Patent Publication Number: US-2021187813-A1

Title: Method for manufacturing an individually sheathed strand, strand thus obtained and strand manufacturing installation

Description:
The present invention concerns the individually protected strands used in civil engineering structures, in particular to prestress or to suspend structure portions. 
     BACKGROUND 
     The individually sheathed strands comprise a group of metal wires twisted together, usually seven in number. The metal wires are frequently subjected to an electrochemical treatment (galvanization, galfan coating, . . . ) procuring some resistance to corrosion. 
     Routinely used are strands individually protected by a plastic material, usually high-density polyethylene (PEHD) or epoxy, sheath that creates a sealed barrier around the metal wires. A filler product, which may be of a number of types (wax, grease, polymer, . . . ), fills the gaps that exist between the metal wires and the individual sheath to strengthen the protection of the strand against corrosion. 
     The filler product either allows the metal wires of the strand to slide relative to its individual sheath (sheathed and greased or sheathed and waxed strand), or to the contrary adhesion to transmit shear forces between the sheath and the strand (coherent strand). 
     In the coherent strand, the filler product is typically a polymer adhering to the metal wires and to the interior of the sheath. The coherent strand is in particular usable when it is necessary to transmit tangential forces from the sheath to the metal wires. See for example EP-A-0 855 471. This is the case for example in the cables supporting suspension bridges in which the load transmitted by each suspension means causes a tangential effort on the cable at the level of the suspension member attachment collar or in diverters installed on the masts of anchored structures. 
     In the sheathed and greased or sheathed and waxed strand, the filler product is a lubricant, which in particular has the advantage of improving the fatigue behavior of the strand by lubricating the contacts between its metal wires. See for example EP-A-1 211 350. Moreover, the greases or waxes employed in the sheathed and greased or sheathed and waxed strands are generally less costly than the resins employed in the coherent strands, with the result that the sheathed and greased or sheathed and waxed strand has a lower unit cost. 
     JP 2005-048405 A discloses a strand aiming to prevent the noise caused by collisions between the metal wires and the sheath. To this end, the strand includes portions where a layer of urethane foam is formed between the group of metal wires and its tubular polyethylene sheath and, alternating along the strand, portions where the sheath is tightened to be in contact with the group of metal wires. 
     EP-A-2 601 344 discloses a strand having portions of coherent type alternating along the strand with portions of coherent type. This strand is produced from a sheathed and waxed strand the sheath of which is removed locally, likewise the wax, before applying the resin procuring the adhesion property. This method, somewhat non-industrial, and therefore relatively costly, leads to discontinuities of the sheath on the finished strand. Moreover, it is difficult in practise to eliminate the wax so that it does not interfere with the required adhesion in the strands of coherent type. The inability to degrease these portions completely and easily limits or prevents goods adhesion of the resin (once hardened) to the steel. 
     An object of the present invention is to propose a reliable and economic technique for producing a strand having one or more portions with adhering sheath and one or more portions where the same sheath extends without needing to adhere to the metal wires. 
     SUMMARY 
     There is proposed a method for manufacturing an individually sheathed strand. This method comprises: 
     conveying a group of metal wires through a die; 
     upstream of the die, applying a first filler product to at least a first portion of the strand; 
     upstream of the die, applying a second filler product to at least a second portion of the strand distinct from the first portion, the second filler product having greater adhesion to the group of metal wires than the first filler product; and 
     extruding a plastic material around the group of metal wires passing through the die, so as to envelop the group of metal wires coated with the first and second filler products in a continuous sheath formed of the extruded plastic material. 
     The method produces a sheathed strand with composite filler under well-controlled industrial conditions. The sheath, generally of constant exterior section, provides reliable and uninterrupted protection of the coated metal wires either by a product procuring adhesion, or by a lubricating product, as required. 
     The invention enables manufacture of a continuous sheath covering all of the portions whilst guaranteeing the required mechanical characteristics in each portion. In particular, the adhesion of the hardened resin is not interfered with by the presence of wax in the coherent strand portions. 
     One particular embodiment of the method comprises: 
     causing the group of metal wires to move along a succession of stations as far as the die, including a filler product application station; 
     spreading the metal wires of the group on entering the filler product application station; 
     selectively applying the first filler product and the second filler product to the spread metallic wires in the filler product application station as a function of the advance of the group of metal wires in the filler product application station; and 
     compacting the metal wires of the group on leaving the filler product application station. 
     The filler product application station may comprise a first part selectively activated to apply the first filler product to the spread metal wires of the group and a second part selectively activated to apply the second filler product to the spread metal wires of the group, the second part being farther upstream than the first part relative to the die. 
     In one embodiment of the filler product application station, the first part comprises nozzles for spraying the first filler product directed convergently toward a line along which the spread metal wires of the group move in the filler product application station. This first part may further comprise a tray for recovering the first filler product under the spray nozzles and a system for recirculating the first filler product recovered in the tray to return it to the spray nozzles. 
     In one embodiment of the filler product application station, the second part of the filler product application station comprises a mixer for mixing a plurality of components and a head for depositing the mixture on the spread metal wires of the group. 
     An air curtain may be blown onto the group of spread metal wires between the first and second parts of the filler product application station. 
     One embodiment of the method comprises heating the group of metal wires ahead of the filler product application station, to a temperature between 80° C. and 140° C. inclusive, for example by induction. 
     One embodiment of the method comprises regulating the temperature of the group of compacted metal wires downstream of the filler product application station, the group of metal wires reaching the die, where the plastic material of the sheath is extruded, at a temperature between 60° C. and 120° C. inclusive. The regulation of the temperature of the group of metal wires downstream of the filler product application station may comprises cooling the group of metal wires to a temperature between 10° C. and 30° C. inclusive, then reheating the group of metal wires to the temperature between 60° C. and 120° C. inclusive. The cooling may be produced by a flow of air and the heating by induction. 
     In one embodiment of the method, the die is part of an extrusion head where there are successively deposited on the group of metal wires an adhesion primer and the plastic material of the sheath. 
     When the extrusion head has passed through it a portion of the strand in which the group of metal wires is coated with the first filler product, a rate of injection of the adhesion primer may be reduced relative to a moment at which the extrusion head has passed through it a portion of the strand in which the group of metal wires is coated with the second filler product. 
     Downstream of the die, the group of metal wires enveloped in the sheath may be cooled in a water bath. 
     Another aspect of the invention relates to an individually sheathed strand comprising: 
     a group of metal wires; 
     a continuous plastic material sheath of constant exterior section; 
     a first filler product contained in the sheath with the group of metal wires in at least a first portion of the strand; and 
     a second filler product contained in the sheath with the group of metal wires in at least a second portion of the strand distinct from the first portion, the second filler product having a greater adhesion to the group of metal wires and the sheath than the first filler product. 
     The individually sheathed strand may have at least one third portion situated between a first portion and a second portion, where a mixture of the first and second filler products is contained in the sheath with the group of metal wires. Each third portion may have a length between 20 cm and 1 m inclusive. 
     A further aspect of the invention relates to an installation for manufacturing an individually sheathed strand, comprising: 
     a die to receive a group of metal wires; 
     on an upstream side of the die, a system for applying a filler product to the group of metal wires; 
     a system for extruding a plastic material around the group of metal wires conveyed through the die so as to envelop the group of metal wires coated with the filler product in a continuous sheath formed by the extruded plastic material; and 
     a controller of the filler product application system for selecting a composition of the filler product applied as a function of the advance of the group of metal wires in the filler product application system from a first product and a second product having a greater adhesion to the group of metal wires and the sheath than the first product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other particular features and advantages of the present invention will become apparent in the following description of nonlimiting embodiments given with reference to the appended drawings, in which: 
         FIG. 1  is a schematic side view of one embodiment of an individually sheathed strand in accordance with the invention; 
         FIGS. 2 and 3  are views of that individually sheathed strand in cross-section on the planes II-II and indicated in  FIG. 1 ; 
         FIG. 4  is a schematic of one example of a manufacturing installation in accordance with the invention; 
         FIG. 5  is a cross-sectional schematic of a part of the installation used to apply wax to the metal wires of strand; and 
         FIG. 6  is a perspective view of the part represented in  FIG. 5 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIGS. 1 to 3  show an individually sheathed strand  10  which, along its length, has in succession portions  11  in which the sheath  15  of the strand does not adhere to the group of metal wires  16  and portions  12  in which the sheath  15  adheres to the group of metal wires  16 . 
     In the example represented, the metal wires  16  of the group are seven in number, with a substantially rectilinear central wire (when the strand  10  is itself rectilinear) and six peripheral wires twisted around the central wire. 
     A flexible filler product fills the interstices with a curvilinear triangle shape section that extend helicoidally around the central wire and also covers the exterior of the peripheral wires. This filler product is contained with the group of metal wires  16  inside the sheath  15 . 
     The sheath  15  is made of PEHD for example. 
     In the portions  11  of the strand  10  the filler product  17  has lubricating properties. It may in particular be a wax or a grease. In the remainder of the present description it is considered that the filler product  17  is a wax, without this limiting the invention. 
     In the portions  12  of the strand  10  the filler product  18  adheres both to the metal wires  16  and to the internal surface of the sheath  15 . It is thus able to resist shear stresses that develop when tangential forces are transmitted from the sheath  15  to the wires  16 . In the remainder of the present description it is considered that the filler product  18  is a resin, without this limiting the invention. 
     The sheath  15  extends continuously around the group of metal wires  16  all along the strand  10 . The portions  11  and the portions  12  alternate along the length of the strand. 
     As  FIG. 1  shows, there may exist between them transition portions  13  in which it is a mixture of the products  17  and  18  (for example wax and resin) that is contained in the sheath  15  with the group of metal wires  16 . The transition portions  13  typically have a length between 20 cm and 3 m inclusive. That length may be from 40 to 50 cm for example. 
     In one application example the individually sheathed strand  10  is part of an anchor of an anchored structure such as a bridge. In some anchored structure designs the anchors are diverted in a saddle mounted on a mast. In that saddle the strands that constitute an anchor are diverted without degrading their mechanical strength. Sufficient friction must exist between the strands and the ducts of the saddle each of which receives a strand for optimum take-up by the mast receiving the saddle of the differential forces to which the anchor is subjected on either side of the mast. In this application, the strands  10  constituting the anchor can be manufactured so that a portion  12  with adhesion is located in the body of the saddle while portions  11  without adhesion are formed on the rest of the strand to constitute the main part of the anchor. In this main part, which generally constitutes the majority of the length of the anchor, the friction that exists between the metal wires  16  of the strands is limited by the lubrication provided by the wax  17 . Furthermore, as the wax  17  is generally less costly than the adhesive resin  18 , the additional cost generated by the necessity to have adhesion at the level of the saddle remains limited. 
     The transition portions  13 , which are the result of the method of manufacturing the individually sheathed strand, do not interfere with the mechanical behavior of the strand. To the contrary, they are advantageous because they avoid a sudden transition between a portion  12  with adhesion and a portion  11  without adhesion. 
     An installation for manufacturing an individually sheathed strand of this kind is represented schematically in  FIG. 4 . 
     The group of metal wires  16  (bare strand possibly galvanized or galfan-coated), manufactured beforehand, is fed onto a drum  20  from which it is unwound to be conveyed along the installation. The finished product is the individually sheathed strand  10  which, once the operations have ended, is wound onto another drum  22  on which it can be stored and transported. 
     To drive the strand along its path in the installation two caterpillar track driving devices  23 ,  24  are provided, for example, one which takes up the bare strand  16  on the drum  20  and the other which pulls on the individually sheathed strand  10  to convey it to the drum  22 . If necessary, other drive devices may be provided along the path. 
     Various treatment stations  25 - 30  extend in succession along the path of the strand in the installation. 
     A first station situated downstream of the drive device  23  is a coil  25  in which the bare strand  16  is heated to a temperature T 1 . The temperature T 1  is typically between 80° C. and 140° C. inclusive. Induction heating is efficient and avoids having to bring a heating element into contact with the metal wires. 
     Then a filler product application station  26  selectively applies wax  17  or resin  18  between and around the metal wires  16  of the strand. 
     The station  26  comprises firstly a spreader  32  that grips the peripheral wires of the strand and applies torsion to them to spread them. The metal wires therefore pass through the station  26  in a spread configuration. After the spreader the station  26  comprises two parts  33 ,  38  for respectively depositing the resin  18  and applying the wax  17 . 
     The part  33  comprises a head  36  directed toward the group of spread wires to deposit thereon one or more resin beads  18 . When thereafter the metal wires are compacted, the resin  18  that has been deposited in a paste-like state is deformed to fill in the internal interstices of the strand and to overflow onto its exterior surface. At the downstream end of the station  26  a shaper  34  verifies that the metal wires  16  have been correctly replaced in their compacted configuration and eliminates the excess resin  18 . 
     The resin  18  is typically a two-component resin. The head  36  is then connected to a mixer  35  that mixes the components, namely the polymer base R of the resin and a hardener H, before sending the mixture to the head  36 . 
     Downstream of the part  33  serving to deposit the resin  18 , the station  26  comprises the part  38  serving to apply the wax  17 . 
     To manufacture a sheathed and waxed strand, the bare strand is usually caused to circulate in a bath of fluidized wax after which the excess wax is removed by a template around the strand coated in this way. However, that method is not appropriate in the context of the method in accordance with the invention because the aim is to prevent the presence of wax in the portions  12  with adhesion of the individually sheathed strand  10 . 
     The part  38  preferably includes a tank  40  into which the wax W is conveyed and heated to its application temperature, at which it is in the fluid state, nozzles  41  for spraying the wax in the fluid state onto the spread metal wires of the strand, a tray  42  under the strand for recovering wax that has dripped and the nozzles  41 , and a recirculation system (not represented) for returning the wax recovered in the tray  42  to the tank  40  and the spray nozzles  41 . 
     The spray nozzles  41  may be formed in a body  44  represented in  FIGS. 5 and 6 . This body  44  includes a bore  45  in which the spread metal wires of the strand circulate. The nozzles  41  are formed in the wall of the bore  45 . In the example represented there are two sets of four nozzles  41  that extend in succession along the path of the strand. In this example each set includes four nozzles that converge toward the direction of movement of this strand, for example being oriented at 90° to one another as represented schematically in  FIG. 5 . In its lower part the bore  45  includes a hole  46  for the evacuation of the excess wax toward the tray  42 . 
     As shown schematically in  FIG. 4 , the part  38  serving to apply the wax is situated in the zone in which the metal wires  16  of the strand are compacted into a compact formation. In particular, the first set of nozzles  41  may be placed at a location where the metal wires  16  are still spread while the second set of nozzles  41  is placed slightly downstream, where the metal wires  16  come to be compacted. This therefore ensures that the wax is not evacuated excessively and that sufficient thereof remains ins the interstices of the strand. The second set of nozzles  41  ensures sufficient coating of the periphery of the strand. 
     On leaving the station  26  the shaper  34  eliminates the excess resin  18  or the excess wax  17 , as appropriate. 
     To prevent the part  33  of the station  26  from interfering with the part  38 , the separation between those two parts may be produced by a compressed air curtain blown by a compressor  48 . 
     The parts  33 ,  38  of the filler product application station  26  are controlled by a controller  50  in order for the flow rate of resin applied by the head  36  and the flow rate of wax sprayed by the nozzles  41  to be selected as a function of the advance of the strand in the station  26 . When it is a portion  11  of the strand  10  that is circulating in the station  26  the controller  50  activates the part  38  and deactivates the part  33 . Conversely, when it is a portion  12  of the strand  10  that is circulating in the station  26  the controller  50  activates the part  33  and deactivates the part  38 . 
     Thus the controller  50  enables close control of the location of the of portions  12  with adhesion and of the portions  11  without adhesion of the individually sheathed strand  10 . It is therefore possible to adjust at will the spatial distribution of the portions  11 ,  12  to adapt the strand  10  to suit the structure which is intended to be equipped with it. 
     In the transitions between the portions  11  and  12  the controller  50  also regulates the quantities of filler products in order to form the transition portions  13  in the required manne. 
     Downstream of the station  26  the temperature of the strand coated with resin or with wax is regulated in the stations  27 ,  28  on the one hand to allow the resin  18  to harden and on the other hand to ensure an appropriate temperature for the approach of the strand into the extrusion head  29 . 
     Firstly, the station  27  is an air-cooling tunnel with a length from 5 m to 10 m, for example, to cool the coated strand to a temperature T 2  between 10° C. and 30° C. inclusive. 
     Thereafter, the station  28  is another coil that heads the metal wires of the strand to a temperature T 3  less than the aforementioned temperature T 1 . The temperature T 3  is for example between 60° C. and 120° C. inclusive. 
     On leaving the coil  28 , the strand penetrates into the extrusion head  29 . The extrusion head  29  includes the die  52  at its downstream end. Just upstream of the die  52  is the injection system for successively introducing, at the periphery of the strand, an adhesion primer B and then polyethylene PE brought to the liquid state in order to form the sheath  15  after cooling. The ducts  53  for injecting the adhesion primer B are situated immediately upstream of the ducts  54  for injecting the polyethylene PE, which are themselves situated immediately upstream of the die  52  in the extrusion head  29 . 
     The primer B ensures the adhesion of the sheath  15  onto the resin  18  deposited in the portions  12  of the strand  10 . In one particular example this is a polymer marketed under the mark Orevac by the company Arkema. In the portions  11  of the strand  10  it is preferable not to stop the flow of the adhesion primer B in the injection system in order not to disturb the latter. This flow rate may be reduced, however, because the adhesion primer B is not necessary in the sections  11 . The primer B adheres to the internal surface of the sheath  15  but does not interfere with the behavior of the lubricating wax in the portions  11 . 
     Downstream of the die  52  the final station of the installation ahead of the caterpillar track driver device  24  is a water bath  30  in which the group of metal wires  16  enveloped in the sheath  15  is cooled to room temperature, i.e. between 10° C. and 30° C. 
     The embodiments described hereinabove are merely an illustration of the present invention. Various modifications may be made to them without departing from the scope of the invention that emerges from the appended claims.