Patent Publication Number: US-2013244817-A1

Title: Tennis racket and method for manufacturing the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority upon Japanese Patent Application No. 2012-40538 filed on Feb. 27, 2012, which is herein incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a tennis racket and a method for manufacturing a tennis racket. 
     2. Related Art 
     A tennis racket includes a frame that surrounds a hitting face for hitting a ball. There has also been proposed a tennis racket in which the frame is formed hollow with the entire hollow portion is uniformly filled with foam material (e.g., see Japanese Patent Application Laid-Open Publication No. 6-105923). 
     The area of the hitting face that is frequently used differs from player to player in some cases. For example, top players (advanced players) tend to frequently use the area of the hitting face at the tip side (side opposite to the grip portion) than the substantially central portion (sweet spot). For this reason, there is demand for an increase in particularly rigidity (strength) and vibration absorption in the portions of the frame that correspond to the area that is frequently used. 
     Incidentally, since a foam material is formed uniformly inside the frame of tennis rackets such as those described above, it has been difficult to increase rigidity and vibration absorption at specific positions in the frame. Increasing rigidity by providing foam material to the entire frame leads to a problem of an increase in repulsive force (due to a decrease in frame flexure) and a decline in control when hitting the ball (ball control), and also leads to a problem of difficulty in weight reduction. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of these circumstances, and an object thereof is to provide a tennis racket that allows improvement in rigidity and vibration absorption at a desired position and allows improvement in ball control and reduction in weight, as well as a method for manufacturing the same. 
     A tennis racket according to a main invention for achieving the aforementioned object is a tennis racket including a frame that surrounds a hitting face for hitting a ball, wherein a foam material is provided inside two side portions of the frame on opposite sides of a predetermined area of the hitting face, and the two side portions of the frame have a strength that is higher than a strength of another portion of the frame. 
     According to the tennis racket of the present invention, rigidity and vibration absorption at a desired position in the frame can be improved, and ball control can be improved and the weight thereof can be reduced as well. 
     Other features of the present invention will become clear from the explanation in the present specification and the description of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a front view of a tennis racket  1  according to an embodiment of the present invention; 
         FIG. 2  is a side view of the tennis racket  1  according to the embodiment of the present invention; 
         FIG. 3A  is a schematic diagram showing a method for manufacturing the tennis racket  1 ; 
         FIG. 3B  is another schematic diagram showing the method for manufacturing the tennis racket  1 ; 
         FIG. 3C  is another schematic diagram showing the method for manufacturing the tennis racket  1 ; 
         FIG. 3D  is another schematic diagram showing the method for manufacturing the tennis racket  1 ; 
         FIG. 4A  is a cross-sectional view taken along A-A in  FIG. 3B ; 
         FIG. 4B  is a cross-sectional view taken along B-B in  FIG. 3B ; 
         FIG. 5  is a schematic diagram showing an aspect of a hardness test performed with the foam material alone; 
         FIG. 6  is a schematic diagram showing an aspect of a test regarding the amount of cross-section deformation of the racket; 
         FIG. 7  is a schematic diagram showing an aspect of a racket bending test; and 
         FIG. 8  is an illustrative diagram of a modified example. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
     Summary of Disclosure 
     At least the following matters will be made clear by the explanation in the present specification and the description of the accompanying drawings. 
     Specifically, the tennis racket that will be made clear is a tennis racket including a frame that surrounds a hitting face for hitting a ball, wherein a foam material is provided inside two side portions of the frame on opposite sides of a predetermined area of the hitting face, and the two side portions of the frame have a strength that is higher than a strength of another portion of the frame. 
     According to such tennis racket, rigidity and vibration absorption at a desired position in the frame can be improved, and ball control can be improved and the weight thereof can be reduced as well. 
     In the above tennis racket, a foam material for filling that is different from the foam material is provided inside the another portion of the frame. Also, the foam material is provided inside the another portion of the frame, and an amount of the foam material in the two side portions is different from an amount of the foam material in the another portion. 
     According to such tennis racket, a hollow portion can be prevented from being made to the frame, and foreign particles can be prevented from entering into the frame. 
     In the above tennis racket, it is desirable that the frame has a frame portion that surrounds the hitting face on one end side, and has a grip portion on another end side, and the predetermined area is an area on the one end side than substantially a middle of the hitting face. 
     According to such tennis racket, optimum hitting characteristics (e.g., control) can be obtained when used by a player who often uses one end side of the hitting face (e.g., a top player). 
     In the above tennis racket, it is desirable that when a position of the one end of the frame portion is set at 0 degrees relative to a center of the hitting face, the foam material is provided in a range of 20 degrees to 60 degrees of the frame portion. 
     According to such tennis racket, it is possible to increase rigidity and vibration absorption when hitting the ball with one end side of the hitting face. 
     Also, a method for manufacturing a tennis racket including a frame having a first strength and a second strength that is higher than the first strength, including the steps of arranging a foam material on a resin sheet in an area that corresponds to the second strength, forming a sheet tube by wrapping the resin sheet, on which the foam material is arranged, into a cylindrical shape, and forming a frame in which the resin sheet and the foam material are integrally molded, by bending the sheet cylinder to be set in a die, and then causing the foam material to undergo foaming. 
     Embodiments 
     Example of Racket Configuration 
     A configuration example of a tennis racket  1  according to the present invention will be described below with reference to  FIGS. 1 and 2 .  FIG. 1  is a front view of the tennis racket  1  according to the present embodiment.  FIG. 2  is a side view of the tennis racket  1  according to the present embodiment. Note that the upper side (with respect to the figure) of the tennis racket  1  shown in  FIGS. 1 and 2  is called the tip side, and the lower side is called the base side. 
     As shown in  FIG. 1 , the tennis racket  1  of the present embodiment includes a frame  10  that is configured by a face portion  12  (corresponding to the frame portion), a shaft portion  13 , and a grip portion  14 . 
     The face portion  12  is provided in the top portion of the tennis racket  1  and is formed in a substantially elliptical shape. Insertion holes (not shown) for inserting a string are provided, in an inner circumferential portion and an outer circumferential portion of the face portion  12  (and a later-described yoke portion  16 ), so as to communicate the inner and outer portions, and therefore a string that is inserted into the insertion hole at the outer circumferential portion can be taken out from the insertion hole at the inner circumferential portion. A plurality of these insertion holes (not shown) are provided in a line along the circumferential direction of the face portion  12 . When a string is strung between the insertion holes, the string segments cross each other in a grid form within the face portion  12 . A substantially elliptical hitting face (face for hitting a ball) is thus formed within the face portion  12 . Also, a sweet spot S shown in  FIG. 1  is a substantially middle area of the hitting face that includes the center C of the hitting face, and the size of this area is dependent on the shape of the hitting face (in other words, the shape of the face portion  12  seen from the front). 
     The shaft portion  13  is a portion that connects the face portion  12  and the grip portion  14 , and is formed so as to branch out into two from the grip portion  14  toward the face portion  12 . Also, the yoke portion  16  that configures a portion of the hitting face is provided at the boundary between the face portion  12  and the shaft portion  13 . 
     The grip portion  14  is the portion that is gripped by the player, and is positioned in the base end portion of the tennis racket  1 . Normally, grip tape  20  is wound around the grip portion  14  of the tennis racket  1 . The grip tape  20  absorbs the shock generated when hitting a ball, and also improves the grip. 
     Note that a fiber-reinforced resin, which is obtained by impregnating fiber with resin such as mainly carbon fiber, glass fiber, organic fiber, ceramic fiber, or the like, is used to form the frame  10  (the face portion  12 , the shaft portion  13 , and the grip portion  14 ) of the tennis racket  1 . In the present embodiment, the frame  10  uses a thermosetting fiber-reinforced resin whose reinforcing fiber is carbon fiber, and as will be described later, the frame  10  is formed by rolling up the fiber-reinforced resin in a sheet form (a carbon sheet) so as to form a hollow bar body, bending the bar body into the shape of the tennis racket  1 , and then fitting the bar body into a predetermined die and performing heat and pressure molding. 
     The frame  10  of the present embodiment is also provided with a foam material  30  in the areas indicated by hatching (referred to hereinafter as hatched portions) in the face portion  12  that surrounds the hitting face. As shown in  FIG. 1 , the portions where the foam material  30  is provided are areas on respective sides (two side portions) of the face portion  12  that are on opposite sides of a predetermined area of the hitting face on the tip side than the sweet spot S. More specifically, assuming that the tip (upper end) position of the face portion  12  is 0 degrees relative to the center C of the hitting face, the foam material  30  is provided inside the area of the face portion  12  that corresponds to the range between angle θ 1  (e.g., 20 degrees) and angle θ 2  (e.g., 60 degrees). Note that the foam material  30  is not provided in portions other than the hatched portions of the frame  10  (referred to hereinafter as non-hatched portions) 
     Also, as shown in  FIG. 2 , the width (length in the normal direction relative to the hitting face) W 2  of the frame  10  in the hatched portions is made greater than the width W 1  of the frame  10  in the non-hatched portions. Specifically, the width W 1  is 20 mm, and the width W 2  is 22 mm. Since the width of the frame  10  in the hatched portions is made greater than that in the non-hatched portions in this way, more of the foam material  30  can be provided in the frame  10 , and the strength can be increased even more. 
     The following describes the reason why the foam material  30  is provided to the frame  10  (the face portion  12 ) only partially in this way. As one example, top players (advanced players) tend to frequently use the area on the tip side of the hitting face than the sweet spot S. For this reason, there is a demand to increase particularly rigidity (strength) and vibration absorption in the areas of the frame  10  that correspond to this area. Since the foam material  30  is provided in these areas in the present embodiment, it is possible to improve rigidity and vibration absorption when hitting a ball with the tip side of the hitting face. 
     Also, if the foam material  30  were uniformly provided in the entire interior of the frame  10 , the overall strength would increase, thus leading to a risk of a decrease in flexure, an increase in repulsive force, and a decline in ball control. Furthermore, the overall weight of the frame  10  would increase, thus making a reduction in weight difficult. In contrast to this, with the present embodiment, the foam material  30  is provided on the two sides (the hatched portions) of the frame  10  (the face portion  12 ) that are on opposite sides of a predetermined area on the tip side than the sweet spot S, and the foam material  30  is not provided in the other areas (the non-hatched portions), thus allowing the face portion  12  to flex to an appropriate amount when hitting a ball. This enables to improve control when hitting a ball. This also makes a reduction in weight possible. 
     Racket Manufacturing Method  
     Present Embodiment 
     A method for manufacturing the tennis racket  1  of the present embodiment will be described below with reference to  FIGS. 3A to 3D .  FIGS. 3A to 3D  are schematic diagrams showing the method for manufacturing the tennis racket  1 . 
     The method for manufacturing the tennis racket  1  of the present embodiment has a foam material arranging step, a sheet tube forming step, a bending step, and a die molding step. 
     &lt;Foam Material Arranging Step&gt; 
     First, as shown in  FIG. 3A , a carbon sheet  44  (which corresponds to a resin sheet) of prepreg fiber-reinforced resin (FRP) mainly made of carbon fiber is prepared, and the foam material  30  is arranged in areas that correspond to the hatched portions in  FIG. 1 . The foam material  30  is obtained by mixing a resin-based adhesive and a thermally expandable foam material, and forming the mixture into a sheet shape. 
     Note that in  FIG. 3A , the midpoint of the carbon sheet  44  in the lengthwise direction (left-right direction in the figure) will become the tip of the frame  10  (the face portion  12 ) of the tennis racket  1 , and therefore positions on the frame  10  of the tennis racket  1  can be specified in advance based on the distance from the midpoint. In the present embodiment, the size of the sheets of the foam material  30  and the positions where they are arranged on the carbon sheet  44  are set such that the foam material  30  is provided to the hatched portions of the frame  10  in  FIG. 1 . 
     &lt;Sheet Tube Forming Step&gt; 
     Next, as shown in  FIG. 3A , a tube  42  is fitted around a mandrel  40 , and the carbon sheet  44  having the foam material  30  arranged thereon is wound around the tube  42 . Note that the mandrel  40  is a round bar-shaped member (cored bar) whose length corresponds to the entire length of the frame  10  (the face portion  12 , the shaft portion  13 , and the grip portion  14 ) of the tennis racket  1 . Also, the tube  42  is a flexible resin tube (e.g., a nylon tube). 
     The mandrel  40  is then removed, and thus a cylindrical tube body (referred to hereinafter as a sheet tube  100 ) is formed as shown in  FIG. 3B . 
       FIG. 4A  is a cross-sectional view taken along A-A in  FIG. 3B , and  FIG. 4B  is a cross-sectional view taken along B-B in  FIG. 3B . As shown in  FIG. 4A , in the A-A cross-section of the sheet tube  100 , the foam material  30  is arranged on the outside of the tube  42 . The carbon sheet  44  is then wound on the outside of the foam material  30 . In contrast, as shown in  FIG. 4B , in the B-B cross-section of the sheet tube  100 , the carbon sheet  44  is wound on the outside of the tube  42 , and the foam material  30  is not provided. 
     &lt;Bending Step&gt; 
     As shown in  FIG. 3C , the sheet tube  100  is bent so as to conform to the shape of the frame  10  (the face portion  12 , the shaft portion  13 , and the grip portion  14 ) of the tennis racket  1 . Also, the yoke portion  16 , which is formed using a configuration similar to that of the sheet tube  100 , is attached in the border portions between the face portion  12  and the shaft portion  13 , and a carbon sheet  18  is wound around the border portions. Note that the foam material  30  maybe provided inside the yoke portion  16 . 
     &lt;Die Molding Step&gt; 
     The sheet tube  100  bent in the shape of the frame  10  and the yoke portion  16  attached to the sheet tube  100  are set in a die  50  as shown in  FIG. 3D . Heating is then performed, and air is introduced into the tube  42  through the opening at the end (base) of the grip portion  14  portion of the sheet tube  100 . The sheet tube  100  undergoes plastic deformation due to being heated, and at this time, the carbon sheet  44  is pushed and spread outward due to expansion of the tube  42 , and the carbon sheet  44  is pressed against the die  50 . The plastically deformed sheet tube  100  is thus molded into a shape that conforms to the die  50 . Note that although the foam material  30  expands (foams) at this time due to being heated, this expansion is suppressed due to being subjected to air pressure from the tube  42 . The foam material  30  then expands when the air pressure inside the tube  42  is released. Accordingly, in the areas (hatched portions) where the foam materials  30  are formed, the tube  42  is pressed into the surrounding foam material  30 , and the interior of the frame  10  is filled with the foam material  30 . This results in the molding of the frame  10  in which the carbon sheet  44  and the foam material  30  are integrated. 
     Comparative Example 
     The following describes a method for manufacturing a tennis racket according to a comparative example. In this comparative example, a liquid-form foam material that has a high expansion ratio (e.g., urethane) is injected into a heat-molded hollow frame through an opening at the base of the grip portion (end portion of the hollow portion), and then is caused to undergo foaming. Due to such foam material (the foam material used in this comparative example being referred to hereinafter as the foam material  30 ′) undergoing foaming, the foam material  30 ′ is uniformly provided inside the entire the frame  10 . 
     In contrast, with the present embodiment, the sheets of the foam material  30  are arranged on the carbon sheet  44  before the sheet tube  100  is formed, thus enabling to provide the foam material  30  easily in arbitrary areas of the frame  10  by changing the size and arrangement position of the foam material  30 . 
     Foam Material Characteristics 
     The following describes a comparison of characteristics (e.g., hardness) of the foam material  30  according to the present embodiment and the foam material  30 ′ according to the comparative example. Note that as described above, the foam material  30  is formed by mixing a resin-based adhesive and a thermally expandable foam material, and the foam material  30 ′ is a highly expandable urethane foam material. 
     The following experiments were performed on cases with the foam materials alone and with them formed inside the frame  10 . The results of these experiments are described below. 
     &lt;Foam Material Hardness Test&gt; 
     Test Method 
       FIG. 5  is a schematic diagram showing aspects of a hardness test performed on the foam material alone. Test pieces (samples) having vertical and horizontal lengths of 20 mm and a height of 5 mm were created using the respective foam materials (alone) in the foamed state, and the amount of deformation when a force of 10 kgf (approximately 98 N) was applied in the direction of the arrow shown in  FIG. 5  was measured. 
     Test Results 
     The amount of deformation was 2.44 mm with the foam material  30 ′, whereas the amount of deformation was 1.78 mm with the foam material  30 . Accordingly, in the case of the foam material alone, it was confirmed that hardness improved approximately 27% with the foam material  30  of the present embodiment compared to the foam material  30 ′ of the comparative example. 
     &lt;Test on Amount of Racket Cross-Section Deformation&gt; 
     Test Method 
       FIG. 6  is a schematic diagram showing aspects of the test on the amount of racket cross-section deformation. 
     The respective foam materials were provided inside the frame  10  (the face portion  12 ), and the amount of deformation when applying a force of 50 kgf (approximately 490 N) toward the interior of the face portion  12  was measured. 
     Test Results 
     The amount of deformation was 0.51 mm with the comparative example (the foam material  30 ′), whereas the amount of deformation was 0.41 mm with the present embodiment (the foam material  30 ). Accordingly, it was confirmed that an improvement of approximately 12% with respect to the amount of racket cross-section deformation was achieved with the foam material  30  of the present embodiment compared to the foam material  30 ′ of the comparative example. 
     &lt;Racket Bending Deformation Test&gt; 
     Test Method 
       FIG. 7  is a schematic diagram showing aspects of a racket bending test. 
     The areas where the respective foam materials were provided in the frames  10  (the face portions  12 ) were cut to a length of 100 mm, and the amount of bending was measured when supporting the two ends and applying a force of 50 kgf (approximately 490 N) to the center. 
     Test Results 
     The amount of bending was 0.73 mm with the comparative example (the foam material  30 ′), whereas the amount of bending was 0.69 mm with the present embodiment (the foam material  30 ). Accordingly, it was confirmed that an improvement of approximately 5% with respect to the amount of racket bending deformation was achieved with the foam material  30  of the present embodiment compared to the foam material  30 ′ of the comparative example. 
     As described above, the tennis racket  1  of the present embodiment includes the frame  10  that surrounds the hitting face for hitting a ball, and the foam material  30  is provided inside (in hatched portions) the frame  10  on the two sides of the frame  10  that are on opposite sides of an area of the hitting face on the tip side than the sweet spot S. In this way, the strength of the areas on the respective sides of the frame  10  is increased. This enables to improve the rigidity and vibration absorption when hitting a ball with the tip side of the hitting face. Also, since the foam material  30  is not provided in the other areas (the non-hatched portions), the face portion  12  can be allowed to flex by an appropriate amount when hitting a ball, and ball control can be improved. It is also possible to achieve a reduction in weight compared to the case of providing the foam material  30  in the entire frame  10 . 
     Variation 
     The following describes a modified example of the present embodiment. In the above-described embodiment, the foam material was not provided in the non-hatched areas (non-hatched portions) (the interior of the frame  10  was hollow in the non-hatched portions) of the frame  10  of the tennis racket shown in  FIG. 1 . However, in this modified example, a foam material (foam material  32 ) different from the foam material  30  is provided in the non-hatched portions. 
     The foam material  32  is a foam material (which corresponds to a foam body for filling) formed into a sheet shape using a material whose hardness is different from that of the foam material  30 . Note that the hardness of the foam material  32  is lower than the hardness of the foam material  30 . 
       FIG. 8  is an illustrative diagram of a method for manufacturing the variation of the present embodiment. 
     As shown in  FIG. 8 , the foam material  30  is arranged on the carbon sheet  44  at positions corresponding to the hatched portions, and the foam material  32  is arranged on the carbon sheet  44  at positions corresponding to the non-hatched portions in  FIG. 1 . The following manufacturing method is similar to that of the above-described embodiment. In this way, whereas the foam material was not provided in the non-hatched portions of the frame  10  of the tennis racket  1  shown in  FIG. 1  (the interior of the frame  10  was hollow in the non-hatched portions) in the above-described embodiment, a foam material (the foam material  32 ) different from the foam material  30  is provided in the non-hatched portions in this modified example. Since the foam material  30  and the foam material  32  have different hardnesses, the strength can be changed according to the position in the frame  10 . 
     With this modified example, the rigidity of the hatched portions can be increased, while the non-hatched portions are prevented from being hollow. This enables to prevent chips, foreign particles, and the like from entering the frame  10  when the holes for the insertion of the string are formed in the face portion  12 , for example. 
     Note that although the foam material  32  is provided to all of the non-hatched portions in this modified example, the modified example is not limited to such. For example, the foam material  32  may be provided on only the tip side of the tennis racket  1  (the range between the two areas where the foam material  30  is provided). 
     Also, the amount of foam material  30  used (the density thereof) in the hatched portions of the tennis racket  1  in  FIG. 1  may be different from the amount of foam material  30  used (the density thereof) in the non-hatched portions. For example, a configuration is possible in which a thinly-formed sheet of the foam material  30  is arranged in the lengthwise direction of the carbon sheet  44 , and then the foam material  30  is further arranged thereon in areas corresponding to the hatched portions (the positions where the foam material  30  is arranged in  FIG. 3A ) . The strength can be changed according to the position in the frame  10  in this case as well. 
     Other Embodiments 
     The above-described embodiment is for facilitating understanding of the present invention, and is not intended to limit the interpretation of the present invention. As a matter of course, the present invention can be changed and modified without departing from spirit the invention, and equivalents thereof are encompassed in the present invention. 
     &lt;Foam Material  30 &gt; 
     In the above-described embodiment, the foam material  30  is obtained by mixing a resin-based adhesive and a thermally expandable foam material, and forming the mixture into a sheet shape, but the present invention is not limited to this. The foam material  30  may be configured from other compositions as long as it can be formed into a sheet shape and expands (foams) due to heat or the like. 
     &lt;Position where Foam Material  30  is Formed&gt; 
     In the above-described embodiment, the foam material  30  was provided to areas on the two sides of the face portion  12  that are on opposite sides of an area on the tip side than the sweet spot S (the areas of the face portion  12  that correspond to the range from angle θ 1  to angle θ 2  in  FIG. 1 ), but the present invention is not limited to such. For example, in a case such as when the sweet spot S is frequently used, the foam material  30  may be provided to the two sides of the face portion  12  that are on opposite sides of the sweet spot S. Also, the area of the hitting face between the sweet spot S and the base side (side closer to the grip portion  14 ) is often used in the case of volleys performed mainly by middle-aged players, for example. In this case, the foam material  30  may be provided in areas on the respective sides of the face portion  12  that are on opposite sides of that area on the base side. Note that in the present embodiment, the areas where the foam material  30  is formed in the frame  10  (the face portion  12 ) are determined by the positions where the foam material  30  is arranged on the carbon sheet  44 , thus making it is possible to easily change the positions where the foam material  30  is formed.