U.S. patent application number 11/798757 was filed with the patent office on 2008-01-17 for racket frame and racket.
This patent application is currently assigned to SRI Sports Limited. Invention is credited to Takeshi Ashino, Kunio Niwa.
Application Number | 20080015063 11/798757 |
Document ID | / |
Family ID | 38949944 |
Filed Date | 2008-01-17 |
United States Patent
Application |
20080015063 |
Kind Code |
A1 |
Ashino; Takeshi ; et
al. |
January 17, 2008 |
Racket frame and racket
Abstract
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.
Inventors: |
Ashino; Takeshi; (Hyogo,
JP) ; Niwa; Kunio; (Hyogo, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
SRI Sports Limited
|
Family ID: |
38949944 |
Appl. No.: |
11/798757 |
Filed: |
May 16, 2007 |
Current U.S.
Class: |
473/524 |
Current CPC
Class: |
A63B 49/022 20151001;
A63B 60/00 20151001; A63B 49/025 20151001; A63B 49/02 20130101 |
Class at
Publication: |
473/524 |
International
Class: |
A63B 49/02 20060101
A63B049/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2006 |
JP |
2006-190971 |
Claims
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.
Description
[0001] This nonprovisional application claims priority under 35
U.S.C. .sctn. 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
[0002] 1. Field of the Invention
[0003] 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.
[0004] 2. Description of the Related Art
[0005] 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 1a of the racket frame 1
to the peripheral side 1b 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 1a of the racket frame 1 to
the peripheral side 1b thereof.
[0006] 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.
[0007] 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 2a 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 2a.
[0008] 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.
[0009] 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 7a formed in penetration through the inner peripheral part
of the racket frame is set larger than the inner diameter r1 of the
outer opening 7b 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 7a. The deformation support point of the
string is disposed at the outer hole 7b. 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.
[0010] 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.
[0011] 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.
[0012] Patent document 1: Japanese Patent Application Laid-Open No.
2001-252376
[0013] Patent document 1: Japanese Patent Application Laid-Open No.
2000-61004
SUMMARY OF THE INVENTION
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.degree..
[0026] 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.degree.. 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.degree.. 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.degree..
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.degree..
[0041] 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.degree.. 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.degree.. 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.degree..
[0042] 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.
[0043] 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.
[0044] 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
[0045] FIG. 1A is a front view showing a racket frame according to
a first embodiment of the present invention.
[0046] FIG. 1B is a side view showing the racket frame shown in
FIG. 1A.
[0047] 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.
[0048] FIG. 3 is an enlarged sectional view showing parallel
longitudinal string holes of the racket frame.
[0049] FIG. 4 is an enlarged sectional view showing parallel
transverse string holes of the racket frame.
[0050] FIG. 5 is a perspective view showing the racket frame and a
string protection member.
[0051] 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.
[0052] FIG. 7 is an enlarged sectional view showing inclined string
holes of the racket of the second embodiment.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] FIG. 11 is a schematic view showing a method of measuring
the coefficient of restitution of a racket.
[0057] FIG. 12 shows a conventional art of inserting strings into
string holes.
[0058] FIG. 13 shows another conventional art.
[0059] FIG. 14 shows still another conventional art.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0060] The embodiments of the present invention will be described
below with reference to the drawings.
[0061] 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.
[0062] FIGS. 1 through 5 show a racket 10 according to a first
embodiment of the present invention.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] 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 21a are formed on inner peripheral sides of the head
part 12 and the yoke part 16, whereas outer holes 21b are formed on
peripheral sides of the head part 12 and the yoke part 16.
[0068] 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.
[0069] 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.degree. to the longitudinal axis L1 of the
racket frame 11. Thus the inner holes 21a are formed on an inner
peripheral surface of the connection portion 17, whereas elliptic
outer holes 21b are formed on the peripheral surface of the throat
part 13.
[0070] 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 22a are formed on the inner peripheral surface of the
head part 12, whereas outer holes 22b are formed on the peripheral
surface of the head part 12.
[0071] The size of the inner hole 21a of the longitudinal string
hole 21 (including the extended longitudinal string hole 21-1) and
that of the inner hole 22a of the transverse string hole 22 are so
set that there is a gap between the longitudinal string 41 passing
through the inner hole 21a and the periphery of the inner hole 21a
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 22a and the periphery of the inner hole 22a
to allow the transverse string 42 to be displaceable in the
gap.
[0072] 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.
[0073] The string protection member 31 is composed of a plurality
of grommets 32 having insertion through-holes 32a 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.
[0074] 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.degree. thereto and with the transverse string hole 22
parallel with the widthwise direction L2 or almost parallel
therewith, namely, inclining at .+-.1.degree. thereto. The total of
the vertical and transverse string holes 21, 22 is denoted as
Ns1.
[0075] 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.degree.) 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.degree.) 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.
[0076] 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 21b 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 22b 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".
[0077] 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.
[0078] 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 21b. 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.
[0079] FIGS. 6 and 7 show a racket of a second embodiment of the
present invention having strings mounted on a racket frame 11
thereof.
[0080] 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).
[0081] 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.
[0082] 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.
[0083] 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.
[0084] 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.degree. thereto.
[0085] 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.
[0086] 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.
[0087] 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.degree.) 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.degree.) with
the stretched direction of the transverse strings 42 passing
therethrough. Therefore Ns2/N2 is 0.88 equal to the above-described
Ns1/N1.
[0088] FIG. 3 shows a racket of the third embodiment.
[0089] 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).
[0090] 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.
[0091] 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.
[0092] 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.
[0093] 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.degree. thereto.
[0094] 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.
[0095] 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.
[0096] In the racket of the third embodiment, the longitudinal
strings 41a and the transverse strings 42a 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.degree.) with the stretched
direction of the longitudinal strings 41a 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.degree.) with the stretched direction of the transverse
strings 42a passing therethrough. Therefore Ns2/N2 is 0.71 equal to
the above-described Ns1/N1.
EXAMPLES
[0097] 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.
[0098] 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.
TABLE-US-00001 TABLE 1 Comparison Example 1 Example 2 Example 3
example 1 Example 4 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
[0099] 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.
[0100] 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.degree., 22.degree.,
30.degree., 45.degree. and 90.degree. 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.degree. for 30 minutes, with an air pressure of 9
kgf/cm.sup.2 kept applied to the inside of the pressurized tube to
form the racket frame of each of the examples and comparison
examples.
[0101] 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
[0102] 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.
[0103] 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
[0104] 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
[0105] 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.
[0106] The other constructions were the same as those of the
examples 1 and 2.
Example 4
[0107] 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
[0108] 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
[0109] 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.
[0110] 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.
[0111] Measurement of Coefficient of Restitution
[0112] 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.
[0113] 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.
[0114] 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.
[0115] 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.
* * * * *