U.S. patent application number 14/195204 was filed with the patent office on 2014-09-11 for racket.
This patent application is currently assigned to YONEX KABUSHIKI KAISHA. The applicant listed for this patent is YONEX KABUSHIKI KAISHA. Invention is credited to Tomoya IWASAWA, Naoto OGAWA, Shinji SAITO.
Application Number | 20140256480 14/195204 |
Document ID | / |
Family ID | 50272300 |
Filed Date | 2014-09-11 |
United States Patent
Application |
20140256480 |
Kind Code |
A1 |
OGAWA; Naoto ; et
al. |
September 11, 2014 |
RACKET
Abstract
A racket including: a handle; an annular frame having a
plurality of through holes, and a groove section provided on an
outer peripheral face of the frame along a circumferential
direction of the frame; a shaft connecting the handle and the
frame; and a string that passes through the through hole to the
outside of the frame, and that is folded back on the groove
section, wherein when either one of a longitudinal direction of the
racket and a width direction perpendicular to the longitudinal
direction on a hitting surface of the racket is defined as a first
direction, a depth of a position at which the string is folded back
at a second position in the first direction is smaller or less than
a depth of a position at which the string is folded back at a first
position in the first direction, the first position being located
on the outer peripheral face of the frame, the second position
being located on the outer peripheral face of the frame and outside
of the first position in the first direction.
Inventors: |
OGAWA; Naoto; (Nagaoka-shi,
JP) ; SAITO; Shinji; (Nagaoka-shi, JP) ;
IWASAWA; Tomoya; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
YONEX KABUSHIKI KAISHA |
Tokyo |
|
JP |
|
|
Assignee: |
YONEX KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
50272300 |
Appl. No.: |
14/195204 |
Filed: |
March 3, 2014 |
Current U.S.
Class: |
473/543 |
Current CPC
Class: |
A63B 2049/0211 20130101;
A63B 49/022 20151001; A63B 60/54 20151001; A63B 49/02 20130101;
A63B 49/028 20151001; A63B 2049/0217 20130101; A63B 51/00
20130101 |
Class at
Publication: |
473/543 |
International
Class: |
A63B 51/00 20060101
A63B051/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 6, 2013 |
JP |
2013-044426 |
Claims
1. A racket comprising: a handle; an annular frame having a
plurality of through holes, and a groove section provided on an
outer peripheral face of the frame along a circumferential
direction of the frame; a shaft connecting the handle and the
frame; and a string that passes through the through hole to the
outside of the frame, and that is folded back on the groove
section, wherein when either one of a longitudinal direction of the
racket and a width direction perpendicular to the longitudinal
direction on a hitting surface of the racket is defined as a first
direction, a depth of a position at which the string is folded back
at a second position in the first direction is smaller or less than
a depth of a position at which the string is folded back at a first
position in the first direction, the first position being located
on the outer peripheral face of the frame, the second position
being located on the outer peripheral face of the frame and outside
of the first position in the first direction.
2. A racket according to claim 1, wherein the position at which the
string is folded back on the outer peripheral face of the frame
becomes shallower in depth as it goes from a center of the frame to
an end in the first direction.
3. A racket according to claim 1, wherein the frame has
substantially an elliptical shape, and a position in an end in the
longitudinal direction on the outer peripheral face of the frame at
which the string is folded back at a center in the width direction
is deeper in depth than a position in an end in the width direction
on the outer peripheral face of the frame at which the string is
folded back at a center in the longitudinal direction.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority upon Japanese Patent
Application No. 2013-44426 filed on Mar. 6, 2013 which is herein
incorporated by reference.
BACKGROUND
[0002] 1. Technical Field
[0003] The invention relates to a racket used in tennis and the
like.
[0004] 2. Related Art
[0005] As a racket used in tennis and the like, there are known
ones including: a handle which a player grips; a frame inside which
a string is strung; and a shaft connects the handle and the frame.
Generally, on the outer peripheral face of the frame, a groove
section is provided along the circumferential direction of the
frame and in the middle in the thickness direction (a direction
perpendicular to a hitting surface). In addition, along the
circumferential direction of the frame, the frame has a plurality
of through holes through which a string passes. The openings of the
through holes are formed in the groove section. When stringing the
string in the frame, the following processes are repeated: the
string passes from the inside to the outside of the frame through a
through hole; the string is folded back along the groove section of
the frame; and the string passes through the next through hole.
Then, in the frame, the portions of the string (longitudinal
strings) along the longitudinal direction of the racket are strung
at intervals in the width direction intersecting with the
longitudinal direction. Also, the portions of the string
(transverse strings) along the width direction are strung at
intervals in the longitudinal direction so as to intersect the
longitudinal strings. Thus, the net-like hitting surface is formed
in the frame (for example, see Japanese Patent Application
Laid-open Publication No. 6-269514).
[0006] The sweet spot, which has a high rebound performance of a
ball and vibrates less when hitting a ball, can be enlarged by
increasing the lengths of longitudinal strings and transverse
strings and by reducing the difference between the lengths of
longitudinal strings arranged in the width direction and/or the
difference between the lengths of transverse strings arranged in
the longitudinal direction. However, if the frame has an annular
shape such as an elliptical shape, the string is folded back along
the outer peripheral face (the groove section) of the curved frame.
Therefore, longitudinal strings and transverse strings passing the
ends of the frame are shorter in length than longitudinal strings
and transverse strings passing the center of the frame.
SUMMARY
[0007] The invention has been made in view of the above issue, and
an advantage thereof is to enlarge the sweet spot of a racket
without changing the shape of the frame.
[0008] An aspect of the invention to achieve the above advantage is
a racket including: a handle; an annular frame having a plurality
of through holes, and a groove section provided on an outer
peripheral face of the frame along a circumferential direction of
the frame; a shaft connecting the handle and the frame; and a
string that passes through the through hole to the outside of the
frame, and that is folded back on the groove section, wherein when
either one of a longitudinal direction of the racket and a width
direction perpendicular to the longitudinal direction on a hitting
surface of the racket is defined as a first direction, a depth of a
position at which the string is folded back at a second position in
the first direction is smaller or less than a depth of a position
at which the string is folded back at a first position in the first
direction, the first position being located on the outer peripheral
face of the frame, the second position being located on the outer
peripheral face of the frame and outside of the first position in
the first direction.
[0009] Other features of this invention will become apparent from
the description in this specification and the attached
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a plan view and a side view of a tennis
racket.
[0011] FIG. 2A is a cross-sectional view of a frame in the
thickness direction, and
[0012] FIG. 2B is a diagram showing how the string is strung in the
frame.
[0013] FIG. 3 is a diagram of a groove section provided in the
frame of the present embodiment.
[0014] FIG. 4 is a diagram of a groove section provided in the
frame of a comparative example.
[0015] FIG. 5 is a diagram of the frame and the groove section of a
modified example.
DESCRIPTION OF EMBODIMENTS
Overview
[0016] From the description in the detailed description of the
invention and the accompanied drawings, at least the following
matters will be apparent.
[0017] A racket including:
[0018] a handle;
[0019] an annular frame having [0020] a plurality of through holes,
and [0021] a groove section provided on an outer peripheral face of
the frame along a circumferential direction of the frame;
[0022] a shaft connecting the handle and the frame; and
[0023] a string [0024] that passes through the through hole to the
outside of the frame, and [0025] that is folded back on the groove
section, wherein
[0026] when either one of a longitudinal direction of the racket
and a width direction perpendicular to the longitudinal direction
on a hitting surface of the racket is defined as a first direction,
[0027] a depth of a position at which the string is folded back at
a second position in the first direction is smaller or less than a
depth of a position at which the string is folded back at a first
position in the first direction, [0028] the first position being
located on the outer peripheral face of the frame, [0029] the
second position being located on the outer peripheral face of the
frame and outside of the first position in the first direction.
[0030] With such a racket, it is possible that, without changing
the shape of the annular frame, the length of the string on any
outer end of the frame (the second position) in the first direction
becomes more equal to the length of the string at the center of the
frame (the first position) in the first direction. As a result,
sweet spot can be enlarged.
[0031] In such a racket, the position at which the string is folded
back on the outer peripheral face of the frame may become shallower
in depth as it goes from a center of the frame to an end in the
first direction.
[0032] With such a racket, it is possible to reduce the difference
in length between strings arranged in a direction intersecting with
the first direction. This makes it possible to reduce the
differences caused by different hitting positions in the frame of
the rebound performance of a ball and the feel, which results in
enlarging the sweet spot.
[0033] In such a racket, the frame may have substantially an
elliptical shape, and a position in an end in the longitudinal
direction on the outer peripheral face of the frame at which the
string is folded back at a center in the width direction may be
deeper in depth than a position in an end in the width direction on
the outer peripheral face of the frame at which the string is
folded back at a center in the longitudinal direction.
[0034] With such a racket, the sweet spot can be further enlarged
while preventing the strength deterioration of the frame. This also
allows the racket to rebound a ball more and to vibrate less.
Racket 1
[0035] An embodiment of a tennis racket will be described below as
an example of a racket according to the invention.
[0036] FIG. 1 is a plan view and a side view of a tennis racket
(hereinafter referred to as a racket 1). The left one is the plan
view and the right one is a side view. FIG. 2A is a cross-sectional
view of a frame 11 in the thickness direction, and FIG. 2B is a
diagram showing how a string 12 is strung in the frame 11. The
racket 1 includes: a head 10 which hits a ball; a handle 20 which a
player grips to hold the racket 1; and a shaft 30 connects the head
10 and the handle 20 in an integrated manner. For the purpose of
illustration, of the longitudinal sides of the racket 1, the side
of the head 10 is called as a tip-end side, and the side of the
handle 20 is called as a handle-end side. On the hitting surface of
the racket 1 (that is, on the plane along the hitting surface), a
direction perpendicular to the longitudinal direction is called as
a width direction. A direction perpendicular to the hitting surface
of the racket 1 is called as a thickness direction.
[0037] The head 10 includes: the frame 11 with an elliptical shape
extending in the longitudinal direction of the racket 1; and the
string 12 strung inside the frame 11. The frame 11 is made by
shaping into an elliptical shape, for example, a hollow rod which
is made of fiber reinforced resin and the like. It should be noted
that the frame 11 may not be hollow and the inside of the frame 11
may be filled with foamed material. As shown in FIG. 2A, in the
outer peripheral face 11a of the frame 11, a groove section 111 is
provided along the circumferential direction of the frame 11 and
the groove section 111 has the middle in the thickness direction
which is concave compared to the both side sections 112. In
addition, the frame 11 has a plurality of string holes 13
(corresponding to through holes) along the circumferential
direction of the frame 11; the string holes 13 extend through from
the inner peripheral face 11b to the outer peripheral face 11a and
are for inserting the string 12.
[0038] As shown in FIG. 2B, the opening of each string hole 13 is
provided on the groove section 111. In order to protect the string
12 and to dampen vibrations caused by hitting a ball, the string 12
is strung in the frame 11 through a grommet 40. The grommet 40
includes: A plurality of tube sections 41; and a strip-shaped base
section 42 which connects the plurality of tube sections 41 to each
other and has communicating holes 43 which communicate the hollow
of each tube section 41. The tip end of each of the tube section 41
is placed into the string hole 13 from the side of the outer
peripheral face 11a of the frame 11. Then, the base section 42 is
embedded in the groove section 111 of the frame 11. In this way,
the grommet 40 is placed in the outer peripheral face 11a of the
frame 11. In the base section 42 of the grommet 40, a groove
section 44 extending from one end of the base section 42 to the
other end is provided in the middle of the thickness direction; the
openings of the communicating holes 43 are provided in the groove
section 44. The thickness of the base section 42 (the thickness
from the surface on which the tube section 41 is placed to the
bottom surface of the groove section 44) is uniform.
[0039] Thus, when stringing the string 12 in the frame 11, the
string 12 passes from the inside of the frame 11 to the outside
through one of the tube sections 41 (the string holes 13) of the
grommet 40, and the string 12 is folded back on the groove section
44 of the grommet 40. In other words, through the groove section 44
of the base section 42 of the grommet 40, the string 12 is folded
back on the groove section 111 of the frame 11. Thereafter, the
string 12 is inserted into the next tube section 41 (string hole
13). This process is repeated. Thus, a plurality of "longitudinal
strings 121", which are the portions of the string 12 extending in
the longitudinal direction, are strung inside the frame 11 at
intervals in the width direction. A plurality of "transverse
strings 122", which are the portions of the string 12 extending in
the width direction, are strung at intervals in the longitudinal
direction so as to intersect the longitudinal strings 121. Thus, a
net-like hitting surface is formed.
[0040] The shaft 30 branches into two sections toward the tip end
in longitudinal direction as seen from the thickness direction, as
shown in the left diagram of FIG. 1. So, an opening 31 extending in
the thickness direction is formed by the branched portions and the
handle end of the frame 11. However, this invention is not limited
thereto. A racket may have an unbranched shaft 30.
Groove Section 111 of Frame 11
[0041] FIG. 3 is a diagram of the groove section 111 provided in
the frame 11 of the present embodiment. FIG. 4 is a diagram of the
groove section 111' provided in the frame 11' of a comparative
example. In each of these diagrams, of the outer peripheral face
11a of the frame 11, a solid line indicates the shape (the outer
peripheral edge) of the top surface 112a of both side sections in
the thickness direction (see FIG. 2A) as seen from the thickness
direction. Another solid line indicates the shape (the inner
peripheral edge) of the inner peripheral face 11b of the frame 11
as seen from the thickness direction. A dotted line indicates the
shape of the bottom surface 111a of the groove section as seen from
the thickness direction.
[0042] The width of the frame 11 of the present embodiment (the
thickness from the inner peripheral edge to the outer peripheral
edge) is substantially uniform through the entire circumference of
the frame 11. Of the outer peripheral face 11a of the frame 11, in
other portions in the longitudinal direction except the handle end,
that is, in other portions except the portions connecting to the
shaft 30 and the portion facing the opening 31, the groove section
111 is provided continuously along the circumferential direction of
the frame 11; the groove section 111 is a section on which the
string 12 passing through the string holes 13 to the outside of the
frame 11 is folded back. The depth of the groove section 111
differs in place. Note that the depth of the groove section 111
means the length from the bottom surface 111a of the groove section
to the outer peripheral edge in the direction perpendicular to the
bottom surface 111a. The groove section 111 is substantially
symmetric about the line along the longitudinal direction passing
the center of the frame 11 in the width direction, and is
substantially symmetric about the line along the width direction
passing the center of the frame 11 in the longitudinal
direction.
[0043] The depth of the groove section 111 will be described
concretely below. The portion of the groove section 111 of the
frame 11 provided on the tip end (hereinafter referred to as "the
top") in the longitudinal direction, in other words, the groove
section 111 within a range in which longitudinal strings 121 are
folded back, gradually becomes shallower in depth as it goes from
the center to the outside (left or right) in the width direction.
For example, in the top of the frame 11, the depth d1 of the groove
section 111 at the center P1 in the width direction is deepest. The
depth d4 of the groove section 111 at the point P4 outside (to the
right of) the center P1 in the width direction is shallower than
the depth d1 at the center P1. The depth d3 of the groove section
111 at the point P3 outside the point P4 in the width direction is
further shallower than the depth d4 at the point P4
(d1>d4>d3).
[0044] Similarly, The portions of the groove section 111 of the
frame 11 provided on the ends (hereinafter referred to as "sides")
in the width direction, in other words, the portions of the groove
section 111 within a range in which transverse strings 122 are
folded back, gradually become shallower in depth as they go from
the center to the outside (the tip end or the handle end) in the
longitudinal direction. For example, in the right side of the frame
11, the depth d2 of the groove section 111 at the longitudinal
center P2 is deepest. The depth d5 of the groove section 111 at the
point P5 outside (to the tip end of) the center P2 in the
longitudinal direction is shallower than the depth d2 at the center
P2. The depth d3 of the groove section 111 at the point P3
longitudinally outside the point P5 is further shallower than the
depth d5 at the point P5 (d2>d5>d3). It should be noted that,
in the present embodiment, the depth d1 of the groove section 111
of the top of the frame 11 at the center P1 in the width direction
is equal to the depth d2 of the groove section 111 of the sides of
the frame 11 at the longitudinal center P2.
[0045] On the other hand, in the frame 11' of the comparative
example shown in FIG. 4, unlike the frame 11 of the present
embodiment, the groove section 111' is uniform in depth.
Specifically, in the top of the frame 11, the depth d1 of the
groove section 111 at the center P1 in the width direction is equal
to the depth d3 of the groove section 111 at the outer end P3 in
the width direction. Also, in the right side of the frame 11, the
depth d2 of the groove section 111 at the longitudinal center P2 is
equal to the depth d3 of the groove section 111 at the longitudinal
outer end P3 (d1=d2=d3). In the comparative example, the elements
except for the depth of the groove section 111' of the frame 11',
such as the range in which the groove section 111' is formed and
the shape of the outer peripheral edge and the inner peripheral
edge, are the same as those of the frame 11 of the present
embodiment.
[0046] The sweet spot, which has a high rebound performance of a
ball and vibrates less when hitting a ball, can be enlarged by
increasing the lengths of the longitudinal strings 121 and the
transverse strings 122 and by reducing the difference between the
lengths of the longitudinal strings 121 arranged in the width
direction and/or the difference between the lengths of the
transverse strings 122 arranged in the longitudinal direction. As
shown in FIG. 4, the length (La) of one longitudinal string 121
means the length in the longitudinal direction from the point on
the handle-end side at which the longitudinal string 121 is folded
back to the point on the tip-end side at which the longitudinal
string 121 is folded back (to the bottom surface 111a of groove
section; more precisely, to the bottom surface of the groove
section 44 in the base section 42 of the grommet 40). Also, the
length (Lb) of one transverse string 122 means the length in the
width direction from the point on the left side at which the
transverse string 122 is folded back to the point on the right side
at which the transverse string 122 is folded back. Therefore, in a
case where the outer peripheral edge has an elliptical (annular)
shape as seen from the thickness direction as in the frame 11 of
the present embodiment and the frame 11' of the comparative
example, if the groove section 111 has constant depth as in the
frame 11' of the comparative example, the string 12 is folded back
along the bottom surface 111a' of the groove section, which is
greatly curved in the same manner as the outer peripheral edge of
the frame 11'. Thus, the longitudinal strings 121 passing both ends
of the frame 11' in the width direction are shorter in length than
the longitudinal strings 121 passing the center, and the transverse
strings 122 passing both ends of the frame 11' in the longitudinal
direction are shorter in length than the transverse strings 122
passing the center. Therefore, in the frame 11' of the comparative
example, only the center serves as the sweet spot, whose area is
not sufficiently large.
[0047] As opposed thereto, in the frame 11 of the present
embodiment, on the outer peripheral face 11a of the top of the
frame 11, compared to the portion of the groove section 111 at a
certain position (e.g. P1) in the width direction, the portion of
the groove section 111 is smaller or less in depth at a position
(e.g. P3) outside of the certain position (e.g. P1) in the width
direction. Therefore, in the present embodiment, the groove section
111 of the top of the frame 11 extending in the width direction has
a lower degree of curving than the outer peripheral edge and the
inner peripheral edge of the top of the elliptical frame 11. This
groove section 111 also has a lower degree of curving than the
groove section 111' of the top of the frame 11' of the comparative
example extending in the width direction. That is, the bottom
surface 111a of the groove section provided in the top of the frame
11 of the present embodiment is an arc-shaped surface the shape of
which is closer to straight line.
[0048] The string 12 is folded back on the groove section 111
provided on the outer peripheral face 11a of the frame 11 through
the base section 42 of the grommet 40, and the base section 42 of
the grommet is uniform in thickness. Thus, on the outer peripheral
face 11a of the top of the frame 11, compared to a certain position
(e.g. P1, corresponding to the first position) in the width
direction (corresponding to the first direction), the depth of the
position where the string 12 is folded back is smaller or less at a
position (e.g. P3) outside of the certain position (P1) in the
width direction. Not that "the depth at the position where the
string 12 is folded back" when stringing the string 12 through the
grommet 40 means the distance from the bottom surface of the base
section 42 of the grommet 40 to the outer peripheral edge in the
direction perpendicular to the bottom surface.
[0049] Similarly, on the outer peripheral face 11a of the sides of
the frame 11, compared to the portion of the groove section 111 at
a certain position (e.g. P2) in the longitudinal direction, the
portion of the groove section 111 is smaller or less in depth at a
position (e.g. P3) outside of the certain position (e.g. P2) in the
longitudinal direction. Therefore, in the present embodiment, the
groove section 111 of the sides of the frame 11 extending in the
longitudinal direction has a lower degree of curving than the outer
peripheral edge and the inner peripheral edge of the sides of the
elliptical frame 11. This groove section 111 also has a lower
degree of curving than the groove section 111' of the sides of the
frame 11' of the comparative example extending in the longitudinal
direction. The bottom surface 111a of the groove section provided
in the sides of the frame 11 of the present embodiment is an
arc-shaped surface the shape of which is closer to straight line.
That is, on the outer peripheral face 11a of the sides of the frame
11, compared to the depth of the position where the string 12 is
folded back at a certain position (e.g. P2, corresponding to the
first position) in the longitudinal direction (corresponding to the
first direction), the depth of the position where the string 12 is
folded back at a position (e.g. P3, corresponding to the second
position) outside of the certain position (e.g. P2) in the
longitudinal direction is smaller or less.
[0050] Therefore, in the top of the frame 11 of the present
embodiment, the portions of the bottom surface 111a of the groove
section which are located on the outer ends in the width direction
(the positions at which the longitudinal strings 121 are folded
back) are located closer to the outer peripheral edge compared to
the frame 11' of the comparative example. Similarly, in the sides
of the frame 11 of the present embodiment, the portions of the
bottom surface 111a of the groove section which are located on the
longitudinal outer ends (the positions at which the transverse
strings 122 are folded back) are located closer to the outer
peripheral edge compared to the frame 11' of the comparative
example. Therefore, in the frame 11 of the present embodiment, the
longitudinal strings 121 passing the outer ends of the frame 11 in
the width direction can be longer than in the frame 11' of the
comparative example. Also, in the frame 11 of the present
embodiment, compared to the frame 11' of the comparative example,
it is possible to reduce the difference in length between the
longitudinal strings 121 passing the outer ends of the frame 11 in
the width direction and the longitudinal strings 121 passing the
center. Also, the transverse strings 122 passing the outer ends of
the frame 11 in the longitudinal direction can be longer. And, it
is possible to reduce the difference in length between the
transverse strings 122 passing the outer ends of the frame 11 in
the longitudinal direction and the transverse strings 122 passing
the center. Therefore, even when hitting a ball with a region far
from the center of the frame 11, the difference is small from when
hitting a ball with the central region. This improves the rebound
performance of a ball and allows the racket to vibrate less when
hitting a ball (achieves the same feel). That is, sweet spot can be
enlarged.
[0051] As a concrete example, consider the following case: the
depth of the groove section 111' of the frame 11' in the
comparative example is 3 mm (a constant); the depth of the groove
section 111 of both sides of the frame 11 in the present embodiment
at the longitudinal center (e.g. P2 in FIG. 3) is 6 mm; the depth
of the groove section 111 at the longitudinal outer end (e.g. P3 in
FIG. 3) is 1 mm; and the depth of the groove section 111 becomes
gradually shallower from the center toward the outer end in the
longitudinal direction. In the frame 11' of the comparative example
of this case, when the length of the transverse string (the 10th
transverse string 122(10) from the tip end) passing the center of
the frame 11' in the longitudinal direction is defined as "100%",
the length of the transverse string (the third transverse string
122(3) from the tip end) passing the tip-end portion of the frame
11' in the longitudinal direction becomes "85%". As opposed
thereto, in the frame 11 of the present embodiment, when the length
of the transverse string 122(10) passing the center of the frame 11
in the longitudinal direction is defined as "100%", the length of
the transverse string 122(3) passing the tip-end portion of the
frame 11 in the longitudinal direction becomes "89%". From this, it
will be seen that, compared to the frame 11' of the comparative
example, the frame 11 of the present embodiment makes it possible
to reduce the difference between the lengths of the transverse
strings 122 arranged in the longitudinal direction.
[0052] Further, with the racket 1 of the present embodiment, the
sweet spot can be enlarged without changing the elliptical shape of
the frame 11. The frame 11 of the present embodiment has an
accurate elliptical shape. However, the invention is not limited
thereto. For example, the frame 11 may be substantially elliptical,
may have perfect circular shape, or may have a shape close to
rectangle. However, compared to a frame whose shape is close to
rectangle, a frame having an elliptical or perfect circular shape
obtains more the effect caused by making the lengths of the
longitudinal strings 121 close to each other or by making the
lengths of the transverse strings 122 close to each other. Also, a
frame having an elliptical or perfect circular shape can reduce air
resistance caused when swinging the racket.
[0053] The difference between the lengths of the longitudinal
strings 121 arranged in the width direction and/or the difference
between the lengths of the transverse strings 122 arranged in the
longitudinal direction are reduced across wide range as the depth
of the groove section 111 of the top of the frame 11 at the center
in the width direction is increased and/or as the depth of the
groove section 111 of the sides of the frame 11 at the center in
the longitudinal direction is increased. However, if the groove
section 111 is considerably deep in depth, the thickness from the
inner peripheral face 11b (inner peripheral edge) of the frame 11
to the bottom surface 111a of the groove section becomes small,
which results in deterioration of the strength of the frame 11.
Therefore, it is preferable that the depth of the groove section
111 is decided in the light of this fact.
[0054] In the racket 1 of the present embodiment, the groove
section 111 is provided on the outer peripheral face 11a of the
frame 11 and is not provided on the inner peripheral face 11b. The
lengths of the longitudinal strings 121 and the transverse strings
122, which affects enlargement of the sweet spot, are a distance in
the longitudinal direction or in the width direction from one-end
side point of the folding back of the string 12 to other-end side
point of the folding back of the string 12. So, even if the groove
section is provided on the inner peripheral face 11b of the frame
11, the groove section does not contribute enlargement of the sweet
spot. Further, providing the groove section 111 on the outer
peripheral face 11a of the frame 11 makes a manufacturing process
of the frame 11 easier than providing a groove section on the inner
peripheral face 11b (for example, a mold for shaping the frame 11
into an ellipse is easy to be manufactured). Furthermore, compared
to the case in which the groove section 111 is provided on the
inner peripheral face 11b of the frame 11, providing the groove
section 111 on the outer peripheral face 11a makes it possible to
reduce stress concentration when hitting a ball, and can increase
the strength of the frame 11.
[0055] In the racket 1 of the present embodiment, in the top of the
frame 11, the portions of the groove section 111 at the outer ends
in the width direction are shallower than the portion of the groove
section 111 at the center; the thickness and the strength from the
inner peripheral edge to the bottom surface 111a of the groove
section increase. Similarly, in the sides of the frame 11, the
portions of the groove section 111 at the outer ends in the
longitudinal direction are shallower than the portion of the groove
section 111 at the center; the thickness and the strength from the
inner peripheral edge to the bottom surface 111a of the groove
section increase. Thus, when hitting a ball, the frame 11 is likely
to be curved inwardly in the longitudinal direction and in the
width direction (the side which the ball hits is defined as the
inside). That is, the frame 11 is likely to deform so as to rap the
ball. This makes it possible to increase the contact area between
the ball and the hitting surface (string 12), which results in
improvement of control and spin.
[0056] If the frame 11 has an elliptical (annular) shape, when the
groove section 111' is uniform in depth as in the frame 11' of the
comparative example, the longitudinal strings 121 become shorter as
they pass through either outer portion of the frame 11 in the width
direction, and the transverse strings 122 become shorter as they
pass through either outer portion of the frame 11 in the
longitudinal direction. Thus, in the racket 1 of the present
embodiment, the portions of the groove section 111 which are
located on the outer peripheral face 11a of the frame 11, that is,
the portions where the string 12 is folded back, are shallower in
depth as they go outwards from the center in the longitudinal
direction or the width direction of the frame 11. That is, in the
top of the frame 11, the portion of the groove section 111 (the
positions at which the string 12 is folded back) becomes shallower
in depth as it goes outwards in the width direction. Also, in side
of the frame 11, the portions of the groove section 111 (the
positions at which the string 12 is folded back) become shallower
in depth as they go outwards in the longitudinal direction. This
makes it possible to reduce the difference between the lengths of
longitudinal strings 121 arranged in the width direction and/or the
difference between the lengths of transverse strings 122 arranged
in the longitudinal direction while increasing the lengths of the
longitudinal strings 121 and the transverse strings 122. This makes
it possible to reduce the differences caused by different hitting
positions in the frame 11 of the rebound performance of a ball or
the feel, which results in enlarging the sweet spot.
[0057] Further, it is preferable that the groove section 111 varies
gradually in depth without varying in steps in depth of the groove
section 111, in other words, without steps in the bottom surface
111a of the groove section. This makes it possible to prevent
stress when hitting a ball from concentrating into steps provided
in the bottom surface 111a of the groove section and to increase
the strength of the frame 11. This also makes it possible to
prevent change of the feel produced only when a ball hits a
corresponding portion within the frame 11 to the steps provided in
the bottom surface 111a of the groove section.
Modified Example
[0058] In the abovementioned embodiment, the base section 42 of the
grommet 40 is uniform in thickness. However, the invention is not
limited thereto. For example, on the outer peripheral face 11a of
the top of the frame 11, the portions of the base section 42 of the
grommet 40 provided in the outside in the width direction may be
thicker than the portions of the base section 42 of the grommet 40
provided in the center in the width direction. Similarly, on the
outer peripheral face 11a of the sides of the frame 11, the
portions of the base section 42 of the grommet 40 provided in the
outside in the longitudinal direction may be thicker in thickness
than the portions of the base section 42 of the grommet 40 provided
in the center in the longitudinal direction.
[0059] Compared to the case in which only the depth of the groove
section 111 of the frame 11 varies, this makes it possible to
further reduce the difference between the lengths of longitudinal
strings 121 arranged in the width direction and also to further
reduce the difference between the lengths of transverse strings 122
arranged in the longitudinal direction. Further, a thick portion of
the base section 42 of the grommet 40 is located far from the frame
11 which is highly rigid, so that elastic deformation of the
grommet 40 easily to occur. Thus, when a ball impacts the hitting
surface, if an force toward the inside of the frame 11 is produced
at the positions where the string 12 is folded back, the force can
be absorbed by the thick grommet 40 (the base section 42). Thus,
though the portions of the string passing in the outer ends in the
width direction or in the longitudinal of the frame 11 are slightly
shorter than the portions of the string passing in the center, the
thick grommet 40 (the base section 42) has a high vibration damping
capacity. Therefore, even when a player hits a ball with a region
far from the center of the frame 11, the player can experience the
feel nearly as much as when hitting a ball with a region of the
center. That is, the sweet spot can be enlarged.
[0060] It should be noted that, if the base section 42 of the
grommet 40 varies in thickness, the groove section 111 of the frame
11 may be uniform in depth. In other words, adjusting the thickness
of the base section 42 of the grommet 40 may achieve the
followings: on the outer peripheral face 11a of the frame 11,
compared to a certain position in the longitudinal direction, the
depth of the position where the string 12 is folded back is
shallower at a position outside of the certain position in the
longitudinal direction; on the outer peripheral face 11a of the
frame 11, compared to a certain position in the width direction,
the depth of the position where the string 12 is folded back is
shallower at a position outside of the certain position in the
width direction.
[0061] FIG. 5 is a diagram of the frame 11 and the groove section
111 of the modified example. The frame 1 of the racket 1 in the
abovementioned embodiment has an elliptical shape. However, the
invention is not limited thereto. For example, as shown in FIG. 5,
the frame 1 may have substantially an elliptical shape in which the
tip end and the handle end in the longitudinal direction have a
lower degree of curving than those of an ellipse.
[0062] Further, in the racket 1 of the abovementioned embodiment,
the depth (d1) of the point (P1 in FIG. 3) of the groove section
111 of the top of the frame 11 at the center in the width direction
is equal to the depth (d2) of the point (P2 in FIG. 3) of the
groove section 111 of the sides of the frame 11 at the center in
the longitudinal direction. However, the invention is not limited
thereto. The groove section 111 may vary in depth. As mentioned
above, the difference between the lengths of longitudinal strings
121 arranged in the width direction and/or the difference between
the lengths of transverse strings 122 arranged in the longitudinal
direction can be reduced as the depth of the groove section 111 of
the top of the frame 11 at the center in the width direction is
increased and/or as the depth of the groove section 111 of the
sides of the frame 11 at the center in the longitudinal direction
is increased. This reduces the thickness of the frame 11 from the
inner peripheral face 11a to the bottom surface 111a of the groove
section, which results in deterioration of the strength of the
frame 11. However, if the frame 11 is substantially in an
elliptical shape (including a perfect elliptical shape), the frame
11 is strong and difficult to deform because the top of the frame
11 is smaller in size than the sides. Further, since longitudinal
strings 121 are longer than transverse strings 122, reducing the
difference between the lengths of longitudinal strings 121 arranged
in the width direction is more effective than reducing the
difference between the lengths of transverse strings 122 arranged
in the longitudinal direction. That is, it is possible to further
enlarge the sweet spot; also this allows the racket to rebound a
ball more and to vibrate less.
[0063] As shown in FIG. 6, it is preferable that, in the frame 11
having substantially an elliptical shape, the depth (D+.alpha.) of
the point (P1 in FIG. 5) of the groove section 111 of the outer
peripheral face 11a of the top (the end in the longitudinal
direction) at the center in the width direction be deeper than the
depth (D) of the point (P2 in FIG. 5) of the groove section 111 of
the outer peripheral face 11a of either side (either end in the
width direction) at the center in the longitudinal direction. That
is, it is preferable the depth at the position where the string 12
is folded back on the outer peripheral face 11a of the top of the
frame 11 at the center in the width direction is deeper than the
depth at the position where the string 12 is folded back on the
outer peripheral face 11a of the sides of the frame 11 at the
center in the longitudinal direction. As a result, the sweet spot
can be further enlarged while preventing the strength deterioration
of the frame 11. This also allows the racket to rebound a ball more
and to vibrate less.
[0064] In the racket 1 of the abovementioned embodiment, all
portions of the groove section 111 located in the top and both
sides of the frame 11 become shallower in depth as they go from the
center to the outer ends. However, the invention is not limited
thereto. For example, at least one portion of the groove section
111 located in the top and both sides of the frame 11 may be
different in depth. Further, in the racket 1 of the abovementioned
embodiment, the groove section 111 is not provided in the handle
end (the portion facing the opening 30) of the frame 11 in the
longitudinal direction. However, the invention is not limited
thereto. For example, the groove section 111 may be provided in the
handle end and the groove section 111 may be shallower in depth as
it goes from the center in the width direction to the outer ends.
Furthermore, in the racket 1 of the abovementioned embodiment, the
groove section 111 varies gradually in depth. However, the
invention is not limited thereto. For example, the groove section
111 may vary in depth by providing steps in the bottom surface 111a
of the groove section.
Other Embodiments
[0065] It should be noted that the aforementioned embodiment is for
facilitating understanding of the invention, and are not limiting
of the invention, and are not to be interpreted as limiting the
invention. The invention can of course be altered and improved
without departing from the gist thereof, and equivalents are
intended to be embraced therein.
[0066] For example, in the abovementioned embodiment, a tennis
racket is taken as an example of a racket associated with the
invention. However, the invention is not limited thereto. For
example, the invention may be applied to a squash racket, a
badminton racket and the like. Also, in the abovementioned
embodiment, the racket 1 in the frame 11 of which the string 12 is
strung is taken as an example of a racket associated with the
invention. However, the invention is not limited thereto. A racket
in which the string 12 is not strung may also be employed.
* * * * *