Racket

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.

The present specification discloses a racket that is suitable for use in, for example, tennis, soft tennis, squash, padel, and badminton.

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.

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.

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.

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

each show a tennis racket. The racketincludes a frame, a grip, an end cap, a grommet, and a string. The racketcan be used in regulation-ball tennis. In, an arrow X represents the width direction of the racket; an arrow Y represents the axial direction of the racket; and an arrow Z represents the thickness direction of the racket. In, the illustration of the grommetand the stringis omitted.

The frameincludes a head, a first throata second throatand a shaft. The headforms the contour of a face(the facewill be described below in detail). The front view shape of the headis substantially an ellipse. The major axis direction of the ellipse coincides with the axial direction Y of the racket. The minor axis direction of the ellipse coincides with the width direction X of the racket. In, reference sign Ch indicates the center of the head. The first throatextends from the head. The second throatextends from the head. The second throatmerges with the first throatat a position away from the head. The shaftextends from the position where the two throatsmerge together. The shaftis continuous with the throats. A portion of the head, the portion being positioned between the two throats, is a yoke. The frameis hollow.

The main material of the frameis a fiber reinforced resin. The fiber reinforced resin includes a resin matrix and a large number of reinforcement fibers. The frameincludes a plurality of fiber reinforced layers. The fiber reinforced layers will be described below in detail.

Examples of the base resin of the frameinclude: 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.

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

As shown in, the headincludes a groove. The grooveis recessed from the outer peripheral surface of the head. The grooveis formed over substantially the entire periphery of the head, except the yoke. The headfurther includes a plurality of holes. The plurality of holesare arranged over substantially the entire periphery of the head.

The gripis formed by a tape wound around the shaft. The gripsuppresses slip between a hand of a player and the racketwhen the racketis swung by the player. As shown in, the grommetincludes a baseand a plurality of pipes. The baseis belt-shaped. Each pipeis integrated with the base. Each piperises from the base. A typical material of the grommetis a synthetic resin that is softer than the frame. The tennis racketmay include a plurality of grommets. The number of pipesof each grommetmay be one.

The grommetis attached to the head. In a state where the grommetis attached to the head, the baseis accommodated in the groove. The basemay partly protrude from the groove. Further, in the state where the grommetis attached to the head, the pipesextend through the respective holes.

As shown in, the stringis stretched on the head. The stringis stretched in the width direction X and the axial direction Y. The stringextends through the pipes. The stringforms a large number of threads. Of the string, portions extending in the width direction X are referred to as transverse threads. Of the string, portions extending in the axial direction Y are referred to as longitudinal threadsThe faceis formed by a plurality of transverse threadsand a plurality of longitudinal threadsThe facegenerally extends along an X-Y plane.shows part of the face. The facemay be formed by two or more strings.

Hereinafter, one example of a method of manufacturing the tennis racketis described with reference to. In this manufacturing method, a mandrel, a tube, and a plurality of prepregsare prepared. Each prepregis 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 prepregsare sequentially wound around the tube. As a result of the winding, the prepregshave a tubular shape.shows a tubular prepregand a sheet-shaped prepregIn, the illustration of the mandrel and the tube is omitted.

By rotating the mandrel, the prepregis wound around the prepregAs a result of the winding, the prepreghas a tubular shape, and thus a layered bodyis obtained. Another prepregis further wound around the layered bodyas necessary. A plurality of sheet-shaped prepregsmay be layered one on top of another, which may be then wound around the mandrel or the prepregIn, an arrow Arepresents the longitudinal direction of the layered body.

After the mandrel is removed from the tube, the tube and the layered bodyare set in a mold. In the mold, gas is injected into the tube, thereby inflating the tube. The prepregsare pressed against the cavity surface of the mold by the inflation. The prepregsare 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.

The holesare drilled in the molded article. The molded article is further subjected to treatments such as surface polishing and painting, and thereby the frameis obtained. Components such as the gripand the grommetare attached to the frame. Further, the stringis stretched on the frame, and thus the tennis racketis completed.

is an enlarged sectional view taken along line V-V of. A cross section shown inextends along a plane that is perpendicular to the axial direction of the frameand that passes the center Ch of the head. The headincludes a first high-elasticity layerand a second high-elasticity layer(see also). Each high-elasticity layeris positioned on the inner side in the headin the thickness direction.

is an enlarged sectional view taken along line VI-VI of.shows the head. The headincludes a plurality of fiber reinforced layers. In the present embodiment, the headincludes a plurality of first fiber reinforced layersa plurality of second fiber reinforced layersand a plurality of third fiber reinforced layersIn the present embodiment, the number of first fiber reinforced layersis five; the number of second fiber reinforced layersis five; and the number of third fiber reinforced layersis four. The first fiber reinforced layersand the second fiber reinforced layersare arranged alternately in the thickness direction of the head(i.e., the vertical direction in). The first high-elasticity layeris formed by the plurality of third fiber reinforced layersThe first high-elasticity layeris positioned on the inner side in the headin the thickness direction (in, the lower side). Although not illustrated, the shape of the second high-elasticity layeris mirror-symmetrical to the shape of the first high-elasticity layer

shows a first prepregfor the first fiber reinforced layersThe first prepregincludes a matrixand a plurality of first reinforcement fibersarranged in parallel. Each first reinforcement fiberis inclined relative to the longitudinal direction A. In, an arrow θa represents an inclination angle (absolute value) of the first reinforcement fiberrelative to the longitudinal direction A. The inclination angle θa is greater than or equal to 30° and less than or equal to 60°. In the present specification, a reinforcement fiberhaving 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 layersinclude bias-type reinforcement fibers.

shows a second prepregfor the second fiber reinforced layers. The second prepregincludes the matrixand a plurality of second reinforcement fibersarranged in parallel. Each second reinforcement fiberis inclined relative to the longitudinal direction A. The direction in which each second reinforcement fiberis inclined is opposite to the direction (shown in) in which each first reinforcement fiberis inclined. In, an arrow Ob represents an inclination angle (absolute value) of the second reinforcement fiberrelative to the longitudinal direction A. The inclination angle Ob is greater than or equal to 30° and less than or equal to 60°. Each second reinforcement fiberis a “bias-type reinforcement fiber”. The second fiber reinforced layersinclude bias-type reinforcement fibers.

shows part of the high-elasticity layer. As previously described, the number of third fiber reinforced layersincluded in the high-elasticity layeris four. These third fiber reinforced layersare obtained by folding a sheet-shaped third prepregin such a manner that the sheet-shaped third prepregis wound around itself.

shows the third prepregfor the high-elasticity layer. The third prepregincludes the matrixand a plurality of third reinforcement fibersarranged in parallel. Each third reinforcement fiberextends in the longitudinal direction Al. Each third reinforcement fiberhas a zero inclination angle (absolute value) relative to the longitudinal direction A. Each third reinforcement fibermay be slightly inclined relative to the longitudinal direction A. In the present specification, a reinforcement fiberhaving an inclination angle (absolute value) of less than or equal to 10° relative to the longitudinal direction Ais referred to as a “straight-type reinforcement fiber”. The third fiber reinforced layersinclude straight-type reinforcement fibers. In other words, the high-elasticity layerincludes the straight-type reinforcement fibers.

A graph inshows a relationship between a side pressure stiffness value Gand a ball-hitting face stiffness value Gof the frame. In this graph, a straight line denoted by reference sign Sis expressed by a mathematical formula shown below.

In the case of the tennis racketplotted on the straight line Sor plotted not on the straight line Sbut above the straight line S, a ratio (G2/G1) is greater than or equal to 3.20. This racketsatisfies a mathematical formula (1) shown below.

In the case of the tennis racketsatisfying 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.

In the case of the tennis racketsatisfying 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 racketwith 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 racketwhose side pressure stiffness value G1 is relatively small has excellent repulsion performance.

In the case of the tennis racketsatisfying 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 racketwith 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 racketwhose ball-hitting face stiffness value G2 is relatively large has excellent repulsion performance.

As previously described, each high-elasticity layeris positioned on the inner side in the headin the thickness direction. At the time of measuring the ball-hitting face stiffness value G2 of the tennis racket, force in the thickness direction (Z-direction) is applied to the head. The force causes the headto bend relative to the shaftin the thickness direction. Since the third reinforcement fibersare straight-type reinforcement fibers, due to the bending, a great tensile stress occurs on the third reinforcement fibersof the high-elasticity layer. The third reinforcement fiberssuppress the bending. The high-elasticity layercontributes to achieving a large ball-hitting face stiffness value G.

At the time of measuring the side pressure stiffness value G1 of the tennis racket, force in the width direction (X-direction) is applied to the head. The force causes the headto bend inward in the width direction. A stress that occurs on the third reinforcement fibersdue to the bending is small. The third reinforcement fibersdo not hinder the bending deformation. The tennis racketincluding the high-elasticity layercan achieve a small side pressure stiffness value G1.

shows a method of measuring the side pressure stiffness value G. In, the tennis racketis placed on a base, which is a rigid body. The width direction X of the racketcoincides with the vertical direction. The axial direction Y of the racketcoincides with the horizontal direction. A plate, which is a rigid body, is lowered, and thereby a load is applied to the racket. A displacement (cm) of the plateis 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 stringis removed from the frame.

In light of repulsion performance, the side pressure stiffness value Gis 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 racketsuitable for practical use is greater than or equal to 20 kgf/cm.

show a method of measuring the ball-hitting face stiffness value G2. In this measurement, a first bara second barand a third barare prepared. The material of these barsis steel. Each barhas a circular cross-sectional shape having a radius of 10.0 mm. Each barextends in the width direction X. The distance between the first barand the third barin the axial direction is 170 mm, and the distance between the third barand the second barin the axial direction is 170 mm. The first baris positioned at the top of the head. The racketis placed on the first barand the second barThe width direction X and the axial direction Y of the racketcoincide with the horizontal direction. The third baris lowered, and thereby a load is applied to the tennis racket. A displacement (cm) of the third baris 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 stringis removed from the frame.

In light of repulsion performance, the ball-hitting face stiffness value Gis 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 racketsuitable for practical use is less than or equal to 500 kgf/cm.

In, an arrow Lh represents the length of each high-elasticity layerin the axial direction. In the present embodiment, the length Lh is a distance in the axial direction from a top Pt of the headto an end Ed of the high-elasticity layer. 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.

In, an arrow Tf represents the thickness of the frame, and an arrow Wf represents the width of the frame. 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 racketis 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 racketsuitable for practical use is less than or equal to 4.0.

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.

In a graph of, a straight line denoted by reference sign Sis expressed by a mathematical formula shown below.

In the case of the tennis racketplotted on the straight line Sor plotted not on the straight line Sbut above the straight line S, the ratio (G2/G1) is greater than or equal to 3.55. In the case of this tennis racket, the side pressure stiffness value G1 is relatively small, and the ball-hitting face stiffness value Gis relatively large. According to findings obtained by the inventor of the present invention, this tennis rackethas more excellent repulsion performance. In other words, the tennis racketsatisfying a mathematical formula shown below has more excellent repulsion performance.

In the graph of, a straight line denoted by reference sign Sis expressed by a mathematical formula shown below.

In the case of the tennis racketplotted on the straight line Sor plotted not on the straight line Sbut above the straight line S, the ratio (G2/G1) is greater than or equal to 3.90. In the case of this tennis racket, the side pressure stiffness value Gis relatively small, and the ball-hitting face stiffness value Gis relatively large. According to findings obtained by the inventor of the present invention, this tennis rackethas more excellent repulsion performance. In other words, the tennis racketsatisfying a mathematical formula shown below has more excellent repulsion performance.

In the graph of, a straight line denoted by reference sign Sis expressed by a mathematical formula shown below.