GB2080128A - Arrows - Google Patents

Abstract

An archery arrow (10) includes an arrow shaft (20) comprising an aluminium tube (26) having an outer layer (28) of high strength fibres in the form of a fibre matrix composite which is applied to the outer surface of the aluminium tube. Alternatively, or in addition, an inner layer of reinforcing fibres may be applied to the inner surface of the aluminium tube. <IMAGE>

Description

SPECIFICATION Arrows This invention relates to arrows.

The design of an arrow requires a delicate balancing of the physical properties and structural characteristics of the arrow shaft. For example, in the physical design of an arrow it is desirable to minimize the weight and the diameter of the shaft to increase velocity and reduce aerodynamic drag. At the same time, it is structurally desirable to maximize arrow stiffness to reduce flexing of the shaft which occurs when the arrow is subjected to the accelerating forces of the archery bow. Excessive flexing may cause permanent arrow shaft distortion, slow the arrow, and result in poor shooting accuracy. Additionally, to further improve shooting accuracy, the shaft must exhibit rapid damping of the bow induced flexural oscillations that result from a phenomenon known as archer's paradox. Accordingly, a shaft having a high elastic recovery rate is desirable.Arrow shafts must also possess sufficient impact resistance to survive the forces at target impact without fracture or permanent deformation.

Throughout the early history of archery, arrows were traditionally fabricated of wood.

In a continuing effort to improve the consistency, durability and accuracy of arrows, modern materials have been employed in their contruction. The use of aluminium and, more recently, fibreglass tubing in the construction of arrow shafts has resulted in significant improvements in arrow design. However, in prior arrow design, an increase in shaft stiffness and elastic recovery rate has required a heavier and larger diameter arrow shaft, thus compromising its velocity and trajectory characteristics.

According to the present invention there is provided an arrow shaft comprising a metal tube having a reinforcing material applied to a surface of the tube.

In a preferred embodiment, the core is formed of a hollow aluminium tube. The aluminium tube is reinforced by an outer layer of high strength fibres which is applied in the form of a fibre matrix composite to the entire outer surface of the aluminium tube. It has been found that this combination of materials provides a tubular construction having physical properties which permit the construction of an improved arrow shaft. Selection and optimization of the physical properties of the arrow are achieved by dimensional variations of the aluminium core and the fibre layer in combination with variations in the thickness and fibre orientation of the fire reinforcing layer.

In an alternative embodiment of the invention, the reinforcing material comprises a layer of reinforcing fibres which is applied to the entire inner surface of the metal tube. This inner reinforcing layer may be used in place of or in addition to an outer reinforcing layer in the construction of an arrow shaft.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a view illustrating an arrow constructed in accordance with the invention; Figure 2 represents an enlarged cross-sectional view of the arrow shaft taken along section line 2-2 of Fig. 1; Figure 3 represents an enlarged partial cutaway perspective view of the arrow shaft of Fig. 1 showing the successibe layers partially removed; Figure 4 represents an enlarged cross-sectional view of the arrow shaft taken as in Fig.

2 and showing another embodiment of the invention; and Figure 5 represents an enlarged cross-sectional view of the arrow shaft taken as in Fig.

2 and showing still another embodiment of the invention.

Referring to Fig. 1 there is shown an archery arrow 10 (without feathers) comprising a shaft 20, a head 22 and a nock 24. In the preferred embodiment, the shaft 20 consists of a hollow aluminium core 26 as shown in Fig. 2, which is formed by successively drawing aluminium alloy tubing until the desired outside diameter, wall thickness and shaft length are achieved. Typical aluminium alloys employed for this application are alloy type 2024 and alloy type 7075. The method of using drawn tubing results in a core with extremely consistent wall thickness and wall diameter over the entire length of the shaft, and is the same method used to produce hollow aluminium arrow shafts in the prior art.

A typical range of dimensions for the aluminium core 26 include lengths from twenty-four to thirty-four inches (approximately 60 to 90 centimetres) outside diameters from two to four tenths of an inch (approximately 5 to 1 Omm), and wall thickness from five to fifteen thousandths of an inch (approximately 1.25 to 3.75 mm). The actual dimensions chosen for a particular arrow design depend upon the ultimate use of the arrow as described hereinafter.

The arrow shaft 20 of the preferred embodiment further includes an outer layer 28 of reinforcing fibres bonded to the outside surface of the aluminium core 26 as shown in Fig. 2. This outer layer 28 may be composed of reinforcement fibres embedded in a resin matrix. Although many different fibre materi als such as graphite, boron, carbon, or glass may be used in this application, it has been found that carbon fibres such as TORN EEL type 300/3K fibres manufactured by Union Carbide, oriented in a thermosetting epoxy resin matrix, such as type 934 resin manufactured by Fiberite Corporation, Winona, Minnesota, provide an optimum fibre matrix composite for this application. An arrow constructed in this manner has excellent elastic recovery and good impact resistance.

Several of the parameters in the design of an arrow are influenced by the type of archery bow used to shoot the arrow. For example, the length of the arrow shaft is a function of the size of the archer and shape of the bow; and the stiffness of the arrow shaft is a function of the length of the arrow shaft and of the force imparted by the bow to the arrow.

A greater bow force requires a stiffer arrow shaft to reduce initial shaft flexing when the bow is released. The stiffness, or spine, of an arrow shaft has traditionally been measured as the deflection of the shaft when depressed by a standard weight at the shaft center. The determination of the required length and stiffness of the arrow shaft in turn dictate the shaft diameter and wall thickness necessary to achieve these requirements.

Fibre reinforced aluminium arrow shafts constructed in accordance with the invention have an outside shaft diameter of from twenty to twenty-five percent less than the outside diameter required of an unreinforced aluminium shaft of comparable length and stiffness.

Fibre reinforcing of the shaft also results in a twenty to twenty-five percent reduction in shaft weight which yields a ten to fifteen percent increase in initial arrow velocity over an unreinforced arrow shaft having the same length and stiffness. A decrease in outside shaft diameter has the further effect of reducing aerodynamic drag during arrow flight, resulting in lower trajectories.

It has also been found that the fibre reinforcing layer acts to rapidly dampen the flexural oscillations and bending modes induced during bow release. It is known to those skilled in the art that the forces on the arrow shaft during bow release cause the shaft to bend and undergo several cycles of oscillation about the axis of final arrow flight. The fibre reinforcing layer absorbs the elastic energy, which causes the shaft oscillations to dampen rapidly so that the arrow assumes true flight attitude within a short distance from the bow.

The result is an improvement in shooting accuracy.

The embodiment of the invention shown in Fig. 2 may be formed by a method which includes the steps of successively drawing aluminium alloy tubing to achieve the required length, outside diameter, and wall thickness for the hollow aluminium core 26 of the arrow shaft 20. The tubing is drawn to a length which is typically one or two inches (25 to 50 cm) longer than the desired final length for the shaft 20. The outer surface of the core 26 is etched chemically to remove any oxide layer, and a thin film of contact adhesive is uniformly applied to the etched surface. A sheet of a fibre matrix composite 28 is then wrapped around the outside of the core 26.

This fibre matrix composite 28 consists of carbon fibres oriented in a thermosetting epoxy resin matrix. The orientation of the fibres relative to the axis of the core 26 may be varied to achieve different reinforcement characteristics for the shaft 20. The sheet of the fibre matrix composite 28 is typically three to four mils thick, the length of the sheet being equal to the length of the core 26 so that a single wrap forms a uniform layer of composite on the outside surface of the core 26. The number of wraps and hence the thickness of the reinforcing composite 28 may be increased to achieve greater stiffness of the shaft 20. One method of varying the orientation of the fibres is to wrap successive layers with a plurality of matrix composite sheets each oriented to provide the desired angle of fibre orientation.For example, a first layer may contain fibres oriented at a 30 angle to the axis of the core 26, and a second layer may contain fibres oriented parallel to the axis of the core 26.

A heat shrinkable plastics film formed of a material such as TEDLAR, manufactured by Dupont Corporation, is then spirally wound over the layers of the composite 28. This assembly is then heated to 1 75on for one hour to heat cure the epoxy resin. During the curing, the spiral wound plastics film shrinks around the resin matrix applying sufficient pressure to eliminate voids and provide a more uniform density to the reinforcing composite 28. At the completion of the cure, the heat shrinkable plastics film is removed and the shaft 20 is centerless ground to the desired final dimensions for the outside diameter. The centreless grinding also assures uniform wall thickness around the circumference of the shaft 20 to produce an arrow 10 which is balanced about its axis. The shaft 20 is cut to its final length by removing excess material from both ends. This ensures that any unevenness of wrap which may occur at the ends of the shaft 20 is discarded. The properties of a particular example of the heat cured fibre reinforcing layer include a Young's modulus of elasticity of 17,100,000 p.s.i. (120 kN/mm2), a flexural strength of 254,000 p.s.i. (1.75 kN/mm2) and a tensile strength of 209,000 p.s.i. (1.45 kN/mm2).

The completed shaft 20 is fitted in a conventional manner with a suitable arrow point 22 and nocking 24 to form the fibre reinforced aluminium arrow 10 shown in Fig. 1.

Fig. 3 illustrates a cut-away perspective view of a shaft 20 constructed in accordance with the embodiment shown in Fig. 2.

A cross section of an alternative embodiment of the invention is shown in Fig. 4. in this configuration the fibre reinforce ment is in the form of an inner layer 30 of reinforcing fibres bonded to the inside surface of the aluminium tube 26. This inner layer 30 may be composed of the same fibre matrix composite discussed heretofore.

The embodiment of the invention shown in Fig. 4 may be formed by producing a hollow tube from the resin matrix composite having suitable dimensions to the inserted into the aluminium tube 26. The assembly is then heat cured to bond the resin matrix layer 30 to the tube 26.

A cross section of another embodiment of the invention is shown in Fig. 5. This configuration represents the combination of the embodiments shown in Figs. 2 and 4 (discussed above) and results in an arrow shaft 20 having an aluminium tube 26, an outer fibre reinforcing layer 28 and an inner fibre reinforcing layer 30. Fig. 3 illustrates a cut-away view of a shaft 20 constructed in accordance with the embodiment shown in Fig. 2.

While the invention is disclosed an particular embodiments thereof are described in detail, it is not intended that the invention be limited solely to these embodiments. Many modifications will occur to those skilled in the art which are within the spirit and scope of the invention.

Claims (11)

1. An arrow shaft comprising a metal tube having a reinforcing material applied to a surface of the tube.

2. An arrow shaft as claimed in claim 1, in which the reinforcing material comprises at least one layer of reinforcing fibres applied to the outside surface of the tube.

3. An arrow shaft as claimed in claim 1 or 2, in which the reinforcing material comprises at least one layer of reinforcing fibres applied to the inside surface of the tube.

4. An arrow shaft as claimed in claim 2 or 3, in which the fibres of the layer, or at least one of the layers, are oriented parallel to the axis of the metal tube.

5. An arrow shaft as claimed in any one of claims 2 to 4, in which the fibres of the layer, or at least one of the layers are oriented at an angle of 30 to the axis of the metal tube.

6. An arrow shaft as claimed in any one of claims 2 to 5, in which the reinforcing fibres, or some of them, are embedded in a resin matrix.

7. An arrow shaft as claimed in claim 6, in which the resin matrix comprises an epoxy resin.

8. An arrow shaft as claimed in any one of claims 2 to 7. in which the fibres. or some of them, are carbon fibres.

9. An arrow shaft as claimed in any one of the preceding claims, in which the metal tube is formed of aluminium.

10. An arrow shaft substantially as described herein with reference to and as shown in Figs. 1 to 3, in Fig. 4 or in Fig. 5 of the accompanying drawings.

11. An arrow including an arrow shaft as claimed in any one of the preceding claims.