Patent Publication Number: US-2019172606-A1

Title: Multicoaxial cable

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2017-231712, filed on Dec. 1, 2017, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a multicoaxial cable. 
     BACKGROUND 
     JP-A-2014-220043 discloses an insulated electric cable including: a core wire formed by stranding a plurality of core members, each of the core members including a conductor and an insulating layer covering the conductor; a sheath that is formed to cover the core wire; and a paper tape that is disposed between the core wire and the sheath in a state of being wrapped around the core wire. 
     SUMMARY 
     However, in the configuration of the insulated electric cable disclosed in JP-A-2014-220043, there is a room for improvement to both realize reduction in diameter and high bending resistance. 
     An object of the present invention is to provide a multicoaxial cable capable of realizing both reduction in diameter and high bending resistance. 
     In order to achieve the object, according to an aspect of the present invention, a multicoaxial cable according to an aspect of the present invention includes: 
     a core wire that includes a first unit formed by twisting two first insulated electric wires, each of the first insulated electric wires including a conductor and an insulating layer, the conductor being formed by stranding a plurality of child stranded conductors, each of the child stranded conductors being formed by stranding a plurality of conductor wires, and the insulating layer being formed to cover the conductor; and 
     a sheath that covers the core wire, wherein 
     a cross-sectional area of the conductor is 1.2 mm 2  to 3.5 mm 2 , 
     the conductor is formed of a hard-drawn copper wire, and 
     an outer diameter of the insulating layer is 2.0 mm to 3.6 mm. 
     In addition, in order to achieve the object, a multicoaxial cable according to another aspect of the present invention includes: 
     a core wire that is formed using a first insulated electric wire and a second insulated electric wire, 
     the first insulated electric wire including a conductor and an insulating layer, the conductor of the first insulated electric wire being formed by stranding a plurality of child stranded conductors, each of the child stranded conductors of the first insulated electric wire being formed by stranding a plurality of conductor wires, and the insulating layer of the first insulated electric wire being formed to cover the conductor, 
     the second insulated electric wire including a conductor and an insulating layer, the conductor of the second insulated electric wire being formed by stranding a plurality of child stranded conductors, each of the child stranded conductors of the second insulated electric wire being formed by stranding a plurality of conductor wires, and the insulating layer of the second insulated electric wire being formed to cover the conductor; and 
     a sheath that covers the core wire, wherein 
     a cross-sectional area of the conductor of the first insulated electric wire is 1.2 mm 2  to 3.5 mm 2 , 
     the conductor of the first insulated electric wire is formed of a hard-drawn copper wire, 
     an outer diameter of the insulating layer of the first insulated electric wire is 2.0 mm to 3.6 mm, 
     a cross-sectional area of the conductor of the second insulated electric wire is 0.13 mm 2  to 0.75 mm 2 , 
     an outer diameter of the insulating layer of the second insulated electric wire is 1.0 mm to 2.2 mm, and 
     the core wire is formed by twisting a second unit with two first insulated electric wires, the second unit being formed by twisting two second insulated electric wires. 
     According to the present invention, a multicoaxial cable capable of realizing both reduction in diameter and high bending resistance can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a configuration of an insulated electric cable according to a first embodiment of the present invention; 
         FIG. 2  is a view illustrating a schematic configuration of a manufacturing device of manufacturing the insulated electric cable according to the first embodiment of the present invention; 
         FIG. 3  is a cross-sectional view illustrating a configuration of an insulated electric cable according to a second embodiment of the present invention; 
         FIG. 4  is a cross-sectional view illustrating a configuration of an insulated electric cable according to a third embodiment of the present invention; and 
         FIG. 5  is a schematic view illustrating an example of a bend test method. 
     
    
    
     DETAILED DESCRIPTION 
     Summary of Embodiment of Present Invention 
     First, the summary of an embodiment of the present invention will be described. 
     (1) A multicoaxial cable according to an embodiment of the present invention includes: 
     a core wire that includes a first unit formed by twisting two first insulated electric wires, each of the first insulated electric wires including a conductor and an insulating layer, the conductor being formed by stranding a plurality of child stranded conductors, each of the child stranded conductors being formed by stranding a plurality of conductor wires, and the insulating layer being formed to cover the conductor; and 
     a sheath that covers the core wire, wherein 
     a cross-sectional area of the conductor is 1.2 mm 2  to 3.5 mm 2 , 
     the conductor is formed of a hard-drawn copper wire, and 
     an outer diameter of the insulating layer is 2.0 mm to 3.6 mm. 
     The multicoaxial cable having the above-described configuration is a small-diameter cable in which the cross-sectional area of the conductor of the first insulated electric wire and the outer diameter of the insulating layer are in the above-described ranges. The conductor of the first insulated electric wire is formed of a hard-drawn copper wire. Therefore, the bending resistance of the cable can be improved. 
     (2) In addition, a multicoaxial cable according to an embodiment of the present invention includes: 
     a core wire that is formed using a first insulated electric wire and a second insulated electric wire, 
     the first insulated electric wire including a conductor and an insulating layer, the conductor of the first insulated electric wire being formed by stranding a plurality of child stranded conductors, each of the child stranded conductors of the first insulated electric wire being formed by stranding a plurality of conductor wires, and the insulating layer of the first insulated electric wire being formed to cover the conductor, 
     the second insulated electric wire including a conductor and an insulating layer, the conductor of the second insulated electric wire being formed by stranding a plurality of child stranded conductors, each of the child stranded conductors of the second insulated electric wire being formed by stranding a plurality of conductor wires, and the insulating layer of the second insulated electric wire being formed to cover the conductor; and 
     a sheath that covers the core wire, wherein 
     a cross-sectional area of the conductor of the first insulated electric wire is 1.2 mm 2  to 3.5 mm 2 , 
     the conductor of the first insulated electric wire is formed of a hard-drawn copper wire, 
     an outer diameter of the insulating layer of the first insulated electric wire is 2.0 mm to 3.6 mm, 
     a cross-sectional area of the conductor of the second insulated electric wire is 0.13 mm 2  to 0.75 mm 2 , 
     an outer diameter of the insulating layer of the second insulated electric wire is 1.0 mm to 2.2 mm, and 
     the core wire is formed by twisting a second unit with two first insulated electric wires, the second unit being formed by twisting two second insulated electric wires. 
     According to this configuration, the multicoaxial cable includes the second unit. The second unit is formed by twisting the two second insulated electric wire. In each of the two second insulated electric wires, the cross-sectional area of the conductor is in a range of 0.13 mm 2  to 0.75 mm 2 , and the outer diameter of the insulating layer is in a range of 1.0 mm to 2.2 mm. With the multicoaxial cable including the second unit, multiple systems can be operated using the single cable. Therefore, the convenience of the cable can be improved. In addition, a reduction in diameter and high bending resistance of the multicoaxial cable can be both realized. 
     (3) In addition, in the multicoaxial cable according to (2) described above, 
     the core wire may further include a third unit that is formed by twisting two third insulated electric wires, 
     each of the third insulated electric wires may include a conductor and an insulating layer, 
     the conductor may have a cross-sectional area of 0.13 mm 2  to 0.75 mm 2 , 
     the insulating layer may be formed to cover the conductor and have an outer diameter of 1.0 mm to 2.2 mm, and 
     the core wire may be formed by stranding the first insulated electric wires, the second unit, and the third unit with each other. 
     According to this configuration, the multicoaxial cable includes the third unit. The third unit is formed by the twisting two third insulated electric wires. In each of the third second insulated electric wires, the cross-sectional area of the conductor is in a range of 0.13 mm 2  to 0.75 mm 2 , and the outer diameter of the insulating layer is in a range of 1.0 mm to 2.2 mm. With the multicoaxial cable including the third unit, multiple types of systems can be operated using the single cable. Therefore, the convenience of the cable can be further improved. In addition, a reduction in diameter and high bending resistance of the multicoaxial cable including the first to third units can be both realized. 
     (4) In addition, in the multicoaxial cable according to any one of (1) to (3) described above, 
     the sheath may include a first cover layer and a second cover layer, 
     the first cover layer may cover a periphery of the core wire, and 
     the second cover layer may cover a periphery of the first cover layer. 
     According to this configuration, the sheath is formed of the two cover layers. As a result, the stranding of the core wire does not appear on the sheath. 
     (5) In addition, in the multicoaxial cable according to (4) described above, 
     the first cover layer may be formed of a material that is more flexible than that of the second cover layer. 
     Since the first cover layer is formed of a material that is more flexible than that of the second cover layer, the cable having high flexibility, bending resistance, and wear resistance can be provided. 
     (6) In addition, in the multicoaxial cable according to (4) or (5) described above, 
     the first cover layer may be formed of a foamed material. 
     With this configuration, the bending resistance can be further improved. 
     (7) In addition, the multicoaxial cable according to any one of (1) to (6) described above may further include: 
     a tape member that is disposed between the core wire and the sheath in a state of being wrapped around a periphery of the core wire. 
     According to this configuration, the tape member is disposed between the core wire and the sheath such that the core wire and the sheath are separated from each other. Therefore, by removing the tape member, the core wire and the sheath can be easily separated from each other to expose the core wire. This way, with this configuration, the workability of an operation of taking the core wire out can be improved. 
     (8) In addition, in the multicoaxial cable according to any one of (1) to (6) described above, 
     powder may be applied to the periphery of the core wire, and 
     the periphery of the core wire to which the powder is applied may be covered with the sheath. 
     According to this configuration, the powder is applied to the periphery of the core wire, and the periphery is covered with the sheath. Therefore, the core wire and the sheath can be easily separated from each other to expose the core wire. 
     (9) In addition, in the multicoaxial cable according to any one of (1) to (8) described above, 
     a stranding pitch at which the child stranded conductors may be stranded is less than a twisting pitch at which the two first insulated electric wires are twisted, and 
     the twisting pitch at which the two first insulated electric wires are twisted may be 4 times or less the stranding pitch at which the child stranded conductors are stranded. 
     With this configuration, the bending resistance of the conductor of the first insulated electric wire can be improved while maintaining the productivity of the multicoaxial cable. 
     Details of Embodiment of Present Invention 
     Hereinafter, examples of the embodiments of the multicoaxial cable according to the present invention will be described in detail with respect to the drawings. 
     First Embodiment 
       FIG. 1  is a cross-sectional view illustrating a configuration of an insulated electric cable  10  (an example of the multicoaxial cable) according to a first embodiment of the present invention. The insulated electric cable  10  is used for, for example, an electromechanical brake mounted on a vehicle, and can be used as a cable for supplying electric power to a motor that drives a brake caliper. In particular, the insulated electric cable  10  is used for an electromechanical parking brake (EPB). 
     As illustrated in  FIG. 1 , the insulated electric cable  10  includes: a core wire  1 ; a tape  6  (an example of the tape member) that is wrapped around the core wire  1 ; and a sheath  7  (an example of the sheath) that covers an outer periphery of the tape  6  wrapped around the core wire  1 . The outer diameter of the insulated electric cable  10  according to the example is in a range of 6 to 12 mm and preferably in a range of 7.0 to 10.5 mm. 
     The core wire  1  is formed by twisting two insulated electric wires  2  (an example of the first insulated electric wire) having substantially the same diameter as each other. That is, the core wire  1  includes a main unit  20  (an example of the first unit) that is formed by twisting the two insulated electric wires  2  with each other. Each of the two insulated electric wires  2  includes a conductor  3  and an insulating layer  4  that is formed to cover an outer periphery of the conductor  3 . 
     The conductor  3  is formed of a plurality of (in this example, seven) child stranded conductors  5 . The child stranded conductors  5  substantially the same structure. Each of the child stranded conductors  5  is formed as a stranded wire that is formed by stranding a plurality of conductor wires having an outer diameter of 0.05 to 0.16 mm as hard-drawn copper wires. The conductor  3  is formed as a stranded wire that is formed by stranding a plurality of child stranded conductors (stranded wires)  5 . The number of wires constituting one child stranded conductor  5  is in a range of 16 to 100 and preferably in a range of 30 to 75. The cross-sectional area of the conductor  3  having the above-described configuration (the total cross-sectional area of the wires) is in a range of 1.2 to 3.5 mm 2 . In addition, the outer diameter of the conductor  3  is in a range of 1.3 to 3.0 mm and preferably in a range of 2.0 to 2.6 mm. 
     As the hard-drawn copper wire constituting the conductor  3  according to the embodiment, the hard-drawn copper wire defined according to JIS C 3101-1994 can be used. The hard-drawn copper wire is obtained by drawing copper at a normal temperature, and is not annealed unlike the annealed copper wire. The hard-drawn copper wire refers to a robust copper wire that is difficult to deform, and is distinguished from an annealed copper wire that is easily deformable. That is, the hard-drawn copper wire refers to a copper wire having a higher breaking strength than the annealed copper wire. 
     For example, the insulating layer  4  may be a polyolefin resin such as polyethylene (for example, low-density polyethylene (LDPE), high-density polyethylene (HDPE), or very-low-density polyethylene (VLDPE), or a mixture thereof), polypropylene, an ethylene-ethyl acrylate copolymer (EEA), or an ethylene-vinyl acetate copolymer (EVA), an olefin resin other than a polyolefin resin, a polyurethane resin, a fluororesin (for example, a tetrafluoroethylene-ethylene copolymer), or a compound obtained by mixing at least two kinds of the above-described compounds with each other. The insulating layer  4  may be formed of, for example, a resin material which is imparted with flame retardancy by being mixed with a flame retardant. In addition, a material constituting the insulating layer  4  may be crosslinked. The thickness of the insulating layer  4  is about 0.2 to 0.6 mm. In order to reduce the diameter of the multicoaxial cable  10 , it is preferable that the thickness of the insulating layer  4  is small. However, in order to increase bending resistance and to maintain wear resistance, it is necessary to secure the thickness in a predetermined range. The outer diameter of the insulated electric wire  2  including the insulating layer  4  is in a range of 2.0 to 3.6 mm. From the viewpoint of improving bending resistance, it is preferable that the insulating layer  4  is formed of a flexible resin material. 
     The insulating layer  4  may have a two-layer structure. In this case, from the viewpoint of improving bendability, it is preferable that an inner layer (a layer positioned immediately outside of the conductor  3 ) is formed of a relatively flexible resin having a Young&#39;s modulus of 700 MPa or lower at 25° C. and that an outer layer is formed of a relatively hard resin having a Young&#39;s modulus of higher than 700 MPa at 25° C. 
     A parent stranding pitch of the conductor  3  (a pitch at which the child stranded conductors  5  are stranded) can be set according to the outer diameter of the conductor  3  and the like. The parent stranding pitch of the conductor  3  is, for example, about 20 to 80 mm. In addition, a twisting pitch at which the two insulated electric wires  2  constituting the main unit  20  are twisted can be set according to the outer diameter of the insulated electric wire  2  and the like. The twisting pitch of the two insulated electric wires  2  is, for example, about 40 to 150 mm. In the embodiment, the parent stranding pitch of the conductor  3  is set to be less than the twisting pitch of the two insulated electric wires  2 , and the twisting pitch of the two insulated electric wires  2  is set to be 4 times or less the parent stranding pitch of the conductor  3 . It is preferable that the twisting pitch of the two insulated electric wires  2  is set to be 1.1 times to 3 times the parent stranding pitch of the conductor  3 . As a result, the bending resistance of the conductor  3  can be improved while maintaining the productivity of the insulated electric cable  10 . 
     The tape  6  is helically wrapped around the outer periphery of the core wire  1  and is disposed between the core wire  1  and an inner sheath  8  described below. The thickness of the tape  6  is in a range of 0.01 to 0.1 mm. As a material of the tape  6 , paper or an artificial fiber formed of a resin material such as polyester may be used. In addition, a wrapping method of the tape  6  may be helical wrapping or longitudinal wrapping. In addition, a wrapping direction of the tape  6  may be opposite to a twisting direction of each of the insulated electric wires  2  of the core wire  1 . By setting the wrapping direction of the tape  6  and the twisting direction of the insulated electric wires  2  to be opposite to each other, the surface of the tape  6  wrapped around the periphery of the core wire  1  is not likely to be uneven, and the outer diameter of the insulated electric cable  10  is likely to be stable. 
       FIG. 1  illustrates a state where the tape  6  is wrapped around the periphery of the core wire  1 . However, the tape is not necessarily wrapped around the periphery of the core wire  1 . The tape  6  is not necessary as long as the sheath  7  can be easily removed to take out the core wire  1 . Instead of the tape  6 , a release agent (for example, powder such as talc) may be interposed between the core wire  1  and the sheath  7 . 
     The sheath  7  has a two-layer structure including an inner sheath  8  (an example of the first cover layer) and an outer sheath  9  (an example of the second cover layer), and is formed to cover the core wire  1  around which the tape  6  is wrapped (hereinafter, also referred to as “tape-wrapped core wire  100 ”). 
     The inner sheath  8  is formed to be extruded on the outer periphery of the tape-wrapped core wire  100  such that the tape-wrapped core wire  100  is covered. As a material constituting the inner sheath  8 , a material having high flexibility is preferable. For example, the material constituting the inner sheath  8  may be a polyolefin resin such as EEA, EVA, polyethylene (for example, very-low-density polyethylene (VLDPE)), polyurethane (for example, a thermoplastic polyurethane (TPU)), a polyurethane elastomer, a polyester elastomer, or a compound obtained by mixing at least two kinds of the above-described compounds with each other. The material constituting the inner sheath  8  may be crosslinked. For example, in a case where a flexible polyolefin resin such as EEA is used, heat resistance (for example, up to 150° C.) required for use in a vehicle can be obtained by crosslinking the resin material. In addition, in order to improve bending resistance, the material constituting the inner sheath  8  may be caused to foam. The thickness of the inner sheath  8  is about 0.2 to 1.0 mm. The outer diameter of the inner sheath  8  is in a range of 6.0 to 11.0 mm and preferably in a range of 7.3 to 9.3 mm. 
     The outer sheath  9  is formed to be extruded on an outer periphery of the inner sheath  8  such that the outer periphery of the inner sheath  8  is covered. As a material constituting the outer sheath  9 , a material having high heat resistance and wear resistance is preferable. As the material constituting the outer sheath  9 , for example, a flame-retardant polyurethane resin may be used. The polyurethane constituting the outer sheath  9  may be crosslinked in order to improve heat resistance. As described above, the outer diameter of the outer sheath  9 , that is, the outer diameter of the insulated electric cable  10  is about 6 to 11 mm. 
     The inner sheath  8  and the outer sheath  9  may be the same material. In this case, the sheath  7  having a two-layer structure has the same appearance as that of the sheath having a single-layer structure. By extruding the same material twice, the outer diameter of the insulated electric cable  10  is likely to be uniform in a length direction thereof. 
     Next, a method of manufacturing the insulated electric cable  10  will be described.  FIG. 2  illustrates a schematic configuration of a manufacturing device  11  for manufacturing the insulated electric cable  10 . As illustrated in  FIG. 2 , the manufacturing device  11  includes two insulated electric wire supply reels  12 , a stranding portion  13 , a tape supply reel  14 , a tape wrapping portion  15 , an inner sheath extruding portion  16 , an outer sheath extruding portion  17 , a cooling portion  18 , and a cable wrapping reel  19 . 
     The insulated electric wire  2  is wrapped around each of the two insulated electric wire supply reels  12 , and the two insulated electric wires  2  are supplied to the stranding portion  13 . In the stranding portion  13 , the supplied two insulated electric wires  2  are twisted with each other to form the core wire  1 . The core wire  1  is transported to the tape wrapping portion  15 . 
     In the tape wrapping portion  15 , the core wire  1  transported from the stranding portion  13  and the tape  6  supplied from the tape supply reel  14  are joined together, and the tape  6  is helically wrapped around the outer periphery of the core wire  1  to form the tape-wrapped core wire  100 . This tape-wrapped core wire  100  is transported to the inner sheath extruding portion  16 . In a case where the tape  6  is not wrapped around the outer periphery of the core wire  1 , the process and the device (tape wrapping portion  15 ) are not necessary. In a case where another release agent, for example, talc is interposed between the core wire  1  and the sheath  7  instead of the tape  6 , a talc applying device is provided instead of the tape wrapping portion  15  so as to apply talc to the core wire  1  when the core wire  1  passes through the talc applying device. 
     The inner sheath extruding portion  16  is connected to a storage portion  16   a  where the resin material is stored. In the inner sheath extruding portion  16 , the resin material supplied from the storage portion  16   a  is extruded on an outer periphery of tape-wrapped core wire  100 . This way, the inner sheath  8  is formed to cover the outer periphery of the tape-wrapped core wire  100 . The tape-wrapped core wire  100  covered with the inner sheath  8  is transported to the outer sheath extruding portion  17 . 
     The outer sheath extruding portion  17  is connected to a storage portion  17   a  where the resin material is stored. In the outer sheath extruding portion  17 , the resin material supplied from the storage portion  17   a  is extruded on the outer periphery of the inner sheath  8  formed by the inner sheath extruding portion  16 . This way, the outer sheath  9  is formed to cover the outer periphery of the inner sheath  8 , and the insulated electric cable  10  covered with the sheath  7  having a two-layer structure including the inner sheath  8  and the outer sheath  9  is formed. The insulated electric cable  10  is transported to the cooling portion  18  such that the sheath  7  is cooled and cured. Next, the insulated electric cable  10  is transported to the cable wrapping reel  19  and wrapped therearound. 
     Incidentally, for example, in an insulated electric cable that is used as a power line of an electromechanical brake of a vehicle, in order to allow a device such as an electromechanical brake to which power is supplied to reliably operate, it is necessary that a resistance value of a conductor is set to be a predetermined value or lower. Therefore, in a configuration of the related art in which a copper alloy wire is used as the conductor included in the insulated electric wire, in order to suppress the resistance value of the conductor, it is necessary that the diameter of the conductor is a predetermined value or more, and there is a room for improvement to realize reduction in diameter. 
     On the other hand, as described above, the insulated electric cable  10  according to the embodiment includes: the core wire  1  that is formed by twisting the two insulated electric wires  2 ; and the sheath  7  that is formed to cover the core wire  1 , in which each of the insulated electric wires  2  includes the conductor  3  and the insulating layer  4 , the conductor  3  is formed by stranding a plurality of child stranded conductors  5 , and the insulating layer  4  is formed to cover the conductor  3 . In the multicoaxial cable  10 , the cross-sectional area of the conductor  3  is 1.2 mm 2  to 3.5 mm 2 , the conductor  3  is formed of a hard-drawn copper wire, and the outer diameter of the insulating layer  4  is 2.0 mm to 3.6 mm. This way, in the insulated electric cable  10  according to the embodiment, the conductor  3  is formed of a hard-drawn copper wire having a higher breaking strength than an annealed copper wire. Therefore, in the insulated electric wire  2  (and the insulated electric cable  10  including the insulated electric wire  2 ), the cross-sectional area of the conductor  3  and the outer diameter of the insulating layer  4  are in the above-described ranges such that reduction in diameter can be realized, and the bending resistance of the insulated electric cable  10  can be improved compared to that of the related art in which the annealed copper wire is used. 
     In addition, the sheath  7  included in the insulated electric cable  10  according to the embodiment includes: the inner sheath  8  that covers the periphery of the core wire  1 ; and the outer sheath  9  that covers the periphery of the inner sheath  8 . This way, by configuring the sheath  7  to have the two-layer structure including the inner sheath  8  and the outer sheath  9 , the shape of a cross-section (a cross-section perpendicular to a cable length direction) of the insulated electric cable  10  can be made to be fixed along the cable length direction. 
     Since the inner sheath  8  is formed of a material that is more flexible than the outer sheath  9 , the insulated electric cable  10  having high flexibility, bending resistance, and wear resistance can be provided. 
     In addition, the insulated electric cable  10  according to the embodiment further includes the tape  6  that is disposed between the core wire  1  and the sheath  7  in a state where the tape  6  is wrapped around the periphery of the core wire  1 . This way, the tape  6  is disposed between the core wire  1  and the sheath  7 , that is, is disposed such that the core wire  1  and the sheath  7  are separated from each other. As a result, by removing the tape  6 , the core wire  1  and the sheath  7  can be easily separated from each other to expose the core wire  1 . Thus, the workability of an operation of taking the core wire  1  (each of the insulated electric wires  2 ) out can be improved. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIG. 3 . Components having the same configurations as those of the first embodiment are represented by the same reference numerals, and the description thereof will not be repeated.  FIG. 3  illustrates a cross-section of an insulated electric cable  30  according to the second embodiment. The insulated electric cable  30  according to the embodiment can be used not only for supplying electric power to an electromechanical brake (for example, an electromechanical parking brake) but also for other uses, for example for transmitting an electric signal from a wheel speed sensor. In addition, the insulated electric cable  30  may be used for transmitting signals from other devices to a vehicle electronic control unit (ECU) or for transmitting signals from a vehicle ECU to devices. 
     As illustrated in  FIG. 3 , the insulated electric cable  30  according to the example is different from the first embodiment, in that a core wire  1 A includes a sub-unit  31  (an example of the second unit) for transmitting a signal for, for example, a wheel speed sensor in addition to the two insulated electric wires  2  (main unit  20 ). 
     The sub-unit  31  is formed by twisting two insulated electric wires  32  (an example of the second insulated electric wire) having a smaller diameter than the insulated electric wire  2  constituting the main unit  20  and having substantially the same diameter as each other. Each of the two insulated electric wires  32  includes a conductor  33  and an insulating layer  34  that is formed to cover an outer periphery of the conductor  33 . 
     The conductor  33  is a stranded wire that is formed by stranding a plurality of conductor wires formed of, for example, a copper alloy wire. The outer diameter of the wire is, for example, 0.05 to 0.15 mm, and the number of wires constituting one conductor  33  is about 40 to 80 and preferably 50 to 70. In addition, the cross-sectional area of the conductor  33  having the above-described configuration is in a range of 0.13 to 0.75 mm 2  and preferably in a range of 0.2 to 0.5 mm 2 . In addition, the outer diameter of the conductor  33  is in a range of 0.5 to 1.0 mm. A material constituting the conductor  33  is not limited to the copper alloy wire, and any material having a predetermined conductivity and flexibility such as a tin-plated annealed copper wire or an annealed copper wire may be used. In addition, as the material constituting the conductor  33 , a hard-drawn copper wire may be used as in the case of the conductor  3 . 
     The insulating layer  34  is formed of, for example, a polyolefin resin. It is preferable that the insulating layer  34  is flame-retardant. In addition, the insulating layer  34  may be a crosslinked resin. The thickness of the insulating layer  34  is about 0.2 to 0.4 mm, and the outer diameter of the insulating layer  34  is about 1.2 to 1.6 mm. The insulating layer  34  may be formed of the same material as that of the insulating layer  4  of the insulated electric wire  2 . For example, the insulating layer  34  may be formed of another material such as a fluororesin or polyurethane. 
     The sub-unit  31  having the above-described configuration and the two insulated electric wires  2  are bunch twisted to form the core wire  1 A. A twisting pitch of the core wire  1 A (the two insulated electric wires  2  and the sub-unit  31 ) may be in the same range as that of the core wire  1 . The tape  6  is wrapped around an outer periphery of the core wire  1 A, and the inner sheath  8  and the outer sheath  9  are extruded on an outer periphery of the tape  6 . As a result, the insulated electric cable  30  is formed. The tape  6  is not necessarily provided, and another release agent may be interposed between the core wire  1 A and the sheath  7  instead of the tape  6 . 
     As described above, the core wire  1 A of the insulated electric cable  30  according to the second embodiment includes the sub-unit  31 , and the sub-unit  31  is formed by twisting the two insulated electric wires  32  in which the cross-sectional area of the conductor  33  is in a range of 0.13 mm 2  to 0.75 mm 2 . The sub-unit  31  is twisted with the two insulated electric wires  2  to form the core wire  1 A. It is preferable that the diameter of a twisted wire of the two insulated electric wires  32  is substantially the same (0.85 times to 1.15 times) as the diameter of one insulated electric wire  2 . It is preferable that the twisted wire of the insulated electric wires  32  and the two insulated electric wires  2  are disposed in an isosceles triangular shape or an equilateral triangular shape as in a cross-section illustrated in  FIG. 3 . As a result, the combination shape of the core wire  1 A is stable in the cable length direction, and the external shape (a circular cross-section in the length direction) of the insulated electric cable  30  is stable in the cable length direction. This way, in the insulated electric cable  30 , the conductor  3  of the insulated electric wire  2  is formed of a hard-drawn copper wire (a copper wire in which the breaking strength is higher than that of the annealed copper wire and in which the resistance value of the conductor is a predetermined or lower despite a small diameter). Therefore, reduction in diameter and high bending resistance of the insulated electric cable  30  can be both realized. In addition, the insulated electric cable  30  including the core wire  1 A can be used not only as a power line used for an electromechanical brake but also as, for example, a four-core insulated electric cable including a signal line through which an electric signal of a sensor or the like is transmitted. This way, with the insulated electric cable  30  according to the second embodiment, two types of systems can be operated using the single cable. Therefore, the convenience of the cable can be improved. 
     Third Embodiment 
     Next, a third embodiment of the present invention will be described with reference to  FIG. 4 . Components having the same configurations as those of the first embodiment and the second embodiment are represented by the same reference numerals, and the description thereof will not be repeated.  FIG. 4  illustrates a cross-section of an insulated electric cable  40  according to the third embodiment. 
     As illustrated in  FIG. 4 , the insulated electric cable  40  according to the example is different from the second embodiment, in that the core wire  1 B includes a sub-unit  41  (for example, an example of the third unit) in addition to the two insulated electric wires  2  constituting the main unit  20  and the sub-unit  31 . 
     The sub-unit  41  is formed by twisting two insulated electric wires  42  (an example of the third insulated electric wire) having a smaller diameter than the insulated electric wire  2  and having substantially the same diameter as each other. Each of the two insulated electric wires  42  includes a conductor  43  and an insulating layer  44  that is formed to cover an outer periphery of the conductor  43 . The configurations of the conductor  43  and the insulating layer  44  of the insulated electric wire  42  are substantially the same as the configurations of the conductor  33  and the insulating layer  34  of the insulated electric wire  32  of the sub-unit  31 , and thus the detailed description thereof will not be repeated. 
     The sub-unit  41  having the above-described configuration, the two insulated electric wires  2 , and the sub-unit  31  are bunch twisted to form the core wire  1 B. A twisting pitch of the core wire  1 B (the two insulated electric wires  2  and the sub-units  31  and  41 ) may be in the same range as that of the core wire  1  or  1 A. The tape  6  is wrapped around an outer periphery of the core wire  1 B, and the inner sheath  8  and the outer sheath  9  are extruded on an outer periphery of the tape  6 . As a result, the insulated electric cable  40  is formed. The third embodiment is the same as the first embodiment or the second embodiment, in that the tape  6  is not necessarily provided and another release agent may be used instead of the tape  6 . 
     It is preferable that the sub-unit  31  and the sub-unit  41  are not adjacent to each other and, as illustrated in  FIG. 4 , are disposed opposite to each other when seen from the two insulated electric wires  2 . As a result, the combination shape of the core wire  1 B is stable in the cable length direction, and the external shape of the insulated electric cable  40  is stable in the cable length direction. 
     As described above, the core wire  1 B of the insulated electric cable  40  according to the third embodiment includes the sub-unit  41  in addition to the sub-unit  31 , and the sub-unit  41  is formed by twisting the two insulated electric wires  42  in which the cross-sectional area of the conductor  43  is in a range of 0.13 to 0.75 mm 2 . The sub-unit  41  is stranded with the main unit  20  and the sub-unit  31  to form the core wire  1 B. This way, in the insulated electric cable  40 , the conductor  3  of the insulated electric wire  2  included in the main unit  20  is formed of a hard-drawn copper wire. Therefore, reduction in diameter and high bending resistance of the insulated electric cable  40  can be both realized. In addition, the insulated electric cable  40  including the core wire  1 B can be used not only as a power line used for an electromechanical brake but also as, for example, a six-core insulated electric cable including a signal line through which an electric signal of a sensor or the like is transmitted. This way, multiple types of systems can be operated using the single cable. Therefore, the convenience of the cable can be improved. 
     The present invention is not limited to the above-described first to third embodiments, and appropriate modifications, improvements, and the like can be made. In addition, the materials, dimensions, numerical values, forms, numbers, disposition positions, and the like of the respective components in the embodiments are arbitrary and are not limited as long as the present invention can be achieved. 
     The insulating layer  4  of the insulated electric wire  2  constituting the main unit  20  may be formed of one resin layer or two resin layers. In order to improve bending resistance, it is preferable that the insulating layer  4  is formed of two resin layers (an inner layer is formed of a resin that is more flexible than an outer layer). In addition, the insulating layer  34  of the insulated electric wire  32  constituting the sub-unit  31  and the insulating layer  44  of the insulated electric wire  42  constituting the sub-unit  41  may also be formed of one layer or two layers. In a case where the insulating layer  4 ,  34 , or  44  is formed of two layers, an inner layer is formed of a relatively flexible material, and an outer layer is relatively hard material. The inner layer can be formed of, for example, a copolymer of ethylene and an a olefin having a carbonyl group, such as EEA, EVA, or EMA, or very-low-density polyethylene The outer layer can be formed of, for example, polyolefin. 
     In addition, in the description of the example of the first embodiment to the third embodiment, the sheath  7  is formed of two layers including the inner sheath  8  and the outer sheath  9 . However, the present invention is not to the examples. For example, the sheath  7  may be formed of only the outer sheath  9  (that is, the sheath  7  may be formed of only one cover layer). In a case where the sheath formed of one layer is required to have wear resistance, it is preferable that the sheath is formed of polyurethane. In a case where high wear resistance is not required, the sheath may be formed of polyethylene (particular preferably, high-density polyethylene), polypropylene, or polyvinyl chloride (preferably, hard polyvinyl chloride). 
     In addition, in the second embodiment and the third embodiment, the sub-unit  31  or  41  is formed by twisting the two insulated electric wires  32  or  42 . However, the present invention is not limited to this example. For example, the sub-unit may be formed by extruding a cover material on the periphery of the insulated electric wire  32  or  42  to cover the periphery. As a result, in a case where the sub-unit  31  or  41  is connected to a connection destination such as a vehicle sensor, molding can be performed without a gap. As the cover material that covers the periphery of the insulated electric wire  32  or  42 , for example, polyurethane, polyethylene, or other polyolefin resins may be used. The cover material that covers the periphery of the insulated electric wire  32  or  42  may be formed of two layers. In a case where the cover material is formed of two layers, an inner layer and an outer layer may be formed of different materials or the same material. The inner layer may be formed of a flexible resin (having a relatively low Young&#39;s modulus), and the outer layer may be formed of a hard resin (having a relatively high Young&#39;s modulus). The cover material may be crosslinked. in addition, a shield layer may be provided around the periphery of the sub-unit  31  or  41 . As the shield layer, a braid formed of a thin metal wire (a copper alloy wire, an annealed copper wire, or a hard-drawn copper wire) may be used, the thin metal wire may be helically wrapped around the periphery of the sub-unit  31  or  41 , or a metal tape (a metal tape may adhere to a resin tape, or a metal may be deposited on a resin tape) may be wrapped around the periphery of the sub-unit  31  or  41 . The metal tape may be used in combination with drain wire. 
     Next, examples of the present invention will be described. The following cables according to Examples 1 to 6 and Comparative Examples 1 to 6 were prepared, and a bending test was performed using each of the cables. 
     Example 1 
     In Example 1, 50 conductor wires having an outer diameter of 0.08 mm which were formed of a hard-drawn copper wire having a higher breaking strength than the annealed copper wire were stranded to form a child stranded conductor (stranded wire)  5 , and  7  child stranded conductors  5  were stranded to form a conductor  3  having an outer diameter 1.9 mm as a stranded wire. The outer periphery of the conductor  3  was covered with the insulating layer  4  formed of polyethylene to form the insulated electric wire  2  having an outer diameter of 2.7 mm. Two insulated electric wires  2  were twisted to form the core wire  1  (twisted pair). The outer periphery of the core wire  1  was covered with the sheath  7  (having a two-layer structure including the inner sheath  8  and the outer sheath  9 ; both the inner sheath  8  and the outer sheath  9  are formed of polyurethane) formed of polyurethane. As a result, the two-core insulated electric cable  10  having an outer diameter 7.7 mm was prepared. The thickness (thickness of the thinnest portion) of the sheath  7  was 1.15 mm. The stranding pitch of the child stranded conductor  5  was 38 mm (parent stranding), and the twisting pitch of the core wire  1  was 85 mm. The allowable current of the insulated electric wire  2  was 9.7 mΩ/m. 
     Example 2 
     16 annealed copper wires having an outer diameter of 0.08 mm were stranded to form a child stranded wire, and 3 child stranded wires are stranded to form a twisted wire. This twisted wire was covered with polyethylene to prepare an insulated electric wire having an outer diameter of 1.4 mm. Two insulated electric wires were twisted to form the sub-unit  31 . The sub-unit  31  and the two insulated electric wires  2  (as in the case of Example 1) were twisted to form the core wire  1 A. The outer periphery of the core wire  1 A was covered with the sheath  7  (having a two-layer structure including the inner sheath  8  and the outer sheath  9 ; both the inner sheath  8  and the outer sheath  9  are formed of polyurethane) formed of polyurethane. As a result, the four-core insulated electric cable  30  (having an outer diameter of 8.6 mm) was prepared. The thickness (thickness of the thinnest portion) of the sheath  7  was 1.15 mm. The stranding pitch of the conductor (the child stranded conductor  5 ) and the twisting pitch of the core wire  1 A were the same as those of Example 1. 
     Example 3 
     The sub-unit  41  having the same configuration as that of the sub-unit  31  was prepared. The two insulated electric wires  2 , the sub-unit  31 , and the sub-unit  41  were stranded to form the core wire  1 B. The outer periphery of the core wire  1 B was covered with the sheath  7  (having the same configuration as that of Examples 1 and 2) formed of polyurethane. As a result, the six-core insulated electric cable  40  (having an outer diameter of 9.3 mm) was prepared. The thickness (thickness of the thinnest portion) of the sheath  7  was 1.15 mm. The stranding pitch of the conductor (the child stranded conductor  5 ) and the stranding pitch of the core wire  1 A were the same as those of Example 1. 
     Example 4 
     A two-core insulated electric cable was prepared, the cable having the same configuration as that of Example 1 except that the insulating layer of the insulated electric wire (electric wire corresponding to the insulated electric wire  2 ) was formed of two layers. In the insulating layer formed of two layers, an inner layer (a layer adjacent to the outer periphery of the conductor) was formed of EVA (a relatively flexible resin), and an outer layer was formed of polyethylene (a relatively hard resin). The total thickness of the insulating layer was the same as the thickness of the insulating layer formed of one layer according to Example 1. 
     Example 5 
     A four-core insulated electric cable was prepared, the cable having the same configuration as that of Example 2 except that the insulating layer of the insulated electric wire (electric wire corresponding to the insulated electric wire  2 ) was formed of two layers. In the insulating layer formed of two layers, an inner layer (a layer adjacent to the outer periphery of the conductor) was formed of EVA (a relatively flexible resin), and an outer layer was formed of polyethylene (a relatively hard resin). The total thickness of the insulating layer was the same as the thickness of the insulating layer formed of one layer according to Example 1 (the same shall be applied to Example 2). 
     Example 6 
     A six-core insulated electric cable was prepared, the cable having the same configuration as that of Example 3 except that the insulating layer of the insulated electric wire (electric wire corresponding to the insulated electric wire  2 ) was formed of two layers. In the insulating layer formed of two layers, an inner layer (a layer adjacent to the outer periphery of the conductor) was formed of EVA (a relatively flexible resin), and an outer layer was formed of polyethylene (a relatively hard resin). The total thickness of the insulating layer was the same as the thickness of the insulating layer formed of one layer according to Example 1 (the same shall be applied to Example 3). 
     Comparative Example 1 
     A two-core insulated electric cable was prepared, the cable having the same configuration as that of Example 1 except that the conductor of the insulated electric wire was formed of an annealed copper wire instead of the hard-drawn copper wire. 
     Comparative Example 2 
     A four-core insulated electric cable was prepared, the cable having the same configuration as that of Example 2 except that the conductor of the insulated electric wire was formed of an annealed copper wire instead of the hard-drawn copper wire. 
     Comparative Example 3 
     A six-core insulated electric cable was prepared, the cable having the same configuration as that of Example 3 except that the conductor of the insulated electric wire was formed of an annealed copper wire instead of the hard-drawn copper wire. 
     Comparative Example 4 
     60 wires having an outer diameter of 0.08 mm which were formed of a copper alloy wire were stranded to form a child stranded conductor (stranded wire)  5 , and  7  child stranded conductors  5  were stranded to form a conductor having an outer diameter 2.1 mm as a stranded wire. The outer periphery of the conductor was covered with an insulating layer formed of polyethylene to form an insulated electric wire having an outer diameter of 2.9 mm. Two insulated electric wires were twisted to form a core wire (twisted pair), and the outer periphery of the core wire was covered with a sheath formed of polyurethane. As a result, an insulated electric cable having an outer diameter of 8.2 mm was prepared. The thickness (thickness of the thinnest portion) of the sheath was 1.15 mm. The outer diameter of the insulated electric cable was larger than that of Example 1 by 6%. 
     Comparative Example 5 
     Two insulated electric wires having the same configuration as that of Comparative Example 4 and a sub-unit having the same configuration as that of Example 2 were stranded and covered with a sheath having the same configuration as that of Example 2. As a result, a four-core insulated electric cable was prepared. The thickness (thickness of the thinnest portion) of the sheath was 1.15 mm. The outer diameter of the insulated electric cable was 9.2 mm which was larger than that of Example 2 by 7%. 
     Comparative Example 6 
     Two insulated electric wires having the same configuration as that of Comparative Example 4 and two sub-units having the same configuration as that of Example 3 were twisted and covered with a sheath having the same configuration as that of Example 3. As a result, a six-core insulated electric cable was prepared. The thickness (thickness of the thinnest portion) of the sheath was 1.15 mm. The outer diameter of the insulated electric cable was 10.0 mm which was larger than that of Example 3 by 7%. 
     In Comparative Examples 4 to 6, the diameter of the child stranded conductor formed of a copper alloy wire is larger than that of the child stranded conductor formed of the hard-drawn copper wire in Example 1. Therefore, the stranding pitch of the child stranded conductor was 45 mm, and the twisting pitch of the core wire was 85 mm. 
     In the insulated electric cables according to Comparative Examples 4 to 6, the allowable current of the insulated electric wire was 9.8 mΩ/m. 
     Bending Test 
     The bending resistance of the insulated electric cable was evaluated based on the result of a bending test defined according to ISO 14572:2011 (E)  5 . 9 . In the bending test, as illustrated in  FIG. 5 , a cable C is caused to pass through a pair of mandrels  61  (the diameter of the mandrel  61  was 40 mm), the cable C was lowered, an upper end of the cable C was held by a chuck  62 , and a 2 kg weight  63  was attached to a lower end of the cable C. In an environment of −30° C., by swinging the chuck  62  like a pendulum along a circumference centering on a gap between the mandrels  61 , the cable C was repeatedly bent to the respective mandrels  61  side in a range of −90° to +90°. The number of times of bending was counted until the conductor of the insulated electric wire (the first insulated electric wire) constituting the cable C broke (a decrease rate of the resistance value of the conductor exceeded 5%). In a case where the cable C started moving from the vertical state and returned to the vertical state again after being bent in a range of +90° to − 90 °, the number of times of bending was increased by one. 
     Test Result 
     In Example 1, the conductor  3  of the two-core insulated electric cable  10  did not break even after performing the bending test 70,000 times. In addition, in each of the insulated electric cables according to Examples 2 and 3, the conductor  3  did not break even after performing the bending test 70,000 times. In each of the insulated electric cables according to Examples 4 to 6, the conductor  3  did not break even after performing the bending test 200,000 times. In Examples 4 to 6, the insulating layer was formed of two layers, the inner layer was formed of a relatively flexible resin, and the outer layer was formed of a relatively hard resin. As a result, the bendability was further improved. 
     On the other hand, in Comparative Example 1, the conductor of the two-core insulated electric cable broke when the number of times of bending was less than 10,000. In addition, in each of the insulated electric cables according to Comparative Examples 2 and 3, the conductor broke when the number of times of bending was less than 10,000. As a result, it was found that the bending resistance of Examples 1 to 6 was higher than that of Comparative Examples 1 to 3. 
     In Comparative Example 4, the conductor of the two-core insulated electric cable did not break even after performing the bending test 100,000 times or more. In addition, in each of the insulated electric cables according to Comparative Examples 5 and 6, the conductor did not break even after performing the bending test 100,000 times or more. As a result, it was found that the bending resistance of Examples 1 to 6 and Comparative Examples 4 to 6 was high. As described above, the outer diameter of the insulated electric cable  10  according to each of Examples 1 to 6 was reduced by 6 to 7% compared to that of each of Comparative Examples 4 to 6. 
     It was found from the above-described results that, in Examples 1 to 6, reduction in diameter and high bending resistance of the cable were able to be achieved.