US20080015063A1 - Racket frame and racket

Info

Abstract

Description

Claims

US20080015063A1

Publication number: US20080015063A1
Application number: US11/798,757
Authority: US
Inventors: Takeshi Ashino; Kunio Niwa
Original assignee: SRI Sports Ltd
Current assignee: Dunlop Sports Co Ltd
Priority date: 2006-07-11
Filing date: 2007-05-16
Publication date: 2008-01-17
Also published as: JP2008017951A

A racket frame including a yoke part disposed between left and right portions of a throat part continuous with a head part and a shaft part; a string-mounting part, including the yoke part and the head part, which surrounds the ball-hitting face; and string holes formed on the string-mounting part. In this construction, supposing that a total of longitudinal string holes whose axial directions are parallel with a line drawn from an apex of the ball-hitting face to a center of a gripping part and transverse string holes whose axial directions are parallel with a widthwise direction, of the racket frame, which is orthogonal to the axial direction of the racket frame is Ns1 and that a total of all of the string holes is N1, a ratio of Ns1 to N1 is set to not less than 0.70 nor more than 1.0.

This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 2006-190971 filed in Japan on Jul. 11, 2006, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a racket frame for use in a tennis racket and a racket on which strings are tensionally mounted to increase the sweet area thereof.
2. Description of the Related Art
As shown in FIG. 12, in a conventional racket frame, string holes 2 are formed in penetration through the string-mounting part constituted of the yoke part and the head part surrounding the ball-hitting face, with the string holes 2 disposed vertically to a tangent to the circular arc-shaped racket frame 1. In this method, it is easy to use a drill in making the string holes 2 in the direction from the inner peripheral side 1 a of the racket frame 1 to the peripheral side 1 b thereof. Further this method allows the length of the string S to be shorter than any other method in the region from the inner peripheral side 1 a of the racket frame 1 to the peripheral side 1 b thereof.
The string to be tensionally fastened to the racket frame is composed of longitudinal strings stretched in parallel or approximately parallel with the axis of the racket frame drawn from the top of the ball-hitting face to the center of the grip and transverse strings stretched orthogonally to the longitudinal strings.
Except the longitudinal string passing through the top of the racket frame 1 and the transverse string passing through both sides of the head part having the longest width, the string-stretched direction is not coincident with the direction in which the string holes 2 are formed in penetration through the racket frame 1. Because the string S is bent in contact with the inner opening 2 a of the string hole 2 disposed on the inner peripheral surface of the racket frame 1, the effective length of the string S is equal to the length between the inner peripheral edges of the opposed inner openings 2 a.
The sweet area of the racket frame can be increased by increasing the effective length of the string. Therefore proposals for increasing the effective length of the string have been hitherto made. For example, in the art disclosed in Japanese Patent Application Laid-Open No. 2001-252376 (patent document 1), as shown in FIG. 13, the grooves 5 are partly formed at the inner peripheral side of the face part 4 of the racket frame 3. The grooves 5 are disposed at the central portion in the thickness direction of the racket frame 3, with the grooves 5 extending in the circumferential direction of the face part 4. According to the disclosure, the sweet area can be increased because the effective length of the deformable string can be made longer than the length between the inner edges of opposed inner openings disposed at the inner peripheral surface of the racket frame 3.
The present applicant proposed the racket frame shown in FIG. 14, as disclosed in Japanese Patent Application Laid-Open No. 2000-61004 (patent document 2). In the art disclosed therein, in at least one part of a plurality of the string holes 7 formed in penetration through the inner peripheral part and peripheral part of the hollow racket frame 6, the inner diameter r2 of the inner opening 7 a formed in penetration through the inner peripheral part of the racket frame is set larger than the inner diameter r1 of the outer opening 7 b formed in penetration through the peripheral part thereof. In this construction, a gap is formed between the string inserted through the string hole 7 and the inner peripheral surface of the inner opening 7 a. The deformation support point of the string is disposed at the outer hole 7 b. Thereby the effective length of the string can be increased by the length between the inner peripheral part of the racket frame and the peripheral part thereof.
In the above-described racket frame 3 disclosed in the patent document 1, the increase amount of the effective length of the string is equal to only a part of the width parallel with the ball-hitting face. This construction does not contribute to an increase of the effective length of the string and is thus incapable of increasing the sweet area effectively.
It is difficult to obtain an outstanding sweet area-increasing effect if the ratio of the total number of the strings having an increased effective length to the total number of all strings is very low. In the racket frames 3 and 6 disclosed in the patent documents 1 and 2 respectively, neither the number of the string holes for increasing the effective length of the string nor the number of strings having the increased effective length is specified.
Patent document 1: Japanese Patent Application Laid-Open No. 2001-252376
Patent document 1: Japanese Patent Application Laid-Open No. 2000-61004

SUMMARY OF THE INVENTION

The present invention has been made in the above-described problem. Therefore it is an object of the present invention to provide a racket frame capable of securely and outstandingly displaying the effect of an increase of the sweet area owing to an increase of the effective length of a string; and a racket including the racket frame.
To achieve the object, the first invention provides a racket frame including a yoke part disposed between left and right portions of a throat part continuous with a head part and a shaft part; a string-mounting part, including the yoke part and the head part, which surrounds the ball-hitting face; and string holes formed on the string-mounting part. In this construction, supposing that a total of longitudinal string holes whose axial directions are parallel with a line drawn from an apex of the ball-hitting face to a center of a gripping part and transverse string holes whose axial directions are parallel with a widthwise direction, of the racket frame, which is orthogonal to the axial direction of the racket frame is Ns1 and that a total of all of the string holes is N1, a ratio of Ns1 to N1 is set to not less than 0.70 nor more than 1.0.
The racket frame of the present invention is formed with a continuous pipe, composed of a laminate of prepregs, which constitutes a peripheral wall surrounding a hollow portion of the racket frame. The string holes are formed in penetration through an inner peripheral wall of the racket frame and a peripheral wall thereof. The hollow portion of the racket frame is present between the inner peripheral wall of the racket frame and the peripheral wall thereof. That is, each of the string holes is constructed of an inner opening disposed at the ball-hitting face side, an outer opening at the peripheral side of the racket frame, and the hollow portion disposed between the inner opening and the outer opening. To smoothly insert the string through the string hole, the sectional area of the inner and outer openings of the string hole is set larger than that of the string.
As described above, the string to be mounted on the racket frame is composed of the longitudinal string and the transverse string tensionally. The longitudinal string is tensionally mounted thereon in parallel or almost parallel with the longitudinal axis thereof, on the ball-hitting face, which is drawn from the apex of the ball-hitting face to the center of the gripping part. The transverse string is tensionally mounted thereon with the transverse string orthogonal to the longitudinal string on the ball-hitting face.
When the axis of the string hole through which the longitudinal string is inserted is set parallel with the longitudinal direction of the racket frame which is coincident with the line drawn from the apex of the ball-hitting face to the center of the gripping part thereof, the axial direction of the longitudinal string hole is coincident with the stretched direction of the longitudinal string. Therefore it is possible to prevent the longitudinal string from contacting the periphery of the inner opening of the longitudinal string hole, with the longitudinal string in contact with the periphery of the outer opening of longitudinal string hole.
Similarly when the axis of the string hole through which the transverse string is inserted is set orthogonally to the longitudinal direction of the longitudinal string hole, the axial direction of the transverse string hole is coincident with the stretched direction of the transverse string. Therefore it is possible to prevent the transverse string from contacting the periphery of the inner opening of the transverse string hole, with the transverse string in contact with the periphery of the outer opening of the transverse string hole.
The string penetrating through the longitudinal string hole or the transverse string hole is fastened to the peripheral surface of the racket frame, with each string in contact with only the periphery of the outer opening of the longitudinal string hole or the transverse string hole. Therefore it is possible to dispose the deformation support point of the string at the outer edge of the outer opening and hence increase the effective length (variable length) of the string.
When the string to be inserted into the longitudinal string hole or the transverse string hole is inserted through a grommet inserted into the longitudinal string hole or the transverse string hole from the outer opening, it is preferable to form a gap between the peripheral surface of the grommet and the edge of the inner opening of the longitudinal string hole or the transverse string hole.
As described above, in the present invention, the ratio of the total Ns1 of the longitudinal and transverse string holes capable of increasing the effective length of the string to the total N1 of all the string holes is set to not less than 0.70 nor more than 1.0.
The conventional racket frame has Ns1/N1 at less than 0.70. Thus the racket frame of the present invention is capable of having a higher restitution performance than the conventional racket frame in the region including the left and right sides of the central portion of the ball-hitting face and the upper and lower sides thereof and hence increasing the sweet area in various directions greatly and in a favorable balance.
The lower limit of Ns1/N1 is set to favorably not less than 0.75, more favorably not less than 0.8, and most favorably not less than 0.88.
As described above, the axis of the longitudinal string hole is set parallel with the longitudinal direction of the racket frame, whereas the transverse string hole is set parallel with the widthwise direction thereof. The “parallel” specified herein means that the intersection angle formed between the axis of the longitudinal string hole and the longitudinal direction of the racket frame and the intersection angle formed between the axis of the transverse string hole and the widthwise direction of the racket frame are not more than ±10°.
It is favorable that the intersection angle of not less than 80% of all the longitudinal and transverse string holes is not more than +5°. It is particularly favorable that the intersection angle of not less than 80% of all the longitudinal and transverse string holes is not more than ±20. It is also favorable that the intersection angle of not less than 80% of each of the longitudinal and transverse string holes is not more than ±5°. It is also favorable that the intersection angle of not less than 90% of all the longitudinal and transverse string holes is not more than ±2°.
String holes formed at the left and right connection portions adjacent to the yoke part and the left and right portions of the throat part are the longitudinal string holes extended longitudinally from the ball-hitting face side of the left and right connection portions to a peripheral surface of the left and right portions of the throat part.
In the conventional racket frame, the string holes formed in the left and right portions of the triangular connection portion adjacent to the yoke part and the left and right portions of the throat part incline to the stretched direction of the longitudinal and transverse strings on the ball-hitting face. On the other hand, in the present invention, the longitudinal string holes are extended from the ball-hitting face side of the left and right connection portions to the peripheral surface of the left and right portions of the throat part in the same direction as the stretched direction of the longitudinal and transverse strings in the ball-hitting face.
It is possible to provide the long longitudinal string by extending it to the throat part and forming the outer opening on the peripheral surface of the throat part. The longitudinal string inserted into the longitudinal string hole is folded and tensionally mounted on the periphery of the racket frame, with the longitudinal string in contact with the outer edge of the outer opening disposed on the peripheral surface of the throat part. Therefore it is possible to greatly increase the effective length of the string.
The longitudinal strings inserted respectively into the longitudinal string holes extended longitudinally from the ball-hitting face side of the left and right connection portions to the throat part are disposed at the left and right sides of the sweet area located at the central portion of the ball-hitting face. Thus the longitudinal string holes are capable of transversely increasing the sweet area to a high extent.
The second invention provides a racket in which strings are fastened to the periphery of the racket frame with the strings stretched inside the ball-hitting face.
The racket of the second invention includes a racket including a yoke parts disposed between left and right portions of a throat part continuous with a head part and a shaft part; a string-mounting part, including the yoke part and the head part, which surrounds the ball-hitting face; and longitudinal strings and transverse strings mounted on the racket frame by stretching the longitudinal strings and the transverse strings on the ball-hitting face, with the longitudinal strings and the transverse strings in penetration through string holes formed on the string-mounting part. In this construction, supposing that a total of longitudinal string holes whose axial directions align with a direction in which the longitudinal strings inserted through the longitudinal string holes are stretched on the ball-hitting face and transverse string holes aligning with a direction in which the transverse strings inserted through the transverse string holes are stretched on the ball-hitting face is Ns2 and that a total of all of the string holes is N2, a ratio of Ns2 to N2 is set to not less than 0.70 nor more than 1.0.
The racket frame of the second invention on which strings are tensionally mounted is substantially the same as that of the first invention. The direction of the sting hole is so set that the axial direction thereof aligns with the stretched direction of the string inside the ball-hitting face.
That is, when the longitudinal string hole and the transverse string hole are formed on the racket frame in the same direction as the direction in which the longitudinal and transverse strings are stretched on the ball-hitting face, with the longitudinal and transverse strings inclined to the longitudinal and transverse string holes, the longitudinal string or the transverse string contacts the periphery of the inner opening of the longitudinal or the transverse string hole. Thereby it is impossible to increase the effective length of the longitudinal and transverse strings intended by the present invention.
Therefore in the second invention, the axial direction of the string hole is specified with respect to the direction in which the string is stretched on the ball-hitting face. That is, the string hole which allows the longitudinal and transverse strings to contact only the outer opening thereof the string hole is defined as the longitudinal and transverse string holes.
Therefore the sectional area of the inner opening of the string hole disposed adjacently to the ball-hitting face is set larger than that of the string inserted therethrough so that the string is inserted therethrough without contact between the inner surface of the inner opening and the string.
As described above, the direction in which the longitudinal and transverse string holes are extended aligns with the stretched direction of the longitudinal and transverse strings on the ball-hitting face. Thus the string inserted through the longitudinal string hole or the transverse string hole is fastened to the peripheral surface of the racket frame, with the string in contact with only the periphery of the outer opening thereof. Therefore it is possible to specify the deformation support point of the string at the outer edge of the outer opening and increase the effective length (variable length) of the string.
The total Ns2 (second invention) of the longitudinal string holes aligning with the direction in which the longitudinal strings inserted through the longitudinal string holes are stretched on the ball-hitting face and the transverse string holes aligning with the direction in which the transverse strings inserted through the transverse string holes are stretched on the ball-hitting face is equal to Ns1 (Ns1=Ns2) of the first invention unless the longitudinal strings and the transverse strings are inserted through the longitudinal string holes and the transverse string holes respectively by inclining the longitudinal strings to the longitudinal string holes and inclining the transverse strings to the transverse string holes.
Similarly to the first invention, the ratio of the total Ns2 of the longitudinal and transverse string holes which increases the effective length to the total N2 of all the string holes is set to not less than 0.70 nor more than 1.0. Thereby the racket of the second invention is capable of having a higher restitution performance than a racket having the Ns2/N2 at less than 0.70 in the region including the left and right sides of the central portion of the ball-hitting face and the upper and lower sides of the central portion thereof.
In the second invention, the axial direction of the longitudinal and transverse string holes align with the direction in which the longitudinal and transverse strings are stretched on the ball-hitting face. The “align” means that similarly to the definition of the first invention, the intersection angle formed between the axis of the longitudinal string hole and the axis of the longitudinal string on the ball-hitting face and the intersection angle formed between the axis of the transverse string hole and the axis of the transverse string on the ball-hitting face are not more than ±10°.
It is favorable that the intersection angle of not less than 80% of all the longitudinal and transverse string holes is not more than +50. It is particularly favorable that the intersection angle of not less than 80% of all the longitudinal and transverse string holes is not more than ±2°. It is also favorable that the intersection angle of not less than 80% of each of the longitudinal and transverse string holes is not more than ±5°. It is also favorable that the intersection angle of not less than 90% of all the longitudinal and transverse string holes is not more than ±2°.
In the second invention, even when the direction in which the longitudinal string is stretched on the ball-hitting face is unparallel with the longitudinal direction of the racket frame or/and even when the direction in which the transverse string is stretched on the ball-hitting face is unparallel with the transverse direction of the racket frame, it is possible to securely increase the effective length of the string by extending the longitudinal and transverse string holes in approximately alignment with the direction in which the longitudinal and transverse strings are stretched.
As described above, in the present invention, the deformation support point of the string is set to the outer edge of the outer opening of the string hole. Further the ratio of the total of the longitudinal and transverse string holes capable of increasing the effective length to the total of all the string holes is set to not less than 0.70 nor more than 1.0. Therefore the racket frame of the present invention is capable of having a higher restitution performance than the conventional racket frame in the region including the left and right sides of the central portion of the ball-hitting face and the upper and lower portions thereof and hence greatly increasing the sweet area in various directions in a favorable balance.
The longitudinal string holes are formed at the left and right connection portions adjacent to the yoke part and the left and right portions of the throat part by extending them longitudinally from the ball-hitting face side of the left and right connection portions to the peripheral surface of the left and right portions of the throat part. Thereby it is possible to dispose the deformation support point of the string at the outer edge of the outer opening of the longitudinal string hole formed at the throat part. This construction increases the variable length of the string inserted through the longitudinal string hole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a front view showing a racket frame according to a first embodiment of the present invention.

FIG. 1B is a side view showing the racket frame shown in FIG. 1A.

FIG. 2 is a main part-depicted front view showing a state in which strings are tensionally mounted on the racket frame shown in FIG. 1.

FIG. 3 is an enlarged sectional view showing parallel longitudinal string holes of the racket frame.

FIG. 4 is an enlarged sectional view showing parallel transverse string holes of the racket frame.

FIG. 5 is a perspective view showing the racket frame and a string protection member.

FIG. 6 is a main part-depicted front view showing a state in which strings are tensionally mounted on a racket of a second embodiment of the present invention.

FIG. 7 is an enlarged sectional view showing inclined string holes of the racket of the second embodiment.

FIG. 8 is a main part-depicted front view showing a state in which strings are tensionally mounted on a racket of a third embodiment of the present invention.

FIG. 9 is a main part-depicted front view showing a state in which strings are tensionally mounted on a racket of an example 4 of the present invention.

FIG. 10 is a main part-depicted front view showing a state in which strings are tensionally mounted on a racket of a comparison example 1.

FIG. 11 is a schematic view showing a method of measuring the coefficient of restitution of a racket.

FIG. 12 shows a conventional art of inserting strings into string holes.

FIG. 13 shows another conventional art.

FIG. 14 shows still another conventional art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be described below with reference to the drawings.
Rackets of the embodiments which will be described below are regulation-ball tennis rackets. In the embodiments described below, the longitudinal string and the transverse string are stretched in parallel with the longitudinal direction of the racket frame and the widthwise direction thereof respectively. The left part and the right part of the racket frame are symmetrical with respect to the longitudinal axis drawn from the apex of the ball-hitting face to the center of the gripping part.
FIGS. 1 through 5 show a racket 10 according to a first embodiment of the present invention.
In the racket 10, as shown in FIGS. 1 and 2, strings are tensionally mounted on a racket frame 11 of the present invention. In the first embodiment, the strings are tensionally fastened to 64 string holes 20. A ball-hitting face F includes 14 longitudinal strings 41 and 18 transverse strings 42. Thus the total of the longitudinal and transverse strings is 32.
The racket frame 11 is composed of a tubular body made of fiber reinforced prepreg sheets. The racket frame 11 has a head part 12, a throat part 11, a shaft part 14, and a grip part 15. These parts are continuously formed. The throat part 13 connecting the head part 12 and the shaft part 14 to each other is bifurcated. A yoke part 16 is formed between a pair of the left and right throat parts 13. A string-stretching part surrounding a ball-hitting face F is constructed of the yoke part 16 and the head part 12.
As shown in FIG. 1, a string groove 18 is circumferentially continuously formed on the head part 12 at the peripheral side thereof. Longitudinal string holes 21 and transverse string holes 22 are formed in penetration through the head part 12 in the range from the bottom surface (peripheral side of racket frame) of the string groove 18 to the inner peripheral surface of the heat part 12 where the head part 12 and the periphery of the ball-hitting face F come in contact with each other. The longitudinal string holes 21 are also formed in penetration through the yoke part 16.
As shown in FIGS. 2 and 3, three longitudinal string holes 21 are formed at left and right triangular connection portions 17 each disposed adjacently to the yoke part 16 and including a transition portion in the range from a left portion or a right portion of the throat part 13 to the head part 12 by extending the longitudinal string holes 21 from the connection portion 17 to the throat part 13. These longitudinal string holes 21 formed at the triangular connection portions 17 are hereinafter referred to as extended longitudinal string holes 21-1 to distinguish them from the longitudinal string holes 21 formed on the head part 12.
As shown in FIG. 3, the longitudinal strings 21 are formed in penetration through the racket frame 11, with axes L3 thereof parallel with a longitudinal axis L1 of the racket frame 11. Thus inner holes 21 a are formed on inner peripheral sides of the head part 12 and the yoke part 16, whereas outer holes 21 b are formed on peripheral sides of the head part 12 and the yoke part 16.
The extended longitudinal string holes 21-1 are formed in penetration through the racket frame 11, with axes L3-1 thereof almost parallel with the longitudinal axis L1 of the racket frame 11.
More specifically, the extended longitudinal string holes 21-1 are formed in penetration through the racket frame 11 in the range from the connection portion 17 to the peripheral surface of the throat part 13 with the extended longitudinal string holes 21-1 inclining at ±1° to the longitudinal axis L1 of the racket frame 11. Thus the inner holes 21 a are formed on an inner peripheral surface of the connection portion 17, whereas elliptic outer holes 21 b are formed on the peripheral surface of the throat part 13.
As shown in FIG. 4, the transverse string holes 22 are formed in penetration through the head part 12 with axes L4 thereof parallel with a widthwise direction L2 of the racket frame 11. Thus inner holes 22 a are formed on the inner peripheral surface of the head part 12, whereas outer holes 22 b are formed on the peripheral surface of the head part 12.
The size of the inner hole 21 a of the longitudinal string hole 21 (including the extended longitudinal string hole 21-1) and that of the inner hole 22 a of the transverse string hole 22 are so set that there is a gap between the longitudinal string 41 passing through the inner hole 21 a and the periphery of the inner hole 21 a to allow the longitudinal string 41 to be displaceable in the gap and that there is a gap between the transverse string 42 passing through the inner hole 22 a and the periphery of the inner hole 22 a to allow the transverse string 42 to be displaceable in the gap.
In tensionally mounting the longitudinal string 41 and the transverse string 42 on the racket frame 11, a string protection member 31, shown in FIG. 5, which has a bumper 33 and a grommet 32 is mounted on the string groove 18, with the string protection member 31 interposed between the strings 41, 42 and the racket frame 11.
The string protection member 31 is composed of a plurality of grommets 32 having insertion through-holes 32 a through which the strings 41, 42 are inserted and the bumper 33 connecting the base portions of the grommets 32 to each other, with the base portions of the grommets 32 projecting inward. The string protection member 31 can be made of thermoplastic resin or thermosetting resin. Nylon resin and urethane resin are favorable because these resins allow the string protection member 31 to have a high strength and flexibility. In this respect, the nylon resin is especially favorable.
In the racket frame 11 having the above-described construction, the total (N1) of the string holes 20 is set to 64. All of the string holes are the vertical and transverse string holes 21, 22 formed in penetration through the racket frame 11, with the longitudinal string hole 21 parallel with the longitudinal axis L1 or almost parallel therewith, namely, inclining at ±1° thereto and with the transverse string hole 22 parallel with the widthwise direction L2 or almost parallel therewith, namely, inclining at ±1° thereto. The total of the vertical and transverse string holes 21, 22 is denoted as Ns1.
In the racket 10 of the first embodiment, the longitudinal strings 41 are mounted on the racket frame 11 by stretching them in the direction parallel with the longitudinal axis L1 of the racket frame 11 and in the direction parallel with the widthwise direction L2 of the racket frame 11 respectively. Therefore on the ball-hitting face F, the axis L3 of each of the longitudinal string holes 21 aligns or almost aligns (inclines at ±1°) with the stretched direction of the longitudinal strings 41 passing therethrough. Similarly on the ball-hitting face F, the axis L4 of each of the transverse string holes 22 aligns or almost aligns (inclines at ±1°) with the stretched direction of the transverse strings 42 passing therethrough. Supposing that the total of the vertical and transverse string holes 21, 22 is Ns2, Ns1=Ns2, and the value of Ns1/N1 and that of Ns2/N2 are 1.0.
Therefore each of the 14 vertical strings 41 stretched on the ball-hitting face F has its deformation support point at its both ends fixed to the outer edge of two (upper and lower) outer openings 21 b of each longitudinal string hole 21. Similarly each of the 18 transverse strings 42 stretched on the ball-hitting face F has its deformation support point at its both ends fixed to the outer edge of two (left and right) outer openings 22 b of the transverse string hole 22. These vertical and transverse strings 41 and 42 are hereinafter referred to as “the string A having its deformation support point at its both ends fixed to the outer edge of the outer openings of the string hole”.
Because the effective length of all of the strings stretched on the ball-hitting face F is set large, the strings have improved restitution performance in a large area in various directions around the center of the ball-hitting face F. Therefore the sweet area can be enlarged in a favorable balance in various directions.
The extended longitudinal string holes 21-1 are formed in the range from the connection portion 17 to the throat part 13, with the extended longitudinal string holes 21-1 approximately parallel with the longitudinal axis L1 of the racket frame 11 so that the longitudinal string 41 comes in contact with only the outer hole 21 b. Therefore the effective length of each longitudinal string can be increased by the length of the corresponding extended longitudinal string hole 21-1. Owing to the formation of the extended longitudinal string holes 21-1, it is possible to greatly enhance the restitution performance of the sweet area at the left and right sides thereof.
FIGS. 6 and 7 show a racket of a second embodiment of the present invention having strings mounted on a racket frame 11 thereof.
In the second embodiment, the number of the longitudinal strings 41 is increased by two by disposing them at the left and right ends of longitudinal strings 41. Thus the total of the number of the longitudinal strings 41 is 16. 16 longitudinal strings 41 and 18 transverse strings 42 are mounted on the racket frame 11 by stretching them in penetration through 68 (N1, N2=68) string holes 20 (21, 22, 23, 24).
As shown in FIGS. 6 and 7, an axis L5 of each of four string holes formed in penetration through the racket frame 11 at the top side and the grip side thereof to receive the left-end longitudinal string 41 and the right-end longitudinal string 41 is vertical to an tangent L6 to the inner peripheral surface of the racket frame 11. The four string holes are denoted as inclined string holes 23.
An axis L7 of each of four string holes formed in penetration through the racket frame 11 at the top side and the grip side thereof to receive the upper-end transverse string 42 and the lower-end transverse string 42 is vertical to an tangent L8 to the inner peripheral surface of the racket frame 11. The four string holes are denoted as inclined string holes 24.
Except the inclined string holes 23, 24, the longitudinal string holes 21 and the transverse string holes 42 through which the longitudinal strings 41 and the transverse strings 42 are inserted respectively are the same as those of the racket of the first embodiment.
Similarly to the first embodiment, three extended longitudinal string holes 21-1 formed in penetration through each of the left and right connection portions 17 are almost parallel with the longitudinal axis L1 of the racket frame 11, i.e., inclines at ±1° thereto.
In the second embodiment, the total N1 of the string holes 20 is 68. The total (Ns1) of the longitudinal and transverse strings 21, 22 is 60. Therefore the value of Ns1/N1 is 0.88.
In the second embodiment, the greater part (30 strings) of all the strings (34 strings) is the string A having its deformation support point at its both ends fixed to the outer edge of the outer openings of the string hole. Therefore the racket frame 11 has an improved restitution performance and a large sweet area.
In the racket 10 of the second embodiment, the longitudinal strings 41 and the transverse strings 42 are mounted on the racket frame 11 by stretching them in the direction parallel with the longitudinal axis L1 of the racket frame 11 and in the direction parallel with the widthwise direction L2 of the racket frame 11 respectively. Therefore on the ball-hitting face F, the axis L3 of each of the longitudinal string holes 21 aligns or almost aligns (inclines at ±1°) with the stretched direction of the longitudinal strings 41 passing therethrough. Similarly on the ball-hitting face F, the axis L4 of each of the transverse string holes 22 aligns or almost aligns (inclines at ±1°) with the stretched direction of the transverse strings 42 passing therethrough. Therefore Ns2/N2 is 0.88 equal to the above-described Ns1/N1.
FIG. 3 shows a racket of the third embodiment.
In the third embodiment, similarly to the second embodiment, the total of the number of the longitudinal strings 41 is 16, and the total of the number of the transverse strings 42 is 18. The 34 longitudinal and transverse strings are mounted on the racket frame 11 by stretching them in penetration through 68 (N1, N2=68) string holes 20 (21, 22, 23, 24).
The racket frame 11 has eight inclined string holes 23, disposed at the top side thereof, into which eight (four left-end strings and four right-end strings) longitudinal strings 41 disposed at the left and right sides of the racket frame 11 are inserted respectively and two inclined string holes 23, disposed at the grip side thereof, into which the left-end and right-end longitudinal strings 41 are inserted respectively.
The racket frame 11 has four inclined string holes 24, disposed at the top side thereof, into which two transverse strings 42 are inserted respectively and six inclined string holes 24, disposed at the grip side thereof, into which three transverse strings 42 are inserted respectively.
Except the inclined string holes 23, 24, the longitudinal string holes 21 and the transverse string holes 22 are the same as those of the first embodiment.
Similarly to the first embodiment, three extended longitudinal string holes 21-1 formed in penetration through each of the left and right connection portions 17 are almost parallel with the longitudinal axis L1 of the racket frame 11, i.e., inclines at ±1° thereto.
In the third embodiment, the total of the string holes is 68 (=N1). The total (Ns1) of the longitudinal and transverse strings 21, 22 is 48. Therefore the value of Ns1/N1 is 0.71.
In the third embodiment, some of 24 strings which exceeds 70% of the total, namely, 34 strings are the string A having its deformation support point at its both ends fixed to the outer edge of the outer openings of the string hole. Some of 24 strings are strings B having deformation support point disposed at its one end fixed to the outer edge of the outer opening of the string hole. Because the variable length of the string A or the string B is long, the racket frame is capable of having improved restitution performance and an increased sweet area.
In the racket of the third embodiment, the longitudinal strings 41 a and the transverse strings 42 a are mounted on the racket frame 11 by stretching them in the direction parallel with the longitudinal axis L1 of the racket frame 11 and in the direction parallel with the widthwise direction L2 of the racket frame 11 respectively. Therefore on the ball-hitting face F, the axis L3 of each of the 22 longitudinal string holes 21 aligns or almost aligns (inclines at ±1°) with the stretched direction of the longitudinal strings 41 a passing therethrough. Similarly on the ball-hitting face F, the axis L4 of each of the 26 transverse string holes 22 aligns or almost aligns (inclines at ±1°) with the stretched direction of the transverse strings 42 a passing therethrough. Therefore Ns2/N2 is 0.71 equal to the above-described Ns1/N1.

EXAMPLES

As shown in table 1, a tennis racket of each of examples 1 through 4 and a comparison example 1 was formed. The total (Ns) of the longitudinal string holes and the transverse string holes, the total (N) of the string holes, the value of Ns/N, and the configurations of the longitudinal string holes formed at the connection portion 17 were differentiated from one another respectively. The coefficient of restitution of each tennis racket was measured at different hitting points on the ball-hitting face F.
The total Ns of the longitudinal and transverse string holes shown in table 1 corresponds to Ns1. The longitudinal and transverse strings were inserted through the longitudinal and transverse string holes respectively without inclining the longitudinal and transverse strings to the axial direction of the longitudinal and transverse string holes. Therefore in any of the racket frame of each of the examples 1 through 4 and the comparison example 1, Ns1=Ns2.

Weight (g)	260	260	260	260	260
Balance (mm)	360	360	360	360	360
Ns	64	60	48	18	54
N	64	68	68	68	68
Ns/N	1.00	0.88	0.71	0.26	0.79
Position of string hole at left and	Throat	Throat	Throat		5 o'clock on	5 o'clock on
right sides of yoke part				clock face	clock face

coefficient	spaced at 0 cm from	0.428	0.426	0.425	0.42	0.421
of	longitudinal axis
restitution	spaced at 18 cm from
	top
	spaced at 3 cm from	0.383	0.377	0.364	0.332	0.345
	longitudinal axis
	spaced at 18 cm from
	top
	spaced at 6 cm from	0.289	0.281	0.275	0.252	0.261
	longitudinal axis
	spaced at 18 cm from
	top

The racket frames of the examples 1 through 4 and the comparison example 1 were made of fiber reinforced thermoplastic resin and hollow and had the same configuration. The area of the ball-hitting face F was set to 110 square inches. The whole length of the racket frame was set to 27 inches (699 mm). The weight of the racket frame and the racket frame balance were set as shown in table 1.
More specifically, prepreg sheets made of the fiber reinforced resin composed of carbon fibers serving as the reinforcing fiber thereof and epoxy resin serving as the matrix thereof were layered at angles of 0°, 22°, 30°, 45° and 90° on a mandrel coated with a pressurized tube made of nylon 66 to form a vertical laminate of the prepreg sheets. After the mandrel was removed from the laminate, the laminate was set in a die. The die was clamped and heated to 150° for 30 minutes, with an air pressure of 9 kgf/cm²kept applied to the inside of the pressurized tube to form the racket frame of each of the examples and comparison examples.
In the racket frame of each of the examples 1 through 4 and the comparison example 1, the longitudinal string was tensionally fastened thereto in parallel with the longitudinal axis L1 thereof whereas the transverse string 42 was tensionally fastened thereto in parallel with the widthwise direction L2 thereof.

Example 1

The racket frame had the same construction as that of the racket frame of the first embodiment. More specifically, the racket frame had 64 string holes consisting of 28 longitudinal string holes 21 and 36 transverse string holes 22. Thus Ns/N was 1.0.
Three longitudinal string holes were formed as the extended longitudinal string holes 21-1 at each of the left and right connection portions 17 adjacent to the yoke part 16 and the left and right portions of the throat part 13 by extending the longitudinal string holes 21 to the peripheral surface of the throat part 13.

Example 2

The racket frame of the example 2 had the same construction as that of the second embodiment. That is, of 68 string holes, four inclined string holes 23 were formed at the top side of the racket frame and the grip side thereof to receive a left-end longitudinal string and a right-end longitudinal string, and four inclined string holes 24 were formed at the left side of the racket frame and the right side thereof to receive an upper-end transverse string and a lower-end transverse string. 60 string holes were formed as longitudinal and transverse strings 21 and 22. The value of Ns1/N1 was 0.88. The other constructions were the same as those of the example 1.

Example 3

The racket frame of the example 3 had the same construction as that of the third embodiment. That is, the racket frame had 68 string holes. The racket frame 11 had eight inclined string holes 23, disposed at the top side thereof, into which four left-side longitudinal strings 41 and four right-side longitudinal strings 41 are inserted respectively; two string holes 23, disposed at the grip side thereof, into which one left-end longitudinal string and one right-end longitudinal string are inserted respectively; four string holes 24 into which two top-side transverse strings are inserted; and six string holes 24 into which three grip-side transverse strings are inserted. 48 string holes were longitudinal and transverse strings 21 and 22. The value of Ns/N was 0.71.
The other constructions were the same as those of the examples 1 and 2.

Example 4

As shown in FIG. 9, of 68 string holes, the racket frame 11 had two inclined string holes 23, disposed at the top side thereof, into which one left-end longitudinal string and one right-end longitudinal string are inserted respectively; and 10 longitudinal string holes, disposed at the grip side thereof, into which five left-side longitudinal strings and five right-side longitudinal strings are inserted respectively; and two string holes 23, disposed at the top side thereof, into which one transverse string is inserted. 54 string holes were longitudinal and transverse strings 21 and 22. The value of Ns/N was 0.79
The inclined string holes 23 formed at each of the left and right connection portions 17 were disposed at 5 o'clock and 7 o'clock, supposing that the ball-hitting face F is regarded as a clock face.

Comparison Example 1

As shown in FIG. 10, of 68 string holes, the racket frame had 18 transverse string holes 22 into which nine transverse strings including fifth transverse string from the uppermost transverse string through 13th transverse string are inserted and 50 inclined string holes 23, 24. The value of Ns/N was 0.26.
The inclined string holes 23 formed at each of the left and right connection portions 17 were disposed at 5 o'clock and 7 o'clock directions, supposing that the ball-hitting face F is regarded as a clock surface.
Measurement of Coefficient of Restitution
As shown in FIG. 11, strings were tensionally mounted on the racket frame of each of the examples and the comparison example at a tensile force of 60 pounds in a vertical direction and 55 pounds in a horizontal direction. The grip part 15 of each tennis racket was fixed in such a way that each tennis racket was free in the vertical direction. A tennis ball launched from a ball launcher at a constant speed of V1 (30 m/sec) collided with the ball-hitting face of the racket frame at points A, B, and C thereof to measure the rebound speed V2 of the tennis ball. The restitution coefficient is obtained by computing the ratio of the rebound speed V2 to the launched speed V1 (V2/V1). The higher the restitution coefficient is, the higher the rebounding performance of the tennis racket is.
The above-described point A is a position spaced at 0 cm from the center of the ball-hitting face, namely, a position spaced at 0 cm from the longitudinal axis L1 of the racket frame and spaced at 18 cm from the top thereof. The above-described point B is a position spaced at 3 cm from the point A in the widthwise direction of the racket frame. The above-described point C is a position spaced at 6 cm from the point A in the widthwise direction of the racket frame.
As shown in table 1, the tennis rackets of the examples 1 through 4 in which the Ns/N was set to not less than 0.70 had higher coefficients of restitution than the tennis racket of the comparison example 1 at any of the hitting points A, B, and C. This is because the tennis rackets of the examples 1 through 4 had a larger number of strings having a large effective length than the tennis racket of the comparison example 1. Therefore in the tennis rackets of the examples 1 through 4, the region having a high restitution performance was not limited to the central portion of the ball-hitting face, but was widened. Particularly the extended longitudinal string holes greatly increase the effective length of the longitudinal strings tensionally fastened to the racket frame at the left and right sides of the ball-hitting face thereof. Therefore the tennis rackets of the examples 1 through 4 had outstandingly improved restitution performance at the left and right sides of the center of the ball-hitting face.
The restitution performance of the tennis racket of the example 3 and that of the tennis racket of the example 4 are compared with each other. The tennis racket of the example 3 had a little higher restitution performance than that of the example 4 at all of the points A through C, although the tennis racket of the example 4 had a larger number of strings having a large effective length than the tennis racket of the example 3. In the tennis racket of the example 3, the extended longitudinal string holes parallel with the longitudinal direction of the racket frame were formed at the connection portion. Thus the restitution performance at the points B and C widthwise spaced at the above-described interval from the center of the ball-hitting face can be more effectively enhanced than the tennis racket of the example 4 in which no extended longitudinal string holes were formed at the connection portion.

Comparison

1. A racket frame comprising: a yoke part disposed between left and right portions of a throat part continuous with a head part and a shaft part; a string-mounting part, including said yoke part and said head part, which surrounds said ball-hitting face; and string holes formed on said string-mounting part,

wherein supposing that a total of longitudinal string holes whose axial directions are parallel with a line drawn from an apex of said ball-hitting face to a center of a gripping part and transverse string holes whose axial directions are parallel with a widthwise direction, of said racket frame, which is orthogonal to said axial direction of said racket frame is Ns1 and that a total of all of said string holes is N1, a ratio of Ns1 to N1 is set to not less than 0.70 nor more than 1.0.

2. The racket according to claim 1, wherein string holes formed at said left and right connection portions adjacent to said yoke part and said left and right portions of said throat part are said longitudinal string holes extended longitudinally from said ball-hitting face side of said left and right connection portions to a peripheral surface of said left and right portions of said throat part.

3. A racket comprising: a yoke part disposed between left and right portions of a throat part continuous with a head part and a shaft part; a string-mounting part, including said yoke part and said head part, which surrounds said ball-hitting face; and longitudinal strings and transverse strings mounted on said racket frame by stretching said longitudinal strings and said transverse strings on said ball-hitting face, with said longitudinal strings and said transverse strings in penetration through string holes formed on said string-mounting part,

wherein supposing that a total of longitudinal string holes whose axial directions align with a direction in which said longitudinal strings inserted through said longitudinal string holes are stretched on said ball-hitting face and transverse string holes aligning with a direction in which said transverse strings inserted through said transverse string holes are stretched on said ball-hitting face is Ns2 and that a total of all of said string holes is N2, a ratio of Ns2 to N2 is set to not less than 0.70 nor more than 1.0.

4. The racket according to claim 3, wherein string holes formed in left and right connection portions adjacent to said yoke part and said left and right portions of said throat part are said longitudinal string holes extended in the same direction as that of said longitudinal strings stretched on said ball-hitting face from a ball-hitting face side of said left and right connection portions to a peripheral surface of said left and right portions of said throat part.

5. The racket according to claim 3, wherein an area of an inner opening of said string hole disposed at said ball-hitting face side is set larger than a sectional area of a string to be inserted through said string hole to allow said string to be inserted into said string hole without contact between an inner surface of said inner opening and said string.

6. The racket according to claim 4, wherein an area of an inner opening of said string hole disposed at said ball-hitting face side is set larger than a sectional area of a string to be inserted through said string hole to allow said string to be inserted into said string hole without contact between an inner surface of said inner opening and said string.