Daniel, Karo
Ofer
Gerald
Emmanuel, Elbaum
Daniel, Karo
Ofer
| 1. | A composite sports racket string comprising an inner core of low durometer plastics material, a sleeve composed of a multiplicity of small diameter metal filaments and an outer jacket of plastics material having a higher durometer than the core material. |
| 2. | A racket string as claimed in Claim 1, wherein the filaments of said sleeve are braided around said core. |
| 3. | A racket string as claimed in Claim 1 or 2, wherein said core is only partly covered by said filaments. |
| 4. | A racket string as claimed in Claim 1, 2 or 3, wherein said sleeve is partly or wholly embedded in said core. |
| 5. | A racket string as claimed in Claim 1, 2, 3 or 4, wherein said sleeve is partly or wholly embedded in said jacket. |
| 6. | A racket string as claimed in Claim 1, 2, or 3, wherein the jacket and the core are bonded by thermal or reactive bonding. |
| 7. | A racket string as claimed in any preceding claim, wherein the core is made of polyethylene. |
| 8. | A racket string as claimed in any preceding claim, wherein said jacket is made of polyurethane. |
| 9. | A racket string as claimed in any preceding claim, wherein the filaments of said sleeve are made of metal alloy. |
| 10. | A racket string as claimed in Claim 9, wherein the metal alloy is stainless steel alloy. |
| 11. | A racket string as claimed in any preceding claim, wherein said jacket is made of multicoloured layers, whereby in use string wear will reveal inner strata of various colours, so that changes in colour will provide visual indication of remaining string life span. |
| 12. | A racket string according to any preceding claim, wherein the core is made with a crosssection of geometrical shape such as to enhance desired specific characteristics of the imparted ball. |
| 13. | A racket string according to Claim 12, wherein said crosssection is characterised by having one plane of symmetry and two poles of different structure. |
| 14. | A sports racket having a racket head which is strung with a racket string as claimed in any one of Claims 1 to 13. 15. |
| 15. | A racket which is strung with a string as claimed in Claim 13, wherein the orientation of said poles is kept constant throughout the threading of said string. |
| 16. | A racket as claimed in Claim 15, wherein the two sides of the racket head have different dynamic properties. |
| 17. | A racket as claimed in Claim 16, wherein the handle or frame of the racket is marked to distinguish the two sides. |
| 18. | A racket as claimed in Claim 16 or 17, wherein the two faces of the racket head are made in contrasting colours. |
| 19. | A tennis racket having a racket head strung with a racket string as claimed in any one of Claims 1 to 13. |
The joint action of the frame and the interwoven strings making up the racket face should provide for:
(a) stability upon ball impact,
(b) evenly distributed forces around the frame,
(c) minimised energy absorption by the strings and racket, (d) maximised area within which ball controllability is maintained, and (e) appropriate surface characteristics to achieve desired spinning of the ball. It is known that with existing strings and racket frames, controllability of the ball is reduced whenever impact on the racket occurs close to the frame and away from the centre of the preferred area referred to as the "sweet spot". The "sweet spot" is the area on the racket face in which the tension of the strings and the distribution of forces upon a ball impact are relatively uniform. "Sweet spot" size of existing rackets is limited. With time and wear inconsistent spacing among the strings appears in the racket face. As a result, the degree of controllability of the ball varies at different points on the racket surface, due to non-uniform distribution of the ball impact forces on the racket surface area. It is also known that upon application of load to a string by the striking ball, a given amount of elongation occurs and upon removing the load, i.e. imparting the ball, the string does not return to its original length, due to permanent elongation, known as "memory". Known strings must be replaced periodically due to tension loss
and "memory effect" which reduce string efficiency. Known strings are further subject to breakage due to repetitive shock loads of striking balls, natural gut being more likely to break than synthetic materials. In the past these problems have been addressed by using animal gut or plastics material with similar characteristics, as the main element to impart the desired characteristics to the ball, while covering, or coating, the natural gut or its replacement with other materials to provide durability and increase life-span of the string.
Existing frame-string systems however, still suffer from a number of disadvantages including :- (a) string memory is high. (b) string life span is limited.
(c) "sweet spot" size is limited and is greatly reduced with use.
(d) relative movement (creep) of the strings at crossing points upon ball impact impairs grid uniformity.
(e) imparted ball direction and spin are difficult to control due to variations in string response at different contact locations.
It is an object of the invention to provide a string for sports rackets, designed to improve the following aspects:
(a) durability,
(b) environmental conditions deterioration effect,
(c) minimization of relative string movement at crossing points upon ball impact,
(d) size of "sweet spot",
(e) minimisation of energy absorption upon impact,
(f) life-span,
(g) efficient application of desired ball characteristics (controllability) .
In accordance with the invention, a sports racket string is provided with an inner core of low durometer plastics
material, a sleeve composed of a multiplicity of small diameter metal filaments, and a plastics material jacket which envelopes the core and sleeve, said jacket being made of plastics material with a higher durometer than the core.
When the striking ball applies load at the contact area with the strings, the jacket, utilising surface texture and material physical characteristics, grips and "envelopes" a portion of the ball surface allowing for greater contact area with the ball. At the same time the jacket is further compressed in the contact area. The sleeve tends to unwind its braided pattern upon impact of the striking ball. Thus predictable elongation of the braided sleeve occurs in the close vicinity of the contact area of ball and strings, with virtually no elongation of the sleeve fibres, while sleeve diameter is subject to decrease in said contact area. The sleeve may be embedded in the inner core, to further limit inner core elongation at the specific contact area vicinity. As a result, deflection and compression of the inner core due to the longitudinal and diametrical changes of the braided sleeve are localised. Alternatively, limitation of string elongation may be achieved by embedding the sleeve into the outer jacket, or into both the jacket and the core. The elongation of the string being limited substantially to the contact area vicinity, the loss of tension known as "memory effect" is greatly reduced. Upon return of the string-system of core-sleeve-jacket to original length and diameter, return forces are applied to the imparted ball by deflection, core depression, and secondary effect of jacket decompression. Compression-set of the composite string of the invention upon impact provides for mechanical fixation, known as friction joint, of strings at their crossing point, preventing movement and slipping of the strings relative to each other upon impact of a striking ball. Constant
spacing of the strings is maintained, and all forces are evenly distributed within and around the ball contact area. As a result a better controllability of the ball at any contact location on the racket is achieved, and effectively a dynamic "sweet spot", covering the whole area of the racket face is provided.
Handling of the ball is highly dependent on core section shape and dimensions. Imparting speed, spinning momentum, and ease of ball direction control are all shape dependent. It is therefore possible to make rackets for various specific purposes with specially adjusted core section shapes and dimensions. It is further possible to make a two sided/double purpose racket allowing for priority characteristic to be controlled differently with opposite racket faces.
By way of example, such a double purpose racket may be achieved by utilising a string with a core having a cross section in the shape of an isosceles triangle, with the base consistently facing the same side of the racket. The triple layer structure of the string of the invention provides such a long life span, that wear of the jacket surface occurs earlier than string rupture. Therefore a colour coding index can be provided so that the exposure of a designated colour indicates a degree of loss of the surface characteristics, and an indication is given of the end of the useful life of the string. Alternatively, the string may be constructed with more than one sleeve, more than one jacket or with additional sleeve and jacket pairs. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figures 1A and 13 are cross-sections of two racket strings, botn made in accordance with the invention, each comprising a core 1, a sleeve 2 and a jacket 3; Figure 2 is an exploded side view of a racket string made
in accordance with the invention, showing a core 1, a braided sleeve 2 and a jacket 3; and
Figure 3 illustrates alternative cross-sections of racket strings made in accordance with the invention. Referring to the drawings, the core 1 is made of a synthetic polymer strand having a designated cross section shape.
The core material properties of hardness, elasticity and tensile strength, provide for elongation and shrinkage upon impact to produce designated rebound force.
The core cross section shape and dimensions may be varied to achieve preferred effects on the imparted tennis ball. The core is subject to elongation at the contact area of the striking ball and its close vicinity, as allowed by the sleeve. The core is further subject to local deflection and compression as a result of the longitudal and diametric changes of the sleeve.
The hardness of the core material has a substantial effect on the strength, flexibility, and elongation. Various degrees of hardness may be used with different models to achieve desired performance, with differing elongation and compression rates.
In a preferred embodiment, the core material is polyethylene, preferably having a hardness defined by SHORE D65.
Alternatively the core material may be any suitable plastics or alloy plastics material selected from the thermo-plastics, thermo-setting plastics, elastomers, reactive plastics materials or natural materials, of any suitable hardness.
The shape of the core cross section is determined by the priority of controlling certain characteristics rather than others. Spinning the ball at desired speed, achieving higher speed, reducing shocks from incoming balls, and accurately redirecting imparted balls are all shape dependent. The cross-section of the core may advantageously be made
in six different shapes according to the higher priority characteristics associated with the different models, viz.
(a) circular; (b) rectangular;
(c) square;
(d) star-shaped;
(e) triangular or
(f) diamond. Alternatively, the cross-section may be V-shaped, pentagonal, hexagonal or any other suitable geometrical design. The core may be either solid or hollow. Figure 3 of the drawings illustrates these various cross- sections. Thus 1 shows a circular cross-section, 2 star- shaped, 3 square, 4 concaved square, 5 triangular, 6 concaved triangular, 7 isosceles triangular, 8 concaved isosceles triangular, 9 rectangular, 10 concaved rectangular, 11 diamond and 12 a concaved diamond cross- section. The dimensions of the core cross-section provide means of determination of the decompression, elongation, and deflection of the core upon ball impact. The cross-section dimension which governs the performance of the core is the largest in any direction. By way of example, this dimension may be the diameter of a circular cross-section such as shown in Figures 1A and IB, or it may be the longest line between any two corners of a iamond-shaped cross-section.
In a preferred embodiment, the core diameter or its largest dimension varies between 0.7-1.lmm for different string models.
Alternatively, the core diameter or its largest dimension may be any value within the range from 0.35mm to 1.35mm. The sleeve 2 is made of small diameter filaments (fibres) which may be braided around the core as shown by way of example in Figure 2 or in accordance with any other pattern. The strands may be arranged in groups as shown by way of example in Figure 13, or they may be evenly
distributed as shown in Figure 1A, or arranged in accordance with any preferred design. The sleeve is used as stability and tension control mechanism, applying its strength and flexibility to the string system. In a preferred embodiment, the sleeve 2 is partially embedded in the core 1. In alternative embodiments, the sleeve 2 may be covering the core 1 and may be embedded into the jacket 3, or it may be completely embedded into the core 1, or it may be embedded in both the core 1 and the jacket 3.
The braided pattern of sleeve 2 in Figure 2 is subject to unwinding upon impact of the striking ball, whereby predictable elongation of the sleeve 2 is limited to the close vicinity of the contact area of the ball and strings, with virtually no elongation of the sleeve fibres. The sleeve is further subject to decrease in diameter in the same contact area.
Deflection of the braided sleeve applies the main imparting force. While elongated and deflected, the braided sleeve applies compression to the core, enabling core decompression which provides the secondary imparting force. The sleeve 2, acting as a stability and tension control mechanism and enhancing the strength of the string, provides for a greater string life span. The sleeve threads pattern determines the flexibility and durability of the string.
In a preferred embodiment, the sleeve is constructed as a braid enveloping the core. Penetration of the sleeve threads into the core material can be up to 80%, preferably 15% of the sleeve thickness.
Alternatively, the sleeve pattern may be any mesh, or spiral pattern, with or without braiding. The sleeve may not penetrate the core material at all or it may penetrate up to 80% of its thickness into the core material. The sleeve threads may be arranged to provide only partial coverage of the core surface, so that surface coverage may vary, for example, between 5% and
95% of the core surface, preferably 20 to 60%. In a preferred embodiment, the sleeve threads material is stainless steel alloy 304.
Alternatively, the sleeve material may be any ferrous or non-ferrous substance, or any metallic or non-metallic alloy.
In a preferred embodiment, the sleeve threads cross- section has a circular shape with a diameter of 0.04mm. Alternatively, the sleeve threads may have other different shapes such as circular, diamond, square, etc. The longest dimension (or diameter, if circular) of the thread cross-section may be between 0.0025-0.11mm. It will be appreciated by those versed in the art that the invention is not limited to the above described embodiments and many various modifications of the sleeve may be made.
The jacket 3 is a distinct structural element of the racket string serving as the direct means for applying to the imparted ball the forces and direction as generated by the player's arm, and enhanced by the core and the sleeve. The jacket material surface characteristics enable a sufficiently firm and stable grip of the ball whereby the forces and their direction are applied with a high degree of efficiency. The jacket material also allows for sufficient compression of the jacket 3 between the ball and the sleeve 2, to increase contact area with the ball and further improve the distribution of forces on the ball surface.
The hardness of the material has a substantial effect on the grip between the ball and the string. Various degrees of hardness may be used with the different models to achieve desired performance.
In a preferred embodiment, the jacket material is polyurethane, of hardness defined by SHORE A60 and A85. Alternatively, the jacket material may be any suitable polymeric material, or any plastics or alloy plastics material selected from the thermo-plastics, thermo-
setting plastics, elastomers, reactive plastics materials, or natural materials of suitable durability and surface characteristics.
The compression set enabled by the respective materials and structure of the string layers results in mechanical fixation, known as friction joint, of the strings to each other at their crossing points upon impact preventing relative movement of the strings.
The application of the compression set concept achieves: (a) mechanical fixation and string slipping prevention,
(b) maintaining constant spacing among strings,
(c) concentration of load and forces of the ball impact at precisely the contact area of ball and strings, and
(d) maximum controllability at the contact area, anywhere on the racket surface.
According to a further aspect of the invention, the jacket is composed of layers of various colours, wherein a colour coding system is utilised to indicate the wear of the outer jacket to a point where its physical characteristics are below the designated standards. The colours applied to the inner strata of the jacket will become visible after a predicted portion of the jacket has been worn. A colour coding index provided with the string allows a visual comparison with the governing colour of the tennis racket string to determine the remaining useful life of the string. The cross-section shape and dimensions of the core determine effectiveness of imparting desired characteristics to the ball. Therefore, in accordance with a further aspect of the invention asymmetrical cross-section core shapes are used to construct: dual- purpose strings. It will be appreciated that rackets which impart characteristics of different degree and intensity to the ball at opposite faces of the racket may enable a better handling of the ball for desired shots.
They also enable the player to surprise his/her opponent by imparting different shots with the same body movements.
It will be appreciated that strings would have to be applied to such rackets while maintaining the proper orientation relative to the racket plane. In a preferred embodiment, an asymmetrical cross-section core is used and a groove or similar mark on the racket- handle or frame indicates to the player the orientation of the racket so that the sides may be alternated in accordance with the player's requirements. Dual-purpose, double-sided strings and rackets may be marked by applying different colours on opposite sides of the racket's strings. The different colours on opposite sides of the string which is stranded properly on the racket give a clean indication to the player as to the side of the racket to be played. At the same time colour coding also provides the player's opponent with information about the side-orientation of the racket so the impending shot can be appreciated and response planned.
While the dual purpose string has been described with respect to one preferred embodiment, it will be appreciated that many variations, modifications and other applications of the invention may be made.