Golf Club Shaft

The present invention relates to golf club shaft made of composite materials, and in particular, a shaft having a complex shape.

Conventional golf club shafts are generally made of steel, metal alloys, or composite materials. They have a slightly conical shape and continuous variation of their section, the maximum dimension being found at the grip or handle, and the minimum dimension at the neck, where the head of the club is attached. This remains the most widely-used shaft geometry.

If one wishes to vary the mechanical properties of the shaft, i.e., in particular, the moment of inertia and the elastic line under torsion and flection, the opportunities for such changes on these shafts are rather limited. The addition of inertia blocks or reinforcements at different places on the shaft is not a satisfactory solution, since one part of the club is made heavier, a generally undesirable effect. One example of an embodiment of this kind is given in Patent No. JP 1-159 879, which describes the, manufacture of a shaft made of composite materials comprising reinforcement zones produced by adding pieces formed from layers of resin-impregnated fiber sheets to the body of the shaft. A second disadvantage of this construction arises from the lack of continuity of the fiber sheets at these reinforcement sites, which appreciably impairs the reproducibility of the mechanical properties from one shaft to another and thus. limits their use by professionals.

Similarly, Patent No. GB 256,049 describes a golf club fitted with a metal shaft on which flexible areas of contraction are produced so as to modify the curve of deformation under flection, and thus to improve the elastic response of the club. While flection properties are, in this case, controlled and optimized, the torsion properties, are poorly controlled, mainly because of the homogeneous, non-fibrous nature of the material used.

It is thus an object of the invention to remedy the above-mentioned disadvantages resulting mainly from the structure and the nature of the materials used, by proposing a golf club shaft incorporating a new design. To this end, the shaft according to the present invention is tubular and manufactured using essentially continuous layers of sheets of fibers impregnated with a plastic material. The shaft is provided over its length with at least one area of enlargement and/or narrowing and is characterized by the fact that the curve of variation of the internal diameter of the shaft as a function of the length,

beginning at the point of the smallest internal diameter,

and extending toward at least one of the ends of the shaft, allows at least one decreasing portion,

The invention will be better understood, and its advantages and properties will more clearly emerge, from the embodiments described below and illustrated by the accompanying drawings.

FIG. 1 shows a golf club on which a shaft according to the prior art is mounted.

FIG. 2 shows a golf club on which a shaft according to the invention is mounted.

FIG. 3 shows in cross-section a shaft according to a first embodiment of the invention.

FIG. 4 is a graph showing the variation of the internal diameter of the shaft as a function of its lenght.

FIGS. 5, 7, and 9 are views similar to FIG. 3 showing other embodiments.

FIGS. 6, 8, and 10 are graphs similar to FIG. 4, showing variations of the internal diameter of the shaft as a function of length, corresponding to the embodiments in FIGS. 5, 7, and 9, respectively.

FIG. 11 is a schematic representaion, in cross-section, of a conventional shaft which is embedded for the performance of flection tests.

FIG. 12 is a view similar to FIG. 11, but of a conventionally reinforced shaft.

FIG. 13 is a view similar to FIG. 11, but of the shaft according to the invention illustrated in FIG. 2.

FIGS. 14 to 19 show the various steps in an example of a process for manufacture of shafts according to the invention.

FIG. 20 shows the shaft of FIG. 5 with mounted grip.

FIG. 21 shows the shaft of FIG. 7 with a filling ring mounted thereon.

As shown in FIG. 1, a golf club 1 generally comprises a head 2, a shaft 3, a grip or handle 4, and possibly an intermediate part 5, called a "hosel," whose main function is to reinforce the head-shaft connection. The shaft 3 is, in conventional practice, a tubular, conical object whose narrowest section is located at the end on which the head 2 of the club is attached. This end is generally termed the "tip" end 31, the other end being the "butt" 32.

FIG. 2 shows a golf club 1 on which a shaft 3 according to the invention is mounted. In this preferred embodiment, the shaft 3 is made of composite materials, and more specifically, continuous layers of sheets of resin-impregnated fibers. Among the fibrous materials used, carbon and/or glass fibers may be mentioned. The resins are normally epoxy thermohardening resins, for example. This shaft has a slightly conical shape which widens toward the handle and is interrupted by a an enlarged area 6.

FIG. 3 is a longitudinal cross-section view illustrating the shaft in FIG. 2. It is provided over its length with an area of enlargement 6 which interrupts the slightly conical generation of the general shape. The smallest internal diameter of the shaft is located at the tip 31, i.e., at the end attached to the head 2 of the club.

FIG. 4 represents the curve of generation of the internal diameter of the shaft as a function of length. It may be noted that the area of enlargement 6 is characterized on the curve by a decreasing portion 61 preceded by an increasing portion 62. Furthermore, the slope of the increasing portion 62 is greater than the average slope of the curve external to the area of enlargement 6. Since the shaft accommodates a slight overall conicity, the curve external to the area of enlargement 6 increases in dimension and has a slight slope extending toward the end of the shaft supporting the handle. The increasing portion 62 and the decreasing portion 61, as shown in FIGS. 3 and 4, are connected by an attachment piece 63 whose slope is substantially equal to that of the curve of the zone of enlargement 6. Advantageously, the slope of this portion 63 can also be approximately zero.

Finally, the shaft in FIG. 3 is formed by a stack of successive, continuous layers of fiber sheets extending mainly from one end to the other of the shaft and having a thickness which varies minimally along the shaft.

In the embodiment illustrated in FIGS. 5 and 6, the tubular shaft 3 incorporates, beginning at the "tip" end 31 having the smallest diameter, a first conical portion, which is illustrated in FIG. 6 by a slight increasing slope beginning at the point of minimum diameter (Dmin.), then an abrupt narrowing 7 on the shaft extending toward the butt end 32, as illustrated on the curve by a sharply decreasing portion 71, followed by an substantially constant portion 72.

This embodiment is particularly advantageous because it allows the incorporation of a grip 4 which covers and fills the narrowed zone 7. The thickness of the grip 4 is preferably chosen so that it does not exceed the depth of the narrowed zone 7, as illustrated in FIG. 20. A grip 4 flush with the rest of the shaft 3 is thus obtained.

Another embodiment of the invention, illustrated in FIGS. 7 and 8, shows a shaft 3 provided over its length with a narrowed zone 7. This zone is characterized on the curve by a decreasing portion 71 preceding an increasing portion 73. Furthermore, time slope of the increasing portion 73 is greater than the average slope of the curve external to said narrowed zone 7. Finally, the decreasing portion 71 and the increasing portion 73 are advantageously connected by a connecting piece 74 having a slope that is substantially zero or equal to that of the curve external to the narrowed zone 7.

Of course, the increasing 73 and decreasing 71 portions may be connected directly without a connecting piece.

In the shaft embodiment shown in FIGS. 7 and 8, it may be advantageous to fill the space formed by the narrowed zone 7 with a filler ring 40, as shown in FIG. 21.

This ring 40 may contribute to the balancing of the club or to its dampening. Depending on the case, the ring 40 may be made of a plastic material, e.g., a material with viscoelastic properties, or of a metal or metal alloy.

The enlarged zone 6 may also derive from a biconical shaft shape, as shown in FIG. 9. The generation of the curve in FIG. 10 shows a first increasing portion 62, to which a second decreasing portion 61 is attached. Portions 61, 62 are, advantageously, substantially linear.

In order to understand the particularly advantageous mechanical properties of the shafts according to the invention, it is easy to use modelling to compare, as an example, the moduli of deflection f corresponding to the vertical movement of the tip end 31 of an embedded shaft having a length D and stressed by means of a predetermined force F. The shaft is embedded at the butt end over a length d1.

(FIG. 11)

This example concerns a conventional shaft produced from a succession of eleven layers of sheets of T300 and M40 pre-impregnated carbon fibers marketed by the TORAY company and having the following characteristics:

Of the eleven layers, five are turned 0.degree. in relation to the longitudinal axis (I, I') of the shaft, three are turned +45.degree., and three are turned -45.degree.. The order, beginning at the interior of the shaft, is: 0, +45, -45, 0, +45, -45, 0, +45, -45, 0, 0).

The conicity of the shaft in relation to axis I, I'is 0.21.degree..

d1 is 102 mm (embedded length) for a total shaft length of 1,057.3 mm.

F is 29.6 under pure flection.

Results:

Deflection f equals 149.3 mm for a shaft weight computed to be 75.6 g.

(FIG. 12)

This example concerns a conventional shaft identical to that in Example I, to which is added an excess thickness of two layers of impregnated fiber sheets so as to create an external zone of enlargement 8. This technique is conventionally applied for strengthening shafts, as described, for example, in Patent No. JP 1-259 879. The excess thickness corresponds to two layers, or 0.34 mm. It is positioned at a distance d2 equal to 298.2 mm from the butt end 32 and has a length d3 of 303.3 mm.

For a force of flection F identical to Example I (29.6 N), a deflection of 125.8 mm is computed for a shaft weight of 81.8 g.

(FIG. 13)

This example is illustrative of an embodiment of the invention. The shaft comprises an enlarged area 6 and is formed from eleven layers of fiber sheets arranged and turned as in Example I, and its properties are identical to the latter. The enlarged area 6 is located at the same place as in Example II (d2, d3 identical to Example II).

The total length of the shaft is also identical to the two preceding examples.

The increase of the internal radius of the shaft in the zone of enlargement 6 remains uniform and equal to 1.44 mm, as compared with the internal radius in the same area of the shaft shown in Example II.

Thus, a deflection f of 125.8 mm is computed, i.e., a deflection equivalent to that in Example II. However, the total weight of the shaft is 78.4 g, i.e., less than the weight of the shaft in Example II.

Thus, a lightened shaft with uniform stiffness under flection is obtained in comparison with a shaft incorporating conventional reinforcement.

Of, course, one prior art solution for modifying stiffness under flection without increasing weight would involve modifying the proportion by weight of the fibers to the pre-impregnated fiber resin or matrix, or changing fiber properties (reference: TORAY's T700 instead of T300); however, these solutions are costly when compared to the solution according to the invention.

An especially advantageous process for the manufacture of shafts according to the invention will now be described by way of example.

This process permits the manufacture of shafts having complex shapes and comprising continuous layers of fiber sheets, and it consists of molding the tubular shaft made of resin-impregnated fibers by exerting pressure at the interior of the shaft, so as to conform the shaft to an external impression. Thus, as shown in FIG. 14, the process consists in producing, prior to the molding stage, a thin latex bladder on a form 10 by soaking the form in a bath 11 of calcium nitrate, and then of latex. After coagulation, the bladder 9 undergoes a baking procedure for approximately 10 minutes at between 70.degree. and 80.degree. C. After cooling, the bladder is arranged on a mandrel 12, as illustrated in FIG. 15, whose length is at least equal to that of the shaft to be manufactured. This technique makes it possible to obtain bladders of reduced thickness i.e., of approximately 0.2 to 0.3 mm.

The following step (FIG. 16) consists in dressing the mandrel 12, covered with bladder 9, with sheets of fibers 13 pre-impregnated with synthetic resins, by winding in preferably continuous multiple layers. A composite structure in the shape of a truncated cone is thus produced. A complex form, such as that illustrated in FIG. 17, is obtained prior to molding. Of course, similar results would be achieved by means of filament winding of one or more yarns pre-impregnated with resin.

Next, in FIG. 18, the mandrel 12 is placed in a mold 14 whose impression 15 will determines the final form of the shaft to be manufactured. Thus, for example, the short area 15a of the mold 14 has a larger section in its central part so as to form the enlargement 6 of the final shaft 3, as shown in FIGS. 2 or 3.

The molding operation is conducted by heating the mold 14 and applying internal pressure which, through gas fed to the interior of the elastic bladder 9, is exerted so as to compact the composite structure 13 on the impression 15 of the mold.

The molding cycle will, of course, vary as a function of the kind and reactivity of the impregnated materials used.

Those skilled in the art will know how to determine the parameters that are operational during the cycle.

Compressed air is preferably used as the molding gas at a pressure of approximately 2.5 to 3 bars. The complex is then cooled and unmolded fairly easily, given the substantial play obtained after compaction between the internal diameter of the shaft 3 and the mandrel. Further, no special surface treatment is required on the shaft finished by use of this process.