racket

Abstract

A racket includes a frame 4. The frame 4 includes a head 14. The head 14 includes a first high-elasticity layer 37a and a second high-elasticity layer 37b. Each of these high-elasticity layers 37 is positioned on an inner side in the head 14 in a thickness direction of the head 14. Each high-elasticity layer 37 includes straight-type reinforcement fibers. In the frame 4, a ratio (G2 / G1) of a ball-hitting face stiffness value G2 to a side pressure stiffness value G1 is greater than or equal to 3.20. In the frame 4, a ratio (Tf / Wf) of a thickness Tf of the frame 4 to a width Wf of the frame 4 is preferably greater than or equal to 2.0.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of Japanese Patent Application No. 2024-086055, filed on May 28, 2024, the entire disclosure of which is incorporated herein by reference.BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0002] The present specification discloses a racket that is suitable for use in, for example, tennis, soft tennis, squash, padel, and badminton.DESCRIPTION OF THE RELATED ART

[0003] In tennis, a ball is hit by a racket. As a result of the hitting, the kinetic energy of the racket is transferred to the ball, and the ball flies. In a case where a ball is hit by a tennis racket having excellent repulsion performance, the ball can fly at a high speed. In a game of tennis, a high flying speed of the ball is advantageous. Japanese Laid-Open Patent Application Publication No. H05-15617 discloses a tennis racket having excellent repulsion performance.

[0004] Tennis players demand for further improvement in repulsion performance. It is an intention of the applicant of the present application to provide a racket having excellent repulsion performance.SUMMARY OF THE INVENTION

[0005] A racket disclosed in the present specification includes a frame including a head. In the frame, a ratio (G2 / G1) of a ball-hitting face stiffness value G2 to a side pressure stiffness value G1 is greater than or equal to 3.20. In this connection it should be noted that the head may form the contour of a face, with a front view shape of the head being at least substantially an ellipse, wherein the ball hitting face stiffness value is measured in a plane extending in parallel to the contour of the face; and / or wherein the side pressure stiffness value G1 is measured in a plane extending perpendicular to the contour of the face at a side of the head.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 is a front view showing a tennis racket according to one embodiment. FIG. 2 is a right-side view showing the tennis racket of FIG. 1. FIG. 3 is an exploded view showing part of the tennis racket of FIG. 1 in an enlarged manner. FIG. 4 is a perspective view showing part of a manufacturing process of the racket of FIG. 1. FIG. 5 is an enlarged sectional view taken along line V-V of FIG. 1. FIG. 6 is an enlarged sectional view taken along line VI-VI of FIG. 5. FIG. 7 is an enlarged view showing part of a prepreg for first fiber reinforced layers of a head of FIG. 6. FIG. 8 is an enlarged view showing part of a prepreg for second fiber reinforced layers of the head of FIG. 6. FIG. 9 is a perspective view showing part of a high-elasticity layer of the head of FIG. 6. FIG. 10 is an enlarged view showing part of a prepreg for the high-elasticity layer of FIG. 9. FIG. 11 is a graph showing a relationship between a side pressure stiffness value and a ball-hitting face stiffness value of a frame of the tennis racket of FIG. 1. FIG. 12 is a front view showing a method of measuring the side pressure stiffness value of the frame of the tennis racket of FIG. 1. FIG. 13A is a plan view showing a method of measuring the ball-hitting face stiffness value of the frame of the tennis racket of FIG. 1, and FIG. 13B is a front view thereof. DETAILED DESCRIPTION

[0007] Hereinafter, preferred embodiments are described in detail with reference to the drawings as necessary.

[0008] FIGS. 1 to 3 each show a tennis racket 2. The racket 2 includes a frame 4, a grip 6, an end cap 8, a grommet 10, and a string 12. The racket 2 can be used in regulation-ball tennis. In FIGS. 1 and 2, an arrow X represents the width direction of the racket 2; an arrow Y represents the axial direction of the racket 2; and an arrow Z represents the thickness direction of the racket 2. In FIG. 2, the illustration of the grommet 10 and the string 12 is omitted.

[0009] The frame 4 includes a head 14, a first throat 16a, a second throat 16b, and a shaft 18. The head 14 forms the contour of a face 20 (the face 20 will be described below in detail). The front view shape of the head 14 is substantially an ellipse. The major axis direction of the ellipse coincides with the axial direction Y of the racket 2. The minor axis direction of the ellipse coincides with the width direction X of the racket 2. In FIG. 1, reference sign Ch indicates the center of the head 14. The first throat 16a extends from the head 14. The second throat 16b extends from the head 14. The second throat 16b merges with the first throat 16a at a position away from the head 14. The shaft 18 extends from the position where the two throats 16 merge together. The shaft 18 is continuous with the throats 16. A portion of the head 14, the portion being positioned between the two throats 16, is a yoke 22. The frame 4 is hollow.

[0010] The main material of the frame 4 is a fiber reinforced resin. The fiber reinforced resin includes a resin matrix and a large number of reinforcement fibers. The frame 4 includes a plurality of fiber reinforced layers. The fiber reinforced layers will be described below in detail.

[0011] Examples of the base resin of the frame 4 include: thermosetting resins such as epoxy resin, bismaleimide resin, polyimide, and phenolic resin; and thermoplastic resins such as polyether ether ketone, polyether sulphone, polyether imide, polyphenylene sulfide, polyamide, and polypropylene. Epoxy resin is a particularly suitable resin for the frame 4.

[0012] Examples of the reinforcement fibers of the frame 4 include carbon fibers, metal fibers, glass fibers, and aramid fibers. Carbon filament fibers are particularly suitable fibers for the frame 4. Multiple types of fibers may be used in combination as the reinforcement fibers.

[0013] As shown in FIGS. 2 and 3, the head 14 includes a groove 24. The groove 24 is recessed from the outer peripheral surface of the head 14. The groove 24 is formed over substantially the entire periphery of the head 14, except the yoke 22. The head 14 further includes a plurality of holes 26. The plurality of holes 26 are arranged over substantially the entire periphery of the head 14.

[0014] The grip 6 is formed by a tape wound around the shaft 18. The grip 6 suppresses slip between a hand of a player and the racket 2 when the racket 2 is swung by the player.

[0015] As shown in FIG. 3, the grommet 10 includes a base 28 and a plurality of pipes 30. The base 28 is belt-shaped. Each pipe 30 is integrated with the base 28. Each pipe 30 rises from the base 28. A typical material of the grommet 10 is a synthetic resin that is softer than the frame 4. The tennis racket 2 may include a plurality of grommets 10. The number of pipes 30 of each grommet 10 may be one.

[0016] The grommet 10 is attached to the head 14. In a state where the grommet 10 is attached to the head 14, the base 28 is accommodated in the groove 24. The base 28 may partly protrude from the groove 24. Further, in the state where the grommet 10 is attached to the head 14, the pipes 30 extend through the respective holes 26.

[0017] As shown in FIG. 1, the string 12 is stretched on the head 14. The string 12 is stretched in the width direction X and the axial direction Y. The string 12 extends through the pipes 30. The string 12 forms a large number of threads 32. Of the string 12, portions extending in the width direction X are referred to as transverse threads 32a. Of the string 12, portions extending in the axial direction Y are referred to as longitudinal threads 32b. The face 20 is formed by a plurality of transverse threads 32a and a plurality of longitudinal threads 32b. The face 20 generally extends along an X-Y plane. FIG. 1 shows part of the face 20. The face 20 may be formed by two or more strings 12.

[0018] Hereinafter, one example of a method of manufacturing the tennis racket 2 is described with reference to FIG. 4. In this manufacturing method, a mandrel, a tube, and a plurality of prepregs 34 are prepared. Each prepreg 34 is made from a plurality of reinforcement fibers arranged in parallel and a matrix resin. In this manufacturing method, first, the mandrel is inserted into the tube. The prepregs 34 are sequentially wound around the tube. As a result of the winding, the prepregs 34 have a tubular shape. FIG. 4 shows a tubular prepreg 34p and a sheet-shaped prepreg 34s. In FIG. 4, the illustration of the mandrel and the tube is omitted.

[0019] By rotating the mandrel, the prepreg 34s is wound around the prepreg 34p. As a result of the winding, the prepreg 34s has a tubular shape, and thus a layered body 36 is obtained. Another prepreg 34 is further wound around the layered body 36 as necessary. A plurality of sheet-shaped prepregs 34s may be layered one on top of another, which may be then wound around the mandrel or the prepreg 34p. In FIG. 4, an arrow A1 represents the longitudinal direction of the layered body 36.

[0020] After the mandrel is removed from the tube, the tube and the layered body 36 are set in a mold. In the mold, gas is injected into the tube, thereby inflating the tube. The prepregs 34 are pressed against the cavity surface of the mold by the inflation. The prepregs 34 are heated to cure the matrix resin. A molded article is obtained by the curing. The molded article has a reverse shape of that of the cavity surface.

[0021] The holes 26 are drilled in the molded article. The molded article is further subjected to treatments such as surface polishing and painting, and thereby the frame 4 is obtained. Components such as the grip 6 and the grommet 10 are attached to the frame 4. Further, the string 12 is stretched on the frame 4, and thus the tennis racket 2 is completed.

[0022] FIG. 5 is an enlarged sectional view taken along line V-V of FIG. 1. A cross section shown in FIG. 5 extends along a plane that is perpendicular to the axial direction of the frame 4 and that passes the center Ch of the head 14. The head 14 includes a first high-elasticity layer 37a and a second high-elasticity layer 37b (see also FIG. 2). Each high-elasticity layer 37 is positioned on the inner side in the head 14 in the thickness direction.

[0023] FIG. 6 is an enlarged sectional view taken along line VI-VI of FIG. 5. FIG. 6 shows the head 14. The head 14 includes a plurality of fiber reinforced layers 38. In the present embodiment, the head 14 includes a plurality of first fiber reinforced layers 38a, a plurality of second fiber reinforced layers 38b, and a plurality of third fiber reinforced layers 38c. In the present embodiment, the number of first fiber reinforced layers 38a is five; the number of second fiber reinforced layers 38b is five; and the number of third fiber reinforced layers 38c is four. The first fiber reinforced layers 38a and the second fiber reinforced layers 38b are arranged alternately in the thickness direction of the head 14 (i.e., the vertical direction in FIG. 6). The first high-elasticity layer 37a is formed by the plurality of third fiber reinforced layers 38c. The first high-elasticity layer 37a is positioned on the inner side in the head 14 in the thickness direction (in FIG. 6, the lower side). Although not illustrated, the shape of the second high-elasticity layer 37b is mirror-symmetrical to the shape of the first high-elasticity layer 37a.

[0024] FIG. 7 shows a first prepreg 34a for the first fiber reinforced layers 38a. The first prepreg 34a includes a matrix 40 and a plurality of first reinforcement fibers 42a arranged in parallel. Each first reinforcement fiber 42a is inclined relative to the longitudinal direction A1. In FIG. 7, an arrow θa represents an inclination angle (absolute value) of the first reinforcement fiber 42a relative to the longitudinal direction A1. The inclination angle θa is greater than or equal to 30° and less than or equal to 60°. In the present specification, a reinforcement fiber 42 having an inclination angle of greater than or equal to 30° and less than or equal to 60° is referred to as a "bias-type reinforcement fiber". The first fiber reinforced layers 38a include bias-type reinforcement fibers.

[0025] FIG. 8 shows a second prepreg 34b for the second fiber reinforced layers 38b. The second prepreg 34b includes the matrix 40 and a plurality of second reinforcement fibers 42b arranged in parallel. Each second reinforcement fiber 42b is inclined relative to the longitudinal direction A1. The direction in which each second reinforcement fiber 42b is inclined is opposite to the direction (shown in FIG. 7) in which each first reinforcement fiber 42a is inclined. In FIG. 8, an arrow θb represents an inclination angle (absolute value) of the second reinforcement fiber 42b relative to the longitudinal direction A1. The inclination angle θb is greater than or equal to 30° and less than or equal to 60°. Each second reinforcement fiber 42b is a "bias-type reinforcement fiber". The second fiber reinforced layers 38b include bias-type reinforcement fibers.

[0026] FIG. 9 shows part of the high-elasticity layer 37. As previously described, the number of third fiber reinforced layers 38c included in the high-elasticity layer 37 is four. These third fiber reinforced layers 38c are obtained by folding a sheet-shaped third prepreg 34c in such a manner that the sheet-shaped third prepreg 34c is wound around itself.

[0027] FIG. 10 shows the third prepreg 34c for the high-elasticity layer 37. The third prepreg 34c includes the matrix 40 and a plurality of third reinforcement fibers 42c arranged in parallel. Each third reinforcement fiber 42c extends in the longitudinal direction A1. Each third reinforcement fiber 42c has a zero inclination angle (absolute value) relative to the longitudinal direction A1. Each third reinforcement fiber 42c may be slightly inclined relative to the longitudinal direction A1. In the present specification, a reinforcement fiber 42 having an inclination angle (absolute value) of less than or equal to 10° relative to the longitudinal direction A1 is referred to as a "straight-type reinforcement fiber". The third fiber reinforced layers 38c include straight-type reinforcement fibers. In other words, the high-elasticity layer 37 includes the straight-type reinforcement fibers.

[0028] A graph in FIG. 11 shows a relationship between a side pressure stiffness value G1 and a ball-hitting face stiffness value G2 of the frame 4. In this graph, a straight line denoted by reference sign S1 is expressed by a mathematical formula shown below. G 2 = 3.20 ⋅ G 1 In the case of the tennis racket 2 plotted on the straight line S1 or plotted not on the straight line S1 but above the straight line S1, a ratio (G2 / G1) is greater than or equal to 3.20. This racket 2 satisfies a mathematical formula (1) shown below. G 2 ≥ 3.20 ⋅ G 1 In the case of the tennis racket 2 satisfying the above mathematical formula (1), the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large.

[0029] In the case of the tennis racket 2 satisfying the mathematical formula (1), as mentioned above, the side pressure stiffness value G1 is relatively small. According to findings obtained by the inventor of the present invention, in a vibration mode excited during a collision of the racket 2 with a tennis ball, the mode amplitude of the tennis ball is relatively large. Therefore, the speed of the tennis ball in the traveling direction at the end of the collision is high. In other words, the racket 2 whose side pressure stiffness value G1 is relatively small has excellent repulsion performance.

[0030] In the case of the tennis racket 2 satisfying the mathematical formula (1), as mentioned above, the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, in a vibration mode excited during a collision of the racket 2 with a tennis ball, the mode amplitude of the tennis ball is relatively large. Therefore, the speed of the tennis ball in the traveling direction at the end of the collision is high. In other words, the racket 2 whose ball-hitting face stiffness value G2 is relatively large has excellent repulsion performance.

[0031] As previously described, each high-elasticity layer 37 is positioned on the inner side in the head 14 in the thickness direction. At the time of measuring the ball-hitting face stiffness value G2 of the tennis racket 2, force in the thickness direction (Z-direction) is applied to the head 14. The force causes the head 14 to bend relative to the shaft 18 in the thickness direction. Since the third reinforcement fibers 42c are straight-type reinforcement fibers, due to the bending, a great tensile stress occurs on the third reinforcement fibers 42c of the high-elasticity layer 37. The third reinforcement fibers 42c suppress the bending. The high-elasticity layer 37 contributes to achieving a large ball-hitting face stiffness value G2.

[0032] At the time of measuring the side pressure stiffness value G1 of the tennis racket 2, force in the width direction (X-direction) is applied to the head 14. The force causes the head 14 to bend inward in the width direction. A stress that occurs on the third reinforcement fibers 42c due to the bending is small. The third reinforcement fibers 42c do not hinder the bending deformation. The tennis racket 2 including the high-elasticity layer 37 can achieve a small side pressure stiffness value G1.

[0033] FIG. 12 shows a method of measuring the side pressure stiffness value G1. In FIG. 12, the tennis racket 2 is placed on a base 44, which is a rigid body. The width direction X of the racket 2 coincides with the vertical direction. The axial direction Y of the racket 2 coincides with the horizontal direction. A plate 46, which is a rigid body, is lowered, and thereby a load is applied to the racket 2. A displacement (cm) of the plate 46 is measured from when the load is 25 kgf to when the load is 50 kgf. The side pressure stiffness value G1 is calculated by dividing the load difference 25 kgf by the displacement (cm). The side pressure stiffness value G1 is measured in a state where the string 12 is removed from the frame 4.

[0034] In light of repulsion performance, the side pressure stiffness value G1 is preferably less than or equal to 90 kgf / cm, more preferably less than or equal to 80 kgf / cm, and particularly preferably less than or equal to 75 kgf / cm. The side pressure stiffness value G1 of the tennis racket 2 suitable for practical use is greater than or equal to 20 kgf / cm.

[0035] FIGS. 13A and 13B show a method of measuring the ball-hitting face stiffness value G2. In this measurement, a first bar 48a, a second bar 48b, and a third bar 48c are prepared. The material of these bars 48 is steel. Each bar 48 has a circular cross-sectional shape having a radius of 10.0 mm. Each bar 48 extends in the width direction X. The distance between the first bar 48a and the third bar 48c in the axial direction is 170 mm, and the distance between the third bar 48c and the second bar 48b in the axial direction is 170 mm. The first bar 48a is positioned at the top of the head 14. The racket 2 is placed on the first bar 48a and the second bar 48b. The width direction X and the axial direction Y of the racket 2 coincide with the horizontal direction. The third bar 48c is lowered, and thereby a load is applied to the tennis racket 2. A displacement (cm) of the third bar 48c is measured from when the load is 25 kgf to when the load is 50 kgf. The ball-hitting face stiffness value G2 is calculated by dividing the load different 25 kgf by the displacement (cm). The ball-hitting face stiffness value G2 is measured in a state where the string 12 is removed from the frame 4.

[0036] In light of repulsion performance, the ball-hitting face stiffness value G2 is preferably greater than or equal to 100 kgf / cm, more preferably greater than or equal to 200 kgf / cm, and particularly preferably greater than or equal to 250 kgf / cm. The ball-hitting face stiffness value G2 of the tennis racket 2 suitable for practical use is less than or equal to 500 kgf / cm.

[0037] In FIG. 2, an arrow Lh represents the length of each high-elasticity layer 37 in the axial direction. In the present embodiment, the length Lh is a distance in the axial direction from a top Pt of the head 14 to an end Ed of the high-elasticity layer 37. The length Lh is preferably greater than or equal to 170 mm, more preferably greater than or equal to 250 mm, and particularly preferably greater than or equal to 340 mm for the reason that a large ball-hitting face stiffness value G2 can be achieved with such setting of the length Lh.

[0038] In FIG. 5, an arrow Tf represents the thickness of the frame 4, and an arrow Wf represents the width of the frame 4. A ratio (Tf / Wf) of the thickness Tf to the width Wf is preferably greater than or equal to 2.0. If the ratio (Tf / Wf) of the tennis racket 2 is within this range, a large ratio (G2 / G1) can be readily achieved. In light of this, the ratio (Tf / Wf) is more preferably greater than or equal to 2.2, yet more preferably greater than or equal to 2.4, and particularly preferably greater than or equal to 2.8. The ratio (Tf / Wf) of the tennis racket 2 suitable for practical use is less than or equal to 4.0.

[0039] The thickness Tf is preferably greater than or equal to 20.0 mm, more preferably greater than or equal to 26.0 mm, yet more preferably greater than or equal to 28.5 mm, and particularly preferably greater than or equal to 33.0 mm for the reason that a large ratio (G2 / G1) can be readily achieved with such setting of the thickness Tf. The thickness Tf of the tennis racket 2 suitable for practical use is less than or equal to 40.0 mm.

[0040] In a graph of FIG. 11, a straight line denoted by reference sign S2 is expressed by a mathematical formula shown below. G 2 = 3.55 ⋅ G 1 In the case of the tennis racket 2 plotted on the straight line S2 or plotted not on the straight line S2 but above the straight line S2, the ratio (G2 / G1) is greater than or equal to 3.55. In the case of this tennis racket 2, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, this tennis racket 2 has more excellent repulsion performance. In other words, the tennis racket 2 satisfying a mathematical formula shown below has more excellent repulsion performance. G 2 ≥ 3.55 ⋅ G 1

[0041] In the graph of FIG. 11, a straight line denoted by reference sign S3 is expressed by a mathematical formula shown below. G 2 = 3.90 ⋅ G 1 In the case of the tennis racket 2 plotted on the straight line S3 or plotted not on the straight line S3 but above the straight line S3, the ratio (G2 / G1) is greater than or equal to 3.90. In the case of this tennis racket 2, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, this tennis racket 2 has more excellent repulsion performance. In other words, the tennis racket 2 satisfying a mathematical formula shown below has more excellent repulsion performance. G 2 ≥ 3.90 ⋅ G 1

[0042] In the graph of FIG. 11, a straight line denoted by reference sign S4 is expressed by a mathematical formula shown below. G 2 = 4.10 ⋅ G 1 In the case of the tennis racket 2 plotted on the straight line S4 or plotted not on the straight line S4 but above the straight line S4, the ratio (G2 / G1) is greater than or equal to 4.10. In the case of this tennis racket 2, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, this tennis racket 2 has extremely excellent repulsion performance. In other words, the tennis racket 2 satisfying a mathematical formula shown below has extremely excellent repulsion performance. G 2 ≥ 4.10 ⋅ G 1[Evaluation][Sample 1]

[0043] A tennis racket model for simulation was fabricated. The specifications of the tennis racket model were as shown below. The width of each of the first prepreg and the second prepreg: 250 mm The thickness Tf of the frame: 29.9 mm The elastic modulus of the reinforcement fibers in the high-elasticity layer: 80 tf / mm 2< The thickness of the high-elasticity layer: 0.825 mm [Samples 2 to 120]

[0044] Tennis racket models of samples 2 to 120 were fabricated in the same manner as the sample 1, except that the specifications of the tennis racket models were varied as shown in Tables 1 to 6 below.[Simulation]

[0045] The side pressure stiffness value G1 and the ball-hitting face stiffness value G2 of each sample were calculated by simulation. Further, a tennis ball was brought into collision with each sample, and the speed of the tennis ball when rebounding off the sample was calculated by simulation. The results are shown in Tables 1 to 6 below. Table 1 Evaluation ResultsPrepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 125033.8800.825120284.295966Sample 225033.8801.650123294.245962Sample 337533.8401.650182444.145953Sample 437533.8800.825192454.275964Sample 537533.8801.650203454.515986Sample 650031.2801.650250614.105950Sample 750033.8400.825240584.145953Sample 850033.8401.650255604.255963Sample 950033.8800.825264594.475983Sample 1050033.8801.650282614.625996Sample 1162531.2801.650297724.135952Sample 1262533.8400.825289704.135952Sample 1362533.8401.650310724.315968Sample 1462533.8800.825315714.445979Sample 1562533.8801.650341734.676000Sample 1675031.2801.650345844.115950Sample 1775033.8400.825339824.135953Sample 1875033.8401.650365844.355971Sample 1975033.8800.825367844.375973Sample 2075033.8801.650400864.655998 Table 2 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 2125033.8400.825107273.965938Sample 2225033.8401.650110274.075948Sample 2337531.2800.825173443.935935Sample 2437531.2801.650181454.025943Sample 2537533.8400.825173443.935935Sample 2650031.2800.825237594.025942Sample 2762531.2800.825279713.935935Sample 2875031.2800.825321823.915933 Table 3 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 2925031.2401.650101283.615906Sample 3025031.2800.825110293.795923Sample 3125031.2801.650113303.775920Sample 3237528.6800.825156443.555901Sample 3337528.6801.650163453.625908Sample 3437531.2400.825157433.655910Sample 3537531.2401.650164443.735917Sample 3637533.800.825128363.565902Sample 3737533.801.650128363.565902Sample 3850028.6800.825211593.585904Sample 3950028.6801.650221603.685913Sample 4050031.2400.825216583.725917Sample 4150031.2401.650227593.855928Sample 4262528.6801.650259713.655910Sample 4362531.2400.825257703.675912Sample 4462531.2401.650271713.825925Sample 4562533.800.825241663.655910Sample 4662533.801.650241663.655910Sample 4775028.6801.650297823.625908Sample 4875031.2400.825298813.685913Sample 4975031.2401.650316833.815924Sample 5075033.800.825293793.715915Sample 5175033.801.650293793.715915 Table 4 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 5225028.6400.82591283.255875Sample 5325028.6401.65093293.215871Sample 5425028.6800.825101293.485895Sample 5525028.6801.650104303.475894Sample 5625031.2400.82598283.505897Sample 5737526801.650146453.245875Sample 5837528.6400.825141433.285878Sample 5937528.6401.650147443.345883Sample 6050026800.825187583.225873Sample 6150026801.650196593.325881Sample 6250028.6400.825192583.315880Sample 6350028.6401.650201593.415889Sample 6450033.800.825190543.525899Sample 6550033.801.650190543.525899Sample 6662526801.650227693.295879Sample 6762528.6400.825225693.265876Sample 6862528.6401.650237703.395887Sample 6962528.6800.825245703.505897Sample 7062531.200.825216663.275877Sample 7162531.201.650216663.275877Sample 7275026801.650257803.215872Sample 7375028.6400.825258803.235873Sample 7475028.6401.650273813.375886Sample 7575028.6800.825279813.445892Sample 7675031.200.825259783.325881Sample 7775031.201.650259783.325881 Table 5 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 782502600.82553242.215783Sample 792502601.65053242.215783Sample 8025026400.82583292.865841Sample 8125026401.65086302.875841Sample 8225026800.82593303.105862Sample 8325026801.65095313.065859Sample 8425028.600.82558232.525811Sample 8525028.601.65058232.525811Sample 8625031.200.82563222.865841Sample 8725031.201.65063222.865841Sample 8825033.800.82567213.195870Sample 8925033.801.65067213.195870Sample 903752600.82593392.385799Sample 913752601.65093392.385799Sample 9237526400.825126432.935847Sample 9337526401.650131442.985851Sample 9437526800.825140443.185869Sample 9537528.600.825106392.725828Sample 9637528.601.650106392.725828Sample 9737531.200.825118383.115862Sample 9837531.201.650118383.115862Sample 995002600.825133542.465806Sample 1005002601.650133542.465806Sample 10150026400.825168572.955848Sample 10250026401.650177583.055858Sample 10350028.600.825154542.855840Sample 10450028.601.650154542.855840Sample 10550031.200.825172543.195869 Table 6 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 10650031.201.650172543.195869Sample 1076252600.825162642.535812Sample 1086252601.650162642.535812Sample 10962526400.825195672.915845Sample 11062526401.650206683.035856Sample 11162526800.825214683.155866Sample 11262528.600.825189652.915845Sample 11362528.601.650189652.915845Sample 1147502600.825191752.555813Sample 1157502601.650191752.555813Sample 11675026400.825221772.875842Sample 11775026401.650234783.005853Sample 11875026800.825240783.085860Sample 11975028.600.825225772.925846Sample 12075028.601.650225772.925846

[0046] In the case of the samples shown in Table 1, the ratio (G2 / G1) is greater than or equal to 4.10. The speed of the ball rebounding off each sample in Table 1 is greater than or equal to 5950 mm / s.

[0047] In the case of the samples shown in Table 2, the ratio (G2 / G1) is greater than or equal to 3.90 and less than 4.10. The speed of the ball rebounding off each sample in Table 2 is greater than or equal to 5930 mm / s and less than 5950 mm / s.

[0048] In the case of the samples shown in Table 3, the ratio (G2 / G1) is greater than or equal to 3.55 and less than 3.90. The speed of the ball rebounding off each sample in Table 3 is greater than or equal to 5900 mm / s and less than 5930 mm / s.

[0049] In the case of the samples shown in Table 4, the ratio (G2 / G1) is greater than or equal to 3.20 and less than 3.55. The speed of the ball rebounding off each sample in Table 4 is greater than or equal to 5870 mm / s and less than 5900 mm / s.

[0050] In the case of the samples shown in Tables 5 and 6, the ratio (G2 / G1) is less than 3.20. The speed of the ball rebounding off each sample in Tables 5 and 6 is less than 5870 mm / s.

[0051] These evaluation results clearly indicate the superiority of the tennis rackets with a large ratio (G2 / G1).[Disclosure Items]

[0052] The following items each disclose a preferred embodiment.[Item 1]

[0053] A racket including a frame including a head. In the frame, a ratio (G2 / G1) of a ball-hitting face stiffness value G2 to a side pressure stiffness value G1 is greater than or equal to 3.20.[Item 2]

[0054] The racket according to item 1, wherein the head includes a high-elasticity layer that is positioned on an inner side in the head in a thickness direction of the head, the high-elasticity layer including straight-type reinforcement fibers.[Item 3]

[0055] The racket according to item 2, wherein the high-elasticity layer is present in a cross section that extends along a plane that is perpendicular to an axial direction of the frame and that passes a center of the head.[Item 4]

[0056] The racket according to any one of items 1 to 3, wherein in a cross section that extends along a plane that is perpendicular to an axial direction of the frame and that passes a center of the head, a ratio (Tf / Wf) of a thickness Tf of the frame to a width Wf of the frame is greater than or equal to 2.0.[Item 5]

[0057] The racket according to any one of items 1 to 4, wherein the ratio (G2 / G1) is greater than or equal to 3.55.[Item 6]

[0058] The racket according to item 5, wherein the ratio (G2 / G1) is greater than or equal to 3.90.[Item 7]

[0059] The racket according to item 6, wherein the ratio (G2 / G1) is greater than or equal to 4.10.[Item 8]

[0060] The racket according to any one of items 1 to 7, wherein the side pressure stiffness value G1 is less than or equal to 90 kgf / cm.[Item 9]

[0061] The racket according to any one of items 1 to 8, wherein the ball-hitting face stiffness value G2 is greater than or equal to 100 kgf / cm.

[0062] The racket as described above is suitable also for use in, for example, soft tennis, squash, padel, and badminton. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention.

Examples

Embodiment Construction

[0007]Hereinafter, preferred embodiments are described in detail with reference to the drawings as necessary.

[0008]FIGS. 1 to 3 each show a tennis racket 2. The racket 2 includes a frame 4, a grip 6, an end cap 8, a grommet 10, and a string 12. The racket 2 can be used in regulation-ball tennis. In FIGS. 1 and 2, an arrow X represents the width direction of the racket 2; an arrow Y represents the axial direction of the racket 2; and an arrow Z represents the thickness direction of the racket 2. In FIG. 2, the illustration of the grommet 10 and the string 12 is omitted.

[0009]The frame 4 includes a head 14, a first throat 16a, a second throat 16b, and a shaft 18. The head 14 forms the contour of a face 20 (the face 20 will be described below in detail). The front view shape of the head 14 is substantially an ellipse. The major axis direction of the ellipse coincides with the axial direction Y of the racket 2. The minor axis direction of the ellipse coincides with the width direction X...

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of Japanese Patent Application No. 2024-086055, filed on May 28, 2024, the entire disclosure of which is incorporated herein by reference.BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0002] The present specification discloses a racket that is suitable for use in, for example, tennis, soft tennis, squash, padel, and badminton.DESCRIPTION OF THE RELATED ART

[0003] In tennis, a ball is hit by a racket. As a result of the hitting, the kinetic energy of the racket is transferred to the ball, and the ball flies. In a case where a ball is hit by a tennis racket having excellent repulsion performance, the ball can fly at a high speed. In a game of tennis, a high flying speed of the ball is advantageous. Japanese Laid-Open Patent Application Publication No. H05-15617 discloses a tennis racket having excellent repulsion performance.

[0004] Tennis players demand for further improvement in repulsion performance. It is an intention of the applicant of the present application to provide a racket having excellent repulsion performance.SUMMARY OF THE INVENTION

[0005] A racket disclosed in the present specification includes a frame including a head. In the frame, a ratio (G2 / G1) of a ball-hitting face stiffness value G2 to a side pressure stiffness value G1 is greater than or equal to 3.20. In this connection it should be noted that the head may form the contour of a face, with a front view shape of the head being at least substantially an ellipse, wherein the ball hitting face stiffness value is measured in a plane extending in parallel to the contour of the face; and / or wherein the side pressure stiffness value G1 is measured in a plane extending perpendicular to the contour of the face at a side of the head.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 is a front view showing a tennis racket according to one embodiment. FIG. 2 is a right-side view showing the tennis racket of FIG. 1. FIG. 3 is an exploded view showing part of the tennis racket of FIG. 1 in an enlarged manner. FIG. 4 is a perspective view showing part of a manufacturing process of the racket of FIG. 1. FIG. 5 is an enlarged sectional view taken along line V-V of FIG. 1. FIG. 6 is an enlarged sectional view taken along line VI-VI of FIG. 5. FIG. 7 is an enlarged view showing part of a prepreg for first fiber reinforced layers of a head of FIG. 6. FIG. 8 is an enlarged view showing part of a prepreg for second fiber reinforced layers of the head of FIG. 6. FIG. 9 is a perspective view showing part of a high-elasticity layer of the head of FIG. 6. FIG. 10 is an enlarged view showing part of a prepreg for the high-elasticity layer of FIG. 9. FIG. 11 is a graph showing a relationship between a side pressure stiffness value and a ball-hitting face stiffness value of a frame of the tennis racket of FIG. 1. FIG. 12 is a front view showing a method of measuring the side pressure stiffness value of the frame of the tennis racket of FIG. 1. FIG. 13A is a plan view showing a method of measuring the ball-hitting face stiffness value of the frame of the tennis racket of FIG. 1, and FIG. 13B is a front view thereof. DETAILED DESCRIPTION

[0007] Hereinafter, preferred embodiments are described in detail with reference to the drawings as necessary.

[0008] FIGS. 1 to 3 each show a tennis racket 2. The racket 2 includes a frame 4, a grip 6, an end cap 8, a grommet 10, and a string 12. The racket 2 can be used in regulation-ball tennis. In FIGS. 1 and 2, an arrow X represents the width direction of the racket 2; an arrow Y represents the axial direction of the racket 2; and an arrow Z represents the thickness direction of the racket 2. In FIG. 2, the illustration of the grommet 10 and the string 12 is omitted.

[0009] The frame 4 includes a head 14, a first throat 16a, a second throat 16b, and a shaft 18. The head 14 forms the contour of a face 20 (the face 20 will be described below in detail). The front view shape of the head 14 is substantially an ellipse. The major axis direction of the ellipse coincides with the axial direction Y of the racket 2. The minor axis direction of the ellipse coincides with the width direction X of the racket 2. In FIG. 1, reference sign Ch indicates the center of the head 14. The first throat 16a extends from the head 14. The second throat 16b extends from the head 14. The second throat 16b merges with the first throat 16a at a position away from the head 14. The shaft 18 extends from the position where the two throats 16 merge together. The shaft 18 is continuous with the throats 16. A portion of the head 14, the portion being positioned between the two throats 16, is a yoke 22. The frame 4 is hollow.

[0010] The main material of the frame 4 is a fiber reinforced resin. The fiber reinforced resin includes a resin matrix and a large number of reinforcement fibers. The frame 4 includes a plurality of fiber reinforced layers. The fiber reinforced layers will be described below in detail.

[0011] Examples of the base resin of the frame 4 include: thermosetting resins such as epoxy resin, bismaleimide resin, polyimide, and phenolic resin; and thermoplastic resins such as polyether ether ketone, polyether sulphone, polyether imide, polyphenylene sulfide, polyamide, and polypropylene. Epoxy resin is a particularly suitable resin for the frame 4.

[0012] Examples of the reinforcement fibers of the frame 4 include carbon fibers, metal fibers, glass fibers, and aramid fibers. Carbon filament fibers are particularly suitable fibers for the frame 4. Multiple types of fibers may be used in combination as the reinforcement fibers.

[0013] As shown in FIGS. 2 and 3, the head 14 includes a groove 24. The groove 24 is recessed from the outer peripheral surface of the head 14. The groove 24 is formed over substantially the entire periphery of the head 14, except the yoke 22. The head 14 further includes a plurality of holes 26. The plurality of holes 26 are arranged over substantially the entire periphery of the head 14.

[0014] The grip 6 is formed by a tape wound around the shaft 18. The grip 6 suppresses slip between a hand of a player and the racket 2 when the racket 2 is swung by the player.

[0015] As shown in FIG. 3, the grommet 10 includes a base 28 and a plurality of pipes 30. The base 28 is belt-shaped. Each pipe 30 is integrated with the base 28. Each pipe 30 rises from the base 28. A typical material of the grommet 10 is a synthetic resin that is softer than the frame 4. The tennis racket 2 may include a plurality of grommets 10. The number of pipes 30 of each grommet 10 may be one.

[0016] The grommet 10 is attached to the head 14. In a state where the grommet 10 is attached to the head 14, the base 28 is accommodated in the groove 24. The base 28 may partly protrude from the groove 24. Further, in the state where the grommet 10 is attached to the head 14, the pipes 30 extend through the respective holes 26.

[0017] As shown in FIG. 1, the string 12 is stretched on the head 14. The string 12 is stretched in the width direction X and the axial direction Y. The string 12 extends through the pipes 30. The string 12 forms a large number of threads 32. Of the string 12, portions extending in the width direction X are referred to as transverse threads 32a. Of the string 12, portions extending in the axial direction Y are referred to as longitudinal threads 32b. The face 20 is formed by a plurality of transverse threads 32a and a plurality of longitudinal threads 32b. The face 20 generally extends along an X-Y plane. FIG. 1 shows part of the face 20. The face 20 may be formed by two or more strings 12.

[0018] Hereinafter, one example of a method of manufacturing the tennis racket 2 is described with reference to FIG. 4. In this manufacturing method, a mandrel, a tube, and a plurality of prepregs 34 are prepared. Each prepreg 34 is made from a plurality of reinforcement fibers arranged in parallel and a matrix resin. In this manufacturing method, first, the mandrel is inserted into the tube. The prepregs 34 are sequentially wound around the tube. As a result of the winding, the prepregs 34 have a tubular shape. FIG. 4 shows a tubular prepreg 34p and a sheet-shaped prepreg 34s. In FIG. 4, the illustration of the mandrel and the tube is omitted.

[0019] By rotating the mandrel, the prepreg 34s is wound around the prepreg 34p. As a result of the winding, the prepreg 34s has a tubular shape, and thus a layered body 36 is obtained. Another prepreg 34 is further wound around the layered body 36 as necessary. A plurality of sheet-shaped prepregs 34s may be layered one on top of another, which may be then wound around the mandrel or the prepreg 34p. In FIG. 4, an arrow A1 represents the longitudinal direction of the layered body 36.

[0020] After the mandrel is removed from the tube, the tube and the layered body 36 are set in a mold. In the mold, gas is injected into the tube, thereby inflating the tube. The prepregs 34 are pressed against the cavity surface of the mold by the inflation. The prepregs 34 are heated to cure the matrix resin. A molded article is obtained by the curing. The molded article has a reverse shape of that of the cavity surface.

[0021] The holes 26 are drilled in the molded article. The molded article is further subjected to treatments such as surface polishing and painting, and thereby the frame 4 is obtained. Components such as the grip 6 and the grommet 10 are attached to the frame 4. Further, the string 12 is stretched on the frame 4, and thus the tennis racket 2 is completed.

[0022] FIG. 5 is an enlarged sectional view taken along line V-V of FIG. 1. A cross section shown in FIG. 5 extends along a plane that is perpendicular to the axial direction of the frame 4 and that passes the center Ch of the head 14. The head 14 includes a first high-elasticity layer 37a and a second high-elasticity layer 37b (see also FIG. 2). Each high-elasticity layer 37 is positioned on the inner side in the head 14 in the thickness direction.

[0023] FIG. 6 is an enlarged sectional view taken along line VI-VI of FIG. 5. FIG. 6 shows the head 14. The head 14 includes a plurality of fiber reinforced layers 38. In the present embodiment, the head 14 includes a plurality of first fiber reinforced layers 38a, a plurality of second fiber reinforced layers 38b, and a plurality of third fiber reinforced layers 38c. In the present embodiment, the number of first fiber reinforced layers 38a is five; the number of second fiber reinforced layers 38b is five; and the number of third fiber reinforced layers 38c is four. The first fiber reinforced layers 38a and the second fiber reinforced layers 38b are arranged alternately in the thickness direction of the head 14 (i.e., the vertical direction in FIG. 6). The first high-elasticity layer 37a is formed by the plurality of third fiber reinforced layers 38c. The first high-elasticity layer 37a is positioned on the inner side in the head 14 in the thickness direction (in FIG. 6, the lower side). Although not illustrated, the shape of the second high-elasticity layer 37b is mirror-symmetrical to the shape of the first high-elasticity layer 37a.

[0024] FIG. 7 shows a first prepreg 34a for the first fiber reinforced layers 38a. The first prepreg 34a includes a matrix 40 and a plurality of first reinforcement fibers 42a arranged in parallel. Each first reinforcement fiber 42a is inclined relative to the longitudinal direction A1. In FIG. 7, an arrow θa represents an inclination angle (absolute value) of the first reinforcement fiber 42a relative to the longitudinal direction A1. The inclination angle θa is greater than or equal to 30° and less than or equal to 60°. In the present specification, a reinforcement fiber 42 having an inclination angle of greater than or equal to 30° and less than or equal to 60° is referred to as a "bias-type reinforcement fiber". The first fiber reinforced layers 38a include bias-type reinforcement fibers.

[0025] FIG. 8 shows a second prepreg 34b for the second fiber reinforced layers 38b. The second prepreg 34b includes the matrix 40 and a plurality of second reinforcement fibers 42b arranged in parallel. Each second reinforcement fiber 42b is inclined relative to the longitudinal direction A1. The direction in which each second reinforcement fiber 42b is inclined is opposite to the direction (shown in FIG. 7) in which each first reinforcement fiber 42a is inclined. In FIG. 8, an arrow θb represents an inclination angle (absolute value) of the second reinforcement fiber 42b relative to the longitudinal direction A1. The inclination angle θb is greater than or equal to 30° and less than or equal to 60°. Each second reinforcement fiber 42b is a "bias-type reinforcement fiber". The second fiber reinforced layers 38b include bias-type reinforcement fibers.

[0026] FIG. 9 shows part of the high-elasticity layer 37. As previously described, the number of third fiber reinforced layers 38c included in the high-elasticity layer 37 is four. These third fiber reinforced layers 38c are obtained by folding a sheet-shaped third prepreg 34c in such a manner that the sheet-shaped third prepreg 34c is wound around itself.

[0027] FIG. 10 shows the third prepreg 34c for the high-elasticity layer 37. The third prepreg 34c includes the matrix 40 and a plurality of third reinforcement fibers 42c arranged in parallel. Each third reinforcement fiber 42c extends in the longitudinal direction A1. Each third reinforcement fiber 42c has a zero inclination angle (absolute value) relative to the longitudinal direction A1. Each third reinforcement fiber 42c may be slightly inclined relative to the longitudinal direction A1. In the present specification, a reinforcement fiber 42 having an inclination angle (absolute value) of less than or equal to 10° relative to the longitudinal direction A1 is referred to as a "straight-type reinforcement fiber". The third fiber reinforced layers 38c include straight-type reinforcement fibers. In other words, the high-elasticity layer 37 includes the straight-type reinforcement fibers.

[0028] A graph in FIG. 11 shows a relationship between a side pressure stiffness value G1 and a ball-hitting face stiffness value G2 of the frame 4. In this graph, a straight line denoted by reference sign S1 is expressed by a mathematical formula shown below. G 2 = 3.20 ⋅ G 1 In the case of the tennis racket 2 plotted on the straight line S1 or plotted not on the straight line S1 but above the straight line S1, a ratio (G2 / G1) is greater than or equal to 3.20. This racket 2 satisfies a mathematical formula (1) shown below. G 2 ≥ 3.20 ⋅ G 1 In the case of the tennis racket 2 satisfying the above mathematical formula (1), the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large.

[0029] In the case of the tennis racket 2 satisfying the mathematical formula (1), as mentioned above, the side pressure stiffness value G1 is relatively small. According to findings obtained by the inventor of the present invention, in a vibration mode excited during a collision of the racket 2 with a tennis ball, the mode amplitude of the tennis ball is relatively large. Therefore, the speed of the tennis ball in the traveling direction at the end of the collision is high. In other words, the racket 2 whose side pressure stiffness value G1 is relatively small has excellent repulsion performance.

[0030] In the case of the tennis racket 2 satisfying the mathematical formula (1), as mentioned above, the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, in a vibration mode excited during a collision of the racket 2 with a tennis ball, the mode amplitude of the tennis ball is relatively large. Therefore, the speed of the tennis ball in the traveling direction at the end of the collision is high. In other words, the racket 2 whose ball-hitting face stiffness value G2 is relatively large has excellent repulsion performance.

[0031] As previously described, each high-elasticity layer 37 is positioned on the inner side in the head 14 in the thickness direction. At the time of measuring the ball-hitting face stiffness value G2 of the tennis racket 2, force in the thickness direction (Z-direction) is applied to the head 14. The force causes the head 14 to bend relative to the shaft 18 in the thickness direction. Since the third reinforcement fibers 42c are straight-type reinforcement fibers, due to the bending, a great tensile stress occurs on the third reinforcement fibers 42c of the high-elasticity layer 37. The third reinforcement fibers 42c suppress the bending. The high-elasticity layer 37 contributes to achieving a large ball-hitting face stiffness value G2.

[0032] At the time of measuring the side pressure stiffness value G1 of the tennis racket 2, force in the width direction (X-direction) is applied to the head 14. The force causes the head 14 to bend inward in the width direction. A stress that occurs on the third reinforcement fibers 42c due to the bending is small. The third reinforcement fibers 42c do not hinder the bending deformation. The tennis racket 2 including the high-elasticity layer 37 can achieve a small side pressure stiffness value G1.

[0033] FIG. 12 shows a method of measuring the side pressure stiffness value G1. In FIG. 12, the tennis racket 2 is placed on a base 44, which is a rigid body. The width direction X of the racket 2 coincides with the vertical direction. The axial direction Y of the racket 2 coincides with the horizontal direction. A plate 46, which is a rigid body, is lowered, and thereby a load is applied to the racket 2. A displacement (cm) of the plate 46 is measured from when the load is 25 kgf to when the load is 50 kgf. The side pressure stiffness value G1 is calculated by dividing the load difference 25 kgf by the displacement (cm). The side pressure stiffness value G1 is measured in a state where the string 12 is removed from the frame 4.

[0034] In light of repulsion performance, the side pressure stiffness value G1 is preferably less than or equal to 90 kgf / cm, more preferably less than or equal to 80 kgf / cm, and particularly preferably less than or equal to 75 kgf / cm. The side pressure stiffness value G1 of the tennis racket 2 suitable for practical use is greater than or equal to 20 kgf / cm.

[0035] FIGS. 13A and 13B show a method of measuring the ball-hitting face stiffness value G2. In this measurement, a first bar 48a, a second bar 48b, and a third bar 48c are prepared. The material of these bars 48 is steel. Each bar 48 has a circular cross-sectional shape having a radius of 10.0 mm. Each bar 48 extends in the width direction X. The distance between the first bar 48a and the third bar 48c in the axial direction is 170 mm, and the distance between the third bar 48c and the second bar 48b in the axial direction is 170 mm. The first bar 48a is positioned at the top of the head 14. The racket 2 is placed on the first bar 48a and the second bar 48b. The width direction X and the axial direction Y of the racket 2 coincide with the horizontal direction. The third bar 48c is lowered, and thereby a load is applied to the tennis racket 2. A displacement (cm) of the third bar 48c is measured from when the load is 25 kgf to when the load is 50 kgf. The ball-hitting face stiffness value G2 is calculated by dividing the load different 25 kgf by the displacement (cm). The ball-hitting face stiffness value G2 is measured in a state where the string 12 is removed from the frame 4.

[0036] In light of repulsion performance, the ball-hitting face stiffness value G2 is preferably greater than or equal to 100 kgf / cm, more preferably greater than or equal to 200 kgf / cm, and particularly preferably greater than or equal to 250 kgf / cm. The ball-hitting face stiffness value G2 of the tennis racket 2 suitable for practical use is less than or equal to 500 kgf / cm.

[0037] In FIG. 2, an arrow Lh represents the length of each high-elasticity layer 37 in the axial direction. In the present embodiment, the length Lh is a distance in the axial direction from a top Pt of the head 14 to an end Ed of the high-elasticity layer 37. The length Lh is preferably greater than or equal to 170 mm, more preferably greater than or equal to 250 mm, and particularly preferably greater than or equal to 340 mm for the reason that a large ball-hitting face stiffness value G2 can be achieved with such setting of the length Lh.

[0038] In FIG. 5, an arrow Tf represents the thickness of the frame 4, and an arrow Wf represents the width of the frame 4. A ratio (Tf / Wf) of the thickness Tf to the width Wf is preferably greater than or equal to 2.0. If the ratio (Tf / Wf) of the tennis racket 2 is within this range, a large ratio (G2 / G1) can be readily achieved. In light of this, the ratio (Tf / Wf) is more preferably greater than or equal to 2.2, yet more preferably greater than or equal to 2.4, and particularly preferably greater than or equal to 2.8. The ratio (Tf / Wf) of the tennis racket 2 suitable for practical use is less than or equal to 4.0.

[0039] The thickness Tf is preferably greater than or equal to 20.0 mm, more preferably greater than or equal to 26.0 mm, yet more preferably greater than or equal to 28.5 mm, and particularly preferably greater than or equal to 33.0 mm for the reason that a large ratio (G2 / G1) can be readily achieved with such setting of the thickness Tf. The thickness Tf of the tennis racket 2 suitable for practical use is less than or equal to 40.0 mm.

[0040] In a graph of FIG. 11, a straight line denoted by reference sign S2 is expressed by a mathematical formula shown below. G 2 = 3.55 ⋅ G 1 In the case of the tennis racket 2 plotted on the straight line S2 or plotted not on the straight line S2 but above the straight line S2, the ratio (G2 / G1) is greater than or equal to 3.55. In the case of this tennis racket 2, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, this tennis racket 2 has more excellent repulsion performance. In other words, the tennis racket 2 satisfying a mathematical formula shown below has more excellent repulsion performance. G 2 ≥ 3.55 ⋅ G 1

[0041] In the graph of FIG. 11, a straight line denoted by reference sign S3 is expressed by a mathematical formula shown below. G 2 = 3.90 ⋅ G 1 In the case of the tennis racket 2 plotted on the straight line S3 or plotted not on the straight line S3 but above the straight line S3, the ratio (G2 / G1) is greater than or equal to 3.90. In the case of this tennis racket 2, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, this tennis racket 2 has more excellent repulsion performance. In other words, the tennis racket 2 satisfying a mathematical formula shown below has more excellent repulsion performance. G 2 ≥ 3.90 ⋅ G 1

[0042] In the graph of FIG. 11, a straight line denoted by reference sign S4 is expressed by a mathematical formula shown below. G 2 = 4.10 ⋅ G 1 In the case of the tennis racket 2 plotted on the straight line S4 or plotted not on the straight line S4 but above the straight line S4, the ratio (G2 / G1) is greater than or equal to 4.10. In the case of this tennis racket 2, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value G2 is relatively large. According to findings obtained by the inventor of the present invention, this tennis racket 2 has extremely excellent repulsion performance. In other words, the tennis racket 2 satisfying a mathematical formula shown below has extremely excellent repulsion performance. G 2 ≥ 4.10 ⋅ G 1[Evaluation][Sample 1]

[0043] A tennis racket model for simulation was fabricated. The specifications of the tennis racket model were as shown below. The width of each of the first prepreg and the second prepreg: 250 mm The thickness Tf of the frame: 29.9 mm The elastic modulus of the reinforcement fibers in the high-elasticity layer: 80 tf / mm 2< The thickness of the high-elasticity layer: 0.825 mm [Samples 2 to 120]

[0044] Tennis racket models of samples 2 to 120 were fabricated in the same manner as the sample 1, except that the specifications of the tennis racket models were varied as shown in Tables 1 to 6 below.[Simulation]

[0045] The side pressure stiffness value G1 and the ball-hitting face stiffness value G2 of each sample were calculated by simulation. Further, a tennis ball was brought into collision with each sample, and the speed of the tennis ball when rebounding off the sample was calculated by simulation. The results are shown in Tables 1 to 6 below. Table 1 Evaluation ResultsPrepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 125033.8800.825120284.295966Sample 225033.8801.650123294.245962Sample 337533.8401.650182444.145953Sample 437533.8800.825192454.275964Sample 537533.8801.650203454.515986Sample 650031.2801.650250614.105950Sample 750033.8400.825240584.145953Sample 850033.8401.650255604.255963Sample 950033.8800.825264594.475983Sample 1050033.8801.650282614.625996Sample 1162531.2801.650297724.135952Sample 1262533.8400.825289704.135952Sample 1362533.8401.650310724.315968Sample 1462533.8800.825315714.445979Sample 1562533.8801.650341734.676000Sample 1675031.2801.650345844.115950Sample 1775033.8400.825339824.135953Sample 1875033.8401.650365844.355971Sample 1975033.8800.825367844.375973Sample 2075033.8801.650400864.655998 Table 2 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 2125033.8400.825107273.965938Sample 2225033.8401.650110274.075948Sample 2337531.2800.825173443.935935Sample 2437531.2801.650181454.025943Sample 2537533.8400.825173443.935935Sample 2650031.2800.825237594.025942Sample 2762531.2800.825279713.935935Sample 2875031.2800.825321823.915933 Table 3 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 2925031.2401.650101283.615906Sample 3025031.2800.825110293.795923Sample 3125031.2801.650113303.775920Sample 3237528.6800.825156443.555901Sample 3337528.6801.650163453.625908Sample 3437531.2400.825157433.655910Sample 3537531.2401.650164443.735917Sample 3637533.800.825128363.565902Sample 3737533.801.650128363.565902Sample 3850028.6800.825211593.585904Sample 3950028.6801.650221603.685913Sample 4050031.2400.825216583.725917Sample 4150031.2401.650227593.855928Sample 4262528.6801.650259713.655910Sample 4362531.2400.825257703.675912Sample 4462531.2401.650271713.825925Sample 4562533.800.825241663.655910Sample 4662533.801.650241663.655910Sample 4775028.6801.650297823.625908Sample 4875031.2400.825298813.685913Sample 4975031.2401.650316833.815924Sample 5075033.800.825293793.715915Sample 5175033.801.650293793.715915 Table 4 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 5225028.6400.82591283.255875Sample 5325028.6401.65093293.215871Sample 5425028.6800.825101293.485895Sample 5525028.6801.650104303.475894Sample 5625031.2400.82598283.505897Sample 5737526801.650146453.245875Sample 5837528.6400.825141433.285878Sample 5937528.6401.650147443.345883Sample 6050026800.825187583.225873Sample 6150026801.650196593.325881Sample 6250028.6400.825192583.315880Sample 6350028.6401.650201593.415889Sample 6450033.800.825190543.525899Sample 6550033.801.650190543.525899Sample 6662526801.650227693.295879Sample 6762528.6400.825225693.265876Sample 6862528.6401.650237703.395887Sample 6962528.6800.825245703.505897Sample 7062531.200.825216663.275877Sample 7162531.201.650216663.275877Sample 7275026801.650257803.215872Sample 7375028.6400.825258803.235873Sample 7475028.6401.650273813.375886Sample 7575028.6800.825279813.445892Sample 7675031.200.825259783.325881Sample 7775031.201.650259783.325881 Table 5 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 782502600.82553242.215783Sample 792502601.65053242.215783Sample 8025026400.82583292.865841Sample 8125026401.65086302.875841Sample 8225026800.82593303.105862Sample 8325026801.65095313.065859Sample 8425028.600.82558232.525811Sample 8525028.601.65058232.525811Sample 8625031.200.82563222.865841Sample 8725031.201.65063222.865841Sample 8825033.800.82567213.195870Sample 8925033.801.65067213.195870Sample 903752600.82593392.385799Sample 913752601.65093392.385799Sample 9237526400.825126432.935847Sample 9337526401.650131442.985851Sample 9437526800.825140443.185869Sample 9537528.600.825106392.725828Sample 9637528.601.650106392.725828Sample 9737531.200.825118383.115862Sample 9837531.201.650118383.115862Sample 995002600.825133542.465806Sample 1005002601.650133542.465806Sample 10150026400.825168572.955848Sample 10250026401.650177583.055858Sample 10350028.600.825154542.855840Sample 10450028.601.650154542.855840Sample 10550031.200.825172543.195869 Table 6 Evaluation Results Prepreg widthThickness TfHigh-elasticity layerStiffnessG2 / G1SpeedmmmmElastic modulus tf / mm 2< Thickness mmG2 kgf / cmG1 kgf / cmmm / sSample 10650031.201.650172543.195869Sample 1076252600.825162642.535812Sample 1086252601.650162642.535812Sample 10962526400.825195672.915845Sample 11062526401.650206683.035856Sample 11162526800.825214683.155866Sample 11262528.600.825189652.915845Sample 11362528.601.650189652.915845Sample 1147502600.825191752.555813Sample 1157502601.650191752.555813Sample 11675026400.825221772.875842Sample 11775026401.650234783.005853Sample 11875026800.825240783.085860Sample 11975028.600.825225772.925846Sample 12075028.601.650225772.925846

[0046] In the case of the samples shown in Table 1, the ratio (G2 / G1) is greater than or equal to 4.10. The speed of the ball rebounding off each sample in Table 1 is greater than or equal to 5950 mm / s.

[0047] In the case of the samples shown in Table 2, the ratio (G2 / G1) is greater than or equal to 3.90 and less than 4.10. The speed of the ball rebounding off each sample in Table 2 is greater than or equal to 5930 mm / s and less than 5950 mm / s.

[0048] In the case of the samples shown in Table 3, the ratio (G2 / G1) is greater than or equal to 3.55 and less than 3.90. The speed of the ball rebounding off each sample in Table 3 is greater than or equal to 5900 mm / s and less than 5930 mm / s.

[0049] In the case of the samples shown in Table 4, the ratio (G2 / G1) is greater than or equal to 3.20 and less than 3.55. The speed of the ball rebounding off each sample in Table 4 is greater than or equal to 5870 mm / s and less than 5900 mm / s.

[0050] In the case of the samples shown in Tables 5 and 6, the ratio (G2 / G1) is less than 3.20. The speed of the ball rebounding off each sample in Tables 5 and 6 is less than 5870 mm / s.

[0051] These evaluation results clearly indicate the superiority of the tennis rackets with a large ratio (G2 / G1).[Disclosure Items]

[0052] The following items each disclose a preferred embodiment.[Item 1]

[0053] A racket including a frame including a head. In the frame, a ratio (G2 / G1) of a ball-hitting face stiffness value G2 to a side pressure stiffness value G1 is greater than or equal to 3.20.[Item 2]

[0054] The racket according to item 1, wherein the head includes a high-elasticity layer that is positioned on an inner side in the head in a thickness direction of the head, the high-elasticity layer including straight-type reinforcement fibers.[Item 3]

[0055] The racket according to item 2, wherein the high-elasticity layer is present in a cross section that extends along a plane that is perpendicular to an axial direction of the frame and that passes a center of the head.[Item 4]

[0056] The racket according to any one of items 1 to 3, wherein in a cross section that extends along a plane that is perpendicular to an axial direction of the frame and that passes a center of the head, a ratio (Tf / Wf) of a thickness Tf of the frame to a width Wf of the frame is greater than or equal to 2.0.[Item 5]

[0057] The racket according to any one of items 1 to 4, wherein the ratio (G2 / G1) is greater than or equal to 3.55.[Item 6]

[0058] The racket according to item 5, wherein the ratio (G2 / G1) is greater than or equal to 3.90.[Item 7]

[0059] The racket according to item 6, wherein the ratio (G2 / G1) is greater than or equal to 4.10.[Item 8]

[0060] The racket according to any one of items 1 to 7, wherein the side pressure stiffness value G1 is less than or equal to 90 kgf / cm.[Item 9]

[0061] The racket according to any one of items 1 to 8, wherein the ball-hitting face stiffness value G2 is greater than or equal to 100 kgf / cm.

[0062] The racket as described above is suitable also for use in, for example, soft tennis, squash, padel, and badminton. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention.

Examples

Embodiment Construction

[0007]Hereinafter, preferred embodiments are described in detail with reference to the drawings as necessary.

[0008]FIGS. 1 to 3 each show a tennis racket 2. The racket 2 includes a frame 4, a grip 6, an end cap 8, a grommet 10, and a string 12. The racket 2 can be used in regulation-ball tennis. In FIGS. 1 and 2, an arrow X represents the width direction of the racket 2; an arrow Y represents the axial direction of the racket 2; and an arrow Z represents the thickness direction of the racket 2. In FIG. 2, the illustration of the grommet 10 and the string 12 is omitted.

[0009]The frame 4 includes a head 14, a first throat 16a, a second throat 16b, and a shaft 18. The head 14 forms the contour of a face 20 (the face 20 will be described below in detail). The front view shape of the head 14 is substantially an ellipse. The major axis direction of the ellipse coincides with the axial direction Y of the racket 2. The minor axis direction of the ellipse coincides with the width direction X...

Claims

1. A racket (2) comprising a frame (4) including a head (14), wherein in the frame (4), a ratio (G2 / G1) of a ball-hitting face stiffness value G2 to a side pressure stiffness value G1 is greater than or equal to 3.20.

2. The racket (2) according to claim 1, wherein the head (14) includes a high-elasticity layer that is positioned on an inner side in the head (14) in a thickness direction of the head (4), the high-elasticity layer including straight-type reinforcement fibers.

3. The racket (2) according to claim 2, wherein the high-elasticity layer is present in a cross section that extends along a plane that is perpendicular to an axial direction of the frame and that passes a center of the head (4).

4. The racket (2) according to claim 1 or 2, wherein in a cross section that extends along a plane that is perpendicular to an axial direction of the frame (4) and that passes a center of the head (4), a ratio (Tf / Wf) of a thickness Tf of the frame (4) to a width Wf of the frame (4) is greater than or equal to 2.0.

5. The racket (2) according to any one of claims 1 to 4, wherein the ratio (G2 / G1) is greater than or equal to 3.55.

6. The racket (2) according to claim 5, wherein the ratio (G2 / G1) is greater than or equal to 3.90.

7. The racket (2) according to claim 6, wherein the ratio (G2 / G1) is greater than or equal to 4.10.

8. The racket (2) according to any one of claims 1 to 7, wherein the side pressure stiffness value G1 is less than or equal to 90 kgf / cm.

9. The racket (2) according to any one of claims 1 to 8, wherein the ball-hitting face stiffness value G2 is greater than or equal to 100 kgf / cm.

10. The racket (2) according to any one of claims 1 to 9, wherein the head (14) forms the contour of a face (20), with a front view shape of the head (14) being at least substantially an ellipse, wherein the ball hitting face stiffness value G2 is measured in a plane extending in parallel to the contour of the face (20).

11. The racket (2) according to any one of claims 1 to 10, wherein the head (14) forms the contour of a face (20), with a front view shape of the head (14) being at least substantially an ellipse, wherein the side pressure stiffness value G1 is measured in a plane extending perpendicular to the contour of the face (20) at a side of the head (14).

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