Abstract:
An archery string is produced of polyethylene monofilaments of a super high molecular weight by multi-staged bundling and twisting in order to lower the elongation percentage, thereby assuring high hitting rate in shooting and better endurance.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an improved archery string, and more particularly relates to improvement in mechanical properties at stretch of an archery string including strands made up of a lot of synthetic monofilament yarns. 
     Most conventional archery strings of the above-described type are made of polyester monofilaments which in general have too high elongation percentage, i.e. the magnitude of elongation of a string under application of a fixed load. Such high elongation percentage impairs repulsion of the string and lowers the initial velocity of arrows at release, thereby lowering hitting rate in shooting. 
     It is thinkable to lower the twist of each strand in order to lower its elongation percentage. Such an expedient, however, reduces the maximum number of use at breakage and impairs endurance of the string. For these reasons, it is not advisable to adjust the elongation percentage of a string by reducing the twist of the strands. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to lower the elongation percentage of an archery string without any ill influence on its endurance. 
     In accordance with the present invention, an archery string includes a plurality of strands bundled together, each strand is made up of a plurality of elementary yarns of a prescribed thickness twisted together, and each elementary yarn is made up of a plurality of polyethylene monofilaments bundled together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of one embodiment of the archery string in accordance with the present invention, and 
     FIG. 2 is a plan view of the relation between the archery string and the archer&#39;s hand at release of an arrow. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One example of the archery string in accordance with the present invention is shown in FIG. 1, in which an archery string 1 is provided with a pair of end loops 1a and 1b for attachment to an archery bow. 
     As the starting material, monofilaments are made of polyethylene of a super high molecular weight. That is, the thickness of the monofilament should preferably be in a range from 5 to 20 deniers, Youngs modulus at stretch should preferably be in a range from 80 to 150 GPa (International Systems SI units) and the specific gravity should preferably be in a range from 0.96 to 0.98. A plurality of such monofilaments are bundled together substantially with no twists to form an elementary yarn of 500 to 1500 deniers thickness. In the case of the illustrated example, three elementary yarns are twisted together without any twists on each yarn to form a strand. The thickness of each elementary yarn and the number of elementary yarns to be twisted together so that the thickness of a resultant strand should preferably be in a range from 18,000 to 28,000 deniers. Such strands are bundled together to form an archery string such as shown in FIG. 1. 
     When three elementary yarns are twisted together to form a strand, the pitch of twists on the strand is given by 10000×(60-120)/thickness of the elementary yarns. Either S and Z twists are employable. Some examples of the pitch of twists are given in Table 1. 
     
                       TABLE 1______________________________________Thickness of theelementary yarn          Number of twists perin denier      meter on the strand______________________________________ 500           120˜2401000            60˜1201500           40˜80______________________________________ 
    
     According to the present invention, the elongation percentage of the archery string is adjusted to the range from 0.08 to 1.2% and, as a consequence, the repulsion of the string at release is greatly enlarged, the string returns forward very quickly and initial velocity of an arrow at release is raised considerably, thereby much improving the hitting rate in shooting. In particular a skilled archer opens fingers very swiftly at release. More specifically as best seen in FIG. 2, shift in hand position from the left to right state is very swift in the case of a skilled archer and, as a consequence, slow forward return of the string would allow itself to move as indicated with a dot line and this curved return of the strings would cause end whip of an arrow. For these reasons, strings with low elongation percentage are in general favoured by skilled archers. 
     In another example of the present invention, a plurality of monofilaments are bundled together substantially with no twists to form an elementary yarn of 500 to 1500 denier thickness. After application of individual twists, two elementary yarns are twisted together to form a strand. A plurality of such strands are bundled together to form an archery string in accordance with the present invention. 
     At application of individual twists on the elementary yarn, the pitch of twists on the elementary yarn is given by 1000 (200-350)/ thickness of the elementary yarns. Whereas the pitch of twists on the strand is given by 1000×(200-350)/ thickness of the elementary yarns. S twists may be used for the individual twists on the elementary yarn in combination with Z twists used for the general twists on the strand. The combination may be reversed. Some samples of the pitch of twists are given in Table 2. 
     
                       TABLE 2______________________________________Thickness of the        Number of twists                     Number of twistselementary yarn        per meter on per meter on thein denier    the strand   elementary yarn______________________________________ 500         400˜700                     400˜7001000         200˜350                     200˜3501500         133˜233                     133˜233______________________________________ 
    
     According to the present invention, subdued elongation percentage assures improved mechanical properties of an archery string with rich endurance.