Patent Publication Number: US-2011068208-A1

Title: Fiber tensioning device and method of making prestressed structures

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This patent application is a continuation-in-part of U.S. patent application Ser. No. 11/797,227 titled “Fiber Tensioning Device” which was filed on May 1, 2007. The contents of U.S. patent application Ser. No. 11/797,227 are incorporated herein by reference herein in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The technology described here may be manufactured, used, or licensed by or for the United States government. 
    
    
     TECHNICAL FIELD 
     The technology described here generally relates to tensioning of fibers and methods of making prestressed structures by winding fibers around a structure. 
     BACKGROUND 
     Filament winding processes have been used to produce a variety of products, such as pressure vessels, pipes, rocket motor casings, tanks, and gun barrels, by winding a continuous fiber or filament onto a rotating mandrel in a pre-determined pattern. These wound structures are often formed from advanced composites, including a combination of fibers, such as carbon, graphite, and/or Kevlar™, and a matrix, such as polyester, epoxy, or vinyl ester. 
     A conventional “wet-winding” process is schematically illustrated in  FIG. 1  where a filament or fiber  2  is supplied by spools  4 . The term “fiber” is used here to broadly include any continuous strand, such as a thread, strand, filament, fibril, string, cord, rope, etc. In  FIG. 1 , the fiber  2  is pulled from one or more spools  4  (or other conventional supply packages) and then passed through a resin bath  6  that impregnates the fiber  2  with a liquid, plastic precursor, such as epoxy. This impregnated fiber  2  is then threaded through a delivery head  8  which may translate and/or rotate in a controlled manner. Upon leaving the delivery head  8 , the fiber  2  is positioned and wound upon a mandrel  10  which is mounted on a winding device  12 . The rotation of the mandrel  10  pulls the fiber  2  from the spools  4  with a relatively small amount of tension (relative to the tensile strength of the fiber) in order to promote proper fiber alignment on the mandrel  10 , and adequate compaction or “de-bulking” to the filament wound article. When the precursor cures (solidifies), the fiber-wound article may, or may not, be removed from the mandrel  10 . 
     Various tensioning techniques are available for providing tension in the fiber  2  in order to promote alignment and compaction in the filament-wound article  10 . For example,  FIGS. 2A and 2B  illustrate static bars  20  which are arranged parallel to each other and typically made of steel. During the winding process, the fiber  2  is threaded around the static bars  20  in a serpentine fashion so that sliding friction between the fiber and the bars imparts tension to the fiber. Although such static bars  20  are generally capable of imparting high levels of tension for an indefinite duration, the abrasion of the fiber  2  caused by sliding over the static bars  20  can reduce the strength of the tensioned fiber. 
     Such static bars  20  are often used in conjunction with creel racks for holding bobbins or spools of “outside-pull” fibers which are unwound from the outside of a bobbin, spool, or other packaging. During filament winding, the package is mounted on the creel, and the fiber is pulled from the outside diameter of the package. Such creel frames typically incorporate either a mechanical, or electro-mechanical, system for applying controlled levels of torque to the spool and, consequently, of tension to the fiber as it is unwound from the spool. 
     U.S. Pat. No. 4,545,548 to Kato et al., is incorporated by reference here in its entirety and discloses an equal tension wire winding device. The device pays out thin wires from a plurality of wire reels and then winds them on a take-up bobbin. The Kato et al. device includes a plurality of revolving shafts, which are juxtaposed next to one another on a base, and support reels upon which the wires are coiled. Two pulleys are mounted on opposite ends of the revolving shafts, and a plurality of braking belts are trained between the neighboring pulleys for producing sliding friction. Rollers engage with the braking belts to adjust the tension in the belts. If some of the revolving shafts rotate at a higher or lower velocity, the associated braking belts will move so that variations in the rotation are suppressed by the neighboring shafts and the tension in the wires paid out from the wire reels is consistently maintained. 
     U.S. Pat. No. 3,770,219 to Hickman discloses an apparatus for forming a prestressing winding on a concrete pipe in which a concrete pipe is supported and rotated about its longitudinal axis as wire is fed to the pipe and wound onto the pipe. The apparatus includes a variable breaking means so as to maintain substantially constant tension to the wire. The wire is wound onto the pipe in a helical pattern while the wire is under tension so as to prestress the wiring and/or the concrete pipe. 
     These and other tensioning devices may suffer from a variety of drawbacks. For example, as the fiber or other filament is pulled from the spool under high tension, the fiber can be damaged as the outermost fiber abrades against the underlying fiber upon which it is wound. This abrasive damage upon is compounded as the tension increases and the normal forge acting on the wrapped fiber increases. Even at relatively low levels of tension, compared with the tensile strength of the fiber, this damage can quickly accumulate until the fiber breaks. 
     SUMMARY 
     The technology described below generally relates to a fiber tensioning device and to a filament winding process for forming a prestressed winding of fibers on a structure. The process includes: winding of a plurality of aligned fibers around a structure in order to compressively prestress the structure; wherein the winding is achieved by a fiber tensioning device for winding a plurality of aligned fibers and increasing the tension in the aligned fibers during the winding process, the fiber tensioning device including: a frame; a plurality of axles rotatably supported by the frame, each axle having a drum that engages the plurality of aligned fibers, each axle also having a sprocket; a chain for coupling each of the sprockets; a brake connected to the chain which opposes the motion of the chain thus increasing the tension in the plurality of aligned fibers that are in contact with the drums; whereby the aligned fibers are wound onto the structure under increased tension thus imparting compressive pressure between the aligned fibers on to the wound structure. 
     The process may further include passing the plurality of aligned fibers through a resin bath or contacting the fibers with a resin before winding of the plurality of aligned fibers around the structure. In certain embodiments, the process also includes curing the resin on the fibers that were wound on the structure. In certain desirable embodiments, the fibers are wound onto said structure in a pattern, more desirably a predetermined predetermined pattern, for example a helical pattern. Exemplary suggested structures include, but are not limited to, a pressure vessel, a pipe, a rocket motor casing, a tank, a gun barrel and so forth. Suggested structures include, but are not limited to, concrete structures, for example a concrete pipe, a concrete tank, a concrete piling and other solid concrete structures and hollow concrete structures. 
     In one desirable embodiment, the fiber tensioning device includes at least four axles wherein each of the four axles is coupled to the chain and is controlled by the brake for increasing the tension in the plurality of aligned fibers that are in contact with the drums attached to the axles. Suggested fibers for use in the process include, but are not limited to, carbon fibers, graphite fibers, fiberglass fibers, nylon fibers, polyaramid fibers and combinations thereof. Suggested resins for use in the process include, but are not limited to, polyesters, epoxy resins, vinyl ester resins and combinations thereof. In certain embodiments, the process is a wet winding process. The fiber tensioning device may also further include friction-enhancing bearings to increase the braking torque in the axles. Desirably, the aligned fibers are wound on to the wound article under higher tension and the brake controls the chain so as to provide an appropriate level or torque to the wheels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of this technology will be described with reference to the following figures which are not necessarily drawn to scale, but use the same reference numerals to designate similar components throughout each of the several views. 
         FIG. 1  is a schematic illustration of a conventional wet-winding process. 
         FIGS. 2A and 2B  are schematic side views of conventional static bars. 
         FIG. 3  is a front orthographic view of an embodiment of a fiber tensioning device. 
         FIG. 4  is a rear orthographic view of the fiber tensioning device shown in  FIG. 3 . 
         FIG. 5  is a front elevation view of the fiber tensioning device shown in  FIG. 3 . 
         FIG. 6  is an orthographic view of another embodiment of a fiber tensioning device. 
         FIG. 7  is a schematic top view of the device shown in  FIG. 6 . 
         FIG. 8  is a schematic cross-sectional view of the device shown in  FIG. 6 . 
         FIG. 9  is a schematic cross-sectional view of another embodiment of the device shown in  FIG. 8 . 
     
    
    
     DESCRIPTION 
       FIGS. 3-5  illustrate various aspects of one exemplary embodiment of a fiber tensioning device  30 . The illustrated fiber tensioning device  30  includes a frame  32  which rotatably supports several axles  34 . Although the illustrated embodiment of the fiber tensioning device  30  includes ten axles  34  arranged in two vertical columns of five axles each, any other number and/or arrangement of axles  34  may also be used. 
     As best shown in  FIG. 3 , each axle  34  includes a drum  36  on one end for engaging the fiber  2  as described in more detail below with respect to  FIG. 5 . Although the drums  36  are illustrated as substantially cylindrical disks, a variety of other configurations may also be used. For example, the drums  36  may also be formed as sheaves, pulleys, mandrels, pins, or bobbins. 
     As best illustrated in  FIG. 4 , the opposite ends of the axles  34  on the back side of the frame  32  are provided with wheels  38  for engaging, or otherwise coupling, to a belt  40 . For example, some or all of the wheels  38  may include a sheave or pulley with a groove for receiving a correspondingly shaped belt. Alternatively, or in addition, some or all of the wheels  38  may include a sprocket, or other type of gear, for engaging with a toothed belt, chain, or other power transmission device. Although a single belt  40  is illustrated in  FIG. 4 , multiple belts may also be provided for some or all of the wheels  38 . 
     The illustrated axles  34  are arranged on flange-mounted friction-reducing bearings  46  which may include any suitable bearing, including, but not limited to ball bearings and journal bearings. Also supported by the frame  32  is a brake  42  for controlling the belt  40  and providing the appropriate level of torque to the wheels  36 . For example, the brake  42  may be supported on the frame  32  by a bracket or other mounting device  50 . Friction-enhancing bearings may also be used to increase the torsion resistance of the axles  34 . 
     As best illustrated in  FIG. 5 , the fiber  2  is drawn from the supply spool  4  under a relatively small tensile force. For example, the spool  4  may be oriented so that the fiber  2  is unwound from the spool as straight as possible through one or more entrance guide elements  44  and onto the first drum  36 . The entrance guide elements  44  help to orient the travel of the fiber  2  such that the fiber contacts the outer diameter of the first drum  36  substantially perpendicular to the rotation of the drum. The fiber is then threaded in a serpentine fashion around some or all of the drums  36 . 
     For a given material in the fiber  2  (including, but not limited to carbon, fiberglass, cotton, nylon, and etc.), the number and arrangement of the drums  36  around which the fiber  2  is threaded can be chosen so as to balance between minimizing the length of the fiber that interacts with the drums  36 , (i.e., using the lowest number of drums) while also maintaining sufficient contact between the fiber  2  and the drums  36  so that slip between the fiber and drums is eliminated. Thus, some or all of the drums  36  may make contact with fiber  2 . The sizes, configuration, and/or number of rolling drums  36  around which the fiber  2  passes may be optimized in order to minimize the length of the fiber  2  upon which the tensioning device  30  acts while maintaining a condition of no-slip. For example, in a typical configuration for the illustrated fiber tensioning device  30 , the fiber  2  may make contact with between four and eight of the disk-shaped drums  36  illustrated in  FIGS. 3-5 . When this no-slip condition is achieved, the forward rotation of the drums  36  is cinematically dictated by the forward motion of the fiber  2  as the fiber is demanded by the rotating mandrel  10  during filament winding. Furthermore, it is precisely this condition of no-slip that affects the relationship between the drag torque (torque acting in the opposite direction of the forward rotation) in the drums  36  and the tension in the fiber  2  as it exits the tensioning device and passes to the mandrel. Specifically, tension in the fiber  2  will equal the drag torque in the drums  36  divided by drum radius. Hence, modulating the drag torque will modulate the fiber tension. In the exemplary embodiment shown in  FIGS. 3-5 , drag torque in the drums  36  is controlled by a magnetic particle brake  42 . The drag torque produced by the magnetic particle brake is controlled by a variable-current electric power supply (not shown). 
     After passing over the drums  36 , the tensioned fiber may be arranged to pass through exit guide elements  48  that direct the fiber  2  to be properly deposited on the mandrel  10  or other structure which may be arranged on a winding device  12  (not shown in  FIGS. 3-5 ). The mandrel  10  will then be driven with sufficient torque in order to overcome the tension in the fiber  2 . 
     Turning now to  FIG. 6 , another fiber tensioning device  30  is illustrated where the fiber  2  is drawn from a supply spool  4  under a relatively small value of tensile force and wound upon mandrel  10  under higher tension. For example, the fiber supply spool  4  may be mounted on a creel (not shown) that is typically supplied with, or suitable for, various filament winding machines and/or processes. In the embodiment shown in  FIG. 6 , the fiber spool  4  is oriented such that the fiber  2  is unwound from the fiber supply spool  4  as straight as possible through one or more entry guide elements  44  and the first of several spacing elements  52 . The fiber  2  then passes over and/or around the first rolling drum  36  and then again through the spacing elements  52  to the second rolling drum  36 . As with the embodiment shown in  FIGS. 3-5 , the drums  36  in  FIG. 6  may be provided with recesses or groves for aiding in positioning the fiber  2  on the drums  36 . Additional drums  36  may also be provided in the embodiment shown in  FIG. 6 . 
     The spacing elements  52  include several axially-aligned sheaves or pulleys arranged substantially parallel to the drums  36  on friction reducing bearings  46 . For example, each sheave may have a groove for receiving a single pass of the fiber  2  between the drums. In this configuration, the drums  36  and/or spacing elements  52  are arranged on the frame  32  substantially parallel to each other and secured on friction reducing bearings  46 . 
     Although the frame  32  shown in  FIG. 6  is arranged in a horizontal configuration, a variety of other configurations may also be used including vertical and/or angled configurations, which is also applicable to the embodiment of  FIG. 3 . For the embodiment illustrated in  FIG. 6 , pairs of horizontally-separated spacing elements  52  are arranged near each of the drums  36  so that each spacing element receives either top or bottom pass of the fiber  2  between the drums  36 . However, the spacing elements  52  may also be horizontally separated between the drums  36  and more or less than two pairs of spacing elements may also be used. For example, a single pair, or a single spacing element  56  might be used midway between the drums  36 . Although the spacing elements  52  are illustrated in  FIG. 6  as axially-aligned sheaves, they may also be configured as other stationary and/or moving guide elements, or other technology for directing the fiber  2  as it travels back and forth between the drums  36 . 
     In addition to, or instead of the illustrated spacing elements  52 , some or all of the drums  36  may be provided with slots or grooves  54  or other surface texturing to perform the same or similar functions as the spacing elements  52 . When provided on the drums  36  from the embodiment illustrated in  FIGS. 3-5 , such grooved spacing elements will also help to position and maintain the fiber  2  on the drum  36 . For example, multiple grooved spacing elements could be provided on the drums  36  in  FIGS. 3-5  for positioning more than one fiber  2  on a drum. 
     Each of the drums  36  in  FIG. 6  serves to direct the fiber  2  back to the other drum  36 . In addition, the rotation of one or both of the drums  36  may be controlled by the brake  42  in order to impart the appropriate tension in the fiber  2 . For example, the tensioning device  30  shown in  FIG. 6  may be provided with wheels and/or belts (not shown in  FIG. 6 ) for coupling the brake  42  to more than one of the drums  36 . For the embodiment illustrated in  FIG. 6 , the brake  42  is directly coupled to one of the drums  36 . 
     After the fiber passes around the first drum  36 , it is directed back toward the second drum  36 . The guide elements  52  may be arranged to shift or index the fiber  2  along the length of the rolling drums  36  during passes between drums  36 . For example; as shown in the schematic top view of  FIG. 7 , one of the top (and/or bottom) guide elements  52  may be shifted along its axis so as to provide the appropriate positioning of the fiber  2  on each of the drums  36 . 
     As shown in the schematic cross-sections of  FIGS. 8 and 9 , the fiber  2  may be passed between the spacing elements  52  in a top-to-top or “oval” configuration as shown in the cross-section of  FIG. 8 , or a top-to-bottom or “figure-8” configuration as shown in the cross-section of  FIG. 9 . In the configuration shown in  FIGS. 8 and 9 , the drums  36  rotate in the same direction, while in the configuration shown in  FIG. 9 , the drums rotate in opposite directions. 
     The present invention also provides a method of making a prestressed structure such as a pressure vessel, a pipe, a rocket motor casing, a tank, a gun barrel, a concrete pipe, a concrete tank, a concrete piling and other solid concrete structures and hollow concrete structures and so forth. The method includes winding fibers around a structure in order to compressively prestress the structure wherein the winding is achieved by a fiber tensioning device described above. Advantageously, the aligned fibers are wound onto the structure under increased tension thus imparting compressive pressure between the aligned fibers on to the wound structure. 
     The process may further include passing the plurality of aligned fibers through a resin bath or contacting the fibers with a resin before winding of the plurality of aligned fibers around the structure. In certain desirable embodiments, the fibers are wound onto said structures in a pattern, more desirably a predetermined pattern, for example a helical pattern. Suggested fibers for use in the process include, but are not limited to, carbon fibers, graphite fibers, fiberglass fibers, polyaramid fibers such as KEVLAR fibers and NOMEX fibers, nylon fibers and combinations thereof. Suggested resins for use in the process to wet the fibers include, but are not limited to, polyesters, epoxy resins, vinyl ester resins and combinations thereof. In certain embodiments, the process is a wet winding process. And in certain embodiments, the process also includes curing the resin on the fibers that were wound on the structure to form a solid, cured winding around the structure. 
     It should be emphasized that the embodiments described above, and particularly any “preferred” embodiments, are merely examples of various implementations that have been set forth here in order to provide a basic understanding of various aspects of the invention. One of ordinary skill will be able to after many of these embodiments without substantially departing from the scope of the invention defined solely by a proper construction of the following claims.