US20030017884A1

US20030017884A1 - Golf club shaft with superelastic tensioning device

Info

Publication number: US20030017884A1
Application number: US09/822,701
Authority: US
Inventors: Brett Masters; Marthinus Schoor; Alexander Jessiman
Original assignee: Mide Technology Corp
Current assignee: Mide Technology Corp
Priority date: 2001-03-30
Filing date: 2001-03-30
Publication date: 2003-01-23

Abstract

A shaft for a golf club or other sporting equipment is disclosed, wherein the shaft is hollow and contains a wire or cable placed under tension therein, the wire being made of a superelastic material. The wire is connected at one end to a variation device such as a cam which varies the tension on the wire and thus the bending stiffness of the golf club. Because the wire is made of a superelastic material, for example Nitinol, it can reversibly elongate in response to pre-tensioning and dynamic stresses encountered during swinging the golf club, in order to counterbalance and accommodate, the stress encountered during normal use of the golf club, thus ensuring a long life and preventing damage to the golf club shaft.

Description

FIELD OF INVENTION

The present invention relates to shape memory alloys, which are materials capable of recovering their original shape after being deformed under stress, and more particularly relates to the use of such materials in sporting equipment such as a golf club or a hockey stick.

BACKGROUND OF THE INVENTION

Shape memory alloys (SMAs) are metal alloy materials that have the ability to return to their original shape after being deformed. All SMAs have two distinct crystal structures, or phases, with the phase present being dependent on the temperature and the amount of stress applied to the SMA. The two phases are martensite, which exists at lower temperatures, and austenite at higher temperatures. The exact structure of these two phases depends on the type of SMA, where the most commonly used type is called Nitinol. Nitinol is a mixture of two component metals, nickel (Ni) and titanium (Ti), which are mixed in an approximate ratio of 55% by weight Ni and 45% by weight Ti, and annealed to form a part in the desired shape.

Shape memory alloys possess two material properties that work together to provide shape memory. The first material property is an austenite to martensite transition in the SMA. This is a solid-to-solid phase transition from an austenite phase with high symmetry (such as a cubic molecular structure) to a martensite phase with lower symmetry (such as tetragonal or monoclinic structures). The second property of a shape memory alloy is the ability of the low-symmetry martensite structure to be deformed by twin boundary motion. A twin boundary is a plane of mirror symmetry in the material. If the twin boundary is mobile, as in certain martensite structures, the motion of the boundary can cause the crystal to rearrange and thus accommodate strain.

The coupling of the above two properties produces two distinct types of mechanical behavior in shape memory alloys. These two behaviors are referred to as “shape memory effect” and “superelasticity.”

The shape memory effect occurs when deformation incurred in the martensite phase via twin boundary motion is recovered by heating the material past a transition temperature to the high temperature austenite phase. The following three-stage model illustrates the changes undergone by a shape change alloy according to this effect:

In stage 1, the alloy is in the austenite phase. As the alloy is cooled below the transition temperature, T _m, the material tends to retain its original shape by inducing twin boundaries that allow the newly deformed (stage 2) crystal structure to occupy approximately the same volume as the stage 1 structure. Now, if stress is applied to the structure, it can deform by twin boundary motion. The twin boundaries move to rearrange the crystalline asymmetry to accommodate strain (thereby reaching stage 3). This rearrangement can occur in several directions, allowing the crystal structure to handle strain in multiple directions. Finally, when the material is re-heated, the asymmetry that permitted strain in the crystal structure disappears in the transformation, and the material recovers to its original (stage 1) shape. The particular orientation of the crystal structure in stage 2 is unimportant, as the material returns to only one structure, i.e. the original austenite structure of stage 1. Hence, such a material exhibits the shape memory effect. Thermally actuated shape memory materials such as Nitinol (NiTi) include an elastic range of up to 8% reversible elongation in some materials, and the yield stress is very low, thus allowing the material to deform easily in the martensite state.

Superelasticity uses the same deformation mechanisms as shape memory, but occurs without a change in temperature. Instead, the transformation is induced by stress alone. Applied stress can overcome the natural driving force which keeps the material at equilibrium in the austenite phase. By applying stress to the material, it can be converted into the martensite phase, and the crystal structure will strain to accommodate the applied stress. When this stress-energy is greater than the chemical driving force of stabilization in the austenite phase, the material will transform to the martensite phase and be subject to a large amount of strain. When the stress is removed, the material returns to its original shape in the austenite phase, since martensite cannot exist above the transition temperature. This superelastic behavior is fully reversible and does not require any change in temperature.

The full stress recovery of a superelastic material can occur with up to approximately 8% elongation in Nitinol (NiTi). Because of this large elastic range, superelastic materials are used in applications such as cardiovascular stents, mobile telephone antennas, and eyeglass frames. Superelastic materials have not previously been used in sporting equipment such as golf clubs or hockey sticks.

U.S. Ser. No. 09/158,172, commonly owned with the present application, discloses a variable stiffness shaft for use in a golf club, and is incorporated by reference in the present disclosure. As taught in the '172 application, a golf club includes a hollow shaft having a wire placed under tension inside the shaft, the tension being adjustable to a desired level. Such an invention is useful for varying the stiffness of the shaft to accommodate individual users and different anticipated levels of stress. However, the internal wire is customarily made of a material that does not have shape memory, such as steel (piano wire), titanium, aluminum, or a corrosion resistant plastic such as nylon, all of which are non-SMAs. Such materials can reversibly elongate by typically 0.33%-0.34%, and even as much as 1% for spring steel, but dynamic strains realized in a golf club shaft commonly range from 0.33% to about 1%, thus resulting in material failure. Accordingly, conventional materials are subject to damage after repeated blows on the golf course.

SUMMARY OF THE INVENTION

A hollow shaft for use in a golf club or other sporting equipment having a shaft is disclosed, wherein the shaft contains a tensioning device comprising a wire or cable made of a superelastic alloy. The tension level of the wire can be varied in order to reduce the bending stiffness of the shaft, in accordance with particular anticipated loads or to accommodate the player's individual stroke. Initially, the wire is pre-tensioned by mechanically tightening the wire using a variation means. As a result of being tightened, the wire elongates by approximately 0-7% (i.e. any amount within the wire's elastic range of up to 8%) against the shaft stiffness, which is known as a pre-stress or pre-tension level of the wire. The shaft is pre-tensioned by an amount less than the maximum strain level of 8% in the wire, so as to accommodate dynamically induced strain encountered during a swing and contact with the ball.

As a result of pre-stressing, the shaft is compressed to the tension level produced by the wire elongation. By pre-compressing the shaft, the bending frequency of the shaft is reduced which reduces the net flex rating and improves performance of the golf club. Shaft pre-compression also tends to offset centrifugally induced shaft tension encountered during a swing, such that, upon impact of the golf club with a ball, a lower net strain level is present in the shaft as compared with uncompressed composite shafts. Thus, when coupled with preset strains, strain levels present in the shaft at impact do not exceed yield and failure strains of the shaft.

During the swing, centrifugal loads of the accelerating golf club head mass result in approximately 50-100 pounds of dynamic tension force being placed on the shaft. On top of this are short-term dynamic stresses and strains in the shaft that result from ball impact. Conventional composite golf club shafts degrade over time when used by hard swingers because the net dynamic strain (i.e. the large dynamic strain that results from swing centripetal acceleration forces coupled with impact dynamic strains) causes the material situated near the hosel end of the shaft to fail. By incorporating in the shaft a wire made of a superelastic shape memory alloy, measured levels of swing and impact-induced dynamic strain, which can reach approximately 0.33-1%, will not result in significant degradation of the shaft due to stress, as substantially all of the stress is absorbed by the wire. Whereas a composite shaft incorporating a wire made of a conventional material produces large stress changes in the shaft which accompany relatively small changes in strain, thus resulting in premature fatigue and failure.

Superelastic wires exhibit a large recoverable strain capability, and can recover approximately 0-8% of strain, or substantially the entire range of deformation produced in the wire. Since the superelastic wire can recover over a large strain range, even for nominal dynamic stresses above the pre-tension amount, the wire made of a superelastic alloy has superior fatigue and failure properties, and is also an extremely hard, corrosion-resistant material.

A preferred superelastic alloy is Nitinol (NiTi), which can reversibly elongate over an elastic range of up to approximately 8%, allowing the golf club to be swung repeatedly without damaging the shaft. As used herein, the terms “shape memory alloy” and “superelastic alloy” refer to a material having (i) an austenite to martensite solid-to-solid phase transition, and (ii) an ability for the martensite structure to be deformed by twin boundary motion. The preferred materials to be used in the present invention are superelastic alloys, which are further defined as materials that undergo the martensite to austenite phase transition without a significant change in temperature. In superelastic alloys, the martensite to austenite transition occurs due to the dynamically applied stress forces which overcome the natural driving force that keeps the material at equilibrium in the austenite phase.

The golf club with a wire made of Nitinol incorporated in the hollow shaft can respond to each swing by returning to its original preset shape. The wire can reversibly elongate under strain over an elastic range of up to approximately 8% of reversible strain. The use of a superelastic alloy in the golf club yields unexpected results in terms of high performance and long-lasting durability of the golf club. As taught in the '172 application, by placing a wire under tension inside a hollow shaft of the club, bending stiffness of the club can be reduced, thereby improving trajectory for each stroke. However, when the internal wire is made of conventional materials, it tends to stretch whenever the total stress exceeds the maximum level of approximately 0.33-1% tolerated by the wire material, and the wire becomes damaged over time. Such stretching and damage is minimized by the use of a wire made of a superelastic alloy, in accordance with the present invention. The applied stress produces elongation in the wire which is well within the recoverable strain of the superelastic wire and which causes only minimal variations in the pre-stressed wire. Thus, a golf club with a hollow shaft incorporating a superelastic wire (e.g. made of Nitinol) demonstrates high stroke performance but also a long term durability not present in golf club shafts made of a conventional material.

The stiffness of the hollow shaft is initially set by using a variation means to adjust the tension on a tensioning device disposed in the hollow shaft. The tensioning device is attached to the shaft at two points. Thus, applying tension to the tensioning device causes compression in the shaft region between the two points. The tensioning device, as explained above, can be a wire or cable made of a superelastic alloy, or a plurality of wires or cables. Many variation means are possible, including a cam, a pinned retractable piece, a lockable lead screw (which may be adjusted by an external actuator), a pump, a sleeve screw, or a “set and forget” displacement actuator. The shaft can have one or more constraint inserts, which can be made, for example, of low density foam, plastic, or an elastomer. The constraint inserts impede dynamic variation of the tensioning device. They can be held in the shaft by compression fit or by adhesive. The sporting equipment can be, for example, a golf club, a tennis racket, a ski pole, a hockey stick, a baseball bat, a fishing pole, a hurling stick, a lacrosse stick, or a vaulting pole.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawing figures wherein like reference characters denote corresponding parts throughout the several views and wherein: [0017]
FIG. 1 is a cross-sectional view of a preferred embodiment of a golf club shaft according to the present invention; [0018]
FIG. 2 is a cross-sectional view of another preferred embodiment of a golf club shaft; [0019]
FIG. 3 is a cross-sectional view of a further preferred embodiment of a golf club shaft; and [0020]
FIGS. [0021] 4A-4F are cross-sectional views of six alternative preferred embodiments of a variation means according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING THE PREFERRED EMBODIMENT(S)

The present invention discloses a golf club or other sporting equipment with a hollow shaft inside the golf club, where a wire or cable is placed under tension, allowing the bending stiffness of the golf club to be varied. In a particular aspect of this invention, the wire is made of a superelastic alloy, such that the wire recovers to its original pre-stressed state after a dynamically applied stress is released (i.e. after the club head strikes the ball). Accordingly, after repeated swings, the wire retains its original shape, with little variation from the pre-tensioned state, and does not suffer significant damage, thus prolonging the life of the golf club. [0022]
Such a design utilizes the well known concept that a beam's bending stiffness can be varied by preloading the beam along its longitudinal axis. The present invention involves the use of this principle to provide a variation mechanism for golf club shafts and other sporting equipment. The preloading device is the wire or cable (mentioned above) which is placed under tension in the hollow shaft of a golf club. Tensioning the wire/cable compresses the shaft along its bending axis, reducing the effective bending stiffness as compared to that of the unloaded shaft. An advantage of the present invention is that it allows production of a shaft having a high torsional stiffness, enabling a rapid response to player wrist turnover action, but having a low bending stiffness, so that the player achieves the best ball trajectory for a given swing speed. A detailed explanation of the above principle is provided later in this description. [0023]
FIG. 1 illustrates a preferred embodiment of a [0024] golf club shaft 10 having a butt end 12 and a hosel end 14, with a tensioning wire or cable 16 secured at both ends of the shaft. A variation mechanism 18 is located at the butt end 12 of the shaft, where it is readily accessible to the user of the club. FIG. 2 shows an alternate preferred embodiment of the golf club shaft, further comprising a wire termination insert 20 in the interior of the shaft. In this embodiment, the wire 16 extends only part way into the shaft, reaching from the butt end 12 to the termination insert 20. FIG. 3 shows a further preferred embodiment of the golf club shaft, wherein the shaft includes a plurality of constraint inserts 22, made for example from low density foam, which restrain lateral motion of the wire 16.
The internal tensioning wire/[0025] cable 16 is preferably made of a superelastic alloy which can reversibly elongate under an applied dynamic stress by an amount in excess of the normal expected strain of approximately 0.33-1% experienced in a golf club upon impact with the ball. A preferred superelastic alloy is Nitinol (NiTi), which exhibits an elastic range of up to 8% deformation, and is therefore able to fully recover from the expected strain experienced in a golf club. Such an elastic range provides significant advantages over conventional metals and alloys which generally exhibit strain levels very close to the expected level of 0.33-1%.

A problem encountered in conventional composite shaft golf clubs is that the dynamic stresses often produce unacceptable strain levels in the

wire

16, leading to premature fatigue and failure. Using such materials, the wire is often pre-tensioned to approximately its elastic limit, resulting in damage if any additional dynamic strain is applied during swinging. Repeated swings can lead to permanent damage of the golf club and a reduced useful life. Use of a superelastic alloy such as Nitinol enables the golf club to be swung repeatedly without damaging the shaft.

TABLE 1


Comparison of Material Properties of Nitinol With Other Materials

						304
	NiTi	6061	Cast	AM100A	4140	Stainless
Material	SMA	Al	Iron	Mg	Steel	Steel	Wood

Recoverable	8%	0.34%	0.23%	0.33%	0.33%	0.11%	0.42%
Strain (%)
Recoverable	˜7000	147	41	137	142	15	51
Energy (J/kg)
Density (kg/m³)	6450	2770	7064	1800	7830	7830	500
Modulus (GPa)	75 aus.	70	90	45	205	197	12.1
	28 mart.
Yield Stress	600	241	250	150	675	215	13.1
(MPa)
Melting Temp. (° C.)	1240	582	—	540	1400	1427	—

As shown in Table 1 above, Nitinol provides properties superior to those of conventional materials and is ideal for use in the wire/[0027] cable 16. The recoverable strain in Nitinol is approximately 8%, which is significantly greater than Al (0.34%), Mg (0.33%), and steel (0.33%). Spring steel (not shown) can recover up to approximately 1% of recoverable strain. Iron and stainless steel, the most common materials for golf club shafts, exhibit even lower percentages of recoverable strain, and thus are not practical for use in a composite shaft, where expected levels of strain can produce approximately 0.33-1% elongation. Even using aluminum, magnesium, or steel in the hollow shaft 10 can result in damage to the shaft because the recoverable strain of such materials is very close to the expected strain experienced in a golf club and can be exceeded whenever a large amount of stress is applied. Wood (0.42%) has a higher percentage of recoverable strain, but is not practical for use in the golf shaft due to its low yield stress.
Also as demonstrated in Table 1, the recoverable energy of Nitinol is extremely high compared to conventional materials, a property which stems from the ability of Nitinol and other superelastic alloys to convert from the austenite to the martensite phase, and then return to its original shape upon returning to the austenite phase. [0028]
While the preferred material for the [0029] wire 16 is a Nitinol alloy, other superelastic alloys and shape memory alloys can be used, including but not limited to mixtures of: nickel and aluminum (Ni—Al), copper and zinc and another element Cu—Zn—X (where the other element X can be silicon (Si), tin (Sn), or aluminum (Al)), copper and zinc (Cu—Zn), copper and tin (Cu—Sn), copper and aluminum and nickel (Cu—Al—Ni), iron and platinum (Fe—Pt), iron and manganese and silicon (Fe—Mn—Si), or manganese and copper (Mn—Cu).
When the [0030] wire 16 is made of one of the above superelastic alloys, e.g. Nitinol, the wire is subject to stress at three different stages: (1) when the wire is pre-tensioned by a desired amount, where the pre-tension amount is within the range of 8% maximum allowable strain so as to allow room for dynamically induced strain occurring during the swing and impact; (2) during the downswing as the result of tension forces loaded onto the shaft by the accelerating club head; and (3) as the golf club strikes the ball. Accordingly, the wire 16 is converted from austenite to martensite during stressing, and the martensite structure deforms by twin boundary motion in an elastic range of up to approximately 8% elongation to accommodate the stress. After the stress is released, the wire 16 returns to its original zero-strain, austenite state. The wire 16 is able to reversibly elongate under strain to accommodate the full amount of stress encountered during a golf swing.
A number of wire/cable tension variation systems are suitable for use with the invention. Several such systems are shown in FIGS. [0031] 4A-4F as applied to a golf club. FIG. 4A depicts a cammed lever variation system having four settings that span the accepted stiffness range for golf club shafts. As a cam 30 is rotated in the direction of the arrow, the tension is increased on the wire 16, resulting in a softening of the shaft. There are four settings for the illustrated cam 30, corresponding to the four faces of the cam, each of which is at a different distance from an attachment point 32 of the wire. As shown, the wire tension is at its lowest setting, corresponding to the stiffest shaft setting. It will be apparent to those skilled in the art that this type of variation system is not limited to a four-faced cam, but can be used with a cam having a different number of faces or with a continuous cam, as long as the variation system is capable of holding its setting. The illustrated cam 30 is held in any of the four settings by pressure on the flat face of the cam; a continuous cam can use a set screw or the like (not shown) to achieve the same end.
FIG. 4B depicts a plunger clevis/cotter pin variation system having four settings that span the accepted stiffness range for golf club shafts. In this arrangement, a [0032] plunger clevis 40 passes through a plug 42, which is attached to the butt end of the shaft 12. The clevis 40 is provided with several holes 44 perpendicular to its axis, and is attached to the wire 16 at point 46. A cotter pin 48 is adapted to slide into any of the holes, thereby varying the position of the plunger clevis 40 relative to the plug 42. It will be apparent to those skilled in the art that this type of variation system is not limited to having four levels, nor are the illustrated shapes of the clevis and cotter pin intended to be limiting.
FIG. 4C depicts a pneumatic or hydraulic variation system, where a constant force is applied to a [0033] piston 50 by a working fluid 52 which applies a constant tension to the wire 16. A pumping mechanism 54 allows a golfer to vary the tension by pumping working fluid 52 through a one-way valve (not shown). The tension can be relieved by opening the valve to allow back flow. This type of system is continuously variable over a range of tensions.
FIG. 4D depicts a lead screw variation system comprising a threaded [0034] lead screw 60, a threaded lock fitting 62, and a guide 64 attached to the butt end 12 of the shaft. The threads of the lead screw 60 engage the lock 62, which can thus be set at any point on the length of the lead screw 60. The lock is held in compression fit with the guide 64, allowing the lead screw to apply a tension to the wire 16. The attachment point 66 is preferably designed not to twist the wire when the lead screw 60 is turned. This type of system is continuously variable over a range of tensions. The lead screw can be turned by hand, or can be activated by an external actuator such as a battery powered electric screwdriver or the like. This type of system is continuously variable over a range of tensions.
FIG. 4E depicts a threaded sleeve variation system. A [0035] plug 70 is affixed to the butt end 12 of the shaft, the plug 70 having an outer thread. An inner threaded turn sleeve 72 is engaged with the plug 70. The turn sleeve supports a head 74, and can be adapted to slip relative to the head at their point of contact 76. The head is connected to the wire 16 at point 78. It will be seen that rotation of the turn sleeve 72 will raise or lower the head 74 to vary the wire tension over a continuous range.
FIG. 4F depicts an active set and forget displacement actuator variation system. A [0036] head 80 is connected to the wire 16 at point 82, and a standard set and forget actuator 84 is inserted between the butt end of the shaft 12 and the head 80. Such actuators are commonly known in the art, and generally are activated by connection of a separate power source. Once the actuator has been set to the desired length (and corresponding wire tension and shaft stiffness), the power source (not shown) can be removed for the swing. This variation system can vary the wire tension over a continuous range.
A detailed explanation will now be provided with respect to behavior of the golf club shaft according to the present invention. The fundamental behavior that this invention leverages is described by the general, unforced, one dimensional equation of motion for a beam, [0037]
(EIw″)″+(Pw′)′+m{overscore (w)}=0 (1)
where EI is the elastic modulus times the shaft cross section area moment of inertia that can vary along the length of the golf club shaft, P is the axial compression applied to the shaft that can also vary along the length of the shaft, m is the mass per unit length that can vary along the length of the shaft, and w is the lateral deflection which is a function of time and position along the length of the shaft. The first term of Equation (1) relates how the beam bending stiffness is affected by inertial loads through the second spatial derivative of the shaft internal moment (the bracketed term that includes second order spatial derivative of the deflection) with respect to the axial coordinate. The second term of Equation (1) is generally small and is typically neglected. The last term in Equation (1) represents inertial load (per unit length) resulting from motion, where force equals mass times the second derivative of the deflection with respect to time. [0038]
When a beam is under compression, the compressive load reduces the apparent bending stiffness of the beam by amplifying the lateral deflection. This effect can be visualized by considering a ruler being compressed along its measurement axis by pressing opposing ends together. When the center of the ruler is slightly perturbed laterally, the deflection is enhanced by the compression and the ruler bends. [0039]
This behavior can be seen mathematically, to first order, by assuming a deflection as a function of time, t, and position on the shaft, x, [0040] $\begin{matrix} w = A \sin (ω t) \sin (\frac{π x}{L}) & (2) \end{matrix}$
where ω is the natural frequency of motion, L is the shaft length and A is an arbitrary constant. This simplifying assumption is a good one since Fourier theory states that any bounded analytic function can be approximated by a series of sines and cosines. Placing this deflection shape into the equation of motion and assuming that the bending stiffness, EI, and mass per unit length, m, can be represented by constant averaged values over the length of the shaft, EI and m, and that P is uniform, yields [0041] $\begin{matrix} [\frac{π^{4}}{L^{4}} \overline{E I} - P \frac{π^{2}}{L^{2}} - \overline{m} ω^{2}] A \sin (ω t) \sin (\frac{π x}{L}) = 0 & (3) \end{matrix}$
which, when solved for the natural frequency by setting the square bracketed term equal to zero, gives [0042] $\begin{matrix} ω = \sqrt{\frac{π^{4} \overline{E I} (1 - \frac{P L^{2}}{π^{2} \overline{E I}})}{m L^{4}}} & (4) \end{matrix}$
To first order, Equation (4) shows how the apparent stiffness, the numerator of the quotient under the square root sign, decreases with increased static compression. For reference, the general buckling load for a uniform beam under uniform compression is [0043] $\begin{matrix} P_{buckling} = \frac{{απ}^{2} \overline{E I}}{L^{2}} & (5) \end{matrix}$
so that a compressive load equal to 20% of the buckling load results in a 20% decrease in stiffness and a corresponding 10% decrease in natural frequency. [0044]
It is important to note that the shaft load P changes during the downswing, that is, the centrifugal force generated by the accelerating club head counters the precompression and results in general shaft stiffening. Without precompression, the centrifugal load would merely serve to stiffen the dynamic behavior of the shaft, since the shaft load P would by definition be negative. [0045]
For beams of general cross sections, moduli, mass per unit length, and preloading that vary with the axial coordinate, expressions analogous to Equations (2)-(5) do not exist in closed form, and full analysis is necessary for accurate predictions. However, the behavior for more complicated geometries and loading is generally similar to the simple case illustrated above. [0046]
This invention may be applied to metal, wood, and composite shafts as long as the shafts are hollow or hollowed to allow the insertion of the invention. The tensioning member may be affixed to any point along the shaft's length. This allows tailoring of the variable stiffness length of the shaft. For full length golf club shaft embodiments a tensioning wire/cable assembly may be integrated into the butt and hosel ends of the shaft. The tensioning device may comprise a single wire or cable, or multiple (twisted or untwisted) wires. [0047]
The wire can be restrained from dynamic vibration within the shaft by filling the enclosed volume, either entirely or partially, with low density foam or a similar material. The wire can also be restrained from dynamic vibration by placing form fitting inserts down the length of the wire and affixing them to the shaft, by glue or compression resulting from expansion, thus allowing the wire to run freely through the insert while restraining its lateral motion. [0048]
Although the invention has been described in detail including the preferred embodiments thereof, such description is for illustrative purposes only, and it is to be understood that changes and variations including improvements may be made by those skilled in the art without departing from the spirit or scope of the following claims. [0049]

Claims

What is claimed is:

1. A sporting device having a variable stiffness shaft, comprising:

a hollow shaft;

a length of a superelastic alloy placed under tension within the hollow shaft and affixed thereto at two points on the shaft, the alloy capable of reversibly elongating to accommodate an applied stress; and

variation means for adjusting the tension of the tensioning device, whereby increasing the tension reduces the bending stiffness of the shaft.

2. The sporting device of claim 1, wherein the superelastic alloy is Nitinol.

3. The sporting device of claim 1, wherein the superelastic alloy can reversibly elongate by up to approximately 8% to accommodate the applied stress.

4. The sporting device of claim 1, wherein the applied stress includes a pre-stress applied to the length of superelastic alloy by the variation means.

5. The sporting device of claim 1, wherein the applied stress includes a dynamic stress produced during a swing.

6. The sporting device of claim 1, wherein the length of superelastic alloy comprises a wire affixed at one end to the shaft, and affixed at the opposing end to the variation means, the variation means transmitting the tension of the wire to the shaft.

7. The sporting device of claim 1, wherein the length of superelastic alloy comprises a plurality of wires affixed at one end to the shaft, and affixed at the opposing end to the variation means, the variation means transmitting the tension of the wires to the shaft.

8. The sporting device of claim 1, wherein the variation means is a cam.

9. The sporting device of claim 1, wherein the variation means is a pinned retractable piece.

10. The sporting device of claim 1, wherein the variation means is a lockable lead screw.

11. The sporting device of claim 9, wherein the lead screw is varied by an external actuator.

12. The sporting device of claim 1, wherein the variation means is a pump.

13. The sporting device of claim 1, wherein the variation means is a sleeve screw.

14. The sporting device of claim 1, wherein the variation means is a displacement actuator powered by an external source.

15. The sporting device of claim 1, and further comprising a constraint insert for preventing dynamic vibration of the length of superelastic alloy.

16. The sporting device of claim 15, wherein the constraint insert comprises a material selected from the group consisting of low density foam, plastic, and elastomers.

17. The sporting device of claim 15, and further comprising a plurality of discrete inserts.

18. The sporting device of claim 15, wherein the constraint insert is a single constraint insert extending substantially along the length of the shaft.

19. The sporting device of claim 15, wherein the constraint insert is held in the shaft by compression.

20. The sporting device of claim 15, wherein the constraint insert is held in the shaft by adhesive.

21. The sporting device of claim 1, wherein the sporting device is selected from the group consisting of a golf club, a tennis racket, a ski pole, a hockey stick, a baseball bat, a fishing pole, a hurling stick, a lacrosse stick, and a vaulting pole.

22. A golf club having a variable stiffness shaft, comprising:

a hollow shaft;

a wire disposed within the hollow shaft and affixed thereto at two points on the shaft, wherein the wire is made of a superelastic alloy which reversibly elongates to accommodate an applied stress; and

variation means for varying the tension of the wire, whereby increasing the tension of the wire reduces the bending stiffness of the shaft.

23. The golf club of claim 22, wherein the superelastic alloy is Nitinol which can reversibly elongate by up to approximately 8%.

24. The sporting device of claim 22, wherein the applied stress includes a pre-stress applied to the length of superelastic alloy by the variation means.

25. The sporting device of claim 22, wherein the applied stress includes a dynamic stress produced during a swing.

24. The golf club of claim 21, wherein the superelastic alloy is selected from the group consisting of nickel and aluminum (Ni—Al); copper and zinc and another element Cu—Zn—X (where the other element X is silicon (Si), tin (Sn), or aluminum (Al)); copper and zinc (Cu—Zn); copper and tin (Cu—Sn); copper and aluminum and nickel (Cu—Al—Ni); iron and platinum (Fe—Pt); iron and manganese and silicon (Fe—Mn—Si); and manganese and copper (Mn—Cu).

25. The golf club of claim 21, wherein the wire is affixed at one end to the shaft, and affixed at the opposing end to the variation means, the variation means transmitting the tension of the wire to the shaft.

26. The golf club of claim 25, wherein the tensioning device comprises a plurality of wires affixed at one end to the shaft, and affixed at the opposing end to the variation means, the variation means transmitting the tension of the wires to the shaft.

27. The golf club of claim 21, wherein the variation means is a cam.