Golf club shaft and manufacturing method thereof

Abstract

The invention relates to a golf club shaft and a manufacturing method thereof. The golf club shaft has a front end portion, a base end portion, an intermediate portion defined between respective longitudinal ends adjoining the front end portion and the base end portion, where the intermediate portion is divided into a first portion and a second portion, the first portion being formed to have a decreasing trend greater than the wall thickness to induce an upward projection in a profile of flexural rigidity, and the second portion being formed to have a decreasing trend greater than the wall thickness to induce a downward projection in a profile of flexural rigidity. And the second portion is located axially adjacent to the first portion between the first portion and the base end portion and is formed to be less than a decreasing trend of the wall thickness to induce a downward protrusion in a distribution curve of flexural rigidity.

Description

Technical Field

[0001] This invention relates to a golf club shaft for increasing ball speed and a method for manufacturing the same. Background Technology

[0002] For example, JP2005-152613A discloses a conventional golf club shaft. This golf club shaft has a front end portion for mounting the head and a base end portion for mounting the grip. The golf club shaft has a flexural stiffness distribution extending along its entire length and including points of maximum flexural stiffness, where the flexural stiffness value is greater than the flexural stiffness value of the corresponding adjacent section on the base end side. Then, the golf club shaft has portions with reduced flexural stiffness in the corresponding sections between these maximum points.

[0003] Therefore, the technology in JP2005-152613A provides a completely smooth, whip-like bend in the golf club shaft, without any localized flex. As a result, it allows the player to swing naturally without any sense of disharmony, thereby increasing head speed and ball flight distance.

[0004] However, traditional golf club shafts have limitations in increasing the speed of the shot. Summary of the Invention

[0005] The purpose of this invention is to provide a golf club shaft that can increase the speed of the ball shot according to the curvature of the golf club shaft.

[0006] To achieve this objective, a first aspect of the invention provides a golf club shaft having a front end portion for attaching a joint thereto, a base end portion for attaching a grip thereto, and a middle portion defined between corresponding ends of the middle portion axially adjacent to the front end portion and the base end portion, having a stepped portion whose outer diameter gradually increases from the end adjacent to the front end portion to the end adjacent to the base end portion, and having a wall thickness that gradually decreases with the increase in the outer diameter of the stepped portion. The middle portion is divided into a first portion and a second portion, the first portion being formed with a wall thickness decreasing trend greater than that of the middle portion to cause an upward bulge in the bending stiffness distribution curve, and the second portion being located between the first portion and the base end portion and axially adjacent to the first portion, and being formed with a wall thickness decreasing trend less than that to cause a downward bulge in the bending stiffness distribution curve.

[0007] Furthermore, a second aspect of the present invention provides a method for manufacturing a golf club shaft. The method involves performing a wall thickness deviation process on a blank tube made of metal, such that the middle portion of the blank tube has a thick-walled portion with relatively thick walls and thin-walled portions with relatively thin walls on both axial sides of the thick-walled portion; and performing a graded processing corresponding to the wall thickness on the blank tube having the thick-walled and thin-walled portions, thereby forming a plurality of stepped portions with gradually increasing outer diameters.

[0008] The first and second aspects of the present invention provide a golf club shaft capable of increasing the speed of a shot based on the curvature of the golf club shaft. Attached Figure Description

[0009] Figure 1 This is a general view showing the appearance of a golf club according to one embodiment of the present invention;

[0010] Figure 2 It is used in Figure 1 A schematic cross-sectional view of the shaft of a golf club;

[0011] Figure 3 It shows the use in Figure 1 A schematic enlarged view of the appearance of the golf club shaft in a golf club;

[0012] Figure 4 It is shown Figure 3 The distribution curve of the outer diameter of the golf club shaft and the curve of the outer diameter of the golf club shaft of the comparative example are shown in the graph.

[0013] Figure 5 It is shown Figure 3 The distribution curve of the bending stiffness of the golf club shaft and the curve of the bending stiffness of the golf club shaft of the comparative example without wall thickness deviation treatment are shown in the graph.

[0014] Figure 6 It is shown Figure 3 The graph shows the distribution curve of the wall thickness of a golf club shaft and the distribution curve of the wall thickness of a comparative example golf club shaft without wall thickness deviation treatment.

[0015] Figure 7 It shows the distribution curve of the outer diameter and Figure 3 A graph showing the relationship between the stepped sections of a golf club shaft;

[0016] Figure 8 This is a cross-sectional view of a blank tube subjected to wall thickness deviation treatment in a method for manufacturing a golf club shaft according to the above embodiment;

[0017] Figure 9(A) is a side view showing the head and its surroundings just before impact with the ball. Figure 9 (B) is a side view showing the head and its surroundings during the impact. Figure 9 (C) is a side view showing the head and its surroundings immediately after the impact;

[0018] Figure 10 (A) is a side view of a golf club according to the comparative example, having a curved section, during the downswing and just before impact with the ball. Figure 10 (B) is a side view showing a golf club with two curved sections during the downswing and just before impact with the ball;

[0019] Figure 11 (A) is a side view showing a golf club without a hardened section in the middle of the swing during the initial and middle phases of the downswing. Figure 11 (B) is a side view showing a golf club with a hardened section in the middle part during the initial and middle phases of a downswing;

[0020] Figure 12 (A) is a side view showing a golf club with a soft tip section striking the ball. Figure 12 (B) is a side view showing a golf club with a hard front end striking a ball;

[0021] Figure 13 (A) is a side view showing sample 1 of a golf club shaft according to the above embodiment. Figure 13 (B) is a cross-sectional view showing the wall thickness of sample 1 of the golf club shaft;

[0022] Figure 14 (A) is a side view showing sample 2 of a golf club shaft according to the above embodiment. Figure 14 (B) is a cross-sectional view showing the wall thickness of sample 2 of the golf club shaft;

[0023] Figure 15 (A) is a side view showing sample 3 of a golf club shaft according to the above embodiment. Figure 15 (B) is a cross-sectional view showing the wall thickness of sample 3 of the golf club shaft;

[0024] Figure 16 (A) is a side view showing sample 4 of a golf club shaft according to the above embodiment. Figure 16 (B) is a cross-sectional view showing the wall thickness of sample 4 of the golf club shaft;

[0025] Figure 17 It is shown Figure 13 The curve of the distribution curve of the bending stiffness of sample 1;

[0026] Figure 18 It is shown Figure 14 The curve of the distribution curve of the bending stiffness of sample 2;

[0027] Figure 19 It is shown Figure 15 The curve of the distribution curve of the bending stiffness of sample 3;

[0028] Figure 20 It is shown Figure 16 The curve of the flexural stiffness distribution of sample 4.

[0029] Figure 21 This is a table showing the specifications of samples of golf club shafts according to embodiments and comparative examples;

[0030] Figure 22 This is a graph showing the distribution curves of the bending stiffness of samples of golf club shafts according to the embodiments and comparative examples; and

[0031] Figure 23 This is a table showing the results of test hits using the golf club shafts according to the embodiments and comparative examples. Detailed Implementation

[0032] The golf club shaft 3 of the embodiment includes a front end portion 9, a base end portion 11, and a middle portion 13. The front end portion 9 is the area for attaching a joint portion 5 thereon. The base end portion 11 is the area for attaching a grip 7 thereon. The middle portion 13 is defined between corresponding ends of the middle portion 13 that are axially adjacent to the front end portion 9 and the base end portion 11. The middle portion 13 has stepped portions S1-S11 whose outer diameter gradually increases from the end adjacent to the front end portion 9 to the end adjacent to the base end portion 11, and has a wall thickness that gradually decreases according to the increase in the outer diameter of the stepped portions S1-S11 or gradually decreases inversely proportional to the increase in the outer diameter of the stepped portions S1-S11. The middle portion 13 is divided into a first portion R1 and a second portion E1.

[0033] The first portion R1 has a decreasing wall thickness, greater than that of the intermediate portion 13. The first portion R1 causes an upward bulge in the bending stiffness distribution curve, which spans from the front end portion 9 to the base end portion 11. The second portion E1 is located axially adjacent to the first portion R1 between the first portion R1 and the base end portion 11, and has a decreasing wall thickness. The step portion S3 in the second portion E1 is preferably shorter axially than the step portions S4-S6. The second portion E1 causes a downward bulge in the bending stiffness distribution curve.

[0034] The difference in outer diameter between adjacent stepped portions S4-S6 in the first part R1 can be smaller than the difference in outer diameter between adjacent stepped portions S3 in the second part E1.

[0035] The middle portion 13 of the golf club shaft 3 may have a third portion E2. The third portion E2 is located axially adjacent to the first portion R1 between the first portion R1 and the front portion 9, and is formed with a decreasing trend less than the wall thickness. The third portion E2 causes a downward bulge in the bending stiffness distribution curve. The stepped portions S7-S11 of the third portion E2 are preferably shorter than the stepped portions S4-S6 of the first portion R1.

[0036] The third part E2 can induce a flat section F in the distribution curve of bending stiffness, which transitions toward the base section 11 with a fixed value of bending stiffness.

[0037] In addition, the golf club shaft 3 may have a fourth section R2. The fourth section R2 is positioned relative to the third section E2 on the side closer to the front section 9, and is significantly greater than the third section E2 in terms of bending stiffness.

[0038] The method for manufacturing the golf club shaft 3 involves performing a wall thickness deviation treatment on a metal blank tube 20, such that the middle portion 13 of the blank tube 20 has a thick-walled portion 25a and thin-walled portions 24a and 24b on both axial sides of the thick-walled portion 25a. Next, a graded processing is performed on the blank tube 20 with the thick-walled portions 25a and the thin-walled portions 24a and 24b according to their wall thickness, thereby forming stepped portions S1-S11 with gradually increasing outer diameters. Preferably, the graded processing results in the stepped portions S1-S11 at the thick-walled portion 25a being relatively longer axially, and the stepped portions S1-S11 at the thin-walled portions 24a and 24b being relatively shorter axially.

[0039] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0040] Figure 1 This is a general view showing the appearance of a golf club according to an embodiment of the present invention.

[0041] A golf club 1 has a shaft 3, a head 5, and a grip 7 attached to the shaft 3. The shaft 3 has a tip portion 9, a base portion 11, and a middle portion 13. The tip portion 9 is the area for attaching the head 5, while the base portion 11 is the area for attaching the grip 7. The middle portion 13 is the area located between the tip portion 9 and the base portion 11.

[0042] A curved portion a is provided in the middle section 13. Although in Figure 1The middle portion 13 provides two curved portions a, but may provide one curved portion or three or more curved portions. Furthermore, the middle portion 13 has uncurved portions b and c that are axially adjacent to the curved portions a.

[0043] The curved portion a is the relatively softer part of the golf club shaft 3, while the non-bending portions b and c are the relatively stiffer parts of the golf club shaft 3. The curved portion a and the non-bending portions b and c are determined by the outer diameter and wall thickness of the golf club shaft 3, as described below.

[0044] It should be noted that the unbendable parts b and c are not the parts that never bend, but rather the parts that are difficult to bend relative to the bent part a. Therefore, the bent part a can be easily bent relative to the unbendable parts b and c.

[0045] Figure 2 It is used in Figure 1 A schematic cross-sectional view of the shaft of a golf club.

[0046] like Figure 1 and Figure 2 As shown, the front end portion 9 is the front end portion of the golf club shaft 3 in the longitudinal or axial direction, and is the area extending from the front end of the golf club shaft 3 within a predetermined range to which the connector portion 5 is attached. In this embodiment, the length of the front end portion 9 is suitably set within approximately 160 mm. The front end portion 9 in this embodiment includes an insertion portion 8, a tapered portion 19, and a portion of a straight portion 17.

[0047] The insertion portion 8 is the part to be inserted into the head 5 and is formed into a tapered shape with the outer diameter gradually increasing towards the base portion 11 of the golf club shaft 3. However, the front end portion 9 can be formed into a straight tube shape with a fixed outer diameter. The tapered portion 19 is axially adjacent to the base end of the insertion portion 8 and is formed into a tapered shape with a taper ratio greater than that of the insertion portion 8. The base end of the insertion portion 8 is an axial end of the insertion portion 8 away from the front end of the golf club shaft 3. The straight portion 17 is adjacent to the base end of the tapered portion 19 and has a fixed outer diameter. The base end of the tapered portion 19 is an axial end of the tapered portion 19 away from the front end of the golf club shaft 3. It should be noted that the front end portion 9 may consist solely of the insertion portion 8. Alternatively, the front end portion 9 may be formed entirely into a straight shape.

[0048] The base portion 11 is the base portion of the golf club shaft 3 in the longitudinal direction, and is the area extending from the base of the golf club shaft 3 within a predetermined range to where the grip 7 is attached. In this embodiment, the length of the base portion 11 is suitably set within approximately 300 mm. Although in this embodiment the base portion 11 is formed as a straight tube shape with a fixed outer diameter, the base portion 11 may also be formed as, for example, a tapered shape that changes slightly toward the outer diameter of the base.

[0049] The intermediate portion 13 is located between the front end portion 9 and the base end portion 11, and has a front end adjacent to the front end portion 9 and a base end adjacent to the base end portion 11. That is, the intermediate portion 13 is defined between the corresponding ends of the intermediate portion 13 that are axially adjacent to the front end portion 9 and the base end portion 11. In this embodiment, the length of the intermediate portion 13 is suitably set within approximately 750 mm. The intermediate portion 13 has stepped portions S1-S11 whose outer diameter gradually increases from its front end to its base end, and has a wall thickness that gradually decreases or decreases inversely proportional to the increase in the outer diameter of the stepped portions S1-S11.

[0050] The reduction in wall thickness of the intermediate portion 13 refers to a reduction in the amount of wall thickness reduction, which is based on or inversely proportional to the increase in outer diameter, and does not include the increase or decrease in wall thickness at the first portion R1, the second portion E1, and the third portion E2, which will be explained later.

[0051] For example, the reduction in wall thickness of the intermediate portion 13 exhibits a constant decreasing trend in the wall thickness distribution curve, and this reduction can occur when the base of the front end portion 9 and the front end of the base end portion 11 are connected by a stepped portion, wherein each stepped portion has a fixed length and adjacent stepped portions have, for example, a fixed difference in outer diameter (see...). Figure 6 (Comparative example). The decreasing trend of wall thickness can be seen on the straight line connecting the base of the front end portion 9 and the front end of the base end portion 11 in the wall thickness distribution curve.

[0052] The middle section 13 includes the remaining portion of the straight section 17 and the stepped section 15.

[0053] In this embodiment, the portion of the straight portion 17 included in the intermediate portion 13 is longer than the portion of the straight portion 17 included in the front end portion 9. However, the straight portion 17 in the intermediate portion 13 can be suitably provided. The stepped portion 15 is axially abutted to the base end of the straight portion 17. The stepped portion 15 is composed of stepped portions S1-S11.

[0054] Based on the outer diameter and wall thickness of the golf club shaft 3, the middle section 13 is divided into a first section R1, a second section E1, and a third section E2.

[0055] Figure 3 It shows the use in Figure 1 A schematic enlarged view of the appearance of a golf club shaft. Figure 4 It is shown Figure 3 The graphs show the distribution curves of the outer diameter of the golf club shafts of the examples and the distribution curves of the outer diameter of the golf club shafts of the comparative examples. Figure 5 It is shown Figure 3 The graph shows the distribution curve of the bending stiffness of the golf club shaft and the distribution curve of the bending stiffness of the golf club shaft of a comparative example without wall thickness deviation treatment. Figure 6 It is shown Figure 3 The graph shows the distribution curve of the wall thickness of a golf club shaft and the distribution curve of the wall thickness of a comparative example golf club shaft without wall thickness deviation treatment. Figure 7 It shows the distribution curve of the outer diameter and Figure 3 A graph showing the relationship between the stepped sections of a golf club shaft. Additionally, Figure 2 and Figure 3 They are different in shape from one another, and schematically show the same structure.

[0056] like Figures 3 to 7 As shown, the first part R1 is formed with a decreasing trend in wall thickness compared to the middle part 13 (see [reference]). Figure 6 (Comparative example), and the stepped portion in the first part R1 is relatively longer in the axial direction (compared to the stepped portion in the second part E1). According to this embodiment, the first part R1 is composed of three stepped portions S4-S6. It should be noted that, depending on the characteristics of the golf club shaft 3, the number and length of the stepped portions constituting the first part R1 can be optional.

[0057] By relatively reducing the number of steps S4-S6 in the first part R1, the steps S4-S6 are relatively longer (compared to the steps in the second part E1). This suppresses changes in the outer diameter of the first part R1. Furthermore, the steps S4-S6 are configured such that the difference in outer diameter between axially adjacent steps is relatively small. This further suppresses changes in the outer diameter of the first part R1.

[0058] Therefore, this embodiment ensures that the area with the thickest wall portion R1 is relatively long. The first portion R1 causes an upward bulge in the bending stiffness distribution curve spanning from the front end portion 9 to the base end portion 11. Thus, the first portion R1 forms the non-bending portion b of the golf club shaft 3. It should be noted that the bulge shape in the bending stiffness distribution curve is based on reference line L1. Reference line L1 is a straight line connecting the starting point of the first portion R1 and the ending point of the second portion E1 in the bending stiffness distribution curve. To cause the first portion R1 to bulge upward in the bending stiffness distribution curve, it is not necessary to make the steps S4-S6 relatively long or reduce the number of steps S4-S6.

[0059] The second part E1 is located in the intermediate part 13, between the first part R1 and the base part 11, and is axially adjacent to the first part R1. The second part E1 is formed with a decreasing wall thickness compared to the intermediate part 13, and the stepped portion in the second part E1 is relatively short axially (compared to the stepped portion of the first part R1). According to this embodiment, the second part E1 is composed of a stepped portion S3. Depending on the characteristics of the golf club shaft 3, etc., the number and length of the stepped portions constituting the second part E1 can be optional.

[0060] The number of steps S1-S3 in the portion extending from the second part E1 to the point just before the base portion 11 is the same as, or may be different from, the number of steps in the first part R1. In the portion extending from the second part E1 to the point just before the base portion 11, steps S1-S3 are shorter than steps S4-S6 in the first part R1, and the variation in outer diameter increases within a shorter axial range. Furthermore, the difference in outer diameter between axially adjacent steps in steps S1-S3 is set to be relatively large (compared to the steps in the first part R1). This further increases the variation in the outer diameter of the second part E1. Additionally, in this embodiment, the difference in outer diameter between axially adjacent steps in the second part E1 is the difference in outer diameter between step S3, which mainly constitutes the second part E1, and steps S2 and S4, which are axially adjacent to step S3.

[0061] Therefore, based on the combination of the wall thickness setting and the variation in the outer diameter in the region with the thinner wall thickness in the second part E1, this embodiment causes a downward bulge in the distribution curve of bending stiffness spanning from the front end portion 9 to the base end portion 11. Thus, the second part E1 forms the curved portion a of the golf club shaft 3. Furthermore, the stepped portions S1 and S2 extending to the base end portion 11 form the non-bending portion c of the golf club shaft 3. To cause the second part E1 to bulge downward in the distribution curve of bending stiffness, the stepped portions S1-S3 need not be relatively short.

[0062] The convex shape of the second part E1 in the bending stiffness distribution curve transitions sharply from the second part E1 to the first part R1 according to the setting of the outer diameter and wall thickness, thereby relatively increasing the bending stiffness of the first part R1.

[0063] The third part E2 is located in the middle part 13, between the first part R1 and the front end part 9, and is axially adjacent to the first part R1. The third part E2 is formed with a decreasing wall thickness compared to the middle part 13, and the stepped portion of the third part E2 is relatively shorter axially (compared to the stepped portion of the first part R1). Furthermore, the number and length of the stepped portions constituting the third part E2 are optional, depending on the characteristics of the golf club shaft 3, etc. Moreover, the size relationship between the stepped portions S1-S11 is represented by S4=S5=S6>S1=S2>S7>S3=S8>S9=S10=S11, and is not limited thereto.

[0064] According to this embodiment, the third part E2 is composed of five stepped sections S7-S11 and a portion of the straight section 17. Furthermore, depending on the characteristics of the golf club shaft 3, the number of stepped sections constituting the third part E2 can be optional.

[0065] In the third part E2, the steps S7-S11 are shorter than the steps S4-S6 of the first part R1, and the variation in outer diameter is increased within a shorter axial range. Furthermore, the steps S7-S11 are configured such that the difference in outer diameter between axially adjacent steps within the steps S7-S11 is relatively large (compared to the steps of the first part R1). This further increases the variation in the outer diameter of the third part E2.

[0066] Therefore, based on this combination of wall thickness setting and outer diameter variation, this embodiment causes a downward bulge in the distribution curve of bending stiffness spanning from the front end portion 9 to the base end portion 11 in the region of the third portion E2 with thin wall thickness. Thus, the third portion E2 forms the curved portion a of the golf club shaft 3.

[0067] The convex shape in the bending stiffness distribution curve is referenced to reference line L2. Reference line L2 is a straight line in the bending stiffness distribution curve connecting the end point of the first part R1 and the starting point of the third part E2.

[0068] The convex shape of the third part E2 in the bending stiffness distribution curve transitions sharply from the third part E2 to the first part R1 according to the setting of the outer diameter and wall thickness, thereby relatively increasing the bending stiffness of the first part R1. In addition, the convex shape of the third part E2 in the bending stiffness distribution curve includes a straight portion F in the bending stiffness distribution curve, which transitions towards the base portion 11 with a fixed value of bending stiffness.

[0069] The golf club shaft 3 of this embodiment includes a fourth portion R2. The fourth portion R2 is located in the front end portion 9, axially adjacent to the third portion E2. The fourth portion R2 corresponds to the portions of the tapered portion 19 and the straight portion 17 in the front end portion 9.

[0070] The flexural stiffness of the fourth part R2 is greater than that of the straight portion F of the third part E2. According to this embodiment, the distribution curve of the flexural stiffness of the fourth part R2 bulges upwards. This bulging shape of the flexural stiffness distribution curve is based on a reference line L3. The reference line L3 is a straight line in the flexural stiffness distribution curve connecting the starting point of the fourth part R2 and the ending point of the third part E2.

[0071] Figure 8 A blank tube subjected to wall thickness deviation treatment is shown in the method of manufacturing a golf club shaft according to the above embodiment. Figure 2 The golf club shaft 3 was executed by Figure 8 The blank tube 20, which has wall thickness deviation treatment, is manufactured by performing graded processing.

[0072] The wall thickness deviation treatment involves forming thick-walled portions 23a and 25a, and thin-walled portions 24a and 24b axially adjacent to the thick-walled portions 25a, on the front end portion 23 and the middle portion 25 of the blank tube 20, which is made of metal (particularly steel). The front end portion 23 of the blank tube 20 corresponds to the front end portion 9 of the golf club shaft 3, and the middle portion 25 of the blank tube 20 corresponds to the middle portion 13 of the golf club shaft 3. Furthermore, the material of the blank tube 20 can be a metal other than steel.

[0073] Specifically, thick-walled portions 23a are formed on portions A and B of the blank tube 20 corresponding to the insertion portion 8 of the front end portion 9 of the golf club shaft 3 and the tapered portion 19, and thick-walled portions 25a are formed on portions D, E, and F corresponding to the first portion R1 of the intermediate portion 13. Portions B, D, and F have inner peripheries formed in a tapered shape.

[0074] The C and G portions of the blank tube 20, corresponding to the third portion E2 and the second portion E1 of the golf club shaft 3, are respectively formed as thin-walled portions 24a and 24b with thinner walls. In addition, the C and G portions are straight tubes, and the C portion is thicker than the G portion in terms of wall thickness.

[0075] Therefore, a wall thickness deviation treatment is performed on the metal billet tube 20, so that the middle portion 25 of the billet tube 20 has a thick-walled portion 25a corresponding to the first portion R1 and thin-walled portions 24a and 24b axially adjacent to the thick-walled portion 25a on both sides of the thick-walled portion 25a, corresponding to the second portion E1 and the third portion E2. Furthermore, the thin-walled portion 24a on the side near the portion corresponding to the front end portion 9 of the golf club shaft 3 (as the front end portion 23 of the billet tube 20) is thicker than the thin-walled portion 24b on the side near the portion corresponding to the base end portion 11 of the golf club shaft 3 (as the base end portion of the billet tube 20).

[0076] The present invention applies to the blank tube 20 that has undergone wall thickness deviation treatment (i.e., as shown in the figure). Figure 3 The blank tube 20 shown, which has a thick-walled portion 25a and thin-walled portions 24a and 24b according to its wall thickness, is subjected to graded processing to obtain, as shown in the figure. Figure 5 The curve showing the distribution of bending stiffness is shown.

[0077] That is, the outer diameter of the stepped portions S1-S11 gradually increases from the front end to the base end of the middle portion 25 of the blank tube 20 through graded processing, such that the stepped portions S4-S6 at the thick-walled portion 25a are relatively long and the stepped portions S1-S3 and S7-S11 at the thin-walled portions 24a and 24b are relatively short. In this embodiment, the difference in outer diameter between each stepped portion in S1-S3 and S7-S11 and the stepped portion axially adjacent to each stepped portion in S1-S3 and S7-S11 is relatively large at the thin-walled portions 24a and 24b. Thus, the golf club shaft 3 is manufactured.

[0078] Therefore, the blank tube 20, which has undergone wall thickness deviation treatment, is subjected to graded processing to form a golf club shaft 3. The golf club shaft 3 has an intermediate portion 13 defined between the corresponding ends that are axially adjacent to the front end portion 9 and the base end portion 11. The intermediate portion 13 of the golf club shaft 3 has stepped portions S1-S11 whose outer diameter gradually increases from the end adjacent to the front end portion 9 to the end adjacent to the base end portion 11. It is divided into a first portion R1, a second portion E1, and a third portion E2 according to the thick-walled portion 25a and the thin-walled portions 24a and 24b of the blank tube 20.

[0079] Figure 9 (A) is a side view showing the head and its surroundings just before impact with the ball. Figure 9 (B) is a side view showing the head and its surroundings during the impact. Figure 9 (C) is a side view showing the head and its surroundings just after the impact. Figure 10 (A) is a side view of a golf club according to the comparative example, having a curved section, during the downswing and just before impact with the ball. Figure 10(B) is a side view showing a golf club with two curved sections during the downswing and just before impact with the ball.

[0080] Head 5 Figure 9 (A) The head velocity Vh that appears just before impact with the ball, such as Figure 9 (B) Hitting the ball like that, and as Figure 9 (C) It presents the state immediately following the impact. For example... Figure 9 (C) In this case, the ball speed immediately after impact is Vb.

[0081] In a golf swing, the shaft 3 bends in the opposite direction to the clubface during the downswing and then bends in the direction of the clubface just before impact with the ball, such as... Figure 10 (A) and Figure 10 As shown in (B).

[0082] like Figure 10 As shown in (A), the golf club 1 with a curved portion a in the middle section 13 bends slightly during the downswing and just before impact. Conversely, as... Figure 10 As shown in (B), the golf club 1, which has two curved sections a in the middle section 13, bends with a large amount of curvature during the downward swing and just before impact.

[0083] Therefore, the golf club 1 with two curved sections a increases the distance the head 5 travels during a fixed time period, thus increasing the head velocity Vh just before impact. This results in an increase in ball velocity Vb.

[0084] Figure 11 (A) is a side view showing a golf club without a hardened section in the middle of the swing during the initial and middle phases of the downswing. Figure 11 (B) is a side view showing a golf club with a hardened section in the middle part during the initial and middle phases of a downswing.

[0085] like Figure 11 As shown in (A), the golf club 1 without a hardened section in the middle portion 13 recovers from the flex delay at the initial stage of the downswing during the middle phase of the downswing. Conversely, as... Figure 11 As shown in (B), the golf club 1 with a hardened portion in the middle section 13 quickly recovers from the bend in the initial phase of the downswing during the middle phase of the downswing.

[0086] Therefore, the golf club 1 with a hardened section in the middle section 13 increases the distance the head 5 travels during a fixed time period, which results in an increase in ball speed Vb and head speed Vh.

[0087] Figure 12 (A) is a side view showing a golf club with a soft tip section striking the ball. Figure 12 (B) is a side view showing a golf club with a hard front end striking the ball.

[0088] like Figure 12 As shown in (A), the golf club 1 with a soft tip portion 9 stores energy based on the bending of the middle portion 13, and consumes the stored energy to deform the tip portion 9 during contact between the face and the ball. Conversely, as Figure 12 As shown in (B), the golf club 1 with a hard front end portion 9 is suppressed from consuming the energy stored in the bending of the middle portion 13 to deform the front end portion 9.

[0089] Therefore, the golf club 1 with a hard front end 9 increases the energy transferred to the ball and increases the ball speed Vb.

[0090] The golf club shaft 3 of this embodiment has a first portion R1, which is a non-bending portion b, and a second portion E1 and a third portion E2, which are bending portions a, on both sides of the first portion R1 along its axial direction. Therefore, the golf club shaft 3 bends more during the downswing and just before impact with the ball, and recovers rapidly from the bend in the middle phase of the downswing. As a result, the golf club shaft 3 increases the ball speed Vb and head speed Vh.

[0091] Furthermore, based on the setting of the outer diameter and wall thickness, the golf club shaft 3 of this embodiment causes the convex shape of the second part E1 and the third part E2 in the distribution curve of bending stiffness, so as to abruptly transition from the second part E1 and the third part E2 to the first part R1 in terms of bending stiffness.

[0092] This results in a relatively increased bending stiffness at the first part R1, and makes the second part E1 and the third part E2 more prone to bending from the areas of the second part E1 and the third part E2 close to the first part R1. Thus, the golf club shaft 3 of this embodiment further ensures increased bending during the downswing and just before impact with the ball, and further ensures rapid recovery from bending in the middle phase of the downswing.

[0093] Furthermore, the golf club shaft 3 of this embodiment has a fourth portion R2, which is the non-bending portion c in the front end portion 9, thereby suppressing the bending of the front end portion 9 during ball-face impact to increase energy transfer and ball speed Vb.

[0094] Based on the above mechanism, samples 1-4 of the golf club shaft 3 for this embodiment were manufactured, and each sample 1-4 was used for test hits.

[0095] Figure 13(A) Figure 14 (A) Figure 15 (A) and Figure 16 (A) is a side view showing samples 1-4 of a golf club shaft according to an embodiment. Figure 13 (B) Figure 14 (B) Figure 15 (B) and Figure 16 (B) is a cross-sectional view showing the wall thickness of samples 1-4 of the golf club shaft. Figures 17 to 20 These are graphs showing the distribution curves of the flexural stiffness of samples 1-4, respectively.

[0096] exist Figure 13 (A) to Figure 16 (B) In samples 1-4, Figure 13 Sample 1 Figure 15 Sample 3 and Figure 16 Each of the middle portions 13 of sample 4 has one unbent portion (first portion R1) and two bent portions (second portion E1 and third portion E2), as shown. Figure 17 , Figure 19 and Figure 20 As shown. Conversely, Figure 14 The middle portion 13 of sample 2 has two unbent portions (first portion R1 and fifth portion R3) and three bent portions (second portion E1, third portion E2 and sixth portion E3), as shown. Figure 18 As shown.

[0097] In sample 1, the first portion R1, the second portion E1, and the third portion E3 are closer to the front end portion 9 relative to the embodiment. Correspondingly, the fourth portion R2 is closer to the front end of the front end portion 9 relative to the embodiment. Furthermore, in the bending stiffness distribution curve, sample 1 causes the convex shape of the first portion R1 to be smaller than the convex shape of this embodiment. Everything else is the same as this embodiment.

[0098] In sample 2, the first part R1, the second part E1 and the third part E3 are located near the front end part 9 relative to the embodiment, and the fifth part R3 and the sixth part E3 are arranged between the first part R1 and the third part E2.

[0099] In the bending stiffness distribution curve, sample 2 causes a smaller convex shape in the first portion R1 than in this embodiment. The fifth portion R3 causes an upward convexity at the same level as the first portion R1 in the bending stiffness distribution curve, and the sixth portion E3 causes a downward convexity at the same level as the second portion E1 in the bending stiffness distribution curve. Everything else is the same as in this embodiment.

[0100] In sample 2, the first part R1, the second part E1, the third part E2, the fourth part R2, the fifth part R3, and the sixth part E3 each have approximately the same length and are shorter than the length of the embodiment.

[0101] Sample 3 has the same structure as the embodiment, but differs slightly in the number of steps and the distribution curve of bending stiffness.

[0102] The front end portion 9 of sample 4 is softened compared to sample 3. The distribution curve of the bending stiffness of sample 4 is similar to that of sample 3.

[0103] Sample 4 has an inclination between the third part E2 and the fourth part R2 that are axially adjacent to each other, and between the third part E2 and the first part R1 that are axially adjacent to each other. This inclination is set to be gentler in the distribution curve of the bending stiffness of sample 4 than that of sample 3.

[0104] Figure 21 These are graphs and tables showing the specifications of samples of golf club shafts according to embodiments and comparative examples. Figure 22 It is a graph showing the distribution curve of the bending stiffness of the golf club shaft according to the embodiment and comparative example.

[0105] Test impact tests were performed on samples 1-4 and the comparative example. Samples 1-4 were constructed as described above, while the comparative example was constructed such that the wall thickness decreased with increasing outer diameter, as... Figure 4 and Figure 6 The dotted lines are shown in the diagram. In this comparative example, as shown... Figure 22 The bending stiffness distribution curves shown do not exhibit any upward or downward protrusions. Furthermore, the bending stiffness distribution curves of the fourth portion R2 of samples 1-3 are higher than those of the front portion 9 of the comparative example.

[0106] like Figure 21 As shown, the specifications of samples 1-4 and the comparative examples are for, for example, a No. 7 iron rod, and are fixed in terms of elasticity and length. Other parameters such as weight, BP (balance point), torque, total weight, and vibration frequency vary depending on the settings of the outer diameter and thickness of samples 1-4 and the comparative examples.

[0107] The test impact was performed by the robot using samples 1-4 and each of the comparative samples, with the connector 5 attached to it. The ball was positioned on the extension at the center of the robot, and the direction of its face was orthogonal to the target direction.

[0108] The heads 5 in samples 1-4 and the comparative example had the same specifications and were used for all test hits, and the point of impact in each head 5 was its center of gravity. The number of test hits was 10 for each sample 1-4 and the comparative example.

[0109] The robot was set to target a head velocity of 40 m / s for the comparative sample, and this setting was used to test-blow all samples 1-4 and the comparative sample.

[0110] Figure 23 This is a table showing the results of test hits using samples of golf club shafts according to the embodiments and comparative examples. Figure 23 In the diagram, the values ​​of BP (ball speed) and flight distance are shown as the results of the test hit.

[0111] The comparative example had a ball speed of Vb = 55 m / s and a flight distance of 168.9 yards. In contrast, samples 1-4 exhibited ball speeds and flight distances exceeding those of the comparative example.

[0112] In particular, Sample 3 exhibited a ball speed Vb of 56.9 m / s and a flight distance of 177.0 yards, exceeding the Comparative Example by 1.4 m / s in ball speed Vb and by 8.1 yards in flight distance.

Claims

1. A golf club shaft comprising: a front end portion for attaching a head thereon; a base end portion for attaching a grip thereon; an intermediate portion defining between respective end portions thereof axially adjoining the front end portion and the base end portion, having steps of gradually increasing outer diameter from an end portion adjoining the front end portion to an end portion adjoining the base end portion, and having a wall thickness gradually decreasing according to the increase of the outer diameter of the steps, wherein the intermediate portion is divided into a first portion and a second portion, the first portion is formed to be greater than a decreasing tendency of the wall thickness of the intermediate portion to cause an upward convex in a distribution curve of bending stiffness spanning from the front end portion to the base end portion, and the second portion is positioned axially adjoining the first portion between the first portion and the base end portion, and is formed to be less than the decreasing tendency of the wall thickness to cause a downward convex in the distribution curve of the bending stiffness.

2. The golf club shaft according to claim 1, wherein a step portion of the second portion is shorter in an axial direction than a step portion of the first portion.

3. The golf club shaft according to claim 1, wherein an outer diameter difference between axially adjacent step portions in the first portion is less than an outer diameter difference between axially adjacent step portions in the second portion.

4. The golf club shaft according to claim 1, wherein the intermediate portion has a third portion divided to be located between the first portion and the front end portion axially adjoining the first portion, and formed to be less than the decreasing tendency of the wall thickness to cause a downward convex in the distribution curve of the bending stiffness.

5. The golf club shaft according to claim 4, wherein a step portion of the third portion is shorter in an axial direction than a step portion of the first portion.

6. The golf club shaft according to claim 4, wherein the third portion causes a flat portion in the distribution curve of the bending stiffness, the flat portion transitions toward the base end portion with a fixed value of the bending stiffness.

7. The golf club shaft according to claim 4, further comprising: a fourth portion located closer to the front end portion relative to the third portion, and greater than the third portion in terms of the bending stiffness.

8. A method of manufacturing a golf club shaft, comprising: performing a wall thickness deviation process on a blank tube made of metal such that an intermediate portion of the blank tube has a thick wall portion of a relatively thick wall thickness and thin wall portions of a relatively thin wall thickness on both axial sides of the thick wall portion; performing a step processing corresponding to the wall thickness on the blank tube with the thick wall portion and the thin wall portions set, thereby forming a plurality of step portions of gradually increasing outer diameter.

9. The method of manufacturing a golf club shaft according to claim 8, wherein in the step processing, a step portion at the thick wall portion is made relatively longer in an axial direction, and a step portion at the thin wall portion is made relatively shorter in the axial direction.

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