Golf shaft and method of manufacturing the same

Abstract

This invention relates to a golf club shaft and a method for manufacturing the same. The golf club shaft has a front end, a base end, and an intermediate portion defined between opposing longitudinal ends adjacent to the front end and the base end. The intermediate portion includes a first portion and a second portion. The first portion is formed with a decreasing wall thickness to create an upward bulge in a bending stiffness distribution curve, and the second portion is located axially adjacent to the first portion between the first portion and the base end, and is formed with a decreasing wall thickness to create a downward bulge in the bending stiffness distribution curve.

Description

Technical Field

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

[0002] A conventional golf club shaft is disclosed in Japanese Patent JP2005-152613A. This golf club shaft has a tip-side front end for mounting the head and a base-side rear end 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 sections on the base 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 patent provides a golf club shaft with a completely smooth, whip-like bend instead of 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 when hitting it, depending on the curvature of the golf club shaft.

[0006] To achieve this objective, a first aspect of the present invention provides a golf club shaft having a front end for attaching a grip, a base end for attaching a handle, and a middle portion. The middle portion is defined at opposite ends axially adjacent to the front end and the base end, and has a plurality of steps whose outer diameter gradually increases from the end adjacent to the front end to the end adjacent to the base end, and has a wall thickness that gradually decreases with increasing outer diameter. The middle portion includes a first portion and a second portion. The first portion is configured such that its wall thickness is greater than the decreasing trend of the middle portion's wall thickness to create an upward bulge in the bending stiffness distribution curve. The second portion is located axially adjacent to the first portion between the first portion and the base end, and is configured such that its wall thickness is less than the decreasing trend of the wall thickness to create a downward bulge in the bending stiffness distribution curve.

[0007] Furthermore, a second embodiment of the present invention provides a method for manufacturing a golf club shaft. The method involves performing a wall thickness deviation process on the middle portion of a metal blank tube to establish a thick-walled portion with relatively thick wall thickness and a thin-walled portion adjacent to the thick-walled portion with relatively thin wall thickness; the blank tube with the thick-walled portion and the thin-walled portion is then subjected to a graded processing corresponding to the wall thickness, thereby forming a plurality of steps with gradually increasing outer diameter.

[0008] The first and second versions of the present invention provide a golf club shaft capable of increasing the speed of the shot according to the curvature of the golf club shaft. Simple Explanation of the Diagram

[0009] Figure 1 is a general view showing the appearance of a golf club according to an embodiment of the present invention; Figure 2 is a schematic cross-sectional view of the golf shaft used in the golf club of Figure 1; Figure 3 is a schematic enlarged view showing the appearance of the golf shaft used in the golf club of Figure 1; Figure 4 is a graph showing the distribution curve of the outer diameter of the golf club shaft in Figure 3 and the distribution curve of the outer diameter of the golf club shaft in the comparative example; Figure 5 is a graph showing the distribution curve of the bending stiffness of the golf club shaft in Figure 3 and the distribution curve of the bending stiffness of the golf club shaft in a comparative example without wall thickness deviation treatment; Figure 6 is a graph showing the wall thickness distribution curve of the golf club shaft in Figure 3 and the wall thickness distribution curve of a comparative example of a golf club shaft without wall thickness deviation treatment; Figure 7 is a graph showing the relationship between the outer diameter distribution curve and the steps of the golf club shaft in Figure 3; Figure 8 is a cross-sectional view of a blank tube subjected to wall thickness deviation treatment in the method of manufacturing a golf club shaft according to the above embodiment; 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 impact; and Figure 9(C) is a side view showing the head and its surroundings immediately after impact. Figure 10(A) is a side view of a golf club with one curved section according to the comparative example during the downswing and just before impact with the ball; Figure 10(B) is a side view of a golf club with two curved sections during the downswing and just before impact with the ball. 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 of the swing during the initial and middle phases of the downswing. Figure 12(A) is a side view showing a golf club with a softer tip striking the ball, and Figure 12(B) is a side view showing a golf club with a stiffer tip striking the ball. Figure 13(A) is a side view showing Embodiment 1 of the golf club shaft according to the above-described embodiment, and Figure 13(B) is a cross-sectional view showing the wall thickness of Embodiment 1 of the golf club shaft; Figure 14(A) is a side view showing Embodiment 2 of the golf club shaft according to the above embodiment, and Figure 14(B) is a cross-sectional view showing the wall thickness of Embodiment 2 of the golf club shaft; Figure 15(A) is a side view showing Embodiment 3 of the golf club shaft according to the above-described embodiment, and Figure 15(B) is a cross-sectional view showing the wall thickness of Embodiment 3 of the golf club shaft; Figure 16(A) is a side view showing Embodiment 4 of the golf club shaft according to the above embodiment, and Figure 16(B) is a cross-sectional view showing the wall thickness of Embodiment 4 of the golf club shaft; Figure 17 is a graph showing the distribution curve of the flexural stiffness of Embodiment 1 in Figure 13; Figure 18 is a graph showing the distribution curve of the flexural stiffness of Embodiment 2 in Figure 14; Figure 19 is a graph showing the distribution curve of the flexural stiffness of Embodiment 3 in Figure 15; Figure 20 is a graph showing the distribution curve of the flexural stiffness of Embodiment 4 in Figure 16; Figure 21 is a specification table showing embodiments of the golf club shaft according to the implementation method and comparative examples; Figure 22 is a graph showing the distribution curves of the bending stiffness of the golf club shaft according to embodiments and comparative examples; and Figure 23 is a table showing the results of test hits using the golf club shafts according to the embodiments and comparative examples. Implementation

[0010] The golf club shaft 3 according to an embodiment of the present invention includes: a front end 9, a base end 11, and a middle portion 13. The front end 9 is the region for attaching a joint portion 5 thereon. The base end 11 is the region for attaching a grip 7 thereon. The opposite ends of the middle portion 13 are defined between the axially adjacent front end 9 and the base end 11. The middle portion 13 has steps S1 to S11 whose outer diameter gradually increases from the end adjacent to the front end 9 to the end adjacent to the base end 11, and has a wall thickness that gradually decreases according to the increase of the outer diameter of the steps S1 to S11 or gradually decreases inversely proportional to the increase of the outer diameter of the steps S1 to S11. The middle portion 13 includes a first portion R1 and a second portion E1.

[0011] The first portion R1 has a decreasing wall thickness, greater than that of the intermediate portion 13. The first portion R1 bulges upwards in the bending stiffness distribution curve, which spans from the front end 9 to the base end 11. The second portion E1 is located axially adjacent to the first portion R1 between the first portion R1 and the base end 11, and has a decreasing wall thickness, less than the first portion R1. Preferably, step S3 in the second portion E1 is shorter axially than steps S4-S6. The second portion E1 bulges downwards in the bending stiffness distribution curve.

[0012] The difference in outer diameter between adjacent steps S4 to S6 in the first part R1 can be less than the difference in outer diameter between adjacent steps S3 in the second part E1.

[0013] The middle section 13 of the golf club shaft 3 may have a third section E2. The third section E2 is located axially adjacent to the first section R1 between the first section R1 and the front end 9, and is formed with a decreasing wall thickness. The third section E2 bulges downwards in the bending stiffness distribution curve. Preferably, the steps S7-S11 of the third section E2 are shorter than the steps S4-S6 of the first section R1.

[0014] The third part E2 can produce a straight section in the distribution curve of bending stiffness, which transitions toward the base end 11 with a fixed value of bending stiffness.

[0015] In addition, the golf club shaft 3 may have a fourth section R2. The fourth section R2 is located on the side closer to the tip 9 relative to the third section E2, and is significantly greater than the third section E2 in terms of bending stiffness.

[0016] The method for manufacturing the golf club shaft 3 involves performing a wall thickness deviation treatment on the middle portion of a metal blank tube 20, resulting in a thick-walled portion 25a and thin-walled portions 24a and 24b on either side of the thick-walled portion 25a along its axial direction. Next, the blank tube 20, with its thick-walled portion 25a and thin-walled portions 24a and 24b, is subjected to graded processing to form a plurality of steps S1 to S11 with gradually increasing outer diameters. Preferably, the graded processing results in relatively long steps S1 to S11 at the thick-walled portion 25a and relatively short steps S1 to S11 at the thin-walled portions 24a and 24b.

[0017] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0018] Figure 1 is an overall view showing the appearance of a golf club according to an embodiment of the present invention.

[0019] 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 9, a base 11, and a middle section 13. The tip 9 is the area on which the head 5 is attached, while the base 11 is the area on which the grip 7 is attached. The middle section 13 is the area located between the tip 9 and the base 11.

[0020] A curved portion a is provided in the intermediate portion 13. Although two curved portions a are shown in FIG. 1, one curved portion or three or more curved portions may be provided. In addition, the intermediate portion 13 has uncurved portions b and c that are axially adjacent to the curved portions a.

[0021] 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.

[0022] It should be noted that the unbendable parts b and c are not the parts that are never bent, 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.

[0023] Figure 2 is a schematic cross-sectional view of the golf shaft used in the golf club of Figure 1.

[0024] As shown in Figures 1 and 2, 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 joint 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.

[0025] 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 end 11 of the golf club shaft 3. However, the front end 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 9 can be composed solely of the insertion portion 8. Furthermore, the front end 9 can also be formed entirely into a straight shape.

[0026] The base end 11 is the base end of the golf club shaft 3 in the longitudinal direction, and is the area extending from the base end of the golf club shaft 3 within a predetermined range to which the grip 7 is attached. In this embodiment, the length of the base end 11 is suitably set within approximately 300 mm. Although in this embodiment the base end 11 is formed as a straight tube shape with a fixed outer diameter, the base end 11 may also be formed as, for example, a tapered shape that changes slightly toward the outer diameter of the base end.

[0027] 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 opposite ends of the intermediate portion 13 are defined between the axially adjacent 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 steps S1 to 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 steps S1 to S11.

[0028] The reduction in wall thickness of the intermediate portion 13 refers to the reduction in the amount of wall thickness reduction, which is reduced according to the increase in outer diameter 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.

[0029] 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 a step is used to connect the base end of the front portion 9 and the front end of the base end portion 11, wherein each step has a fixed length and adjacent steps have, for example, a fixed difference in outer diameter (see the comparative example in Figure 6). The decreasing trend in wall thickness can be observed on a straight line connecting the base end of the front portion 9 and the front end of the base end portion 11 in the wall thickness distribution curve.

[0030] The middle part 13 includes the remaining part of the straight part 17 and the stepped part 15.

[0031] In this embodiment, the portion of the straight portion 17 contained in the intermediate portion 13 is longer than the portion of the straight portion 17 contained 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 adjacent to the base end of the straight portion 17. The stepped portion 15 is composed of steps S1 to S11.

[0032] Based on the outer diameter and wall thickness of the golf club shaft 3, the middle section 13 has a first part R1, a second part E1, and a third part E2.

[0033] Figure 3 is a schematic enlarged view showing the appearance of the golf club shaft used in the golf club of Figure 1. Figure 4 is a graph showing the outer diameter distribution curve of the golf club shaft of Figure 3 and the outer diameter distribution curve of the comparative example golf club shaft. Figure 5 is a graph showing the bending stiffness distribution curve of the golf club shaft of Figure 3 and the bending stiffness distribution curve of the comparative example golf club shaft without wall thickness deviation treatment. Figure 6 is a graph showing the wall thickness distribution curve of the golf club shaft of Figure 3 and the wall thickness distribution curve of the comparative example golf club shaft without wall thickness deviation treatment. Figure 7 is a graph showing the relationship between the outer diameter distribution curve and the step of the golf club shaft of Figure 3. Furthermore, Figures 2 and 3 are different in shape from each other and schematically show the same structure.

[0034] As shown in Figures 3 to 7, the first part R1 is formed with a decreasing wall thickness compared to the middle part 13 (see the comparative example in Figure 6), and the steps in the first part R1 are relatively longer in the axial direction (compared to the steps in the second part E1). According to this embodiment, the first part R1 is composed of three steps S4 to S6. It should be noted that, depending on the characteristics of the golf club shaft 3, the number and length of the steps constituting the first part R1 can be selected.

[0035] By relatively reducing the number of steps S4-S6 in the first part R1, 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, 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.

[0036] Therefore, this embodiment ensures that the area with the thickest wall portion R1 is relatively long. The first portion R1 bulges upward in the bending stiffness distribution curve spanning from the front end 9 to the base end 11. Thus, the first portion R1 forms the non-bending portion b of the golf club shaft 3. It should be noted that the bulging shape in the bending stiffness distribution curve is based on the auxiliary line L1. The auxiliary 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. In order to produce the upward bulge of the first portion R1 in the bending stiffness distribution curve, it is not necessary to make the steps S4 to S6 relatively long or reduce the number of steps S4 to S6.

[0037] The second part E1 is located in the middle part 13, between the first part R1 and the base end 11, and is axially adjacent to the first part R1. The second part E1 is formed with a decreasing wall thickness compared to the middle part 13, and the steps in the second part E1 are relatively short axially (compared to the steps in the first part R1). According to this embodiment, the second part E1 is composed of a single step S3. Depending on the characteristics of the golf club shaft 3, etc., the number and length of the steps constituting the second part E1 can be selected.

[0038] The number of steps S1 to S3 in the portion extending from the second part E1 to the point just before the base end 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 end 11, steps S1 to S3 are shorter than steps S4 to 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 S1 to 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.

[0039] Therefore, based on the combination of the wall thickness setting and the variation in the outer diameter of the region with the thinner wall thickness in the second part E1, this embodiment produces a downward bulge in the distribution curve of bending stiffness spanning from the front end 9 to the base end 11. Thus, the second part E1 forms the curved portion a of the golf club shaft 3. Furthermore, steps S1 and S2, continuing to the base end 11, form the non-bending portion c of the golf club shaft 3. To produce the downward bulge of the second part E1 in the distribution curve of bending stiffness, it is not necessary to make steps S1 to S3 relatively short.

[0040] 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.

[0041] The third part E2 is located in the middle part 13, between the first part R1 and the front end 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 steps of the third part E2 are relatively shorter axially (compared to the steps of the first part R1). In addition, the number and length of the steps constituting the third part E2 can be selected according to the characteristics of the golf club shaft 3, etc. Furthermore, the size relationship between steps S1 to S11 is represented by S4=S5=S6>S1=S2>S7>S3=S8>S9=S10=S11, and is not limited thereto.

[0042] According to this embodiment, the third part E2 is composed of five steps S7 to S11 and a portion of the straight section 17. Furthermore, depending on the characteristics of the golf club shaft 3, the number of steps constituting the third part E2 can be selected.

[0043] In the third part E2, steps S7~S11 are shorter than steps S4~S6 in the first part R1, and the variation in outer diameter is increased within a shorter axial range. Furthermore, steps S7~S11 are configured such that the difference in outer diameter between axially adjacent steps in steps S7~S11 is relatively large (compared to the steps in the first part R1). This further increases the variation in the outer diameter of the third part E2.

[0044] Therefore, based on this combination of wall thickness setting and outer diameter variation, this embodiment produces a downward bulge in the distribution curve of bending stiffness spanning from the front end 9 to the base end 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.

[0045] The convex shape in the bending stiffness distribution curve is referenced to the auxiliary line L2. The auxiliary 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.

[0046] 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 in the bending stiffness distribution curve, which transitions towards the base end 11 with a fixed value of bending stiffness.

[0047] 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.

[0048] The flexural stiffness of the fourth part R2 is greater than that of the straight portion of the third part E2. According to this embodiment, the flexural stiffness distribution curve of the fourth part R2 bulges upwards. This bulging shape of the flexural stiffness distribution curve is based on an auxiliary line L3. The auxiliary 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.

[0049] Figure 8 shows a blank tube undergoing wall thickness deviation treatment in the method of manufacturing a golf club shaft according to the above embodiment. The golf club shaft 3 of Figure 2 is manufactured by performing graded processing on the blank tube 20 that has undergone the wall thickness deviation treatment shown in Figure 8.

[0050] 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 23 and middle portion 25 of the blank tube 20, which is made of metal (particularly steel). The front end 23 of the blank tube 20 corresponds to the front end 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.

[0051] Specifically, thick-walled portions 23a are formed on portions A and B of the blank tube 20 corresponding to the insertion portion 8 and the tapered portion 19 of the front end 9 of the golf club shaft 3, 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.

[0052] The C and G sections of the blank tube 20, corresponding to the third part E2 and the second part E1 of the golf club shaft 3, are formed as thin-walled sections 24a and 24b, respectively. Furthermore, sections C and G are straight tubes, and section C is thicker than section G in terms of wall thickness.

[0053] Therefore, a wall thickness deviation treatment is performed on the metal billet tube 20, such 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 and 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 9 of the golf club shaft 3 (as the front end 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 11 of the golf club shaft 3 (as the base end of the billet tube 20).

[0054] The present invention performs graded processing on the blank tube 20 that has undergone wall thickness deviation treatment (that is, the blank tube 20 with thick wall portion 25a and thin wall portion 24a and 24b according to the wall thickness as shown in FIG3) to obtain the bending stiffness distribution curve shown in FIG5.

[0055] That is, the step-processing forms steps S1 to S11 that gradually increase in outer diameter from the front end of the middle portion 25 of the blank tube 20 to the base end, such that steps S4 to S6 at the thick-walled portion 25a are relatively long and steps S1 to S3 and S7 to S11 at the thin-walled portions 24a and 24b are relatively short. In this embodiment, the difference in outer diameter between each step in steps S1 to S3 and S7 to S11 and the step axially adjacent to each step in steps S1 to S3 and S7 to S11 is relatively large at the thin-walled portions 24a and 24b. Thus, the golf club shaft 3 is manufactured.

[0056] 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 9 and the base end 11. The intermediate portion 13 of the golf club shaft 3 has steps S1 to S11 where the outer diameter gradually increases from the end adjacent to the front end 9 to the end adjacent to the base end 11. Furthermore, the thick-walled portion 25a and the thin-walled portions 24a and 24b of the blank tube 20 have a first portion R1, a second portion E1, and a third portion E2.

[0057] 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 at impact; Figure 9(C) is a side view showing the head and its surroundings just before impact. Figure 10(A) is a side view showing a golf club with one curved section according to the comparative example 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.

[0058] The head 5 is shown in Figure 9(A) with a head velocity Vh just before impacting the ball, impacts the ball as shown in Figure 9(B), and is shown in Figure 9(C) immediately after impact. As shown in Figure 9(C), the ball velocity (speed) immediately after impact is Vb.

[0059] In a golf swing, the shaft 3 of the golf club bends in the opposite direction to the direction of the clubface as it swings down and bends in the direction of the clubface just before impacting the ball, as shown in Figure 10(A) and Figure 10(B).

[0060] As shown in Figure 10(A), a golf club 1 with one curved portion a in the middle section 13 bends slightly during the downswing and just before impact. Conversely, as shown in Figure 10(B), a golf club 1 with two curved portions a in the middle section 13 bends significantly during the downswing and just before impact.

[0061] 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.

[0062] 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 of the swing during the initial and middle phases of the downswing.

[0063] As shown in Figure 11(A), a golf club 1 without a hardened portion in the middle section 13 recovers from the flex of the initial phase of the downswing with a delayed recovery during the middle phase of the downswing. Conversely, as shown in Figure 11(B), a golf club 1 with a hardened portion in the middle section 13 recovers from the flex of the initial phase of the downswing with a rapid recovery during the middle phase of the downswing.

[0064] Therefore, the golf club 1 with a hardened section in the middle 13 increases the distance the head 5 travels during a fixed time period, which leads to an increase in ball speed Vb and head speed Vh.

[0065] Figure 12(A) is a side view showing a golf club with a softer tip striking the ball; Figure 12(B) is a side view showing a golf club with a stiffer tip striking the ball.

[0066] As shown in Figure 12(A), the golf club 1 with a softer tip 9 stores energy based on the bending of the middle portion 13 and consumes the stored energy to deform the tip 9 during contact between the face and the ball. Conversely, as shown in Figure 12(B), the golf club 1 with a stiffer tip 9 is inhibited from consuming the energy stored based on the bending of the middle portion 13 to deform the tip 9.

[0067] Therefore, the stiffer tip 9 of the golf club 1 increases the energy transferred to the ball and increases the ball speed Vb.

[0068] 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.

[0069] Furthermore, based on the setting of the outer diameter and wall thickness, the golf club shaft 3 of this embodiment produces a 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.

[0070] 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. Therefore, 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.

[0071] 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 9, thereby suppressing the bending of the front end 9 during ball-face impact to increase energy transfer and ball speed Vb.

[0072] Based on the above mechanism, samples 1 to 4 of the golf club shaft 3 for this embodiment were manufactured, and each sample 1 to 4 was used for test hits.

[0073] Figures 13(A), 14(A), 15(A), and 16(A) are side views showing samples 1-4 of golf club shafts according to the embodiments. Figures 13(B), 14(B), 15(B), and 16(B) are cross-sectional views showing the wall thickness of samples 1-4 of the golf club shafts. Figures 17 to 20 are graphs showing the distribution curves of the bending stiffness of samples 1-4.

[0074] In samples 1-4 of Figures 13(A) to 16(B), the middle portion 13 of sample 1 in Figure 13, sample 3 in Figure 15, and sample 4 in Figure 16 each has one unbent portion (first portion R1) and two bent portions (second portion E1 and third portion E2), as shown in Figures 17, 19, and 20. Conversely, the middle portion 13 of sample 2 in Figure 14 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 in Figure 18.

[0075] In sample 1, the first portion R1, the second portion E1, and the third portion E2 are located closer to the front end 9 relative to the embodiment. Correspondingly, the fourth portion R2 is located closer to the front end 9 relative to the embodiment. Furthermore, in the bending stiffness distribution curve, sample 1 produces a smaller convex shape for the first portion R1 than the convex shape of this embodiment. Everything else is the same as this embodiment.

[0076] In sample 2, the first part R1, the second part E1 and the third part E2 are located near the front end 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.

[0077] In the bending stiffness distribution curve, sample 2 produces a smaller convex shape in the first part R1 than in this embodiment. The fifth part R3 produces an upward convexity at the same level as the first part R1 in the bending stiffness distribution curve, and the sixth part E3 produces a downward convexity at the same level as the second part E1 in the bending stiffness distribution curve. Everything else is the same as in this embodiment.

[0078] 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.

[0079] Sample 3 has the same structure as the embodiment, but differs slightly in the number of steps and the distribution curve of bending stiffness.

[0080] Sample 4 was configured to soften the front end 9 relative to Sample 3. The distribution curve of the bending stiffness of Sample 4 is similar to that of Sample 3.

[0081] 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 than that of sample 3 in the distribution curve of the bending stiffness of sample 4.

[0082] Figure 21 is a graph and table showing the specifications of the golf club shaft samples according to the embodiment and comparative example. Figure 22 is a graph showing the distribution curve of the bending stiffness of the golf club shaft according to the embodiment and comparative example.

[0083] 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 shown by the dotted lines in Figures 4 and 6. In this comparative example, there were no upward or downward bulges in the bending stiffness distribution curve shown in Figure 22. Furthermore, the bending stiffness distribution curve of the fourth portion R2 of samples 1-3 was higher than that of the bending stiffness distribution curve of the front end 9 of the comparative example.

[0084] As shown in Figure 21, 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 outer diameter and thickness settings of samples 1-4 and the comparative examples.

[0085] 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.

[0086] The heads 5 in samples 1-4 and the comparative example had the same specifications and were used for all test strikes, and the striking point in each head 5 was its center of gravity. The number of test strikes was 10 for each sample 1-4 and the comparative example.

[0087] 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.

[0088] Figure 23 is a table showing the results of test hits using samples of golf club shafts according to the embodiments and comparative examples. In Figure 23, the values ​​of BS (ball speed) and flight distance are shown as test hit results.

[0089] 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.

[0090] 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.

[0091] 1: Golf clubs 3: Golf club shaft 5: Head 7: Grip 8: Insertion section 9: Front end 11: Base end 13: Middle section 15: Step section 17: Straight section 19: Conical part 20:Blank tube 23: Front end 23a: Thick-walled section 25: Middle section 25a: Thick-walled section 24a, 24b: Thin-walled portion a: curved part b, c: Unbent parts BP: Equilibrium Point BS: Ball Speed L1, L2, L3: Auxiliary lines R1: Part 1 R2: Part Four R3: Part 5 E1: Part Two E2: Part Three E3: Part Six S1~S11: Steps Vh: Head speed Vb: Ball speed (striking speed)

Claims

1. A golf club shaft, comprising: The front end is used to attach the connector. The base portion is used to attach the handle; The intermediate portion, defined at its two opposite ends axially adjacent to the front end portion and the base end portion, has a plurality of steps with an outer diameter that gradually increases from the end adjacent to the front end portion to the end adjacent to the base end portion, and has a wall thickness that gradually decreases with the increase of the outer diameter. The intermediate portion comprises a first portion, a second portion, and a third portion. The first portion is formed with a wall thickness greater than the decreasing wall thickness of the intermediate portion to create an upward convexity in a bending stiffness distribution curve that spans from the front end portion to the base end portion. The second portion is located axially adjacent to the first portion between the first portion and the base end portion and is formed with a wall thickness less than the decreasing wall thickness to create a downward convexity in the bending stiffness distribution curve. The third portion is divided and located axially adjacent to the first portion between the first portion and the front end portion, and is formed with a wall thickness less than the decreasing wall thickness to create a downward convexity in the bending stiffness distribution curve.

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

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

4. The golf club shaft according to claim 2, wherein, The difference in outer diameter between axially adjacent steps in the first part is less than the difference in outer diameter between axially adjacent steps in the second part.

5. The golf club shaft according to any one of claims 1 to 4, wherein, The step in the third part is shorter in the axial direction than the step in the first part.

6. The golf club shaft according to claim 4, wherein, The third part produces a straight section in the distribution curve of the bending stiffness, the straight section transitioning towards the base portion with a fixed value of the bending stiffness.

7. The golf club shaft according to claim 4, further comprising: The fourth part is located on the side closer to the front end relative to the third part, and has greater bending stiffness than the third part.

8. A method for manufacturing a golf club shaft according to any one of claims 1 to 4, wherein, A wall thickness deviation process is performed on the middle portion of a metal blank tube corresponding to the middle portion of the golf club shaft, thereby setting a thick-walled portion with a relatively thick wall thickness corresponding to the first portion and a thin-walled portion with a relatively thin wall thickness adjacent to the axial sides of the thick-walled portion and corresponding to the second and third portions. The thin-walled portion on one side of the portion corresponding to the front end is thicker than the thin-walled portion on one side of the portion corresponding to the base end. The blank tube with the thick-walled portion and the thin-walled portion is subjected to a graded processing corresponding to the wall thickness, thereby forming the golf club shaft. The golf club shaft has: a plurality of steps whose outer diameter gradually increases from the end adjacent to the front end to the end adjacent to the base end; and the middle portion, which corresponds to the thick-walled portion and the thin-walled portion of the blank tube, and respectively has the first portion, the second portion and the third portion.

9. The method for manufacturing a golf club shaft according to claim 8, wherein, In the graded processing, the steps are made relatively long at the thick-walled portion and relatively short at the thin-walled portion.

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