Racket

This application claims priority to and the benefit of Japanese Patent Application No. 2024-086056, 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 that has 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. Japanese Laid-Open Patent Application Publication No. 2001-61996 discloses a tennis racket having excellent repulsion performance and an excellent feel at impact.

Tennis players demand not only high repulsion performance but also high spin performance and face stability for tennis rackets. It is an intention of the applicant of the present application to provide a racket having an excellent balance among repulsion performance, spin performance, and face stability.

In a racket disclosed by the present specification, an in-plane stiffness index Gi, which is a ratio of a top pressure stiffness value Git (kgf/cm) to a side pressure stiffness value Gis (kgf/cm), is greater than or equal to 1.40; an out-of-plane stiffness index Go, which is a product of a throat stiffness value Gos (kgf/cm) and a ball-hitting face stiffness value Goh (kgf/cm), is greater than or equal to 60000 and less than or equal to 85000; a moment of inertia Mi about an axis of the racket is greater than or equal to 13500 g cmand less than or equal to 15000 g cm; and an index of inertia Ii is greater than or equal to 0.150, which is calculated by the following mathematical formula: Ii=Mi/(Wr·Lc) (where Wr is a mass (g) of the racket, and Lc is a distance (mm) from a grip end of the racket to a center of gravity of the racket).

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

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. 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 resin 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 in the circumferential direction of the head.

The gripis wound around the shaft. The gripis formed by a tape. 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. The material of the grommetis typically 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. 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. As a result of the winding, a layered bodyis obtained. In, an arrow Arepresents the longitudinal direction of the layered body. Another prepregis further wound around the layered bodyif necessary.

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. The racketincludes a plurality of fiber reinforced layers.

shows a first prepregThe 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 θrepresents 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 20° and less than or equal to 60°. In the present specification, a reinforcement fiberhaving an inclination angle of greater than or equal to 20° and less than or equal to 60° is referred to as a “bias-type reinforcement fiber”.

shows a second prepregThe 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 20° and less than or equal to 60°. Each second reinforcement fiberis a “bias-type reinforcement fiber”.

shows a third prepregThe third prepregincludes the matrixand a plurality of third reinforcement fibersarranged in parallel. Each third reinforcement fiberextends in the longitudinal direction A. 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 frameincludes fiber reinforced layers including bias-type reinforcement fibers and fiber reinforced layers including straight-type reinforcement fibers.

The tennis rackethas a proper in-plane stiffness index Gi. The in-plane stiffness index Gi is the ratio of a top pressure stiffness value Git (kgf/cm) to a side pressure stiffness value Gis (kgf/cm). The in-plane stiffness index Gi is calculated by a mathematical formula shown below.

shows a method of measuring the top pressure stiffness value Git. In, the tennis racketis fixed to a support. The supportincludes a spacer. The yokeis placed on the spacer. The width direction X of the racketcoincides with the horizontal direction. The axial direction Y of the racketcoincides with the vertical direction. A plate, which is a rigid body, is in contact with the top of the racket. The plateis 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 top pressure stiffness value Git (kgf/cm) is calculated by dividing the load difference 25 kgf by the displacement (cm). The top pressure stiffness value Git is measured in a state where the stringis removed from the frame.

In light of repulsion performance, the top pressure stiffness value Git is preferably greater than or equal to 60 kgf/cm, more preferably greater than or equal to 70 kgf/cm, and particularly preferably greater than or equal to 80 kgf/cm. In light of control performance, the top pressure stiffness value Git is preferably less than or equal to 110 kgf/cm, more preferably less than or equal to 100 kgf/cm, and particularly preferably less than or equal to 90 kgf/cm.

shows a method of measuring the side pressure stiffness value Gis. 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 Gis (kgf/cm) is calculated by dividing the load difference 25 kgf by the displacement (cm). The side pressure stiffness value Gis is measured in a state where the stringis removed from the frame.

In light of repulsion performance, the side pressure stiffness value Gis is preferably greater than or equal to 45 kgf/cm, more preferably greater than or equal to 50 kgf/cm, and particularly preferably greater than or equal to 60 kgf/cm. In light of control performance, the side pressure stiffness value Gis is preferably less than or equal to 100 kgf/cm, more preferably less than or equal to 90 kgf/cm, and particularly preferably less than or equal to 80 kgf/cm.

Preferably, the in-plane stiffness index Gi is greater than or equal to 1.40. The tennis racketwith the in-plane stiffness index Gi being greater than or equal to 1.40 has excellent spin performance. It is presumed that the reason why this rackethas excellent spin performance is that upon impact with a tennis ball, deflection of the headin the axial direction Y is suppressed. It is presumed that in the case of the headwith less deflection in the axial direction Y, the real length of the longitudinal threadsin the axial direction upon impact with a tennis ball is greater. Upon impact with a tennis ball, the longitudinal threadsdeform and then restore, and it is presumed that the deformation and the restoration of the longitudinal threadsimpart great rotational force to the tennis ball. In light of spin performance, the in-plane stiffness index Gi is more preferably greater than or equal to 1.45, and particularly preferably greater than or equal to 1.50. The upper limit of the range of the in-plane stiffness index Gi of the tennis racketsuitable for practical use is 1.80.

The tennis rackethas a proper out-of-plane stiffness index Go. The out-of-plane stiffness index Go is a product of a throat stiffness value Gos (kgf/cm) and a ball-hitting face stiffness value Goh (kgf/cm). The out-of-plane stiffness index Go is calculated by a mathematical formula shown below.

show a method of measuring the throat stiffness value Gos. 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 5.0 mm. Each barextends in the width direction X. The distance between the first barand the third barin the axial direction Y is 100 mm, and the distance between the third barand the second barin the axial direction Y is 100 mm. The position of the first baris shifted toward the headside from one end Pof each throat. The position of the second baris shifted toward the gripside from the other end Pof each throat. 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 racket. A displacement (cm) of the third baris measured from when the load is 25 kgf to when the load is 50 kgf. The throat stiffness value Gos (kgf/cm) is calculated by dividing the load difference 25 kgf by the displacement (cm). The throat stiffness value Gos is measured in a state where the stringis removed from the frame.

In light of repulsion performance, the throat stiffness value Gos is preferably greater than or equal to 350 kgf/cm, more preferably greater than or equal to 370 kgf/cm, and particularly preferably greater than or equal to 400 kgf/cm. In light of control performance, the throat stiffness value Gos is preferably less than or equal to 480 kgf/cm, more preferably less than or equal to 460 kgf/cm, and particularly preferably less than or equal to 440 kgf/cm.

show a method of measuring the ball-hitting face stiffness value Goh. 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 Y is 170 mm, and the distance between the third barand the second barin the axial direction Y is 170 mm. The first baris positioned at the top Pt 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 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 Goh (kgf/cm) is calculated by dividing the load difference 25 kgf by the displacement (cm). The ball-hitting face stiffness value Goh is measured in a state where the stringis removed from the frame.

In light of repulsion performance, the ball-hitting face stiffness value Goh is preferably greater than or equal to 100 kgf/cm, more preferably greater than or equal to 110 kgf/cm, and particularly preferably greater than or equal to 120 kgf/cm. In light of control performance, the ball-hitting face stiffness value Goh is preferably less than or equal to 170 kgf/cm, more preferably less than or equal to 160 kgf/cm, and particularly preferably less than or equal to 150 kgf/cm.

The out-of-plane stiffness index Go is preferably greater than or equal to 60000 and less than or equal to 85000. The tennis racketwith the out-of-plane stiffness index Go being greater than or equal to 60000 has excellent repulsion performance. In light of this, the out-of-plane stiffness index Go is more preferably greater than or equal to 65000, and particularly preferably greater than or equal to 70000. The tennis racketwith the out-of-plane stiffness index Go being less than or equal to 85000 has an excellent feel at impact and excellent control performance. In light of this, the out-of-plane stiffness index Go is more preferably less than or equal to 83000, and particularly preferably less than or equal to 80000.

The tennis rackethas a proper moment of inertia Mi about the axis Y.shows a method of measuring the moment of inertia Mi. In this measurement, a part of the racket, the part being positioned 15 mm away from the grip end, is fixed to a cord. The racketis suspended by the cord. The axial direction Y of the racketcoincides with the vertical direction. The racketis rotated about the vertical direction Y. period Tc (sec) of the rotation is measured, and the moment of inertia Mi (g·cm) is calculated based on a mathematical formula shown below.

The moment of inertia Mi is measured in a state where the stringis removed from the frame.

The moment of inertia Mi is preferably greater than or equal to 13500 g·cmand less than or equal to 15000 g·cm. In the case of the tennis racketwith the moment of inertia Mi being greater than or equal to 13500 g·cm, a change in the angle of the facethat occurs when a tennis ball collides with the faceon a position that is not the sweet spot is small. In other words, the racketwith the moment of inertia Mi being greater than or equal to 13500 g·cmhas excellent face stability. In light of this, the moment of inertia Mi is more preferably greater than or equal to 13800 g·cm, and particularly preferably greater than or equal to 14000 g·cm. With the tennis racketwith the moment of inertia Mi being less than or equal to 15000 g·cm, a sharp feel at impact can be obtained. In light of this, the moment of inertia Mi is more preferably less than or equal to 14900 g·cm, and particularly preferably less than or equal to 14800 g·cm.

The tennis rackethas a proper index of inertia Ii. The index of inertia Ii is calculated by a mathematical formula shown below.

In this mathematical formula, Wr is the mass (g) of the racket, and Lc is the distance (mm) from the end of the gripto the center of gravity of the racket.

The index of inertia Ii is preferably greater than or equal to 0.150. The tennis racketwith the index of inertia Ii being greater than or equal to 0.150 has excellent face stability. In light of this, the index of inertia Ii is more preferably greater than or equal to 0.152, and particularly preferably greater than or equal to 0.153. The upper limit of the range of the index of inertia Ii of the tennis racketsuitable for practical use is 0.180.

The mass of the tennis racketis preferably greater than or equal to 260 g and less than or equal to 320 g. The rackethaving a mass of greater than or equal to 260 g has excellent repulsion performance. In light of this, the mass of the tennis racketis more preferably greater than or equal to 270 g, and particularly preferably greater than or equal to 285 g. The rackethaving a mass of less than or equal to 320 g has excellent spin performance. In light of this, the mass of the tennis racketis more preferably less than or equal to 310 g, and particularly preferably less than or equal to 305 g. The mass is measured in a state where the stringis removed from the frame.

The tennis racketachieves all of the following: the in-plane stiffness index Gi being greater than or equal to 1.40; the out-of-plane stiffness index Go being greater than or equal to 60000 and less than or equal to 85000; the moment of inertia Mi being greater than or equal to 13500 g·cmand less than or equal to 15000 g·cm; and the index of inertia Ii being greater than or equal to 0.150. This tennis rackethas an excellent balance among repulsion performance, spin performance, and face stability.

As previously described, the tennis racketincludes bias-type reinforcement fibers. The bias-type reinforcement fibers can contribute to achieving the in-plane stiffness index Gi being greater than or equal to 1.40 and the out-of-plane stiffness index Go being greater than or equal to 60000 and less than or equal to 85000. In particular, the reinforcement fiberswith an inclination angle (θa or θb) having an absolute value of greater than or equal to 40° and less than or equal to 50° can contribute to the in-plane stiffness index Gi and the out-of-plane stiffness index Go. The ratio of the reinforcement fiberswith an inclination angle having an absolute value of greater than or equal to 40° and less than or equal to 50° to all of the reinforcement fibersincluded in the frameis preferably greater than or equal to 8% by mass, more preferably greater than or equal to 10% by mass, and particularly preferably greater than or equal to 12% by mass. This ratio is preferably less than or equal to 30% by mass, more preferably less than or equal to 25% by mass, and particularly preferably less than or equal to 20% by mass.

The in-plane stiffness index Gi being greater than or equal to 1.40 and the out-of-plane stiffness index Go being greater than or equal to 60000 and less than or equal to 85000 can be achieved through adjustments of the material, thickness, density, etc. of the reinforcement fibers.