GOLF CLUB HEAD

FIELD OF THE INVENTION

The present invention relates to a golf club head having excellent rebound performance.

BACKGROUND ART

Heretofore, various golf club heads designed to increase a carry distance have been proposed.

For example, the following patent document 1 discloses a golf club head comprising a head main body, an elastic layer composed of a rubber-like elastic material disposed in a face portion thereof, and a fiber reinforced resin layer disposed on the surface of the elastic layer. Such golf club head is expected to exhibit such effect that, when hitting a ball, the elastic layer causes compressive deformation over a wide range through the fiber reinforced resin layer, and the repulsion is increased.

[Patent document 1] Japanese Patent Application Publication No. 5-305161

SUMMARY OF THE INVENTION
Problems to be Resolved by the Invention

In the golf club head of the patent document 1, the club face which comes into direct contact with a ball is made of a fiber reinforced resin.

In general, a friction coefficient between a fiber reinforced resin and a ball is small. According to various tests heretofore made, it was discovered that if the friction coefficient between a club face and a ball is smaller, the hit ball has higher spin. (cf. JSEA journal SPORTS ENGINEERING No. 1 issued 31 Jul. 2006, “Recent study on Golf gears” author Masahide ONUKI)

Therefore, in the golf club head of the patent document 1, there is a possibility that the carry or flying distance of the hit ball becomes insufficient due to the high spin.

The reason why the elastic layer of the patent document 1 is deformed over a wide range when hit by a ball is supposed that the fiber reinforced resin layer (a) has a certain degree of rigidity as shown in FIG. 13(A) and FIG. 13(B) which show the face portion before and when hit by the ball.

On the other hand, for improving the rebound performance of such club head, it is considered to be effectual to configure the elastic layer so as to deform more locally when hit by a ball. Accordingly, in the art disclosed in the patent document 1, there is a room for improvement in the rebound performance of the club head.

The present invention was made with the view toward the above-mentioned problem, and a primary object of the present invention is to provide a golf club head having excellent rebound performance and being capable of achieving sufficient flying distances.

Means of Solving the Problems

According to the present invention, a golf club head comprises a face portion having a club face for hitting a ball and comprising a composite part,

wherein

the composite part having a layered structure comprising: a thin surficial layer made of a metal material for contacting with a ball; an intermediate elastic layer composed of a rubber-like elastic material disposed inside the thin surficial layer; and a back support layer made of a metal material disposed inside the intermediate elastic layer,

wherein

in a front view of the club head, the intermediate elastic layer includes a center of the club face, and

the edge surface of the intermediate elastic layer is at least partially exposed at the outer surface of the club head.

In the golf club head of the present invention, it may be possible that, in the front view of the club head, the intermediate elastic layer extends in a first direction, and

the edge surface of the intermediate elastic layer exposed at the outer surface of the club head is positioned in the vicinity of one end or alternatively each end of the intermediate elastic layer in the first direction.

In the golf club head of the present invention, it may be possible that the composite part comprises a side support wall for preventing the intermediate elastic layer from running out in a direction orthogonal to the first direction.

In the golf club head of the present invention, it is preferable that the thickness of the thin surficial layer is 0.1 to 1.0 mm.

In the golf club head of the present invention, it may be possible that the Young's modulus of the rubber-like elastic material is 10 MPa to 1 GPa.

In the golf club head of the present invention, it may be possible that the Young's modulus of the rubber-like elastic material is 100 MPa to 500 MPa.

In the golf club head of the present invention, it may be possible that the first direction is the up-and-down direction of the club head.

In the golf club head of the present invention, it may be possible that the first direction is the toe-heel direction of the club head.

In the golf club head of the present invention, it may be possible that the first direction is an oblique direction inclined downwards towards the heel from the toe of the club head.

In the golf club head of the present invention, it is preferable that the natural frequency in a primary mode of the club head when the center of the club face is supported or fixed is 800 to 1600 HZ.

In the golf club head of the present invention, it is preferable that the ratio feg/fc is 1.0 to 1.3, wherein fc is the natural frequency in a primary mode of the club head 1 when the center of the club face is supported or fixed, and feg is the natural frequency in a primary mode of the club head 1 when a peripheral edge portion of the club face corresponding to the composite part is supported of fixed.

According to the present invention, the face portion of the golf club head comprises the composite part. The composite part comprises the thin surficial layer made of the metal material for contacting with a ball, the intermediate elastic layer composed of the rubber-like elastic material disposed inside the thin surficial layer, and the back support layer made of the metal material disposed inside the intermediate elastic layer. And, in the front view of the club head, the intermediate elastic layer of the composite part includes the club face's center, and the edge surface of the intermediate elastic layer is at least partially exposed at the outer surface of the club head.

When a ball hits the composite part of the golf club head of the present invention, the ball first hits the thin surficial layer made of the metal material. Since the metal material has a higher friction coefficient against the ball in comparison with a resin material, the ball is prevented from having excess spin.

Since the thin surficial layer has a small thickness, the thin surficial layer can be deformed locally when hit by the ball. Accordingly, the intermediate elastic body disposed on the inside thereof can make large local elastic deformation. Thereby, the repulsion of the club head is improved.

Since the intermediate elastic layer of the composite part is arranged so as to include the center of the club face in the front view of the club head, a hit ball having low spin and high rebound performance can be obtained at the most preferable ball hitting position.

Further, the intermediate elastic layer is partially exposed at the outer surface of the club head. The composite part including such intermediate elastic layer can provide a high-repulsive ball-hitting area extending to the periphery of the face portion.

In the composite part of the golf club head of the present invention, therefore, the high rebound performance is exhibited in a wide hitting area while reducing the ball spin. As a result, it is possible to achieve a long carry distance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a golf club head as an embodiment of the present invention.

FIG. 2(A) is a front view of the golf club head of FIG. 1 under its standard state.

FIG. 2(B) is a cross sectional view taken along line A-A in FIG. 2(A).

FIG. 3 is a plan view of the golf club head of FIG. 1 in its standard state.

FIG. 4 is a cross sectional view taken along line B-B in FIG. 2.

FIG. 5 is an exploded perspective view of a head main body of the golf club head in this embodiment.

FIG. 6(A) is a front view of a golf club head in its standard state showing another embodiment of the present invention.

FIG. 6(B) is a cross sectional view thereof taken along line A-A in FIG. 6(A).

FIG. 7(A) is a front view of a golf club head in its standard state showing still another embodiment of the present invention.

FIG. 7(B) is a cross sectional view thereof taken along line A-A in FIG. 7(A).

FIG. 8 is a front view of a golf club head in its standard state showing still another embodiment of the present invention.

FIG. 9 is a front view of a golf club head in its standard state showing still another embodiment of the present invention.

FIG. 10(A) is a perspective view of a iron-type golf club head as another embodiment of the present invention.

FIG. 10(B) is a vertical cross sectional view thereof.

FIG. 11(A) is a schematic cross sectional view showing the composite part before hit by a ball.

FIG. 11(B) is a schematic cross sectional view showing the composite part when hitting a ball.

FIG. 12 is a graph showing ball spin rates and club face materials.

FIG. 13(A) is a schematic cross sectional view showing a face portion of the prior art golf club head before hit by a ball.

FIG. 13(B) is a schematic cross sectional view showing the face portion of the prior art golf club head when hit by the ball.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detail in conjunction with accompanying drawings.

In this application, dimensions, positions, directions and the like relating to the club head refer to those under a standard state of the club head unless otherwise noted.

Here, the standard state of the club head is such that the club head is set on a horizontal plane so that the axis of the club shaft (not shown) is inclined at the specified lie angle while keeping the axis on a vertical plane, and the club face forms the specified loft angle with respect to the horizontal plane. Incidentally, in the case of the club head alone, the center line of the shaft inserting hole 6a can be used instead of the axis of the club shaft.

In FIGS. 1, 2(A) and 2(B) showing a first embodiment of the present invention, a club head 1 according to the present invention is a wood-type golf club head provided with a hollow (i) therein.

The club head 1 comprises a face portion 2, a crown portion 3, a sole portion 4, a side portion 5, and a hosel portion 6.

The face portion 2 has a club face 2a for hitting a ball. The crown portion 3 forms the upper surface of the club head 1. The sole portion 4 forms the bottom face of the club head 1. The side portion 5 extends between the crown portion 3 and the sole portion 4 and extends between the toe-side edge of the club face 2a and the heel-side edge of the club face 2a through the back side of the club head 1.

The hosel portion 6 has a shaft inserting hole 6a into which a tip end of a shaft (not shown) is inserted. For example, the hosel portion 6 is tubular and disposed in a heel side of the crown portion.

In this embodiment, each of the crown portion 3, the sole portion 4, the side portion 5 and the hosel portion 6 is made of a metal material. But, at least one of them may be partially made of a fiber reinforced resin.

The club face 2a comprises a composite part 7 capable of exhibiting high rebound performance with respect to a ball. The composite part 7 is, as shown in FIG. 2(B), layered by a thin surficial layer 8, an intermediate elastic layer 9 made of a rubber-like elastic material, and a back support layer 10.

The thin surficial layer 8 is for directly contacting a ball, and forms a part of the club face 2a.

The thin surficial layer 8 is made of a metal material, wherein titanium, titanium alloys, stainless steels, aluminum alloys and the like can be used for example. Without limited thereto, various metal materials may be used.

In comparison with resin materials, the metal material has a higher friction coefficient against a ball, therefore, the thin surficial layer 8 can create lower spin on the hit ball.

FIGS. 11(A) and 11(B) are diagrams showing the composite part 7 before and when hit by a ball respectively.

It is preferable that the thin surficial layer 8 is constructed as thin as possible so as to have such rigidity that, when a ball hits the composite part 7, as shown in FIG. 11(B), only a part of the thin surficial layer 8 which part is hit by the ball is deformed locally.

In other words, it is preferable that the thin surficial layer 8 has an extremely small thickness which can causes, on the intermediate elastic layer 9, substantially the same deformation as that when the ball directly hits the intermediate elastic layer 9.

Thereby, the thin surficial layer 8 allows the intermediate elastic layer 9 to cause larger local compressive elastic deformation when hit by a ball.

Meanwhile, the thin surficial layer 8 is required to have durability capable of withstanding a large number of shots, therefore, it is preferable that the thickness t1 of the thin surficial layer 8 is set in a range of from about 0.1 mm to about 1.0 mm for example.

The intermediate elastic layer 9 contacts with the inner surface (surface on the hollow side) of the thin surficial layer 8. The intermediate elastic layer 9 is made of a rubber-like elastic material, for example, rubber, elastomer, resin or the like. Preferably, an ionomer resin is used as the rubber-like elastic material.

When hit by a ball, the intermediate elastic layer 9 makes compressive elastic deformation according to the local deformation of the thin surficial layer 8. If the deformation of the intermediate elastic layer 9 is excessively large, there is a possibility that the durability of the composite part 7 is deteriorated. In this light, it is desirable that the Young's modulus of the rubber-like elastic material is set in a range of not less than 10 MPa, more preferably not less than 50 MPa, still more preferably not less than 140 MPa, but, not more than 1 GPa, more preferably not more than 800 MPa, still more preferably not more than 560 MPa, yet still more preferably not more than 300 MPa.

Preferably, the rubber-like elastic material of the intermediate elastic layer 9 has a rebound resilience of not less than 50%, more preferably not less than 60%. Thereby, it is possible to further improve the rebound performance of the club head 1.

Here, the rebound resilience is the LUPKE rebound resilience determined by Japanese Industrial Standard K 6255 “Rubber, vulcanized or thermoplastic-Determination of rebound resilience”.

The intermediate elastic layer 9 is arranged so that the intermediate elastic layer 9 includes a center C of the club face, and the edge surface of the intermediate elastic layer 9 is partly exposed at the outer surface of the club head.

Here, the center C of the club face is a point on the club face 2a which point is, in the front view of the club head as shown in FIG. 2(A), positioned at the intersecting point between a vertical line bisecting the maximum dimension of the club face 2a in the toe-heel direction and a horizontal line bisecting the maximum dimension of the club face 2a in the up-and-down direction.

The intermediate elastic layer 9 extends in a first direction D1. Preferably, the edge surface of the intermediate elastic layer 9 at its both ends in the first direction D1 is exposed at the outer surface of the club head.

For example, if the first direction D1 of the intermediate elastic layer 9 is the up-and-down direction of the club head 1, then the intermediate elastic layer 9 extends from the crown portion 3 to the sole portion 4, while including the center C of the club face.

The upper edge surface 9a and the lower edge surface 9b of the intermediate elastic layer 9 (namely, the edge surface at both ends in the first direction D1) are exposed at the outer surface of the club head in the crown portion 3 and the sole portion 4, respectively, excepting the club face 2a.

The back support layer 10 of the metal material contacts with the inner surface (surface on the hollow side) of the intermediate elastic layer 9 as shown in FIG. 2(B).

The upper edge 10a of the back support layer 10 joins the crown portion 3. The lower edge 10b of the back support layer 10 joins the sole portion 4.

The back support layer 10 supports the intermediate elastic layer 9 from behind, and prevents backward motions of the intermediate elastic layer 9 when hit by a ball.

Owing to such action, the impactive force when hit by a ball is received by the intermediate elastic layer 9, and the intermediate elastic layer 9 makes large compressive elastic deformation.

In order that the back support layer 10 has sufficient rigidity, the thickness of the back support layer 10 is preferably more than that of the thin surficial layer 8.

When a ball is hit by the composite part 7, the ball first contacts with the thin surficial layer 8 made of the metal material.

As described above, in comparison with the resin materials, the metal material has a higher friction coefficient with respect to a golf ball, and the hit ball is prevented from having excessive spin.

The thin surficial layer 8 hit by a ball is locally deformed, and accordingly, the intermediate elastic layer 9 is also locally largely elastically deformed. Thus, the rebound performance of the club head 1 is improved.

Further, in the front view of the club head as shown in FIG. 2(A), the intermediate elastic layer 9 is arranged so as to include the center C of the club face, thereby the high rebound performance can be provided at the center C of the club face which is a preferable hitting position.

Furthermore, the upper edge surface 9a and the lower edge surface 9b of the intermediate elastic layer 9 are exposed at the outer surface of the club head in the crown portion 3 and the sole portion 4, respectively, therefore, even if the ball hits a peripheral edge portion of the club face 2a such as an upper edge portion or a lower edge portion of the club face 2a corresponding to the composite part 7, the face portion 2 can largely elastically deform, and high rebound performance can be obtained.

Such function is advantageous to the club head 1 designed as a fairway wood frequently used to hit a ball set directly on the ground.

As described above, the composite part 7 of the club head 1 in this embodiment exhibits high rebound performance over a wide ball-hitting area, while restricting the spin rate of the ball, therefore. it is possible to provide a long carry distance.

In the face portion 2 of the club head 1 in this embodiment, the composite part 7 is provided in its central region in the toe-heel direction.

The composite part 7 constitutes a part of the face portion 2 as explained above. The width W of the composite part 7 measured orthogonally to the first direction D1 is set in a range of not less than 5 mm, preferably not less than 10 mm in view of the contact area with the hit ball.

In contrast, a part of the face portion 2 on the toe-side and on the heel-side of the composite part 7 is constructed as a non-composite part 11 made of a metal material only.

The non-composite part 11 preferably has a thickness more than that of the thin surficial layer 8.

It is preferable that the composite part 7 further includes a side support wall 12 in order to prevent the intermediate elastic layer 9, which is deformed by hitting a ball, from running out toward the toe or heel (toward the non-composite part 11 side).

The side support wall 12 functions to prevent the deformed intermediate elastic layer 9 from running out in the orthogonal direction to the first direction D1 in the front view of the club head.

In this embodiment, the side support wall 12 includes a first side support wall 12a and a second side support wall 12b.

The first side support wall 12a connects between the thin surficial layer 8 and the back support layer 10 so as to support the toe-side edge surface 9c of the intermediate elastic layer 9. The upper edge of the first side support wall 12a is integrally connected to the crown portion 3, and the lower edge of the first side support wall 12a is integrally connected to the sole portion 4 (not shown).

Similarly, the second side support wall 12b connects between the thin surficial layer 8 and the back support layer 10 so as to support the heel-side edge surface 9d of the intermediate elastic layer 9. The upper edge of the second side support wall 12b is integrally connected to the crown portion 3, and the lower edge of the second side support wall 12b is integrally connected to the sole portion 4 (not shown).

The side support wall 12 constructed as above can prevent the intermediate elastic layer 9 from running out in the orthogonal direction to the first direction D1 in the front view of the club head. Thereby, the impactive force when hit by a ball can be effectively converted to larger compressive elastic deformation of the intermediate elastic layer 9 in the front-back direction, and the rebound performance of the club head 1 can be further improved.

In order that the intermediate elastic layer 9 can withstand the above-described large deformation when hit by a ball, the thickness t2 of the intermediate elastic layer 9 is preferably set in a range of not less than 1 mm, more preferably not less than 3 mm, still more preferably not less than 5 mm.

If the thickness t2 of the intermediate elastic layer 9 is excessively large, there is a possibility that the mass of the club head 1 is excessively increased. In this light, the thickness t2 of the intermediate elastic layer 9 is preferably set in a range of not more than 30 mm, more preferably not more than 20 mm.

In this embodiment, it is preferable that the natural frequency in a primary mode of the club head 1 when the center c of the club face is supported or fixed is adjusted to about 800 Hz to about 1600 Hz, more preferably about 1000 Hz to about 1400 Hz.

The natural frequency in a primary mode of the club head means the lowest frequency in an eigenmode.

Incidentally, an eigenmode is a natural vibration of a system such that various parts all move sinusoidally at the same frequency. There can be phase differences for different parts of the system.

Such natural frequency can be obtained by a modal analysis on the club head. As to the modal analysis, either a test analysis, namely, experimental modal analysis or a simulation analysis can be employed. In the test analysis, an object (club head) is vibrated, and an eigenmode is obtained. In the simulation analysis, an eigenmode is obtained through a numerical analysis method utilizing a computer such as a finite element method. Preferably, giving a restraint condition such that a position in the club face is supported or fixed, such modal analysis is carried out.

The popular golf balls has a natural frequency in a primary mode of about 800 Hz to about 1200 Hz.

According to the frequency matching theory, the natural frequency in a primary mode of the club head 1 is set in a range (1000 Hz to 1400 Hz) approximate to that of the balls, but slightly higher than that of the balls. Thereby, it is possible to further improve the rebound performance of the club head 1.

It is preferable that the natural frequency in a primary mode of the club head 1 is adjusted by mainly changing the Young's modulus and/or the thickness of the intermediate elastic layer 9.

Incidentally, the impedance matching theory is described in detail for example in Japanese patent publication No. 4-56630, and The Japan society of Mechanical Engineers, collected papers vol. 67 No. 656 (2001-4) “Design of Golf Club Head with High Restitution Performance”.

Preferably, a ratio feg/fc is set in a range of from 1.0 to 1.3, more preferably in a range of from 1.0 to 1.2,

wherein

the “fc” is the natural frequency in a primary mode of the club head 1 when the center C of the club face is supported or fixed, and

the “feg” is the natural frequency in a primary mode of the club head 1 when a peripheral edge portion of the club face corresponding to the composite part 7 (in the FIG. 1 embodiment, an upper edge portion or a lower edge portion of the club face 2a) is supported or fixed.

Thereby, the high-repulsive ball-hitting area can be obtained in a wide range from the club face's center C to the club face's peripheral edge.

It is important that the ratio feg/fc does not fall below 1.0.

In general, a club head is designed such that the coefficient of restitution comes closer to the upper limit regulated by the golf rule as far as possible so that the flying distance of a ball becomes longest when the ball is hit at the center C of the club face.

Therefore, if a club head has the ratio feg/fc of 1.0, there is a possibility that the club head infringes the golf rule when measured at a club face's peripheral edge portion. Accordingly, it is not preferable that the ratio feg/fc falls below 1.0.

In the above description, in order to quantitatively express the rebound performance of the club head 1, the natural frequency in a primary mode of the club head is used. However, in order to express the rebound performance, the characteristic time may be used instead of or together with the natural frequency in a primary mode.

Preferably, the characteristic time of the club head 1 at the center C of the club face is set in a range of from 200 to 257 microseconds.

Incidentally, the characteristic time means the “Characteristic Time” measured by the Pendulum Test specified in “Technical Description of the Pendulum Test” appended to “Notice To manufacturers” issued from the USGA on Feb. 24, 2003. The characteristic time is the actual amount of time a golf ball stayed in contact with the face of a club head (driver). The longer the characteristic time, the better the rebound performance.

A maximum characteristic time of 257 microseconds (inclusive of a maximum test tolerance of 18 microseconds) is allowed in the golf rule.

By setting the characteristic time within the range of from 200 to 257 microseconds, the club head 1 can exert higher rebound performance within the golf rule.

Particularly, it is preferable that the difference of the characteristic time of the head when a club face's peripheral edge portion corresponding to the composite part 7 is supported or fixed from

the characteristic time of the head when the club face's center C is supported or fixed

is a positive value and not more than 50 microseconds. Thereby, the high-repulsive ball-hitting area can be obtained in a wide range as previously described.

The club head 1 as described above can be manufactured in a variety of methods.

For example, as shown in FIG. 5, a head main body 1a excluding the intermediate elastic layer 9 is first manufactured by joining two or more metal parts. Then, the rubber-like elastic material is disposed between the thin surficial layer 8 and the back support layer 10 which layers are included in the head main body 1a.

As an example, it is possible to pre-shape the rubber-like elastic material into an elastic body and fix it to the head main body 1a.

As another example, it is possible to cast the rubber-like elastic material, which is in liquid form or in a plasticized state, into the head main body 1a and harden it by a suitable manner, e.g. vulcanizing, cross-linking, cooling, etc.

FIGS. 6(A), 6(B), 7(A), 7(B), 8, 9, 10(A) and 10(B) show further embodiments of the present invention, wherein parts and positions corresponding to those of the former first embodiment are provided with the same reference numbers, and their descriptions are omitted.

In the embodiment shown in FIGS. 6(A) and 6(B), the edge surface of the intermediate elastic layer 9 at only one of the ends in the first direction D1 is exposed at the outer surface of the club head. More specifically, only the lower edge surface 9b of the intermediate elastic layer 9 is exposed at the outer surface of the club head.

In a fairway wood frequently used to hit a ball set directly on the ground, there is a tendency that the ball is hit at a sole portion 4 side of the club face 2a.

In this embodiment shown in FIGS. 6(A) and 6(8), the club face from its center C to its lower edge (the edge on the sole portion 4 side) is formed by the composite part 7, and the rebound performance is increased there. Therefore, the head 1 in this embodiment is preferably designed as a fairway wood.

In this case, the carry distance is effectively increased with a less volume of rubber-like elastic material (the intermediate elastic layer 9).

In this embodiment, the upper edge surface 9a of the intermediate elastic layer 9 is terminated at a position above the center c of the club face, without being exposed at the outer surface of the club head.

In this case, it is preferable that the upper edge surface 9a of the intermediate elastic layer 9 is prevented from running out in the upward direction by the use of an upper support wall 15. The upper support wall 15 extends between the back support layer 10 and the thin surficial layer 8 so as support the upper edge surface 9a of the intermediate elastic layer 9.

Thereby, the deformation of the thin surficial layer 8 is effectively converted to compressive elastic deformation in the front-back direction, of the intermediate elastic layer 9, and the head can exhibit excellent rebound performance.

In the embodiment shown in FIGS. 7(A) and 7(B), the above-mentioned first direction D1 of the intermediate elastic layer 9 is the toe-heel direction of the club head 1.

The intermediate elastic layer 9 extends from the toe-side edge to the heel-side edge of the side portion 5, passing through the center c of the club face of the face portion.

The toe-side edge surface 9c and the heel-side edge surface 9d, namely, both edge surfaces of the intermediate elastic layer 9 in the first direction D1 are exposed at the outer surface of the club head.

In this embodiment, the toe-side edge surface 9c and the heel-side edge surface 9d of the intermediate elastic layer 9 are exposed in the side portion 5.

In the embodiment shown in FIGS. 7(A) and 7(B), the upper edge surface 9a of the intermediate elastic layer 9 terminates on the crown portion 3 side of the center c of the club face, without being exposed at the outer surface of the club head.

It is preferable that the upper edge surface 9a of the intermediate elastic layer 9 is supported by an upper support wall 15 for preventing its running out in the upward direction. The upper support wall 15 extends between the back support layer 10 and the thin surficial layer 8 so as to support the upper edge surface 9a of the intermediate elastic layer 9.

Also the lower edge surface 9b of the intermediate elastic layer 9 terminates on the sole portion 4 side of the center C of the club face, without being exposed at the outer surface of the club head.

It is preferable that the lower edge surface 9b of the intermediate elastic layer 9 is supported by a lower support wall 16 for preventing its running out in the downward direction. The lower support wall 16 extends between the back support layer 10 and the thin surficial layer 8 so as to support the lower edge surface 9b of the intermediate elastic layer 9.

In the embodiment shown in FIGS. 7(A) and 7(B), the high repulsive ball hitting region (composite part 7) can be formed in a wide range in the toe-heel direction of the face portion 2.

As a modification (not shown) of the intermediate elastic layer 9, it may be possible to terminate only one of the toe-side edge surface 9c and the heel-side edge surface 9d, without being exposed in the side portion 5.

In the embodiment shown in FIG. 8, the above-mentioned first direction D1 of the composite part 7 is an oblique direction inclined downwards toward the heel from the toe of the club head 1. Such first direction D1 approximates the distribution of ball hitting positions of the average golfers. In the club head in this embodiment, therefore, long flying distances can be obtained steadily even if ball hitting positions are off-centered along the first direction D1. In this embodiment too, the edge surfaces of the intermediate elastic layer 9 not exposed at the outer surface of the club head are prevented from running out in the orthogonal direction to the first direction D1 in the front view of the club head.

In the embodiment shown in FIG. 9, excepting an edge surface near the hosel portion, the most part of the edge surface of the intermediate elastic layer 9 is exposed at the outer surface of the club head. Such composite part 7 is desirable in that the high repulsive ball hitting region is formed in a wider range of the face portion 2.

In this embodiment too, the edge surface of the intermediate elastic layer 9 not exposed at the outer surface of the club head is prevented from running out by the use of a support wall as explained above.

As a modification (not shown) of the intermediate elastic layer 9, it is also possible to circularly continuously expose the entire edge surface of the intermediate elastic layer 9 at the outer surface of the club head.

The embodiment shown in FIG. 10 is an iron-type golf club head 1 differently from the former embodiments.

In this embodiment, the first direction D1 is the up-and-down direction of the club head 1, and

the intermediate elastic layer 9 is exposed at the upper surface and bottom face of the club head.

Such club head 1 can exhibit high rebound performance at the center of the club face as well as near the upper edge and the lower edge of the club face 2a.

As a modification (not shown) of the intermediate elastic layer 9, it is also possible to expose the intermediate elastic layer 9 in a toe side and/or a heel side of the club head.

While description has been made of preferable embodiments of the present invention, the present invention can be embodied in various forms without being limited to these embodiments.

Comparison Tests

In order to confirm the advantageous effects of the present invention, club heads for a fairway wood (#3) were experimentally manufactured according to the specifications listed in Table 1 and tested as follows.

Working Example

The club heads as working examples according to the present invention were based on the structure shown in FIGS. 1 to 4. The component parts of each head are as follows.

- thin surficial layer: titanium alloy (Ti-6Al-4v)
- intermediate elastic layer: ionomer resin
- back support layer: maraging steel, Young's modulus 210 GPa, thickness 2.0 mm
- other parts of club head: same as back support layer
- club head mass: about 214 grams

Comparative Example 1

comparative example 1 had a structure which was basically the same as those of working examples, excepting the face portion not provided with the composite part.

The face portion was made of a titanium alloy Ti-6Al-4v plate whose thickness was 2.5 mm in its central area and 1.5 mm its peripheral area. Other than the face portion, the club head was made of the maraging steel having a Young's modulus of 210 GPa similarly to working examples. The mass of the club head was about 190.7 grams.

Comparative Example 2

Comparative example 2 had a structure which was basically the same as those of working examples, excepting the thin surficial layer made of a carbon fiber reinforced resin.

- thin surficial layer: carbon fiber reinforced epoxide resin. thickness 2 mm
- intermediate elastic layer: ionomer resin, Young's modulus 280 MPa, specific gravity 8.3, thickness 5 mm
- back support layer: maraging steel, Young's modulus 210 GPa
- other parts of club head: same as back support layer
- club head mass: about 214 grams Comparative Example 3

Comparative example 3 was basically the same as comparative example 2, excepting that

the thin surficial layer had a thickness of 4 mm,

the intermediate elastic layer had a thickness of 6.6 mm, and

the intermediate elastic layer had a Young's modulus of 5 MPa. Comparative example 3 was prepared to simulate the club head disclosed in the patent document 1.

In the comparison test, by the use of a finite element computer simulation, the natural frequency in a primary mode of each club head when each of the following positions of the club face was supported or fixed was computed.

- fs: natural frequency at the sweet spot
- fc: natural frequency at the center of the club face
- fc5: natural frequency at 5 mm downward from the center of the club face
- feg: natural frequency at 10 mm downward from the center of the club face (namely, club face's peripheral edge portion)
  
  In the computer simulation, the above-mentioned support or fixation of each position was made by giving a boundary condition to define as being immovable, all of node points of the finite elements existing in a 10 mm diameter circle centered on each position.

If the natural frequency in a primary mode of a club head when a ball hitting position of the club face is supported or fixed coincides with the natural frequency of the ball when a position of the ball is supported or fixed, then the rebound performance of the club head becomes highest.

If the club head's natural frequency becomes excessively lower than the ball's natural frequency, then the rebound performance sharply decreases.

Therefore, when designing a club having high rebound performance, it is desirable that the club head has a natural frequency in a primary mode in the range of from 1000 to 1400 Hz which range approximates to and slightly higher than the range 800 to 1200 Hz of the natural frequencies of the balls.

TABLE 1

comparative
comparative
comparative
working
working
working

Club head
example 1
example 2
example 3
example 1
example 2
example 3

surficial layer

thickness t1 (mm)
2 (*1)
2 (*2)
4 (*2)
0.5
0.2
1.0

intermediate elastic layer

thickness t2 (mm)
0
5
6.6
5
5
5

Young's modulus (MPa)
—
280
5
280
280
280

natural frequency (Hz)

fs
1835
1524
782
1096
959
1288

fc
1603
1661
794
1092
951
1297

fc5
1805
1700
783
1182
1023
1410

feg
3061
1590
748
1252
1094
1443

ratio feg/fc
1.9
1.0
0.9
1.1
1.2
1.1

working
working
working
working
working
working

Club head
example 4
example 5
example 6
example 7
example 8
example 9

surficial layer

thickness t1 (mm)
0.5
0.5
0.5
0.5
0.5
0.5

intermediate elastic layer

thickness t2 (mm)
2
4
10
20
5
5

Young's modulus (MPa)
280
280
280
280
140
560

natural frequency (Hz)

fs
1269
1147
986
859
890
1332

fc
1230
1128
1027
878
897
1326

fc5
1341
1223
1059
892
950
1458

feg
1586
1331
1052
849
970
1602

ratio feg/fc
1.3
1.2
1.0
1.0
1.1
1.2

(*1) titanium alloy (Ti—6Al—4V)

(*2) CFRP

From the test results, it was confirmed that the club heads as working examples were improved in the rebound performance in a wide area including the sweet spot and the center of the club face as well as their downward positions. Meanwhile, the club head as Comparative example 3 showed a natural frequency fc lower than 800 Hz, therefore, it can not be expected that such club head exhibits sufficient rebound performance.

Further, the natural frequency feg at the club face's peripheral edge portion became lower than the natural frequency fc at the center of the club face (namely, the coefficient of restitution became higher). In this case, if the coefficient of restitution at the center of the club face is set as being closer to the golf rule's upper limit as far as possible, there is a possibility that such club head infringes the golf rule with respect to the coefficient of restitution at the club face's peripheral edge portion as previously mentioned.

FIG. 12 is a graph showing ball spin rates and club face materials. Plates made of various materials and having substantially identical surface roughness (ten point average roughness of not more than 1 micrometers) were prepared. Each plate was inclined so as to correspond to a loft angle of 15 degrees. A golf ball shot from an airgun was hit at each plate at a speed of 30 m/s, and the rebound ball was measured for the spin rate.

As shown in FIG. 12, the spin rates of the metal materials are low in comparison with the resin materials. The reason for this is supposed to be a difference in surface energy between a metal and a resin, but it is not yet explained exactly.

Anyhow, in the club head according to the present invention, since the surface of the composite part is made of a metal material, it is possible to provide high rebound performance as confirmed by Table 1 and a low spin rate as confirmed by FIG. 12. Consequently, owing to a synergistic effect of these, long flying distances can be obtained.

REFERENCE SIGNS LIST

- 1 club head
- 2 face portion
- 2
  a club face
- 3 crown portion
- 4 sole portion
- 5 side portion
- 6 hosel portion
- 7 composite part
- 8 thin surficial layer
- 9 intermediate elastic layer
- 10 back support layer
- 12 side support wall
- C club face's center

GOLF CLUB HEAD

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Priority Claims (1)