EP4098332A1

EP4098332A1 - Badminton racket

Info

Publication number: EP4098332A1
Application number: EP21756867.4A
Authority: EP
Inventors: Wataru Kimizuka
Original assignee: Sumitomo Rubber Industries Ltd
Current assignee: Sumitomo Rubber Industries Ltd
Priority date: 2020-02-18
Filing date: 2021-01-19
Publication date: 2022-12-07
Also published as: EP4098332A4; WO2021166519A1; JP2021129634A; JP7424096B2

Abstract

A badminton racket 2 includes a shaft 4, a frame 6, a grip 8, and a string 10. A flexural rigidity value EI(2) of the shaft 4 at a second measurement point located at a distance of 75 mm from the grip 8 is smaller than a flexural rigidity value EI(1) of the shaft 4 at a first measurement point located at a distance of 35 mm from the grip 8 and a flexural rigidity value EI(4) of the shaft 4 at a fourth measurement point located at a distance of 155 mm from the grip 8. A flexural rigidity value EI(3) of the shaft 4 at a third measurement point located at a distance of 115 mm from the grip 8 is smaller than the flexural rigidity value EI(1) and the flexural rigidity value EI(4).

Description

Technical Field

The present invention relates to a badminton racket. In particular, the present invention relates to improvements to the shaft of the racket.

Background Art

A badminton racket includes a frame, a string, and a shaft. The player shots a shuttlecock with the racket. The shaft is deformed upon the shot.
Various attempts have been made to optimize the shaft deformation behavior. Japanese Laid-Open Patent Application Publication No. 2001-70481 discloses a racket including a shaft including two tubes made of different materials.

Citation List

Patent Literature

PTL 1: Japanese Laid-Open Patent Application Publication No. 2001-70481

Summary of Invention

Technical Problem

In a badminton game, the player performs various types of shots. The player performs shots such as smash, lob, cut, and clear.
The smash is a shot by which the shuttlecock is moved quickly to the court of the opponent player. For the smash, the player is required to have the skill to allow the shuttlecock to flight at a high speed. When the player is one who frequently uses the smash, the player wants to achieve high-speed flight of the shuttlecock in the smash.
In the smash, the shaft is significantly deflected in the in-plane direction and in the out-of-plane direction. The player wants to achieve high-speed flight of the shuttlecock also for shots which are other than the smash and in which the shaft is deflected in both the in-plane and out-of-plane directions.
An object of the present invention is to provide a badminton racket suitable for shots in which the shaft of the racket is significantly deflected in both the in-plane and out-of-plane directions.

Solution to Problem

A badminton racket according to the present invention includes:

a grip;
a shaft having a portion inserted into the grip, the inserted portion including a butt end of the shaft and a vicinity of the butt end; and
a frame attached to the shaft in a vicinity of a tip end of the shaft. A flexural rigidity value EI(2) of the shaft at a second measurement point located at a distance of 75 mm from the grip is smaller than a flexural rigidity value EI(1) of the shaft at a first measurement point located at a distance of 35 mm from the grip and a flexural rigidity value EI(4) of the shaft at a fourth measurement point located at a distance of 155 mm from the grip. A flexural rigidity value EI(3) of the shaft at a third measurement point located at a distance of 115 mm from the grip is smaller than the flexural rigidity value EI(1) and the flexural rigidity value EI(4).

Preferably, a ratio (EI(2)/EI(1)) of the flexural rigidity value EI(2) to the flexural rigidity value EI(1) is not greater than 0.95. Preferably, a ratio (EI(2)/EI(4)) of the flexural rigidity value EI(2) to the flexural rigidity value EI(4) is not greater than 0.95.
Preferably, a ratio (EI(3)/EI(1)) of the flexural rigidity value EI(3) to the flexural rigidity value EI(1) is not greater than 0.95. Preferably, a ratio (EI(3)/EI(4)) of the flexural rigidity value EI(3) to the flexural rigidity value EI(4) is not greater than 0.95.
Preferably, a difference (EI(2) - EI(1)) between the flexural rigidity value EI(2) and the flexural rigidity value EI(1) is not greater than -0.30 Nm². Preferably, a difference (EI(2) - EI(4)) between the flexural rigidity value EI(2) and the flexural rigidity value EI(4) is not greater than -0.30 Nm².
Preferably, a difference (EI(3) - EI(1)) between the flexural rigidity value EI(3) and the flexural rigidity value EI(1) is not greater than -0.30 Nm². Preferably, a difference (EI(3) - EI(4)) between the flexural rigidity value EI(3) and the flexural rigidity value EI(4) is not greater than -0.30 Nm².
The shaft may have a hollow structure. Preferably, an inner diameter of the shaft is substantially constant from the first measurement point to the fourth measurement point.
Preferably, an outer diameter of the shaft is substantially constant from the first measurement point to the fourth measurement point.
Preferably, a ratio (W2/W1) of a weight W2 of the shaft in a zone from the second measurement point to the third measurement point to a weight W1 of the shaft in a zone from the first measurement point to the second measurement point is not less than 0.95 and not greater than 1.05. Preferably, a ratio (W2/W3) of the weight W2 of the shaft in the zone from the second measurement point to the third measurement point to a weight W3 of the shaft in a zone from the third measurement point to the fourth measurement point is not less than 0.95 and not greater than 1.05.
Preferably, the shaft includes:

(1) a fiber-reinforced layer located in a zone including the first measurement point and not including the third measurement point in an axial direction of the shaft, the fiber-reinforced layer including a plurality of reinforcing fibers oriented substantially in the axial direction; and
(2) another fiber-reinforced layer located in a zone not including the second measurement point but including the fourth measurement point in the axial direction, the other fiber-reinforced layer including a plurality of reinforcing fibers oriented substantially in the axial direction.

Advantageous Effects of Invention

A player using the badminton racket according to the present invention can easily perform shots in which the shaft is significantly deflected in both the in-plane and out-of-plane directions. The racket can contribute to winning badminton games.

Brief Description of Drawings

FIG. 1 is a front view showing a badminton racket according to one embodiment of the present invention.
FIG. 2 is a right side view showing the racket of FIG. 1.
FIG. 3 is an enlarged cross-sectional view showing a part of the shaft of the racket of FIG. 1.
FIG. 4 is an enlarged cross-sectional view taken along the line IV-IV of FIG. 3.
FIG. 5 is a developed view showing prepregs for the shaft of the racket of FIG. 1.
FIG. 6 is a schematic diagram showing a method of measuring a flexural rigidity value EI of the shaft of the racket of FIG. 1.
FIG. 7 is a graph showing a flexural rigidity distribution of the shaft of the racket of FIG. 1.
FIG. 8 is a graph showing a flexural rigidity distribution of a shaft of a racket according to Comparative Example.
FIG. 9 is a graph showing a flexural rigidity distribution of a shaft of a racket according to Example 2 of the present invention.
FIG. 10 is a graph showing a flexural rigidity distribution of a shaft of a racket according to Example 3 of the present invention.

Description of Embodiments

The following will describe in detail the present invention based on preferred embodiments with appropriate reference to the drawings.
FIGS. 1 and 2 show a badminton racket 2. The racket 2 includes a shaft 4, a frame 6, a grip 8, and a string 10. In FIGS. 1 and 2, the arrow X represents the width direction, the arrow Y represents the axial direction, and the arrow Z represents the thickness direction.
The shaft 4 includes a butt portion 12, a middle portion 14, and a tip portion 16. The shaft 4 further includes a butt end 18 and a tip end 20. The shaft 4 is hollow. The shaft 4 is formed from a fiber-reinforced resin. The fiber-reinforced resin includes a resin matrix and a large number of reinforcing fibers. The shaft 4 includes a plurality of fiber-reinforced layers (which will be described in detail later).
Examples of the base resin of the shaft 4 include: thermosetting resins such as epoxy resin, bismaleimide resin, polyimide, and phenol resin; and thermoplastic resins such as polyether ether ketone, polyethersulfone, polyetherimide, polyphenylene sulfide, polyamide, and polypropylene. Epoxy resin is particularly suitable as the resin of the shaft 4.
Examples of the reinforcing fibers of the shaft 4 include carbon fibers, metal fibers, glass fibers, and aramid fibers. Carbon fibers are particularly suitable as the fibers of the shaft 4. Different kinds of fibers may be used in combination.
The frame 6 is ring-shaped and hollow. The frame 6 is formed from a fiber-reinforced resin. Any of the resins as mentioned for the base resin of the shaft 4 can be used as the base resin of the fiber-reinforced resin. Any of the fibers as mentioned for the reinforcing fibers of the shaft 4 can be used as the reinforcing fibers of the fiber-reinforced resin. The frame 6 is firmly joined to the tip end 20 of the shaft 4.
The grip 8 includes a hole 21 extending in the axial direction (Y direction). The butt end 18 and its vicinity of the shaft 4 are inserted into the hole 21. The inner peripheral surface of the hole 21 and the outer peripheral surface of the shaft 4 are bonded with an adhesive.
The string 10 is strung on the frame 6. The string 10 is placed along the width direction X and the axial direction Y. The portions of the string 10 that extend along the width direction X are referred to as transverse threads 22. The portions of the string 10 that extend along the axial direction Y are referred to as longitudinal threads 24. The plurality of transverse threads 22 and the plurality of longitudinal threads 24 form a face 26. The face 26 extends generally along the X-Y plane.
In FIG. 1, the reference sign L represents the length of the exposed portion of the shaft 4. The length L is usually not less than 150 mm and not greater than 210 mm.
FIG. 3 is an enlarged cross-sectional view showing a part of the shaft 4 of the racket 2 of FIG. 1. FIG. 4 is an enlarged cross-sectional view taken along the line IV-IV of FIG. 3. As previously stated, the shaft 4 is hollow. As shown in FIG. 4, the cross-section of the shaft 4 is circular. In other words, the shaft 4 is in the shape of a cylindrical tube.
In FIGS. 3 and 4, the arrow Di represents the inner diameter of the shaft 4. The inner diameter Di is typically not less than 3 mm and not greater than 10 mm. In FIGS. 3 and 4, the arrow Do represents the outer diameter of the shaft 4. The outer diameter Do is typically not less than 5 mm and not greater than 15 mm.
As previously stated, the shaft 4 is formed from a fiber-reinforced resin. The shaft 4 can be produced by a sheet winding method. In the sheet winding method, a plurality of prepregs are wound on a mandrel. Each prepreg includes a plurality of fibers and a matrix resin. The matrix resin is uncured.
FIG. 5 is a developed view showing a prepreg arrangement for the shaft 4 of the racket 2 of FIG. 1. The prepreg arrangement includes 11 prepregs (sheets). Specifically, the prepreg arrangement includes a first sheet S1, a second sheet S2, a third sheet S3, a fourth sheet S4, a fifth sheet S5, a sixth sheet S6, a seventh sheet S7, an eighth sheet S8, a ninth sheet S9, a tenth sheet S10, and an eleventh sheet 11. A plurality of fiber-reinforced layers are formed from the prepregs by a method described later. Specifically, a first fiber-reinforced layer is formed from the first sheet S1, a second fiber-reinforced layer is formed from the second sheet S2, a third fiber-reinforced layer is formed from the third sheet S3, a fourth fiber-reinforced layer is formed from the fourth sheet S4, a fifth fiber-reinforced layer is formed from the fifth sheet S5, a sixth fiber-reinforced layer is formed from the sixth sheet S6, a seventh fiber-reinforced layer is formed from the seventh sheet S7, an eighth fiber-reinforced layer is formed from the eighth sheet S8, a ninth fiber-reinforced layer is formed from the ninth sheet S9, a tenth fiber-reinforced layer is formed from the tenth sheet S10, and an eleventh fiber-reinforced layer is formed from the eleventh sheet S11.
The left-right direction in FIG. 5 is the axial direction of the shaft 4. In FIG. 5, the locations of the butt end 18 and tip end 20 are indicated by arrows. In FIG. 5, the locations of four measurement points P1, P2, P3, and P4 described later are also indicated by arrows. In FIG. 5, the reduction scale in the left-right direction (axial direction) is not equal to the reduction scale in the up-down direction.
The first sheet S1 extends over the entirety of the shaft 4. The first sheet S1 is generally rectangular. The first sheet S1 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is inclined with respect to the axial direction. The direction in which each carbon fiber extends forms an angle of not less than 30° and not greater than 60° with the axial direction. In the present embodiment, the angle is 45°. The first sheet S1 is 95 mm wide and 340 mm long.
The second sheet S2 extends over the entirety of the shaft 4. The second sheet S2 is generally rectangular. The second sheet S2 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is inclined with respect to the axial direction. The direction in which each carbon fiber extends forms an angle of not less than -60° and not greater than -30° with the axial direction. In the present embodiment, the angle is -45°. The second sheet S2 is 95 mm wide and 340 mm long.
The direction of the inclination of the carbon fibers in the second sheet S2 is opposite to the direction of the inclination of the carbon fibers in the first sheet S1. Thus, the direction of the inclination of the carbon fibers in the second fiber-reinforced layer is opposite to the direction of the inclination of the carbon fibers in the first fiber-reinforced layer. In the shaft 4, a bias structure is achieved by the first and second fiber-reinforced layers. The first and second fiber-reinforced layers contribute to the flexural rigidity and torsional rigidity of the shaft 4. The first and second fiber-reinforced layers particularly contribute to the torsional rigidity of the shaft 4.
The third sheet S3 is localized in the middle portion of the shaft 4. The third sheet S3 is generally in the shape of a parallelogram. The third sheet S3 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is inclined with respect to the axial direction. The direction in which each carbon fiber extends forms an angle of not less than 30° and not greater than 60° with the axial direction. In the present embodiment, the angle is 45°. The third sheet S3 is 25 mm wide and 70 mm long.
The fourth sheet S4 is localized in the middle portion of the shaft 4. In the axial direction, the location of the fourth sheet S4 is the same as the location of the third sheet S3. The fourth sheet S4 is generally in the shape of a parallelogram. The fourth sheet S4 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is inclined with respect to the axial direction. The direction in which each carbon fiber extends forms an angle of not less than -60° and not greater than -30° with the axial direction. In the present embodiment, the angle is -45°. The fourth sheet S4 is 25 mm wide and 70 mm long.
The direction of the inclination of the carbon fibers in the fourth sheet S4 is opposite to the direction of the inclination of the carbon fibers in the third sheet S3. Thus, the direction of the inclination of the carbon fibers in the fourth fiber-reinforced layer is opposite to the direction of the inclination of the carbon fibers in the third fiber-reinforced layer. In the shaft 4, a bias structure is achieved by the third and fourth fiber-reinforced layers. The third and fourth fiber-reinforced layers contribute to the flexural rigidity and torsional rigidity of the middle portion 14. The third and fourth fiber-reinforced layers particularly contribute to the torsional rigidity of the middle portion 14.
The fifth sheet S5 is localized towards the tip end 20 of the shaft 4. The fifth sheet S5 is generally trapezoidal. The fifth sheet S5 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is the same as the axial direction. In other words, the direction in which each carbon fiber extends forms an angle of substantially 0° with the axial direction. The fifth sheet S5 is 50 mm wide, 105 mm long on the upper base, and 115 mm long on the lower base.
As stated above, the carbon fibers contained in the fifth sheet S5 are oriented substantially in the axial direction. Thus, the carbon fibers are oriented substantially in the axial direction also in the fifth fiber-reinforced layer. The structure in which the carbon fibers are oriented substantially in the axial direction is herein referred to as a "straight structure". The fifth fiber-reinforced layer has a straight structure. Upon deflection of the shaft 4, the carbon fibers are subjected to a strong tension. The tension acts to prevent further deflection of the shaft 4. In other words, the carbon fibers contribute to the flexural rigidity of the shaft 4. As shown in FIG. 5, the fifth sheet S5 is located in a zone not including the first and second measurement points P1 and P2 but including the third and fourth measurement points P3 and P4 in the axial direction. Thus, the fifth fiber-reinforced layer is also located in the zone not including the first and second measurement points P1 and P2 but including the third and fourth measurement points P3 and P4 in the axial direction. The fifth fiber-reinforced layer particularly contributes to the flexural rigidity of the tip portion 16.
The sixth sheet S6 is localized towards the butt end 18 of the shaft 4. The sixth sheet S6 is generally trapezoidal. The sixth sheet S6 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is the same as the axial direction. In other words, the direction in which each carbon fiber extends forms an angle of substantially 0° with the axial direction. The sixth sheet S6 is 50 mm wide, 155 mm long on the upper base, and 165 mm long on the lower base.
As stated above, the carbon fibers contained in the sixth sheet S6 are oriented substantially in the axial direction. Thus, the carbon fibers are oriented substantially in the axial direction also in the sixth fiber-reinforced layer. The sixth fiber-reinforced layer has a straight structure. Upon deflection of the shaft 4, the carbon fibers are subjected to a strong tension. The tension acts to prevent further deflection of the shaft 4. In other words, the carbon fibers contribute to the flexural rigidity of the shaft 4. As shown in FIG. 5, the sixth sheet S6 is located in a zone including the first and second measurement points P1 and P2 and not including the third and fourth measurement points P3 and P4 in the axial direction. Thus, the sixth fiber-reinforced layer is also located in the zone including the first and second measurement points P1 and P2 and not including the third and fourth measurement points P3 and P4 in the axial direction. The sixth fiber-reinforced layer particularly contributes to the flexural rigidity of the butt portion 12.
The seventh sheet S7 is localized in the middle portion 14 of the shaft 4. The seventh sheet S7 is generally in the shape of a parallelogram. The seventh sheet S7 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is inclined with respect to the axial direction. The direction in which each carbon fiber extends forms an angle of not less than 30° and not greater than 60° with the axial direction. In the present embodiment, the angle is 45°. The seventh sheet S7 is 25 mm wide and 110 mm long.
The eight sheet S8 is localized in the middle portion of the shaft 4. In the axial direction, the location of the eighth sheet S8 is the same as the location of the seventh sheet S7. The eighth sheet S8 is generally in the shape of a parallelogram. The eighth sheet S8 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is inclined with respect to the axial direction. The direction in which each carbon fiber extends forms an angle of not less than -60° and not greater than -30° with the axial direction. In the present embodiment, the angle is -45°. The eighth sheet S8 is 25 mm wide and 110 mm long.
The direction of the inclination of the carbon fibers in the eighth sheet S8 is opposite to the direction of the inclination of the carbon fibers in the seventh sheet S7. Thus, the direction of the inclination of the carbon fibers in the eighth fiber-reinforced layer is opposite to the direction of the inclination of the carbon fibers in the seventh fiber-reinforced layer. In the shaft 4, a bias structure is achieved by the seventh and eighth fiber-reinforced layers. The seventh and eighth fiber-reinforced layers contribute to the flexural rigidity and torsional rigidity of the middle portion 14. The seventh and eighth fiber-reinforced layers particularly contribute to the torsional rigidity of the middle portion 14.
The ninth sheet S9 is localized towards the tip end 20 of the shaft 4. The ninth sheet S9 is generally trapezoidal. The ninth sheet S9 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is the same as the axial direction. In other words, the direction in which each carbon fiber extends forms an angle of substantially 0° with the axial direction. The ninth sheet S9 is 50 mm wide, 85 mm long on the upper base, and 95 mm long on the lower base.
As stated above, the carbon fibers contained in the ninth sheet S9 are oriented substantially in the axial direction. Thus, the carbon fibers are oriented substantially in the axial direction also in the ninth fiber-reinforced layer. The ninth fiber-reinforced layer has a straight structure. Upon deflection of the shaft 4, the carbon fibers are subjected to a strong tension. The tension acts to prevent further deflection of the shaft 4. In other words, the carbon fibers contribute to the flexural rigidity of the shaft 4. As shown in FIG. 5, the ninth sheet S9 is located in a zone not including the first, second, and third measurement points P1, P2, and P3 but including the fourth measurement point P4 in the axial direction. Thus, the ninth fiber-reinforced layer is also located in the zone not including the first, second, and third measurement points P1, P2, and P3 but including the fourth measurement point P4 in the axial direction. The ninth fiber-reinforced layer particularly contributes to the flexural rigidity of the tip portion 16.
The tenth sheet S10 is localized towards the butt end 18 of the shaft 4. The tenth sheet S10 is generally trapezoidal. The tenth sheet S10 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is the same as the axial direction. In other words, the direction in which each carbon fiber extends forms an angle of substantially 0° with the axial direction. The tenth sheet S10 is 50 mm wide, 135 mm long on the upper base, and 145 mm long on the lower base.
As stated above, the carbon fibers contained in the tenth sheet S10 are oriented substantially in the axial direction. Thus, the carbon fibers are oriented substantially in the axial direction also in the tenth fiber-reinforced layer. The tenth fiber-reinforced layer has a straight structure. Upon deflection of the shaft 4, the carbon fibers are subjected to a strong tension. The tension acts to prevent further deflection of the shaft 4. In other words, the carbon fibers contribute to the flexural rigidity of the shaft 4. As shown in FIG. 5, the tenth sheet S10 is located in a zone including the first measurement point P1 and not including the second, third, and fourth measurement points P2, P3, and P4 in the axial direction. Thus, the tenth fiber-reinforced layer is also located in the zone including the first measurement point P1 and not including the second, third, and fourth measurement points P2, P3, and P4 in the axial direction. The tenth fiber-reinforced layer particularly contributes to the flexural rigidity of the butt portion 12.
The eleventh sheet S11 extends over the entirety of the shaft 4. The eleventh sheet S11 is generally rectangular. The eleventh sheet S11 includes a plurality of carbon fibers arranged in parallel. The direction in which each of the carbon fibers extends is the same as the axial direction. In other words, the direction in which each carbon fiber extends forms an angle of substantially 0° with the axial direction. The eleventh sheet S11 is 30 mm wide and 340 mm long.
As stated above, the carbon fibers contained in the eleventh sheet S11 are oriented substantially in the axial direction. Thus, the carbon fibers are oriented substantially in the axial direction also in the eleventh fiber-reinforced layer. The eleventh fiber-reinforced layer has a straight structure. Upon deflection of the shaft 4, the carbon fibers are subjected to a strong tension. The tension acts to prevent further deflection of the shaft 4. In other words, the carbon fibers contribute to the flexural rigidity of the shaft 4.
In the shaft 4, the first, second, and eleventh fiber-reinforced layers extend from the butt end 18 to the tip end 20. These fiber-reinforced layers can contribute to the durability of the shaft 4.
In production of the shaft 4, the sheets shown in FIG. 5 are sequentially wound on a mandrel. A stack of the first and second sheets S1 and S2 may be wound on the mandrel. A stack of the third and fourth sheets S3 and S4 may be wound on the mandrel. A stack of the seventh and eighth sheets S7 and S8 may be wound on the mandrel. In addition to these sheets, another sheet may be wound on the mandrel. An example of the other sheet is a sheet containing glass fibers.
A wrapping tape is further wound on the above sheets. The mandrel, prepregs (sheets S1 to S11), and wrapping tape are heated by means such as an oven. The heating fluidizes the matrix resin. Further heating induces a curing reaction of the resin, as a result of which a shaped product is obtained. The shaped product is subjected to various processes such as end face machining, polishing, and painting, and thus the shaft 4 is completed.
FIG. 6 is a schematic diagram showing a method of measuring a flexural rigidity value EI of the shaft 4 of the racket 2 of FIG. 1. FIG. 6 shows the measurement at a measurement point P located at a distance L1 from the grip 8. In this measurement, the shaft 4 is supported from below at first and second support points 28 and 30. The distance from the measurement point P to the first support point 28 is 30 mm. The distance from the measurement point P to the second support point 30 is 30 mm. The measurement is performed using a universal testing machine (manufactured by INTESCO Co., Ltd. under the product name "2020"). The testing machine includes an indenter 32. The indenter 32 is in the shape of a hemisphere. The radius of curvature of the hemisphere is 20 mm. The indenter 32 gradually descends at a speed of 2 mm/min. The indenter 32 comes into contact with the measurement point P and presses the shaft 4. The pressing causes gradual deflection of the shaft 4. The amount of deflection B (m) of the shaft 4 is measured at the moment when the load applied to the shaft 4 by the indenter 32 reaches 100 N. The amount of deflection B is substituted into the following equation to calculate the flexural rigidity value EI (Nm²). $EI = F \cdot L 2^{3} / (48 \cdot B)$
In this equation, F is the load (N), L2 is the distance (m) between the two support points, and B is the amount of deflection (m). In the present embodiment, the load F is 100 N, and the distance L2 is 0.06 m. The flexural rigidity value EI of the shaft 4 may be measured in the absence of the grip 8 and frame 6.
In the present embodiment, the flexural rigidity values EI are measured at the first, second, third, and fourth measurement points P1, P2, P3, and P4. The distance L1 from the grip 8 to each measurement point is as follows.

First measurement point P1: 35 mm
Second measurement point P2: 75 mm
Third measurement point P3: 115 mm
Fourth measurement point P4: 155 mm

In the shaft 4 having the prepreg arrangement shown in FIG. 5, the flexural rigidity value EI(1) at the first measurement point P1 is 5.67 Nm², the flexural rigidity value EI(2) at the second measurement point P2 is 3.16 Nm², the flexural rigidity value EI(3) at the third measurement point P3 is 3.50 Nm², and the flexural rigidity value EI(4) at the fourth measurement point P4 is 5.05 Nm². The flexural rigidity distribution of the shaft 4 is shown in the graph of FIG. 7.
In the shaft 4, the flexural rigidity value EI(2) at the second measurement point P2 is smaller than the flexural rigidity value EI(1) at the first measurement point P1 and the flexural rigidity value EI(4) at the fourth measurement point P4. Furthermore, in the shaft 4, the flexural rigidity value EI(3) at the third measurement point P3 is smaller than the flexural rigidity value EI(1) at the first measurement point P1 and the flexural rigidity value EI(4) at the fourth measurement point P4. In the shaft 4, the following four inequality relations are established.

EI(2) < EI(1)
EI(2) < EI(4)
EI(3) < EI(1)
EI(3) < EI(4)

As shown in FIG. 7, the shaft 4 has a rigidity distribution with a downward-convex profile.
As previously stated, the fifth fiber-reinforced layer is located in a zone not including the first and second measurement points P1 and P2 but including the third and fourth measurement points P3 and P4 in the axial direction. The sixth fiber-reinforced layer is located in a zone including the first and second measurement points P1 and P2 and not including the third and fourth measurement points P3 and P4 in the axial direction. The ninth fiber-reinforced layer is located in a zone not including the first, second, and third measurement points P1, P2, and P3 but including the fourth measurement point P4 in the axial direction. The tenth fiber-reinforced layer is located in a zone including the first measurement point P1 and not including the second, third, and fourth measurement points P2, P3, and P4 in the axial direction. There is no fiber-reinforced layer having a straight structure and located in a zone not including the first measurement point P1 but including the second and third measurement points P2 and P3. There is no fiber-reinforced layer having a straight structure and located in a zone including the second and third measurement points P2 and P3 and not including the fourth measurement point P4. This layer structure can attain a flexural rigidity distribution with a downward-convex profile.
Another layer structure can also attain a flexural rigidity distribution with a downward-convex profile. Local presence of fiber-reinforced layers having a straight structure in the butt portion 12 and tip portion 16 can result in a flexural rigidity distribution with a downward-convex profile.
According to the findings obtained by the present inventors, the shaft 4 having a flexural rigidity distribution with a downward-convex profile is suitable for the smash. The player who performs the smash using the racket 2 can allow the shuttlecock to fly at a high speed.
The reason why the racket 2 according to the present invention is suitable for the smash is that the flexural rigidity distribution shown in FIG. 7 matches the deformation behavior of the shaft 4 in the smash. In the smash, the shaft 4 is significantly deflected in the in-plane direction (direction along the X-Y plane) and in the out-of-plane direction (Z direction). The racket 2 according to the present invention is suitable also for shots which are other than the smash and in which the shaft 4 is significantly deflected in both the in-plane and out-of-plane directions.
The flexural rigidity distribution can be adjusted by changing the location of the prepregs, the number of the prepregs, the width of the prepregs, the length of the prepregs, the angle of the fibers, the weight per unit area of the fibers, and the elastic modulus of the fibers.
From the viewpoint of the stability of the trajectory, the ratio (EI(2)/EI(1)) of the flexural rigidity value EI(2) to the flexural rigidity value EI(1) is preferably not greater than 0.95, more preferably not greater than 0.75, and particularly preferably not greater than 0.65. From the viewpoint of ease of production of the shaft 4, the ratio is preferably not less than 0.30.
From the viewpoint of the stability of the trajectory, the ratio (EI(2)/EI(4)) of the flexural rigidity value EI(2) to the flexural rigidity value EI(4) is preferably not greater than 0.95, more preferably not greater than 0.84, and particularly preferably not greater than 0.75. From the viewpoint of ease of production of the shaft 4, the ratio is preferably not less than 0.30.
From the viewpoint of the stability of the trajectory, the ratio (EI(3)/EI(1)) of the flexural rigidity value EI(3) to the flexural rigidity value EI(1) is preferably not greater than 0.95, more preferably not greater than 0.80, and particularly preferably not greater than 0.70. From the viewpoint of ease of production of the shaft 4, the ratio is preferably not less than 0.30.
From the viewpoint of the stability of the trajectory, the ratio (EI(3)/EI(4)) of the flexural rigidity value EI(3) to the flexural rigidity value EI(4) is preferably not greater than 0.95, more preferably not greater than 0.89, and particularly preferably not greater than 0.79. From the viewpoint of ease of production of the shaft 4, the ratio is preferably not less than 0.30.
The ratio (EI(1)/EI(4)) is preferably not less than 0.5 and not greater than 2.0. The ratio (EI(2)/EI(3)) is preferably not less than 0.5 and not greater than 2.0.
From the viewpoint of the stability of the trajectory, the difference (EI(2) - EI(1)) between the flexural rigidity value EI(2) and the flexural rigidity value EI(1) is preferably not greater than -0.30 Nm², more preferably not greater than -1.25 Nm², and particularly preferably not greater than -1.75 Nm². From the viewpoint of ease of production of the shaft 4, the difference is preferably not less than -5.0 Nm².
From the viewpoint of the stability of the trajectory, the difference (EI(2) - EI(4)) between the flexural rigidity value EI(2) and the flexural rigidity value EI(4) is preferably not greater than -0.30 Nm², more preferably not greater than -0.73 Nm², and particularly preferably not greater than -1.20 Nm². From the viewpoint of ease of production of the shaft 4, the difference is preferably not less than -5.0 Nm².
From the viewpoint of the stability of the trajectory, the difference (EI(3) - EI(1)) between the flexural rigidity value EI(3) and the flexural rigidity value EI(1) is preferably not greater than -0.30 Nm², more preferably not greater than -1.03 Nm², and particularly preferably not greater than -1.50 Nm². From the viewpoint of ease of production of the shaft 4, the difference is preferably not less than -5.0 Nm².
From the viewpoint of the stability of the trajectory, the difference (EI(3) - EI(4)) between the flexural rigidity value EI(3) and the flexural rigidity value EI(4) is preferably not greater than -0.30 Nm², more preferably not greater than -0.51 Nm², and particularly preferably not greater than -1.00 Nm². From the viewpoint of ease of production of the shaft 4, the difference is preferably not less than -5.0 Nm².
Preferred ranges of the flexural rigidity values EI are as follows.

EI(1): Not less than 3.5 Nm² and not greater than 7.5 Nm²
EI(2): Not less than 1.0 Nm² and not greater than 5.0 Nm²
EI(3): Not less than 1.0 Nm² and not greater than 5.0 Nm²
EI(4): Not less than 3.5 Nm² and not greater than 7.5 Nm²

FIG. 3 shows the first, second, third, and fourth measurement points P1, P2, P3, and P4. As is clear from FIG. 3, the inner diameter Di of the shaft 4 is substantially constant from the first measurement point P1 to the fourth measurement point P4. The shaft 4 can be produced using a mandrel having a simple shape. In the production of the shaft 4, the winding of the prepregs is easy. The shaft 4 may have a slight variation in the inner diameter Di due to, for example, production error. The ratio (Di1/Di2) of the maximum inner diameter Di1 to the minimum inner diameter Di2 in the zone from the first measurement point P1 to the fourth measurement point P4 is preferably not greater than 1.10, more preferably not greater than 1.05, and particularly preferably not greater than 1.03. The ratio (Di1/Di2) is ideally 1.00.
As is clear from FIG. 3, the outer diameter Do of the shaft 4 is substantially constant from the first measurement point P1 to the fourth measurement point P4. The shaft 4 can be produced using a mandrel having a simple shape. In the production of the shaft 4, the winding of the prepregs is easy. The shaft 4 may have a slight variation in the outer diameter Do due to, for example, production error. The ratio (Do1/Do2) of the maximum outer diameter Do1 to the minimum outer diameter Do2 in the zone from the first measurement point P1 to the fourth measurement point P4 is preferably not greater than 1.10, more preferably not greater than 1.05, and particularly preferably not greater than 1.03. The ratio (Do1/Di2) is ideally 1.00.
The ratio (W2/W1) of the weight W2 of the shaft 4 in the zone from the second measurement point P2 to the second measurement point P3 to the weight W1 of the shaft 4 in the zone from the first measurement point P1 to the second measurement point P2 is preferably not less than 0.95 and not greater than 1.05. Furthermore, the ratio (W2/W3) of the weight W2 to the weight W3 of the shaft 4 in the zone from the third measurement point P3 to the fourth measurement point P4 is preferably not less than 0.95 and not greater than 1.05. Such a shaft 4 is free from a significant local increase in weight. The player can swing the racket 2 including the shaft 4 without feeling discomfort. From this viewpoint, the ratios (W2/W1) and (W2/W3) are more preferably not less than 0.97 and not greater than 1.03 and particularly preferably not less than 0.98 and not greater than 1.02.
In the shaft 4, the flexural rigidity distribution is not adjusted by thickness adjustment. In the shaft 4, the flexural rigidity distribution is not adjusted by opening formation. Furthermore, the shaft 4 does not have any joint between tubes made of different materials. The shaft 4 is less likely to suffer stress concentration. The shaft 4 has excellent durability.

Examples

Hereinafter, the effect of the present invention will be demonstrated by examples. The present invention should not be restrictively interpreted based on the description of the examples.

[Example 1]

A badminton racket as shown in FIGS. 1 to 6 was produced. The flexural rigidity values EI of the racket are shown in Table 1 below and in FIG. 7.

[Examples 2 and 3 and Comparative Example]

Badminton rackets of Examples 2 and 3 and Comparative Example were obtained in the same manner as the badminton racket of Example 1, except for changing the prepreg arrangement. The flexural rigidity values EI of the rackets are shown in Table 1 below and in FIGS. 8 to 10.

[Practical Test]

A shuttlecock was launched by a launching machine. A player was caused to perform smash on the shuttlecock, and a video image of the trajectory of the shuttlecock was captured. The image was analyzed to calculate the speed of the shuttlecock. The measurement was conducted six times, and an average speed Vave was determined. The results are shown in Table 1 below.

[Table 1]

Table 1 Evaluation Results

	Comparative Example	Example 2	Example 1	Example 3
Flexural rigidity distribution	FIG. 8	FIG. 9	FIG. 7	FIG. 10
EI(1) (Nm²)	4.36	5.05	5.67	6.22
EI(2) (Nm²)	4.45	3.80	3.16	2.72
EI(3) (Nm²)	4.36	4.02	3.50	3.11
EI(4) (Nm²)	4.08	4.53	5.05	5.53
EI(2)/EI(1)	1.02	0.75	0.56	0.44
EI(3)/EI(1)	1.00	0.80	0.62	0.50
EI(2)/EI(4)	1.09	0.84	0.63	0.49
EI(3)/EI(4)	1.07	0.89	0.69	0.56
EI(2) - EI(1) (Nm²)	0.09	-1.25	-2.51	-3.50
EI(3) - EI(1) (Nm²)	0.00	-1.03	-2.17	-3.11
EI(2) - EI(4) (Nm²)	0.37	-0.73	-1.89	-2.81
EI(3) - EI(4) (Nm²)	0.28	-0.51	-1.55	-2.42
Vave (m/s)	19.7	21.0	21.6	22.4

As is clear from Table 1, the speed of the shuttlecock in the smash is high when any of the badminton rackets of Examples is used. The evaluation results demonstrate the superiority of the present invention.

Industrial Applicability

The badminton racket according to the present invention is suitable for players who play in a style involving frequent use of smash. The racket is suitable also for players who play in other styles.

Reference Signs List

2: badminton racket
4: shaft
6: frame
8: grip
10: string
12: butt portion
14: middle portion
16: tip portion
18: butt end
20: tip end
26: face
S1: first sheet
S2: second sheet
S3: third sheet
S4: fourth sheet
S5: fifth sheet
S6: sixth sheet
S7: seventh sheet
S8: eighth sheet

Claims

A badminton racket comprising:
a grip;

a shaft having a portion inserted into the grip, the inserted portion including a butt end of the shaft and a vicinity of the butt end; and

a frame attached to the shaft in a vicinity of a tip end of the shaft, wherein

a flexural rigidity value EI(2) of the shaft at a second measurement point located at a distance of 75 mm from the grip is smaller than a flexural rigidity value EI(1) of the shaft at a first measurement point located at a distance of 35 mm from the grip and a flexural rigidity value EI(4) of the shaft at a fourth measurement point located at a distance of 155 mm from the grip, and

a flexural rigidity value EI(3) of the shaft at a third measurement point located at a distance of 115 mm from the grip is smaller than the flexural rigidity value EI(1) and the flexural rigidity value EI(4).
The racket according to claim 1, wherein a ratio (EI(2)/EI(1)) of the flexural rigidity value EI(2) to the flexural rigidity value EI(1) is not greater than 0.95.
The racket according to claim 1 or 2, wherein a ratio (EI(2)/EI(4)) of the flexural rigidity value EI(2) to the flexural rigidity value EI(4) is not greater than 0.95.
The racket according to any one of claims 1 to 3, wherein a ratio (EI(3)/EI(1)) of the flexural rigidity value EI(3) to the flexural rigidity value EI(1) is not greater than 0.95.
The racket according to any one of claims 1 to 4, wherein a ratio (EI(3)/EI(4)) of the flexural rigidity value EI(3) to the flexural rigidity value EI(4) is not greater than 0.95.
The racket according to any one of claims 1 to 5, wherein a difference (EI(2) - EI(1)) between the flexural rigidity value EI(2) and the flexural rigidity value EI(1) is not greater than -0.30 Nm².
The racket according to any one of claims 1 to 6, wherein a difference (EI(2) - EI(4)) between the flexural rigidity value EI(2) and the flexural rigidity value EI(4) is not greater than -0.30 Nm².
The racket according to any one of claims 1 to 7, wherein a difference (EI(3) - EI(1)) between the flexural rigidity value EI(3) and the flexural rigidity value EI(1) is not greater than -0.30 Nm².
The racket according to any one of claims 1 to 8, wherein a difference (EI(3) - EI(4)) between the flexural rigidity value EI(3) and the flexural rigidity value EI(4) is not greater than -0.30 Nm².
The racket according to any one of claims 1 to 9, wherein
the shaft has a hollow structure, and

an inner diameter of the shaft is substantially constant from the first measurement point to the fourth measurement point.
The racket according to any one of claims 1 to 10, wherein an outer diameter of the shaft is substantially constant from the first measurement point to the fourth measurement point.
The racket according to any one of claims 1 to 11, wherein
a ratio (W2/W1) of a weight W2 of the shaft in a zone from the second measurement point to the third measurement point to a weight W1 of the shaft in a zone from the first measurement point to the second measurement point is not less than 0.95 and not greater than 1.05, and

a ratio (W2/W3) of the weight W2 of the shaft in the zone from the second measurement point to the third measurement point to a weight W3 of the shaft in a zone from the third measurement point to the fourth measurement point is not less than 0.95 and not greater than 1.05.
The racket according to any one of claims 1 to 12, wherein the shaft includes:
(1) a fiber-reinforced layer located in a zone including the first measurement point and not including the third measurement point in an axial direction of the shaft, the fiber-reinforced layer including a plurality of reinforcing fibers oriented substantially in the axial direction; and

(2) another fiber-reinforced layer located in a zone not including the second measurement point but including the fourth measurement point in the axial direction, the other fiber-reinforced layer including a plurality of reinforcing fibers oriented substantially in the axial direction.