Double-walled rectangular conduit with excellent wire passing properties and buckling resistance, connection structure of double-walled rectangular conduit using the same, and method for determining mass per unit length of double-walled rectangular conduit and tensile yield stress at which it does not buckle under its own weight

Abstract

To provide a double-wall square conduit pipe, etc. which can suppress the buckling of an inner pipe in a cantilever state even if using a normal polyolefin resin without using a large friction-resistance ethylene system copolymer and a thermoplastic elastomer.SOLUTION: A double-wall square conduit pipe 1 is mainly constituted of an inner pipe 9 and an outer pipe 7. The outer pipe 7 has a large-diameter part 3 having a relatively-large diameter, and a small-diameter part 5 having a relatively-small diameter. In a cross section of the outer pipe 7 vertical to a pipe axial direction, the large-diameter part 3 is formed into a substantially-rectangular cross section shape, and a cross section of the small-diameter part 5 is formed into a circular cross section shape. The inner pipe 9 is disposed inside the outer pipe 7. A cross section shape of the inner pipe 9 vertical to a pipe axial direction is circular. At this time, the buckling of the inner pipe 9 can be suppressed by satisfying a formula of σy>16w L2(d+2t) / (π ((d+2t)4-d4))(σy: the tension yield stress of a material of the inner pipe 9, w: a mass per unit length of the conduit pipe, L: a length of the conduit pipe, d: an inside diameter of the inner pipe 9, and t: a wall thickness of the inner pipe 9).SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a double-walled rectangular conduit having excellent wire passing properties and buckling resistance, a connection structure of the double-walled rectangular conduit using the same, and a method for determining the mass per unit length of the double-walled rectangular conduit and the tensile yield stress of an inner tube that does not buckle under its own weight. [Background technology]

[0002] Conventionally, round spiral corrugated conduits have been widely used, but for example, rectangular conduits with alternating rectangular and circular cross sections have been proposed as underground wire protection conduits. Rectangular conduits can be tightly packed together to form multi-row, multi-stage conduits, so they have the advantage of requiring less excavation of soil and being compactly housed in a small space during conduit installation work. For this reason, rectangular conduits are widely used, especially in large cities. Furthermore, as part of disaster prevention measures in recent years, cable installation methods using rectangular conduits are gaining attention as an alternative to cable installation on utility poles.

[0003] As described later, such a conduit has a problem that when a communication cable or the like is inserted into the conduit, it gets caught on a step on the inner surface, making it difficult to pass the cable through. In order to solve this problem, a double-walled rectangular conduit with an inner tube provided inside the conduit has been proposed. However, it has been found that when a double-walled rectangular conduit is used, unlike a conventional double-walled corrugated pipe, the gap between the large diameter parts, which are the unfused double walls, is large, and this causes the inner tube to buckle during handling, resulting in a large amount of protrusion of the inner tube into the inside of the pipe, and thus reducing the ability to pass the cable through. As a result of extensive research into preventing this buckling, the inventors have found that this problem can be solved by using a material that satisfies certain conditions for the inner tube material, and have thus made the present invention.

[0004] As a conventional rectangular conduit, for example, a rectangular synthetic resin corrugated pipe has been proposed in which the pipe wall is formed in an annular uneven shape, and further, the unevenly shaped concave and convex portions are formed alternately and continuously, with the cross-sectional shape of the convex portions of the uneven shape being a roughly square shape with arc-shaped corners, and the cross-sectional shape of the concave portions being circular (Patent Document 1).

[0005] Patent Document 1 aims to solve the problem that conventional conduits with a circular cross section are often shipped wound around a drum, which causes some tendency for the tube to curl, and in addition to the tube having a circular cross section, the tube is unstable when piping due to the tendency for the tube to curl, and it is difficult to maintain parallelism even when piping the tubes in parallel, making insertion difficult. Patent Document 1 makes it possible to piping multiple rows and multiple stages in close contact with each other using a rectangular corrugated resin tube that is easy to position, does not easily shift position even after piping, and is easy to maintain the piping position.

[0006] On the other hand, such a conduit may cause a problem that when a communication cable or the like is inserted into the conduit, it gets caught on a step on the inner surface, making it impossible to pass the cable through. For example, in Patent Document 1, if the length of the large diameter portion is long, there is a problem that when the cable is pulled into the conduit, the cable gets caught on a step, reducing the ease of passing the cable through.

[0007] In response to this, a double-walled conduit has been proposed in which an inner tube is provided inside the conduit to make it easier to pass a communication cable through the conduit. The double-walled conduit includes a conduit with a circular outer tube shape and a rectangular conduit. For example, a double-walled rectangular resin corrugated tube has been proposed as a double-walled rectangular conduit, which includes an outer tube with an outer surface shape formed by alternating square cross-sectional wall portions and circular cross-sectional wall portions in the axial direction of the tube, and an inner tube with an inner surface shape formed into a substantially linear circular cylindrical surface (Patent Document 2).

[0008] Also, in order to balance the rigidity and flexibility of the electric conduit, a corrugated double pipe with a trapezoidal cross section has been proposed, which is made up of a circular inner layer pipe with a smooth inner surface made of polypropylene, polyethylene, and a flexibility imparting agent, and a corrugated outer layer pipe made of polypropylene (Patent Document 3). Patent Document 3 describes that rigidity is ensured by using polypropylene for the outer pipe, and flexibility is ensured by using a rubber material or a thermoplastic elastomer as a flexibility imparting agent in a polymer blend with any synthetic resin material as the constituent material of the inner pipe.

[0009] Here, in the case of a corrugated double-walled conduit with a circular cross section, the pitch between the inner and outer tubes in the axial direction of the conduit is generally small, and the cross section is also circular, so stress is less likely to concentrate than in the double-walled rectangular conduit of Patent Document 2, and wrinkles and buckling are less likely to occur in the inner tube of the conduit. Even if buckling does occur, the amount of protrusion of the inner tube is not so large that it does not pose a problem.

[0010] In contrast, in the case of a rectangular double-walled conduit in which rectangular and circular cross sections are alternately formed, the rectangular section protrudes significantly from the circular section in order to ensure stability when stacked, and the axial length of the large diameter section must be increased to enable blow molding of the large diameter section. Furthermore, since the outer tube and the inner tube are fused and restrained in the small diameter section, stress relief due to three-dimensional deformation of the outer tube at this location cannot be expected. As a result, the inner tube of the conduit is prone to buckling, and when this occurs, the inner tube protrudes significantly.

[0011] For example, the double-walled rectangular conduit of Patent Document 2 can be stacked in multiple rows and columns like conventional rectangular conduit, has superior workability compared to conduits with a circular cross section, and has an inner tube disposed inside an outer tube. Therefore, it is expected to have excellent wire passing properties, but if a normal polyolefin resin is used for the inner tube, there is a possibility that the inner tube will buckle during handling during transportation and installation.

[0012] As a method for preventing wrinkles in the inner pipe of a double pipe, there is a method of using a hard synthetic resin for the outer pipe of a synthetic resin double pipe and an ethylene copolymer or a thermoplastic elastomer for the inner pipe (Patent Document 4). Patent Document 4 is related to a fluid pipe, but in the case of a fluid pipe, even small wrinkles can easily become a problem because wrinkles occurring in the inner pipe affect the flow resistance. According to Patent Document 4, it is possible to obtain a synthetic resin double pipe that can prevent wrinkles from occurring on the inside of the inner pipe and has good durability and moldability. Even if wrinkles and buckling on the inside of the inner pipe can be prevented by using an ethylene copolymer, a thermoplastic elastomer, or a rubber material for the inner pipe as in Patent Document 4, when such a soft resin or thermoplastic elastomer is used, there is a problem that friction resistance increases when passing wires through the electric conduit of the present invention. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 8-219333 [Patent Document 2] Japanese Patent Application Publication No. 9-280430 [Patent Document 3] JP 2007-139043 A [Patent Document 4] JP 2002-257269 A Summary of the Invention [Problem to be solved by the invention]

[0014] Here, in the double-walled rectangular conduit, buckling is considered to occur because the cross-sectional shape is irregular and the distance in the axial direction of the large diameter part is large. In the case of a conventional double-walled corrugated pipe, buckling is unlikely to occur, and even if buckling occurs, the amount of protrusion of the inner pipe is small, so it is considered not to be a big problem. On the other hand, in the double-walled rectangular conduit, buckling of the inner pipe actually becomes a problem when loading the product onto a truck or the like during transportation, or when working on site while supporting one end of the double-walled rectangular conduit. In such work, one end is supported and the other end hangs down, so that the double-walled rectangular conduit becomes a cantilever beam, and buckling of the inner pipe occurs. Here, if the center of the double-walled rectangular conduit is held, buckling of the double-walled rectangular conduit is unlikely to occur, but it is practically difficult to handle the product in this way during work on site, and buckling actually occurs in many cases.

[0015] If buckling occurs in the inner pipe in this way, during actual construction site work, when pulling wires, the inner pipe will bend and protrude toward the center of the pipe at the buckled point, causing the cable to get caught at the buckled point, increasing the resistance to pulling the cable and resulting in problems with pulling the cable.

[0016] As described above, the rectangular conduit of Patent Document 1 has a rectangular cross-section as the large diameter section, which allows for piping in a multi-strand, multi-tier stacked structure. However, there is a large step between the circular cross-section section and the rectangular cross-section section, which can cause problems with passing the cable through the tube when the cable is pulled through the step.

[0017] Moreover, the conduit of Patent Document 2 is an invention in which an inner tube is disposed inside a rectangular conduit in order to improve the wire-passing properties of a normal single-walled rectangular conduit as described in Patent Document 1. However, when a product of a certain length is supported under cantilever conditions, buckling occurs, resulting in a problem of reduced wire-passing properties.

[0018] In addition, the corrugated double-walled conduit described in Patent Document 3 has a double-walled pipe structure, but buckling is not taken into consideration. The corrugated double-walled conduit described in Patent Document 3 has a trapezoidal corrugated cross section in the longitudinal direction of the product, but the pipe product shape is circular, and it is a corrugated thin-walled double-walled conduit with a circular cross section. Therefore, the cross section perpendicular to the pipe axis of the product has a circular cross section, which makes it difficult to stack in multiple rows and layers. Furthermore, in the case of Patent Document 3, buckling is less likely to occur by using a polyolefin resin with a rubber material or a thermoplastic elastomer added as the inner pipe material.

[0019] The synthetic resin double pipe of Patent Document 4 is a fluid pipe having a corrugated shape in which the outer pipe protrudes in a trapezoidal shape and the circular cross section is formed in a spiral shape, as in Patent Document 3, and Patent Document 4 has a structure different from that of a double-walled rectangular electric conduit in which the large diameter part is substantially rectangular and the small diameter part and the large diameter part have different irregular cross-sectional diameter shapes. Therefore, as in Patent Document 3, the surface of the inner pipe may swell, but the inner pipe buckling does not occur as in the electric conduit with a rectangular cross-sectional structure. In addition, an ethylene copolymer and a thermoplastic elastomer are used for the inner pipe, which prevents the generation of wrinkles in the inner pipe, but when these inner pipe materials are used as an electric conduit, friction resistance during wiring becomes large and problems occur.

[0020] The present invention has been made in consideration of such problems, and has as its object to provide a double-walled rectangular conduit that does not cause buckling of the inner tube under cantilever conditions even when using ordinary polyolefin resin, without using ethylene-based copolymers, thermoplastic elastomers, or rubber components with high frictional resistance, a connection structure for the double-walled rectangular conduit, and a method for determining the mass per unit length of the double-walled rectangular conduit and the tensile yield stress of the inner tube that will not buckle under its own weight. [Means for solving the problem]

[0021] In order to achieve the above-mentioned object, a first invention is a double-walled rectangular conduit having an outer tube and an inner tube, the outer tube has a large diameter portion having a relatively large diameter and a small diameter portion having a relatively small diameter, the large diameter portion and the small diameter portion are alternately and repeatedly formed in an axial direction of the outer tube, the large diameter portion has a substantially rectangular cross-sectional shape and the small diameter portion has a circular cross-sectional shape in a cross section perpendicular to the axial direction of the outer tube, the inner tube is disposed inside the outer tube, the cross-sectional shape perpendicular to the axial direction of the inner tube is circular, an inner peripheral surface of the outer tube and an outer peripheral surface of the inner tube are fused together in the small diameter portion, and a space is formed between the inner peripheral surface of the outer tube and the outer peripheral surface of the inner tube in the large diameter portion, σ y >16w·L 2 (d+2t) / (π·((d+2t) 4 -d 4 )) (σ y : tensile yield stress of the material of the inner tube, w: mass per unit length of the conduit, L: length of the conduit, d: inner diameter of the inner tube, t: wall thickness of the inner tube) The double-wall rectangular conduit is characterized in that it satisfies the above requirements.

[0022] It is desirable that the length L of the electric conduit is 2000 mm to 6000 mm, the inner diameter d of the inner tube is 30 mm to 200 mm, and the wall thickness t of the inner tube is 0.4 mm to 3 mm.

[0023] The mass w per unit length of the conduit is 200 to 7000 g / m, and 0.08×d 2 +4.55d+14< w < 0.15×d 2 +11.71d+84 It is desirable to satisfy the following.

[0024] In the compression test of JIS C3653 Appendix 3, it is desirable that the deflection δ = (D1 - D2) / D1 × 100% (D1: outer diameter of the conduit before compression, D2: outer diameter of the conduit after compression) be 3.5% or less.

[0025] The outer pipe is usually made of polyolefin resin, taking into consideration strength, corrosion resistance, moisture absorption, cost, etc. Polyethylene resin and polypropylene resin can be used as the polyolefin resin. For example, high density polyethylene, medium density polyethylene, high density polyethylene, various grades of polypropylene, etc. can be used as the polyethylene resin. Here, the polyolefin resin to be used may be appropriately selected in consideration of the pressure resistance required when buried underground according to the product dimensions of the double-walled rectangular electric conduit. Weathering agents and flame retardants may also be added as necessary.

[0026] The outer tube and the inner tube are preferably made of a resin having mutual compatibility. The reason for this is that in the double-wall rectangular electric conduit of the present invention, the inner surface of the small diameter part of the outer tube and the outer surface of the small diameter part of the inner tube are fused to each other at the small diameter part of the outer tube during the manufacturing of the double-wall rectangular electric conduit, so it is preferable that the outer tube and the inner tube have mutual compatibility. In this case, if the inner tube and the outer tube are firmly fused with a resin having high compatibility, the compressive strength and rigidity in the tube axis direction of the double-wall rectangular electric conduit can be increased.

[0027] The inner tube is preferably made of a polyolefin resin that does not contain an ethylene copolymer, a thermoplastic elastomer, or a polyolefin-based modified resin containing a rubber component. The reason for using a polyolefin resin for the inner tube is that it is desired to have high compatibility with the outer tube resin in consideration of strength, corrosion resistance, moisture absorption, cost, etc. Therefore, it is preferable to use the same type of polyolefin resin for the outer tube and the inner tube, and the polyethylene resin may be high density polyethylene, medium density polyethylene, high density polyethylene, or various grades of polypropylene resin.

[0028] In the present invention, the objective is to prevent buckling during handling of the double-wall rectangular conduit without impairing its wire passing property, and therefore the polyolefin resin used in the present invention does not include so-called polyolefin-based modified resins that contain ethylene copolymers, thermoplastic elastomers, or rubber components that have high frictional resistance.

[0029] In order to improve compatibility with the outer tube resin, the inner tube resin may further contain a predetermined amount of an acid-modified polyethylene resin or an acid-modified polypropylene resin. The inner tube may contain an acid-modified resin in addition to the polyolefin resin, and the content of the acid-modified resin may be 0.5 parts by mass or more and 10 parts by mass or less when the content of the polyolefin resin is 100 parts.

[0030] The yield stress of the inner pipe material may be greater than the yield stress of the outer pipe material. Here, the double-walled rectangular electric conduit of the present invention uses a material containing an ethylene copolymer, a thermoplastic elastomer, or a rubber component having a large friction resistance so as not to reduce wireability, and is not intended to prevent wrinkles or flexibility of the inner pipe, but rather to prevent buckling during handling, so there is no need to lower the yield stress of the inner pipe material. Since double-walled rectangular electric conduits are usually premised on linear installation, it is not preferable for the double-walled rectangular electric conduit to have excessive flexibility. Therefore, from the viewpoint of suppressing flexibility and improving buckling resistance, it is desirable for the yield stress of the inner pipe material to be high, and the yield stress of the inner pipe material to be higher than the yield stress of the outer pipe material.

[0031] The wall thickness of the inner tube at a portion corresponding to the large diameter portion of the outer tube is thicker than the wall thickness of the inner tube at a portion fused to the small diameter portion of the outer tube, and the inner tube may be formed so that thick-walled portions and thin-walled portions of the inner tube have alternating wall thickness variations.

[0032] Any of the joint parts may be formed at at least one end or both ends of the double-wall rectangular conduit. For example, a male joint part may be formed as a joint part at one end of the double-wall rectangular conduit, and a female joint part may be formed as a joint part at the other end. In this way, if joint parts are formed at one or both ends of the double-wall rectangular conduit, a conduit of the double-wall rectangular conduit can be formed by connecting these joint parts to each other.

[0033] According to the first aspect of the present invention, by providing a substantially square large diameter portion, the double-wall rectangular conduit can be stably arranged when stacked. Also, since the inner pipe is provided, cables such as electric wires, power cables, and communication cables do not get caught in the step between the large diameter portion and the small diameter portion when passing the wires.

[0034] In addition, the tensile yield stress σ of the material of the inner tube of the double-wall rectangular conduit y By setting the mass per unit length of the conduit (unit mass) w, the length of the conduit L, the inner diameter d of the inner tube, and the wall thickness t of the inner tube to have a specified relationship, it is possible to prevent buckling of the inner tube during handling.

[0035] Considering practical aspects such as manufacturability, transportation efficiency, and workability of the conduit, it is particularly desirable that the length L of the conduit be 2000mm to 6000mm, the inner diameter d of the inner tube be 30mm to 200mm, and the wall thickness t of the inner tube be 0.4mm to 3mm.More preferably, the length L of the conduit is 3000mm to 5000mm, the wall thickness t of the inner tube is 0.8mm to 1.5mm, and the inner diameter d of the inner tube is 50mm to 150mm.

[0036] The mass w per unit length of the conduit is not related to the length of the conduit, but taking into account the buckling resistance and the results of the compression test described later, the mass w per unit length of the conduit can be set to 200 to 7000 g / m. 2 +4.55d+14< w <0.15×d 2 By satisfying +11.71d+84, the amount of deflection in a compression test described later can be suppressed within a predetermined range. Furthermore, it is preferable that the mass w per unit length of the electric conduit is 300 to 5000 g / m. Here, 200 g / m is the minimum value of the pipe mass that can suppress the amount of deflection in a compression test within a predetermined range when the inner diameter d is the minimum dimension of 30 mm, and 7000 g / m is the maximum value of the pipe mass that can suppress the amount of deflection in a compression test within a predetermined range when the inner diameter d is the maximum dimension of 200 mm.

[0037] In addition, the above-mentioned relational equation between the inner diameter d and the mass w per unit length of the conduit expresses the relationship of the mass w per unit length of the conduit according to the change in the size of each product inner diameter d, and by adding a specified product inner diameter, it is possible to find the upper and lower limits of the mass w per unit length of the conduit at that inner diameter.

[0038] In addition, if the deflection δ in the compression test of JIS C3653 Appendix 3 is 3.5% or less, the double-wall rectangular conduit can be prevented from being crushed by earth pressure when buried underground.

[0039] Furthermore, if the outer tube is made of polyolefin resin, it can be manufactured by a general manufacturing method. In this case, if the outer tube and the inner tube are made of resins that are compatible with each other, the outer tube and the inner tube can be reliably fused to each other at the small diameter portion. Since a general polyolefin resin that does not contain a so-called polyolefin modified resin that contains an ethylene copolymer, a thermoplastic elastomer, or a rubber component can be used as such an inner tube, the wire-passing resistance can be reduced compared to the case where an ethylene copolymer or a thermoplastic elastomer, which has a large friction resistance, is used.

[0040] In addition, when the polyolefin resin contains an acid-modified resin, the content of the acid-modified resin can be 0.5 parts by mass or more and 10 parts by mass or less, when the content of the above-mentioned base resin excluding the acid-modified resin is taken as 100 parts. In order to increase the fusion strength, if it is 0.5 parts by mass or less, the effect is insufficient, and if it exceeds 10 parts by mass, the effect is saturated and the cost becomes high.

[0041] Furthermore, by making the yield stress of the material of the inner tube greater than the yield stress of the material of the outer tube, not only is the flexibility suppressed, but buckling of the inner tube can also be suppressed more reliably.

[0042] Furthermore, by making the thickness of the inner tube at the portion corresponding to the large diameter portion of the outer tube thicker than the thickness of the inner tube at the portion corresponding to the small diameter portion of the outer tube, and forming the thick-walled and thin-walled portions to have alternating thickness variations, it is possible to improve the buckling resistance when the ends of the double-walled rectangular conduit are supported horizontally.

[0043] The reason for this is that, as can be seen from the formula for calculating the buckling strength, the location where buckling of the inner pipe occurs is not the location corresponding to the small diameter part of the outer pipe, but the location corresponding to the large diameter part of the outer pipe. By increasing the wall thickness of the inner pipe at the location corresponding to the large diameter part of the outer pipe, the rigidity of the location where buckling of the inner pipe occurs can be improved, and the buckling resistance of the double-walled rectangular electric conduit can be improved. In addition, such an effect can be obtained by forming the inner pipe such that the wall thickness of the unfused part of the inner pipe corresponding to the large diameter part of the outer pipe is thicker than the wall thickness of the inner pipe at the fused part corresponding to the small diameter part of the outer pipe, and the thick part and the thin part have a wall thickness difference alternately. In addition, since a part of the wall thickness of the inner pipe can be thinned, the weight of the entire inner pipe can be reduced, and the buckling resistance of the double-walled rectangular electric conduit can be improved.

[0044] Furthermore, if any joint portion is formed at least at one end or at both ends of the double-wall rectangular conduit, the double-wall rectangular conduits can be easily connected to each other to form a conduit.

[0045] A second invention is a connection structure for double-wall rectangular conduits, characterized in that the double-wall rectangular conduits according to the first invention are connected to each other.

[0046] A joint portion may be formed at at least one end of the double-wall rectangular conduit, and the double-wall rectangular conduits may be connected to each other at the joint portion.

[0047] The joint portion may be a male joint or a female joint.

[0048] The double-walled rectangular conduits are connected to each other by a pipe joint, and the pipe joint has a tubular member and a spring-like locking piece disposed inside the tubular member, and is provided from an end thereof with: The small diameter portion,The end portions of the double-wall rectangular conduit, in which the large diameter portion, the small diameter portion, and the large diameter portion are formed in this order, are inserted from both sides of the tubular member, respectively, into the substantially rectangular tubular member. Leave , than the large diameter portion closer to the opposing portion of the double-wall rectangular conduit, On the side farther from the opposing portion of the double-wall rectangular conduit The locking piece is supported, A mechanism may be provided for bringing the locking piece into contact with a side surface of the large diameter portion closer to the opposing portion of the double-wall rectangular conduit.

[0049] The double-wall rectangular conduits are connected to each other by a pipe joint, the pipe joint being formed by a joint body and a Π-shaped fixing member, the Π-shaped fixing member having a top surface portion and two legs protruding downward and connected to the top surface portion so as to be approximately perpendicular to each other, The small diameter portion, The ends of the double-walled rectangular conduit having the large diameter portion, the small diameter portion, and the large diameter portion formed in this order may be inserted from both sides of the joint body, a notch may be formed above a position of the joint body corresponding to the small diameter portion, and the Π-type fixing member may be inserted into the notch from above, so that movement of the large diameter portion is restricted by the Π-type fixing member, thereby connecting the double-walled rectangular conduits to each other.

[0050] According to the second invention, a conduit can be formed in which a plurality of double-wall rectangular conduits are connected.

[0051] In this case, if a joint portion is formed at at least one end or both ends of the double-walled rectangular conduit, the double-walled rectangular conduits can be connected to each other at the joint portion, making it easy to form the conduit.

[0052] On the other hand, double-walled rectangular conduits can also be connected to each other using a pipe joint. In this case, if the pipe joint has a cylindrical member and a spring-like locking piece disposed inside the cylindrical member, the ends of the double-walled rectangular conduit can be inserted into both sides of the cylindrical member to be fixed to the double-walled rectangular conduit. This makes it easy to fix the double-walled rectangular conduit to the pipe joint.

[0053] In addition, the pipe joint is formed by a joint body and a Π-type fixing member, and the ends of the double-wall rectangular electric conduit are inserted from both sides of the joint body, and the Π-type fixing member is inserted into the joint body from above, so that the movement of the large diameter part of the double-wall rectangular electric conduit can be restricted by the Π-type fixing member. Even when such a pipe joint is used, the double-wall rectangular electric conduit can be fixed to the double-wall rectangular electric conduit simply by inserting the Π-type fixing member in a state where the double-wall rectangular electric conduit is inserted into the joint body. Therefore, the double-wall rectangular electric conduit and the pipe joint can be easily fixed to each other.

[0054] A third invention is a method for determining a mass per unit length of a double-walled rectangular conduit having an outer tube and an inner tube and a tensile yield stress at which the conduit does not buckle under its own weight, the outer tube having a large diameter portion having a relatively large diameter and a small diameter portion having a relatively small diameter, the large diameter portion and the small diameter portion are alternately and repeatedly formed in an axial direction of the outer tube, the large diameter portion has a substantially rectangular cross-sectional shape and the small diameter portion has a circular cross-sectional shape in a cross section perpendicular to the axial direction of the outer tube, the inner tube is disposed inside the outer tube, the cross-sectional shape perpendicular to the axial direction of the inner tube is circular, an inner circumferential surface of the outer tube and an outer circumferential surface of the inner tube are fused to each other in the small diameter portion, and the inner circumferential surface of the outer tube is fused to the outer circumferential surface of the inner tube in the large diameter portion. a space is formed between the surface and the outer circumferential surface of the inner tube; σ y >16w·L 2 (d+2t) / (π·((d+2t) 4 -d 4 )) (σ y : tensile yield stress of the material of the inner tube, w: unit mass of the electric conduit, L: length of the electric conduit, d: inner diameter of the inner tube, t: wall thickness of the inner tube) So that σ y This method determines the mass per unit length of a double-walled rectangular conduit and the tensile yield stress at which the conduit does not buckle under its own weight, characterized by determining the relationship between w and

[0055] According to the third aspect of the present invention, it is possible to obtain a double-walled rectangular conduit that does not buckle under its own weight when handled. Effect of the Invention

[0056] According to the present invention, it is possible to provide a double-walled rectangular conduit having excellent wireability and buckling resistance, which does not cause buckling of the inner tube under cantilever conditions even when a normal polyolefin resin is used, without using an ethylene-based copolymer or a thermoplastic elastomer which have high frictional resistance, a connection structure for the double-walled rectangular conduit using the same, and a method for determining the mass per unit length of the double-walled rectangular conduit and the tensile yield stress at which the double-walled rectangular conduit does not buckle under its own weight. [Brief description of the drawings]

[0057] [Figure 1] FIG. 2 is a perspective view showing a double-wall rectangular conduit 1. [Diagram 2] FIG. 2A is a side view showing a double-wall rectangular conduit 1, and FIG. 2B is a cross-sectional view of the double-wall rectangular conduit 1 in the axial direction. [Diagram 3] FIG. 1 is a partially enlarged view of an axial cross section of a double-walled rectangular conduit in which the inner tube wall thickness of the portion corresponding to the large diameter portion is thicker than the inner tube wall thickness of the portion fused to the small diameter portion of the outer tube, and thick-walled and thin-walled portions are alternately formed to have a difference in wall thickness. [Figure 4] FIG. 2A is a side view showing a double-wall rectangular conduit 1a, and FIG. 2B is a cross-sectional view in the axial direction of the double-wall rectangular conduit 1a. [Diagram 5] 4(a) and (b) are diagrams showing a method of connecting two double-wall rectangular conduits 1b with each other using a pipe joint 45. FIG. [Figure 6] FIG. [Figure 7] 4A is an axial cross-sectional view showing a state in which a double-walled rectangular electric conduit is inserted into a pipe joint 45a, and FIG. 4B is a cross-sectional view perpendicular to the axial direction showing a state in which a Π-shaped fixing member 55 is inserted. [Figure 8] FIG. 4A is a diagram showing a manufacturing method of a double-wall rectangular conduit 1, and FIG. 4B is an enlarged cross-sectional view of a portion Z in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] Hereinafter, a double-wall rectangular conduit 1 according to an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a perspective view of the double-wall rectangular conduit 1, Fig. 2(a) is a side view showing the double-wall rectangular conduit 1, and Fig. 2(b) is a cross-sectional view of the double-wall rectangular conduit 1 in the pipe axis direction.

[0059] The double-walled rectangular conduit 1 is mainly composed of an inner pipe 9 and an outer pipe 7. The outer pipe 7 has a large diameter portion 3 having a relatively large diameter and a small diameter portion 5 having a relatively small diameter, and the large diameter portion 3 and the small diameter portion 5 are alternately and repeatedly formed in the axial direction of the outer pipe 7. That is, on the outer circumferential surface of the double-walled rectangular conduit 1, a plurality of large diameter portions 3 and small diameter portions 5 are alternately and repeatedly formed in the axial direction of the outer pipe 7, and the diameter (length of one side) of the large diameter portion 3 is larger than the outer diameter of the small diameter portion 5. The cross-sectional shape of the large diameter portion 3 perpendicular to the axial direction of the outer pipe 7 is a substantially rectangular cross-sectional shape with rounded corners. The cross-sectional shape of the small diameter portion 5 perpendicular to the axial direction of the outer pipe 7 is a circular cross-sectional shape.

[0060] An inner pipe 9 is disposed inside the outer pipe 7. The cross section of the inner pipe 9 perpendicular to the pipe axis direction is circular. In the small diameter portion 5, the inner peripheral surface of the outer pipe 7 and the outer peripheral surface of the inner pipe 9 are fused to each other and integrated. In addition, in the large diameter portion 3, a space is formed between the inner peripheral surface of the outer pipe 7 and the outer peripheral surface of the inner pipe 9. By providing the large diameter portion 3 having a substantially square shape in this way, the double-walled rectangular conduit 1 can be stably arranged when stacked. Details of the configuration of each part of the double-walled rectangular conduit 1 that can suppress buckling will be described later.

[0061] The inner pipe 9 disposed inside the outer pipe 7 may be configured as shown in the cross section of a double-walled rectangular conduit as shown in Fig. 3. In Fig. 3, the wall thickness of the inner pipe 9 at the portion corresponding to the large diameter portion 3 of the outer pipe 7 is thicker than the wall thickness of the inner pipe 9 at the portion fused to the small diameter portion 5 of the outer pipe 7, and the wall thickness (t2) of the inner pipe thick portion 92 and the wall thickness (t1) of the inner pipe thin portion 91 have alternate thickness differences and are formed to satisfy the relationship t2>t1. Details of a method for obtaining a double-walled rectangular conduit in which the wall thickness of the inner pipe 9 at the portion corresponding to the large diameter portion 3 of the outer pipe 7 is formed thicker than the wall thickness of the inner pipe 9 at the portion fused to the small diameter portion 5 of the outer pipe 7 will be described later. In this way, by using a double-walled rectangular conduit in which the thickness of the inner tube 9 at the portion corresponding to the large diameter portion 3 of the outer tube 7 is thicker than the thickness of the inner tube 9 at the portion fused to the small diameter portion 5 of the outer tube 7, and which is formed so as to have alternating thickness variations, it is possible to improve the buckling resistance when the ends of the double-walled rectangular conduit are supported horizontally.

[0062] As described above, the inner pipe 9 is disposed inside the outer pipe 7 and is integrated by fusion at a position corresponding to the small diameter portion 5 of the outer pipe 7. For this reason, it is desirable that the outer pipe 7 and the inner pipe 9 are made of resins having mutual compatibility. For example, it is desirable that the outer pipe 7 is made of a synthetic resin such as polyolefin resin. Similarly, it is desirable that the inner pipe 9 is made of a synthetic resin such as polyolefin resin. It is desirable that the yield stress of the material of the inner pipe 9 is greater than the yield stress of the material of the outer pipe 7. In this way, it is possible to obtain an effect of suppressing excessive flexibility of the double-walled rectangular conduit and further acting in a favorable direction from the viewpoint of suppressing buckling resistance.

[0063] The ends of the double-wall rectangular conduit 1 can be connected to each other by a pipe joint. A double-wall rectangular conduit connection structure in which multiple double-wall rectangular conduits 1 are connected to each other can provide a conduit of the double-wall rectangular conduit 1.

[0064] Also, instead of using a pipe joint, a joint part may be formed at the end of the double-wall rectangular electric conduit. FIG. 4(a) is a side view showing the double-wall rectangular electric conduit 1a, and FIG. 4(b) is a cross-sectional view in the pipe axis direction of the double-wall rectangular electric conduit 1a. The double-wall rectangular electric conduit 1a has a structure substantially similar to that of the double-wall rectangular electric conduit 1, but a male joint 11 is provided at one end of the outer tube 7 of the double-wall rectangular electric conduit 1a, and a female joint 13 is provided at the other end. The male joint 11 and the female joint 13 become a connection part with another double-wall rectangular electric conduit 1a. Here, FIG. 4(a) and FIG. 4(b) show the male joint and the female joint of the double-wall rectangular electric conduit 1a, but the double-wall rectangular electric conduit of FIG. 3 may have a similar joint structure, although not shown.

[0065] The maximum outer diameter of the female joint 13 is smaller than the diameter (length of one side) of the large diameter portion 3. Therefore, when viewed from the end portion on the female joint 13 side, the female joint 13 does not protrude beyond the outer periphery of the large diameter portion 3. Therefore, when the double-wall rectangular electric conduits 1a are stacked with the large diameter portions 3 in contact with each other, the female joints 13 do not interfere with the adjacent double-wall rectangular electric conduits 1a.

[0066] The joint portion may be formed at least at one end of the double-wall rectangular electric conduit 1a. In this case, the joint portion may be either the male joint 11 or the female joint 13. In this manner, by connecting the double-wall rectangular electric conduits 1a to each other with the joint portion, a connection structure of the double-wall rectangular electric conduit can be obtained. The male joint 11 and the female joint 13 may have a known structure.

[0067] As a method for connecting the double-walled rectangular conduits 1 to each other with a pipe joint, for example, a pipe joint 45 shown in Fig. 5(a) and Fig. 5(b) can be used. The examples shown in Fig. 5(a) and Fig. 5(b) show a connection example of a double-walled rectangular conduit 1b in which a protrusion 43 is formed on the outer periphery of the small diameter portion 5. The double-walled rectangular conduit 1b has a configuration substantially similar to that of the double-walled rectangular conduit 1, but can be easily cut along the groove between the pair of protrusions 43 formed on the small diameter portion 5. In this way, the double-walled rectangular conduit 1b can be separated into two double-walled rectangular conduits by cutting it at the small diameter portion 5.

[0068] Next, a connection structure of double-walled rectangular electric conduit 1b using pipe joint 45 will be described. Figures 5(a) and 5(b) are diagrams showing a method of connecting double-walled rectangular electric conduit 1b to each other using pipe joint 45. Pipe joint 45 is made of a cylindrical member 47 whose diameter is reduced approximately at the center in the axial direction. Note that the cylindrical member 47 has a substantially circular reduced diameter portion at the center and other portions on both sides thereof have a substantially rectangular shape.

[0069] The rectangular cross-sectional portion of the pipe joint 45 is provided with a locking piece 49 that protrudes toward the center in the axial direction. The locking piece 49 has spring properties and is formed so that the amount of inward protrusion gradually increases toward the center in the axial direction. The locking pieces 49 are disposed at approximately symmetrical positions on either side of the reduced diameter portion in the center in the axial direction. The locking pieces 49 are also disposed at multiple locations (for example, at least two opposing locations) in the circumferential direction of the tubular member 47.

[0070] First, as shown in FIG. 5(a), the ends (small diameter parts 5) of the double-walled rectangular electric conduit 1b are arranged facing each other. The double-walled rectangular electric conduit 1b are inserted (arrow I in the figure) from the openings at both ends of the pipe joint 45 (cylindrical member 47), and the double-walled rectangular electric conduit 1b are arranged facing each other by the facing parts 51, so that the large diameter parts 3 of the double-walled rectangular electric conduit 1b come into contact with the locking pieces 49. Furthermore, by pushing the double-walled rectangular electric conduit 1b into the double-walled rectangular electric conduit 1b from the openings at both ends of the pipe joint 45, the locking pieces 49 overcome the large diameter parts 3 and return to their original state by spring properties, as shown in FIG. 5(b). That is, a mechanism is provided in the substantially rectangular cylindrical member 47 to abut the locking pieces 49 supported on the side farther from the facing part of the double-walled rectangular electric conduit 1b against the side of the large diameter part 3 on the side closer to the facing part of the double-walled rectangular electric conduit 1b.

[0071] Therefore, the tip of the locking piece 49 comes into contact with the side surface of each large diameter portion 3 to lock the double-wall rectangular electric conduit 1b. In this manner, a connection structure 50 for the double-wall rectangular electric conduit 1b is formed using the pipe joint 45. When such a pipe joint 45 is used, the double-wall rectangular electric conduit 1b is supported by the locking pieces 49 in the pipe joint 45, so that the double-wall rectangular electric conduit 1b does not come out of the pipe joint 45 even if a pulling force acts in a direction to move the double-wall rectangular electric conduit 1b away from the pipe joint 45.

[0072] The structure of the pipe joint is not limited to the examples shown in Figures 5(a) and 5(b). For example, pipe joint 45a shown in Figure 6 may be used instead of pipe joint 45. Pipe joint 45a mainly comprises joint body 53 and a pair of Π-shaped fixing members 55.

[0073] The Π-shaped fixing member 55 comprises a top surface portion 57 and a pair of legs 59. The legs 59 are connected to the vicinity of both ends of the top surface portion 57 so as to be substantially perpendicular to the top surface portion 57 and protrude downward. An engagement portion 61 is formed on each of the legs 59 facing outward. The engagement portions 61 are formed so that their lower portions protrude in both directions.

[0074] At both ends of the joint body 53, substantially rectangular tubular portions 65 are formed symmetrically in the pipe axis direction. A notch 67 is formed and opens at the top of the tubular portion 65. An engagement step 69 is formed on the inner surface of the tubular portion 65 at a position corresponding to the notch 67 of the joint body 53. The engagement step 69 is formed by a protrusion that protrudes inward substantially parallel to the pipe axis direction of the joint body 53.

[0075] Openings 63 are provided on both sides of the bottom of the tubular portion 65 of the joint body 53. The openings 63 are formed at positions corresponding to the notches 67 with respect to the axial direction of the joint body 53. The openings 63 allow both legs 59 of the Π-shaped fixing member 55 to be inserted therein.

[0076] Next, a method for connecting the double-walled rectangular electric conduit 1b using the pipe joint 45a will be described. First, the double-walled rectangular electric conduit 1b is inserted from both sides of the joint body 53 of the pipe joint 45 with the ends facing each other. Inside the tubular part 65 of the joint body 53 on the inner side of the notch 67, a waterproof member 71 is arranged so as to surround the entire inner circumference of the tubular part 65.

[0077] 7(a) is a cross-sectional view in the tube axis direction showing a state in which the double-walled rectangular conduit 1b is inserted into both sides of the joint body 53. At least the large diameter portion 3, the small diameter portion 5, and the large diameter portion 3 of the double-walled rectangular conduit 1b are arranged in this order in the cylindrical portion 65 of the joint body 53, and the small diameter portion 5 is located at the position of the notch portion 67 of the cylindrical portion 65. Also, the inner surface of the joint body 53 and the outer surface of the large diameter portion 3 of the double-walled rectangular conduit 1b are in close contact with each other via the water-stopping member 71, thereby ensuring water-stopping performance.

[0078] Next, the Π-type fixing member 55 is inserted into the notch 67 from above the joint body 53. FIG. 7(b) is a cross-sectional view perpendicular to the pipe axis direction, showing the state after the Π-type fixing member 55 has been inserted. When the Π-type fixing member 55 is inserted into the notch 67 from above, the tips of both legs 59 of the Π-type fixing member 55 are inserted into the opening 63 provided in the bottom of the tubular part 65 of the joint body 53. At this time, the engagement part 61 and the engagement step part 69 engage with each other, and the Π-type fixing member 55 is fixed to the joint body 53.

[0079] When the Π-type fixing member 55 is inserted into the notch 67, the inner peripheral surface of the top surface 57 of the Π-type fixing member 55 is smaller than the large diameter portion 3 of the double-wall rectangular electric conduit 1b inside and larger than the small diameter portion 5. Therefore, when the double-wall rectangular electric conduit 1b is pulled out from the joint body 53 with the Π-type fixing member 55 fixed to the joint body 53, a part of the upper part of the inner surface of the top surface 57 of the Π-type fixing member 55 and both foot portions 59 abut against and engage with the side surface of the large diameter portion 3 of the double-wall rectangular electric conduit 1b. In this way, the double-wall rectangular electric conduit 1b and the pipe joint 45a are engaged in the pipe axial direction, so that the double-wall rectangular electric conduit 1b does not come out of the joint body 53. That is, the movement of the large diameter portion 3 is restricted by the Π-shaped fixing member 55, and the double-walled rectangular conduits are connected to each other, so that a connection structure 50a between the double-walled rectangular conduits 1b using the pipe joint 45a can be formed.

[0080] The above-mentioned connection structures 50, 50a can be applied not only to connections between double-walled rectangular conduits, but also, for example, as connection structures with handholes and connection adapters for connecting different types of conduits.

[0081] Next, a method for manufacturing the double-walled rectangular conduit 1 will be described. Fig. 8(a) is a schematic diagram showing a double-walled rectangular conduit manufacturing apparatus 80. In the following description, the double-walled rectangular conduit 1 will be described, but the double-walled rectangular conduits 1a and 1b can also be manufactured in the same manner. The double-walled rectangular conduit manufacturing apparatus 80 is mainly composed of extruders 81a and 81b, caterpillar tracks 82a and 82b, etc.

[0082] The endless tracks 82a, 82b are composed of multiple dies 85, etc. that rotate in opposite directions (directions of arrow R in the figure). The endless tracks 82a, 82b move in the same direction and at the same speed relative to the feed direction of the double-wall rectangular electric conduit manufacturing apparatus 80, and have forming sections that face each other. In the forming sections, the respective dies 85, etc. face each other and are in close contact with each other, and the dies 85 move at the same speed in the feed direction.

[0083] The die 85 is a half-cylinder member split into two halves, the inner surface of which is formed with concave and convex portions corresponding to the outer shapes of the large diameter portion and the small diameter portion of the double-wall rectangular conduit 1. A pair of dies 85 are fitted together facing each other to form a cylindrical member.

[0084] Fig. 8(b) is a cross-sectional view of part Z in Fig. 8(a). Extruders 81a and 81b and an extrusion head 83 are arranged on the upstream side in the feed direction of a double-wall rectangular electric conduit manufacturing apparatus 80. An end of the extrusion head 83 is inserted into a cylindrical portion where dies 85 face each other and are in close contact with each other.

[0085] In the extrusion head 83, a flow path 87a through which the resin for forming the outer tube 7 connected to the extruder 81a flows and a flow path 87b through which the resin for forming the inner tube 9 connected to the extruder 81b flows are arranged coaxially, and the resin is extruded from each extruder into the flow path (arrows T and V in the figure). For example, in a cross section perpendicular to the axial direction of the extrusion head 83, the flow path 87b is arranged near the center of the extrusion head 83, and the flow path 87a is formed in a ring shape so as to surround the periphery of the flow path 87b.

[0086] From the tip of the flow path 87a through which the outer tube resin flows, a cylindrical base tube 89a (parison) is extruded between the butted dies 85 in synchronization with the moving speed of the dies 85, and the cylindrical base tube 89a is sent to the cylindrical space formed by the pair of dies 85.

[0087] The die 85 has a plurality of holes through which the air inside is sucked in from the outside (in the direction of the arrow W in the figure). Compressed air is sent to a flow path 87c between the flow paths 87a and 87b (in the direction of the arrow U in the figure), thereby preventing the pressure inside the blank tube 89a from becoming negative.

[0088] Because the air between the die 85 and the blank tube 89a is sucked out of the die 85, the blank tube 89a is pressed against the inner surface of the die 85. Therefore, in the blank tube 89a, large diameter portions 3 are formed in the recesses of the die 85 and small diameter portions 5 are formed in the protrusions thereof according to the shape of the inner surface of the die 85. In other words, the tubular molten resin extruded from the outer tube nozzle (the tip of the flow path 87a) is vacuum-sucked and gradually pressed against the inner surface of the caterpillar-type divided die 85, and is finally pressed against the inner surface of the die 85 to form the outer tube 7.

[0089] A cylindrical blank tube 89b (parison) is similarly extruded from the annular tip of flow path 87b, through which the inner tube resin flows, closer to the tip of extrusion head 83 than the formation portion of the outer tube 7. The extrusion speed of blank tube 89b and the moving speed of die 85 are synchronized. Blank tube 89b is sent between the inner surface of previously formed outer tube 7 and the tip of extrusion head 83, and inner tube 9 is formed into a cylindrical shape, and the outer surface of inner tube 9 is fused to the inner surface of small diameter section 5 of outer tube 7. Thereafter, cooling is performed to produce double-walled rectangular conduit 1.

[0090] At this time, if the protruding pressure of the resin discharged from the extrusion head 83 during resin extrusion of the inner tube 9 is increased, the thickness of the inner tube 9 at the position corresponding to the large diameter portion of the outer tube 7 increases with the increase in protruding pressure because there are no obstacles outside the inner tube resin. On the other hand, the inner tube 9 at the position corresponding to the small diameter portion of the outer tube 7 is pressed by the small diameter portion of the outer tube 7 and fused to the small diameter portion of the outer tube 7, so that the thickness of the inner tube 9 at the position corresponding to the small diameter portion of the outer tube 7 is thinner than the thickness of the inner tube 9 at the position corresponding to the large diameter portion of the outer tube 7.

[0091] In order to form the thick-walled portion 92 and the thin-walled portion 91 of the inner tube at the position corresponding to the large diameter portion of the outer tube and the position corresponding to the small diameter portion of the outer tube so that they have thickness differences alternately in the tube axial direction, the discharge pressure of the resin forming the inner tube during extrusion may be set to a pressure such that the thickness of the blank tube 89b is thicker than the clearance between the small diameter portion of the outer tube 7 and the inner tube mold. Here, the overall manufacturing range of the inner tube resin is 0.4 mm to 3.0 mm, but the thickness of the inner tube resin can be adjusted by adjusting the protruding pressure of the inner tube resin. For example, when the thickness of the inner tube resin at the position corresponding to the small diameter portion of the outer tube is 1.0 mm, the inner tube thickness at the position corresponding to the large diameter portion can be adjusted within the range of 1.0 mm to 1.6 mm, but the inner tube thickness at the position corresponding to the large diameter portion is preferably 1.6 mm or less, more preferably 1.5 mm or less, in consideration of the thickness stability of this portion, die wear and damage, mass production stability, etc.

[0092] If dies for forming male joints 11 and female joints 13 are provided at both front and rear ends of a die having a repeating shape of large diameter section 3 and small diameter section 5, male joints 11 and female joints 13 can be formed at both ends of a tube of a predetermined length. When multiple double-wall rectangular conduits are continuously formed, the dies for forming male joints 11 and female joints 13 are arranged adjacent to each other. In this case, a transition section connecting male joint 11 and female joint 13 is provided as a section for adjusting the difference in outer diameter of the tip of both. The transition section and unnecessary inner tube 9 inside female joint 13 are then cut off.

[0093] Next, the dimensions and other configurations of the double-walled rectangular conduit 1 will be described in more detail. As described above, the double-walled rectangular conduit 1 may buckle when it is handled. However, since the cross-sectional structure of the double-walled rectangular conduit is complicated, it has not been clear what measures should be taken to prevent this. In response to this, the inventors have conducted various studies on the causes of buckling in a double-walled rectangular conduit in which rectangular and circular cross sections are alternately formed, and have found that wrinkles and buckling in the double-walled rectangular conduit occur only in the inner tube 9, and therefore there is a certain relationship with the yield stress of the inner tube 9.

[0094] In this case, in consideration of the fact that buckling does not occur in the outer pipe 7, but occurs only in the inner pipe 9, the influence of the outer pipe 7 on the buckling of the inner pipe 9 of the double-walled rectangular electric conduit was examined as a mass load. In this case, since the outer pipe 7 has a continuous periodic structure, the weight of the outer pipe 7 is assumed to be a uniformly distributed load in the longitudinal direction, and the mass of the outer pipe 7 is superimposed on the mass of the inner pipe 9 (per unit length The study was conducted based on the equation for a cantilever beam of the inner tube of a circular tube, taking into account the mass increase per unit area.

[0095] As a result, the relationship between the maximum bending stress of the inner pipe 9 and the yield stress of the inner pipe 9 was obtained from the diameter of the inner pipe 9, the outer diameter of the inner pipe 9, and the mass per unit length of the double-walled rectangular electric conduit, which is the sum of the masses of the inner pipe 9 and the outer pipe 7, and the product length of the double-walled rectangular electric conduit, and the result obtained was compared with the verification test results of a cantilever beam experiment on an actual product to obtain the relationship between the buckling stress of the inner pipe 9 and the yield stress of the inner pipe 9, which will be described later. This will be explained in detail below.

[0096] As mentioned above, when a double-wall rectangular conduit is handled by supporting its ends, it buckles. If we consider this as a cantilever beam model and consider the effect of its own weight as a uniform load in the longitudinal direction, the balance of the moment gives the following: M+wL×L / 2=0 (1) Here, M is the moment generated at the support, w is the unit mass of the entire conduit, and L is the length of the conduit.

[0097] On the other hand, from the equation for the bending stress of the beam, the maximum bending stress σ acting on the inner pipe 9 is σ=M y / I (2) The second moment of area is calculated by the formula for the case where the inner pipe 9 is a hollow pipe. I=π (D 4 -d 4 ) / 64 (3) Here, D is the outer diameter of the inner tube 9, d is the inner diameter of the inner tube 9, and y=D / 2.

[0098] From equations (1) to (3), the maximum surface bending stress σ acting on the inner pipe 9 is σ=16w L 2 (d+2t) / (π·((d+2t) 4 -d 4 )) Here, t is the wall thickness of the inner pipe 9, and D=d+2t. This maximum surface bending stress is the tensile yield stress σ y If it is smaller than this, the occurrence of buckling of the inner tube 9 can be suppressed.

[0099] That is, σ y >16w·L 2 (d+2t) / (π·((d+2t) 4 -d 4 )) (4) (σ y : tensile yield stress of the material of the inner tube 9, w: mass per unit length of the electric conduit, L: length of the electric conduit, d: inner diameter of the inner tube 9, t: wall thickness of the inner tube 9) By satisfying the above condition, buckling of the inner tube 9 can be suppressed. The mass per unit length of the conduit varies depending on the size of the conduit. However, the mass w per unit length of the double-walled rectangular conduit having the outer tube 7 and the inner tube 9 and the tensile yield stress σ at which the conduit does not buckle under its own weight can be calculated by the above-mentioned method. y The relationship can be determined.

[0100] Here, the length L of the double-walled rectangular conduit is not particularly limited, but is desirably 2000 mm to 6000 mm, more preferably 3000 to 5000 mm, and even more preferably 3000 to 4000 mm. If the length L of the double-walled rectangular conduit is short, buckling is unlikely to occur, and at the same time, the number of connections required to form a conduit increases, which is undesirable. If the length L of the double-walled rectangular conduit is too long, buckling is likely to occur, and the weight of each double-walled rectangular conduit is heavy, making it difficult to handle, and also making it difficult to load on a truck or the like during transportation.

[0101] Furthermore, the wall thickness t of the inner tube 9 is set appropriately, but is preferably 0.4 mm to 3 mm. If the wall thickness t of the inner tube 9 is too thin, not only will the tube be prone to buckling, but it will also be difficult to obtain a stable wall thickness, and holes will be prone to appear in the inner tube during manufacturing. On the other hand, if the wall thickness t of the inner tube 9 is too thick, the effect against buckling will saturate, and the mass will increase.

[0102] The inner diameter d of the inner tube 9 is determined by the practical dimensions of the product, but is preferably 30 mm to 200 mm, and more preferably 50 mm to 150 mm. If the inner diameter d of the inner tube 9 is too small, the cable will not easily pass through and will be prone to buckling. On the other hand, if the inner diameter d of the inner tube 9 is too large, manufacturing will be difficult and the weight will increase.

[0103] In addition, double-wall rectangular conduits are often buried underground, so they are often required to have a certain level of buried strength. For this reason, the following tests are performed in the compression test of JIS C3653 Appendix 3: Deflection δ = (D1 - D2) / D1 × 100% (5) (D1: outer diameter of the conduit before compression, D2: outer diameter of the conduit after compression) However, it is desirable that it be 3.5% or more.

[0104] More specifically, in JIS C3653, Appendix 3, a compression test specimen is cut from the product to a length of 250 mm. After the test specimen and the testing device are kept at a temperature of 20±2°C, the sample is sandwiched between two flat steel tubes at that temperature, and a compressive load is applied to the sample in the direction perpendicular to the tube axis at a speed of 20 mm per minute. The compressive load is based on the design load described in JIS C3653, Appendix 3, and the maximum stress generated in the tube when buried is estimated by stress analysis, and the compressive load at which the maximum stress generated when the tube is sandwiched between two flat plates and compressed is equivalent is found by stress analysis, and this is used as the test load.

[0105] In this case, it is desirable that the deflection δ calculated by formula (5) during the above test is 3.5% or less and that no cracks or breaks occur in any part. By satisfying these conditions, appropriate buried strength can be ensured. EXAMPLES

[0106] Next, the suitability of the buckling occurrence condition according to formula (4) using a cantilever beam as a model is examined. In the above example, the calculation of the maximum bending stress at the time of buckling under the cantilever beam condition of a circular pipe is applied to the double-walled rectangular electric conduit 1. In this case, when comparing the double-walled rectangular electric conduit 1 with a single circular pipe, the double-walled rectangular electric conduit 1 is different in that an inner pipe 9 is arranged inside an outer pipe 7 in which a substantially rectangular large diameter portion 3 and a small diameter portion 5 formed of a circular cross section are alternately and repeatedly formed, and the inner pipe 9 and the outer pipe 7 are fused together at the small diameter portion 5. For this reason, the buckling behavior of the double-walled rectangular electric conduit 1 under the cantilever beam condition appears to be more complicated due to the complexity of its cross-sectional shape.

[0107] Therefore, it is necessary to evaluate buckling by taking into account both the structural elements of the outer pipe 7 and the inner pipe 9. However, as a result of observing the double-walled rectangular conduit 1 in which buckling occurred, it was confirmed that buckling occurred in the inner pipe 9 located in the large diameter part 3 of the outer pipe 7 near the support part of the double-walled rectangular conduit 1 which satisfies the cantilever beam condition, but buckling did not occur in the fused part between the inner pipe 9 and the outer pipe 7 or in the outer pipe 7.

[0108] Meanwhile, as described above, the double-wall rectangular conduit 1 is formed by sucking a blank tube extruded to a predetermined thickness into a die that moves on an endless track in sync with the blank tube. In this way, the outer tube 7 is periodically formed into a predetermined shape in the tube longitudinal direction, so if the large diameter portion 3 and the small diameter portion 5 are considered to be one pitch, the mass variation in the tube axial direction within the pitch is not so large. For this reason, it is estimated that there is no problem even if the load distribution of the outer tube 7 is considered to be approximately uniform as a whole.

[0109] Therefore, it was confirmed that by considering that the outer pipe 7 does not contribute to structural reinforcement and that only the load of the outer pipe 7 is applied to the inner pipe 9, and by using the mass per unit length of the inner and outer pipes combined as the pipe mass, equation (4) can be applied to evaluate the buckling of double-walled rectangular conduit.

[0110] First, double-walled rectangular conduits were manufactured using materials with a yield stress of 25 MPa and materials with a yield stress of 8 MPa for the inner tube. The unit mass of the double-walled rectangular conduit using the material with a yield stress of 25 MPa was 573 g / m, the inner diameter of the inner tube was 49.2 mm, and the wall thickness of the inner tube was 0.91 mm. The unit mass of the double-walled rectangular conduit using the material with a yield stress of 8 MPa was 586 g / m, the inner diameter of the inner tube was 49.1 mm, and the wall thickness of the inner tube was 0.84 mm.

[0111] The test was carried out under the condition of a simple cantilever beam, and the length of the beam at which buckling occurs was determined under the test conditions. The above evaluation was carried out under the condition of a cantilever beam, and the results of the calculation using formula (4) and the experiment were compared for the buckling position under the cantilever beam condition. The results are shown in Table 1.

[0112] [Table 1]

[0113] As shown in Table 1, when a material with a yield stress of 25 MPa and a material with a yield stress of 8 MPa were used, the calculation results using formula (4) and the experimental values ​​under the test conditions were almost the same. From this, it was confirmed that the occurrence of buckling in a double-walled rectangular conduit can be evaluated by assuming that the mass of the outer pipe 7 is evenly distributed in the axial direction of the pipe, adding the mass of the outer pipe 7 to the mass of the inner pipe 9, and using formula (4), which is an evaluation formula for the bending load of a cantilever beam of a circular pipe, under this condition.

[0114] Next, double-wall rectangular conduits of various shapes were manufactured and evaluated for the occurrence of buckling, etc. Table 2 shows the various conditions and the evaluation results.

[0115] [Table 2]

[0116] The materials for the inner pipe were selected from polypropylene (PP), high density polyethylene (HDPE), and low density polyethylene (LDPE). The material for the outer pipe was all HDPE. The "Unit weight formula pass / fail" in the table indicates whether the unit weight of the entire pipe is 0.08 × d 2 +4.55d+14< w <0.15×d 2 The ones that satisfy the relationship of +11.71d+84 are marked with "○", and those that do not are marked with "△". In the "Formula Pass / Fail" column in the table, those that satisfy the relationship of the above-mentioned formula (4) are marked with "○", and those that do not are marked with "×".

[0117] For "buckling" in the table, with transportation in mind, if the inner tube does not buckle when lifting the end of the conduit, then it is marked with an "O" and if it does buckle, then it is marked with an "X". Also, for "transportation efficiency" in the table, if two or more tubes can be loaded in series on a 10-ton truck and they can also be loaded on a 4-ton truck, then it is marked with an "◎", if they can be loaded on a 10-ton truck or a 4-ton truck regardless of the number of tubes, then it is marked with an "O", and if they cannot be loaded on a 4-ton truck, then it is marked with a "△". The loading platform of a 10-ton truck is 9670 mm, and that of a 4-ton truck is 6705 mm.

[0118] In the table, the "total pipe mass" is indicated as "◎" if one piece is less than 10 kg, "〇" if it is between 10 kg and 20 kg, and "△" if it is 20 kg or more. In the table, the "number of connections" is indicated as "◎" if the number of connections when laying a 10m pipe is 2 or less, "〇" if it is 4 or less, and "△" if it is 5 or more. In the table, the "buried strength" is indicated as "〇" if the deflection δ calculated by formula (5) in the compression test of JIS C3653 is 3.5% or less and there are no cracks or breaks in any part, and "×" if the deflection δ exceeds 3.5%.

[0119] The results show that in Examples 1 to 25, which satisfied formula (4), the occurrence of buckling was suppressed, while in Comparative Examples 1 to 8, which did not satisfy formula (4), buckling occurred.

[0120] In addition, in Examples 16 and 17 and Comparative Examples 2, 4, and 6, which have a pipe length of 5000 mm or more, the pipes are too long, so the transport efficiency was rated as "good", and in Example 18, which has a pipe length of 7000 mm, the transport efficiency was rated as "good". In Examples 2, 8, and 15, which have a pipe length of 3000 mm or less, the number of connections was rated as "good", and in particular, in Example 14, which has a pipe length of 1000 mm or less, the number of connections was rated as "good". In Example 25, in which the wall thickness of the outer pipe was thinned to reduce the mass of the outer pipe, the buried strength was rated as "bad".

[0121] Although not specifically shown here, in the case of a double-walled rectangular conduit in which the wall thickness of the inner pipe 9 at the portion corresponding to the large diameter portion is thicker than the wall thickness of the inner pipe 9 at the portion fused to the small diameter portion of the outer pipe, and the inner pipe thick-walled portion 92 and the inner pipe thin-walled portion 91 are alternately formed to have a difference in wall thickness, the results in Table 2 can be applied if the wall thickness t2 of the inner pipe thick-walled portion 92 is considered to be the wall thickness t of the inner pipe 9. Furthermore, since the inner pipe 9 actually has the inner pipe thin-walled portion 91, the total weight of the pipe body per given length is lighter than in the case of a double-walled rectangular conduit having a normal inner pipe with a uniform thickness equivalent to the inner pipe thick-walled portion, and it goes without saying that this is more advantageous in terms of buckling resistance.

[0122] Although the embodiment of the present invention has been described above with reference to the attached drawings, the technical scope of the present invention is not limited to the above-described embodiment. It is clear that a person skilled in the art can think of various modifications or alterations within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention. [Explanation of symbols]

[0123] 1, 1a, 1b...Double-wall rectangular conduit 3: Large diameter section 5……Small diameter part 7……Outer tube 9……Inner pipe 91……Inner pipe thin wall part 92……Thick walled part of inner pipe 11……Male joint 13……Female joint 43………Protrusion 45, 45a………Pipe fittings 47... Cylindrical member 49...Latch piece 50, 50a…Connection structure 51.... Opposing part 53....Coupling body 55..... Π type fixing member 57……Top section 59....Both feet 61... Engagement portion 63...Opening 65... Cylindrical section 67...Notch 69……Engagement stepped portion 71...Water-stopping member 80....Double-wall rectangular conduit manufacturing equipment 81a, 81b...Extruder 82a, 82b……Endless track 83...Extrusion head 85………Mold 87a, 87b, 87c...flow path 89a, 89b……Main pipe 90……Mold for inner pipe

Claims

1. A double-wall rectangular conduit having an outer tube and an inner tube, the outer tube has a large diameter portion having a relatively large diameter and a small diameter portion having a relatively small diameter, the large diameter portion and the small diameter portion being alternately and repeatedly formed in a tube axial direction of the outer tube, In a cross section perpendicular to a tube axis direction of the outer tube, the large diameter portion has a substantially rectangular cross-sectional shape, and the small diameter portion has a circular cross-sectional shape, the inner tube is disposed inside the outer tube, and a cross-sectional shape of the inner tube perpendicular to a tube axis direction is circular, In the small diameter portion, an inner peripheral surface of the outer tube and an outer peripheral surface of the inner tube are fused to each other, and in the large diameter portion, a space is formed between the inner peripheral surface of the outer tube and the outer peripheral surface of the inner tube, s ｙ >16A・R ２ (++2t) / (π・ (++2t) ４ -F ４ )) (σ ｙ : tensile yield stress of the material of the inner tube, w: mass per unit length of the electric conduit, L: length of the electric conduit, d: inner diameter of the inner tube, t: wall thickness of the inner tube) A double-wall rectangular conduit characterized by satisfying the above.

2. The double-walled rectangular conduit according to claim 1, characterized in that the length L of the conduit is 2000 mm to 6000 mm, the inner diameter d of the inner tube is 30 mm to 200 mm, and the wall thickness t of the inner tube is 0.4 mm to 3 mm.

3. The mass w per unit length of the conduit is 200 to 7000 g / m, and is 0.08×d ２ +4.55d+14<w<0.15×d ２ 3. The double-wall rectangular conduit according to claim 2, wherein the following is satisfied: +11.71d+84.

4. The double-wall rectangular conduit according to claim 3, characterized in that the deflection δ = (D1 - D2) / D1 × 100% (D1: outer diameter of the conduit before compression, D2: outer diameter of the conduit after compression) in the compression test of JIS C3653 Appendix 3 is 3.5% or more.

5. 5. The double-wall rectangular conduit according to claim 1, wherein the outer tube is made of a polyolefin resin.

6. 6. The double-wall rectangular conduit according to claim 5, wherein the outer tube and the inner tube are made of resins compatible with each other.

7. 7. The double-wall rectangular conduit according to claim 6, wherein the inner tube is made of a polyolefin resin that does not contain so-called polyolefin modified resins such as ethylene copolymers, thermoplastic elastomers, and rubber components.

8. The double-walled rectangular conduit according to claim 7, characterized in that the inner tube contains, in addition to the polyolefin resin, an acid-modified resin, and a content of the acid-modified resin is 0.5 parts by mass or more and 10 parts by mass or less when a content of the polyolefin resin is 100 parts.

9. 9. The double-wall rectangular conduit according to claim 1, wherein a yield stress of a material of the inner pipe is greater than a yield stress of a material of the outer pipe.

10. 10. The double-walled rectangular conduit according to claim 1, wherein a wall thickness of the inner tube at a portion corresponding to the large diameter portion of the outer tube is thicker than a wall thickness of the inner tube at a portion fused to the small diameter portion of the outer tube, and the inner tube is formed so that thick-walled portions and thin-walled portions of the inner tube have alternating wall thickness variations.

11. 11. The double-wall rectangular conduit according to claim 1, wherein a joint portion is formed at at least one end of the double-wall rectangular conduit.

12. A connection structure for double-wall rectangular conduits, comprising the double-wall rectangular conduits according to claim 11 connected to each other.

13. The connection structure of the double-wall rectangular conduit according to claim 11, characterized in that a joint portion is formed at at least one end of the double-wall rectangular conduit, and the double-wall rectangular conduits are connected to each other at the joint portion.

14. 12. The connection structure of a double-wall rectangular electric conduit according to claim 11, wherein the joint portion is a male joint or a female joint.

15. The double-wall rectangular conduits are connected to each other by a pipe joint, The pipe joint includes a tubular member and a spring-like locking piece disposed inside the tubular member, The end of the double-wall rectangular conduit, in which the small diameter portion, the large diameter portion, the small diameter portion, and the large diameter portion are formed in this order from the end, is inserted from both sides of the cylindrical member, 11. The connection structure for a double-wall rectangular conduit according to claim 10, characterized in that, within the approximately rectangular tubular member, the locking piece is supported on a side farther from the opposing portion of the double-wall rectangular conduit than the large diameter portion on a side closer to the opposing portion of the double-wall rectangular conduit, and a mechanism is provided for abutting the locking piece against a side surface of the large diameter portion on a side closer to the opposing portion of the double-wall rectangular conduit.

16. The double-wall rectangular conduits are connected to each other by a pipe joint, The pipe joint is formed by a joint body and a Π-shaped fixing member, The Π-shaped fixing member has a top surface portion and two legs that are connected to the top surface portion so as to be substantially perpendicular to the top surface portion and protrude downward, The end of the double-wall rectangular conduit, in which the small diameter portion, the large diameter portion, the small diameter portion, and the large diameter portion are formed in this order, is inserted from both sides of the joint body, and a notch portion is formed above a position of the joint body corresponding to the small diameter portion, The connection structure of double-wall rectangular conduit according to claim 10, characterized in that the Π-shaped fixing member is inserted into the cutout portion from above, and the movement of the large diameter portion is restricted by the Π-shaped fixing member, thereby connecting the double-wall rectangular conduits to each other.

17. A method for determining the mass per unit length of a double-wall rectangular conduit having an outer tube and an inner tube and the tensile yield stress at which the conduit does not buckle under its own weight, comprising: the outer tube has a large diameter portion having a relatively large diameter and a small diameter portion having a relatively small diameter, the large diameter portion and the small diameter portion being alternately and repeatedly formed in a tube axial direction of the outer tube, In a cross section perpendicular to a tube axis direction of the outer tube, the large diameter portion has a substantially rectangular cross-sectional shape, and the small diameter portion has a circular cross-sectional shape, the inner tube is disposed inside the outer tube, and a cross-sectional shape of the inner tube perpendicular to a tube axis direction is circular, In the small diameter portion, an inner peripheral surface of the outer tube and an outer peripheral surface of the inner tube are fused to each other, and in the large diameter portion, a space is formed between the inner peripheral surface of the outer tube and the outer peripheral surface of the inner tube, s ｙ >16A・R ２ (++2t) / (π・ (++2t) ４ -F ４ )) (σ ｙ : tensile yield stress of the material of the inner tube, w: unit mass of the electric conduit, L: length of the electric conduit, d: inner diameter of the inner tube, t: wall thickness of the inner tube) So that σ ｙ A method for determining the mass per unit length of a double-walled rectangular conduit and the tensile yield stress at which the conduit does not buckle under its own weight, comprising determining the relationship between w and

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