FIBER TENSIONING DEVICE AND METHOD OF MAKING PRESTRESSED STRUCTURES

Abstract

A process for forming a prestressed structure that includes: winding a plurality of fibers around a structure in order to compressively prestress the structure; wherein the winding is achieved by a fiber tensioning device that includes: 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 said drums; and 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.

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.

Claims

1. A filament winding process for forming a prestressed winding of fibers on a structure, the process comprising:

winding of a plurality of aligned fibers around a structure in order to compressively prestress said structure;

wherein said 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 comprising:

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 said drums; and

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.

2. The process of claim 1 further comprising passing the plurality of aligned fibers through a resin bath before winding of the plurality of aligned fibers around the structure.

3. The process of claim 1 further comprising contacting the plurality of aligned fibers with a resin before winding of the plurality of aligned fibers around the structure.

4. The process of claim 2 further comprising curing the resin on the fibers that were wound on the structure.

5. The process of claim 3 further comprising curing the resin on the fibers that were wound on the structure.

6. The process of claim 1 wherein said plurality of fibers is wound onto said structure in a pattern.

7. The process of claim 1 wherein said plurality of fibers is wound onto said structure in a predetermined pattern.

8. The process of claim 1 wherein said plurality of fibers is wound onto said structure in a helical pattern.

9. The process of claim 1 wherein said structure is a pressure vessel, a pipe, a rocket motor casing, a tank, or a gun barrel.

10. The process of claim 1 wherein said structure is a concrete structure.

11. The process of claim 1 wherein said structure is a concrete pipe, a concrete tank, a concrete piling or other solid or hollow concrete structure.

12. The process of claim 1 wherein said fiber tensioning device con prising at least four axles wherein each of the four axles is coupled to said chain and is controlled by said brake for increasing the tension in the plurality of aligned fibers that are in contact with said drums attached to said axles.

13. The process of claim 1 wherein said plurality of aligned fibers are selected from the group consisting of carbon fibers, graphite fibers, fiberglass fibers, nylon fibers, polyaramid fibers and combinations thereof.

14. The process of claim 2 wherein said resin is selected from the group consisting of polyesters, epoxy resins, vinyl ester resins and combinations thereof.

15. The process of claim 1 wherein the process is a wet winding process.

16. The process of claim 1 wherein said fiber tensioning device further comprises friction-enhancing bearings to increase the braking torque in the axles.

17. The process of claim 1 wherein the aligned fibers are wound on to the wound article under higher tension.

18. The process of claim 1 wherein the brake controls the chain so as to provide an appropriate level or torque to the wheels.

19. The process of claim 1 wherein the brake modulates the fiber tension.

20. The process of claim 1 wherein the brake modulates the drag torque.