Patent Application
20070131841
The present invention relates generally to archery bows and more particularly pertains to an archery bow limb, an archery bow limb compression mold and methods for manufacturing archery bow limbs.
Archery bow limbs perform the important function of storing energy when the archer draws the bowstring. When the bowstring is drawn, the pre-stressed bow limbs, which are typically made of resilient material, are further flexed to store additional energy. When the bowstring is released, the stored energy propels the arrow. In conventional compound bows, the limb is typically formed of a single element with a substantially rectangular cross section, where one end is attached to the bow handle and the other end has a limb tip slot formed therein, in which a rotational member such as a wheel, cam or pulley is mounted.
Reinforced glass fiber materials have been utilized in archery bow limbs for a number of years. In some instances, the limb profile is machined from extruded solid glass fiber billets, and in other instances the limb profile is machined from pre-formed compression molded billets, which in some cases may be pre-formed to such near net shape that only secondary machining operations are required to remove excess material from the limb tip area and from the butt section, where the limb is joined to the handle. The machining operations can result in the severing of load bearing fibers which reduces the maximum limb operating stress level and the fatigue life of the limbs. In particular, machining out the limb tip slot can sever numerous fibers.
In certain preferred embodiments, the present invention is concerned with an archery bow limb compression mold assembly, a method for manufacturing compression molded archery bow limbs using the mold assembly, and the archery bow limbs produced thereby.
In a preferred embodiment of the present invention, a compression mold for forming an archery bow limb includes a mold assembly defining a mold cavity. The mold cavity receives a resin and fiber slug and is defined in the shape of an archery bow limb. The mold cavity defines a limb tip area with outer limb tip portions. The mold assembly includes a protrusion formed in the mold cavity. The protrusion preferably has a crown and is configured to urge glass fibers in the slug into outer limb tip portions in the archery bow limb.
In another preferred embodiment of the present invention, a mold for forming an archery bow limb comprises a mold assembly defining a mold cavity. The mold cavity receives a slug including resin and glass fibers. The mold assembly defines the mold cavity in the shape of an archery bow limb. The mold further includes means for urging the glass fibers into outer limb tip portions at a distal tip of an archery bow limb.
In yet another preferred embodiment of the present invention, a method of manufacturing an archery limb for an archery bow comprises placing a moldable slug in a compression mold assembly defining an archery bow limb profile. The moldable slug preferably includes a plurality of longitudinally oriented glass fibers and resin. The mold assembly preferably urges the glass fibers in an area defining a limb tip into outer limb tip sections. The method further includes compressing the slug into the mold assembly to form the shape of an archery bow limb with separated fibers and curing the glass fiber slug.
It is an object of this invention to provide an archery bow limb mold configuration, methods of manufacturing compression molded archery bow limbs, and to provide archery bow limbs.
Other objects and attendant advantages of this invention will be readily appreciated as the same become more clearly understood by references to the following detailed description when considered in connection with the accompanying drawings.
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations, modifications, and further applications of the principles of the invention being contemplated as would normally occur to one skilled in the art to which the invention relates.
In preferred embodiments, a compression mold for forming an archery bow limb comprises a mold assembly. The mold assembly defines a mold cavity, in the shape of an archery bow limb, for receiving a resin and fiber slug. The mold cavity defines a limb tip area with outer limb tip sections. The mold assembly further comprises a protrusion formed in the mold cavity. The protrusion preferably urges the fibers in the slug in the limb tip area into the outer limb tip sections. The protrusion defines a depression in the rough limb which can be ground to form a limb tip slot in the archery bow limb.
A method according to the present invention forms a limb for use in an archery bow. A preferred method comprises inserting a moldable slug including resin and a plurality of longitudinally oriented fibers into a bow limb profiling mold. A typical mold consists of two halves, a first half containing one or more female cavities and a second half having one or more mating male sections. In preferred embodiments of the present invention, the first half of the mold cavity includes a protrusion to form a depression in the limb and urge the fibers in that portion toward outer sections of the mold cavity. Heat and pressure are applied for initial curing. The limb is then removed from the mold, flashed and post-cured. The depression is ground to form a slot in the bow limb tip.
Bowstring 34 includes upper end 28 and lower end 30 which are fed-out from pulleys 16 and 18 when the bow is drawn. Bowstring 34 is mounted around pulleys 16 and 18 as is known in the art. Anchor cable 32 preferably extends from an eccentric pulley on one limb, for example axle 20, to the extremities of the opposing bow limb, for example axle 22. The opposed upper bow limb 12 and lower bow limb 14 are relatively short and will characteristically have high spring rates.
When the bowstring 34 is drawn, it causes eccentric pulleys 16 and 18 at each end of the bow to rotate, feeding out cable and bending limb portions 12 and 14 inward, causing additional energy to be stored therein. When the bowstring 34 is released with an arrow engaged to the bowstring, the limb portions 12 and 14 return to their rest position, causing the eccentric pulleys 16 and 18 to rotate in the opposite direction, to take up the bowstring 34 and launch the arrow with an amount of energy proportional to the energy initially stored in the bow limbs. Bow 10 is described for illustration and context and is not intended to be limiting. The present invention can be used with dual-cam compound bows, or can be used with single-cam bows as described for example in U.S. Pat. No. 5,368,006 to McPherson, hereby incorporated herein by reference, or can be used in other pulley/cam arrangements. The present invention can also be used in other types of bows, which are considered conventional for purposes of the present invention.
Illustrated in
Referring to
Both the glass fiber and the resins used in this process are well known in the art. In certain preferred embodiments, the resin is a suitable plastic resin matrix material. Suitable materials include glass fiber filaments which are packaged in spools and sold by Pittsburgh Plate Glass Corp. under the designation No. 712-218, to be employed with Shell 826 epoxy resin and a suitable heat activated catalyst such as Lindride 6K manufactured by Lindow Chemical Company. It has been found that the range of suitable glass fiber to resin ratios by weight is from 60% to 75% which is the equivalent of a glass fiber to resin ratio by volume in the range of 42% to 59%.
When slug 36 is in suitable condition to be molded, it is inserted into the mold assembly 42 illustrated in
Preferably the mold cavity defines a butt section 70 for a limb, a hinge section 72 for a limb, and a tip section 74 for a limb. In one option (not shown), the mold defines a split area in the butt section to enable the limb to be attached to a bolt on a bow riser. In preferred embodiments, tip section 74 includes a protrusion 52 having a crown and defining a partial height raised or split area to form a depression or reduced height limb tip portion which can be ground down to form slot 25 for a pulley to be mounted.
Protrusion 52 preferably includes an apex or summit area 65 rising to a peak or upper portion. In some embodiments, the apex or summit area is a rounded segment. In certain other embodiments, the protrusion includes a plurality of straight, tapered segments. Additionally, the protrusion can include a combination of rounded segments and straight segments together defining a tapered height of the protrusion. The protrusion preferably runs longitudinally for a portion of the length of the mold cavity. In certain preferred embodiments, the protrusion includes a length corresponding to a desired slot depth in the archery bow limb tip.
The protrusion divides the cavity at tip section 74, leaving two outer limb tip sections 67 of cavity 46. The two outer limb tip sections of the cavity will define the two outer limb tip portions of limb tip section 174.
Using the prior art mold illustrated in
In preferred embodiments, the protrusion separates and urges a substantial majority of the glass fibers in the slug from the middle width area toward the two side or fork sections of the cavity, packing the glass fibers into the outer limb tip portions. This may occur during the compression step. Separating and urging of the glass fibers may also occur when the slug is placed in the mold cavity. It should be appreciated that, in certain embodiments, a minimal amount of glass fibers will remain in the depression and may be severed during machining of the limb tip slot.
In an alternative embodiment, at least one half of the mold, preferably the female cavity 5 section, is formed with a rib cavity (not shown) to define a rib portion in the hinge section of the limb. A limb formed with such a mold would integrally include the rib portion. In such embodiments, the rib portion of the limb increases the sectional modulus of the limb, i.e., the limb's spine or stiffness is increased. This also allows thinner limb tips, reducing the amount of mass moved as the limbs recoil from a drawn to brace position. The lower mass in the tips also 10 reduces the moment of inertia, enabling the limbs to react more quickly and at a higher frequency resulting in higher arrow velocity.
In manufacturing the limb, the initial curing of the slug 36 occurs when slug 36 is inserted and compressed into the mold assembly 42 which has been heated to an operating temperature of approximately 300 degrees to 350 degrees Fahrenheit. Slug 36 is preferably maintained in the closed mold assembly 42 at this temperature for a period of 5 to 10 minutes, whereby slug 36 is set to assume the profile determined by the mold assembly 42. Slug 36 is then removed from the mold assembly 42 and the uncured glass fiber filaments forming the tail 38′ are severed (see
In a preferred feature, side edges 178 of the formed limb 12 are molded with a radius along their length by radiused lower corners 59 in lower mold 44. This is provided to avoid having to machine grind or cut stress-inducing sharp corners. By molding in this radius, the fiber filaments are uncut, continuous and protectively sealed in the typically stressed comer areas.
In certain preferred embodiments, the butt 170, hinge 172 and tip 174 sections of limb 12 all have a constant cross-sectional total area. Preferably the cross-sections of the mold are formed in calculated dimensions to maintain a substantially constant cross-sectional area along the length of the mold and in resulting limbs. A cross-section of the mold's hinge section 72 is shown in
As illustrated in
In certain preferred embodiments, the tapered apex or summit area of the protrusion is formed in the mold to disperse the glass fibers. The protrusion is preferably configured to divide the fibers along a centerline of the mold cavity at the tip section and urge the fibers away from the centerline, thereby guiding the fibers towards the outer portions of the mold cavity, such as the two outer limb tip sections. As illustrated in
In preferred embodiments, the glass fibers are urged into the two outer limb tip sections of the mold cavity, thereby creating an increased fiber density in the outer limb tip portions as compared to the remainder of the bow limb. In some embodiments, the glass-to-resin ratio is constant in all areas of the limb except for the tip section, where a higher glass-to-resin ratio occurs. A certain amount of resin is necessary in the glass-to-resin ratio, as is known in the art, to make certain that the slug forms into a bow limb as desired. The ratio is preferably calculated before the mold process begins to ensure that the required amount of resin remains in the two outer limb tip sections of the mold cavity.
While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.
