WO1991011549A1

WO1991011549A1 - Ridged racquet string

Info

Publication number: WO1991011549A1
Application number: PCT/US1990/001698
Authority: WO
Inventors: Sam Hsin-Shun Chen; Tseng Yeng Lin
Original assignee: Chen Sam Hsin Shun; Tseng Yeng Lin
Priority date: 1990-01-26
Filing date: 1990-03-30
Publication date: 1991-08-08
Also published as: EP0465605A1; CA2049307A1; EP0465605A4; AU634937B2; AU5568190A

Abstract

A string (10) for a sports racquet, such as a tennis racquet string, is comprised of a string core (20) having a plurality of external ridges (40) bonded thereon for aiding in imparting spin on a ball and for increasing the cross-over contact area between strings. The ridges are parallel to one another and extend axially with the string. Preferably, the string and ridges can be circumscribed by a maximum diameter circle (90), preferably of 1.70 millimeters. The process for making the ridged string includes passing a string core (20) through a bath (72) of resin (71), such as nylon, for coating and glueing the core and then through a die (74) shaped so as to form the ridges (40).

Description

RIDGED RACQUET STRING

BACKGROUND OF THE INVENTION

Elslύ Ωl iiie 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 such an improved string.

Background of the Invention

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 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 damping, that is low energy loss 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. However, thin strings may stretch and are more easily broken. On the other hand, thick strings are 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 example, 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 of 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 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 ball has been thought to enhance imparting spin on the ball.

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 the 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.

Synthetic strings treated in the above-described manners tend to have poor dimensional stability and are reduced in strength and elasticity resulting in tension loss during play. Further, some are inferior in durability because they exhibit surface aberrations, wearingε or breakages due to degredation of the resins, and abrasion, peeling or denaturing of the treating substances. Moreover, since the above-men ioned treatments constitute additional steps in manufacture, there is an increase in production costs.

Another proposed method to increase string friction is the use of string of polygonal cross-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 str ings may have a thin, evenly _dis_tri_bu_ted coating around the core to provide protection 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 desi rable to have an improved sports racquet string having much better spin-imparting characteristics than a conventional round string and w hi ch a chi eves thi s w i thout si gni f i cant lo ss 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.

SUMMARY OF THE INVENTION

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 1.00 - 1.30 millimeters. Preferably, 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. 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.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a perspective view of a segment of a prefered embodiment of the ridged racquet string of the invention.

Figure 2 is 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 of the ridged string of Figure 3 shown passing over a similar string, such as while strung on a racquet.

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 pre_ferre_d em_bodiment of the ridged racquet string of the invention; this one having four triangular ridges.

Figure 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.

DETAILED DESCRIPTION OF THE 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 10 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.

Turning now to Figure 2, the ridged string 10 of Figure 1 is shown in cross-section with the addition of showing the string 10 as it crosses 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 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.

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 comprising an inner core 24 and and an outer core 28.

An inner core can be a thick extruded monof ilament 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 five hundred to three thousand fibers.

Many synthetic materials are available for such use including nylon, polyester, kevlar, zyex (polyether ether keton) , 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 opposite directions.

As best seen in Figure 5, the string core 20 is processed by passing it through a bath and die apparatus, denoted generally as 70. String core 20 is immersed in 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. Returning 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. Ridges 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 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 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. Too 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 strength and prevent breakage of the ridges.

Also in Figure 2, ridged string 10 is shown crossing over itself or another similar ridged string 10a having ridge 40a as strung on a racquet. The string contact ridges 42 increase the contact with the cross string 10a. 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 is 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.

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 10 from rotating when it encounters the ball. Imparting spin to the ball applies rotational forces to the string. The better the string resists these rotational forces the greater should be the spin on the ball.

Also, in the five ridge configuration, outermost ridge 41 and both of the adjacent ridges are in good position to impart spin on a ball.

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 "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 10a', 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 10'a such aε it would in a strung racquet. In this exemplary embodiment, the string core 20' is made ^• also of construction well-known in the art. Core 20' is comprised of a multiplicity of continuous large monof ilamentε 30 which are twiεted and glued together to form the core 20'. There iε no inner or outer core aε in the exemplary five ridged εtring 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 aε 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 iε simplified in that it does not show the true intermeshing of the εtringε 10*,10a' and their ridges 40',40a' in the strung condition. At cross-over, the string cores and ridges indent one another and the εtring cores and ridges protrude into the concavities between the ridges and core of the cross-over string. This intermeεhing preventε rotation of the εtrings 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 aε illustrated and described heretofore with regard to Figures 1 - 4. Glue, εuch aε nylon reεin 71, εurrounds core 20 and forms ridges 40". Figure 6, illustrates a core diameter iε 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 iε lesε than ninety degrees. Too εmall 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 iε 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 εtring contact ridgeε that will contact the cross-over string. Considerable deformation of the εtring 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 εtringε at the croεs-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 ridgeε 44" remain in position to impart maximum rotational force on the ball. The lack of rotation greatly reduces wear of the εtrings at the crosε-over point.

In the four ridge example of Figure 6 , ball contact r i dgeε 44 " are in par ti cula rly good poεi tion to impart spin on a ball com i ng f r om ei ther di recti on relative to the st ring. That i ε a ball upon whi ch spin i ε impo εed w ill encounte r one of the contact r idge s f i rst and w i th much greater f or ce than the other ridge whi ch i ε only incidentally contacted. Figure 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 εtring 10'" of Figure 6 haε an inner core 20 of 1.00 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 ridgeε of the cross-over string to prevent rotation. Ball contact ridge 44"' imparts spin on a ball approaching from either direction.

The string 10" with four ridges appears to be 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 string need not rotate while stringing the racquet. That iε, at the εeceeding cross¬ over point, the ball contact ridges simply become the string contact ridgeε and vice versa. This allows the ball contact strings to always be in the desired positon. For contra example, the three ridged string needs to be rotated between crosε-over points.

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 characteristicε.

Preliminary testing indicates that the ridged 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 εtring at cross-over.

Although the exemplary embodiments illustrate a fiber inner core 24 with an outer core 28 of multiple large monof ilamentε 29 helically wrapped around it, and core 20' of a multiplicity of large monof ilamentε, aε will be seen, the invention is applicable other core structures, such as an core of a single large monofilament surrounded by a wrapping.

As used herein, the words "string core" alone without further limitation applies to the εtring εtructure being inεerted into the reεin bath and die apparatuε 70 and includes "core-lesε" string such aε εtring formed by twiεting together large monof ilamentε, or having a synthetic "fiber" core only with no outer core. The invention iε alεo adaptable for uεe with natural fibers, such as gut, forming the εtring core.

Also, although in the drawings the strings have been shown in cross-section at a string crosε-over point and ridges have been designated aε "ball contact ridgeε" or aε "string contact ridges"; it is reiterated that a given ridge may change from one to the other at adjacent crosε- over points and , in fact, thiε seems desirable as the string does not thereby need to rotate.

Although particular embodiments of the invention have been illustrated and described, various changes may be made in the form, construction and arrangement of the elements herein without sacrificing any of itε advantageε. Therefore, it iε to be understood that all matter herein iε to be interpreted aε illustrative and not in any limiting senεe^'and it iε intended to cover in the appended claimε εuch modifications and changes aε come within the true spirit and scope of the invention.

Claims

1. A string for a εportε racquet comprising: an elastic, resilient εtring core of high tenεile εtrength; and a plurality of axially extending parallel ridgeε of εynthetic material bonded to said string core; such that in cross-section: said string core iε substantially circular with a diameter of 1.00 - 1.30 millimeters; and said ridges are radial protrusions on said core, the outermost projections of which may be circumscribed by a circle of 1.70 millimeters diameter.

2. The string of Claim 1 wherein said ridgeε are greater than 0.075 millimeterε in height.

3. The εtring of Claim 1 wherein said ridgeε are six in number.

4. The string of Claim 3 wherein said ridgeε are evenly spaced about said string core.

5. The string of Claim 1 wherein said ridgeε are five in number.

6. The εtring of Claim 5 wherein εaid ridges are evenly spaced about εaid string core.

7. The string of Claim 1 wherein said ridges in crosε-εection are arc-εhaped.

8. The εtring of Claim 1 wherein said ridgeε in cross-section are generally arc-shaped with a ridge arc radius of 0.075 millimeterε or greater.

9. The εtring of Claim 1 wherein said ridgeε in croεε-section are εubstantially rectangular.

10. The string of Claim 1 wherein εaid ridgeε in croεε-section are εubεtantially rectangular with a width of twice the height.

11. The string of Claim 1 wherein said ridges are three to eight in number.

12. The string of Claim 1 wherein said ridgeε are evenly spaced about said string core.

13. The string of Claim 1 wherein said ridges in cross- section are substantially triangular.

14. The string of Claim 13 wherein said ridges are four in number.

15. The string of CLaim 13 wherein said ridgeε have an apex angle of leεs than ninety degreeε.

16. The string of Claim 13 wherein said ridges are evenly spaced about εaid string core.

17. The string of Claim 13 wherein said ridges are three in number.

18. The string of Claim 1 wherein said string core comprises: an inner core of twiεted and glued monofilament strands; and an outer core comprising a plurality of larger diameter strands helically wound about said inner core.

19. A string for a sportε racquet compriεing: an elaεtic, reεilient εtring core of high tenεile strength; and a plurality of axially extending parallel ridgeε of εynthetic material bonded to εaid εtring core; such that in crosε-section: said string core is substantially circular with a diameter of 1.00 - 1.35 millimeterε; and εaid ridgeε are radial protrusions on said core and have a height of greater than 0.075 millimeters.

20. The εtring of Claim 19 wherein said ridgeε are four in number and are evenly spaced about said εtring.

21. The εtring of Claim 19 wherein εaid ridgeε are substantially triangular in crosε-section.

22. The εtring of Claim 21 wherein said ridgeε have an apex angle of less than ninty degrees.

23. A method for the production of a ridged string for a sports racquet comprising: forming a string core of elastic, resilient, tensilely strong, material; processing said string core so as to create a plurality of axially extending parallel ridgeε of synthetic material bonded to said string core including passing the εtring core through a bath of the ridge material and through and extruεion die.