Patent Publication Number: US-2023141640-A1

Title: Racket

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Japanese Patent Application No. 2021-182156, filed on Nov. 8, 2021, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a racket for use in tennis or the like. 
     2. Description of the Related Art 
     A tennis racket includes a frame and strings. In the tennis racket, the strings are typically passed through holes via a grommet. A proposal relating to the shape of the grommet is disclosed in Japanese Laid-Open Patent Application Publication No. 2015-217192. 
     SUMMARY OF THE INVENTION 
     A tennis player tries to hit the ball at the center of the face of the racket. However, in tennis playing, the ball is often hit at a point away from the center of the face. When the ball is hit at a point below the center of the face (point toward the ground), the ball flies at a small launch angle due to a change in the angle of the face. The small launch angle leads to a low trajectory of the ball. The ball flying in a low trajectory is likely to fail to pass over the net. 
     The present applicant aims to provide a racket capable of producing a stable trajectory of a ball even in the event that the ball is hit at a point away from the center of the face of the racket. 
     A racket according to the present disclosure includes: 
     (1) a frame; 
     (2) a grommet mounted on the frame, the grommet including a plurality of tubular parts; and 
     (3) strings forming a face of the racket. 
     Each of the tubular parts includes a through hole through which a corresponding one of the strings is passed. The through hole includes a base opening, a side wall, and a tip opening. At least one of the plurality of tubular parts includes a non-circular tip opening. Assuming an imaginary dividing plane that is perpendicular to the face and that divides the non-circular tip opening into a first opening portion and a second opening portion shorter than the first opening portion in a direction parallel to the face, an inside dimension of the first opening portion in a direction perpendicular to the face increases gradually from one end of the non-circular tip opening in the direction parallel to the face to the dividing plane. In the tubular part including the non-circular tip opening, the string passes through the first opening portion and is in contact with the side wall from the base opening to the tip opening. 
     With the use of the racket according to the present disclosure, a stable trajectory can be achieved even in the event that the ball is hit at a point away from the center of the face. 
     The above and further objects, features and advantages of the present disclosure will be more apparent from the following detailed description of preferred embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a front view showing a racket according to a first embodiment of the present disclosure. 
         FIG.  2    is an enlarged view showing a portion of the racket of  FIG.  1   . 
         FIG.  3    is an exploded view showing a portion of the racket of  FIG.  1   . 
         FIG.  4    is an enlarged perspective view showing a portion of a grommet of the racket of  FIG.  3   . 
         FIG.  5 A  is an enlarged cross-sectional view of a portion of the grommet of  FIG.  4    as viewed from the front. 
         FIG.  5 B  is a cross-sectional view taken along the line B-B of  FIG.  5 A . 
         FIG.  5 C  is a view for illustrating the shape of the tip opening of  FIG.  5 B . 
         FIG.  6 A  is an enlarged cross-sectional view of a portion of the grommet of  FIG.  4    as viewed from the front. 
         FIG.  6 B  is a cross-sectional view taken along the line B-B of  FIG.  6 A . 
         FIG.  7    is an enlarged cross-sectional view of the vicinity of the top of the racket of  FIG.  1    as viewed from the front. 
         FIG.  8    is a view of the vicinity of the top of the racket of  FIG.  1    as viewed in an axial direction Y of the racket. 
         FIG.  9    is a front view showing the racket of  FIG.  1    along with a ball. 
         FIG.  10 A  is an enlarged cross-sectional view of the vicinity of the top of the racket of  FIG.  9    as viewed from the front. 
         FIG.  10 B  is a view for illustrating how a string moves upon contact of the ball with the face. 
         FIG.  11    is a view for illustrating a grommet according to Comparative Example 1. 
         FIG.  12    is a view for illustrating a grommet according to Comparative Example 2. 
         FIG.  13    is a view of a portion of a grommet of a racket according to a second embodiment of the present disclosure. 
         FIG.  14    is a view for illustrating how a string moves upon contact of a ball with the face of the racket of  FIG.  13   . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, the present disclosure will be described in detail based on preferred embodiments with appropriate reference to the drawings. 
     First Embodiment 
       FIGS.  1  to  3    show a tennis racket  2 . The tennis racket  2  includes a frame  4 , a grip  6 , grommets  8 , and strings  10 . The tennis racket  2  is usable for regular tennis. In the drawings, the arrow X represents the width direction of the tennis racket  2 , and the arrow Y represents the axial direction of the tennis racket  2 . The reference sign CL represents the centerline of the tennis racket  2 . The tennis racket  2  is symmetrical about the centerline CL. 
     The frame  4  includes a head  12 , two throats  14 , and a shaft  16 . The head  12  defines the outline of the face  17  (described in detail later). The head  12  as viewed from the front is generally shaped as an ellipse. The direction of the major axis of the ellipse coincides with the axial direction Y of the tennis racket  2 . The direction of the minor axis of the ellipse coincides with the width direction X of the tennis racket  2 . One end of each throat  14  is continuous with the head  12 . The throat  14  joins the other throat  14  in the vicinity of the other end. The throat  14  extends from the head  12  to the shaft  16 . The shaft  16  extends from the point where the two throats  14  join each other. The shaft  16  is continuous and integral with the throats  14 . The portion of the head  12  that is located between the two throats  14  is a yoke  18 . The head  12  may have a shape other than an elliptical shape. 
     The frame  4  is formed of a pipe. That is, the frame  4  is hollow. The pipe is made of a fiber-reinforced resin material. The matrix resin of the fiber-reinforced resin material is a thermosetting resin. The thermosetting resin is typically an epoxy resin. The fibers of the fiber-reinforced resin material are typically carbon fibers. The fibers are continuous fibers. 
     The grip  6  is formed by a tape wound around the shaft  16 . The grip  6  reduces slip between the hand(s) of the player and the tennis racket  2  when the player swings the tennis racket  2 . 
     As shown in  FIG.  3   , the tennis racket  2  includes a first grommet  8   a , two second grommets  8   b , and a third grommet  8   c . Each grommet  8  includes a base  20  and a plurality of tubular parts  100 . Each tubular part  100  is integral with the base  20 . The grommet  8  is typically made of a synthetic resin material softer than the material of the frame  4 . 
     As shown by the arrow A 1  in  FIG.  3   , the first grommet  8   a  is mounted around the top of the head  12 . With the first grommet  8   a  mounted on the head  12 , each tubular part  100  of the first grommet  8   a  extends through a corresponding one of holes (not shown) of the head  12 . As shown by the arrow A 2  in  FIG.  3   , each second grommet  8   b  is mounted on a corresponding one of the sides of the head  12 . With the second grommet  8   b  mounted on the head  12 , each tubular part  100  of the second grommet  8   b  extends through a corresponding one of holes (not shown) of the head  12 . As shown by the arrow A 3  in  FIG.  3   , the third grommet  8   c  is mounted on the yoke  18 . With the third grommet  8   c  mounted on the yoke  18 , each tubular part  100  of the third grommet  8   c  extends through a corresponding one of holes (not shown) of the head  12 . 
     The strings  10  are strung on the head  12 . The strings  10  are strung along the width direction X and the axial direction Y. The strings  10  that extend along the width direction X may be referred to as “transverse strings  10   a ”. The strings  10  that extend along the axial direction Y may be referred to as “longitudinal strings  10   b ”. The plurality of transverse strings  10   a  and the plurality of longitudinal strings  10   b  form the face  17  (see  FIG.  1   ). The face  17  extends generally along the X-Y plane. 
       FIG.  4    is an enlarged perspective view showing a portion of the grommet  8  of the tennis racket  2  of  FIG.  3   . The reference sign CP represents a plane passing through the centerline CL of the tennis racket  2  and perpendicular to the width direction X of the tennis racket  2  (this plane will be referred to as “reference plane” hereinafter). The tennis racket  2  is symmetrical about the reference plane CP. As previously stated, the grommet  8  includes the base  20  and the plurality of tubular parts  100 . 
     The tubular part  100  has a through hole  24  ( 24   a  or  24   b ) through which the string  10  is passed. The plurality of tubular parts  100  include a plurality of tubular parts  100   a  each of which has a through hole  24   a  having a generally triangular cross-section and a plurality of tubular parts  100   b  each of which has a through hole  24   b  having an elliptical cross-section. 
       FIG.  5 A  is a cross-sectional view taken along a plane including the axis of the tubular part  100   a .  FIG.  5 B  is a cross-sectional view taken along a plane perpendicular to the axis of the tubular part  100   a . The through hole  24   a  includes a base opening  26   a , a side wall  28   a , and a tip opening  30   a . As previously stated, the through hole  24   a  has a generally triangular cross-section. The cross-section of the through hole  24   a  is congruent from the base opening  26   a  to the tip opening  30   a . Thus, the tip opening  30   a  is generally triangular. 
     In  FIGS.  5 A and  5 B , the string  10  is shown by a dashed-double dotted line. In the tubular part  100   a  having the tip opening  30   a , the string  10  is in contact with the side wall  28   a  from the base opening  26   a  to the tip opening  30   a  which is generally triangular. 
     The shape of the tip opening  30   a  will be described in more detail with reference to  FIG.  5 C . In  FIG.  5 C , the edges of the tip opening  30   a  as viewed in the direction in which the through hole  24   a  extends are shown by a bold solid line. The direction in which the through hole  24   a  extends is parallel to the axial direction of the tubular part  100   a . The tip opening  30   a  is symmetrical about a given plane R parallel to the face  17  (this plane R will be referred to as “face plane R” hereinafter). The tip opening  30   a  is shaped as a triangle with rounded corners. The tip opening  30   a  has three rounded vertices. The face plane R crosses one of the three vertices. Each of the vertices is formed by a curve but may be formed as a point. 
     In  FIG.  5 C , an imaginary dividing plane S dividing the tip opening  30   a  into a first opening portion  31  and a second opening portion  32  in a direction parallel to the face  17  is shown by a dashed-double dotted line. The dividing plane S is perpendicular to the face  17 . The dividing plane S is parallel to the direction in which the through hole  24   a  extends. The length L 1  of the first opening portion  31  in the direction parallel to the face  17  is greater than the length L 2  of the second opening portion  32  in the direction parallel to the face  17 . 
     One end of the tip opening  30   a  in the direction parallel to the face  17  is referred to as a “first end P 1 ”, which is an end of the first opening portion  31  as well. The other end of the tip opening  30   a  in the direction parallel to the face  17  is referred to as a “second end P 2 ”, which is an end of the second opening portion  32  as well. The length L 1  of the first opening portion  31  is a distance between the first end P 1  and the dividing plane S in the direction parallel to the face  17 . The length L 2  of the second opening portion  32  is a distance between the second end P 2  and the dividing plane S in the direction parallel to the face  17 . 
     The first opening portion  31  is generally shaped as a triangle having a vertex at the first end P 1  of the tip opening  30   a  in the direction parallel to the face  17 . The inside dimension Lh of the first opening portion  31  in a direction perpendicular to the face  17  increases gradually from the first end P 1  to the dividing plane S. In the present embodiment, as shown in  FIG.  5 C , the imaginary dividing plane S is at a location where the inside dimension Lh of the tip opening  30   a  in the direction perpendicular to the face  17  reaches a maximum. 
     As shown in  FIG.  5 C , each of the two edges of the first opening portion  31  that extend from the first end P 1  to the dividing plane S includes a straight portion  33  that is straight when viewed in the direction in which the through hole  24   a  extends. The two straight portions  33  are symmetrical to each other about the face plane R. The ratio of the length L 3  of the straight portion  33  in the direction parallel to the face  17  to the length L 1  of the first opening portion  31  in the direction parallel to the face  17  (L 3 /L 1 ) is 0.3 or more, preferably 0.5 or more, and more preferably 0.7 or more. 
     The angle θ between extensions of the two straight portions  33  is 120 degrees or less and preferably 90 degrees or less. The magnitude of the angle θ may differ from one tubular part  100   a  to another. This will be described in detail later. 
     As shown by a dashed-double dotted line in  FIGS.  5 A and  5 B , the string  10  is placed to pass through the first opening portion  31  (see  FIG.  5 C ). More specifically, when viewed in the direction in which the through hole  24   a  extends, the string  10  is in contact with that rounded vertex of the tip opening  30   a  which includes the first end P 1 . As the cross-section of the through hole  24   a  is congruent from the base opening  26   a  to the tip opening  30   a , the string  10  can be in contact with the side wall  28   a  from the base opening  26   a  to the tip opening  30   a . As is clear from  FIGS.  5 A and  5 B , the length L 1  of the first opening portion  31  in the direction parallel to the face  17  is significantly greater than the diameter of the string  10 . In the vicinity of the dividing plane S, the length of the first opening portion  31  in the direction perpendicular to the face  17  is significantly greater than the diameter of the string  10 , while in the vicinity of the first end P 1 , the length of the first opening portion  31  in the direction perpendicular to the face  17  is not significantly greater than the diameter of the string  10 . The tubular part  100   a  permits the movement of the string  10  toward the dividing plane S. 
       FIG.  6 A  is a cross-sectional view taken along a plane including the axis of the tubular part  100   b .  FIG.  6 B  is a cross-sectional view taken along a plane perpendicular to the axis of the tubular part  100   b . The through hole  24   b  includes a base opening  26   b , a side wall  28   b , and a tip opening  30   b . As previously stated, the through hole  24   b  has an elliptical cross-section. The cross-section of the through hole  24   b  is congruent from the base opening  26   a  to the tip opening  30   a . Thus, the tip opening  30   b  is elliptical. The direction of the minor axis of the ellipse of the tip opening  30   b  coincides with the direction parallel to the face  17  (the left-right direction in  FIG.  6 B ). 
     In  FIGS.  6 A and  6 B , the string  10  is shown by a dashed-double dotted line. The string  10  is placed to pass through the center of the ellipse of the tip opening  30   b . More specifically, the minor axis of the ellipse of the tip opening  30   b  crosses the string  10  when viewed in the direction in which the through hole  24   b  extends. For example, the string  10  may be in contact with the side wall  28   b  in the direction parallel to the face  17  from the base opening  26   b  to the tip opening  30   b . As is clear from  FIGS.  6 A and  6 B , the inside dimension of the tip opening  30   b  in the direction perpendicular to the face  17  is significantly greater than the diameter of the string  10 . The tubular part  100   b  permits the movement of the string  10  mainly in the direction perpendicular to the face  17 . 
     As shown in  FIG.  2   , the tennis racket  2  includes a first tubular part  101 , a second tubular part  102 , a third tubular part  103 , a fourth tubular part  104 , a fifth tubular part  105 , a sixth tubular part  106 , a seventh tubular part  107 , an eighth tubular part  108 , a ninth tubular part  109 , a tenth tubular part  110 , an eleventh tubular part  111 , a twelfth tubular part  112 , a thirteenth tubular part  113 , a fourteenth tubular part  114 , a fifteenth tubular part  115 , a sixteenth tubular part  116 , a seventeenth tubular part  117 , an eighteenth tubular part  118 , a nineteenth tubular part  119 , a twelfth tubular part  120 , a twenty-first tubular part  121 , a twenty-second tubular part  122 , a twenty-third tubular part  123 , a twenty-fourth tubular part  124 , a twenty-fifth tubular part  125 , a twenty-sixth tubular part  126 , a twenty-seventh tubular part  127 , a twenty-eighth tubular part  128 , a twenty-ninth tubular part  129 , a thirtieth tubular part  130 , a thirty-first tubular part  131 , a thirty-second tubular part  132 , a thirty-third tubular part  133 , a thirty-fourth tubular part  134 , and a thirty-fifth tubular part  135 . In  FIG.  2 ,  35    tubular parts  100  are shown. As previously stated, the tennis racket  2  is symmetrical about the centerline CL. Thus, the number of the tubular parts  100  in the tennis racket  2  is 70. 
     As shown in  FIG.  2   , the longitudinal strings  10   b  are passed through the first tubular part  101 , second tubular part  102 , third tubular part  103 , fourth tubular part  104 , fifth tubular part  105 , sixth tubular part  106 , seventh tubular part  107 , ninth tubular part  109 , twenty-seventh tubular part  127 , twenty-ninth tubular part  129 , thirtieth tubular part  130 , thirty-first tubular part  131 , thirty-second tubular part  132 , thirty-third tubular part  133 , thirty-fourth tubular part  134 , and thirty-fifth tubular part  135 . The transverse strings  10   a  are passed through the eighth tubular part  108 , tenth tubular part  110 , eleventh tubular part  111 , twelfth tubular part  112 , thirteenth tubular part  113 , fourteenth tubular part  114 , fifteenth tubular part  115 , sixteenth tubular part  116 , seventeenth tubular part  117 , eighteenth tubular part  118 , nineteenth tubular part  119 , twelfth tubular part  120 , twenty-first tubular part  121 , twenty-second tubular part  122 , twenty-third tubular part  123 , twenty-fourth tubular part  124 , twenty-fifth tubular part  125 , twenty-sixth tubular part  126 , and twenty-eighth tubular part  128 . 
     In the present embodiment, each of the second, fourth, and sixth tubular parts  102 ,  104 , and  106  has the through hole  24   a  having a generally triangular cross-section (see  FIG.  5   ). Each of the other tubular parts  100  has the through hole  24   b  having an elliptical cross-section (see  FIG.  6   ). Any of the tubular parts  100  other than the second, fourth, and sixth tubular parts  102 ,  104 , and  106  may have the through hole  24   a  having a generally triangular cross-section or have a through hole having a circular cross-section. 
       FIG.  7    is an enlarged view of the vicinity of the top of the racket  2  of  FIG.  1    as viewed from the front.  FIG.  8    is a view of the vicinity of the top of the racket  2  of  FIG.  1    as viewed in the axial direction Y. In  FIGS.  7  and  8   , the frame  4  and the transverse strings  10   a  are omitted. In  FIG.  7   , there are shown the base  20 , first tubular part  101 , second tubular part  102 , third tubular part  103 , and fourth tubular part  104 . In  FIG.  8   , there are shown the base  20 , first tubular part  101 , second tubular part  102 , third tubular part  103 , fourth tubular part  104 , fifth tubular part  105 , and sixth tubular part  106 . Each of the first, third, and fifth tubular parts  101 ,  103 , and  105  has the through hole  24   b  having an elliptical cross-section (see  FIG.  6   ). Each of the second, fourth, and sixth tubular parts  102 ,  104 , and  106  has the through hole  24   a  having a generally triangular cross-section (see  FIG.  5   ). In each of the second, fourth, and sixth tubular parts  102 ,  104 , and  106 , the string  10  is in contact with the left wall surface (inner one of the opposite wall surfaces in the width direction) of the side wall  28   a  of the through hole  24   a.    
     As shown in  FIG.  8   , the second, fourth, and sixth tubular parts  102 ,  104 , and  106  are identical in that each of them has the generally triangular tip opening  30   a ; however, the generally triangular shape slightly differs among the second, fourth, and sixth tubular parts  102 ,  104 , and  106 . Specifically, in the second tubular part  102 , the angle θ 1  between extensions of the two straight portions  33  included in the edges of the tip opening  30   a  is 30 degrees. In the fourth tubular part  104 , the angle θ 2  between extensions of the two straight portions  33  included in the edges of the tip opening  30   a  is 60 degrees. In the sixth tubular part  106 , the angle θ 3  between extensions of the two straight portions  33  included in the edges of the tip opening  30   a  is 90 degrees. That is, the closer the tubular part is to the centerline CL of the racket  2 , the smaller is the angle θ between extensions of the two straight portions (θ 1 &lt;θ 2 &lt;θ 3 ). The angle θ between extensions of the two straight portions  33  included in the edges of the tip opening  30   a  may be the same for the second, fourth, and sixth tubular parts  102 ,  104 , and  106 . 
     In the tip opening  30   a  of each of the second, fourth, and sixth tubular parts  102 ,  104 , and  106 , the first opening portion  31  is closer to the centerline CL of the racket  2  than the second opening portion  32 . 
       FIG.  9    shows the tennis racket  2  along with a tennis ball B.  FIG.  9    illustrates the moment of impact between the tennis racket  2  and the tennis ball B. In  FIG.  9   , the tennis ball B collides with the face  17  at a point below the centerline CL (point toward the ground G). In this state, the player swings the tennis racket  2  forward and upward. 
     As a result of the swing, the longitudinal string  10   b  passed through the second tubular part  102  is subjected to a force acting perpendicular to the face  17  and outward in the width direction. In  FIG.  10 A , the longitudinal string  10   b  moved by the force is shown. In  FIG.  10 B , the direction in which the ball B contacts the face  17  is shown by the arrow B 1 , and the direction in which the longitudinal string  10   b  moves upon contact of the ball B with the face  17  is shown by the arrow depicted inside the through hole  24   a . The longitudinal string  10   b  is deformed without being disturbed by the second tubular part  102  and moves outward in the width direction. In the tip opening  30   a , the longitudinal string  10   b  is pushed by the ball B and moves toward the dividing plane S along the edge of the first opening portion  31  that is opposite from the ball B. After that, the longitudinal string  10   b  is restored to its original shape. Likewise, the longitudinal strings  10   b  passed through the fourth tubular part  104  ( 100   a ) and sixth tubular part  106  ( 100   a ) are deformed and then restored to their original shapes. The deformation and restoration of the longitudinal strings  10   b  ensure a long time of contact between the tennis racket  2  and the tennis ball B. With the use of the tennis racket  2 , the tennis ball B is shot at a large launch angle. With the use of the tennis racket  2 , a high trajectory of the tennis ball B can be achieved even in the event that the tennis ball B is hit at a point below the centerline CL. 
     As previously stated, the string  10  is in contact with the inner one of the opposite wall surfaces of the side wall  28   a  of the through hole  24   a  in the width direction. Thus, when the tennis ball B collides with the face  17  at a point above the centerline CL, the deformation of the longitudinal string  10   b  is not facilitated by the generally triangular through hole  24   a . As such, a long time of contact is not achieved, and correction of the trajectory does not occur. With the use of the racket  2 , there is little difference in trajectory between collision of the tennis ball B with the face  17  at a point above the centerline CL and collision of the tennis ball B with the face  17  at a point below the centerline CL. 
     As previously stated, the string  10  is pushed by the ball B and moves toward the dividing plane S along the edge of the first opening portion  31  that is opposite from the ball B. As the movement direction of the string  10  is restricted in this manner, a stable trajectory can be achieved with reduced variations in both the launch angle and flight distance. As the tip opening  30   a  is symmetrical about a plane parallel to the face  17 , a stable trajectory can be achieved regardless of which side of the face  17  the ball B collides with. 
     In the first tubular part  101 , the tip opening  30   b  is shaped as an ellipse having a minor axis the direction of which coincides with the direction parallel to the face  17 . Thus, the longitudinal string  10   b  passed through the first tubular part  101  is deformed in the direction perpendicular to the face  17  under the action of the pressing force applied from the tennis ball B. The first tubular part  101  does not hinder this deformation. However, the first tubular part  101  hinders the deformation of the string  10  in the direction parallel to the face  17 . Thus, the string  10  passed through the through hole  24   b  of the first tubular part  101  and the string  10  passed through the through hole  24   a  of the second tubular part  102  adjacent to the first tubular part  101  move away from each other under the action of the pressing force applied from the tennis ball B. Thus, the distance between the two strings  10  is increased upon contact of the ball B with the face  17 , and this makes it easier to spin the ball B. 
     Likewise, the distance between the string  10  passed through the through hole  24   b  of the third tubular part  103  and the string  10  passed through the through hole  24   a  of the fourth tubular part  104  is increased under the action of the pressing force applied from the tennis ball B. The distance between the string  10  passed through the through hole  24   b  of the fifth tubular part  105  and the string  10  passed through the through hole  24   a  of the sixth tubular part  106  is also increased under the action of the pressing force applied from the tennis ball B. 
     The edges of the first opening portion  31  of each of the second, fourth, and sixth tubular parts  102 ,  104 , and  106  include the two straight portions  33  which are symmetrical to each other about a plane parallel to the face  17 . Extensions of the two straight portions  33  in each of the second, fourth, and sixth tubular parts  102 ,  104 , and  106  form an angle θ with each other, and the closer the tubular part is to the centerline CL of the racket  2 , the smaller the angle θ between the extensions is (that is, θ 1 &lt;θ 2 &lt;θ 3 ). This allows the string  10  to move in an appropriate direction in each tubular part. In general, the flight distance of the ball decreases as the ball hitting point becomes more distant from the center of the face of the racket. However, in the present embodiment, the angle θ between extensions of the two straight portions  33  increases with increasing distance from the tubular part to the centerline CL of the racket  2 , so that the farther the tubular part is from the centerline CL, the more easily the string  10  can move in a direction perpendicular to a ball hitting plane. Thus, a desired flight distance is likely to be achieved even in the event that the ball is hit at a point away from the centerline CL of the racket  2 . 
     In order to endow the tennis racket  2  with performance intended by the designer, the tubular part  100   a  may be formed such that the string  10  is in contact with the outer one of the opposite wall surfaces in the width direction. 
     In the tennis racket  2 , each of the tubular parts  100   a  located in the vicinity of the top of the tennis racket  2  (second, fourth, sixth tubular parts  102 ,  104 , and  106 ) has the through hole  24   a  having a generally triangular cross-section. Any of the tubular parts  100  located on the yoke  18  may have the through hole  24   a  having a generally triangular cross-section. Any of the tubular parts  100  located on the sides of the head  12  may have the through hole  24   a  having a generally triangular cross-section. Any of the transverse strings  10   a  may be passed through the tubular part  100  having the through hole  24   a  having a generally triangular cross-section. In any case, the height of trajectory exhibited upon collision of the tennis ball B with a given area of the face  17  can be increased. 
     In the tennis racket  2 , as previously stated, each of the second, fourth, and sixth tubular parts  102 ,  104 , and  106  has the through hole  24   a  having a generally triangular cross-section. Being symmetrical about the centerline CL, the tennis racket  2  includes two second tubular parts  102 , two fourth tubular parts  104 , and two sixth tubular parts  106 . Thus, the total number N of the tubular parts  100   a  having the through hole  24   a , which has a generally triangular cross-section and in which the string  10   b  is in contact with the inner one of the opposite wall surfaces in the width direction, is six. The total number N need not be 6. For example, the total number N is preferably from 2 to 16 and more preferably from 4 to 12. 
     Evaluation Test 
     An evaluation test was conducted in which the effect of the racket  2  including the tubular part  100   a  having the through hole  24   a  having a generally triangular cross-section was evaluated by comparing the racket  2  with other rackets. In the evaluation test, three types of rackets, i.e., rackets of Example, Comparative Example 1, and Comparative Example 2 were produced first. 
     A racket including second, fourth, and sixth tubular parts was produced as the racket of Example. Each of the second, fourth, and sixth tubular parts had a through hole  24   a  having a generally triangular cross-section, like the second, fourth, and sixth tubular parts  102 ,  104 , and  106  described in the above embodiment. 
       FIG.  11    shows a second tubular part  200  of the racket of Comparative Example 1. The through hole  201  of the second tubular part  200  has a circular cross-section. The cross-section of the through hole  201  is congruent from the base opening to the tip opening. The diameter of the through hole  201  is only slightly greater than the diameter of the string  10 , and the string  10  is hardly movable in the through hole  201 . In Comparative Example 1, the cross-sections of the through holes of the fourth and sixth tubular parts have the same shape as the cross-section of the through hole  201  of the second tubular part  200 . 
       FIG.  12    shows a second tubular part  300  of the racket of Comparative Example 2. The through hole  301  of the second tubular part  300  has a circular cross-section. The cross-section of the through hole  301  is congruent from the base opening to the tip opening. The diameter of the through hole  301  is significantly greater than the diameter of the string  10 . The diameter of the through hole  301  is two or more times the diameter of the string  10 . In the tubular part  300 , the string  10  is in contact with the inner one of the opposite wall surfaces of the side wall of the through hole  301  in the width direction. The tubular part  300  restricts the string  10  from moving inward along the width direction in the through hole  301 , and does not restrict the movement of the string  10  in other directions. In Comparative Example 2, the cross-sections of the through holes of the fourth and sixth tubular parts have the same shape as the cross-section of the through hole  301  of the second tubular part  300 . 
     In each of the rackets of Example, Comparative Example 1, and Comparative Example 2, the through holes of tubular parts other than the second, fourth, and sixth tubular parts had the same circular cross-section. Specifically, the shape of the through holes of tubular parts other than the second, fourth, and sixth tubular parts was the same as the shape of the through hole  201  shown in  FIG.  11   . 
     After production of the three types of rackets of Example, Comparative Example 1, and Comparative Example 2 as described above, players were allowed to play rallies using the different types of rackets. The ball trajectories were tracked for each racket, and the average and standard deviation of the launch angle of the ball were calculated. The average and standard deviation of the flight distance of the ball were also calculated. The calculation results are listed in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Comparative 
                 Comparative 
               
               
                   
                 Example 
                 Example 1 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Launch 
                 Average (°) 
                 10.5 
                 9.9 
                 9.6 
               
               
                 angle 
                 Standard deviation 
                 0.64 
                 1.31 
                 1.50 
               
               
                 Flight 
                 Average (m) 
                 24.4 
                 23.9 
                 23.9 
               
               
                 distance 
                 Standard deviation 
                 0.90 
                 1.58 
                 1.99 
               
               
                   
               
            
           
         
       
     
     As seen from Table 1, the average of the launch angle was greater in Example 1 than in Comparative Examples 1 and 2. The average of the flight distance was also greater in Example than in Comparative Examples 1 and 2. This verifies that the racket of Example is superior to the rackets of Comparative Examples 1 and 2 in both the launch angle and flight distance. 
     Additionally, the standard deviation from the average of the launch angle was smaller in Example than in Comparative Examples 1 and 2, and the standard deviation from the average of the flight distance was also smaller in Example than in Comparative Examples 1 and 2. This verifies that the racket of Example produces a more stable trajectory with smaller variations in both the launch angle and flight distance than the rackets of Comparative Examples 1 and 2. 
     Second Embodiment 
       FIG.  13    shows a portion of a grommet  40  of a tennis racket according to a second embodiment. This tennis racket has the same structural components as the tennis racket  2  shown in  FIGS.  1  to  12   , except for the grommet  40 . In the following description, the structural components of the grommet  40  that are the same as those of the grommet of the first embodiment are denoted by the same reference signs and will not be described in detail. 
     The grommet  40  includes a base  20  and tubular parts  100 . Each tubular part  100  has a through hole  24  thorough which a string  10  is passed. The cross-section of the through hole  24  is congruent from the base opening to the tip opening. The plurality of tubular parts  100  include tubular parts  100   a  and  100   c  each of which has a through hole  24   a  or  24   c  having a cross-section shaped as described in the first embodiment. Each of the through holes  24   a  and  24   c  has a generally triangular cross-section just as does the through hole  24   a  described in the first embodiment. The orientation of the generally triangular shape differs between the through holes  24   a  and the through holes  24   c.    
     Specifically, as shown in  FIG.  13   , the through hole  24   c  of the tubular part  100   c  has a cross-section that is symmetrical to that of the through hole  24   a  of the tubular part  100   a  about a line perpendicular to the face  17 . The cross-section of the through hole  24   a  and the cross-section of the through hole  24   c  are in different orientations but are congruent to each other. The tubular parts  100   a  and the tubular parts  100   c  alternate with one another. 
     The angle between extensions of the two straight portions included in the edges of the tip opening is the same for all of the tubular parts  100   a  and  100   c  shown in  FIG.  13   . However, the angle between extensions of the two straight portions included in the edges of the tip opening may differ for each tubular part  100   a  or  100   c . The magnitude of the angle between extensions of the two straight portions may be chosen as appropriate. 
     The present embodiment differs from the first embodiment also in that in the tubular parts  100   a  and  100   c , the first opening portion  31  need not be closer to the centerline CL of the racket  2  than the second opening portion  32 . For example, when the first opening portion  31  is closer to the centerline CL of the racket  2  than the second opening portion  32  in one of the two tubular parts  100   a  and  100   c  adjacent to each other, the first opening portion  31  may be farther from the centerline CL of the racket  2  than the second opening portion  32  in the other of the adjacent tubular parts  100   a  and  100   c.    
     The grommet  40  is mounted around the top of the head  12 . The grommet  40  may be mounted on one of the sides of the head  12  or on the yoke  18 . 
       FIG.  14    is a view for illustrating how the string  10  moves upon contact of a ball with the face of the racket of  FIG.  13   .  FIG.  14    shows one tubular part  100   a  and one tubular part  100   c  which are adjacent to each other. 
     The present embodiment, like the first embodiment, has the advantage of increasing both the launch angle and flight distance. Additionally, the present embodiment, like the first embodiment, has the advantage of producing a stable trajectory with reduced variations in both the launch angle and flight distance. 
     Furthermore, in the present embodiment, the string  10  passed through the through hole  24   a  of the tubular part  100   a  and the string  10  passed through the through hole  24   a  of the tubular part  100   c  adjacent to the tubular part  100   a  move away from each other. Thus, the maximum value of the distance between the two adjacent strings  10  is increased upon contact of the ball with the face. This makes it easier to spin the ball. 
     Other Embodiments 
     Although in the above embodiments the imaginary dividing plane S is at a location where the inside dimension of the tip opening  30   a  in the direction perpendicular to the face  17  reaches a maximum, the imaginary dividing plane S is not limited to being at this location. The dividing plane may be defined at any location, provided that the first opening portion is longer than the second opening portion in the direction parallel to the face and that the inside dimension of the first opening portion in the direction perpendicular to the face increases gradually from one end of the first opening portion in the direction parallel to the face to the dividing plane. 
     Although in the above embodiments the cross-section of the through hole is congruent from the base opening to the tip opening, the cross-section of the through hole need not be congruent from the base opening to the tip opening. For example, the through hole may have a generally triangular tip opening as described in the above embodiments and a base opening having a shape other than a generally triangular shape, such as a circular base opening. For example, the through hole may have a generally triangular tip opening as described in the above embodiments, and the cross-section of the through hole may change continuously or stepwise toward the base opening. 
     Although in the above first embodiment the direction of the minor axis of the ellipse of the tip opening  30   b  coincides with the direction parallel to the face  17  (the left-right direction in  FIG.  6 B ), the minor axis of the ellipse of the tip opening  30   b  may cross the direction parallel to the face  17  (the left-right direction in  FIG.  6 B ). The tip opening  30   b  may be circular. In this case, the tip opening  30   b  may be, for example, an opening as shown in  FIG.  11    which permits little movement of the string or an opening as shown in  FIG.  12    which permits movement of the string. 
     The tip opening including the first and second opening portions need not be generally triangular. The tip opening may be generally shaped as a rectangle composed of a generally triangular first opening portion and a generally triangular second opening portion that is shorter than the generally triangular first opening portion in the direction parallel to the face. The tip opening may be fan-shaped. The tip opening may be asymmetrical about a plane parallel to the face. 
     [Disclosed Items] 
     The following items disclose preferred embodiments. 
     [Item 1] 
     A racket including: a frame; a grommet mounted on the frame, the grommet including a plurality of tubular parts; and strings forming a face of the racket, wherein: each of the tubular parts includes a through hole through which a corresponding one of the strings is passed; the through hole includes a base opening, a side wall, and a tip opening; at least one of the plurality of tubular parts includes a non-circular tip opening; assuming an imaginary dividing plane that is perpendicular to the face and that divides the non-circular tip opening into a first opening portion and a second opening portion shorter than the first opening portion in a direction parallel to the face, an inside dimension of the first opening portion in a direction perpendicular to the face increases gradually from one end of the non-circular tip opening in the direction parallel to the face to the dividing plane; and in the tubular part including the non-circular tip opening, the string passes through the first opening portion and is in contact with the side wall from the base opening to the tip opening. 
     [Item 2] 
     The racket according to item 1, wherein the first opening portion is generally shaped as a triangle having a vertex at the one end of the non-circular tip opening. 
     [Item 3] 
     The racket according to item 1 or 2, wherein the first opening portion is closer to a centerline of the racket than the second opening portion. 
     [Item 4] 
     The racket according to any one of items 1 to 3, wherein the first opening portion includes an edge extending from the one end to the dividing plane, the edge including a straight portion that is straight when viewed in a direction in which the through hole extends, and a ratio of a length of the straight portion in the direction parallel to the face to a length of the first opening portion in the direction parallel to the face is 0.3 or more. 
     [Item 5] 
     The racket according to any one of items 1 to 4, wherein the string passed through the tubular part including the non-circular tip opening is a longitudinal string. 
     [Item 6] 
     The racket according to item 5, wherein the tubular part including the non-circular tip opening is located in the vicinity of a top of the frame. 
     [Item 7] 
     The racket according to any one of items 1 to 6, wherein: the plurality of tubular parts include two or more tubular parts each of which includes the non-circular tip opening, the two or more tubular parts being located in the vicinity of a top of the frame; edges of the first opening portion of each of the two or more tubular parts include two straight portions each of which is straight when viewed in a direction in which the through hole extends, the two straight portions being symmetrical to each other about a plane parallel to the face; extensions of the two straight portions in each of the two or more tubular parts form an angle with each other; and the closer the tubular part is to a centerline of the racket, the smaller the angle between the extensions is. 
     The racket according to the present disclosure can be used in various kinds of sports such as soft tennis, squash, and badminton. 
     From the foregoing description, numerous modifications and other embodiments of the present disclosure are obvious to those skilled in the art. Accordingly, the foregoing description is to be construed as illustrative only, and is provided for the purpose of teaching those skilled in the art the best mode for carrying out the present disclosure. The structural and/or functional details may be substantially modified without departing from the scope of the present disclosure.