Higher overall flex golf shaft

ABSTRACT

A composite golf club shaft is constructed having a reduced butt diameter of 0.400 to 0.560 inches in diameter. The shaft tapers without intentional discontinuities from the reduced-diameter butt section to a cylindrical tip portion having a standard tip diameter adapted to be attached to the hosel of a club head. By reducing the diameter of the butt portion of the shaft while maintaining the shaft free of substantial discontinuities, lead-lag flexure is increased uniformly, thereby improving the controllability of the shaft, and without inducing the artificial bending modes found in bubble shafts and other shaft configurations having intentional discontinuities.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

DETAILED DESCRIPTION

As indicated above, in hitting a golf ball with a golf club, deflection in the golf club shaft may adversely affect the angle at which the face of the golf club head strikes the golf ball. As the club is accelerated during the stroke, the face of the club is deflected from its static angle by four principal forces acting on the club head. With reference to FIG. 1, at the beginning of the stroke, the gross body acceleration of the club head in the plane of the swing lags the gross body acceleration of the butt portion , causing the shaft to bend in the characteristic “lead-lag” principal bending mode in the plane of the swing. Lead-lag bending is a critical feature of golf shaft design, for a substantial portion of the energy delivered to the golf ball at impact is energy stored in the form of lead-lag bending that is released as the shaft straightens during the period of time the club is in contact with the golf ball. With reference to FIG. 2, a second important force acting on a club head having a center of mass is the centrifugal force acting on the golf club head as the golfer swings the club head through an arc at velocities approaching 80 miles per hour or more. Particularly the force produces a moment, which tends to rotate the club head into a toe-down position. With reference to FIG. 3, the same force acting on the club head also produces a moment , which tends to rotate the club head in the plane of the swing in a counterclockwise direction (when viewed from a position opposite the golfer), thereby increasing the effective loft angle of the club head at impact with golf ball . Finally, as shown in FIG. 4, the gross body acceleration of the club head produces a force acting on the center of mass of the club head produces a longitudinal moment that tends to open the face of the golf club head as the club strikes the golf ball , thereby increasing the slice at impact. Although each of the above forces and their effect on the club head at impact are described above as if they were steady state forces and deflections, in reality each of the above forces are dynamic during the stroke, and each is resisted by the elastic properties of the club itself to produce complex vibration modes throughout the shaft. Accordingly, static compensation for any of the above forces will not yield predictable results. Moreover, as is evident from the foregoing discussion, a change in design or materials intended to improve one characteristic often has untoward deleterious side effects on one or more other important characteristics of the shaft.

Historically, the bending profile of a golf shaft has been described in terms of its flex point and kick point. The flex point of a shaft was determined experimentally by providing a cantilever support for the shaft and suspending a weight at the opposite end. The kick point was determined experimentally by compressing the shaft axially a fixed distance to induce a first buckling mode in the shaft and determining the location of the apex of the eccentricity. The location of the flex point or kick point was meant to illustrate the point about which the golf club shaft would bend during the golf swing and further to describe the resultant trajectory of the golf ball in flight. For a given degree of flexure, a low flex/bend point (i.e. near the club head) generally indicated a high loft angle and high ball flight, whereas a high flex point (i.e. near the butt) generally indicated a low loft and low trajectory.

Recent studies indicate, however, that traditional methods of measuring flex point and kick point and attempting to correlate these points to a predicted trajectory are extremely limited. A recent study conducted by a prominent equipment distributor on over 800 different composite and steel shafts discovered that the kick/flex points of all shafts differed by only about ½ inch between shafts of similar construction, and only about 1 inch between all composite and steel shafts evaluated. In short, over 800 shafts, when subjected to a series of static tests, resulting in almost identical bend profiles. Prior art attempts to control the location of the flex point, such as the “bubble” shafts of U.S. Pat. No. 5,439,219, while succeeding to some degree in moving the flex/kick point desirably closer to the butt portion, fail to provide for any substantial degree of flexure in the butt portion. Moreover, as illustrated in FIG. 2B, introduction of an intentional discontinuity , in effect a hinge in the shaft, produces additional bending degree of freedom in the shaft, which causes more complex bending modes in the shaft, thus leading to a loss of controllability.

A tapered composite shaft incorporating features of the present invention is composed of a series of sheets or flags of composite material laid-up on a mandrel defining the inside diameter of the shaft as shown in FIG. A. The mandrel includes a tip end and a butt end . As shown more clearly in FIG. 5B, mandrel comprises a shallow taper from the tip end to an intermediate point . A steeper taper proceeds from intermediate point to the lowermost end of butt portion . In the illustrative embodiment of FIG. 5B, the diameter of mandrel at the butt portion is between 0.450 inches and 0.460 inches. Accordingly, a shaft constructed on mandrel having a butt diameter of the preferred 0.540 outside diameter would necessarily include a wall thickness of from 0.040 to 0.045 inches.

A “parallel-taper-parallel” composite shaft incorporating features of the present invention is composed of a series of sheets or flags of composite material laid-up on a mandrel defining the inside diameter of the shaft as shown in FIG. A. The mandrel includes a tip end and a butt end . As shown more clearly in FIG. 7B, mandrel comprises a cylindrical portion from the tip end to an intermediate point . A taper proceeds from intermediate point to the lowermost end of cylindrical butt portion . In the illustrative embodiment of FIG. 5B, the diameter of mandrel at the butt portion is a constant 0.460 inches.

Preferably the composite material used in construction the shaft comprises a nonwoven graphite fiber material suspended in a synthetic resin, such as a modified epoxy resin well known in the art. The composite material typically comprises about 30-35% resin by weight. As shown in FIG. 6, preferably, the innermost layers of material comprise two layers of graphite-epoxy , extending the full length of the shaft. The composite material has an elastic modulus of 33 million psi and an ultimate tensile strength of about 500,000 psi, known in the industry as “33/500” graphite epoxy. The fibers of the inner two layers , are oriented at angles of +45° and −45° respectively relative to the axis of the shaft so as to provide maximum resistance to torsion. The next two layers comprise tip insert flags, , , comprising two additional layers of 33/500 graphite epoxy extending approximately 10 inches from the tip. The fibers in the tip insert flags , are oriented longitudinally along the axis of the shaft. The outer two layers , comprise two fill-length flags of 33/500 graphite epoxy with fibers also oriented longitudinally along the axis of the shaft. A sacrificial flag of 33/500 or other material may be included at the extreme butt end of the shaft to assist in removing the shaft from the mandrel . The sacrificial flag is removed as the finished shaft is trimmed to length.

An alternative embodiment as shown in FIG. 8 includes the innermost layers of material comprising two layers of a graphite epoxy material , having an elastic modulus of 55 million psi and an ultimate tensile strength of approximately 700,000 psi, known in the industry as 50/700 graphite epoxy, extending the full length of the shaft with the fibers of the inner two layers oriented at angles of +45° and −45° respectively relative to the axis of the shaft. The next layer comprises a full length flag of a the same 50/700 material, with the fibers of this layer oriented longitudinally along the axis of the shaft. The next layer comprises a tip insert layer of 33/500 graphite epoxy extending approximately 20 inches from the tip. The fibers in the tip insert flags are also oriented longitudinally along the axis of the shaft. The outer layer then comprises a full-length flag of 33/500 graphite epoxy with fibers also oriented longitudinally along the axis of the shaft. As with the embodiment of FIG. 6, sacrificial flag of 33/500 or other material may be included at the extreme butt end of the shaft to assist in removing the shaft from the mandrel. The sacrificial layer is removed as the finished shaft is trimmed to length.

As shown in FIGS. 9A and 9B, the finished shafts , have a standard nominal tip diameter of 0.370 inches, or 0.335-0.400 inch in diameter for woods and 0.330-0.390 inch in diameter for irons. The tip portion extends about 1 to 6 inches in length from the tip of the shaft. The shaft then tapers to the maximum outside diameter at the butt end of from 0.400-0.560 inches in diameter, preferably from 0.450 to 0.550 and most preferably from 0.520 to 0.540 inches in diameter. Shafts having a butt diameter significantly greater than 0.560 inch do not exhibit a significant degree of overall flex improvement over prior art shafts; and shafts having a butt diameter significantly below 0.400 are prone to breakage.

Special grips molded to standard outside contours but having a smaller inside diameter to conform to the reduced diameter butt are fitted to the finished shaft. Preferably the grips are molded of synthetic foam rubber or other suitable cushioning material. The inherently thicker wall of the special grips provides additional shock isolation to the user, thereby further increasing comfort and feel of the golf shaft. The total finished length of the shaft is typically from 35-47 inches overall. A standard club head may be fitted to the shaft to complete the golf club assembly.

As can be ascertained from the foregoing, because the shaft is constructed with a relatively conventional number of graphite epoxy layers, the resultant shaft has a wall thickness that is within normal tolerances of shafts of similar construction, however, because of the reduced inside diameter of the mandrel or , the resulting outside diameter is reduced from an industry standard of 0.600-0.620 to the above referenced 0.400-0.560 inch. As is well known, the cross sectional moment of inertia of a round section is equal to the fourth power of the diameter. Accordingly, even the minimal reduction of the cross section from the minimum prior art diameter of 0.600 to the preferred diameter of 0.520-0.540 inch results in a 30% decrease in the cross sectional moment of inertia of the shaft and a corresponding increase in the shaft flexure. More moderate increases in flexure may be accomplished by reducing the outside diameter less dramatically, for example to 0.560 inches. Conversely, shafts constructed according to the present invention with smaller mandrels and smaller finished diameters exhibit even more dramatic increases in flexure.

As shown in FIG. 10, in a standard cantilever test, a composite shaft constructed according to the foregoing description having a butt outside diameter of 0.540 inches deflects over 1.5 inches more than a similarly constructed shaft having a butt outside diameter of 0.620 inches. More dramatic, however, is the shift in flex/kick point. As shown in FIG. 11, as a result of the reduced diameter butt configuration of the shaft, the shaft constructed in accordance with the principals of the present invention has a kick point that is over 3 inches nearer the butt end of the shaft than the similarly constructed shaft having a butt outside diameter of 0.620 inches.

The higher flex/kick point of the reduced diameter butt design of the present invention results in an increase in the desirable lead-lag bending without losing the ability to return the club face to square at impact. The increased flexibility in the butt portion of the shaft permits the user to better control the bend, part of which is occurring at the user's hands. Moreover, the increased lead-lag bending is accomplished without the high torque associated with soft shafts and without artificial discontinuities and impedance mismatching, which induces additional modes of vibration in bubble shafts and other similar designs.

Although certain preferred embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the true spirit and scope of the invention. For example, although the illustrative embodiments comprise lay-ups of graphite epoxy flags, a shaft comprising a filament wound composite material, such as that described in ENGINEERED MATERIALS HANDBOOK VOL I (COMPOSITES) © 1987 ASM International, at pp. 503-07 (incorporated herein by reference) would be within the scope of the present invention. Accordingly, it is intended that the invention shall be limited only to the extent required by the appended claims and the rules and principles of applicable law.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing Figs. in which like references designate like elements and, in which:

FIG. 1 illustrates a first bending mode of a golf shaft in the plane of the swing;

FIG. 2A illustrates a first bending mode of a golf shaft transverse to the plane of the swing;

FIG. 2B illustrates a higher order bending mode of a golf shaft transverse to the plane of the swing;

FIG. 3 illustrates an additional bending mode of a golf shaft in the plane of the swing;

FIGS. 4A and 4B illustrate a torsional bending mode of a golf shaft;

FIGS. 5A and 5B illustrate a mandrel for constructing a golf shaft in accordance with principles of the present invention;

FIG. 6 illustrates a method of constructing a golf shaft in accordance with principles of the present invention;

FIGS. 7A and 7B illustrate a mandrel for constructing a golf shaft in accordance with principles of the present invention;

FIG. 8 illustrates a method of constructing a golf shaft in accordance with principles of the present invention;

FIGS. 9A and 9B show illustrative embodiments of golf shafts constructed in accordance with principles of the present invention;

FIG. 10 is a graphical representation of the flex of a shaft constructed in accordance with principles of the present invention;

FIG. 11 is a graphical representation of the kick point of a shaft constructed in accordance with principles of the present invention.

CLAIMS

1. A golf club shaft comprising: an elongated tubular shaft comprising a plurality of layers of fibers imbedded in a synthetic resin, said elongated tubular shaft having a butt end of relatively larger cross sectional diameter tapering without intervening discontinuities to a tip end of relatively smaller diameter, said tip end having an outside diameter between 0.330 and 0.400 inches; said butt end having an outside diameter of 0.400 to 0.540 inches.

2. The golf club shaft of claim 1, wherein: said butt end has a wall thickness of between 0.04 and 0.045 inches.

3. The golf club shaft of claim 1, wherein: a length of the shaft is from about 35-47 inches.

4. The composite golf club shaft of claim 1, further comprising: two inner layers of graphite fibers embedded in epoxy, said inner layers having fibers oriented at angles of +45° and −45° respectively relative to the axis of the shaft.

5. The composite of golf club shaft of claim 1, further comprising: an intermediate layer of graphite fibers embedded in epoxy, said graphite fibers being oriented longitudinal to the axis of the shaft.

6. A composite golf club shaft comprising: an elongated tubular shaft comprising a plurality of layers of fiber embedded in a synthetic resin, said elongated tubular shaft having a butt section comprising a substantially cylindrical cross section of relatively larger cross section, which transitions without intervening discontinuities to a tapered intermediate section, said tapered intermediate section tapering without intervening discontinuities to a relatively smaller diameter tip section; said tip section including a portion having an outside diameter of between 0.330 and 0.400 inches; said butt section having an outside diameter between 0.400 to 0.540 inches, said butt section diameter displacing a kick point above a center point of the composite golf club shaft.

7. The golf club shaft of claim 6, wherein: said butt section has a wall thickness between 0.04 and 0.045 inches.

8. The golf club shaft of claim 6, wherein: the golf club shaft has a length between 35 and 47 inches.

9. The composite golf club shaft of claim 6, further comprising: two inner layers of graphite fibers imbedded in epoxy, said inner layers having fibers oriented at angles of +45° and −45° respectively relative to the axis of the shaft.

10. The composite golf club shaft of claim 6, further comprising: an intermediate layer of graphite fibers embedded in epoxy, said graphite fibers being oriented longitudinal to the axis of the shaft.

11. A golf club shaft comprising: an elongated tubular shaft, said elongated tubular shaft having a butt section comprising a substantially cylindrical cross section having an outside diameter of between 0.400 and 0.560 inches, which transitions without intervening discontinuities to a tapered intermediate section, said tapered intermediate section transitioning without intervening discontinuities to a relatively smaller diameter tip section, said tip section having an outside diameter of between 0.330 and 0.400 inches, and said tapered intermediate section having a more significant taper than both said butt and tip sections.

12. The shaft of claim 11 wherein said tip section and said butt section include parallel sidewalls.

13. A golf club shaft comprising: an elongated tubular shaft having a length of between about 35 and 47 inches, said elongated tubular shaft having a butt section of relatively larger cross sectional diameter tapering without intervening discontinuities to a tip section of relatively smaller diameter, said tip section having an outside diameter between 0.330 and 0.400 inches; said butt section having at least one portion with an outside diameter of between 0.400 and 0.540 inches, said butt section diameter displacing a kick point toward the butt end of the shaft.

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