The present invention relates to archery bows, and more particularly to an improved composite bow limb construction and method of making same.
High performance compound bows and crossbows typically employ limbs composed of fiber reinforced polymer composite materials. The fiber reinforcement is typically oriented in a direction generally parallel with the limb's length for optimum flexural rigidity and strength. Bias, transverse, through the thickness and other off-axis reinforcements, if present, often represent a small percentage of the limb's total reinforcement.
Most composite limbs have a thickness profile that varies along the limb length. Limb thickness is typically greatest in high stress areas such as the tip end where string load is introduced and the butt where the limb loads are transferred to the bow's riser. The thinnest limb section is typically located between tip end and butt end, most often near the tip end where the lowest bending stiffness and greatest flexion is desired.
The final limb thickness profile is obtained using one or a combination of several well known manufacturing methods such as compression molding fiber and resin into the desired geometry, bonding prefabricated layers together to achieve the desired geometry, or machining precured fiber reinforced polymer composite bar stock into the desired geometry.
An aperture is typically machined through the width of an otherwise solid limb tip end to accommodate a string cam axle or other string mounting component. The machining operation severs primary load bearing fibers and locally weakens the structure. Additional material must be incorporated above and below the aperture location as well as between the aperture and tip end to adequately support static and dynamic string loads. The extra tip end material reduces flexibility and adds undesirable weight, both of which negatively affect bow efficiency; a stiffer tip end reduces limb flexion and shortens the power stroke or energy storage potential and the extra tip end weight increases limb inertia which results in less energy transfer to the arrow or bolt.
The general object of the present invention is to provide a more efficient bow limb construction for greater energy storage and transfer of stored energy.
Embodiments of the present invention relate to bow limbs containing continuous fiber reinforced polymer composite materials and more specifically to bow limbs in which at least a portion of the continuous reinforcing fibers embedded in a polymer matrix extend from the limb body into the tip end and form a loop therein before returning into the limb body. The tip end loop interior is sized and molded to accommodate a string cam axle or other string mounting component.
The fiber reinforced tip end loop surrounds and supports a portion of the string cam axle or other string mounting component's perimeter and efficiently transfer string loads into the limb body without the need for additional tip end reinforcements above and below the aperture as well as between the aperture and the tip end. The resulting low profile limb tip end is more flexible for greater energy storage and has less inertia for more efficient energy transfer to the bow projectile.
In another aspect of the present invention the tip end loop interior is sized to include a bushing for a string cam axle or other string mounting component. The bushing provides additional constraint and effectively transfers load into the tip end loop via direct contact with a portion of the loop perimeter.
In another aspect of the present invention the remaining tip end loop interior volume remains hollow for maximum weight savings or is filled in part or whole with a supporting core or vibration damping material or a combination of these design features. A supporting core inside the tip end loop maintains the tip end loop shape under load. The core also constrains and supports a portion of a string cam axle or other string mounting component's perimeter. A vibration damping material residing inside the tip end loop dampens limb vibrations for greater shooting comfort.
In yet another aspect of the present invention crack arrestors are incorporated to resist peel and shear induced failure at the tip end loop crotch and along the interface where the loop legs merge and transition into the limb body. A toughened adhesive is placed between the merging loop legs to increase the interface's peel and shear strength. A V-shaped structure is co-cured with or adhesively bonded into the loop crotch to mitigate peel and shear induced failures. These crack arrestors are used individually or in combination to improve limb tip end durability.
Enlarged side views of the solid limb tip end 122 are illustrated in
A method of making the limb according to one embodiment of the present invention utilizes unidirectional fiberglass (e.g., E-glass) reinforced epoxy prepreg with a fiber areal weight of approximately 250 g/m2 (7.37 oz/yd2) and an uncured resin content of approximately 38% by weight. Prepreg layers with the unidirectional fibers oriented parallel to the limb's length are cut into strips approximately 152.4 mm (6.00 in) wide and having various lengths. A plurality of main prepreg layers (e.g., approximately 9 layers) 150 wrap around a mandrel (e.g., wedge shaped) 161 and extend the entire limb length as illustrated in
The prepreg layers 150, 152 are tacked together using low heat and contact pressure or contact pressure only to build up a near net shape limb preform 160 over a wedge shaped mandrel 161 as illustrated in
The limb preform 160 and wedge shaped mandrel 161 are then packed in a compression mold 172 as illustrated in
Section A-A in
The packed mold 172 is then transferred to a heated platen press. The press platens compress the packed mold 172 and consolidate the limb preform 160 as it cures for approximately 20 min at approximately 135° C. (275° F.). After demolding the molded limb blank 181 from the compression mold 172 the wedge shaped mandrel 161 is extracted from the molded limb blank 181 as illustrated in
In other embodiments the individual limbs may be cut from the molded limb blank 181 using, for example, a rotary bit or a water jet cutter. In such embodiments the cut limb width may be constant or vary along the limb's length.
Referring again to the illustration in
Individual limbs may undergo additional machining operations such as creating slots in the tip end, trimming to a final length, creating notches in the butt end, rounding outer edges, etc. For example, in some embodiments a portion of the limb tip end may be machined away to produce the two limb tip ends 125 of a split limb 121. Each limb tip end 125 may be approximately 11.12 mm (0.438 in) wide but may be between 6.35 mm (0.250 in) and 25.40 mm (1.000 in) wide. In other examples a portion of the limb butt end may also machined away to produce a notch for mounting on the bow riser. The butt end 127 notch may be approximately 9.53 mm (0.375 in) wide but may be between 6.35 mm (0.250 in) and 19.05 mm (0.750 in) wide.
In other embodiments, the limb fiber reinforcements may be selected from a group including but are not limited to glass (e.g., E-glass, S-glass, etc.), carbon, aramid (e.g., Kevlar, Technora), polyethylene (e.g., Spectra), polypropylene (e.g., Innegra S), polyamide, cellulose (e.g., hemp, flax), basalt, and liquid crystal polymer fibers, and combinations thereof.
In other embodiments, the limb resin may be selected from a group including but not limited to epoxies, polyesters, vinyl esters, thermoset polyurethanes, polyamides, polypropylenes, polyethylenes, thermoplastic polyurethanes, polyethylene terephthalates, polyphenylene sulfides, polyetheretherketones, and combinations thereof. The limb resin cure time and temperature may differ depending on the selected resin or resin combination.
In other embodiments, the number, length, location, fiber orientation, fiber areal weight, and resin content of each prepreg layer may differ to achieve the desired molded limb thickness profile and performance. In other embodiments, layer lengths and start locations may be modified to shift the limb flex point further from or closer to the limb tip end.
In other embodiments, two or more different fiber/resin prepreg systems may be used in the limb preform assembly to achieve the desired molded limb performance. In other embodiments, several fiberglass/epoxy prepreg layers may be replaced by stiffer and stronger carbon/epoxy prepreg layers to produce a lighter limb construction. In other embodiments, liquid crystal polymer fiber/epoxy prepreg layers may be combined with fiberglass/epoxy prepreg layers to produce a limb construction that vibrates less after shooting.
According to one embodiment, a tip end loop 190 may be centered on a limb body's centerline 191 to reduce peel and shear forces acting on a loop crotch 192 as illustrated in the enlarged side view shown in
With continued reference to
The prefabricated bushing material may be selected from a material group including but not limited to fiber reinforced polymers, filled and unfilled polymers (e.g., polyimides, polyamides, UHMW polyethylenes, acetals, PTFEs, PEEK, polyimides, etc.), woods, metals (e.g., bronze, brass, aluminum, etc.), graphite/metal alloys, and combinations thereof.
With continued reference to
In other embodiments, the tip end loop bushing 211 may be prefabricated and press fit or adhesively bonded in the hollow interior of a molded tip end loop 210.
In other embodiments, the limb preform may be assembled by wrapping main prepreg layers around a prefabricated wedge shaped core 221 molded from fiber reinforced polymer composite prepreg layers. After compression molding the limb blank, an aperture 223 is machined through the core to accommodate a string cam axle 224.
In other embodiments, the prefabricated core 221 may be selected from the group including but not limited to fiber reinforced polymers, filled polymers, unfilled polymers, polymeric foams, polymeric honeycombs, woods, metals, metallic honeycombs, and combinations thereof.
In other embodiments, the core 221 may be prefabricated and adhesively bonded into the hollow interior of a molded limb tip end loop 220.
In other embodiments, the core may occupy only a portion of the hollow interior of a molded tip end loop.
In other embodiments, the vibration damping insert 231 material may be selected from the material group including but not limited to other elastomers such as other thermoset or thermoplastic elastomers, rubbers (e.g., natural, polyisoprene, neoprene, nitrile, butyl polyurethane, polybutadiene, silicone, EPDMs, etc.), elastomeric foams, gels, and combinations thereof. In other embodiments, the insert 231 may be a compound of two or more polymers or two or more discrete materials such as one material molded over a second material.
Referring again to an illustration in
In other embodiments, a tip end loop may include crack arrestors to mitigate peel and shear induced failures at the loop crotch and along the merged loop leg interface. One such tip end loop 240 contains a toughened adhesive interlaminar crack arrestor 242 extending from the loop crotch 241 at least partially into the limb body as illustrated in
In other embodiments, the toughened adhesive may be selected from the group including but not limited to silicones, polyurethanes, methacrylates, and polysulfides. In other embodiments, the toughened adhesive may be fiber reinforced.
With continued reference to
In other embodiments, a wedge shaped crack arrestor 244 may be prefabricated and adhesively bonded into the molded tip end loop 243. The prefabricated crack arrestor 244 material may be selected from the group including but not limited to fiber reinforced polymers, filled polymers, unfilled polymers, woods, metals, and combinations thereof.
In other embodiments, a toughened adhesive crack arrestor 242 and wedge shaped crack arrestor 244 may both be used to mitigate peel and shear induced failures.
In other embodiments, reinforcing fibers may be inserted through the limb thickness prior to molding to strengthen the loop leg interface properties.
In other embodiments, a tip end loop may include two or more of the aforementioned tip end loop design features.
In other embodiments, a portion of a tip end loop may include one or more of the aforementioned tip end loop design features with at least a portion of the tip end loop interior remaining hollow.
In another method of making a limb of the present invention the limb preform may be fabricated by wrapping the main prepreg layers over a wedge shaped rubber mandrel. Additional prepreg layers are interleaved between the main prepreg layers to build limb preform thickness. Rubber thermal expansion during the compression molding cycle helps consolidate of the tip end loop prepreg layers against the mold cavity. The wedge shaped rubber mandrel is pressed or pulled out of the molded limb blank tip end loop interior and the aforementioned machining operations are used to produce individual limbs.
In another method of making a limb of the present invention a nylon or rubber bladder may be slid over a wedge shaped mandrel. Prepreg layers are then wrapped over the bladder covered mandrel. Additional prepreg layers are interleaved between the main prepreg layers to build limb preform thickness. The bladder is sealed and inflated during the compression molding cycle to consolidate the tip end loop prepreg layers against the mold cavity as the limb preform cures. The wedge shaped mandrel and bladder are pressed or pulled out of the molded limb blank tip end loop interior and the aforementioned machining operations are used to produce individual limbs.
In another method of making a limb of the present invention the limb preform may be fabricated by wrapping the main prepreg layers over a sacrificial wedge shaped mandrel made from a material such as a eutectic salt. Additional prepreg layers are interleaved between the main prepreg layers to build limb preform thickness. After compression molding the sacrificial mandrel is machined and or washed out of the molded limb blank tip end loop interior and the aforementioned machining operations are used to produce individual limbs.
In another method of making a limb of the present invention dry unidirectional fiberglass fabric reinforcement may be laid up over a wedge shaped mandrel and held in place with a tackifier such as a spray adhesive. Additional dry reinforcements are interleaved between the main dry reinforcement layers and tacked in place to build limb preform thickness. The dry limb preform is placed in a clamshell mold with the desired limb geometry and infused with a low viscosity epoxy infusion resin. After demolding the limb blank the wedge shaped mandrel is pressed or pulled out of the tip end loop interior and the aforementioned machining operations are used to produce individual limbs.
In other configurations the dry limb preform fiber form may be selected from the group including but not limited to fiber tow, mat, and woven fabric, and combinations thereof.
In other configurations the dry limb preform fiber type may be selected from the group including but are not limited to glass (e.g., E-glass, S-glass, etc.), carbon, aramid (e.g., Kevlar, Technora, etc.), polyethylene (e.g., Spectra), polypropylene (e.g., Innegra S), polyamide, cellulose (e.g., hemp, flax), basalt, and liquid crystal polymer fibers, and combinations thereof.
In other configurations the dry limb preform infusion resin may be selected from the group including but not limited to other thermoset resins (e.g., polyesters, vinyl esters, polyurethanes, etc.) and thermoplastic resins, and combinations thereof.
In another method of making a limb of the present invention the limb's main prepreg layers may be wrapped around a long central core 252 and form a loop at the tip end of a limb preform 250 as illustrated in the enlarged side view in
In another method of making a limb of the present invention the limb's main prepreg layers may be wrapped around a prefabricated core and form a loop at the tip end of the limb preform. The prefabricated core is a cured laminate comprised of fiberglass reinforced epoxy composite layers. The assembled limb preform is compression molded to consolidate and cure the limb prepreg layers as well as bond them to the prefabricated core. After demolding and cutting the limb blank to obtain an individual limb an aperture is drilled through the core just inside the tip end loop to accommodate a string cam axle or other string mounting component.
In other configurations the prefabricated core material may be selected from the group including but not limited to other fiber reinforced polymers, filled polymers, unfilled polymers, polymeric foams, polymeric honeycombs, woods, metals, metallic honeycombs, and combinations thereof.
In another method of making a limb of the present invention the limb preform may be assembled and fabricated by wrapping thermoplastic prepreg layers over a wedge shaped mandrel and melt fusing each new layer to the previously consolidated layer or in the case of the very first layer to itself. Additional thermoplastic prepreg layers are interleaved between the main prepreg layers and melt fused to the previously consolidated layer to build limb preform thickness. After completing the layup assembly the wedge shaped mandrel is pressed or pulled out of the limb blank tip end loop interior and the aforementioned machining operations are used to produce individual limbs.
In other configurations the thermoplastic composite limb preform may be compression molded in a heated platen press to ensure full consolidation of all prepreg layers.
Although the present invention has been illustrated and described herein, those skilled in the art will recognize that various modifications and material equivalents may be substituted without departing from the scope of the invention.
This application claims the benefit of U.S. Provisional Patent Application No. 62/429,474, filed Dec. 2, 2016, entitled “COMPOSITE BOW LIMB”.
Number | Date | Country | |
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62429474 | Dec 2016 | US |