Abstract:
A fiber cable for helicopter rescue winches includes a plurality of load-bearing synthetic-fiber strands braided with one another, at least one electrically conductive insert, and a wear indicator providing a visual check of a state of the fiber cable, where the load-bearing synthetic-fiber strands are encased in a radial direction by a friction-reducing stable fiber layer, an inner cable jacket, and outer cable jacket.

Description:
Priority is claimed to German Patent Application No. 10 2007 042 680.3, filed on Sep. 10, 2007, the entire disclosure of which is incorporated by reference herein. 
     The present invention relates to a fiber cable made of high-strength synthetic fibers for a helicopter rescue winch. 
     BACKGROUND 
     Steel cables made of special steel having the material number 1.4314, in a 19×7 configuration, are used at present as the standard cable for helicopter rescue winches. The cables are exposed to large loads during operation. A disadvantage in this context is that the special-steel cables are susceptible to torsional, flexural, and kinking loads. This results in a short duration of use (usually limited to a maximum of 1,500 load cycles) for special-steel cables. Because special-steel cables furthermore have poor damage detectability, costly inspections at short maintenance intervals are necessary in order to check that the cable is undamaged. Further disadvantages of special-steel cables are inherent rotation behavior under load, susceptibility to corrosive media, and relatively high weight. Special-steel cables are also difficult to clean because of their relatively rough surface. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to further develop a cable for a helicopter winch so as to provide a cable having a longer duration of use, easy damage detectability, and/or a lower cable weight, while avoiding the aforesaid disadvantages. 
     The present invention provides a cable for the helicopter winch embodied as a fiber cable made of synthetic fibers, and encompassing multiple load-bearing synthetic-fiber strands braided with one another, at least one electrically conductive insert, and a wear indicator for visual checking of the fiber cable. 
     An advantage of the cable from multiple load-bearing synthetic-fiber strands braided with one another according to the present invention, is that the cable has a low weight, very little elongation under load, high fracture resistance, no inherent rotational torque, and good spliceability. Because plastic fibers are outstanding electrical insulators, the cable is equipped with an electrically conductive insert. This is necessary so that differences in electrical potential between the helicopter and the ground can be equalized. The potential difference occurs as a result of friction of the rotor blades against air molecules, which produces a static charge on the helicopter on the order of 10 kV to 100 kV. Equalization of this electrical potential is necessary in order to prevent an electric shock to persons being conveyed with the winch into the helicopter or from the helicopter to the ground. Because the cable according to the present invention furthermore comprises a wear indicator, damage to the fiber cable is detectable by a simple visual check. 
     According to a first embodiment of the invention, the load-bearing synthetic-fiber strands are encased, viewed in the radial direction, by a staple fiber layer, an inner cable jacket colored with a signal color, and an outer cable jacket. The required electrically conductive insert is embodied in fiber form in the present case and is braided into the staple fiber layer. The forces acting on the cable are carried exclusively by the cable core, i.e. by the load-bearing synthetic-fiber strands that are braided with one another. The purpose of the electrically conductive staple fiber layer arranged between the inner cable jacket and the load-bearing synthetic-fiber strands is to reduce friction between the cable core and cable jacket. As a wear indicator, the inner cable jacket is colored using a signal color, for example orange. This makes a wear indicator available in simple fashion, since in the event of damage to the outer cable jacket, the signal color of the inner cable jacket becomes visible so that cable damage is easily detectable. This construction is advantageous in particular because of the good adhesion between jacket and core, and the good protection of the cable core. 
     According to a second embodiment of the invention, the load-bearing synthetic-fiber strands are encased, viewed in the radial direction, by a staple fiber layer colored with a signal color, and an outer cable jacket. The electrically conductive insert is once again embodied in fiber form and is braided into the staple fiber layer colored with a signal color. Advantageously, in the present case the staple fiber layer serves on the one hand to inhibit friction between the cable jacket and cable core, and on the other hand as a wear indicator in order to indicate damage to the outer jacket. The cable jacket also protects the load-bearing cable core from abrasion and UV radiation. 
     According to a third embodiment of the invention, the load-bearing synthetic-fiber strands are encased, viewed in the radial direction, by a staple fiber layer colored with a signal color, and an outer cable jacket. The required electrically conductive insert is once again embodied in fiber form and is braided into the outer cable jacket. Corresponding to the previous embodiment, the staple fiber layer once again serves as a wear indicator in the event of damage to the outer cable jacket, and to inhibit friction between the cable core and cable jacket. The fiber-shaped electrically conductive insert braided into the cable jacket provides electrical conductivity for the cable structure, as already stated, and at the same time contributes to a reduction in wear resulting from abrasion of the synthetic fibers. 
     The embodiments presented above of the cable according to the present invention for a helicopter winch are preferably impregnated with a flexible resin system. This has the effect of sealing the cable against the penetration of water and dirt, i.e. in particular ensures easier cleaning of the cable. 
     According to a fourth embodiment of the invention the electrically conductive insert is embodied, viewed in the radial direction, as a wire forming the cable core, around which the load-bearing synthetic-fiber strands are braided; the outer periphery of the fiber cable is equipped with a colored coating. Corresponding to the embodiments already described, in this case as well only the synthetic-fiber strands braided with one another are load-bearing, whereas the wire forming the cable core simply ensures the necessary electrical conductivity of the cable. The colored coating once again enables easy visual checking of the cable, since the corresponding location would be easy to detect in the event of damage. 
     According to a fifth embodiment of the invention, the electrically conductive insert encompasses multiple wires, the number of wires corresponding to the number of load-bearing synthetic-fiber strands, and one wire being braided into each of the synthetic-fiber strands. Corresponding to the previous embodiment, the wear indicator is once again embodied as a colored coating. 
     It is also conceivable, in the context of the fourth and fifth embodiments of the cable according to the present invention for a helicopter winch, for the wear indicator to be embodied in such a way that each of the load-bearing synthetic-fiber strands is equipped with a colored coating. 
     In embodiments four and five, the cable is preferably encased in a further enveloping surface with high temperature resistance, for example aramid or Zylon©. This has the advantage that the provision of this enveloping surface guarantees short-term temperature resistance up to 300° C. 
     In order to inhibit the penetration of dirt and water, this enveloping surface is advantageously impregnated with a flexible resin system. 
     In embodiments four and five, the wires are sheathed with a plastic casing. This has the effect of ensuring sufficient protection of the wires from chemical influences. 
     Preferably, the cable comprises eight or twelve load-bearing synthetic-fiber strands braided with one another, and the synthetic-fiber strands are made from aramid, Dyneema©, Vectran©, or Zylon©. 
     Because of its good electrical conductivity, the electrically conductive insert is preferably made from copper. 
     Further advantages, features, and possible applications of the present invention are evident from the description below in conjunction with the exemplifying embodiments presented in the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described below in further detail with reference to exemplifying embodiments. 
       The terms and associated reference characters used in the List of Reference Characters set forth below are used in the Description, the Claims, the Abstract, and the drawings. In the drawings: 
         FIG. 1  is a schematic sectioned depiction of a first embodiment of the cable according to the present invention for a helicopter winch; 
         FIG. 2  is a schematic sectioned depiction of a second embodiment of the cable according to the present invention; 
         FIG. 3  is a schematic sectioned depiction of a third embodiment of the cable according to the present invention; 
         FIG. 4  is a schematic depiction of a fourth embodiment of the cable according to the present invention; and 
         FIG. 5  is a schematic depiction of a fifth embodiment of the cable according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In order to avoid repetitions, in the description that follows and in the Figures, identical components and constituents are labeled with identical reference characters unless further differentiation is necessary or advisable. 
     The cable for a helicopter winch, depicted more or less schematically in a sectioned view in  FIG. 1  and labeled in its entirety with the reference number  10 , encompasses twelve load-bearing synthetic-fiber strands  12  braided with one another. Synthetic-fiber strands  12  are in the present case made from Dyneema©. 
     These twelve braided Dyneema© synthetic-fiber strands  12  constitute the actual cable core. A staple fiber layer  14  is arranged around this cable core. A thin layer of copper wires is braided into staple fiber layer  14  as an electrically conductive insert  16 , in order to ensure the necessary electrical conductivity for cable  10 . 
     Staple fiber layer  14  is surrounded, viewed in radial direction r, by an inner cable jacket  18  and by an outer cable jacket  20  encasing inner cable jacket  18 . Inner cable jacket  18  and outer cable jacket  20  are each made of synthetic fibers. 
     Inner cable jacket  18  is furthermore colored with a signal color, in the present case orange. Inner cable jacket  18  thus serves as a wear indicator, since in the event of damage to outer cable jacket  20 , inner cable jacket  18  becomes visible so that cable damage can easily be detected visually. 
     Outer cable jacket  20  is furthermore impregnated with a flexible polyurethane resin system in order to prevent the penetration of water and dirt. 
     The adhesion of jacket and core, and the protection of the cable core, are extremely high with this construction. 
     In the embodiment of the invention depicted in  FIG. 2  as well, twelve load-bearing synthetic-fiber strands  12  braided with one another form the core of the cable structure. Arranged around the cable core is a staple fiber layer  14  into which an electrically conductive insert  16  in the form of copper fibers is once again braided, in order to ensure electrical conductivity for cable  10 . 
     Staple fiber layer  14  is additionally colored with a signal color, for example orange. Staple fiber layer  14  is in turn surrounded by an outer cable jacket  20 . In contrast to the embodiment depicted in  FIG. 1 , in this case staple fiber layer  14  performs two functions: on the one hand it serves to inhibit friction between the cable jacket and cable core, and on the other hand it serves as a wear indicator in order to indicate damage to outer jacket  20 . 
     Corresponding to the embodiment described in  FIG. 1 , the outer cable jacket is once again sealed with a flexible polyurethane resin system in order to prevent the penetration of dirt and water. 
     In the embodiment depicted in  FIG. 3 , cable  10  once again comprises a cable core made of Dyneema, made up of twelve load-bearing synthetic-fiber strands  12  braided with one another. The cable core is enclosed by a staple fiber layer  14  colored with a signal color, and by an outer cable jacket  20 . Electrically conductive insert  16  is braided into outer cable jacket  20  in the form of copper fibers. 
     Staple fiber layer  14 , colored with the signal color, serves to indicate wear in the event of damage to outer cable  20 , and to inhibit friction between the cable core and cable jacket. The copper fibers introduced into outer cable jacket  20  in order to impart electrical conductivity to the cable structure also contribute, simultaneously, to a reduction in wear due to abrasion of the synthetic fibers. Corresponding to the first and second embodiments, outer cable jacket  20  is once against sealed with a flexible resin system to prevent penetration of water and dirt. 
     The embodiment of the invention depicted in  FIG. 4  comprises, as an electrically conductive insert, a single wire  22  forming the cable core, around which the twelve load-bearing synthetic-fiber strands  12  made of Dyneema are braided. Once again, only synthetic-fiber strands  12  that are braided with one another are load-bearing. 
     The cable is additionally equipped with a colored coating  24 , in the present case embodied as a polyurethane coating; and wire  22  is encased in a plastic sheath  26 . While plastic sheath  26  protects the wire from chemical influences, the colored coating  24  serves as a wear indicator, since corresponding abrasion of the colored coating  24  enables easy visual checking of the cable. Coating  24  also, however, ensures the requisite coefficient of friction that is required so that a corresponding preload can be applied to cable  10  in a preload unit. 
     According to the last embodiment depicted in  FIG. 5 , the electrical conductivity of cable  10  is implemented by way of copper wires  22  braided into the individual cable strands  12 . To protect the copper conductors from chemical influences, they are once again encased in a plastic sheath  26 , similarly to electrical conductors. 
     To ensure sufficient temperature resistance, the embodiments of cable  10  presented in  FIGS. 4 and 5  can be equipped with an additional casing made of a material having high temperature resistance. This casing could be made, for example, of Zylon© or aramid. These types of fiber have very high decomposition temperatures and exhibit poor thermal conductivity, thus ensuring short-term (&lt;5 sec) temperature resistance at up to 300° C. To decrease wear caused by abrasion and light, it is advisable to coat this casing with a polyurethane resin. 
     LIST OF REFERENCE CHARACTERS 
     
         
           10  Cable 
           12  Synthetic-fiber strands 
           14  Staple fiber layer/wear indicator 
           16  Electrically conductive insert 
           18  Inner cable jacket/wear indicator 
           20  Outer cable jacket 
           22  Wire 
           24  Coating/wear indicator 
           26  Wire sheath 
         r Radial direction