Golf shaft and manufacturing method thereof

Abstract

To provide a golf shaft capable of improving an effect of increasing a ball speed by bending of the golf shaft.SOLUTION: A distal end portion 9 to which the head 5 is attached, a proximal end portion 11 to which the grip 7 is attached, an intermediate portion 13 having a plurality of step portions S1 to S11 whose outer diameters gradually increase from an end portion adjacent to the distal end portion 9 to an end portion adjacent to the proximal end portion 11 between the distal end portion 9 and the proximal end portion 11 and whose thickness gradually decreases as the outer diameter increases, and a first partial R1 which is formed to have a thickness larger than a decrease tendency of the thickness in the intermediate portion 13 and whose bending rigidity distribution from the distal end portion 9 to the proximal end portion 11 is convex upward; And a second partial side R1 positioned adjacent to the first partial side R1 between the first partial side E1 and the base end part 11 in the intermediate part 13, formed smaller in thickness than the reduction tendency of the thickness, and having a bending rigidity distribution projecting downward.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a golf shaft and method of manufacture that increases ball speed. [Background technology]

[0002] A conventional golf shaft and manufacturing method thereof includes, for example, a golf club shaft (also referred to as a "golf shaft") described in Patent Document 1.

[0003] The golf shaft described in Patent Document 1 has maximum bending stiffness points that are greater in bending stiffness than adjacent areas on the butt side along the entire length from the tip end on the tip side where the head is attached to the butt side where the grip is attached, and has reduced bending stiffness areas in the areas sandwiched between these maximum bending stiffness points.

[0004] As a result, the technology of Patent Document 1 achieves a smooth, whip-like flex in the shaft overall, rather than locally, which results in a natural swing without any discomfort, which increases head speed and increases the distance the ball travels.

[0005] However, the above-mentioned conventional golf shafts have a limited effect on increasing ball speed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-152613 Summary of the Invention [Problem to be solved by the invention]

[0007] The problem to be solved is that there is a limit to the effect of increasing ball speed by flexing the golf shaft. [Means for solving the problem]

[0008] The present invention provides a golf shaft comprising: a tip portion for attaching a head; a base portion for attaching a grip; a middle portion between the tip portion and the base portion, which has a plurality of steps whose outer diameter gradually increases from the end adjacent to the tip portion to the end adjacent to the base portion and whose thickness gradually decreases as the outer diameter increases; a first portion in the middle portion which is thicker than the decreasing trend of the thickness so that the bending stiffness distribution from the tip portion to the base portion is convex upward; and a second portion in the middle portion between the first portion and the base portion, which is adjacent to the first portion, and which is thicker than the decreasing trend of the thickness so that the bending stiffness distribution is convex downward.

[0009] The present invention also provides a method for manufacturing a golf shaft, which comprises: forming a relatively thick-walled portion and a thin-walled portion adjacent to the thick-walled portion in the middle of a metal mother tube by means of thickness deviation processing; and forming a plurality of steps whose outer diameters gradually increase by means of stepping processing according to the thickness of the mother tube having the thick-walled portion and the thin-walled portion set therein. [Effects of the Invention]

[0010] According to the present invention, it is possible to improve the effect of increasing ball speed due to the bending of the shaft. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is an overall view showing the appearance of a golf club according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view of a golf shaft used in the golf club of FIG. [Figure 3] FIG. 3 is a schematic enlarged view showing the appearance of a golf shaft used in the golf club of FIG. [Figure 4] FIG. 4 is a graph showing the outer diameter distribution of the golf shaft of FIG. 3 together with a comparative example. [Figure 5]FIG. 5 is a graph showing the bending rigidity distribution of the golf shaft of FIG. 3 together with the bending rigidity distribution of a comparative example in which thickness deviation processing is not performed. [Figure 6] FIG. 6 is a graph showing the thickness distribution of the golf shaft of FIG. 3 together with the thickness distribution of a comparative example in which there is no thickness deviation processing. [Figure 7] FIG. 7 is a graph showing the relationship between the outer diameter distribution and the step portion of the golf shaft of FIG. [Figure 8] FIG. 8 is a cross-sectional view of a blank tube after being subjected to thickness deviation processing in a manufacturing method for a golf shaft according to an embodiment. [Figure 9] 9A is a side view of the head and its surroundings immediately before impact with the ball, FIG. 9B is a side view of the head and its surroundings at the moment of impact, and FIG. 9C is a side view of the head and its surroundings immediately after impact. [Figure 10] FIG. 10(A) is a side view showing a golf club according to a comparative example having one flexible portion during the downswing and immediately before impact, and FIG. 10(B) is a side view showing a golf club having two flexible portions during the downswing and immediately before impact. [Figure 11] Figure 11(A) is a side view showing the initial and middle stages of the downswing of a golf club with no hard spots in the middle, and Figure 11(B) is a side view showing the initial and middle stages of the downswing of a golf club with a hard spot in the middle. [Figure 12] FIG. 12(A) is a side view showing the state of a golf club with a soft tip at the time of impact, and FIG. 12(B) is a side view showing the state of a golf club with a hard tip at the time of impact. [Figure 13] FIG. 13(A) is a side view of Sample 1 of the golf shaft of the example, and FIG. 13(B) is a cross-sectional view showing the thickness of Sample 1. [Figure 14] FIG. 14(A) is a side view of Sample 2 of the golf shaft of the example, and FIG. 14(B) is a cross-sectional view showing the thickness of Sample 2. [Figure 15] FIG. 15(A) is a side view of Sample 3 of the golf shaft of the example, and FIG. 15(B) is a cross-sectional view showing the thickness of Sample 3. [Figure 16] FIG. 16(A) is a side view of Sample 4 of the golf shaft of the example, and FIG. 16(B) is a cross-sectional view showing the wall thickness of Sample 4. [Figure 17] FIG. 17 is a graph showing the bending rigidity distribution of Sample 1 of FIG. [Figure 18] FIG. 18 is a graph showing the bending rigidity distribution of Sample 2 of FIG. [Figure 19] FIG. 19 is a graph showing the bending stiffness distribution of Sample 3 of FIG. [Figure 20] FIG. 20 is a graph showing the bending rigidity distribution of Sample 4 of FIG. [Figure 21] FIG. 21 is a table showing the specifications of golf shaft samples according to Examples and Comparative Examples. [Figure 22] FIG. 22 is a graph showing the bending rigidity distribution of golf shaft samples according to Examples and Comparative Examples. [Figure 23] FIG. 23 is a chart showing the test hit results of the golf shaft samples according to the examples and the comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[0012] The goal of improving the effect of increasing ball speed through the flex of the golf shaft was achieved by adjusting the thickness of the middle part of the golf shaft and the stepped part.

[0013] The golf shaft 3 has a tip portion 9, a base portion 11, and an intermediate portion 13. The tip portion 9 is the region where the head 5 is attached. The base portion 11 is the region where the grip 7 is attached. The intermediate portion 13 is located between the tip portion 9 and the base portion 11, and has multiple step portions S1 to S11 whose outer diameter gradually increases from the end adjacent to the tip portion 9 to the end adjacent to the base portion 11, and whose wall thickness gradually decreases as the outer diameter increases. The intermediate portion 13 has a first portion R1 and a second portion E1.

[0014] The first portion R1 is formed to have a greater thickness than the tendency for the thickness to decrease. The first portion R1 has a bending rigidity distribution that is convex upward from the distal end 9 to the proximal end 11. The second portion E1 is located adjacent to the first portion R1 between the first portion R1 and the proximal end 11, and is formed to have a smaller thickness than the tendency for the thickness to decrease. The steps S1 to S3 of the second portion E1 are preferably shorter than the steps S4 to S6 of the first portion R1. The second portion E1 has a bending rigidity distribution that is convex downward.

[0015] The step portions S4 to S6 of the first portion R1 may have a smaller difference in outer diameter than the step portions S1 to S3 of the second portion E1.

[0016] The intermediate portion 13 of the golf shaft 3 may also have a third portion E2. The third portion E2 is located adjacent to the first portion R1 between the tip portion 9 and the first portion R1 in the intermediate portion 13, and is formed with a thickness that is smaller than the decreasing trend in thickness. The third portion E2 has a bending stiffness distribution that is downwardly convex. It is preferable that the steps S7 to S11 of the third portion E2 are shorter than the steps S4 to S6 of the first portion R1.

[0017] The third portion E2 may have a flat portion F having a flexural rigidity distribution in which the flexural rigidity transitions to a constant value toward the base end 11.

[0018] The golf shaft 3 may also have a fourth portion R2. The fourth portion R2 is located closer to the tip portion 9 than the third portion E2, and has higher rigidity overall than the third portion E2.

[0019] The method for manufacturing the golf shaft 3 involves forming a relatively thick portion 25a and thin portions 24a and 24b adjacent to this thick portion 25a by applying thickness deviation processing to the intermediate portion 13 of a metal mother tube 20. Next, the mother tube 20, with the thick portion 25a and thin portions 24a and 24b set therein, is subjected to stepping processing according to the wall thickness to form multiple steps S1-S11 whose outer diameters gradually increase. At this time, it is preferable to make the steps S1-S11 relatively long at the thick portion 25a and relatively short at the thin portions 24a and 24b. [Example]

[0020] [Golf shaft] FIG. 1 is an overall view showing the appearance of a golf club according to an embodiment of the present invention.

[0021] The golf club 1 has a golf shaft 3 to which a head 5 and a grip 7 are attached. The golf shaft 3 has a tip portion 9, a base portion 11, and a middle portion 13. The tip portion 9 is the area to which the head 5 is attached, and the base portion 11 is the area to which the grip 7 is attached. The middle portion 13 is the area located between the tip portion 9 and the base portion 11.

[0022] A flexible portion a is provided in this middle portion 13. Although there are two flexible portions a in Fig. 1, it is also possible to have one or three or more flexible portions a. Furthermore, the middle portion 13 is provided with non-flexible portions b and c adjacent to the flexible portion a.

[0023] The flexible portion a is a relatively soft portion of the golf shaft 3, and the non-flexible portions b and c are relatively hard portions of the golf shaft 3. The flexible portion a and the non-flexible portions b and c are set by the outer diameter and thickness of the golf shaft 3, as will be described later.

[0024] Note that the non-flexible portions b and c do not mean that they do not bend at all, but rather that they do not bend relatively compared to the flexible portion a. Therefore, the flexible portion a means that it bends relatively compared to the non-flexible portions b and c.

[0025] FIG. 2 is a schematic cross-sectional view of a golf shaft used in the golf club of FIG.

[0026] 1 and 2, the tip portion 9 is the tip portion in the longitudinal direction of the golf shaft 3, and refers to a predetermined area from the tip of the golf shaft 3 to which the head 5 is attached. The length of the tip portion 9 in this embodiment is set appropriately up to about 160 mm. The tip portion 9 in this embodiment includes the insertion portion 8, the tapered portion 19, and a part of the straight portion 17.

[0027] The insertion portion 8 is the portion that is inserted into the head 5, and is formed in a tapered shape with the outer diameter gradually increasing toward the base end 11 of the golf shaft 3. However, the tip portion 9 may be formed in a straight tube shape with a constant outer diameter. The tapered portion 19 is adjacent to the base end of the insertion portion 8 and has a larger taper rate than the insertion portion 8. The straight portion 17 is adjacent to the base end of the tapered portion 19 and has a constant outer diameter. Note that the tip portion 9 may be composed of only the insertion portion 8. Alternatively, the tip portion 9 may have a straight shape as a whole.

[0028] The base end 11 is the base end in the length direction of the golf shaft 3, and refers to the area where the grip 7 is attached, which is a predetermined range from the base end of the golf shaft 3. The length of the base end 11 in this embodiment is set appropriately up to about 300 mm. In this embodiment, the base end 11 is formed in a straight tube shape with a constant outer diameter, but it may also be tapered so that the outer diameter changes slightly toward the base end.

[0029] The intermediate portion 13 is located between the distal end 9 and the proximal end 11, and has a distal end adjacent to the distal end 9 and a proximal end adjacent to the proximal end 11. In this embodiment, the length of the intermediate portion 13 is set appropriately up to about 750 mm. The intermediate portion 13 has a plurality of step portions S1 to S11 whose outer diameter gradually increases from the distal end to the proximal end, and whose wall thickness gradually decreases as the outer diameter increases.

[0030] The reduction in thickness of the intermediate portion 13 here means a reduction in thickness in accordance with an increase in outer diameter, and does not include increases or decreases in thickness in the first portion R1, second portion E1, and third portion E2 described below.

[0031] The decrease in thickness of this intermediate portion 13 has a constant decreasing trend in the thickness distribution, and can be, for example, a decrease when connecting the base end of distal portion 9 and the tip of proximal portion 11 at a step with a constant difference in length and outer diameter (see the comparative example in Figure 6). The decreasing trend in thickness may also be a linear decrease in thickness when connecting the base end of distal portion 9 and the tip of proximal portion 11 in the thickness distribution.

[0032] The intermediate portion 13 includes the remaining part of the straight portion 17 and a step portion 15 .

[0033] In this embodiment, a portion of the straight portion 17 included in the intermediate portion 13 is longer than a portion of the straight portion 17 included in the tip portion 9. However, the length of the straight portion 17 in the intermediate portion 13 can be set as appropriate. A step portion 15 is adjacent to the base end of this straight portion 17. The step portion 15 is composed of a plurality of step portions S1 to S11.

[0034] The intermediate portion 13 has a first portion R1, a second portion E1, and a third portion E2 depending on the settings of the outer diameter and thickness.

[0035] FIG. 3 is a schematic enlarged view showing the appearance of a golf shaft used in the golf club of FIG. 1. FIG. 4 is a graph showing the outer diameter distribution of the golf shaft of FIG. 3 together with a comparative example. FIG. 5 is a graph showing the bending rigidity distribution of the golf shaft of FIG. 3 together with the bending rigidity distribution of a comparative example without thickness variation processing. FIG. 6 is a graph showing the thickness distribution of the golf shaft of FIG. 3 together with the thickness distribution of a comparative example without thickness variation processing. FIG. 7 is a graph showing the relationship between the outer diameter distribution and the step portion of the golf shaft of FIG. 3. Note that although the shapes of FIGS. 2 and 3 do not completely match, they are schematic illustrations of the same structure.

[0036] As shown in Figures 3 to 7, the first portion R1 has a greater thickness in the middle portion 13 than the decreasing tendency of the thickness (comparison example of Figure 6), and the step portion is formed relatively long (compared to the second portion E1). In this embodiment, the first portion R1 is composed of three step portions S4 to S6. The length and number of the step portions constituting the first portion R1 are optional depending on the characteristics of the golf shaft 3, etc.

[0037] In the first portion R1, the number of steps S4 to S6 is relatively small, and therefore each of the steps S4 to S6 is formed relatively long (compared to the second portion E1). This suppresses fluctuations in the outer diameter of the first portion R1. Furthermore, the differences in the outer diameters of the steps S4 to S6 are set to be relatively small. This also suppresses fluctuations in the outer diameter of the first portion R1.

[0038] Therefore, in this embodiment, the first portion R1 can have a long area where the thickness is large. The first portion R1 has a flexural rigidity distribution that is convex upward from the tip end 9 to the base end 11. As a result, the first portion R1 constitutes the inflexible portion b of the golf shaft 3. The convex shape of the flexural rigidity distribution is relative to the reference line L1. The reference line L1 is the straight line in the flexural rigidity distribution that connects the start point of the first portion R1 and the end point of the second portion E1. In order to make the flexural rigidity distribution of the first portion R1 convex upward, it is not necessary to make the steps S4 to S6 relatively long or to reduce the number of the steps S4 to S6.

[0039] The second portion E1 is located adjacent to the first portion R1 in the intermediate portion 13, between the first portion R1 and the base end portion 11. The second portion E1 has a smaller thickness than the decreasing tendency of the thickness in the intermediate portion 13, and the step portion is formed to be relatively short (compared to the first portion R1). In this embodiment, the second portion E1 is formed by a single step portion S3. The length and number of the step portions constituting the second portion E1 are optional depending on the characteristics of the golf shaft 3, etc.

[0040] The number of steps S1-S3 in the second portion E1 is the same as that in the first portion R1, but may be greater than that in the first portion R1. In the second portion E1, the steps S1-S3 are shorter than the steps S4-S6 in the first portion R1, so the variation in outer diameter can be increased over a short range. Furthermore, the difference in outer diameter between the steps S1-S3 is set to be relatively large (compared to the first portion R1). From this perspective, the variation in outer diameter can be increased in the second portion E1.

[0041] Therefore, in this embodiment, in the region where the thickness of the second portion E1 is set small, the combination of this thickness setting and the variation in outer diameter causes the bending stiffness distribution from the tip end 9 to the base end 11 to be convex downward. As a result, the second portion E1 constitutes the flexible portion a of the golf shaft 3. Note that the step portions S1 and S2 are portions that continue to the base end 11 and constitute the non-flexible portion c of the golf shaft 3. In order to make the bending stiffness distribution of the second portion E1 convex downward, it is not necessary to make the step portions S1 to S3 relatively short.

[0042] The convex shape of the bending rigidity distribution of the second portion E1 can be made to transition sharply from the second portion E1 to the first portion R1 by setting the outer diameter and thickness, and the first portion R1 can be made larger.

[0043] The third portion E2 is located adjacent to the first portion R1 in the intermediate portion 13, between the first portion R1 and the tip portion 9. The third portion E2 has a smaller thickness than the decreasing tendency of the thickness in the intermediate portion 13, and the steps are formed to be relatively short (compared to the first portion R1). The length and number of the steps constituting the third portion E2 are arbitrary depending on the characteristics of the golf shaft 3, etc. The size relationship of the steps S1 to S11 is, but is not limited to, S4 = S5 = S6 > S1 = S2 > S7 > S3 = S8 > S9 = S10 = S11.

[0044] In this embodiment, the third portion E2 is composed of five step portions S7 to S11 and a part of the straight portion 17. The number of step portions constituting the third portion E2 is arbitrary depending on the characteristics of the golf shaft 3, etc.

[0045] The third portion E2 has steps S7-S11 that are shorter than the steps S4-S6 of the first portion R1, allowing for a larger variation in outer diameter within a shorter range. Furthermore, the steps S7-S11 are designed to have a relatively larger difference in outer diameter (compared to the first portion R1). This also allows for a larger variation in outer diameter within the third portion E2.

[0046] Therefore, in this embodiment, in the region where the thickness of the third portion E2 is set small, the combination of this thickness setting and the variation in outer diameter results in a downwardly convex bending rigidity distribution from the tip end 9 to the base end 11. Therefore, the third portion E2 constitutes the flexible portion a of the golf shaft 3.

[0047] The convex shape of the bending stiffness distribution here is relative to a reference line L2, which is a straight line connecting the end point of the first portion R1 and the start point of the third portion E2 in the bending stiffness distribution.

[0048] The convex shape of the bending rigidity distribution of the third portion E2 can be made to transition sharply from the third portion E2 to the first portion R1 by setting the outer diameter and thickness, thereby making it possible to increase the first portion R1. The convex shape of the bending rigidity distribution of the third portion E2 has a flat portion F of the bending rigidity distribution where the bending rigidity distribution transitions uniformly toward the base end 11.

[0049] The golf shaft 3 of this embodiment includes a fourth portion R2. The fourth portion R2 is located adjacent to the third portion E2 in the tip portion 9. The fourth portion R2 corresponds to a portion of the tapered portion 19 and the straight portion 17 of the tip portion 9.

[0050] The bending rigidity of the fourth portion R2 is greater than that of the flat portion F of the third portion E2. In this embodiment, the bending rigidity distribution of the fourth portion R2 is convex upward. The convex shape of the bending rigidity distribution here is relative to the reference line L3. The reference line L3 is the straight line connecting the start point of the fourth portion R2 and the end point of the third portion E2 in the bending rigidity distribution.

[0051] [Manufacturing method of golf shafts] 8 is a schematic diagram showing a raw tube that has been processed to have a non-uniform wall thickness in a manufacturing method of a golf shaft according to the embodiment. The golf shaft 3 of FIG. 2 is manufactured by subjecting a raw tube 20 that has been processed to have a non-uniform wall thickness in FIG. 8 to stepping processing.

[0052] In the thickness deviation processing, relatively thick portions 23a and 25a are provided at the tip portion 23 and the middle portion 25 of a metal, particularly a steel, mother tube 20, and thin portions 24a and 24b are provided adjacent to the thick portion 25a. Note that the material of the mother tube 20 may be a metal other than steel.

[0053] Specifically, thick-walled sections 23a are provided in sections A and B of the mother tube 20, which correspond to the insertion section 8 and tapered section 19 of the tip 9 of the golf shaft 3, and thick-walled sections 25a are provided in sections D, E, and F, which correspond to the first portion R1 of the intermediate section 13. Sections B, D, and F are formed in a tapered shape.

[0054] The C and G sections of the mother tube 20, which correspond to the third section E2 and the second section E1 of the golf shaft 3, respectively, are thin-walled sections 24a and 24b. The C and G sections are straight tubular, and the C section is thicker than the G section.

[0055] The unbalanced wall thickness processed raw tube 20 is subjected to stepping processing according to the wall thickness as shown in FIG. 3, thereby obtaining a bending rigidity distribution as shown in FIG.

[0056] That is, stepping creates multiple steps S1-S11 whose outer diameters gradually increase, with steps S4-S6 being relatively long in thick-walled portion 25a and steps S1-S3 and S7-S11 being relatively short in thin-walled portions 24a and 24b. In this embodiment, the difference in outer diameter between steps S1-S3 and S7-S11 in thin-walled portions 24a and 24b is made relatively large. In this manner, the golf shaft 3 shown in FIG. 3 is manufactured.

[0057] [Mechanism for improving ball speed (head speed)] Fig. 9(A) is a side view of the head and its surroundings showing the state immediately before impact with the ball, Fig. 9(B) is a side view of the state at the moment of impact, and Fig. 9(C) is a side view of the state immediately after impact. Fig. 10(A) is a side view of a golf club according to a comparative example having one flexible portion during the downswing and immediately before impact, and Fig. 10(B) is a side view of a golf club having two flexible portions during the downswing and immediately before impact.

[0058] The head speed of the head 5 immediately before impact in Figure 9(A) is Vh, and passes through the moment of impact in Figure 9(B) to reach the point immediately after impact in Figure 9(C). The ball speed immediately after impact is Vb.

[0059] As shown in FIG. 10, in a golf swing, the golf shaft 3 bends in the opposite direction to the direction of the club face during the downswing, and then bends in the direction the club face is facing just before impact.

[0060] As shown in Figure 10(A), the golf club 1 having one bending portion a in the middle portion 13 bends little during the downswing and immediately before impact. In contrast, as shown in Figure 10(B), the golf club 1 having two bending portions a in the middle portion 13 bends much during both the downswing and immediately before impact.

[0061] Therefore, in the golf club 1 with two bending portions a, the moving distance of the head 5 increases in the same time, so the head speed Vh immediately before impact increases, and as a result, the ball speed Vb also increases.

[0062] Figure 11(A) is a side view showing the initial and middle stages of the downswing of a golf club with no hard spots in the middle, and Figure 11(B) is a side view showing the initial and middle stages of the downswing of a golf club with a hard spot in the middle.

[0063] In the case of a golf club 1 that does not have a stiff portion in the middle portion 13 as shown in Figure 11(A), the recovery from the bending in the middle of the downswing is delayed compared to the bending in the early part of the downswing. In contrast, in the case of a golf club 1 that has a stiff portion in the middle portion 13 as shown in Figure 11(B), the recovery from the bending in the middle of the downswing is quick compared to the bending in the early part of the downswing.

[0064] Therefore, in the golf club 1 having a hard portion in the middle portion 13, the moving distance of the head 5 increases in the same time period, and therefore the ball speed Vb and the head speed Vh increase.

[0065] FIG. 12(A) is a side view showing the state of a golf club with a soft tip at the time of impact, and FIG. 12(B) is a side view showing the state of a golf club with a hard tip at the time of impact.

[0066] In a golf club 1 with a soft tip portion 9 as shown in Figure 12(A), the energy based on the bending of the middle portion 13 is used to deform the tip portion 9 while in contact with the ball and is lost. In contrast, in a golf club 1 with a hard tip portion 9 as shown in Figure 12(B), the loss of energy based on the bending of the middle portion 13 due to deformation of the tip portion 9 is suppressed.

[0067] Therefore, the golf club 1 with a hard tip portion 9 transmits more energy to the ball, and the ball speed Vb increases.

[0068] The golf shaft 3 of this embodiment has a first portion R1, which is a non-flexible portion b, in the middle section 13, and second and third portions E1 and E2, which are flexible portions a, on both sides of the first portion R1. This allows the golf shaft 3 to bend more greatly during the downswing and immediately before impact, and also quickly recovers from the bending in the middle of the downswing. As a result, the golf shaft 3 can increase the ball speed Vb and head speed Vh.

[0069] Furthermore, in the golf shaft 3 of this embodiment, the convex shape of the bending rigidity distribution of the second portion E1 and the third portion E2 allows the rigidity to transition suddenly from the second portion E1 and the third portion E2 to the first portion R1 by setting the outer diameter and thickness.

[0070] As a result, the first portion R1 can be increased, and the second portion E1 and the third portion E2 can be made to bend easily from the region close to the first portion R1. As a result, the golf shaft 3 of this embodiment can more reliably increase bending during the downswing and immediately before impact, and can quickly return to its original position from the middle of the downswing.

[0071] Furthermore, the golf shaft 3 of this embodiment has a fourth portion R2, which is a portion c that does not bend at the tip portion 9, so that bending of the tip portion 9 during impact is suppressed, improving energy transmission and thereby increasing the ball speed Vb.

[0072] [sample] Samples 1 to 4 were made of the golf shaft 3 of this example based on the above mechanism and test shots were carried out.

[0073] Figures 13(A), 14(A), 15(A), and 16(A) are side views of Samples 1 to 4 of the golf shafts of the examples, respectively. Figures 13(B), 14(B), 15(B), and 16(B) are cross-sectional views showing the wall thicknesses of Samples 1 to 4, respectively. Figures 17 to 20 are graphs showing the bending rigidity distributions of Samples 1 to 4, respectively.

[0074] 13(A) to 16(B), the intermediate portion 13 of Sample 1 of Fig. 13, Sample 3 of Fig. 15, and Sample 4 of Fig. 16 has one non-flexible portion (first portion R1) and two flexible portions (second and third portions E1 and E2) as shown in Fig. 17, Fig. 19, and Fig. 20, respectively. On the other hand, the intermediate portion 13 of Sample 2 of Fig. 14 has two non-flexible portions (first portion R1 and fifth portion R3) and three flexible portions (second, third, and sixth portions E1, E2, and E3) as shown in Fig. 18.

[0075] In Sample 1, the first portion R1, the second portion E1, and the third portion E2 are located closer to the tip portion 9 than in Example 1. Accordingly, the fourth portion R2 is also located closer to the tip of the tip portion 9 than in Example 1. Also, in Sample 1, the convex shape of the first portion R1 is smaller than in Example 1. Otherwise, the basic configuration of Sample 1 is the same as that of Example 1.

[0076] In Sample 2, compared to Example 1, the first portion R1, the second portion E1, and the third portion E2 are closer to the tip portion 9, and the fifth portion R3 and the sixth portion E3 are set between the first portion R1 and the third portion E2.

[0077] The first portion R1 of Sample 2 has a smaller convex shape than that of Example 1. The fifth portion R3 has an upward convex shape similar to that of the first portion R1, and the sixth portion E3 has a downward convex shape similar to that of the second portion E1. Otherwise, the basic configuration of Sample 2 is the same as that of Example 1.

[0078] The first portion R1, the second portion E1, the third portion E2, the fourth portion R2, the fifth portion R3, and the sixth portion E3 of Sample 2 are approximately equal in length and shorter than those of Example 1.

[0079] Sample 3 is almost the same as Example 1, although there are some differences in the number of steps and the rigidity distribution.

[0080] Sample 4 has a tip 9 that is softer than Sample 3. The bending rigidity distribution of Sample 4 is close to that of Sample 3.

[0081] However, in Sample 4, the gradient of the bending rigidity distribution is set to be gentler than in Sample 3 between the adjacent third portion E2 and fourth portion R2 and between the third portion E2 and first portion R1.

[0082] [Trial test] 21 and 22 are charts showing the specifications and bending rigidity distribution of golf shaft samples according to Examples and Comparative Examples, respectively.

[0083] Hitting tests were conducted on Samples 1 to 4 and the Comparative Example. Samples 1 to 4 have the above-described configuration, while the Comparative Example has a configuration in which the wall thickness decreases as the outer diameter increases, as shown by the dashed lines in Figures 4 and 6. In this Comparative Example, the bending rigidity distribution does not have any convex or concave portions, as shown in Figure 22. Furthermore, the bending rigidity distribution of the fourth portion R2 of Samples 1 to 3 is higher than the bending rigidity distribution at the tip portion 9 of the Comparative Example.

[0084] As shown in Figure 21, the specifications of Samples 1 to 4 and the Comparative Example Sample are those used in a 7-iron, and the flex and overall length are the same. Other factors such as weight, BP (balance point), torque, total weight, balance, and frequency vary depending on the outer diameter and thickness settings of Samples 1 to 4 and the Comparative Example Sample.

[0085] The test shots were carried out by a robot with heads 5 attached to Samples 1 to 4 and the comparative sample. The ball was positioned on an extension of the center of the robot, and the face was oriented perpendicular to the target direction.

[0086] The same head 5 was used in all test shots for Samples 1 to 4 and the comparative example, and the impact point was the center of gravity of the head 5. Ten balls were hit for each test shot.

[0087] The robot was set so that the target head speed of 40 m / s was achieved using the comparative sample, and all samples 1 to 4 and the comparative sample were tested using this setting.

[0088] Fig. 23 is a chart showing the test shot results of the golf shaft samples according to the examples and the comparative examples, in which BS (ball speed) and carry are displayed as the test shot results.

[0089] The ball speed of the comparative example was Vb=55.5 m / s and carry=168.9 yards, while samples 1 to 4 all exceeded the values ​​of the comparative example.

[0090] In particular, Sample 3 had a ball speed Vb of 56.9 m / s and a carry of 177.0 yards, which was 1.4 m / s faster and 8.1 yards faster than the comparative example. [Explanation of symbols]

[0091] 1. Golf club 3 Golf shafts 5 heads 7 Grip 9 Tip 11 Proximal end 13 Middle section 23a, 25a Thick wall part R1 First part E1 Second part E2 Third Part R2 Fourth Part

Claims

1. a tip portion for attaching a head; a base end portion for attaching a grip; an intermediate portion having a plurality of stepped portions whose outer diameter gradually increases from an end adjacent to the distal end to an end adjacent to the proximal end between the distal end and the proximal end, and whose wall thickness gradually decreases as the outer diameter increases; a first portion in which the thickness is greater than the decreasing tendency of the thickness in the intermediate portion, and the bending rigidity distribution from the distal end portion to the proximal end portion is convex upward; a second portion located adjacent to the first portion between the first portion and the base end portion in the intermediate portion, the second portion having a thickness smaller than the decreasing tendency of the thickness, and the bending rigidity distribution being downwardly convex; Golf shaft.

2. 2. The golf shaft of claim 1, the step of the second portion is shorter than the step of the first portion; Golf shaft.

3. 3. The golf shaft of claim 1 or 2, The step portion of the first portion has a smaller difference in outer diameter than the step portion of the second portion. Golf shaft.

4. 3. The golf shaft of claim 1 or 2, a third portion located adjacent to the first portion between the first portion and the tip portion in the intermediate portion, the third portion having a thickness smaller than the decreasing tendency of the thickness, and the bending rigidity distribution being downwardly convex; Golf shaft.

5. 5. The golf shaft of claim 4, the step of the third portion is shorter than the step of the first portion; Golf shaft.

6. 5. The golf shaft of claim 4, the third portion has a flat portion of the bending stiffness distribution in which bending stiffness transitions to a constant value toward the base end portion; Golf shaft.

7. 5. The golf shaft of claim 4, a fourth portion located closer to the tip end than the third portion and having higher rigidity than the third portion. Golf shaft.

8. a relatively thick-walled portion and a relatively thin-walled portion adjacent to the thick-walled portion are formed in an intermediate portion of a metal mother tube by means of thickness deviation processing; forming a plurality of step portions whose outer diameters gradually increase by performing stepping processing according to the wall thickness on the mother tube having the thick-wall portion and the thin-wall portion set therein; A method for manufacturing a golf shaft.

9. 9. The method of manufacturing a golf shaft according to claim 8, The stepping process forms the step portion relatively long in the thick-walled portion and relatively short in the thin-walled portion. A method for manufacturing a golf shaft.

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