Power coaxial cable

Abstract

To provide a power coaxial cable that exhibits excellent properties in terms of both inductance and flexibility.SOLUTION: A power coaxial cable comprises: a core wire including a central conductor and an inner coating covering the central conductor; a plurality of outer conductors helically wound around the outer periphery of the core wire; and an outer coating covering the plurality of outer conductors. The ratio (Xi:ΣXo) of a cross-sectional area (Xi: mm2) of the central conductor to a total value (ΣXo: mm2) of cross-sectional areas of the individual outer conductors (Xo: mm2) is from 10:9 to 9:10, and the number of the outer conductors is 8 or more and 20 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a power coaxial cable for transmitting electric power. [Background technology]

[0002] Conventionally, when flexibility and bendability are required for electric wires for transmitting power, cables known as cab-tire cables have been used. Cab-tire cables have a double coating of an insulating coating on the conductor and a thick rubber sheath, making it difficult to achieve sufficient flexibility. Therefore, when even greater flexibility is required, twisted wires, which are simply twisted together, are used, each coated with a highly flexible insulating coating such as silicone rubber. Such flexibility and bendability are required not only for power transmission cables but also for coaxial cables used to transmit high-frequency signals in the kilohertz to gigahertz bands (see Patent Document 1 below). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-180907 Summary of the Invention [Problem to be solved by the invention]

[0004] Whenever a current flows through a conductor, not just an electric wire, a magnetic field is generated in response to changes in the current, which can cause problems due to inductance. Generally, inductance in electric wires used for power transmission can cause transmission loss, waveform distortion, and surges. Compared to twisted wires, which are simply made by twisting multiple electric wires together, coaxial cables are advantageous for power transmission in reducing inductance and preventing such problems. However, it is difficult to achieve good inductance and flexibility with coaxial cables. Therefore, an object of the present invention is to provide a power coaxial cable that has good inductance and flexibility. [Means for solving the problem]

[0005] In order to solve the above problems, the present invention provides: a center wire having a center conductor and an inner coating covering the center conductor; A power coaxial cable comprising a plurality of outer conductors wound helically around the outer periphery of the center wire, and an outer covering covering the plurality of outer conductors, The cross-sectional area of ​​the central conductor (Xi: mm 2 ) and the cross-sectional area (Xo: mm 2 ) total value (ΣXo:mm 2 ) ratio (Xi:ΣXo) is 10:9 to 9:10, The number of the outer conductors is 8 or more and 20 or less. [Effects of the Invention]

[0006] According to the present invention, since it is a coaxial cable, it is possible to reduce inductance compared to a twisted wire in which multiple electric wires are twisted together, and by setting the number of outer conductors within a specific range, it is possible to achieve good flexibility, and it is possible to provide a power coaxial cable with good characteristics in both inductance and flexibility. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic cross-sectional view of a power coaxial cable according to one embodiment. [Figure 2] FIG. 2 is a schematic front view, partially cut away, of a power coaxial cable according to one embodiment. [Figure 3] FIG. 3 is a schematic diagram showing the relationship between the central conductor and the outer conductor of the power coaxial cable used as a simulation model in the example. [Figure 4a] FIG. 4a is a diagram showing the relationship between the number of outer conductors provided around the central conductor of a power coaxial cable and the diameter of a layer central circle passing through the center of the outer conductor. [Figure 4b] FIG. 4b is a diagram showing the relationship between the number of outer conductors provided around the central conductor of a power coaxial cable and the diameter of a layer central circle passing through the center of the outer conductor. [Figure 5] FIG. 5 is a diagram showing the relationship between the number of outer conductors provided in a power coaxial cable and the inductance observed when current is passed through the power coaxial cable. [Figure 6] FIG. 6 is a diagram showing the relationship between the number of outer conductors provided in a power coaxial cable and the conductor resistance observed when electricity is passed through the power coaxial cable. [Figure 7] FIG. 7 is a diagram showing the relationship between the number of outer conductors provided in a power coaxial cable and the magnetic field strength observed when current is passed through the power coaxial cable. DETAILED DESCRIPTION OF THE INVENTION

[0008] An embodiment of the invention relating to a power coaxial cable will be described below with reference to the drawings. As shown in Figures 1 and 2, a power coaxial cable 100 includes a center wire 10 having a center conductor 11 and an inner coating 12 covering the center conductor 11, a plurality of outer conductors 21 wound helically around the center wire 10, and an outer coating 22 covering the plurality of outer conductors 21.

[0009] The power coaxial cable 100 of this embodiment has a cable length direction (hereinafter simply referred to as the "length direction DL"), which is the direction in which the power coaxial cable 100 extends. The power coaxial cable 100 shown in FIG. 1 is a round wire, with a cross-sectional shape (outline shape) that is approximately circular when cut along a plane perpendicular to the length direction DL. The power coaxial cable 100 has an imaginary center axis AX that extends in the length direction DL along a center point BC of the cross-sectional shape. The center point BC of the power coaxial cable 100 coincides with the center point of the center line 10. Therefore, the imaginary center axis AX extends in the length direction DL through the center of the center line 10. The power coaxial cable 100 of this embodiment has a circumferential direction DR that is a direction going around the imaginary center axis AX, and a radial direction DD that passes through the imaginary center axis AX and is perpendicular to the imaginary center axis AX.

[0010] The power coaxial cable 100 is preferably relatively small in size to more significantly exhibit its excellent flexibility. The power coaxial cable 100 has a thickness (diameter) of, for example, 25 mm or less. The thickness of the power coaxial cable 100 may be 20 mm or less, or may be 15 mm or less. The power coaxial cable 100 has a thickness (diameter) of, for example, 1 mm or more.

[0011] The power coaxial cable 100 is suitable as a wiring material for devices that repeatedly bend, such as robot arms, and devices that repeatedly expand and contract (reciprocate), such as air cylinders, and can be used in locations where bending and stretching occurs repeatedly as the device operates. The power coaxial cable 100 is suitable for use in articulated robots that are connected via a joint and include one member and another member extending from the joint, with the one member and the other member being rotatable around an axis passing through the joint, and can be used to extend from the one member along the other member via the joint. The power coaxial cable 100 can be used to connect a DC power supply to a motor driver (inverter) and / or to a motor driver (inverter) and a motor in a power unit (configuration of DC power supply-motor driver (inverter)-motor) that makes up an articulated robot.

[0012] The central conductor 11 is a stranded wire made up of a plurality of stranded wires. More specifically, the central conductor 11 of this embodiment is a concentric stranded wire that includes a plurality of child stranded conductors 111, each of which is made up of a plurality of stranded wires, and is formed by parent twisting the plurality of child stranded conductors 111.

[0013] The outer conductor 21 in the embodiment illustrated in the figures is a twisted wire in which a plurality of wires are twisted together. In the power coaxial cable 100, a plurality of outer conductors 21 are wound spirally around the center wire 10 so as to be parallel to one another, and the plurality of outer conductors 21 are arranged at approximately equal intervals in the circumferential direction DR. The number of outer conductors 21 used in one power coaxial cable 100 is, for example, 8 or more and 20 or less. In addition, when the cross-sectional area of ​​each outer conductor 21 is "Xo (mm 2 )" and when n (n=8 to 20) outer conductors 21 are used in one power coaxial cable 100, the cross-sectional area (Xo: mm 2 ) total value (ΣXo=n×Xo:mm 2 ) is the cross-sectional area (Xi: mm 2 ) and the relationship shown in the following conditional expression (1) is preferably satisfied. Xi:ΣXo=10:9~9:10 (1) In addition, when the central conductor 11 is a parent stranded conductor (composite stranded wire) composed of a plurality of child stranded conductors 111, the cross-sectional area (Xi: mm 2 ) is calculated by adding up the cross-sectional areas of the individual stranded conductors 111.

[0014] Cross-sectional area of ​​the outer conductor 21 (Xo: mm 2 ) total value (ΣXo=n×Xo:mm 2 ) and the cross-sectional area of ​​the central conductor 11 (Xi: mm 2 ) may have the relationship shown in (2) below, or may have the relationship shown in (3). Xi:ΣXo=100:92~92:100 ···(2) Xi:ΣXo=100:95~95:100 ···(3)

[0015] In the power coaxial cable 100, the cross-sectional area of ​​the outer conductor 21 (Xo: mm 2 ) total value (ΣXo=n×Xo:mm 2 ) and the cross-sectional area of ​​the central conductor 11 (Xi: mm 2 ) shows a close relationship, which prevents the electrical resistance from becoming higher than necessary.

[0016] As described above, the power coaxial cable 100 has 8 to 20 outer conductors 21. Therefore, the power coaxial cable 100 can keep its inductance lower than that of a twisted wire or the like. By having a predetermined number of outer conductors 21, the power coaxial cable 100 can suppress local concentration of Joule heat generated from the outer conductors 21 and can suppress the occurrence of local potential differences between the outer conductors 21 and the central conductor 11, allowing the thicknesses of the inner coating 12 and the outer coating 22 to be thin. Furthermore, because the conductor diameter of the outer conductors 21 is also reduced, the finished outer diameter of the power coaxial cable 100 can also be reduced, and the power coaxial cable 100 exhibits good flexibility.

[0017] The child stranded conductors 111 and the outer conductor 21 constituting the central conductor 11 may be bunched or concentrically twisted. The child stranded conductors 111 and the outer conductor 21 are preferably bunched stranded wires because they are more flexible when made up of many thin wires.

[0018] The twist of the wires in the child twisted conductors 111 and the outer conductor 21 that make up the center conductor 11 may be either S twist or Z twist. The parent twist in the center conductor 11 may also be either S twist or Z twist. The direction in which the outer conductors 21 are twisted around the center wire 10 may also be either S twist or Z twist. In this embodiment, the outer conductors 21 are not crossed like in a braid, and all of the twisting directions of the outer conductors 21 around the center wire 10 are unified to either S twist or Z twist, thereby providing the power coaxial cable 100 with excellent flexibility.

[0019] The child twisting direction and parent twisting direction in the central conductor 11 may be the same or different. That is, these twisting directions may be the same or opposite. The twisting direction of the wires of the outer conductor 21 (the twisting direction of the outer conductor itself) and the twisting direction of the outer conductor 21 around the center wire 10 may be the same or opposite.

[0020] The conductor diameter of the center conductor 11 may be, for example, 0.6 mm or more. The conductor diameter of the center conductor 11 may be, for example, 0.8 mm or more, or 1.0 mm or more. The conductor diameter of the center conductor 11 may be, for example, 15 mm or less. The conductor diameter of the center conductor 11 may be, for example, 12 mm or less, or 10 mm or less, or 8 mm or less. The conductor diameter of the center conductor 11 may be, for example, 6 mm or less, or 5 mm or less.

[0021] Cross-sectional area of ​​the central conductor 11 (Xi: mm 2 ) is, for example, 0.2 mm 2 The cross-sectional area of ​​the central conductor 11 (Xi: mm 2 ) is 0.5mm 2 may be 0.8 mm 2 The cross-sectional area of ​​the central conductor 11 may be, for example, 150 mm 2 The cross-sectional area of ​​the central conductor 11 (Xi: mm 2 ) is 110mm 2 May be less than 90mm 2 May be less than 70mm 2 May be less than 50mm 2 May be less than 30mm 2 The cross-sectional area of ​​the central conductor 11 (Xi: mm 2 ) can be calculated according to the following formula (A) using the wire thickness (diameter: Dw (mm)) and the number of wires (n). Xi=n×π(Dw / 2) 2 (A)

[0022] The conductor diameter of the child stranded conductors 111 constituting the central conductor 11 may be, for example, 0.2 mm or more. The conductor diameter of the child stranded conductors 111 may be, for example, 0.3 mm or more, or 0.4 mm or more. The conductor diameter of the child stranded conductors 111 may be, for example, 5 mm or less. The conductor diameter of the child stranded conductors 111 may be 4 mm or less, or 3 mm or less.

[0023] The cross-sectional area of ​​the child stranded conductor 111 can be, for example, 1 / 7, 1 / 12, 1 / 19, etc., of the cross-sectional area of ​​the central conductor 11. The cross-sectional area of ​​the child stranded conductor 111 can be, for example, 0.035 mm 2 The cross-sectional area of ​​the child stranded conductor 111 can be 0.05 mm 2 It may be more than 0.08mm 2 It may be more than 0.1 mm 2 The cross-sectional area of ​​the twisted conductor 111 may be, for example, 22 mm 2 The cross-sectional area of ​​the child strand conductor 111 can be 18 mm 2 May be less than 14mm 2 May be less than 8mm 2 It may be the following:

[0024] The wires constituting the central conductor 11 may have a thickness of, for example, 0.05 mm or more. The wires may have a thickness of, for example, 0.08 mm or more, or 0.1 mm or more. The wires may have a thickness of, for example, 0.45 mm or less. The wires may have a thickness of, for example, 0.32 mm or less, or 0.26 mm or less.

[0025] The outer conductor 21 can also be made of wires having the same thickness as above. The wires constituting the central conductor 11 and the outer conductor 21 can be, for example, copper wires or aluminum wires. The wires can be plated wires or unplated wires. The wires can be, for example, unplated copper wires.

[0026] The outer conductor 21 may have a conductor diameter of, for example, 0.2 mm or more. The conductor diameter of the outer conductor 21 may be, for example, 0.3 mm or more, or 0.4 mm or more. The conductor diameter of the outer conductor 21 may be, for example, 5 mm or less. The conductor diameter of the outer conductor 21 may be 4 mm or less, or 3 mm or less.

[0027] The cross-sectional area of ​​each outer conductor 21 is, for example, 0.035 mm 2 The cross-sectional area of ​​the outer conductor 21 can be 0.05 mm 2 It may be more than 0.08mm 2 It may be more than 0.1 mm 2 The cross-sectional area of ​​the outer conductor 21 may be, for example, 22 mm 2 The cross-sectional area of ​​the outer conductor 21 can be 18 mm 2 May be less than 14mm 2 May be less than 8mm 2 It may be the following:

[0028] A shorter twist pitch in the child stranded conductors 111 and the outer conductor 21 makes the wires less likely to come apart, improving handleability, and also makes it easier to reduce the diameter because the wires are tightly packed together. On the other hand, a longer twist pitch can be advantageous in providing the power coaxial cable 100 with excellent flexibility. These twist pitches can be, for example, 5 times or more the conductor diameter. The twist pitch in the child stranded conductors 111 and the outer conductor 21 may be 7 times or more, or even 9 times or more, the conductor diameter. The twist pitch in the child stranded conductors 111 and the outer conductor 21 may be, for example, 50 times or less the conductor diameter. These twist pitches may be 40 times or less, or even 30 times or less, the conductor diameter.

[0029] A longer pitch of the parent twist in the center conductor 11 can be advantageous in terms of providing excellent flexibility to the power coaxial cable 100, but an excessively long pitch can reduce handleability and make it difficult to reduce the diameter. The pitch of the parent twist in the center conductor 11 can be, for example, 5 times or more the conductor diameter. The pitch of the parent twist in the center conductor 11 may be 7 times or more, or 9 times or more the conductor diameter. The pitch of the parent twist in the center conductor 11 can be, for example, 50 times or less the conductor diameter. The pitch of the parent twist in the center conductor 11 may be 40 times or less, or 30 times or less the conductor diameter.

[0030] The ratio (L1 / L2), hereinafter also referred to as the "twist pitch multiple") of the winding pitch of the outer conductor 21 around the center wire 10 (the longitudinal distance L1 that the outer conductor 21 travels to make one full turn around the center wire 10) to the layer core diameter (diameter L2) can be, for example, 5 times or more. The winding pitch of the outer conductor 21 around the center wire 10 may be 7 times or more, or even 9 times or more, the layer core diameter (diameter L2). The twist pitch multiple (L1 / L2) can be, for example, 50 times or less. The twist pitch multiple (L1 / L2) may be 40 times or less, or may be 30 times or less. The layer core diameter (L2) can be calculated by adding the conductor diameter of the outer conductor 21 to the diameter of the center wire 10.

[0031] The number of outer conductors 21 is preferably large in order to facilitate uniform arrangement of the outer conductors 21 on the outer peripheral surface of the center wire 10 and reduce inductance. Increasing the number of outer conductors 21 allows the conductor diameter of each outer conductor 21 to be small, which is advantageous for reducing the diameter of the power coaxial cable 100 and makes it easier to impart high flexibility to the power coaxial cable 100. The number of outer conductors 21 may be, for example, 9 or more, 10 or more, or 11 or more.

[0032] The power coaxial cable 100 of this embodiment not only reduces the number of parts, thereby enabling reductions in material costs and manufacturing costs, but also has the advantage of being able to reduce the diameter, because the outer conductor 21 is not wrapped in tape or the like, and the outer conductor 21 is in direct contact with both the inner coating 12 and the outer coating 22. That is, in the radial direction DD of the power coaxial cable 100, the inner part of the outer conductor 21 contacts the outer peripheral surface of the inner coating 12 from the outside, and the outer part of the outer conductor 21 contacts the inner peripheral surface of the outer coating 22 from the inside, so that each of the multiple outer conductors 21 is sandwiched between the inner coating 12 and the outer coating 22 from the inside and outside. When the power coaxial cable 100 is bent, slippage occurs between the wires constituting the outer conductor 21, causing the multiple wires twisted together in a round shape to spread in the circumferential direction DR and flatten the outer conductor 21, which functions to provide flexibility to the power coaxial cable 100. At this time, the movement of the wires in contact with the outer peripheral surface of the inner coating 12 and the wires in contact with the inner peripheral surface of the outer coating 22 is restricted, so if the number of outer conductors 21 is increased and the diameter of the outer conductors 21 is made too thin, it may actually reduce the flexibility of the power coaxial cable 100. Therefore, the number of outer conductors 21 may be, for example, 19 or less, 18 or less, or 17 or less.

[0033] As described above, by setting the number of outer conductors 21 to be between 8 and 20, it is possible to reduce the inductance that occurs when current is passed through the power coaxial cable 100. The outer conductors 21 are preferably arranged evenly in the circumferential direction, and when the number of outer conductors 21 is n, it is preferable that the angles (θr) formed by the n imaginary line segments connecting the center of each outer conductor 21 and the center of the power coaxial cable 100 all fall within the following range. [0.9×(360 / n)]≦θr≦[(360 / n) / 0.9]

[0034] In order to reduce inductance, it is preferable to arrange the outer conductors 21 closely, and it is preferable that adjacent outer conductors are in close contact with each other. However, in order to provide the power coaxial cable 100 with better flexibility, it is preferable that the outer conductors are not in close contact with each other. That is, in order to provide the power coaxial cable 100 with better flexibility, it is preferable to provide a gap between adjacent outer conductors 21 in the circumferential direction DR.

[0035] Here, because the outer conductor 21 is twisted spirally around the center line 10, the cross section of the power coaxial cable 100 taken along a plane perpendicular to the longitudinal direction DL is cut at an angle. The cross section of the outer conductor 21 cut at an angle has a radial dimension ("Ld" in FIG. 1) that is basically the same as the conductor diameter (Y2: mm) of the outer conductor 21 (Ld = Y2), but a circumferential dimension ("Lr" in FIG. 1, hereinafter also referred to as "circumferential width") that is slightly longer than the conductor diameter (Y2: mm) (Lr > Y2). That is, the cross section of the outer conductor 21 is, strictly speaking, elliptical, and more specifically, has a minor axis in the radial direction DD and a major axis in the circumferential direction DR. In this case, the dimension in the circumferential direction DR (circumferential width: Lr) can be calculated using the following formula (X) from the twist angle ("θ" in Figure 2) relative to the center line 10 and the conductor diameter (Y2: mm). Lr=Y2÷cosθ (X) If the total circumferential width (Lr: mm) for the number (n) of outer conductors 21 ("ΣLr: mm" = n × Lr) is set to be shorter than the circumferential length "Lc (mm)" of the layer center circle QC passing through the center of the outer conductor 21, a gap will be formed between the outer conductors 21. The diameter (layer core diameter) of the layer center circle QC is usually the sum of the diameter of the center line 10 and the conductor diameter of the outer conductor 21. Therefore, the circumferential length (Lc: mm) of the layer center circle QC can be calculated by multiplying it by pi.

[0036] As described above, the gap between adjacent outer conductors 21 in the circumferential direction DR can be adjusted by the conductor diameter (Y2), twist angle (θ), and number (n) of the outer conductors 21. The proportion of the total value (ΣLr: mm) of the circumferential widths (Lr: mm) to the circumferential length (Lc: mm) of the layer-center circle QC can be, for example, 95% or less. The proportion of the total value (ΣLr: mm) of the circumferential widths (Lr: mm) to the circumferential length (Lc: mm) of the layer-center circle QC can be, for example, 90% or less, 85% or less, 80% or less, or 75% or less. The proportion of the total value (ΣLr: mm) of the circumferential widths (Lr: mm) to the circumferential length (Lc: mm) of the layer-center circle QC can be, for example, 50% or more.

[0037] In this embodiment, a portion of the inner coating 12 penetrates between adjacent child stranded conductors 111 in the circumferential direction DR at the outer periphery of the center conductor 11. Similarly, in this embodiment, a portion of the outer coating 22 penetrates between two adjacent outer conductors 21 in the circumferential direction DR from the outside in the radial direction DD. That is, each of the inner coating 12 and the outer coating 22 has a protrusion 22a that protrudes inward in the radial direction DD. The protrusion 22a restricts the movement of the outer coating 22 in the circumferential direction DR, making it less likely that a change in current due to inductance will occur even when the cable is bent during current flow. If the protrusion 22a penetrates too deeply, the flexibility of the power coaxial cable 100 may not be fully exhibited. Therefore, it is preferable that the penetration depth of the protrusion 22a toward the inside in the radial direction DD of the power coaxial cable 100 be slightly beyond the layer center circle QC or remain radially outward of the layer center circle QC.

[0038] The inner coating 12 can be coated on the central conductor 11 using an extruder. Specifically, the center wire 10 can be produced by heating and melting and kneading the resin composition constituting the inner coating 12 in the extruder while the central conductor 11 is being drawn through a nipple set on the crosshead of the extruder at a predetermined speed, and then extruding the resulting molten mixture through a die opening around the nipple to coat the central conductor 11 and then cooling. At this time, the tip of the nipple is slightly retracted from the die, and the molten mixture is extruded while applying resin pressure to the central conductor 11, thereby performing solid extrusion, which allows part of the inner coating 12 to penetrate between the stranded conductors 111. The outer coating 22 can be formed in a similar manner. The outer conductor 22 is formed by drawing the central wire 10, which has multiple outer conductors 21 spirally wound around it, through a nipple while the molten mixture for the outer coating 22 is extruded through the die, and solid extrusion is performed during this process to form the outer coating 22 with protrusions 22a.

[0039] If the molten mixture penetrates into the fine spaces between the conductor strands and other small spaces during the manufacturing process, the power coaxial cable 100 will not exhibit sufficient flexibility. Therefore, it is preferable that the resin composition constituting the inner coating 12 and the resin composition constituting the outer coating 22 have a relatively high melt viscosity. For this reason, it is preferable that the resin composition contain a resin that has a higher melt viscosity and lower elasticity than common resins such as polyethylene and polyvinyl chloride. A thermoplastic elastomer is suitable as such a resin.

[0040] Examples of thermoplastic elastomers suitable for inclusion in the resin compositions constituting the inner coating 12 and the outer coating 22 include polyester-based elastomers, polystyrene-based elastomers, polyolefin-based elastomers, polyamide-based elastomers, and polyurethane-based elastomers. Among these, polyester-based elastomers are preferred due to their excellent flexibility and abrasion resistance. As the polyester-based thermoplastic elastomer, polyester-ether type block copolymers in which the hard segments are aromatic polyester blocks and the soft segments are aliphatic polyether blocks are more preferred, and those in which the aliphatic polyether blocks contain polyalkylene ether glycol are even more preferred.

[0041] The aromatic polyester block may contain, for example, an aromatic dicarboxylic acid and a diol as constituent units, and examples of the dicarboxylic acid constituting the aromatic polyester block include phthalic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenyletherdicarboxylic acid, diphenylsulfonedicarboxylic acid, etc. Examples of the diol include aliphatic diols such as ethylene glycol, propylene glycol, trimethylene glycol, tetramethylene glycol, and hexamethylene glycol, alicyclic diols such as cyclohexanediol and cyclohexanedimethanol, and aromatic diols such as dihydroxybiphenyl and bishydroxyethoxyphenylpropane.

[0042] Examples of the polyalkylene ether glycol constituting the aliphatic polyether block include polyalkylene ether glycols having 1 to 8 carbon atoms, preferably 2 to 6 carbon atoms, such as polyethylene glycol, polypropylene glycol, polytrimethylene ether glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, block or random copolymers of ethylene oxide and propylene oxide, and block or random copolymers of ethylene oxide and tetrahydrofuran, and among these, polytetramethylene ether glycol is preferred.

[0043] The above-mentioned thermoplastic elastomers and resin compositions containing such thermoplastic elastomers preferably have a melt mass-flow rate (MFR) of 10 g / 10 min or less. The melt mass-flow rate (MFR) of the thermoplastic elastomers and resin compositions may be 8 g / 10 min or less, or may be 6 g / 10 min or less. The melt mass-flow rate (MFR) of the thermoplastic elastomers and resin compositions is preferably 0.1 g / 10 min or more, since this allows for a reduction in the power consumption of the motor and heater of the extruder. The melt mass-flow rate (MFR) of the thermoplastic elastomers and resin compositions may be 0.5 g / 10 min or more, or may be 1.0 g / 10 min or more. The melt mass-flow rate (MFR) can be measured, for example, based on Method A of JIS K 7210-1:2014 "Plastics - Determination of melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics - Part 1: Standard test method", and can be measured under conditions of a temperature of 230°C and a load of 21.16 N.

[0044] To prevent the outer coating 22 from excessively penetrating toward the center line 10, an inclusion may be disposed in the gap between adjacent outer conductors 21 in the circumferential direction. The inclusion is preferably a flexible material such as PP yarn or resin foam. The inclusion may be a flexible elastomer composition such as a watertight compound, or a jelly compound.

[0045] The resin compositions constituting the inner coating 12 and the outer coating 22 may contain a lubricant. Examples of lubricants include fatty acid-based lubricants and wax-based lubricants. Examples of fatty acid-based lubricants include fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, oleic acid, and erucic acid, as well as derivatives thereof, including fatty acid metal salts, fatty acid amides, and fatty acid esters. Examples of wax-based lubricants include oil-based waxes such as hydrogenated castor oil, polyethylene wax, and paraffin wax.

[0046] The amount of lubricant used in typical wire coating materials is typically about 0.5 parts by mass per 100 parts by mass of resin, and is used to prevent bleed-out. However, in this embodiment, the lubricant may be used in a proportion of 0.5 parts by mass or more to intentionally cause bleed-out, in order to improve the slipperiness between the inner coating 12 and the center conductor 11, or between the outer conductor 21 and the outer coating 22 or between the inner coating 12 and the outer coating 22. Bleed-out of the lubricant can be confirmed visually by checking for the presence of an oily or powdery substance on the surface of the inner coating 12 or the outer coating 22, and FT-IR or other methods can be used to confirm that the oily or powdery substance is the lubricant. It is preferable that the power coaxial cable 100 exhibits lubricant bleed-out after being left at typical room temperature (e.g., 23°C, 50% RH) for 30 days.

[0047] The lubricant can be used in a proportion of 0.6 parts by mass or more per 100 parts by mass of the resin contained in the resin composition. The proportion of the lubricant per 100 parts by mass of the resin may be 0.8 parts by mass or more, 1.0 parts by mass or more, or even 1.2 parts by mass or more. The proportion of the lubricant per 100 parts by mass of the resin is, for example, 5 parts by mass or less. The proportion of the lubricant per 100 parts by mass of the resin may be 4 parts by mass or less, or even 3 parts by mass or less.

[0048] The resin compositions constituting the inner coating 12 and the outer coating 22 may contain components other than the thermoplastic elastomer and lubricant. Examples of such components include common rubber and plastic chemicals such as antioxidants and weathering agents. The resin compositions may also contain, for example, pigments and inorganic fillers. To ensure that the flexibility of the thermoplastic elastomer is exerted in the inner coating 12 and the outer coating 22, the resin compositions preferably contain 90% by mass or more of the thermoplastic elastomer, and more preferably 95% by mass or more of the thermoplastic elastomer.

[0049] The inner coating 12 and the outer coating 22 can be provided so as to have a thickness of, for example, 1.2 mm or less. If the inner coating 12 or the outer coating 22 has a protrusion, the thickness of the inner coating 12 or the outer coating 22 refers to the thickness at a location where no protrusion is formed. Therefore, the thickness of the inner coating 12 means a value calculated by subtracting the conductor diameter of the center conductor 11 from the outer diameter of the center wire 10 and multiplying this value by 1 / 2. The thickness of the outer coating 22 can be calculated in a similar manner by subtracting the outer diameter of the center wire 10 and twice the conductor diameter of the outer conductor 21 from the outer diameter of the outer coating 22 and multiplying this value by 1 / 2.

[0050] The thickness of the inner coating 12 and the outer coating 22 may be 1.0 mm or less, 0.9 mm or less, or 0.8 mm or less. The thickness of the inner coating 12 and the outer coating 22 is, for example, 0.2 mm or more. Even with the same finished outer diameter, making the inner coating 12 thinner than the outer coating 22 may be more advantageous in terms of providing flexibility to the power coaxial cable 100 than making the inner coating 12 thicker than the outer coating 22. The thickness of the inner coating 12 is preferably 0.05 mm or more thinner than the outer coating 22, and more preferably 0.10 mm or more thinner. The difference in thickness between the inner coating 12 and the outer coating 22 may be, for example, 0.50 mm or less.

[0051] In order to provide excellent flexibility to the power coaxial cable 100, the resin composition constituting the inner coating 12 and the outer coating 22 preferably has a 100% modulus (M100) of 1.5 MPa or less, as measured in accordance with JIS K 6251:2017 "Vulcanized rubber and thermoplastic rubber - Determination of tensile properties" (No. 1 dumbbell test piece, tensile speed 500 mm / min). The 100% modulus of the resin composition may be 1.2 MPa or less, or may be 1.0 MPa or less. The 100% modulus of the resin composition is, for example, 0.3 MPa or more.

[0052] The resin composition constituting the inner coating 12 and the outer coating 22 preferably has a Type A durometer hardness (instantaneous value) of 50 or less, as measured in accordance with JIS K 6253-3:2012 "Vulcanized rubber and thermoplastic rubber - Determination of hardness - Part 3: Durometer hardness." In particular, the inner coating 12 has a low surface hardness, which allows the outer conductor 21 to be embedded into the outer peripheral surface, preventing the outer conductor 21 from shifting position. The Type A durometer hardness (instantaneous value) of the resin composition may be 48 or less, or may be 46 or less. The Type A durometer hardness (instantaneous value) of the resin composition is, for example, 30 or more.

[0053] The power coaxial cable 100 of this embodiment has excellent flexibility due to the configuration described above, and is therefore particularly useful for transmitting power in articulated robots and the like, as described above, but the power coaxial cable 100 can also be used widely for a variety of other applications in addition to such applications. As described above, this specification includes the following disclosure, but the above disclosure is merely a specific example, and the power coaxial cable 100 of the present invention can be modified in various ways that are not specifically exemplified above.

[0054] (1) a center wire having a center conductor and an inner coating covering the center conductor; A power coaxial cable comprising a plurality of outer conductors wound helically around the outer periphery of the center wire, and an outer covering covering the plurality of outer conductors, The cross-sectional area of ​​the central conductor (Xi: mm 2 ) and the cross-sectional area (Xo: mm 2 ) total value (ΣXo:mm 2 ) ratio (Xi:ΣXo) is 10:9 to 9:10, A power coaxial cable in which the number of outer conductors is 8 or more and 20 or less.

[0055] (2) Calculate the circumferential dimension (Lr: mm) of the outer conductor in a cross section perpendicular to the cable length direction, When the individual dimensions (Lr: mm) of the plurality of outer conductors are added up to calculate the total value (ΣLr: mm), The power coaxial cable according to (1), wherein the total value (ΣLr: mm) is smaller than the circumferential length (Lc: mm) of a layer central circle passing through the centers of the plurality of outer conductors.

[0056] (3) The power coaxial cable according to (1) or (2), wherein the inner coating is made of a resin composition containing a thermoplastic elastomer.

[0057] (4) The power coaxial cable according to any one of (1) to (3), wherein the central conductor is a composite stranded wire in which a plurality of stranded wires are twisted together. [Example]

[0058] The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples. <Evaluation 1: Effect of dividing the outer conductor into multiple pieces> (Example 1: 8 sq power coaxial cable) The resin composition for the inner and outer coatings contained 2 parts by mass of a lubricant (stearic acid) for 100 parts by mass of thermoplastic elastomer (a polyester elastomer having an aliphatic polyether block (soft segment) containing polyalkylene ether glycol and an aromatic polyester block (hard segment)). The resin composition had a 100% modulus of 0.9 MPa and a Type A durometer hardness (instantaneous value) of 44.

[0059] The central conductor was a composite conductor (conductor diameter 4.0 mm) made of 101 stranded 0.12 mm wires, each with a diameter of 4.0 mm, and seven stranded conductors.

[0060] The central conductor was coated with a 0.6 mm thick inner coating using an extruder to produce a 5.2 mm thick center wire, and an outer conductor (0.12 mm x 45 strands, conductor diameter 0.7 mm) was spirally twisted around the center wire, and then a 0.8 mm thick outer coating was coated on top of that by solid extrusion to produce an 8.2 mm thick power coaxial cable in which part of the outer coating penetrated between the outer conductors (having protrusions). The detailed configuration of the power coaxial cable of Example 1 is shown in Table 1.

[0061] (Reference example) Two wires each having the same composite conductor (7 strands / 101 / 0.12:8 sq) as the central conductor of Example 1 and coated with a 1.1 mm thick silicone rubber coating were prepared and twisted together to produce a twisted wire.

[0062] The mass per unit length of both specimens was measured, and their inductance was measured using an LCR meter. The flexibility of both specimens was also evaluated. The results are shown in Table 1.

[0063] [Table 1]

[0064] As described above, the power coaxial cable of Example 1 has flexibility equivalent to that of a twisted wire, and has reduced inductance compared to a twisted wire.

[0065] (Examples 2 and 3) As shown in Table 2, power coaxial cables were produced in the same manner as in Example 1, except that the wire size was reduced from 8 sq to 1.25 sq (Example 2) and 3.5 sq (Example 3).

[0066] The flexibility of the produced power coaxial cables was evaluated in three ways: a bending test, a twisting test, and bending stress, as described below. (Bending test) A power coaxial cable was cut to a specified length and used as a sample. When viewed from the front, the sample was oriented vertically, and the upper half was bent alternately 90 degrees to the left and then 90 degrees to the right. The number of times until the conductor broke was measured at a repetition rate of 60 round trips per minute. (Twist test) A power coaxial cable cut to a length of 25 mm was used as a sample. Both ends of the sample were held, and while one end was fixed, the other end was repeatedly twisted 90 degrees clockwise and counterclockwise (total twisting angle 180 degrees), and the number of times until the conductor broke was measured at a repetition rate of 60 round trips per minute. (bending stress) Three power coaxial cables were cut to a predetermined length and prepared as samples, and bending stress was measured in a three-point bending test. The three-point bending test was performed with a support point spacing of 50 mm and a compression speed of 10 mm / min. Measurements were performed four times for each of the three samples. The four measurements were performed by rotating the sample on the support table 90 degrees in the circumferential direction. The results are shown in Table 2.

[0067] [Table 2]

[0068] <Evaluation 2 (Evaluation of the influence of the number of divisions of the outer conductor)> In Evaluation 1, we examined the flexibility and inductance when a conductor with the same cross-sectional area as the central conductor was divided into multiple strands and arranged around the central conductor as an outer conductor. In this evaluation 2, as in Example 1 of evaluation 1, the cross-sectional area of ​​the central conductor was 8 sq (8 mm 2 ) and the evaluation was carried out using a power coaxial cable as a model, in which the inner coating covering the central conductor was 0.6 mm thick. However, in this evaluation 2, the model was one in which the central conductor and each of the outer conductors were solid wires. As shown in FIG. 3, the number of outer conductors was varied from 1 to 22, and the evaluation was carried out on power coaxial cables in which the outer conductors were arranged around the central conductor at equal intervals in the circumferential direction. First, the core diameter (the diameter of the core circle passing through the center of the outer conductor) of each power coaxial cable was calculated.

[0069] The results are shown in Figures 4a and 4b. Figure 4b is a graph of the results of Figure 4a, and it can be seen from this graph that when the number of outer conductors is less than 8, the core diameter increases rapidly. In other words, from the results shown in Figures 4a and 4b, it can be understood that by increasing the number of outer conductors to 8 or more, it becomes easier to reduce the diameter of the power coaxial cable and to impart excellent flexibility to the power coaxial cable.

[0070] Furthermore, Figure 4b shows that when the number of outer conductors is 20 or more, the effect of reducing the core diameter converges, and it is difficult to expect any further reduction in diameter even if the number of outer conductors is increased beyond 20. Furthermore, if the number of outer conductors exceeds 20, the distance between adjacent outer conductors in the circumferential direction becomes too close, which may actually reduce the flexibility of the power coaxial cable. Therefore, by taking into consideration Figure 4b, it can be seen that keeping the number of outer conductors to 20 or less is advantageous for achieving excellent flexibility in the power coaxial cable.

[0071] Next, the inductance and conductor resistance when a current is applied to the power coaxial cable shown in FIG. 3 at a frequency of 100 kHz were determined by simulation. The simulation conditions are as shown in Table 3 below.

[0072] [Table 3]

[0073] The results of the simulation are shown in FIGS. As shown in Figures 5 to 7, when the number of outer conductors is increased and the number reaches eight or more, the circumferential change in the magnetic field strength generated between the central conductor and the outer conductor is suppressed, and the inductance and conductor resistance show better values ​​than before (when the number is less than eight). Furthermore, it can be seen from Figures 5 and 6 that the effects of reducing inductance and conductor resistance tend to converge when the number of outer conductors is 20 or more, just like the effect of reducing the core diameter.

[0074] As described above, it was confirmed that power coaxial cables that satisfy the specifications of the present invention have excellent flexibility, as exemplified by the power coaxial cables of Examples 1 to 3. Furthermore, it can be understood from the above results that the present invention provides a power coaxial cable that has excellent properties in both inductance and flexibility. [Explanation of symbols]

[0075] 10: center wire, 11: center conductor, 12: inner coating, 21: outer conductor; 22: outer coating; 100: power coaxial cable, 111: child twisted conductor, AX: virtual center axis, BC: center, DD: radial direction, DL: longitudinal direction, DR: circumferential direction.

Claims

1. a center wire having a center conductor and an inner coating covering the center conductor; A power coaxial cable comprising a plurality of outer conductors wound helically around the outer periphery of the center wire, and an outer covering covering the plurality of outer conductors, The cross-sectional area of ​​the central conductor (Xi: mm 2 ) and the cross-sectional area of ​​each of the plurality of outer conductors (Xo: mm 2 ) the total value (ΣXo: mm 2 ) the ratio (Xi:ΣXo) of 10:9 to 9:10, A power coaxial cable in which the number of outer conductors is 8 or more and 20 or less.

2. Calculate the circumferential dimension (Lr: mm) of the outer conductor in a cross section perpendicular to the cable length direction, When the individual dimensions (Lr: mm) of the plurality of outer conductors are added up to calculate a total value (ΣLr: mm), 2. The power coaxial cable according to claim 1, wherein the total value (ΣLr: mm) is smaller than the circumferential length (Lc: mm) of a layer center circle passing through the centers of the plurality of outer conductors.

3. 2. The power coaxial cable according to claim 1, wherein the inner coating is made of a resin composition containing a thermoplastic elastomer.

4. 2. The power coaxial cable according to claim 1, wherein the central conductor is a composite stranded wire formed by twisting together a plurality of stranded wires.

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