Note: Descriptions are shown in the official language in which they were submitted.
WO 91/1 15~9 PCTtUS90/01698
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RIDGED RACQU13T STRING
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BACRGROUND OF T13E INVENTION
Ei~l~ Qf the Invention
This invention relates in general to a sports
racquet string and more specifically involves a string
configuration that imparts more spin on the ball and to a
method of manufacturing s~ch an improved string.
Background Qf the Inventlon
The traditional and most popular cross-section of a
sports racquet string is round. Such strings are made
typically from natural gut (animal fiber) or from
synthetic material,-such as nylon. Conventionally,
strings are constructed by twisting many fine filaments
of these materials together, with or without a center
20 filament, into a round core strand and then by passing `
the core strand through a round die to apply an outer
layer coating.
In general, it is desirable that a string exhibit
small damp~ng, that is low energy los~ and high
resilience, and good elasticity, that is a low modulus of
elasticity. These elements contribute to the playability -
of the string. It is also desirable that the string be
sufficiently durable.
The diameter of the string is very important as it
affects the durability and playability of the string.
Generally, thin strings have superior playability. Thin
strings exhibit high resilience and good elasticity, and
they maintain longer contact with the ball for greater
;control. ~owever, thin strings may stretch and are more
easily broken. On the other hand, thick strings are
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WO91/11549 PCT/~'S90/0169X
28~ ; 2
stronger and more durable but lack the playability of
thin strings.
An additional important characteristic of a string
is its ability to impart spin on the ball. For ex,ample,
in the game of tennis, a player standing behind the
baseline would have to have a height of about six foot
seven to see any of the opponent's court without looking
thru the net. This means that most hard-hit balls
passing over the net and not having forward spin will
land out of bounds over the opponent's baseline. Ball
spin affects the ball's flight characteristics. When a
ball leaves the racquet string bed spinning forward, i.e.
rotating forward on top, it's flight path will tend to
curve downward, and it will land earlier and bounce
lower. With good top spin, a player can hit a given ball
much harder and still have the ball land in. When a ball
leaves the racquet string bed spinning backward, i.e.
rotating forward on bottom, its flight path is flatter;
it will tend to land further and bounce higher. Thus, if
the player can control spin, the player can control to
some degree the trajectory of the ball to advantage.
Again, string characteristics largely determine the
amount of spin that can be imparted on the ball. As
previously mentioned, the amount of string elongation and
resilience determines the amount of time the strings are
in contact with the ball. Generally, the thinner the
string, the~ greater the contact time. When the ball
impacts on the racquet face, the ball remains in contact
with the string bed for about three to five thousandths
o~f a second. ~During this time, the player is able to
impose more control over the direction of ball return and
is able to impart spin to the ball to control its flight
characteristics.
To put spin on the ball, the ball is struck with the
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W091/11549 PCT/~'S90/01698
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-racket face at an angle to the flight path and the racket
face is moved in the plane of the face. Increasing the
friction between the strings and the hall has been
thought to enhance imparting spin on the ball.
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Synthetic fiber strings, in particular, are
excessively smooth in their outer surface and tend to
slip over the ball. Many measures have been taken to
enhance friction including: roughening t`ne outer surface
of the string such as by grinding with abrasives, surface
coating the string with frictional or rubbery substances, -
twisting or braiding fiber multifiliments, and winding of
silk yarns around the string core.
15Synthetic strings treated in the above-described
manners tend to have poor dimensional stability and are
reduced in strength and elasticity resulting in tension
1088 during play. Further, some are inferior in
durability because they exhibit surface aberrations,
wearings or breakages due to degredation of the resins,
and abrasion, peeling or denaturing of the treating
substances. Moreover, since the above-mentioned
treatment6 constitute additional 6teps in manufacture,
there is an increase in production costs.
Another proposed method to increase string friction
is the use of string of polygonal c~oss-section whereby
the sharp corners resulting at the juncture of the faces
of the polygon are the spin enhancing portions. Two
types of polygonal-shaped string have been proposed.
Reta, U.S. Patent No. 4,805,393 proposes the use of a
-multi-sided cross-sectional configuration string; Wells,
U.S~ Patent No. 4,860,531, proposes a polygonal coating
over a round central core.
; Traditional round strings may have a thin, evenly
distributed coatin5 around the core to provide protection
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WO91/11549 PCT/US90/01698
to the core strands which provide the tensile strength
and playability of the string.
As two round strings .cross over one another, due to
the very small contact area they weaken one another by
indenting one another and by cutting one another in a
sawing action as the strings move relative to one another
during play. ;
Therefore, it is desirable to have an improved
sports racquet string having much better spin-imparting
characteristics than a conventional round string and
which achieves this without significant loss in
playability.
It is further desirable, that such a string reduce
the weakening characteristics of string cross-over and
therefore be more durable than conventional strings.
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S~M~ARY OF T~ INV~RTION
According to the invention, a string for a sports
racquet, such as a tennis racquet string, is comprised of
a core having a plurality of ridges thereon for aiding in
imparting spin on a ball. The ridges are parallel to one
another and extend axially with the string. Preferably,
the inner core has a diameter of l.00 - 1.30 millimeters.
ireferably, the ridges are of approximately 0.25
millimeters in height. Preferably, the ridges can be
circumscribed by a circle of 1.70 millimeters.
` ~ The process for making the ridged string includes
passing a string core through a bath of resin, such as
:
nylon, for coating and glueing the core and then through
a die shaped so as to form the ridges.
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WO91/11s4s ~ O~ PCT/US90/01698
Preferably the size, spacing, and number of the
ridges increase contact area between strings at the
cross-over points and add to the stability to prevent
string rotation and movement relative to one another.
Other features and many attendant advantages of the
invention will become more apparent upon a reading of the
following detailed description together with the drawings
in which like reference numerals refer to like parts
throughout.
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BRIFF DESCRIPTION OF T~E DRAHING
Figure 1 is a perspective view of a segment of aprefered embodiment of the ridged racquet string of the
invention.
Figure 2 i8 a slightly enlarged cross-sectional view
of the string of Figure 1 shown passing over a similar
string, such as while strung on a racquet.
Figure 3 is a perspective view of a segment of an
alternate exemplary embodiment of the ridged racquet
string of the invention.
Figure 4 is a slightly enlarged cross-sectional view
i~ 30 of the ridged string of Figure 3 shown passing over a
~ similar string, such as while strung on a racquet.
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Figure 5 is a view of the extrusion step in the
manufacture of the ridged string of the invention.
- ` Figure 6 is a cross-sectional view of an alternate ~ -
preferred embodiment of the ridged racquet string of the
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WO91/lIS49 PCT/US90/01698
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invention; this one having four triangular ridges.
Fig~re 7 is a cross-sectional view of another
alternate exemplary embodiment of the ridged racquet
string of the invention; this one having three triangular
ridges.
DXTAILED D~SCRIPTION OF T~E INVENTION
With reference now to the drawing, and more
particularly to Figure 1 thereof, there is shown, in
perspective view, a segment of a prefered embodiment of
the ridged sports racquet string, denoted generally as
10, of the present invention. Ridged string lC includes
a core, denoted generally as 20, and a plurality of
ridges 40 bonded to the core 20. Ridges 40 run axially
with length of the string and are parallel to one
another.
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Turning now to Figure 2, the ridged string 10 of
Figure 1 i8 shown in cross-6ection with the addition of
showing the string 10 as it cros6e6 over a similar string
10a such as it would in a strung racquet. The cross-over
point of-Figure 2 is simplified in that it does not show
the true intermeshing of the strings 10,10a and their
ridges 40,40a in the strung condition. At cross-over,
the string cores and ridges indent one another and the
string cores and ridges protrudé into the concavities
between the ridges and core of the cross-over string.
This intermeshing prevents rotation of the strings
relative to one another.
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In the prefered embodiment shown, the string core 20
is made of construction well-known in the art. Core 20
is of synthetic material and is of composite construction
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wo91/1l5~9 PCT/~'S90/0169X
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comprising an inner core 24 and and an outer core 28.
An inner core can be a thick extruded monofilament
or be of the "fiber" type. Inner core 24, shown,is of
the "fiber" type and comprises a multiplicity of of small
diameter monofilament fibers or strands which are twisted
and glued together to form the inner core structure. The
small monofilament fibers need not be continuous as the
friction of the package prevents slippage. In cross-
section, a fiber inner core typically contains fivehundred to three thousand fibers.
Many synthetic materials are available for such use
including nylon, polyester, kevlar, zyex
(polyetheretherketon), boron, and graphite fiber.
Various glue compositions are well-known in the art. A
common glue for synthetic fiber is nylon resin. After
glueing, inner core 24 is cured, such as at 150 degrees
C. for two minutes.
Inner core 24 is helically wrapped by a layer of
larger strands 29 which form an outer core 28. If the
outer core is formed by two or more layers of large
strands, then adjacent layers are helically wound in
25 opposite directions. -~
As best seen in Figure 5, the string core 20 is
processed by passing i* through a bath and die apparatus,
denoted generally as 70. String core 20 is immersed in
30 a bath 72 of suitable molten ridge forming material, such -
as nylon 71, and, upon exiting the bath 72 is passed thru
a die 74 which leaves the desired nylon ridges 40
remaining. The nylon also acts as a glue, fills voids in
the core, and forms a thin coating over the core. String
10 is than cooled, such as by water at 20 degrees C. The
resulting string can be further processed, such as for
moisture control or thermosetting, as desired.
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WO91/1l549 PCT/US90/01698
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Ret~lrning once more to Figure 2, the results of the
bath and die processing can be seen in cross-section.
- Nylon 71 from bath 72 acts as a glue and coating for the
core 20. Extrusion die 74 has left ridges 40 bonded to
the outside of the core and extending parallel axially
with the string. In the exemplary embodiment of Figure
2, five evenly spaced ridges are shown. Ridges 40 are
arc-shaped and project radially outward from core 20.
Ridge~ 40 impart more spin on a struck ball.
Preferably, ridges 40 may be circumscribed by a
maximum diameter circle 90, shown in dashed line. Figure
2 is an enlarged view of a sting having a core 20 of 1.20
millimeters and a maximum diameter circle of 1.45
millimeters. Preferably, the largest maximum diameter
circle is 1.70 millimeters.
The height, shape, and spacing of ridges 40 are
important. Unless the maximum diameter circle 90 has a
diameter 0.15 millimeters greater than the core, so that
the ridges are at least 0.075 millimeters in height, the
ridges are too small to be effective. Ridge heights of
about 0.25 millimeters have been found to produce good
25 results. A core diameter of 1.00 - 1.30 millimeters with
a maximum diameter circle of 1.35 - 1.70 millimeters has
been found to impart superior spin without detracting
from the other aspects of playability and therefore seems
preferable.
The ridge radius is the outer arc of the ridge 40.
A ridge radius R of the difference between the radius of
the maximum diameter circle 90 and the radius of the core
20 has been found to provide a desirable combination of
good playing characteristics and ridge longevity. If the
ridges 40 are too pointed on the outer end or too narrow
in width, they are quickly worn down or damaged by play
WO91/11s4s PCT/US90/01698
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and they loose their effectiveness. A large ridge radius
is not as effective at spin enhancement and the added
thickness of coating makes the string stiffer and less
playable. ~oo large a ridge height makes the string
difficult to string on a racquet and also adds to the
string stiffness.
Preferably, between ridges,the glue coating over
outer core 28 is of just sufficient thickness to prevent
abrasion of the core structure during play and increase
the contact area of the cross-over point, and therefore
between ridges the outside of the coated string assumes
the arc of the string core. The area where the ridges
join the inner core coating is faired to add strenqth and
prevent breakage of the ridges.
Also in Figure 2, ridged string lO is shown crossing
over itself or another similar ridged string lOa having
ridge 40a as strung on a racquet. The string contact
ridges 42 increase the contact with the cross string lOa.
The larger contact area reduces the extent to which the
strings will indent one another at the crossover point.
The indenting tends to weaken the strings and lowers
elasticity. There i8 a sewing effect caused by the
strings moving over one another as the ball is hit.
Sewing also wears, cuts and weakens strings. The larger
contact area with the five ridge configuration lessens
the sewing effect.
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The crossover points are the highest points on the
face of the racquet and exert the greatest forces on the
ball. With the five ridge configuration, at string cross-
overs the strings assume the position shown in Figure 2.
This is a very stable position and is thought to prevent
string lO from rotating when it encounters the ball.
Imparting spin to the ball applies rotational forces to
the string. The better the string resists these
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WO91/11549 PCI/l,lS90/01698
2~4~ 7
rotational forces the greater should be the spin on the
ball.
Also, in the five ridge configuration, o~termost
ridge 41 and both of the adjacent ridges are in good
position to impart spin on a ball.
Alsoj although in the drawings the strings have been
shown in cross-section at a string cross-over point and
ridges have been designated as "outermost ridges~ and
"ball contact ridges" or as "string contact ridges"; it
should be clear that a given ridge may change from one to
the other at adjacent cross-over points and , in fact,
this seems desirable as the string does not thereby need
to rotate.
Turning now to Figures 3 and 4 there is illustrated
an alternate exemplary embodiment of ridged racquet
string, denoted generally as 10', of the invention; this
one having six ridges. Figure 3 is a perspective view of
a segment of a six ridged string. Figure 4 is a slightly
enlarged cross-sectional view of the six ridged string
10' of Figure 3 shown passing over a similar string lOa',
such as while strung on a racquet.
The six ridged string 10' includes a core, denoted
generally as 20', and a plurality of ridges 40' bonded to
the core 20'. Ridges 40' are preferably evenly spaced,
run axially with length of the string and are parallel to
one another. Six ridged string 10' is produced the same
- as the five ridged string except by use of a different
die. "`
Turning now to Figure 4, the six ridged string 10'
of Figure 3 is shown in cross-section with the addition
of showing the string 10' as it crosses over a similar
string lO'a such as it would in a strung racquet. In
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wO91/ll549 PCT/~IS90/01698
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this exemplary embodiment, the string core 20' is made
also of construc~ion well-known in the art. Core 20' is
comprised of a multiplicity of continuous large
monofilaments 30 which are twisted and glued together to
form the core 20'. There is no inner or outer core as in
the exemplary five ridged string 10. ~-
Figure 4 illustrates a core diameter of 1.30
millimeters and a maximum diameter circle 90' of 1.45
millimeters.
In the exemplary embodiment shown, ridges 40' are
tooth-like in cross-section and project radially outward
from core 20'. Preferably, toothed ridges 40' are about
twice as wide as they are high for prevention of wear and
breakage.
Six ridged string 10' also produces a large cross-
over contact area which reduced indentation and sewing.
The two contact ridges 42' also help the string to resist
rotation upon ball impact and upon imparting spin. The
cross-over point of Figure 4 is simplified in that it
does not show the true interme~hing of the strings
lO',lOa' and their ridges 40',40a' in the strung
condition. At cross-over, the string cores and ridges
indent one another and the string cores and ridges
protrude into the concavities between the ridges and core
of the cross-over string. This intermeshing prevents
rotation of the strings relative to one another.
~`Figure 6 illustrates another prefered embodiment of
the invention. Figure 6 is a cross-sectional view of a
racquet string 10" having four spaced ridges 40".
Core 20 is of conventional nature, such as illustrated
;35 and described heretofore with regard to Figures 1 - 4.
Glue, such as nylon resin 71, surrounds core 20 and forms
rldges 40". Figure 6, illustrates a core diameter is
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WO91/11549 PCT/US90/01698
20~ `7 12
1.20 millimeters, and a maximum diameter circle of 1.70.
Thus, ridges 40" are 0.25 millimeters in height. Ridges
40", as illustrated, are triangular in shape with an
apex angle of sixty degrees. Preferably, the apex angle
S is less than ninety degrees. Too small an apex angle
leads to rapid wear of the ridge, and an apex angle of
about sixty degrees appears to be optimum. A truncated
cone-shaped ridge of the same height wears better than
the pointed ridge illustrated. Further testing is
required to determine, for a given maximum diameter
circle, under what conditions each produces the superior
result.
As strung, two of the ridges 40", designated here
as 42", are string contact ridges that will contact th~
cross-over tring. Considerable deformation of the
string core and ridges occurs at cross-over. String
contact ridges 42" interlock with the cross-over string
string contact ridges and prevent the string 10" from
rotation upon ball contact. The intermeshing of the
ridges of the cross-over strings at the cross-over point
is thought to reduce relative string movement. The
intermeshing of string contact ridges 42" prevent the
string from rotation upon ball contact such that the ball
contact ridges 44" remain in position to impart maximum
rotational force on the ball. The lac~ of rotation
greatly reduces wear of the strings at the cross-over
point.
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In the four ridge example of Figure 6, ball contact
ridges 44" are in particularly good position to impart
spin on a ball coming from either direction relative to
the string. That is a ball upon which spin i9 imposed
will encounter one of the contact ridges first and with
much greater force than the other ridge which is only
incidentally contacted.
W09l/ll549 PCT/US90/01698
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Fig~re 7 illustrates yet another exemplary
embodiment of the ridged racquet string of the invention.
Figure 7 is a cross-sectional view similar to Figure 6
with a change in diameters and in number of ridges. The
string 10"' of Figure 6 has an inner core 20 of l.00
millimeters and three ridges 40"' , each of height 0.275 ~ -
millimeters, such that the string may be circumscribed by
maximum diameter circle 90"' of 1.55 millimeters.
String contact ridges 42"' interlock with the string
contact ridges of the cross-over string to prevent
rotation. Ball contact ridge 44"' imparts spin on a
ball approaching from either direction.
The string lO" with fo~r ridges appears to be
lS particularly suited for several reasons. First, two of
the ridges are always idealy spaced for intermeshing with
the cross-over string, for providing a wide intermeshed
contact base to prevent rotation. Second, the ball
contact ridges are~thought to be located so as to provide
maximum ball contact spin, i.e. at the outer "corners" of
the string. Third, the 6tring need not rotate while
str1nging the racguet. That is, at the seceeding cross-
over point, the ball contact ridges simply become the
strlng contact ridges and vice versa. This allows the
2S ball contact strings to always be in the desired positon.
For contra example, the three ridged string needs to be
rotated between cross-over point~.
The ridged racquet string of the present invention
will impart much more spin on the ball than conventional
` round or polygonal strings without noticeable loss of
other desirable playability characteristics.
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Preliminary testing indicates that the ridged
~5 racquet string of the present invention will last over
fifty percent longer than a similar round string. This
appears to result primarily from the non-rotation of the
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WO 91/1 1549 PCI/I,'S90/01698
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string at cross-over.
Although the exemplary embodiments illustrate a
fiber inner core 24 with an outer core 28 of multiple
large monofilaments 29 helically wrapped around it, and
core 20' of a multiplicity of large monofilaments, as
will be seen, the invention is applicable other core ~-
structures, such as an core of a single large
monofilament surrounded by a wrapping.
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As used herein, the words rstring core" alone
without further limitation applies to the string
structure being inserted into the resin bath and die
apparatus 70 and includes "core-Iess" string such as
string formed by twisting together large monofilaments,
or having a synthetic "fiber" core only with no outer
core. The invention is also adaptable for use with
natural fibers, such as gut, forming the string core.
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Also, although in the drawings the strings have been
shown in cross-section at a string cross-over point and
ridges have been designated as ~ball contact ridges" or
as ~string contact ridges~; it is reiterated that a given
ridge may change from one to the other at adjacent cross- i
over points and , in fact, this seems desirable as the
string does not thereby need to rotate.
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Although particular embodiments of the invention
have been illustrated and described, various changes may
30~ be made in the form, construction and arrangement of the
elements herein without ~sacrificing any of its
advantages. Therefore, it is to be understood that all
matter herein is to bé interpreted as illustrative and
not in any limiting sense and it is intended to cover in
the appended claims such modifications and changes as
come within the true spirit and scope of the invention.
