EP4371630A1

EP4371630A1 - Golf shaft and manufacturing method for same

Info

Publication number: EP4371630A1
Application number: EP22841674.9A
Authority: EP
Inventors: Kosuke Fujiwara
Original assignee: NHK Spring Co Ltd
Current assignee: NHK Spring Co Ltd
Priority date: 2021-07-13
Filing date: 2022-03-01
Publication date: 2024-05-22
Also published as: JP2023012237A; CN117440854A; WO2023286324A1; KR20230162105A; TW202302189A; JP7142274B1; TWI801205B

Abstract

Provided is a golf shaft capable of realizing a sufficient low trajectory and low spin. The golf shaft has a rigidity distribution in which rigidity is gradually increased from a tip end section 5 to a butt end section 7, and, in the rigidity distribution, has an intermediate point P at which inclination is changed in an intermediate section 9 between the tip end section 5 and the butt end section 7, a difference in rigidity between the tip end section 5 and the intermediate point P is 3.00 kgf·m²-5.00kgf·m², and a difference in rigidity between the intermediate point P and the butt end section 7 is 2.00kgf·m² or less.

Description

FIELD OF THE INVENTION

The present invention relates to a golf shaft realizing a low trajectory and low spin and a manufacturing method for the same.

BACKGROUND OF THE INVENTION

In a golf club, it is known that carry of a ball is improved by a higher trajectory and lower spin. Such a ball with a high trajectory and low spin is realized by adjustment of rigidity distribution of a shaft (golf shaft) for a golf club like, for example, Patent document 1.
The ball with the high trajectory and the low spin is affected by wind and may not be improved in carry. In this case, it is advantageous to lower a trajectory to make a ball be a low trajectory and low spin.
Although the ball with the low trajectory and the low spin is supposedly realized by improving rigidity of a tip end of a golf shaft in general, it is insufficient to simply improve the rigidity of the tip end.

CITATION LIST

PATENT LITERATURE

PATENT LITERATURE 1: JP2014-33831A

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

A problem to be solved is that a sufficient low trajectory and sufficient low spin are not realized.

MEANS FOR SOLVING THE PROBLEM

The present invention provides a golf shaft, having a rigidity distribution in which rigidity gradually increases from a tip end section to a butt end section, wherein the rigidity distribution has an intermediate point at which inclination is changed in an intermediate section between the tip end section and the butt end section, a difference in rigidity between the tip end section and the intermediate point is 3.00 kgf·m²-5.00kgf·m², and a difference in rigidity between the intermediate point and the butt end section is 2.00kgf·m² or less.
Further, the present invention provides a manufacturing method of a golf shaft having a rigidity distribution in which rigidity gradually increases from a tip end section to a butt end section, comprising extending a length of the butt end section in an axial direction by one or more adjustment steps with respect to a basic shape, and eliminating the adjustment steps corresponding to an amount of the extending of the length of the butt end section with respect to the basic shape, the basic shape in which an intermediate section between the tip end section and the butt end section has a main body, a plurality of adjustment steps having different outer diameters located on a tip end side relative to the main body, and a straight portion having a constant outer diameter located on the tip end side relative to the adjustment steps.

EFFECT OF THE INVENTION

According to the golf shaft of the present invention, the rigidity at the tip end section and the butt end section are set appropriately in relation to the intermediate point to realize a sufficient low trajectory and sufficient low spin.
According to the manufacturing method for a golf shaft, a plurality of flexes are easily developed in golf shafts in which rigidity gradually increases from a tip end section to a butt end section including a golf shaft realizing a sufficient low trajectory and sufficient low spin.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] FIG. 1 is a side view of a golf shaft according to an embodiment 1 of the present invention.
[FIG. 2] FIG. 2 is a graph illustrating rigidity distribution of the golf shaft of FIG. 1.
[FIG. 3] FIG. 3 is a table illustrating hitting angles and spin amounts of the golf shaft of FIG. 1 together with comparative examples.
[FIG. 4] FIG. 4 is a graph illustrating the rigidity distribution of FIG. 2 together with rigidity distributions of the comparative examples.
[FIG. 5] FIG. 5 is a side view illustrating a golf shaft having a basic shape used for a manufacturing method of the golf shaft according to the embodiment 1 of the present invention.
[FIG. 6] FIGs. 6(A) to (D) are side views illustrating golf shafts having different flexes.
[FIG. 7] FIGs. 7(A) and (B) are tables illustrating rigidity at tip end sections, intermediate points, and butt end sections of the golf shafts of FIGs. 6(A)-(D) together with corresponding rigidity of the comparative examples.

EMBODIMENT FOR CARRYING OUT THE INVENTION

The object of realizing a sufficient low trajectory and sufficient low spin is accomplished by a rigidity distribution of a golf shaft.
Namely, a golf shaft (1) has a rigidity distribution in which rigidity gradually increases from a tip end section (5) to a butt end section (7). The rigidity distribution has an intermediate point (P) at which inclination is changed in an intermediate section (9) between the tip end section (5) and the butt end section (7), a difference in rigidity between the tip end section (5) and the intermediate point (P) is 3.00 kgf·m²-5.00kgf·m², and a difference in rigidity between the intermediate point (P) and the butt end section (7) is 2.00kgf·m² or less.
The intermediate point (P) may be located in a range having a distance from a tip end of the golf shaft (1) being 40%-60% of an entire length.
The golf shaft (1) is formed into a stepped shape exhibited between the tip end section (5) and the butt end section (7) so that an outer diameter is gradually increased, and the stepped shape has a difference in outer diameter between adjacent steps (13) from the intermediate point (P) to the butt end section (7) being smaller than a difference in outer diameter between adjacent steps (13) from the tip end section (5) to the intermediate point (P).
A manufacturing method for a golf shaft is a method of manufacturing the golf shaft (1) having a rigidity distribution in which rigidity gradually increases from a tip end section (5) to a butt end section (7). The manufacturing method extends a length of the butt end section (7) in an axial direction by one or more adjustment steps (13) with respect to a basic shape, and eliminates the adjustment steps (13) corresponding to an amount of the extending of the length of the butt end section (7) with respect to the basic shape, the basic shape in which an intermediate section (9) between the tip end section (5) and the butt end section (7) has a main body (11), a plurality of adjustment steps (13) having different outer diameters located on a tip end side relative to the main body (11), and a straight portion (15) having a constant outer diameter and located on the tip end side relative to the adjustment steps (13).
In the manufacturing method, an outer diameter of the straight portion (15) may be made equivalent to a largest outer diameter in the eliminated adjustment steps (13).

Embodiment 1

[Structure of Golf Shaft]

FIG. 1 is a side view of a golf shaft according to an embodiment 1 of the present invention.
A golf shaft 1 of the present embodiment is a metal shaft and is made of a metal hollow tube. The golf shaft 1 has an outer diameter being a stepped shape as a whole and being gradually increased. A sectional shape of the golf shaft 1 in a traverse section is a circular shape. It should be noted that the outer diameter of the golf shaft 1 may not be a stepped shape and may be other shapes of which part or a whole is a tapered shape or the like. In addition, the traverse sectional shape of the golf shaft 1 may be another shape such as oval.
A length of the golf shaft 1 is 37-41 inches (about 93.98-104.14cm). The golf shaft 1 may be, however, set shorter than 37 inches or longer than 41 inches. Further, although material of the golf shaft 1 is steel, it may be other material. The other material is other metal such as aluminum, titanium, or alloy thereof, non-metal such as CFRP, composite material that is a combination of metal and non-metal, or the like.
The golf shaft 1 is composed of a tip end section 5, a butt end section 7, and an intermediate section 9.
The tip end section 5 is a tip end portion in an axial direction and is a region having a given range from a tip end of the golf shaft 1 in the axial direction. The tip end section 5 of the present embodiment is a portion to which a head of a golf club is attached. The tip end section 5 is formed into a tapered shape having an outer diameter slightly gradually increases toward a butt end of the golf shaft 1. The tip end section 5 may be, however, formed into a straight shape having a constant outer diameter.
The butt end section 7 is a butt end portion of the golf shaft 1 in the axial direction and is a region having a given range from the butt end of the golf shaft 1 in the axial direction. The butt end section 7 of the present embodiment is a portion to which a grip of the golf club is attached. Although the butt end section 7 is formed into a straight shape having a constant outer diameter, it may be a tapered shape having an outer diameter is slightly gradually varied toward the butt end.
The intermediate section 9 is a portion located between the tip end section 5 and the butt end section 7, and is composed of a main body 11, a plurality of adjustment steps 13, a straight portion 15, and a tapered portion 16.
The main body 11 is a portion entirely formed into the stepped shape and is composed of a plurality of steps 17. The plurality of the steps 17 are gradually increased in outer diameter toward the butt end. In each step 17, the outer diameter is constant and the outer diameter may be gradually increased toward the butt end.
In the stepped shape of the main body 11, a difference in outer diameter between adjacent steps 13 from the intermediate point P to the butt end section 7 is smaller than a difference in outer diameter between adjacent steps 13 from the tip end section 5 to the intermediate point P (see FIG. 2) explained later. If the intermediate point P is located in the middle of a step 17, it is sufficient to make a difference in outer diameter between this step 17 and a subsequently or precedingly located step 17 as a beginning smaller than a difference in outer diameter between previous steps 17. Further, the steps 17 from the intermediate point P to the butt end section 7 are set longer than the steps from the tip end section 5 to the intermediate point P in axial dimension.
The main body 11 is a portion that is not changed in shape regardless of variations of the golf shaft 1 in flex. The flex of the golf shaft 1 is rigidity of the golf shaft 1 and a plurality of flexes are developed even in the same golf shaft 1 in general.
The plurality of the adjustment steps 13 are steps to adjust the golf shaft 1 in flex. The number of the adjustment steps 13 is reduced as the flex becomes harder as explained later. The golf shaft 1 may, therefore, have no adjustment steps 13 according to the flex. In addition, the adjustment steps 13 may not be provided regardless of the flex.
The plurality of the adjustment steps 13 are located on the tip end side relative to the main body 11 and have different outer diameters. According to the present embodiment, the adjustment steps 13 entirely exhibit a stepped shape adjoining to the main body 11 and is continuous to the stepped shape of the main body 11. These adjustment steps 13 are gradually increased in outer diameter toward the butt end similarly to the steps 17 of the main body 11.
It should be noted that the adjustment steps 13 are not necessarily adjoining to the main body 11 and tapered portions, steps or the like may be interposed between the adjustment steps 13 and the main body 11.
An outer diameter of each adjustment step 13 is constant. The outer diameter of the adjustment step 13 may be, however, gradually increased toward the butt end. The plurality of the adjustment steps 13 have the same axial length. The axial lengths of the plurality of the adjustment steps 13 may be, however, different from each other according to flex development.
The straight portion 15 is a portion located on the tip end side relative to the adjustment steps 13 and having a constant outer diameter. In addition, the straight portion 15 may be interposed between the tip end section 5 and the main body 9 of the intermediate section 9 if the adjustment steps 13 are omitted.
The straight portion 15 of the present embodiment is interposed between the tip end section 5 and the adjustment steps 13, an outer diameter of which is smaller than the smallest one of the adjustment steps 13 and larger than of the tip end section 5. The outer diameter of the straight portion 15 is set larger as the flex becomes harder as explained later. In addition, although the straight portion 15 is adjoining to the adjustment step 13, tapered portions, steps or the like may be interposed between the adjustment step 13 and the straight portion 15.
The tapered portion 16 is a portion having a tapered shape arranged between the straight portion 15 and the tip end section 5, to absorb a difference in diameter between the straight portion 15 and the tip end section 5.
The golf shaft 1 with the above structure has a rigidity distribution in which rigidity gradually increases from the tip end section 5 to the butt end section 7. It should be noted that the gradually increasing of the rigidity means that, in the rigidity distribution, inclination of approximation straight line of a segment from the tip end section 5 to the butt end section 7, inclination of a segment connecting the tip end section 5 and the butt end section 7, an average of inclinations of a segment from the tip end section 5 to the butt end section 7 or the like is positive. In that sense, the gradually increasing of the rigidity includes a case in which the rigidity is barely reduced between the tip end section 5 and the butt end section 7.
In the case that the rigidity is reduced, a permissible reduction amount of the rigidity is 10% or less of a difference in rigidity between the tip end section 5 and the butt end section 7 in order to realize a sufficient low trajectory and sufficient low spin. The reduction amount is a difference in rigidity between a peak on the tip end side and a valley on the butt end side that are adjacent to each other in the rigidity distribution.
FIG. 2 is a graph illustrating rigidity distribution of the golf shaft 1. In addition, in FIG. 2, the golf shaft 1 has the length of 41 inches and the weight of 110g and is lighter than a general golf shaft having the same length.
As the rigidity distribution of FIG. 2, in the golf shaft 1 of the present embodiment, the rigidity at the tip end section 5 and the butt end section 7 is constant and the rigidity at the intermediate section 9 being a portion spanning between the tip end section 5 and the butt end section 7 is gradually increased. The rigidity distribution is set according to the stepped shape and wall thickness of the golf shaft 1 and is adjusted according to settings of the axial length of the butt end section 7, the number of the adjustment steps 13, and the outer diameter of the straight portion 15 as explained later.
In the rigidity distribution, the intermediate section 9 has the intermediate point P at which inclination is changed. This inclination means inclination on the tip end side and inclination on the butt end side relative to the intermediate point P as a center. In addition, the intermediate point P means not only a point at which the inclination is changed on the rigidity distribution but also an axial part of the golf shaft 1 corresponding to the point in the present specification.
The inclination on the tip end side is inclination of approximation straight line of a segment from the tip end section 5 to the intermediate point P, inclination of a segment connecting the tip end section 5 and the intermediate point P, an average of inclinations of the segment from the tip end section 5 to the intermediate section 9 or the like. The inclination on the butt end side is inclination of approximation straight line of a segment from the intermediate point P to the butt end section 7, inclination of a segment connecting the intermediate point P and the butt end section 7, an average of inclinations of the segment from the intermediate section 9 to the butt end section 7 or the like.
The inclination on the tip end side is larger than the inclination on the butt end side. This is adjustable according to the difference in outer diameter and axial dimension between the adjacent steps 17 of the stepped shape.
In the rigidity distribution, a difference in rigidity between the tip end section 5 and the intermediate point P is 3.00 kgf·m²-5.00kgf·m² and the difference in rigidity between the intermediate point P and the butt end section 7 is 2.00kgf·m² or less. Although the rigidity at the tip end section 5 is rigidity at a position of 50mm from the tip end in this embodiment, it may be an average value, a maximum value, a minimum value or the like in the tip end section 5 if the rigidity of the tip end section 5 is varied. The rigidity of the butt end section 7 is rigidity at any position of the butt end section 7. If the rigidity of the butt end section 7 is varied in the axial direction, it may be, however, an average value, a maximum value, a minimum value or the like.
The intermediate point P is preferably located in a range having a distance from the tip end of the golf shaft 1 being 40%-60% of the entire length of the golf shaft 1. The rigidity of the intermediate point P is preferably 5.40kgf·m²-8.00kgf·m². More preferably, the rigidity of the tip end section 5 is 2.20 kgf·m² or more and the rigidity of the butt end section 7 is 9.00kgf·m² or less. According to the present embodiment, the position of the intermediate point P is a position at 50% of the entire length and the rigidity of the intermediate point P is 6.06kgf·m². The rigidity of the tip end section 5 is 2.30kgf·m² and the rigidity of the butt end section 7.39kgf·m². In addition, the position and the rigidity of the intermediate point P, the rigidity of the tip end section 5 and the butt end section 7 may employ values other than the above.
Further, in the present embodiment, the rigidity is set so as not to exceed 4.00kgf·m² from the tip end to a position at 20% of the entire length.
With this rigidity distribution, the golf shaft 1 of the present embodiment has the rigidity of the tip end section 5 set higher and the rigidity of the butt end section 7 set lower in relation to the intermediate point P, thereby delaying in bending and reducing a hitting angle. Further, the rigidity of the intermediate point P is high to reduce a spin amount. As this result, the golf shaft 1 of the present embodiment realizes the sufficient low trajectory and the sufficient low spin.
FIG. 3 is a table illustrating hitting angles and spin amounts of the golf shaft 1 of the present embodiment together with comparative examples, and FIG. 4 is a graph illustrating the rigidity distribution of FIG. 2 together with rigidity distributions of the comparative examples. Only the comparative example 3 has a length of 39 inches in order to match a number for a golf club with of the embodiment and the other comparative examples.
The comparative example 1 is for a low trajectory and non-low spin, the comparative example 2 is for a high trajectory and low spin, the comparative example 3 is for a general trajectory and general spin.
The comparative example 1 is set higher in rigidity on the tip end section side and lower in rigidity on the butt end in comparison with the comparative example 3. The comparative example 2 is set higher in rigidity of an entire area spanning from the intermediate section to the butt end section in comparison with the comparative example 3.
In contrast, the present embodiment is set higher in rigidity from the tip end section to the intermediate section and lower in rigidity from the intermediate section to the butt end section in comparison with the comparative example 3. Further, in the present embodiment, the rigidity of the intermediate section 9 is positioned between of the comparative examples 1 and 2.
With the difference, the present embodiment reduces the spin amount while equalizing the hitting angle in comparison with the comparative example 1 in which the rigidity on the tip end section side is simply improved. Further, the spin amount is further reduced relative to the comparative example 2 for the low spin.

[Manufacturing Method for Golf Shaft]

The manufacturing method for the golf shaft 1 of the present embodiment, for example, rolls a plate material to form a pipe body, inner and outer diameters of which are constant. To the pipe body, stepping process is performed and the wall thickness is set.
At this time, the present embodiment sets the axial length of the butt end section 7, the number of the adjustment steps 13 and the outer diameter of the straight portion 15, thereby adjusting the flex.
The adjustment of the flex is performed relatively to a basic shape of FIG. 5. FIG. 5 is a side view illustrating the basic shape for the manufacturing method for the golf shaft 1 of the present embodiment.
In the basic shape, the intermediate section 9 has the main body 11, a plurality of the adjustment steps 13, and the straight portion 15. According to the present embodiment, the basic shape has three adjustment steps 13 and, by that, the butt end section 7 is set shorter than the golf shaft 1 of FIG. 1. Further, the basic shape has no tapered portion 16. The others are the same as of the golf shaft 1 of FIG. 1. It should be noted that the number of the adjustment steps 13 and a dimension of each part and the like in the basic shape may be changed according to characteristics of the golf shaft 1.
The adjustment of the flex is performed by, with respect to the basic shape, extending the length of the butt end section 7 in the axial direction by one or more adjustment steps 13 and eliminating the adjustment steps 13 corresponding to an amount of the extending of the length of the butt end section 7. With this, the flex is adjusted without changing the rigidity distribution of the main body 11.
Further, the outer diameter of the straight portion 15 is made equivalent to a largest outer diameter in the eliminated adjustment steps 13. With this, the rigidity on the tip end side is increased to suppress increase of the inclination of the rigidity distribution from the tip end section 5 to the intermediate point P. At this time, the tapered portion 16 is formed to absorb the difference in outer diameter between the tip end section 5 and the straight portion 15.
It should be noted that the adjustment of the flex (manufacturing method) of the present embodiment is applicable to a golf shaft other than the golf shaft 1 for the low trajectory and the low spin. In the other golf shaft, increase of inclination in the rigidity distribution from a tip end section 5 to an intermediate point P may not be suppressed at the time of adjustment of a flex. In this case, an outer diameter of a straight portion 15 is not necessarily changed at the time of the adjustment of the flex and may remain the basic shape.
FIGs. 6(A) to (D) are side views illustrating golf shafts 1 having different flexes for the low trajectory and low spin. The golf shafts 1 of FIGs. 6(A) to (D) are hardened in flex in this order and are R, S, X, and TX, respectively.
The golf shaft 1 of FIG. 6(A) is the same as the basic shape of FIG. 5. The golf shaft 1 of FIG. 6(B) is the same as the golf shaft 1 of FIG. 1 and is one in which the adjustment step 13 with the smallest diameter is eliminated relatively to the basic shape. The golf shaft 1 of FIG. 6(C) is one in which two adjustment steps 13 from the smallest in diameter are eliminated and the golf shaft 1 of FIG. 6(D) is one in which all the adjustment steps 13 are eliminated.
In the golf shafts 1 of FIG. 6(B)-FIG. 6(D), the lengths of the butt end section 7 are elongated in the axial direction by the one, the two, and the three adjustment steps 13, respectively. Further, in the golf shafts 1 of FIG. 6(B)-FIG. 6(D), the outer diameters of the straight portions 15 are set identically with the outer diameters of the first step, the second step and the third step of the adjustment steps 13, respectively.
With the settings of the outer diameters, the difference in outer diameter between the tip end section 5 and the straight portion 15 is increased in the golf shafts 1 of FIG. 6(B)-FIG. 6(D), and accordingly the tapered portion 16 is interposed between the straight portion 15 and the tip end section 5. According to the present embodiment, the tapered portion 16 is set longer in the axial direction as the difference in outer diameter between the straight portion 15 and the tip end section 5 is increased. This suppresses increase of a taper angle of the tapered portion 16.
FIG. 7(A) and FIG. 7(B) are tables illustrating rigidity at the tip end sections, theintermediate points, and the butt end sections of the golf shafts 1 of FIG. 6(A)-FIG. 6(D) together with corresponding rigidity of the comparative examples, FIG. 7(A) is a case of 41 inches and FIG. 7(B) is a case of 37 inches. In FIGs. 7, the golf shafts 1 of FIGs. 6(A)-(D) are indicated as R, S, X, and TX, respectively. Intermediate-tip indicates the difference in rigidity between the intermediate point P and the tip end section 5, and butt-intermediate indicates the difference in rigidity between the butt end section 7 and the intermediate point P. A unit of numerical values in FIGs. 7 is kgf·m².
As illustrated in FIGs. 7, every golf shaft 1 of the present embodiment is made to have the intermediate-tip being 3.00kgf·m²-5.00kgf·m² and the butt-intermediate being 2.00kgf·m² or less. In contrast, the comparative examples 1 and 2 do not meet simultaneously the intermediate-tip being 3.00kgf·m²-5.00kgf·m² and the butt-intermediate being 2.00kgf·m² or less. As this result, the comparative examples 1 and 2 do not realizes the low trajectory and the low spin as illustrated in FIG. 3.

[Effect of Embodiment 1]

As mentioned above, the golf shaft 1 of the present embodiment, in the rigidity distribution gradually increasing from the tip end section 5 to the butt end section 7, has the intermediate point P at which the inclination is changed in the intermediate section 9 between the tip end section 5 and the butt end section 7, the difference in rigidity between the tip end section 5 and the intermediate point P of the intermediate section 9 being 3.00kgf·m²-5.00kgf ·m² and the difference in rigidity between the intermediate point P of the intermediate section 9 and the butt end section 7 being 2.00kgf·m² or less.
Accordingly, the golf shaft 1 of the present embodiment has the rigidity of the tip end section 5 set higher and the rigidity of the butt end section 7 set lower in relation to the intermediate point P, thereby surely reducing the hitting angle and the spin amount. As this result, the golf shaft 1 of the present embodiment realizes the sufficient low trajectory and the sufficient low spin.
Further, since the intermediate point P is located in the range having the distance from the tip end of the golf shaft 1 being 40%-60% of the entire length of the golf shaft 1, so that the golf shaft 1 more surely realizes the low trajectory and the low spin.
The golf shaft 1 has the stepped shape exhibited between the tip end section 5 and the butt end section 7 so that the outer diameter is gradually increased, and the stepped shape has the difference in outer diameter between the adjacent steps 17 from the intermediate point P to the butt end section 7 being smaller than the difference in outer diameter between the adjacent steps 17 from the tip end section 5 to the intermediate point P.
Accordingly, the golf shaft 1 capable of realizing the low trajectory and the low spin is easily and surely obtained.
The manufacturing method for the golf shaft 1 of the present embodiment extends the length of the butt end section 7 in the axial direction by one or more adjustment steps 13 with respect to the basic shape, and eliminates the adjustment steps corresponding to the amount of the extending of the length of the butt end section 7 with respect to the basic shape, to manufacture the golf shafts 1 with the different flexes.
With this, the present embodiment performs the flex development without changing of the rigidity distribution of the main body 11. Further, the present embodiment discriminates the flex according to the number of the adjustment steps 13 of the manufactured golf shaft 1 in appearance. Further, the planed flex development is easily performed according to setting in length and diameter of the adjustment steps 13.
Furthermore, the present embodiment makes the outer diameter of the straight portion 15 equivalent to the largest outer diameter in the eliminated adjustment steps 13. The flex development is, therefore, easily performed in the golf shaft 1 for the low trajectory and the low spin.

DESCRIPTION OF NUMERALS

1: Golf shaft
5: Tip end section
7: Butt end section
9: Intermediate section
11: Main body
13: Adjustment steps
15: Straight portion
P: Intermediate point

Claims

A golf shaft having a rigidity distribution in which rigidity gradually increases from a tip end section to a butt end section, wherein
the rigidity distribution has an intermediate point at which inclination is changed in an intermediate section between the tip end section and the butt end section, a difference in rigidity between the tip end section and the intermediate point is 3.00 kgf·m²-5.00kgf·m², and a difference in rigidity between the intermediate point and the butt end section is 2.00kgf· m² or less.
The golf shaft according to claim 1, wherein
the intermediate point is located in a range having a distance from a tip end being 40%-60% of an entire length.
The golf shaft according to claim 1 or 2, wherein
a stepped shape is exhibited between the tip end section and the butt end section so that an outer diameter is gradually increased, and

in the stepped shape, a difference in outer diameter between adjacent steps from the intermediate point to the butt end section is smaller than a difference in outer diameter between adjacent steps from the tip end section to the intermediate point.
A manufacturing method of a golf shaft having a rigidity distribution in which rigidity gradually increases from a tip end section to a butt end section, comprising:
extending a length of the butt end section in an axial direction by one or more adjustment steps with respect to a basic shape, and eliminating the adjustment steps corresponding to an amount of the extending of the length of the butt end section with respect to the basic shape, the basic shape in which an intermediate section between the tip end section and the butt end section has a main body, a plurality of adjustment steps having different outer diameters located on a tip end side relative to the main body, and a straight portion having a constant outer diameter and located on the tip end side relative to the adjustment steps.
The manufacturing method for a golf shaft according to claim 4, wherein
an outer diameter of the straight portion is made equivalent to a largest outer diameter in the eliminated adjustment steps.