GB2234760A - Improvements in strings for sports rackets - Google Patents

Abstract

A composite string for use in sports rackets comprises a core of animal gut with a covering formed by at least one helically wound or braided layer of synthetic polymer fibres. The covering is not adhered to the core, but is such that its transverse dimensions decrease on tensile longitudinal extension of the string, thereby forming a static frictional bond with the core at a certain string tension. The string may also be provided with an external coating of synthetic polymeric material.

Description

IMPROVEMENTS IN STRINGS FOR SPORTS RACKETS This invention relates to strings, especially for use in sports rackets, of the type comprising a core of animal gut and an external wrapping or braiding of polymeric material, and to methods of manufacture thereof.

With the advent of synthetic thermoplastics, it has been found that materials such as nylon or other synthetic polymers are suitable for the construction of strings for sports rackets. Indeed, strings constructed from such materials possess a number of advantages over more traditional animal gut strings, being more resistant to the effects of abrasion with balls, racket frame and interwoven strings, and also being impervious to climatic changes. The effects of moisture and/or changes in humidity are particularly damaging for strings constructed from animal gut, since the individual filaments from which such strings are constructed to expand when moist and contract on drying, causing a loss of resiliency and a tendency to break prematurely.This expansion or contraction also causes the string as a whole respectively to attempt to untwist or twist further, in turn causing the bonded filaments to become separated, thus weakening the overall structure.

It has also been found that in order to string a socalled over-size tennis racket so as to provide suitable playing characteristics, a string tension of about 310-380 N (70-85 pounds) is required as opposed to about 220-290 N (50-65 pounds) for a standard racket. Animal gut is unsuitable for stringing such rackets because of the high tensions required for the stringing process and in the portions of the string which form the exterior of the bends thereof around the racket frame, which can often be much higher than the resultant string tension, and because of the considerable abrasion with the racket frame during stringing.

However, strings constructed from animal gut do possess certain advantages over their synthetic counterparts, resulting from the tensile properties of the gut. These advantages are high resiliency and low plasticity; that is to say, good tension holding properties as compared with synthetic strings.

A further advantage of animal gut strings, somewhat difficult to define, but nevertheless a result of the tensile properties of the gut, is that the playing characteristics thereof are almost universally preferred by racket sports players to those of synthetic polymer strings.

Indeed, many tennis players consider that the advantages imparted by the characteristics of animal gut strings outweigh the disadvantages of susceptibility to climatic changes and abrasion with balls, racket frame and interwoven strings, and continue to string their rackets with gut.

It has been proposed by Salsky et al, in U.S. Patent No.4391088, to construct a composite string for sports rackets comprising a gut core covered with filaments of aramid and impregnated with at least one coating of waterresistant, vapour-impermeable, flexible smooth adhesive polymeric resin, which adheres the aramid to the gut core.

This construction is suitable for stringing at high tension, is resistant to abrasion and climatic changes, and is intended to reproduce the playing characteristics of animal gut strings. Nevertheless, it is found that the properties of the filamentary aramid coating adhered to the gut core have an effect on the properties of the composite string as a whole, such that the playing characteristics thereof do not reproduce those of animal gut as closely as may be desired.

The object of this invention is to provide a string, especially for use in sports rackets, which is suitable for stringing at high tensions, possesses high resiliency and low plasticity, may be resistant to the effects of abrasion and climatic changes, and which reproduces the playing characteristics of animal gut almost exactly in badminton, squash and standard tennis rackets, and substantially better than known synthetic or composite strings in mid-size and over-size tennis rackets.A further object of this invention is to provide a method of manufacture of such a string According to one aspect of the invention, there is provided a composite string, especially for use in sports rackets, comprising a core of animal gut with a covering of synthetic polymeric material, said covering not being adhered to said core, but being such that its transverse dimensions decrease on tensile longitudinal extension of said composite string, a static frictional bond between said covering and said core being effected at, and maintained above a certain tension.

In the case of badminton, squash and standard tennis rackets, the tension at which said static frictional bond is effected is preferably slightly less than the required resultant string tension in order for protection against abrasion and climatic changes to be afforded to said core, but for said covering to have little effect on the playing characteristics of said core due to said covering's being substantially un-stressed.

In the case of mid-size and over-size tennis rackets, the tension at which said static frictional bond is effected is preferably such that suitable protection is afforded to said core, by said covering, against over-stressing and consequential breaking during stringing, and also such that said covering is minimally stressed at the resultant string tension so as to affect the playing characteristics of said core as little as possible.

In practice it has been found that substantially the same tension at which said static frictional bond is effected is suitable in both the above cases, and that preferably said tension is between about 155 N (35 pounds) and about 245 N (55 pounds), and most preferably about 196 N (44 pounds).

It is preferred that said covering of synthetic polymeric material is constructed by helically winding or braiding at least one layer of synthetic polymeric fibres around said core. Where more than one layer of helically wound fibres is employed, it is preferred that the fibres of alternate layers be wound in opposite senses. It is further preferred that such helically wound or braided fibres be bonded together by an adhesive, which adhesive is not suitable for adhering said fibres to said core.

Suitable materials for said synthetic polymer fibres are polyamides, for example nylons, or polyesters, for example polyethylene terephthalate.

To provide further protection against both abrasion and climatic changes, the entire composite string may be coated with a polyamide, for example nylon, or a polyester, for example polyethylene terephthalate.

According to a second aspect of the invention, there is provided a method of construction of a string, especially for sports rackets, comprising applying an adhesive to a core of animal gut, helically winding or braiding at least one layer of synthetic polymeric fibres around said core, and drying said adhesive, said adhesive being suitable for bonding together said fibres, but not for bonding said fibres to said core.

If more than one layer of helically wound fibres is employed, it is preferred that the fibres of alternate layers be wound in opposite senses.

It is preferred that the step of drying said adhesive is performed by heating the combination of said core, said adhesive and said fibres, preferably to between about 60 0C and 110 C, possibly by passing said combination through a tube having an internal temperature in this range, which in addition to drying said adhesive, effects second order transition of said polymeric fibres. Most preferably, this step is performed with said combination under tension.

Preferably, said adhesive is in liquid form, for exmaple nylon in a solvent. A suitable solvent is a mixture of carbolic acid and benzyl alcohol. Most preferably, said adhesive is a mixture of about 15% by weight of nylon, about 75% by weight of carbolic acid, and about 10% by weight of benzyl alcohol.

According to a further aspect of the invention, there is provided a method of construction of a string, especially for sports rackets, as described above, and further comprising the step of coating the combination of said core, said adhesive and said fibres, after drying of said adhesive, with a polyamide, for example nylon, or a polyester, for example polyethylene terephthalate.

Preferably, the step of coating said combination is performed by extrusion.

A string according to this invention may be constructed from a gut core of about 0.7mm diameter, two braided layers of synthetic polymeric fibres, each having a thickness of about 0.15mm, and a final coating of a polymer having a thickness of about 0.04mm. This yields a composite string with a diameter of about 1.38mm.

Such a string would typically have a breaking load of about 620 N (140 pounds). This compares favourably with about 575 N (130 pounds) for a gut string of similar diameter. Both these loads correspond to a strain of about 20%. However, since the portions of the composite string which form the exterior of the bends thereof around the racket frame consist of synthetic polymeric fibres, which have a higher breaking strain than natural gut, the composite string is far more suitable for stringing at high tension than is pure gut.

A nylon string having a diameter similar to the above would typically have a breaking load of about 735 N (165 pounds).

Another property of a composite string according to this invention and constructed as above, is that, subject to fluctuations near the load at which the static frictional bond is effected, the curve of load versus extension thereof is substantially linear and, particularly in the working range of about 220 - 380 N (50 - 85 pounds), conforms to that of gut very closely. In contrast, the curve of load versus extension of nylon has a characteristic "S" shape.

A further advantageous property of a composite string according to this invention, and constructed as above, is that the tension holding properties thereof are similar to those of gut. For example, typical percentage tension losses for gut strings and for composite strings constructed as above are about 4.3t and 4.6e respectively after ten days, and no further loss after four weeks. For nylon however, the losses are about 8.1% after ten days, and a further 1.8% after four weeks, giving a total of 9.9% after four weeks.

It will of course be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Claims (32)

CLAIMS:

1. A composite string comprising a core of animal gut with a covering of synthetic polymeric material, wherein said covering is not adhered to said core, but is such that its transverse dimensions decrease on tensile longitudinal extension of said string, and wherein a static frictional bond between said covering and said core is effected at and maintained above a certain string tension.

2. A string according to claim 1 wherein said tension at which said static frictional bond is effected is between about 155 N (35 pounds) and about 245 N (55 pounds).

3. A string according to claim 1 or 2 wherein said tension at which said static frictional bond is effected is about 196 N (44 pounds).

4. A string according to any one of the preceding claims wherein said covering comprises at least one layer of synthetic polymeric fibres helically wound around said core.

5. A string according to claim 4 wherein said fibres in alternate layers are wound in opposite senses.

6. A string according to any one of claims 1-3 wherein said covering comprises at least one layer of synthetic polymeric fibres braided around said core.

7. A string according to any one of claims 4-6 wherein said fibres are bonded together by an adhesive.

8. A string according to any one of claims 1-7 wherein said synthetic polymeric material is a polyamide.

9. A string according to any one of claims 1-8 wherein said synthetic polymeric material is a nylon.

10. A string according to any one of claims 1-7 wherein said synthetic polymeric material is a polyester.

11. A string according to any one of claims 1-7 or 10 wherein said synthetic polymeric material is polyethylene terephthalate.

12. A string according to any one of the preceding claims further comprising an external coating of a polyamide.

13. A string according to claim 12 wherein said coating polyamide is a nylon.

14. A string according to any one of claims 1-11 further comprising an external coating of a polyester.

15. A string according to claim 14 wherein said coating polyester is polyethylene terephthalate.

16. A method of construction of a string, especially for use in- sports rackets, comprising the steps of applying an adhesive to a core of animal gut, helically winding at least one layer of synthetic polymer fibres around said core, and arying said adhesive, wherein said adhesive is suitable for bonding said fibres together, but not for bonding said fibres to said core.

17. A method according to claim 16 wherein said fibres in alternate layers are wound in opposite senses.

18. A method of construction of a string, especially for use in sports rackets, comprising the steps of applying an adhesive to a core of animal gut, braiding at least one layer of synthetic polymer fibres around said core, and drying said adhesive, wherein said adhesive is suitable for bonding said fibres together, but not for bonding said fibres to said core.

19. A method according to any one of claims 16-18 wherein said step of drying said adhesive is effected by heating the combination of said core, said adhesive and said fibres to between about 60 0C and about 1100C, thereby causing second order transition of said polymer fibres.

20. A method according to claim 19 wherein said step of drying said adhesive is effected by passing said combination through a tube having an internal temperature between about 60 0C and about 1100C.

21. A method according to claim 19 or 20 wherein said step of drying said adhesive is effected with said combination under tension.

22. A method according to any one of claims 16-21 wherein said adhesive-is in liquid form.

23. A method according to any one of claims 16-22 wherein said adhesive comprises a nylon in a solvent.

24. A method according to claim 23 wherein said solvent is a mixture of carbolic acid and benzyl alcohol.

25. A method according to claim 23 or claim 24 wherein said adhesive comprises about 15% by weight of nylon, about 75% by weight of carbolic acid and about 10% by weight of benzyl alcohol.

26. A method according to any one of claims 16-25 further comprising the step of coating the combination of said gut, said adhesive and said fibres with a polyamide.

27. A method according to claim 26 wherein said coating polyamide is a nylon.

28. A method according to any one of claims 16-25 further comprising the step of coating the combination of said gut, said adhesive and said fibres with a polyester.

29. A method according to claim 28 wherein said coating polyester is polyethylene terephthalate.

30. A method according to any one of claims 26-29 wherein said step of coating said combination is effected by extrusion.

31. A string, especially for use in sports rackets, substantially as described herein.

32. A method of construction of a string, especially for use in sports rackets, substantially as described herein.