Abstract:
This invention relates generally to the design and manufacture of high performance, composite, tubular structures. More specifically, the invention relates to a high performance, composite, tubular structure that utilizes an integral pattern of ribs on the internal diameter (“ID”) or outer diameter (“OD”) surface of the tube. The present invention provides high performance, composite, tubular structures that are both lighter and stiffer than conventional tubes. In general, the present invention incorporates unconventional features into the design of tubular structures, to greatly enhance performance. For example, in accordance with one preferred embodiment of the present invention, tubular structures are enhanced by incorporating small, stabilizing, raised ribs on the ID of the tubes.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 60/211,904 entitled “Net Molding of High Performance Composite Tubular Structures,” filed Jun. 16, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention relates generally to the design and manufacture of high performance, composite, tubular structures. More specifically, the invention relates to high performance, composite, tubular structures that utilize an integral pattern of reinforcing ribs on the inner diameter (“ID”) or outer diameter (“OD”) surface of the tube. 
     2. Background of the Prior Art 
     Thin-walled, high performance, tubular structures have a wide variety of practical uses, such as for graphite composite golf shafts, arrows, bats, ski poles, hockey sticks, bicycle parts and many other applications. Current state of the art, high performance, tubular structures are constructed by various methods and from various materials. Designers of such tubular structures satisfy certain design criteria (such as strength, stiffness, weight and torsional behavior) by varying material types (fibers/resins), orientations of fiber directions and geometric proportions of the tube itself Another way designers have sought to improve high performance tubes is by developing new manufacturing techniques. 
     Using one manufacturing method, tubular structures are made by rolling material, such as pre-impregnated sheets of fiber/resin (“prepreg”), onto a “mandrel.” The rolled layers of prepreg are then consolidated against the outer surface of the mandrel (called the “ID control surface”) by wrapping the prepreg layers with shrink tape and curing via elevated temperature. FIG. 1 is a simplified diagram of this method, which involves wrapping layers of prepreg  108  around a mandrel  102  and wrapping a layer of shrink wrap material  104  around the layers of prepreg  108 . Through the application of heat, the shrink wrap material  104  contracts, providing external compaction pressure  106  such that the layers of prepreg  108  are consolidated and cured to form a tubular structure. 
     FIGS. 2 a  and  2   b  are copies of magnified, cross sectional photographs of “flag wrapped” (FIG. 2 a ) and “filament wound” (FIG. 2 b ) high performance tubular structures made by the method described in FIG.  1 . FIGS. 2 a  and  2   b  readily demonstrate wall irregularities  202 ,  204  in tubular structures which frequently result from conventional manufacturing techniques. 
     The standard flag wrapping and filament wound processes for manufacturing high performance tubes have several drawbacks due to the fact that, during consolidation/curing, the diameter of the tube is reduced. This reduction in diameter typically makes the final OD surface rough and irregular, thus requiring secondary finishing by centerless grinding and sanding. Grinding and sanding make the OD surface of the tubular structure uniform and smooth so that it can be painted to yield a cosmetically acceptable finish. However, the grinding/sanding process also typically cuts and abrades the outermost fibers of a tubular structure. Because these outermost fibers are the most highly stressed due to their location (i.e., σmax=MC/I, where “C” is the distance to the outside layer), the grinding/sanding process usually reduces the structural integrity of a tubular structure. 
     One variation on the flag wrapping and filament wound techniques for making high performance tubular structures is to consolidate them from the inside, rather than the outside, thus yielding a “net molded” outer surface. This technique uses a female mold, rather than a grinding/sanding process, and the resulting outermost fibers are less distorted during consolidation/cure and are also not cut or abraded during grinding/sanding. The net molding technique also allows for the use of higher, more uniform, consolidation pressures than the conventional, shrink-wrap, flag wrapping and filament wound techniques. Higher consolidation pressures result in higher integrity laminates with fewer voids and, therefore, greater tubular strength. 
     Though the net molding technique may be an improvement over the shrink wrapping techniques, prior art methods for producing high performance, composite, tubular structures are still limited. One problem, aside from the wall irregularities discussed above, is the inability of prior art tubular structures to attain optimal wall thinness while retaining sufficient tubular strength. Whichever technique is chosen for manufacture, a designer typically strives to produce a tubular structure with a uniform, consistent, well-consolidated wall thickness, with undamaged, undistorted composite fibers. A designer also typically tries to make the wall of the tubular structure as thin as possible, to decrease the weight of the tube, while attaining sufficient wall stiffness and strength to enable the structure to be used for its intended purpose. For example, as the wall of a tubular structure is made thinner, its overall stiffness and strength usually decrease. A fundamental failure mode, such as buckling, of a tubular structure may result if the wall of the structure is too thin. A tube that buckles (typically from compression) cannot achieve its maximum strength. Buckling, in turn, usually leads to further structural failures, such as local fiber breaking and premature catastrophic structure failure. 
     Structural failure is especially likely if a tubular structure is bent when used for its intended purpose. FIGS. 3 a  and  3   b , for example, show a tubular structure  302  with arrows representing tension  304  and compression  306  forces which might occur with bending  308 . The combination of tension  304  on one side and compression  306  on the other side of a tubular structure  302  may cause deflection  310  of the structure, as shown in FIG. 3 b . The stiffness of the wall of a tubular structure  302 , determined by such factors as the material used to make the tube and the thickness of the wall of the tube, determines how much deflection  310  occurs when the tubular structure is loaded with bending forces. If deflection  310  reaches a certain point, a situation of exponential decay is reached, wherein the stresses present at the wall section increase exponentially until the wall eventually buckles catastrophically. Because instability is inherent in ultra-thin walls of tubular structures, designers generally must use thicker walls than are desirable, in order to achieve adequate stiffness (which translates to adequate stability). Therefore, using prior art methods to produce high performance, composite, tubular structures, the goal of optimal lightness is sacrificed somewhat to achieve requisite stiffness and strength. Accordingly, a long-felt need exists for a high performance, composite tubular structure, and a method for producing that structure, which will combine optimal wall thinness with optimal resistance to buckling and buckling-related stress. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies the needs described above by providing high performance, composite, tubular structures that are lighter and/or more resistant to buckling-related stress than conventional tubes. In general, the present invention incorporates features into the design of tubular structures to enhance performance. 
     For example, in accordance with one preferred embodiment of the present invention, tubular structures are enhanced by incorporating small, stabilizing, raised ribs on the ID or OD of the tubes. These ribs enable designers to optimize the tubes&#39; inertial properties (area mass moments of inertia) to achieve lighter weight, greater stiffness, increased strength or some combination of all three. The ribs may be configured in a variety of shapes and sizes, but are typically helical or circular, parallel or non-parallel, and/or may travel in opposite directions and cross over one another. In accordance with various aspects of the present invention, the ribs may also be hollow. Hollow ribs optionally allow specific materials that are different from the rest of the tubular structure to be included within the ribs. Thus, it will be readily apparent to one skilled in the art that countless combinations and variations of ribs according to one embodiment of the present invention are possible. Like I-beams used in construction, the integral ribs allow designers of tubular structures to use lesser amounts of material, thus optimizing wall thinness, white simultaneously maintaining wall stability and, therefore, strength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional diagram of the standard flag wrapping and filament wound processes for producing a high performance, composite, tubular structure. 
     FIGS. 2 a  and  2   b  are copies of magnified, cross-sectional pictures of tubular structures produced by the prior art flag wrapping and filament wound processes, respectively. 
     FIGS. 3 a  and  3   b  are longitudinal views of a tubular structure with applied forces of tension and compression and resulting deflection. 
     FIGS. 4 a ,  4   b  and  4   c  show, in cross-section, a method for fabricating an expandable, elastomeric tube using a “male” mold, which may be used in the manufacture of a high performance, composite, tubular structure with integrated ribs. 
     FIGS. 5 a ,  5   b  and  5   c  show, in cross-section, a method for fabricating an expandable, elastomeric tube using a “female” mold, which may be used in the manufacture of a high performance, composite, tubular structure with integrated ribs. 
     FIGS. 6 a  and  6   b  show, in cross-section, a method and apparatus for manufacturing high performance, composite, tubular structures with integrated ribs. 
     FIG. 7 is a longitudinal, cutaway view of a high performance, composite, tubular structure with integrated ribs on the ID of the structure. 
     FIG. 8 a  is a magnified, cross-sectional view of part of a high performance, composite, tubular structure, with a rib on the ID of the structure made from a veil layer and integral metal. 
     FIG. 8 b  is a magnified, cross-sectional view of part of a high performance, composite, tubular structure, with a hollow rib on the ID of the structure. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following descriptions are of exemplary embodiments of the invention only, and are not intended to limit the scope, applicability or configuration of the invention in any way. Rather, the following description is intended to provide convenient illustrations for implementing various embodiments of the invention. As will become apparent, various changes may be made in the function and arrangement of the elements described in these embodiments without departing from the sprit and scope of the invention. For example, as described herein, integral ribs in accordance with the present invention are described as having a helical configuration, though, as mentioned above, various alternative configurations of ribs may likewise be used and still fall within the ambit of the appended claims. 
     That being said, with reference to FIGS. 4 a - 4   c  and  5   a - 5   c  and in accordance with one exemplary embodiment of the present invention, an expandable, elastomeric tube  408 , with an outer surface  416 , comprises the innermost component of the mechanism used to manufacture a tubular structure. FIGS. 4 a - 4   c  show, in cross-section, one non-limiting structure used for fabricating an expandable, elastomeric tube  408  with a “male” mold  402 . Male mold  402  can be made of any material commonly used by those skilled in the art for making molds. The surface  406  of male mold  402 , according to the presently described embodiment, has a linear pattern of bumps  414 . In one embodiment, bumps  414  have a helical pattern. 
     According to one embodiment, to fabricate expandable, elastomeric tube  408 , male mold  402  is covered with a thin elastomeric membrane  404 . Elastomeric membrane  404  may be composed of latex or any other substance suitable for manufacturing elastomeric tube  408 . In one embodiment, male mold  402  may be dipped in liquid latex. Liquid latex is then dried to create elastomeric membrane  404  and elastomeric membrane  404  is pulled off of male mold  402 . If elastomeric membrane  404  is pulled off of male mold  402  inside out, as is depicted in FIG. 4 c , then the pattern of bumps  414  on male mold  402  creates a pattern of grooves  420  on outer surface  416  of expandable, elastomeric tube  408 . 
     FIGS. 5 a - 5   c  show, in cross-section, another non-limiting structure used for fabricating an expandable, elastomeric tube  408  with a “female” mold  502 . Female mold  502  can be made of any material commonly used by those skilled in the art for making molds. The inner surface  506  of female mold  502 , according to the presently described embodiment, has a linear pattern of grooves  514 . In one embodiment, grooves  514  have a helical pattern. 
     According to one embodiment, to fabricate expandable, elastomeric tube  408 , inner surface  506  of female mold  502  is covered with a thin elastomeric membrane  504 . Elastomeric membrane  504  may be composed of latex or any other substance suitable for manufacturing elastomeric tube  408 . In one embodiment, female mold  502  may be dipped in liquid latex. Liquid latex is then dried to create elastomeric membrane  504  and female mold  502  is pulled off of elastomeric membrane  504 . If elastomeric membrane  504  is not turned inside out, as depicted in FIG. 5 c , the pattern of grooves  514  on female mold  502  creates the pattern of grooves  420  on outer surface  416  of expandable, elastomeric tube  408 . 
     In accordance with one embodiment of the present invention, expandable, elastomeric tube  408  is suitably comprised of molded plastic pieces formed from conventional, thermoplastic polymers such as polyethylene or the like, and from conventional plastic molding techniques such as blow molding, injection molding, rotational molding, thermoforming, or the like. Expandable, elastomeric tube  408  may also be composed of rubber-type membranes, such as latex or silicone. Preferably, any material used for the elastomeric tube  408  is rigid enough to perform the “mandrel function” during lay-up operations. For example, preferably the material possesses characteristics which allow elastomeric tube  408  to expand during an elevated temperature cure (to permit part consolidation), while substantially simultaneously maintaining pressurization integrity. It will be understood by those skilled in the art that other materials and techniques may be used to produce equally effective elastomeric tubes  408 . For example, a vacuum-forming technique may be used, wherein a sheet of polystyrene is placed around male mold  402 , polystyrene and male mold  402  are placed in a vacuum bag, and heat and vacuum pressure are applied to cure the polystyrene around male mold  402 . The foregoing examples are for exemplary purposes, and are not exclusive. 
     FIGS. 6 a  and  6   b  show, in cross-section, an apparatus for manufacturing a tubular structure  618  with integrated ribs  620 , in accordance with the present invention. The apparatus includes expandable, elastomeric tube  408 , with outer surface  416 , manufactured as described above and as illustrated in FIGS. 4 a - 4   c  and  5   a - 5   c . Strands of fiber  630 , the fiber being fiberglass, graphite, or any other suitable material, are wrapped around elastomeric tube  408  so that strands of fibers  630  lie in grooves  420  of IML  416 . Layers of prepreg  612  are placed around the circumference of expandable, elastomeric tube  408  and strands of fibers  630 . Prepreg  612  may be standard modulus (33 Msi) graphite fiber with epoxy resin, but likewise may be any other prepreg material commonly known or as yet unknown to those skilled in the art for making high performance tubular structures, such as fiberglass, aramid, boron or thermoplastic or thermosetting resins. Multiple layers of prepreg  612  may be overlaid with fiber orientations at their junctions of any angle, from 0 to 90 degrees. Fiber orientations at the junctions of the layers  612  may effect stiffness and strength of the resulting tubular structure  618 . Also, different reinforcement fibers may be used in different layers  612  to fabricate the tubular structure  618 . For example, layers of graphite, fiberglass and aramid may be used to create a composite. The number and thickness of layers of prepreg  612  chosen depend on the characteristics of lightness, stiffness and strength the designer seeks to attain. 
     According to one embodiment of the present invention, a mandrel (not pictured in FIGS. 6 a  and  6   b ) may be placed inside expandable, elastomeric tube  408 , to begin the consolidation process of strands of fibers  630  and layers of prepreg  612 . The mandrel may be made of any hard substance, such as wood or metal. Typically, it is inserted into elastomeric tube  408 , and the mandrel, tube  408 , strands of fibers  630  and layers of prepreg  612  are manually rolled back and forth on a flat surface to initiate the consolidation and reduce the bulk of the layers  612 . 
     Next, the mandrel is removed and expandable, elastomeric tube  408 , covered by strands of fibers  630  and layers of prepreg  612 , is placed into an external, curing mold  616 . Elastomeric tube  408 , strands of fibers  630 , prepreg layers  612  and curing mold  616  are placed in a press, or any device suitable for pressurization and heat curing. For example, methods for pressurization might include using pressurized gas, pressurized liquid, or heat-expandable foams, pastes or beads. The consolidation and curing process involves inflating expandable, elastomeric tube  408  while heating. In one embodiment, elastomeric tube  408 , strands of fibers  630 , prepreg layers  612  and curing mold  616  are heated to approximately 200° F. to 400° F. and preferably 250° F. to 350° F., and more preferably approximately 300° F., and are pressurized to approximately 50 to 150 psi, and preferably 75 to 125 psi and more preferably approximately 100 psi. Expansion of elastomeric tube  408  applies pressure (represented by arrows within elastomeric tube  408  in FIG. 6 a ) against prepreg layers  612  and consolidates layers  612  against the inner surface  640  of curing mold  616 . In accordance with one embodiment of the present invention, the process depicted in FIGS. 6 a  and  6   b  provides a substantially airtight seal between expandable, elastomeric tube  408 , strands of fibers  630  and layers of prepreg  612 , suitably allowing prepreg layers  612  to be consolidated against inner surface  640  of external, curing mold  616  via pressure applied by expandable, elastomeric tube  408 . 
     FIG. 6 b  shows expanded elastomeric tube  408  and a fully-consolidated tubular structure  618  with integrated ribs  620  containing strands of fibers  630 . During the consolidation/curing process, outer surface  416  of elastomeric tube  408 , with its pattern of grooves  420 , presses a pattern of integrated ribs  620  onto the ID (also called the “inner mold line” or “IML”  624 ) of tubular structure  618 . Integrated ribs  620  make tubular structures  618  stiffer and allow for optimization of wall stability and thinness. In another embodiment of the present invention (not depicted in FIGS. 6 a  and  6   b ), integrated ribs  620  may be placed on the OD (also called the “outer mold line” or “OML”  642 ) of tubular structure  408 . To do so, a pattern of grooves is built into the inner surface  640  of external curing mold  616  and the procedure just described for making tubular structure  618  is carried out. In addition to improved structural properties and performance, placing integrated ribs  620  on OML  642  gives tubular structures  618  the added benefit of a unique, visible, external design. 
     According to one embodiment, after curing, tubular structure  618  and elastomeric tube  408  are removed from curing mold  616  and elastomeric tube  408  is removed from tubular structure  618 . Removal may be accomplished by solvent extraction, manual extraction, or any other feasible means for removing elastomeric tube  408 . In accordance with another embodiment, elastomeric tube  408  is left inside tubular structure  618 . Leaving elastomeric tube  408  inside tubular structure  618  may provide a secondary function, such as damping vibrations in tubular structure  618 . With reference now to FIG. 7, a longitudinal, cutaway view of a high performance, composite, tubular structure  618 , manufactured in accordance with one embodiment of the present invention, illustrates integrated ribs  620  on the IML  624 . In the embodiment depicted in FIG. 7, integrated ribs  620  are aligned in a pattern of two sets of helical lines. One set of ribs  620  is arranged in lines oriented at an angle of approximately 45 degrees, relative to the longitudinal axis of tubular structure  618 . The other set of ribs  620  is arranged in lines oriented at an angle of approximately −45 degrees, relative to the longitudinal axis. Thus, the two sets of integrated ribs  620  are perpendicular to each other. In accordance with one aspect of the present invention, the process allows the orientation of integrated ribs  620  to be varied to achieve desired effects, such as improving stiffness by adjusting the angle of orientation of ribs  620  in relation to the longitudinal axis of tubular structure  618 . For example, ribs  620  may be oriented at +/−45° to the longitudinal axis, as depicted in FIG.  7 . Alternatively, ribs  620  could be oriented at angles other than +/−45° or could be non-parallel, or could be oriented such than no ribs  620  cross one another. Of course, the foregoing orientations are for exemplary purposes only and it will be apparent to those skilled in the art that any angles of orientation and patterns for ribs  620  may be used and that different angles and patterns will provide tubular structures  618  with different characteristics of stiffness and resistance to stress. 
     In accordance with various embodiments of the present invention, integrated ribs  620  of high performance, composite, tubular structures may be manufactured from multiple different materials and in a variety of ways, to better achieve desired characteristics. For example, as shown in FIG. 8 a , an integral metal reinforcement  802  may be used to reinforce rib  620 . Integral metal reinforcement  802  may be incorporated into layers of prepreg used to construct tubular structure  618 , such that it will create ribs  620  according to the pattern dictated by IML  416  of expandable, elastomeric tube  408  (shown in FIGS. 4,  5  and  6 ). Incorporated in this way, integral metal  802  is covered by a “veil layer”  806  of prepreg. Outer shell  812  of tubular structure  618 , may be composed of any of a number of different materials, such as graphite, steel, aluminum, titanium or metal matrix components. Veil layer  806  comprises any suitable material having flexible/conformability properties such as graphite, fiberglass and the like. It will be evident to those skilled in the art that veil layer  806  and outer shell  812  of tubular structure  618  may be made of any material suitable for making tubular structures  618 . Additionally, veil layer  806  and outer shell  812  may be made of different materials. 
     Integral metal reinforcement  802  in ribs  620  may be a different material from that used for the rest of tubular structure  618 . For example, integral metal reinforcement  802  may be composed of graphite, fiberglass, Spectra, Kevlar or a discontinuous/chopped fiber. Different integral metals  802  will give tubular structures  618  different characteristics of stiffness, strength, weight, prevention of structural failure, electrical conductivity and the like. Of course, other alternative materials used for integral metal reinforcement  802 , such as composites (e.g., different fibers or resins), metals (e.g., copper, aluminum, steel, titanium), plastics, ceramics or any other suitable material for reinforcement known to those skilled in the art may be used. 
     As illustrated in FIG. 8 b  and in accordance with another aspect of the present invention, a potentially advantageous type of integrated rib  620  is one with a hollow space  810 . Integrated ribs  620  with hollow spaces  810  may be manufactured by molding dissolvable cores into layers of prepreg  612  used to construct tubular structure  618 . This is similar to the process of incorporating integral metal  802  into layers of prepreg  612 . After consolidation and curing of tubular structure  618 , the dissolvable core that formed ribs  620  can be dissolved, leaving hollow spaces  810 . Ribs  620  with hollow spaces  810  will likely give a tubular structure  618  more stiffness for a given weight than a tubular structure without ribs. Hollow spaces  810  may also be filled with electronic wires or actuators. A plausible variation is an integral system of sensors and actuators (such as piezoelectric) to create “smart” tubular structures  618  that bend or otherwise react according to predetermined values. 
     The potential benefit of manufacturing high performance, composite, tubular structures  618  with integrated ribs  620 , in accordance with the present invention, is the ability to “tailor” the inertia properties of the tubular structures more precisely than can be accomplished with conventional methods. Thus, forces that will be placed on tubular structures  618 , such as tension  304  and compression  306  (shown in FIGS. 3 a  and  3   b ), can be accounted for directly by using different orientations, patterns and/or materials for ribs  620 . Furthermore, integrated ribs  620  accomplish the goals of added strength and stiffness without requiring thicker, heavier tubular structure  618  walls. This allows manufacturers to design ultra-light tubular structures  618  which flex longitudinally but resist buckling and buckling-related stress and, thus, have excellent torque control and stability. Furthermore, in addition to improved structural and performance properties, integrated ribs  620  placed on OML  642  of a tubular structures  618  give the structures a unique, visible, aesthetic design. Such improved tubular structures  618  with integrated ribs  620  may be used to manufacture golf shafts, arrows, bats, ski poles, hockey sticks, or any other article of manufacture requiring a high-performance tubular structure  618 . Additionally, such tubular structures  618  may have a cross-sectional shape that is round, square, hexagonal or any other suitable shape and, due to the flexibility of expandable, elastomeric tube  408 , non-straight tubular structures  618  may be designed and produced. 
     Lastly, as mentioned above, various principles of the invention have been described only as illustrative embodiments, and many combinations and modifications of the above-described structures, arrangements, proportions, elements, materials and components may be used in the practice of the invention. For example, methods and apparatuses not specifically described may be varied and particularly adapted for a specific environment and operating requirement without departing from those principles.