. Further, the joint member includes a first hole into which the outer pipe is inserted and a second hole into which the inner pipe is inserted, wherein the second hole has a diameter that is smaller than a diameter of the first hole, and wherein the first and second holes are bored to be aligned in an axial direction. Moreover, in the joint member, a first-pressure connecting hole communicates with the first hold and a second-pressure connecting hole communicates with the second hole, such that a pressure in the first-pressure connecting hole is lower than a pressure in the second-pressure connecting hole.

Description:
CROSS-REFERENCE TO THE RELATED APPLICATIONS 
     This application is based on and claims the priority benefit of Japanese Patent Application No. 2008-049046, filed on Feb. 29, 2008, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a double pipe connection structure and a method for connecting a double pipe, and particularly, relates to a double pipe connection structure and a method for connecting a double pipe in which caulking is used for connection of the double pipe. 
     2. Description of the Related Art 
     Conventionally, brazing has been usually used for connection of a double pipe. However, in this case, there have been problems that connection work takes time and effort, a degree of difficulty in connection is higher, process costs are comparatively high, and the like. 
     In this regard, for example, Japanese Patent Application Publication No. 2004-270928, or the like, has proposed a method of connecting a double pipe using caulking without brazing. 
     The related art disclosed in Japanese Patent Application Publication No. 2004-270928 provides a structure in which a bypass inner pipe having an O-ring mounted on an outer periphery thereof is inserted into an end of an inner pipe, an end part of this bypass inner pipe is inserted into a small diameter hole in a branch joint, an outer pipe having an O-ring mounted thereon is inserted into a large diameter hole coaxial with the small diameter hole in the branch joint, and a main body part that forms the large diameter hole is caulked to a depressed groove part formed on an outer periphery of the outer pipe. 
     However, in the above-mentioned prior art, since the bypass inner pipe formed separately from the double pipe is mounted on the double pipe, the bypass inner pipe causes increase in the number of components and increase in cost. 
     In addition, in order to mount the bypass inner pipe, the diameter of the inner pipe is expanded. Furthermore, in order to mount the O-ring on the outer pipe, the O-ring is mounted on the depressed groove formed by depressing a general part of the outer pipe inward. 
     For this reason, a flow passage area between the outer pipe and the inner pipe is narrowed, and flow resistance of a fluid flowing in the outer pipe increases. 
     Moreover, the outer pipe has a fixed diameter, a diameter of a large diameter hole in a joint member is limited, and flexibility of connection is low. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above-mentioned conventional problems. An object of the present invention is to provide a double pipe connection structure and a method for connecting a double pipe that allows reduction in cost by reducing the number of components, and reduction in flow resistance, and improvement in connection flexibility. 
     In order to accomplish the aforementioned object, a double pipe connection structure according to one embodiment of the present invention includes: a double pipe which is formed in a dual structure having an inner pipe and an outer pipe coaxially disposed therein, and which has a supporting rib formed integrally therewith in an axial direction, the supporting rib connecting the inner pipe to the outer pipe; a joint member including a large diameter hole into which the outer pipe can be inserted, and a small diameter hole into which the inner pipe can inserted, the large diameter hole and small diameter hole being bored to be aligned in the axial direction, a large-diameter-hole side communicating path is connected to the large diameter hole, and a small-diameter-hole side communicating path is connected to the small diameter hole; an inner pipe end portion formed by removing the outer pipe and the supporting rib at a distal end portion of the double pipe so as to project from a distal end of the outer pipe; an inner pipe side O-ring mounted on the inner pipe end portion around a periphery thereof; an outer pipe expanded part detached from the supporting rib at the distal end portion of the outer pipe and having a diameter expanded more than a general part of the outer pipe; and an outer pipe side O-ring mounted on the outer pipe expanded part around a periphery thereof. 
     The inner pipe end portion is inserted into the small diameter hole while the inner pipe side O-ring is press-contacted with an inner periphery of the small diameter hole, and a periphery of an open end portion of the large diameter hole in the joint member is caulked to the outer pipe expanded part so that the outer pipe is coupled with the joint member, with the outer pipe expanded part being inserted into the large diameter hole while the outer pipe side O-ring is press-contacted with an inner periphery of the large diameter hole. 
     Moreover, a method for connecting a double pipe according to one embodiment of the present invention includes: in an end portion of a double pipe which is formed to have a dual structure where an inner pipe and an outer pipe are coaxially disposed, and which has a supporting rib connecting inner pipe to the outer pipe and being formed integrally in an axial direction. 
     the method includes the steps of: removing the outer pipe and the supporting rib to form an inner pipe end portion projected from a distal end of the outer pipe; detaching a remaining end portion of the outer pipe from the supporting rib to form an outer pipe expanded part whose diameter is expanded more than that of a general part of the outer pipe; mounting an inner pipe side O-ring on the inner pipe end portion around a periphery thereof, and mounting an outer pipe side O-ring on the outer pipe expanded part around a periphery thereof; inserting the double pipe into a joint member in such a way that the inner pipe end portion is inserted into the small diameter hole while the inner pipe side O-ring is press-contacted with an inner periphery of the small diameter hole, and that the outer pipe end portion is inserted into the large diameter hole while the outer pipe side O-ring is press-contacted with an inner periphery of the large diameter hole, the joint member having a large diameter hole into which the outer pipe can be inserted and a small diameter hole into which the inner pipe can be inserted, the large diameter hole and the small diameter hole being bored to be aligned in an axial direction; and caulking a periphery of an open end portion of the large diameter hole in the joint member to couple the double pipe to the joint member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a double pipe connection structure A according to embodiment 1 of the present invention, with a part thereof being cut off; 
         FIG. 2  is an overall system diagram showing an air-conditioner ACU for a vehicle to which the double pipe connection structure A according to embodiment 1 is applied; 
         FIG. 3  is a sectional view showing the double pipe connection structure A according to embodiment 1; 
         FIG. 4  is a sectional view showing a double pipe of the double pipe connection structure A according to embodiment 1; 
         FIG. 5  is a sectional view showing the double pipe of the double pipe connection structure A according to embodiment 1; 
         FIG. 6  is a perspective view showing a state before performing a previous process of the double pipe used for the double pipe connection structure A according to embodiment 1; 
         FIG. 7  is a perspective view illustrating a process to remove an outer pipe and a supporting rib of the double pipe used for the double pipe connection structure A according to embodiment 1; 
         FIG. 8  is a perspective view illustrating a process to expand a diameter of the outer pipe of the double pipe used for the double pipe connection structure A according to embodiment 1; 
         FIG. 9  is a perspective view showing a state where the previous process of the double pipe used for the double pipe connection structure A according to embodiment 1 is completed; 
         FIG. 10  is a perspective view showing a manner that the double pipe  4  is inserted into a joint member in the double pipe connection structure A according to embodiment 1; and 
         FIG. 11  is an overall system diagram showing other air-conditioner ACU 2  for a vehicle to which the double pipe connection structure A according to embodiment 1 is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are described in detail hereinafter with reference to the accompanying drawings. 
     [Embodiment 1] 
       FIGS. 1 to 10  illustrate a double pipe connection structure A according to an embodiment 1 of the present invention. 
       FIG. 2  illustrates a piping structure of an air-conditioner ACU for a vehicle to which the double pipe connection structure A is applied. 
     This air-conditioner ACU for a vehicle includes a front seat side air-conditioning unit  1  that sends air to a front seat side of the vehicle (not shown), and a backseat side air-conditioning unit  2  that sends air to a backseat side of the vehicle (not shown). 
     A front evaporator  11  is installed in the front seat side air-conditioning unit  1 , and a rear evaporator  21  is installed in the backseat side air-conditioning unit  2 . 
     Each of the evaporators  11  and  21  is connected in parallel with a refrigerating cycle  3 . As known, this refrigerating cycle  3  includes a conduit line  35  in which a fluid sequentially circulates a compressor  31  that is driven by an engine for running (not shown) and compresses a refrigerant; a condenser  32  that cools the refrigerant turned into a high pressure gas by the compressor  31  to obtain a saturated liquid; a liquid tank  33  for performing gas liquid separation of the refrigerant; expansion valves  34   a  and  34   b  that turn the refrigerant into a steam of low temperature and low pressure; and the evaporators  11  and  21 . Other than the compressor driven by the engine, a compressor driven by electricity can also be used. 
     In the conduit line  35  of the refrigerating cycle  3  thus configured, in the case of the present embodiment 1, double pipes  4  and  4  are used for parts where a high-pressure refrigerant passage  35   a  extending from the liquid tank  33  to the expansion valves  34   a  and  34   b  is disposed in parallel with a low-pressure refrigerant passage  35   b  extending from each of the front evaporator  11  and the rear evaporator  21  to the compressor  31 . 
     Double pipe connection structures A and A according to the embodiment 1 are used for a piping connection part of both ends of this double pipe  4 . 
     Next, the double pipe connection structure A is described. Since the four of the double pipe connection structures A shown in  FIG. 2  have the same structure, among these, description is given on the double pipe connection structure A (shown as A (a) in  FIG. 2 ), as a representative. The double pipe connection structure A is used for connection between the double pipe  4  that supplies the refrigerant to the rear evaporator  21  and a high-pressure piping  351  that forms the high-pressure refrigerant passage  35   a  as well as a low-pressure piping  352  that forms the low-pressure refrigerant passage  35   b  in the conduit line  35  of the refrigerating cycle  3  as described above. 
     The double pipe  4  used for this double pipe connection structure A is, for example, a metal extrusion-molded product made of aluminum. As shown in  FIGS. 5 and 6 , the double pipe  4  includes an inner pipe  41 , an outer pipe  42  provided coaxial with this inner pipe  41  on the outer side of the inner pipe  41 , and three supporting ribs  43 ,  43 , and  43  erected in a radial direction between the outer pipe  42  and the inner pipe  41  and integrally formed to connect the outer pipe  42  and the inner pipe  41 . 
     A joint member  5  shown in  FIG. 1  is interposed between the double pipe  4  and the high-pressure piping  351  as well as the low-pressure piping  352 . This joint member  5  is a metal casted product made of aluminum or the like. The joint member  5  includes a main body  51 , a large diameter hole  52 , a small diameter hole  53 , a high-pressure connecting hole  54  (communicating path on the small diameter hole side), and a low-pressure connecting hole  55  (communicating path on the large diameter hole side). 
     The main body  51  is formed in a shape where two approximately rectangular parallelepipeds each having a different thickness are connected, as shown in  FIG. 1 . 
     The large diameter hole  52  is bored from a side surface  51   a  of the main body  51 , which is on the right side in  FIG. 1 , to an intermediate part of the main body  51 . The large diameter hole  52  is formed to have an inner diameter larger than an outer diameter of the outer pipe  42  of the double pipe  4 . 
     The small diameter hole  53  is continuously formed at an end of the large diameter hole  52  coaxial with the large diameter hole  52 . The small diameter hole  53  is formed to have an inner diameter larger than an outer diameter of the inner pipe  41  of the double pipe  4 . 
     The high-pressure connecting hole  54  is bored so as to communicate a side surface  51   b  of the main body  51 , which is above in  FIG. 1 , with an end of the small diameter hole  53 . An opening  54   a  of the high-pressure connecting hole  54  is connected with the high-pressure piping  351 . 
     The low-pressure connecting hole  55  is bored so as to communicate the side surface  51   b  of the body  51 , which is above in  FIG. 1 , with the end of the large diameter hole  52 . An opening  55   a  of the low-pressure connecting hole  55  is connected with the low-pressure piping  352 . 
     A cylindrical collar part  51   c  projected from the side surface  51   a  is formed at an open end portion of the large diameter hole  52 . 
     The aforementioned double pipe  4  is subjected to a pre-processing for use in the double pipe connection structure A according to the present embodiment 1. This pre-processing is described. 
     The pre-processing consists of an outer pipe and supporting rib removing process, an outer pipe diameter expanding process, an inner pipe diameter expanding process, a depressed groove forming process, and an O-ring mounting process. 
     First, in the outer pipe and supporting rib removing process, the outer pipe  42  and supporting rib  43  at a distal end portion of the double pipe  4  are cut. Then, the double pipe  4  in a state after extrusion molding shown in  FIG. 6  is processed to have a shape that an inner pipe end portion  41   a  is projected from the end of the outer pipe  42 , as shown in  FIG. 7 . 
     In the next outer pipe diameter expanding process, first, an inner periphery of the outer pipe  42  and an end in an outer radial direction of the supporting rib  43  are detached from each other at the distal end portion of the outer pipe  42  of the double pipe  4  processed to have the shape of  FIG. 7 . Next, as shown in  FIG. 8 , the diameter of the outer pipe end portion detached from the supporting rib  43  is expanded to form an outer pipe expanded part  42   a . This deformation accompanied with formation of the outer pipe expanded part  42   a  causes a position of the end of the outer pipe  42  to displace in a proximal direction of the double pipe  4 . Consequently, the supporting rib  43  is projected from the distal end of the outer pipe expanded part  42   a  by the amount of displacement, as shown in  FIG. 4 . 
     In the following inner pipe diameter expanding process, in the inner pipe end portion  41   a , the diameter of the distal end portion is expanded to form an inner pipe expanded part  41   b  shown in  FIG. 9 . 
     In the next depressed groove forming process, the inner pipe expanded part  41   b  is depressed in an inner radial direction to form an inner pipe depressed groove  41   c  shown in  FIG. 4  around the periphery of the inner pipe expanded part  41   b , and the outer pipe expanded part  42   a  is depressed in an inner radial direction to form an outer pipe depressed groove  42   c  shown in  FIG. 4  around the periphery of the outer pipe expanded part  42   a  (see  FIG. 4 ). 
     At this time, an inner diameter of the part in which the inner pipe depressed groove  41   c  is formed is formed approximately equal to an inner diameter of a general part of the inner pipe end portion  41   a . In the case of the outer pipe depressed groove  42   c , the supporting rib  43  exists in the inner radial direction of the outer pipe depressed groove  42   c . Accordingly, an inner diameter of a part in which the outer pipe depressed groove  42   c  is formed is formed equal to an inner diameter of a general part of the outer pipe  42 , or not less than that. 
     Simultaneously with the processing to form the outer pipe depressed groove  42   c , a bead part  42   d  is formed to be projected in the outer radial direction around the periphery of a proximal end of the outer pipe expanded part  42   a . This bead part  42   d  is formed to be larger than an inner diameter of the opening edge of the large diameter hole  52  and smaller than an inner diameter of a collar part  51   c.    
     At the time when completing the above-mentioned processes, the double pipe  4  is in a state shown in  FIG. 4 . 
     In the following O-ring mounting process, an inner pipe side O-ring  61  is mounted on the inner pipe depressed groove  41   c , and an outer pipe side O-ring  62  is mounted on the outer pipe depressed groove  42   c.    
     The double pipe  4  after subjected to the above-mentioned previous process is in a state shown in  FIG. 9 . 
     Next, the double pipe  4  after subjected to the previous process is connected with the joint member  5 . 
     In this case, as shown in  FIG. 10 , the inner pipe end portion  41   a  of the double pipe  4  is inserted into the large diameter hole  52  of the side surface  51   a  of the joint member  5 . Then, as shown in  FIG. 3 , the inner pipe expanded part  41   b  at the distal end of the inner pipe  41  is inserted into the small diameter hole  53  while the inner pipe side O-ring  61  is compressed and deformed. Simultaneously, the outer pipe expanded part  42   a  is inserted into the large diameter hole  52  while the outer pipe side O-ring  62  is compressed and deformed. This insertion is performed until the bead part  42   d  of the outer pipe  42  abuts against a stepped part  51   d  formed because of a difference of the diameter between the opening of the large diameter hole  52  and the collar part  51   c.    
     Thus, an amount of insertion into the large diameter hole  52  of the joint member  5  is the amount until the bead part  42   d  abuts against the stepped part  51   d  of the joint member  5 , as mentioned above. Therefore, control of this insertion amount is easy and reliable, and workability is excellent. 
     Finally, as shown in  FIGS. 1 and 3 , the collar part  51   c  of the joint member  5  is caulked so as to cover the bead part  42   d.    
     In the double pipe connection structure A according to the embodiment 1 thus connected, the high-pressure refrigerant supplied from the high-pressure piping  351  is introduced into the small diameter hole  53  from the high-pressure connecting hole  54  in the joint member  5 . Subsequently, the high-pressure refrigerant passes through the inner pipe  41  of the double pipe  4 , and flows to the expansion valve  34   b.    
     Moreover, the refrigerant, which has passed through the rear evaporator  21  to have a low-pressure, passes through a space between the outer pipe  42  and the inner pipe  41  of the double pipe  4 . The refrigerant flows into the large diameter hole  52  of the joint member  5 , and subsequently, flows from the large diameter hole  52  through the low-pressure connecting hole  55  to the low-pressure piping  352 . 
     At this time, the inner-pipe side O-ring  61  mounted on the outer periphery of the inner pipe expanded part  41   b  of the inner pipe  4  securely seals the space between the small diameter hole  53  in which the high-pressure refrigerant flows and the large diameter hole  52  in which the low-pressure refrigerant flows. 
     Additionally, the outer pipe side O-ring  62  mounted on the outer pipe expanded part  42   a  of the outer pipe  42  securely seals the space between the outside and the large diameter hole  52  in which the low-pressure refrigerant flows. 
     In this case, in comparison with sealing by brazing, fluctuation depending on workers, fluctuation in melting of a brazing material or the like can be avoided, thereby making it possible to obtain secure and reliable sealing performance. 
     Moreover, the inner pipe depressed groove  41   c  is formed at the inner pipe expanded part  41   b , and formed to have the inner diameter approximately equal to the inner diameter of the general part of the inner pipe end portion  41   a . For this reason, in comparison with a case where the inner pipe depressed groove  41   c  is formed at the inner pipe end portion  41   a,  a part of the inner pipe depressed groove  41   c  can be prevented from causing flow resistance when the refrigerant flows through the inner pipe  41 . 
     Similarly, the outer pipe depressed groove  42   c  is formed at the outer pipe expanded part  42   a , and formed to have the inner diameter not less than the inner diameter of the general part of the outer pipe  42 . For this reason, in comparison with a case where the outer pipe depressed groove is formed by reducing the diameter of the inner diameter part of the general part of the outer pipe  42 , a part between the outer pipe  42  and the inner pipe  41  can be prevented from causing flow resistance when the refrigerant flows between the outer pipe  42  and the inner pipe  41 . 
     As mentioned above, since the flow resistance of the refrigerant can be suppressed, circulation of the refrigerant can be stabilized and air conditioning performance can be improved. 
     As described above, effects to be listed can be obtained in the double pipe connection structure A according to the present embodiment 1 as follows. 
     a) Since no additional component is used for connection between the double pipe  4  and the joint member  5 , the number of components can be reduced and cost reduction can be attained in comparison with a case where an additional component is used. 
     b) Sealing between the low-pressure part and high-pressure part of the refrigerant is performed by the inner pipe side O-ring  61 , and sealing between the low-pressure part and the outside is performed by the outer pipe side O-ring  62 . For this reason, sealing performance can be stabilized compared with sealing by brazing. 
     c) The outer pipe expanded part  42   a  is formed at the distal end portion of the outer pipe  42 , and the outer periphery of the outer pipe expanded part  42   a  is inserted into the large diameter hole  52  of the joint member  5 . For this reason, when multiple specifications are provided in an inner diameter dimension of the large diameter hole  52  of the joint member  5 , it is possible to form the outer diameter of the outer pipe expanded part  42   a  depending on the inner diameter, and to obtain excellent connection flexibility. 
     d) Since the outer pipe  42  is detached from the supporting rib  43  and the supporting rib  43  is left integral with the inner pipe  41  in formation of the outer pipe expanded part  42   a , rigidity of the inner pipe  41  and rigidity of the outer pipe  42  can be secured in comparison with a case where the supporting rib  43  is removed in a range that the outer pipe expanded part  42   a  is formed. Accordingly, support rigidity and durability can be improved, and simultaneously, maintaining performance of coaxiality of the inner pipe  41  and the outer pipe  43  can be improved. 
     e) The inner pipe depressed groove  41   c  is formed at the inner pipe expanded part  41   b  formed by expanding the inner diameter of the inner pipe  41 , and the inner diameter of the inner pipe expanded part  41   b  is formed approximately equal to the inner diameter of the general part of the inner pipe end portion  41   a . For this reason, in comparison with a case where the inner pipe depressed groove is formed by reducing the diameter of the general part of the inner pipe  41 , a part of the inner pipe depressed groove  41   c  can be prevented from causing flow resistance. Thereby, it is possible to attain stabilization of flow of the refrigerant and to improve air conditioning performance. 
     f) The outer pipe depressed groove  42   c  is formed at the outer pipe expanded part  42   a  formed by expanding the inner diameter of the outer pipe  42 , and the inner diameter of the outer pipe expanded part  42   a  is not less than the inner diameter of the general part of the outer pipe  42 . For this reason, in comparison with a case where the outer pipe depressed groove is formed by reducing the diameter of the general part of the outer pipe  42 , it is possible to suppress flow resistance of the refrigerant, to attain stabilization of flow of the refrigerant, and to improve the air conditioning performance. 
     g) When the outer pipe side O-ring  62  is inserted into the large diameter hole  52  by a predetermined amount at the time of insertion of the double pipe  4  into the large diameter hole  52  of the joint member  5 , the bead part  42   d  abuts against the stepped part  51   d  of the opening of the large diameter hole  52 , so that further insertion is restricted. Therefore, control of the amount of insertion of the double pipe is easy and reliable, and workability is excellent. 
     h) The collar part  51   c  of the joint member  5  is caulked to the bead part  42   d  projected from the outer pipe  42  in the outer radial direction. For this reason, it is easy to ensure strength of the caulked part in the axial direction. 
     [Embodiment 2] 
     Next, on the basis of  FIG. 11 , description is given on an embodiment 2 of the present invention. 
     In this embodiment 2, as an example, the double pipe connection structure A according to the embodiment 1 is used for an air-conditioner ACU 2  for a vehicle different from that shown in the embodiment 1. In other words, the air-conditioner ACU 2  for the vehicle shown in the embodiment 2 is used, as an example, in a general structure in which only one air-conditioning unit  201  is mounted in the vehicle. Unlike the embodiment 1, the backseat side air-conditioning unit  2 , and the high-pressure piping  351  as well as the low-pressure piping  352  connected thereto are removed. Other structures are the same as those in the embodiment 1. 
     Thus, the present invention can also be used for the general air-conditioner ACU 2  for the vehicle in which only one air-conditioning unit  201  is mounted. 
     As mentioned above, the preferable embodiments 1 and 2 according to the present invention has been explained in detail with reference to the drawings. However, the present invention is not limited to these embodiments 1 and 2, and includes modifications in design without departing from the scope of the present invention. 
     For example, although an example in which the present invention is used for an air-conditioner for a vehicle is shown in the embodiments 1 and 2, the present invention can, of course, be used for any air-conditioner other than a vehicle that has a structure where the double pipe is connected to the joint member. Moreover, the present invention can be used for industrial machines or the like other than air-conditioners. 
     Although an example in which the inner pipe side O-ring  61  and the outer pipe side O-ring  62  are respectively mounted on the inner pipe depressed groove  41   c  and the outer pipe depressed groove  42   c  has been shown in the embodiment 1, the present invention is not limited to an arrangement in which the O-ring is mounted on the depressed groove formed by deforming the outer pipe and the inner pipe into the inner radial direction, as mentioned above. In other words, a pair of protrusions formed by protruding the outer pipe and the inner pipe in the outer radial direction may be formed, and the O-ring may be mounted on the pair of protrusions. This case has an advantage in a viewpoint of flow resistance since the inner diameter of the part on which the O-ring is mounted is not reduced. 
     Although an example in which the inner pipe expanded part  41   b  is formed at the inner pipe end portion  41   a  and the inner pipe depressed groove  41   c  is formed at this inner pipe expanded part  41   b  has been shown in the embodiment 1, the present invention is not limited to this. For example, the inner pipe end portion  41   a  may be formed to have the same diameter as that of other part of the inner pipe  41 , and the inner pipe depressed groove may be formed at this part. 
     Although in the embodiment 1, the bead part  42   d  is formed around the periphery of the outer pipe expanded part  42   a , and this bead part  42   d  is abutted against the stepped part  51   d  of the opening part of the large diameter hole  52  of the joint member  5 , the present invention is not limited to this. For example, without forming the bead part around the periphery, the distal end of the outer pipe expanded part  42   a  is abutted against the step formed at the large diameter hole  52 , and the collar part  51   c  of the joint member  5  may be caulked to the stepped part formed by the outer pipe expanded part  42   a  and the general part. 
     Moreover, although the bead part  42   d  is abutted against the stepped part  51   d  in the embodiment 1, for an arrangement to control the amount of insertion as mentioned above, a projection may be provided around the periphery of the outer pipe expanded part  42   a . Alternatively, multiple projections may be radially formed, not around the periphery of the outer pipe expanded part  42   a    
     As mentioned above, since no additional component is used for connection between the double pipe and the joint member in the double pipe connection structure according to the present invention, the number of the components can be reduced and cost reduction can be attained in comparison with the case where an additional component is used. 
     In the present invention, sealing between the small diameter hole side and the large diameter hole side is performed by the inner pipe side O-ring, and the outer pipe side O-ring performs sealing between the large diameter hole and the outside. For this reason, sealing performance can be stabilized compared with sealing by brazing. 
     Moreover, the outer pipe expanded part is formed at the distal end portion of the outer pipe, and this outer pipe expanded part is inserted into the inner diameter of the large diameter hole of the joint member. For this reason, when multiple specifications are provided in the inner diameter of the major diameter of the joint member, the outer diameter of the outer pipe expanded part can be formed depending on the inner diameter, and different specifications can be dealt with, thus obtaining excellent connection flexibility. 
     In addition, since the outer pipe is detached from the supporting rib and the supporting rib is left integral with the inner pipe in formation of the outer pipe expanded part, in the range in which the outer pipe expanded part is formed, the rigidity of the inner pipe and the rigidity of the outer pipe are secured in comparison with the case where the supporting rib is removed. Accordingly, support rigidity and durability can be improved, and simultaneously, the maintaining performance of the coaxiality of the inner pipe and the outer pipe can be improved. 
     Moreover, since the outer pipe side O-ring is mounted on the outer pipe depressed groove formed at the outer pipe expanded part whose diameter is expanded more than that of the general part, the inner diameter of the outer pipe expanded part can be expanded and flow resistance of the fluid can be suppressed by an amount of expansion, compared with a case where the outer pipe depressed groove is formed at the general part of the outer pipe. 
     Furthermore, since the inner pipe side O-ring is mounted on the inner pipe depressed groove formed at the inner pipe expanded part whose diameter is expanded more than that the inner pipe end portion, the inner diameter of the inner pipe expanded part can be expanded and flow resistance of the fluid can be suppressed by an amount of expansion, compared with a case where the inner pipe depressed groove is formed at the general part of the inner pipe. 
     Additionally, at the time of insertion of the double pipe into the large diameter hole of the joint member, when the outer pipe side O-ring is inserted in the large diameter hole by the predetermined amount, the bead part abuts against the opening edge part of the large diameter hole so that further insertion is restricted. In this state, the edge part of the large diameter hole of the joint member is caulked to the bead part. 
     Therefore, control of the amount of insertion of the double pipe is easy and reliable, and workability is excellent. Additionally, since a caulking part on the side of the joint member is caulked to the bead part projected from the outer pipe end portion, it is easy to increase connection strength because of strong caulking effect in the axial direction. 
     The double pipe connection structure can be obtained by sequentially performing the outer pipe and supporting rib removing process, the outer pipe diameter expanding process, the O-ring mounting process, the double pipe inserting process, and the caulking process. 
     With the double pipe connection structure obtained through these processes, it is possible to attain cost reduction, stabilization of sealing performance, and improvement in connection flexibility, support rigidity and durability, as mentioned above. 
     Furthermore, since the diameter is expanded in the outer pipe diameter expansion process after detaching the distal end portion of the outer pipe from the supporting rib, the diameter can be easily expanded, and simultaneously, deformation of the inner pipe in this diameter expanding process can be prevented.