Patent Publication Number: US-2015083458-A1

Title: Multi-core cable

Description:
TECHNICAL FIELD 
     The present invention relates to a multi-core cable for assembling plural small-diameter electric wires used in a medical device, a measuring device, etc. 
     BACKGROUND ART 
     A multi-core cable for bundling plural small-diameter insulated electric wires or coaxial electric wires (hereinafter called small-diameter electric wires) is used for a cable, which is to be twisted and bent, for a measuring device or a medical device of an ultrasonograph etc. This kind of known cable is a multi-core cable in which plural small-diameter electric wires are stranded to form one unit and the plural units are further bundled by a wrapping tape etc. to form an electric wire assembly (also called an assembled core) and an outer periphery of the electric wire assembly is covered with a shielding layer made of braided conductive wires and the outside of the shielding layer is coated with a sheath (for example, see Patent Document 1). 
     PRIOR ART DOCUMENT   
     Patent Document   
     Patent Document 1: JP-A-11-329094 
     DISCLOSURE OF THE INVENTION   
     Problems That the Invention Is to Solve   
     In the multi-core cable disclosed in Patent Document 1 described above, a metal thin wire is wound on a fiber core to forma shielding strand and the shielding strands are braided to form a shielding layer and thereby, the multi-core cable has high flexibility and good shielding characteristics can be maintained stably for a long time without causing a break in the cable or a fray in the shielding layer due to bends or tension. 
     However, the multi-core cable disclosed in Patent Document 1 only checks up to bend for 100 turns. Also, in the case of braided normal tin-plated copper alloy strands to form the shielding layer, when the number of bends exceeds 200000, the strands become worn to turn black and become brittle to fracture. 
     In recent years, good twisting resistance as well as better bending resistance and tensile strength is required for a cable, which is to be twisted and bent, for a measuring device or a medical device of an ultrasonograph etc. 
     The invention has been implemented in view of the actual circumstances described above, and provides a small-diameter multi-core cable having better twisting resistance and bending resistance. 
     SUMMARY OF THE INVENTION 
     Means for Solving the Problems   
     A multi-core cable of the invention capable of solving the problem described above is the multi-core cable in which plural electric wire units made of stranded plural small-diameter electric wires are bundled to form an electric wire assembly and a shielding layer made of braided conducting wires is arranged on an outer periphery of the electric wire assembly and an outside of the shielding layer is covered with a sheath made of resin. The multi-core cable is characterized in that the conducting wire of the braided shielding layer is made of copper alloy wires given silver plating and a diameter of the conducting wire is 0.04 mm to 0.15 mm and a conductor elongation rate of the conducting wire is 0.8% or more and a thickness of the silver plating is 0.6 μm or more. 
     In the multi-core cable of the invention, the diameter of the conducting wire is preferably 0.08 mm to 0.15 mm. 
     Also, in the multi-core cable of the invention, a thickness of the sheath is preferably 0.6 mm to 1.0 mm and a distance between the electric wire assembly and an inner surface of the sheath is preferably 0.1 mm to 0.5 mm. 
     Advantage of the Invention   
     The multi-core cable according to the invention described above can withstand more than 550000 twists and bends, and has high long-term reliability, and can better improve noise resistance with respect to shielding characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagram showing an outline of a multi-core cable according to the invention. 
         FIG. 1B  is a diagram showing a cross section of the multi-core cable. 
         FIG. 2  is a diagram describing a twisting and bending test method of the multi-core cable. 
         FIG. 3  is a table showing an evaluation result of the multi-core cable according to the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Mode for Carrying Out the Invention   
     An embodiment of the invention will be described with reference to  FIG. 1 . A multi-core cable according to the invention is used in, for example, connection of a medical device etc., such as an ultrasonograph etc.  FIG. 1A  is a diagram showing an outline of the multi-core cable, and  FIG. 1B  is a diagram showing a cross section of the cable. In  FIG. 1 , numeral  10  shows a multi-core cable, and numeral  11  shows a small-diameter electric wire, and numeral  12  shows an electric wire unit, and numeral  13  shows a wrapping tape, and numeral  14  shows an electric wire assembly, and numeral  15  shows a shielding layer, and numeral  15   a  shows a conducting wire, and numeral  16  shows a sheath. 
     The multi-core cable  10  is constructed by, for example, twisting the plural small-diameter electric wires  11  to form the electric wire unit  12  made of a predetermined number of small-diameter electric wires and further assembling and bundling the plural electric wire units  12  using a wrapping tape  13  etc. to form the electric wire assembly  14  (also called an electric wire core) as shown in  FIG. 1A . Then, the shielding layer  15  made of braided conducting wires  15   a  is arranged on an outer periphery of the electric wire assembly  14  bundled by the wrapping tape  13 , and the outside of the shielding layer  15  is covered with the sheath (also called an outer coat)  16  made of resin. 
     The small-diameter electric wire  11  is made of an insulated electric wire in which a conductor is coated with an insulator or a coaxial electric wire in which the outside of an insulated electric wire is covered with an outer conductor and the outside of the outer conductor is covered with a jacket and, for example, the small-diameter electric wire with an electric wire outside diameter of 0.35 mm or less is used. When the small-diameter electric wire  11  is the insulated electric wire, a conductor with thinner than that of AWG32 is used in a signal conductor, and when the small-diameter electric wire  11  is the coaxial electric wire, a conductor with thinner than that of AWG40 is used in the signal conductor. 
     The electric wire unit  12  includes one or both of these small-diameter insulated electric wires and coaxial electric wires and is obtained by twisting the plural electric wires. 
     The plural electric wire units  12  are mutually twisted or not twisted, and are bundled and assembled by being wrapped using the wrapping tape  13  such as a fluorine resin to form the electric wire assembly  14 . The shielding layer  15  is arranged on the outer periphery of the electric wire assembly  14 , and a noise signal from the outside is prevented from entering the cable and also, a noise signal of the outside is prevented from being diffused from the inside of the cable. 
     In the invention, the shielding layer  15  described above is formed by braiding the conducting wires  15   a  in which a copper alloy wire is given silver plating. The braided shielding layer  15  does not include insulating fiber or the like, for example, resin, and is formed of only the conducting wires  15   a  of the silver-plated copper alloy wires. In addition, the silver plating is softer than the copper alloy wire, and has good lubricity, high-frequency characteristics, low contact resistance, solderability, etc. 
     The copper alloy wire with a diameter of the wire (diameter of the wire) of about 0.04 mm to 0.15 mm and a conductor elongation rate of 0.8% or more is used as the conducting wire  15   a  of the shielding layer  15 . Then, a surface of its copper alloy wire is given silver plating with a thickness of 0.6 μm or more, and the copper alloy wire is braided at, for example, a braid angle of 65° to 80° to form the shielding layer  15 . It is particularly preferable to use the copper alloy wire with the diameter of the wire of 0.08 mm to 0.15 mm as the conducting wire  15   a.    
     The sheath  16  is means for electrically insulating and also mechanically protecting the electric wire assembly  14  on which the shielding layer  15  is arranged. The sheath  16  has a thickness of about 0.6 mm to 1.0 mm, and is formed of a relatively soft resin such as flame-retardant polyethylene or polyvinyl chloride. Also, the sheath  16  can move in a longitudinal direction since the electric wire assembly  14  is relatively loosely covered with the sheath  16 . As a result, a gap G between the electric wire assembly  14  and an inner surface of the sheath  16  is preferably set at 0.1 mm to 0.5 mm on average. 
     In the multi-core cable  10  constructed as described above, as shown by an evaluation result described below, even when the number of twists and bends exceeds 550000 in a twisting and bending test shown in  FIG. 2 , shielding characteristics are well maintained without causing a break in the cable or a fray, and long-term reliability is high. 
       FIG. 2  is a diagram showing a twisting test method of the multi-core cable described above, and a multi-core cable  10 ′ for test with a predetermined length was inserted between a pair of mandrels  21 , and the upper end of the multi-core cable  10 ′ was gripped by a chuck  22  and the lower end of the multi-core cable  10 ′ was drooped by a weight  23  with a load of 5 N. Then, the multi-core cable  10 ′ was bent ±135° like a pendulum to both sides of the pair of mandrels  21  while twisting the chuck  22  ±180° around the axis of the multi-core cable  10 ′. It was tested whether or not a fray or a break in the cable was caused in a shielding layer of the multi-core cable  10  as a result of performing the twists and bends with frequency 10 times per minute. 
       FIG. 3  is a table showing a test result of the multi-core cable by the twisting test described above. In addition, for specimens 1 to 6, a conducting wire of a shielding layer was a silver-plated copper alloy wire and for specimen 7, the conducting wire was a tin-plated copper alloy wire. Also, a diameter of the wire of the conducting wire of the braided shielding layer of the multi-core cable tested was set at 0.08 mm for specimens 1 to 3 and 7, and was set at 0.03 mm for specimen 4, and was set at 0.04 mm for specimen 5, and was set at 0.1 mm for specimen 6. A conductor elongation rate of the conducting wire was set at 0.8% for specimens 1, 2, 4 and 5, and was set at 0.7% for specimen 3, and was set at 2.0% for specimens 6 and 7. A thickness of plating which the conducting wire was given was set at 0.6 μm for specimens 1, 3 to 5 and 7, and was set at 0.5 μm for specimen 2, and was set at 1.2 μm for specimen 6. Also, a distance between the electric wire assembly and an inner surface of the sheath was set at 0.1 mm to 0.5 mm as described in  FIG. 3 . 
     As the above result, 550000 twists and bends can be cleared for specimens 1, 5 and 6, but 400000 twists and bends cannot be cleared for specimens 2 to 4, and 200000 twists and bends cannot be cleared for specimen 7. 
     And, the diameter of the wire of the conducting wire of the braided shielding layer is preferably smaller from the standpoint of reduction in diameter of the cable, but the evaluation result of specimens 4 and 5 described above shows that the diameter of the wire is desirably 0.04 mm or more. When an outside diameter of the cable is too large, handling is not good. In this respect, an outside diameter of the conducting wire of the shielding layer is also limited to a certain level of size, and is preferably 0.15 mm or less. The test result of specimens 1 and 3 shows that the conductor elongation rate of the conducting wire is desirably 0.8% or more and has no upper limit in respect of durability, and the conductor elongation rate is determined by balance of conductivity. 
     Also, from the evaluation result of specimens 1 and 2, the thickness of silver plating of the conducting wire is desirably 0.6 μm or more and has no upper limit from the result of specimen 6, but the sufficient thickness is probably substantially 2 μm or less. 
     Also, the electric wire assembly is loosely covered with the sheath and in order to prevent the conducting wire of the shielding layer from fraying or being broken due to twists and bends of the cable, the distance between the electric wire assembly and the inner surface of the sheath is preferably 0.1 mm or more. However, the distance is desirably set at 0.5 mm or less from the standpoint of reduction in diameter of the cable. 
     The present application is based on Japanese patent application (patent application No. 2012-104446) filed on May 1, 2012, and the contents of the patent application are hereby incorporated by reference.