Abstract:
A racquet in which the area of the string which manifests relatively true rebound of the ball, the so-called &#34;sweet spot&#34;, is maximized. To achieve this condition the strings are so tensioned or so structured that the deflection produced by impact of the ball near the outer periphery of the racquet is substantially equal to the deflection near the center of the racquet. The strings may be subjected to lower tension near the outer periphery than near the center of the racquet or they may be of smaller thickness near the outer periphery than near the center or they may be differently irradiated. So as to achieve the different tensions with strings of a given diameter a plurality of separate short string lengths sections may be used or one or two long strings may be used and differently tensioned for each length. So that they maintain the set tension. Where long strings are used, the string sections of different tension are wedged in certain of the holes in the racquet frame, through which the string sections or string lengths are threaded or otherwise fastened.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to stringed racquets for tennis, squash, racquet ball or the like in which in play a racquet strikes a ball. The invention has particular relationship to stringing of the racquets so as to improve the properties of the racquets in play. 
     Racquets are commonly strung with tensioned string of natural animal gut or synthetic materials or metal wire. The natural gut is made from beef or lamb intestines, and the synthetic materials are made of polymeric materials such as nylon (polyamides or polymeric composites). Natural gut is normally considered to have better playing characteristics; however, it is more costly, affected by weather, and has a short life. Usually a racquet is strung with one continuous length of string (about 33 feet long in the case of a tennis racquet). The string is subject to the same tension, typically of about 55 pounds, throughout. Typically, the string has a diameter of about 0.040 to 0.060 inch. In some cases two separate string lengths may be used as shorter lengths than the 33 feet are more readily manufactured and with shorter lengths stringing is facilitated for some racquets. However, where a racquet is strung with two lengths of string, both are usually subject to the same tension. 
     An important factor in playing performance is the area of the &#34;sweet spot&#34; or the center section of the racquet which exhibits a relatively true rebound to the ball. It is desirable that this &#34;sweet spot&#34; be of as large as practicable area as balls struck outside of the &#34;sweet spot&#34; do not bounce truely and there is a fault when a ball is struck in this outside area. In an attempt to enhance the area of the &#34;sweet spot,&#34; stringers have strung racquets so that the strings are diagonal or so that there are missing rows of strings near the top, bottom and sides, i.e., around the periphery of the racquet. Neither of these expedients has been sufficiently successful to receive widespread adoption. Stringers have also attempted to string portions of the racquets at different tensions; but, this was not successful because the tensions could not be maintained at the initial desired settings during play. 
     It is an object of this invention to maximize effectively the area of racquet which manifests relatively true rebound. 
     SUMMARY OF THE INVENTION 
     Computer studies of the manner in which the strings of a prior art racquet responds to impact of the ball during play have revealed that frequent faults or mishits result when the ball is struck by the strings along the periphery of the racquet because the strings in this area are the shortest and deflect less for like impacts of the ball than the strings in the center of the racquet. To maximize the area of the &#34;sweet spot&#34; of a racquet, it is necessary to maximize the area of the racquet over which the strings have substantially the same deflection, regardless of the differences in string lengths, string locations, spacings between strings, etc. In the practice of this invention, a racquet is provided which has independent string sections or string lengths throughout the string area; each string section or string length being composed of string whose properties and/or structures are such as to achieve, and maintain throughout the life of the racquet, substantially equal deflection for like impacts of the ball. As used in this application the expression &#34;string section&#34; means a section of longer string, for example, where a racquet is strung with one or two long strings, a string section means a section less than the whole of a long string. A string section at a given tension may extend between a pair of holes in the frame of the racquet or between a plurality of pairs of holes. As used in this application a &#34;string length&#34; means a length of string separate from other lengths where a racquet is strung with separate short lengths of string. In the practice of this invention in one of its aspects a racquet is strung with string lengths, each threaded through a number of holes and each subject to different tension. Each string length may be threaded through opposite holes of the frame or through a plurality of pairs of opposite holes. The string lengths are threaded through opposite holes in the usual way with one set longitudinally of the racquet from top to bottom along the direction of the handle, like a warp, and the other set transversely of the racquet perpendicular to the handle and interlaced with the longitudinal strings like a woof. In both cases longer string lengths pass across the center of the racquet and shorter string lengths pass across the periphery of the racquet. In each case the tension is substantially higher for the longer string lengths than for the shorter string lengths. The deflection for like impacts by a ball of a string length subject to lower tension is greater than for string lengths subject to higher tension. The tensions are so adjusted in the practice of this invention that the deflection of the shorter string lengths under an impact is substantially the same as the deflection of the longer string lengths for a like impact. It is essential that each string length be securely locked in position by being wedged in one of the holes through which it is threaded. The tension may vary progressively from the highest magnitude near the center of the racquet to the lowest magnitude near the periphery. 
     The deflection may also be set as described above by providing string lengths of different modulus of elasticity, higher modulus for the string lengths near the center of the racquet and lower modulus for string lengths near the periphery. The modulus of elasticity may be set as desired by appropriate selection of string lengths of different materials such as different amides, graphite composites, etc. The modulus of elasticity may also be varied by irradiation as disclosed in U.S. Pat. No. 4,015,133 Ferrari (Table I--Column 3, line 65). 
     The deflection may also be set as required by providing string lengths of different cross-sectional area; i.e., usually different diameters as the strings are of circular cross section. The string lengths which pass through the center of the racquet are of greater cross-sectional area than the string lengths around its periphery. 
     While racquets strung with different string lengths in the practice of this invention are satisfactory, their demand on string and on time of stringing is higher than for racquets strung with a single or two strings. In accordance with this invention a racquet is provided which is strung with one or two strings but with the string or each string in different string sections, each subject to different tension. The string sections near the center of the racquet are subjected to higher tension and those near the periphery of the racquet to lower tension. This aspect of the invention arises from the realization that prior-art attempts to vary the tension were unsuccessful because the string sections under different tension of prior-art racquets were not isolated. Under the impacts of the ball, the tensions of the different sections tended to equalize soon after the racquet was put into use. In the practice of this invention, each string section under a predetermined tension is mechanically locked in position so that it is mechanically isolated from other sections. Specifically, the string for each string section is threaded through holes in the frame, tensioned and then locked by a tapered plug, retainer ring, grooved pin or the like. Where the tensioned string must be temporarily held, as where two strings, one longitudinal and the other transverse, passes through a hole, the string is held by a clamp. 
     In the practice of this invention strings whose surfaces have a high coefficient of friction, for example, strings with roughened surfaces, may be used. The increased friction would increase the resistance of the strings to movement where the strings cross and also in the holes of the frame. The tension would then remain as set. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a fragmental view in front elevation and partly in section showing a racquet in accordance with this invention formed with separate string lengths under different tensions; 
     FIG. 1A is a fragmental view in section of the portion of the racquet shown in FIG. 1 in the Circle IA; 
     FIG. 2 is a view in perspective showing the manner in which the predetermined tension is maintained temporarily in one of the string sections threaded through a hole of the frame of the racquet shown in FIG. 1 through which two string sections are threaded. 
     FIG. 3 is a fragmental view in front elevation and partly in section showing a racquet in accordance with this invention formed with separate string of different cross-sectional area; 
     FIG. 3A is a fragmental view in section enlarged of the portion of the racquet shown in Circle IIIA of FIG. 3; 
     FIG. 4 is a view in front elevation and partly in section showing a racquet in accordance with this invention formed of two long strings subdivided into different string sections under different tension; 
     FIG. 5 is a view in section taken along line V--V of FIG. 4; and 
     FIG. 6 is a view partly in section of the portion of the racquet shown in FIG. 3 in the Circle VI VI of FIG. 4. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The apparatus shown in FIG. 1 is a racquet 11 including a frame 13 and a handle 15. The frame 13 has the conventional holes 17 and 18 (FIG. 1A). Longitudinal separate string lengths 19a and 19b and transverse separate string lengths 21a, and 21b are threaded through opposite pairs of the holes 17 and 18. The holes 17 accommodate the ends of single string lengths 19a or 21a and the holes 18 accommodate the ends of two string lengths 19b and 21b. The string lengths 19a are threaded through holes 17 disposed oppositely longitudinally of the racquet 11 across the center portion of the racquet and the string lengths 19b are threaded through holes 18 disposed oppositely longitudinally of the racquet 11 near the periphery of the racquet. Each string length 19a or 19b has a knot 23 at the trailing end which prevents it from being pulled through the adjacent hole 17 or 18. At the leading end each string length is pulled through the opposite hole 17 or 18 and subjected to tension and then locked where the hole 17 accommodates the leading end of a single string length or temporarily held in position where the hole 18 accommodates the leading ends of two string lengths. The leading end of the string 19b is held by the clamping pin 27 (FIG. 2) of a clamp 29 which is screwed into the frame 31 to hold the leading end tightly. The clamp 29 is displaced from the hole 18 so that the leading end of the other string length 21b which passes through the hole 18 may be readily threaded through the hole. Where the leading end of a string length 19a is pulled through a single hole 17, it is permanently locked by a tapered plug or wedge 25 after being tensioned. Where a string length, usually 19b, is threaded through a hole 18 accommodating the leading ends of two string lengths, it is temporarily held by the clamp. 
     The string lengths 21a extend transversely across the center portion of the racquet 11 and the string lengths 21b extend transversely near the peripheral portions of the racquet. The string lengths 21a and 21b are threaded through holes 17 and 18 disposed oppositely transversely of the racquet. Each string length 21a or 21b is knotted at the trailing end to prevent it from being pulled through the trailing hole when tensioned. At the leading end each string length 21a or 21b is tensioned and locked in the leading hole. Each string length 21a which passes through a hole 17 that accommodates the leading end of a single string length is locked by the tapered plug or wedge 25 which is secured in the holes 17. Each string length, usually 21b, which passes through a hole 18 that accommodates the ends of two string lengths, is threaded through a hole in which a tensioned longitudinal string length is already being held by the clamp 29. Once string length 21b is threaded through hole 18, it is tensioned. While this string length 21b is in tension, the plug 25 (FIG. 2) is secured in hole 18 locking the ends of string lengths 19b and 21b in the hole 18 and clamp 29 is removed. 
     The string lengths 19a and 21a near the center portion of the racquet 11 are subjected to higher tension than the string lengths 19b and 21b near the periphery of the racquet. At the periphery near the transverse center of the racquet 11 the longitudinal string lengths 19b are subject to the low tension while the transverse string lengths 21a which interlace with these string lengths 19b are subject to high tension. Likewise at the periphery near the longitudinal center of the racquet the transverse string lengths 21b are subject to lower tension while the longitudinal string lengths 19a are subject to high tension. In the quadrant regions displaced about 30° to 60° from the longitudinal and transverse centers and remote from the axis of the racquet both the string lengths 19b and 21b are subject to lower tensions and near the axis of the racquet both the string lengths 19a and 21a are subject to higher tensions. The tensions are so set in the racquet shown in FIGS. 1 and 2 that for like impacts of a ball the deflection of the string is substantially uniform throughout the face of the racquet. 
     The apparatus shown in FIG. 3 is a racquet 41 including a frame 43 provided with conventional holes. This racquet 41 is strung with string lengths 45a and 45b and 47a and 47b of different cross-sectional area. The string lengths 45a and 47a near the center of the racquet are of greater cross-sectional area and the string lengths 45b and 47b near the periphery of the racquet 41 are of smaller cross-sectional area. The cross-sectional area for each of the string lengths 45a and 45b and 47a and 47b may decrease progressively from the center portion of the racquet to the peripheral portion. Each of the string lengths 45a, 45b, 47a, 47b is threaded and held in the same way as the string lengths of the racquet shown in FIG. 1. The trailing end of each string length has a knot which prevent it from being pulled through the trailing hole. The leading end of each string length 45a through 47b is threaded through the leading hole tensioned and locked in the hole by a tapered plug. Where a hole accommodates the ends of two string lengths, the end of the first string length to be threaded is held by a clamp similar to the clamp 29 of FIG. 2. The tension for all string lengths 45a through 47b may be the same or it may vary. The tension and cross-sectional areas are set so that the deflection of the strings throughout the face of the racquet for like impulses is substantially the same. 
     FIGS. 4, 5 and 6 show a racquet 51 having a frame 53 with conventional holes 55 and 56. The holes 55 and 56 terminate in grooves 58 centrally in the external rim of the racquet 51. The holes 55 accommodate single strings and the holes 56, two strings. The racquet 51 is threaded with two continuous strings, of unequal length. The longer is subdivided into longitudinal, mechanically isolated string sections 57a and 57b and the shorter is subdivided into transverse mechanically isolated string sections 59a and 59b. The string sections 57a and 59a are near the center of the face of the racquet 51 and the string sections 57b and 59b are near the periphery of the racquet. 
     Usually, the longer string is strung first. This string is folded over so that it has the form of a U. It is then threaded through the opposite holes on each side of the longitudinal center line with the legs of the U passing through the holes 55 adjacent the handle 61 and the apex of the U in the groove 58 across the frame 53 remote from the handle 61. The apex of the U is then clamped and the string is threaded through the holes 55 on one side of the longitudinal center line or the other. Let it be assumed that initial threading is on the right of the longitudinal center line. The string section 57a1 on the right which was threaded through the hole 55 to the right of and adjacent to the longitudinal center line is then tensioned and is secured in this hole by a plug 63 (FIG. 5). Section 57a1 constitutes a string section. Its tension is the maximum tension applied to the string. Once section 57a1 is physically secured, the string is threaded through the hole 55 to the right of the hole in which the string is locked and through the opposite hole 55 remote longitudinally from the handle forming string section 57a2. This section 57a2 is now subjected to tension smaller than the tension of 57a1 and locked by a tapered plug 63 in the last hole through which it was threaded. The threading and locking of the string alternately in holes adjacent the handle and remote from the handle continues and additional string sections are formed. String sections 57b1, 57b2, etc., are threaded through holes 56 near to and remote from the handle 61, which accommodate two strings and after being tensioned are temporarily secured by clamps such as is shown in FIG. 2. The last longitudinal string section 57b1 on the right after being tensioned and secured is tied by a knot just above the inner surface of the frame to an adjacent section. Usually this knot is adjacent the handle 61. After the longitudinal string sections 57a1, 57a2, etc. and 57b1, 57b2 are completely threaded and locked on the right of the longitudinal center line, the corresponding string sections on the left are threaded and locked. 
     Next the transverse string section 59b and 59a are threaded. These string sections are threaded progressively, with the shorter of the long strings, from the part of the frame 53 remote from the handle 61 to the part of the frame nearest the handle. The end of the shorter of the long strings is tied securely to one of the longitudinal strings 57a or 57b on the right and is then threaded through opposite holes 56 transversely of the frame 53, first through the hole 56 on the right and then through the hole 56 on the left. After being threaded through the hole on the left, the string 59b is appropriately tensioned and locked by a tapered plug 63. The hole 56 contains string 57b of a longitudinal string section. The plug 63 engages both strings 57b and 59b and secures both. If string 57b is at this time clamped, the clamp may be removed. If string 57b which passes through the hole 56 on the right is clamped, a plug 63 is inserted in this hole to lock both strings. The transverse section 59b1 is thus formed. The string is now threaded through the opposite transverse holes 56 on the left and right next remotest the handle. The string is now tensioned locked on the right and on the left, if necessary, by plugs 63 and clamps are removed. String section 59b2 is thus formed. The threading continues. Ultimately the string is threaded through opposite transverse holes 55 which accommodate only one string and tensioned and locked. Then the string is again threaded through opposite transverse holes 56, nearer the handle 61, which accommodate two strings. These string sections are locked as described above. Ultimately the section 59b1 nearest the handle is formed. Typically after this last section is formed, the string is locked in a hole 56 and tied to one of the longitudinal strings. A racquet the tension of whose strings vary is thus formed. In the practice of this invention the strings in the center of the racquet have higher tension and the strings along the periphery of the racquet have lower tension such that throughout the deflection of strings for like impacts of the ball is the same. Desirably the tension of the strings should decrease progressively from the center to the periphery or the racquet 51. 
     EXAMPLE 
     Computer analyses were performed to determine optimum tensions for each string segment in a racquet for three radically different racquets--wood, steel and graphite composite. The analyses showed string tensions as low as 26 pounds and as high as 82 pounds. The results for a graphite composite racquet are presented in Table I below: 
     
                       TABLE I______________________________________STRING TENSIONS REQUIRED TO MAXIMIZE&#34;SWEET SPOT&#34;Longitudinal Strings           Transverse Strings(Tension in pounds)           (Tension in pounds)______________________________________Left      42        Remote from Handle                                38     46                         43     49                         47     52                         50     54                         53     56                         56     58                         58to        59        to               60     60                         62     60                         60     59                         58     58                         58     56                         53     54                         50     52                         47     49                         43     46        Handle           48Right     42______________________________________ 
    
     Four racquets 11 as shown in FIG. 1 of the improved stringing in accordance wth this invention were strung and subjected to field comparison tests. Of the four racquets, two had a wood frame, one a steel frame, and one a graphite composite frame. One wood frame was strung with gut and one with NYLON string; the steel frame was strung with NYLON string and the graphite composite with gut. The tension in the strings was as shown in Table I, but varied somewhat. The four racquets were compared with four like racquets with conventional stringing. 
     The field test was performed by 16 tennis players (4 teaching professionals, 10 top amateurs, and 2 average players). The players were asked to evaluate the two racquets with regard to the size of the &#34;sweet spot&#34; and behavior of off-center hits without any knowledge of what racquet had the improved string and which racquet had the conventional stringing. The results are presented in Table II below: 
     
                       TABLE II______________________________________FIELD TEST RESULTS             Con-              No             ventional                      Improved Differ-Racquets          Stringing                      Stringing                               ence______________________________________#1 - Wilson Kramer Wood                 1        12     3(strung with beef gut)#2 - Wilson T2000 steel                 0        11     5(strung with nylon strings)#3 - Yamaha graphite composite                 3        11     2(strung with beef gut)#4 - Wilson Kramer Wood                 0        14     2(strung with nylon strings)______________________________________ 
    
     While preferred embodiments of this invention have been disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.