Patent Publication Number: US-8119916-B2

Title: Flexible cable having a dual layer jacket

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/156,675, filed Mar. 2, 2009. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to flexible cables and, in particular, multi-conductor cables for use in a mechanical cable track. 
     BACKGROUND OF THE INVENTION 
     Cables are made in various ways, using materials and processes suitable for the internal and external mechanical, environmental and Listing Agency standards and requirements. Combinations of conductors are also assembled, using various methods to produce constructions with unique properties and performance characteristics, including those necessary to survive flexing applications. This area of practice and these methods are all well documented. 
     The prior art includes mechanical cable tracks that house various electrical as well as hydraulic lines used to carry power from one point to another on construction equipment. Specifically, lift devices of the kind used to lift a worker to some height and allow specific tasks to be performed. These tasks, along with the control of the unit itself, require the use of various single and multi-conductor cables. Multiple electrical conductors under one protective jacket is an efficient means of bundling the number of wires needed in a compact design, as well as providing efficient means of connecting the cables. The flexible track space is minimized, for cost and space reasons, so efficient use of that space is important. Since the track provides the power and control of the unit, and the unit is run by one person from the basket, durability and reliability of the cables are critical. 
       FIG. 1  is an example of a prior art lift device. The typical components include a base unit, an articulated boom having the mechanical cable track, and a worker platform. 
     Applicant has conducted extensive research and development as to the superior construction of a cable in such an environment as described herein. 
     In one prior art embodiment, the track application involves link type tracks which the industry refers to as “C” tracks.  FIG. 2  shows the links of a simulated mechanical cable track connected at the pivot points, with bars extending across to connect the links. The links are designed to facilitate relatively small radius bends (see  FIG. 3 ) and are used to allow for continuous operation of the device during the lifting/extension and maneuvering sequences. The track houses various cables to provide power and control connections between a base unit and some device (such as a basket or cage for a worker) at the end of the extended “arm” or boom. The links of the track include pivot joints on each end and these links are attached side to side by flat plates, bars or rollers creating a “track or link”. These tracks travel in two directions and one plane. They have an extension and contraction mode. During the extension mode, the cable contacts the inner track link connection device/method (i.e., the flat plate, bar or roller) of the chain links and at the pivot of the link or chain there is considerable contact, rubbing, or wiping of the cable against the device. This occurs at each pivot or contact point of the track. Tracks are used in varying lengths depending on the reach or extension needed. This “smooth surface” abrasion is particularly abusive to materials like rubber (CPE) and/or Neoprene, and these materials, while they have substantial tensile modulus properties, break down and wear out fairly quickly (e.g., less than 15,000 track cycles), thereby exposing the insulated conductors. Conversely, typical thermoplastic elastomers and PVC&#39;s typically exhibit lower tensile modulus properties combined with a lower surface coefficient of friction, allowing them to perform well in smooth surface abrasion contact conditions, but they are generally not robust enough to prevent the transfer of the track wiping effect. 
     During the contraction mode the cable is allowed to relax. However, no reversal of the forces implied on the cable occurs. Therefore, the stresses on the cable are only and always in one direction, e.g., the extension mode of the unit. Conventional wisdom would attempt to describe the force applied to the cable as torsional in nature. This false conclusion is suggested after observation of the cable. In particular, the cables take on a twisted or ropey appearance which occurs when cables experience excessive torsional load or forces. However, the applicant has discovered that the force applied is not torsional. The force can best be described as a wiping or “milking” force applied to the cables outer contact surface, such as seen in  FIG. 4 . If the force were torsional in nature, the conductors would exhibit a regular twisting shape with the conductor lay length being reduced uniformly along the length exposed to the force. Instead, the applicant has discovered that what occurs is a distinct and consistent change in lay length that is not evenly distributed along the length. The impact is observed as only occurring near the pivot end of the track where the cable is contacted by the flat plate, bar or roller of the link connection. It is believed that the elongation properties of the jacket allow for the displacement (stretching) of the jacket and subsequent transfer of force to the conductor layer. Since the contact occurs over the entire width of the track blade (e.g., approximately 3 inches), the pressure wipes against the conductors influencing the lay and creating the rope or twisting effect, such as seen in  FIGS. 5 and 6 . Materials that are less susceptible to stretching or elongation (i.e., have high tensile modulus properties) cannot be effectively used as an outer jacket material as they fracture or wear out under the regular contact and wiping of the track or are not flexible enough to be installed and used with the relatively small bend radius of typical C tracks. 
     The prior art includes multi-conductor cables produced with a conductor lay length to allow the cable to withstand repeated flexing. In particular, the conductor lay or spiral allows the conductor to avoid being stressed in the same place and in the same plane repetitively. However, if the conductor is subjected to a tightening of the lay, such that the conductor exhibits what the industry refers to as a “Z” kink, the conductors will be effectively locked in a position. As a result, the conductor will be subjected to damage. The damage is a result of the copper strands being subjected to flexing and stressing that causes the conductor to be work-hardened and to lose elongation. The loss of elongation and work hardening leads to conductor breakage and electrical failure. 
     Applicant conducted research into the impact of wiping upon a cable, with multiple conductors and made with a specific lay length. In particular, after track testing, the lay length can be re-measured and the effects recorded. What was found by Applicant was a lengthening of lay followed abruptly by a reduction in lay length. The effects are also visible on the outside of the cable. That is, the cable assumes a twisting or rope like appearance. This appearance is actually the result of a lengthening of lay length in one spot followed by a tightening or accumulating of lay length in an adjacent spot. These intervals of tightening and accumulating will repeat along the length of the cable that has experienced the track effect and will not occur where the same cable length has not experienced this contact. Where contra-helical conductor layers are utilized, the force (track wiping) can be transferred from the outer conductor layer to the layer just underneath it, since the layers are wound in opposite directions, the outer layer can force the inner layer conductors to buckle (this has been observed in actual track testing). In the most extreme circumstances of the “tightening” (or more accurately the accumulation or reduction) of the lay, the effect is so extreme as to create a bunching up of the conductors. Where no lay length is evident, the conductors cannot wipe down any further and the conductors can be the subject of damage as a result of this bend. The industry refers to this as a “Z” kink.  FIG. 7  shows sample conductors exhibiting such features. The jacket has been removed to better demonstrate the effect on the conductors.  FIG. 7A  shows the lengthening of the lay followed by the reduction in conductor lay. The G 1  sample included only a single layer pressure extruded jacket. The G- 3  sample in  FIG. 7B  is the same conductor combination however wherein the inner and outer layer concept was utilized, as taught by the present invention, and as further described below. 
     SUMMARY OF THE INVENTION 
     The present invention provides a flexible multi-conductor cable and a method of manufacturing a flexible multi-conductor cable, wherein the cable is adapted for use, particularly, in a mechanical cable track. The material of the cable is preferably selected so that the cable is capable of surviving the external physical requirements of a mechanical cable track, as well as to: prevent the transfer of the wiping effect onto the conductors; allow low friction intimate contact with insulated conductors and conductor layers; prevent compression of the inner conductor layers creating the opportunity for Z kinking by a single conductor or multiple conductors; and be suitable for smooth surface abrasion applications. In one embodiment, the cable includes 18 AWG or larger conductors for power and control applications. The cable is designed to withstand prolonged exposure to −40° C. temperatures with no movement of the cable followed by repeated extension and contraction cycles. Further, the cable is designed to withstand UV exposure, weather, dust and dirt, concrete, and the casual oil or grease contact. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front elevated view of a prior art lifting device. 
         FIG. 2  is a perspective elevated view of a portion of a mechanical cable track of a lifting device, together with a plurality of prior art cables each having a plurality of conductors. 
         FIG. 3  is a perspective view of a portion of the mechanical cable track and prior art cables of  FIG. 2 . 
         FIG. 4  is a perspective side view of a portion of the mechanical cable track and prior art cables of  FIG. 2 . 
         FIG. 5  is a perspective end view of a portion of the mechanical cable track and prior art cables of  FIG. 2 . 
         FIG. 6  is a partial perspective elevated end view of a portion of the mechanical cable track and prior art cables of  FIG. 2 . 
         FIG. 7A  is a plan view of prior art cable, with the jacket removed to expose the plurality of conductors G 1 . 
         FIG. 7B  is a plan view of a cable in accordance with the present invention, with the jacket removed to expose the plurality of conductors G 3 . 
         FIG. 8  is a cross-section of a cable in accordance with one embodiment of the present invention. 
         FIG. 9  is a side view of a cable in accordance with another embodiment of the present invention. 
         FIG. 10  is a cross-section of the cable of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     As noted above,  FIG. 1  shows one example of a prior art lifting device  10 . The typical embodiment includes a base unit  12 , an articulated boom  14  which includes a mechanical cable track, and a worker platform  16 . The worker platform  16  will accommodate one or more workers. As there is no operator located in the base unit  12 , a control system  18  is located at the worker platform  16  so that the workers may operate the lift device from the worker platform  16 . Thus, a cable system providing control and power extends between the base unit  12  and the worker platform  16 .  FIGS. 2-6  illustrate a portion of such a prior art cable system extending along a cable track. The cable system extends along the length of the articulated boom  14 . 
     The articulated boom  14  is shown in  FIG. 1  to be in an extended position. It will be appreciated that the articulated boom  14  may be collapsed or folded upon itself with the worker platform  16  located just above the base unit  12 . 
       FIG. 8  shows one embodiment of a cable  20  in accordance with the present invention for use in the above noted cable system. The cable  20  includes a dual layer jacket  22 . The jacket  22  includes an outer layer  24  and an inner layer  26 . The cable  20  further includes a plurality of insulated conductors  28 . The insulated conductors  28  may provide for power and control applications. 
     The dual layer jacket  22  includes a material which resists track abrading on the outer layer  24  of the jacket  22  and a material which resists stretching on the inner layer  26  of the jacket  22 . Both materials include a formula of thermoplastic elastomers (TPE). However, the properties of the inner layer  26  and the outer layer  24  differ in order to achieve the objective of the present invention. In particular, the material of the inner layer  26  is provided with a high tensile modulus to resist stretching forces externally applied to the cable. In comparison, the material of the outer layer  24  is provided with a low tensile modulus to avoid breakdown and cracking as the cable is wiped or rubbed against external surfaces. In one embodiment, the tensile modulus of the inner layer  26  is 1572 psi and the tensile modulus of the outer layer  24  is 1232 psi. It will be appreciated that in this just noted embodiment, the tensile modulus of the inner layer  26  is 27.6% greater in comparison to the outer layer  24 . 
     The tensile and elongation properties of the inner layer  26  and outer layer  24  are chosen to withstand the environment of the noted application. 
     By co-extruding the two materials together, the finished cable  20  is able to withstand all subject forces and exposures. Since the materials are similar in base chemistry, no bonding agents or bonding layers are necessary. Since the materials are substantial in their specific properties, there is no need for additional layers of materials or other means required to obtain suitable cable performance. For instance, it is not necessary to include inner jacket layers, binders, braids or other mechanical components. Some binders or wraps may be used for holding one group together during the manufacturing process, or to reduce friction between members, but these additions are not required to improve the cables ability to withstand the wiping of the track. The combination of materials is so resistant that good cable geometry and design are not required. That is, the absence of spaces between components, contra-helical conductor layers and perfect conductor count are not required. For example, in the embodiment shown in  FIG. 8 , the “tubed” jacketed cable  20  has no concentric layers, and the gaps or open interstices are clearly visible, with no pressure extrusion to “trap” the conductors  28 . However, certainly a pressure extruded application would also benefit from the present invention. 
     The following chart shows the properties of the inner layer  26  and outer layer  24  of the jacket  22 , for one embodiment of the present invention. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                   
                 Outer Layer (24) 
                 Inner Layer (26) 
                   
               
               
                   
                 Lower Tensile 
                 Higher Tensile 
                 Difference in 
               
               
                 Property 
                 Modulus 
                 Modulus 
                 properties 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Tensile 
                 2179 
                 Pa 
                 2393 
                 Pa 
                 9.8% increase 
               
               
                 Strength 
               
            
           
           
               
               
               
               
            
               
                 Elongation 
                 323% 
                 340% 
                 5.3% increase 
               
            
           
           
               
               
               
               
               
               
            
               
                 Tensile 
                 1232 
                 psi 
                 1572 
                 psi 
                 27.6% increase  
               
               
                 Modulus 
               
            
           
           
               
               
               
               
               
            
               
                 Hardness 
                 @15 sec 71 A 
                 82 
                 A 
                 N/A 
               
               
                   
                 Instant 82 A 
               
            
           
           
               
               
               
               
               
               
            
               
                 Temperature 
                 105 
                 deg C. 
                 105 
                 deg C. 
                 Same 
               
               
                 Rating 
               
            
           
           
               
               
               
               
            
               
                 Polymer 
                 TPE 
                 TPE 
                 Same 
               
            
           
           
               
               
               
               
               
               
            
               
                 Brittle 
                 −49 
                 C. 
                 −49 
                 C. 
                 Same 
               
               
                 Temperature 
               
               
                   
               
            
           
         
       
     
       FIG. 9  shows another embodiment of the cable of the present invention. The cable  40  is shown to include an outer layer  42  of twelve conductors, an inner layer  44  of six conductors and a central pair  46 . The dual layer jacket  48  is also shown. 
       FIG. 10  is a cross sectional view of the cable  40  of  FIG. 9 . The outer layer  50  and inner layer  52  are shown. It will also be appreciated that the embodiment shown is a pressure extruded application about the insulated conductors. However, the present invention is equally applicable to a “tubed” jacket application about the insulated conductors. It should also be noted that there are no inner layer of extruded material within the interstices formed by the conductors. 
     It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention, but it is understood that this application is limited only by the scope of the appended claims.