Abstract:
The ball sport or ball game racquet, in particular tennis racquet, having a racquet frame for holding stringing and a racquet shaft extending from the racquet frame and having one racquet grip, wherein the racquet frame and the racquet shaft both are formed from a hollow frame profile with a plurality of walls. In at least one of the walls of the racquet frame and/or of the racquet shaft, at least one spacing layer is provided at least in some sections for increasing the stability of the ball sport racquet.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a ball sport or ball game racquet, in particular a tennis racquet, with a racquet frame for holding the stringing and with a shaft extending from the racquet frame and with a racquet grip. The racquet frame and the racquet shaft both feature a hollow frame profile made up of a plurality of walls or wall sections. The invention also relates to a method for manufacturing a hollow frame profile for such a ball sport racquet.  
         [0002]     Modern ball sport or ball game racquets, in particular tennis racquets, consist among other things of a racquet frame and a racquet shaft extending from and connected with this racquet frame. The racquet frame and the racquet shaft are manufactured from an outwardly closed hollow frame profile, which normally is made from a plurality of prepreg layers that are pressed together to form closed hollow frame profile.  
         [0003]     Prepreg layers are layers of synthetic or plastic material reinforced by fibers, such as carbon fibers, glass fibers and also other fibers, which are pre-impregnated with resin. Prepreg layers can be designed as woven prepreg layers, which are pre-impregnated in form of unidirectional and multi-axial layers. Depending on the requirements for the component to be manufactured, various resins are used, especially epoxide or epoxy resins and phenolic resins, both of which hardly flow at room temperature. In order to manufacture a hollow frame profile from such prepreg layers, the prepreg layers are placed in cold condition in a pre-defined mold and hardened under pressure and application of heat. Especially the application of heat causes the resin to liquefy temporarily so that it impregnates the fibers before it begins to harden.  
         [0004]     Various methods, in particular the blow molding methods, are known for manufacturing a racquet frame and racquet shaft featuring a hollow frame profile with a pre-defined wall thickness and fiber orientation and a defined outer contour and surface. In the blow molding method, a plurality of prepreg layers or prepreg layer windings are placed carefully and load-oriented on or around a core, which for example is covered with a tube, and pressed under high pressure into a female mold and hardened to achieve a defined surface and outer contour. In heavily stressed zones, it is possible to include additional prepreg layers at particular locations, in order to achieve different wall thicknesses while maintaining the same outer diameter, if required.  
         [0005]     Furthermore, the wall thicknesses and the outer diameters of the hollow frame profile have a significant influence on the playability of a tennis racquet, so that especially tennis racquets for amateur and hobby use have a very high and wide hollow frame profile with a wall thickness of less than 1 mm in order to achieve a very high degree of stiffness in combination with low weight (less than 200 g). The hollow frame profile of the racquet frame and of the racquet shaft comprises for example four opposing walls in pairs, which can have one left and right opposing wall, each with a height of up to 30 mm or more and one upper and lower opposing wall, each with a width of up to 15 mm or more, to form a honeycomb cross section.  
         [0006]     Individually, neither the upper and lower walls nor the left and right walls of the hollow frame profile are stiff; rather, they receive the necessary stiffness by suitably connecting the corresponding walls.  
         [0007]     It is an object of the present invention is to provide for a ball sport racquet of the type mentioned at the outset featuring a sufficiently high degree of stiffness despite a high and wide hollow frame profile.  
       SUMMARY OF THE INVENTION  
       [0008]     This object is achieved by a ball sport or ball game racquet, in particular tennis racquet, with a racquet frame for holding the stringing and with a shaft extending from the racquet frame and with a racquet grip, wherein the racquet frame and the racquet shaft both feature a hollow frame profile made up of a plurality of walls or wall sections, and wherein at least one of the walls of the racquet frame and/or of the racquet shaft features at least in some sections at least one spacing layer for increasing the stability of the ball sport racquet.  
         [0009]     The increase of stability is not or only to a small extend obtained by the stability or strength of the material of the spacing layer itself, but mainly by the fact that prepreg layers or the windings of the wall or wall section are spaced from another by the spacing layer and that by that the moment of inertia of the wall and the stiffness the wall are increased without substantially increasing the weight of the ball sport racquet.  
         [0010]     The use of a spacing layer in at least one of the walls of the hollow frame profile of a ball sport racquet according to the invention, on at least one or more wall sections, significantly increases the stiffness of the overall racquet frame. Preferably the spacing layer is provided in-between the wall layers forming the wall or wall section, that means in-between two windings in the shaft area and in the upper and lower wall sections of the hollow frame profile, if the hollow frame profile is made by winding the prepreg material around a core.  
         [0011]     The spacing layer is manufactured from a porous or honeycomb material, which is for example known from airplane construction. Alternatively, various foam materials can be used for manufacturing the spacing layer, due to their low volume weight. The density of the material used should be considerably below the density of the prepreg layers, which have a density of approx. 1.9 g/cm 3 . The materials used preferably have a density of less than 1.0 g/cm 3  while also featuring a high degree of compressive strength. This is of particular importance, since when applying the blow molding method for manufacturing the hollow frame profile of the tennis racquet, pressures of more than 8 bar can occur, so that the material used as the spacing layer must be able to withstand such high pressures during the production process. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Exemplary embodiments of the invention are described below with reference to the drawings, in which:  
         [0013]      FIG. 1  shows a simplified representation in top view of a ball sport racquet in the form of a tennis racquet;  
         [0014]      FIG. 2  shows a simplified representation in cross section corresponding to the line  1 - 1  of  FIG. 1 ;  
         [0015]      FIG. 3  shows a simplified representation in cross section corresponding to the line  11 - 11  of  FIG. 1 ;  
         [0016]      FIG. 4  shows a simplified representation of shaft area of the ball sport racquet in the form of a tennis racquet; and  
         [0017]      FIG. 5  shows a simplified representation of single selected production steps for a hollow frame profile manufactured according to the blow molding process. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     The tennis racquet depicted in the drawings and generally designated  1  consists essentially of a racquet frame  2  forming the racquet head, of the stringing  3  located there, which comprises a plurality of crisscrossing string lengths  4 ′,  4 ″ or strings  4 , and of a racquet shaft  6  connecting to the racquet frame  2  by means of a throat or heart  5  and on which (shaft) a racquet grip  7  is provided. The throat  5  is connected with the racquet frame  2  in the connecting areas  5 ′,  5 ″.  
         [0019]     The racquet frame  2  features an outwardly closed hollow frame profile  8 , which is manufactured for example from a plurality of prepreg layers according to the blow molding process.  
         [0020]     To exemplify the structure of the hollow frame profile  8  of the racquet frame  2  and of the racquet shaft  6 ,  FIG. 2  shows a cross section corresponding to the line  1 - 1  of  FIG. 1 . The hollow frame profile  8  features by way of example an oval cross section flattened on two opposing sides each being at a distance form the stinging and parallel or nearly parallel with the stringing. The hollow frame profile  8  features a hollow space  9 , which is surrounded by the walls of a plurality of prepreg layers or windings  10 .  
         [0021]     The hollow frame profile  8  in the depicted embodiment consists of one upper and lower, opposing flat or nearly flat wall sections  8 . 1 ,  8 . 2  and one left and right, likewise opposing wall sections  8 . 3 ,  8 . 4 . In alternative embodiments, additional walls or wall sections can be provided for forming the hollow frame profile  8 . The walls  8 . 1  and  8 . 2  are located on different sides of the strings  3  or stringing  4 , each with the greater distance from these strings. The hollow frame profile  8  has for example a wall thickness W of approximately 1.0 mm. The upper and lower wall sections  8 . 1 ,  8 . 2  are parallel to each other in the depicted embodiment and are separated from each other by the hollow space  9 . The left and right wall sections  8 . 3 ,  8 . 4  extend in the shape of a circular arc in order to form the honeycomb cross section, wherein the inner circular arc sections of the left and right wall sections  8 . 3 ,  8 . 4  face the hollow space  9 . The wall sections  8 . 1 - 8 . 4  merge with each other.  
         [0022]      FIG. 3  shows a cross section corresponding to the line  11 - 11  of  FIG. 1  through one of the two hollow frame profiles  8  of the throat  5 . The upper and lower wall sections  8 . 1 ,  8 . 2  of the hollow frame profile  8  feature an upper and lower spacing layer  11 ,  12 , which are embedded between one upper and lower prepreg winding layer  10 . 1 ,  10 . 2  in the upper and lower wall sections  8 . 1  and  8 . 2 .  
         [0023]     The spacing layer  11 ,  12  is manufactured from a honeycomb material  11 . 1 ,  12 . 1 , which is for example known from airplane construction. Alternatively, a foam material with a low volume weight can be used. In a preferred embodiment, the density of the material  11 . 1 ,  12 . 1  is less than 1.0 g/cm 3  and therefore considerably less than the density of the prepreg layers  10  (approx. 1.9 g/cm 3 ). In addition, the material  11 . 1 ,  12 . 1  of the spacing layers  11 ,  12  has a compressive strength greater than 8.0×10 5  N/m 2  in order to be able to withstand the pressures of more than 8 bar occurring during the production process when the blow molding process is used to manufacture the hollow frame profile  8 .  
         [0024]      FIG. 4  shows the shaft area of the tennis racquet  1 , in particular the throat  5  with one first and second hollow frame profile  8   a ,  8   b , both of which are connected via the racquet shaft  6 . In addition to the second hollow frame profile  8   b , the figure depicts a spacing layer  11  extending over the depicted hollow frame profile  8   a ,  8   b  and possessing the same curvature and which is inserted during the manufacturing process for the tennis racquet  1  for example into the upper wall section  8 . 1  of the second hollow frame profile  8   b  to increase the stiffness of the tennis racquet  1 , particularly in the shaft area. The insertion point of the spacing layer  11  in the throat  5  of the shaft area of the tennis racquet  1  is indicated by two arrows, which point to the upper wall section  8 . 1  of the second hollow frame profile  8   b.    
         [0025]     The racquet shaft  6  also features a hollow frame profile  13 , which consists of upper and lower walls or wall sections  13 . 1 ,  13 . 2  and left and right walls or wall sections  13 . 3 ,  13 . 4 . Furthermore, the hollow frame profile  13  of the racquet shaft  6  also features one first and second connecting section  14 ,  15 , by means of which the first and second hollow frame profile  8   a ,  8   b  of the throat  5  are connected with the racquet shaft  6 . The first and second connection sections  14 ,  15  are provided on the end of the racquet shaft  6  facing the racquet head and are connected with each other by means of a preferably semi-circular connecting element  16 , which closes the hollow space  17  of the hollow frame profile  13  of the racquet shaft  6  toward the outside.  
         [0026]     In a preferred embodiment, the spacing layer  11  is inserted into the upper wall section  6 . 1  of the hollow frame profile  8   a  of the throat  5  so that it projects at least partially over the second connecting section  15  into the racquet shaft  6  and therefore is integrated into the upper wall section  13 . 1  of the hollow frame profile  13  of the racquet shaft  6 . Similar to  FIG. 3 , a plurality of first and second spacing layers  11 ,  12  can be provide both in the upper and in the lower wall sections  6 . 1 ,  6 . 2  of the hollow frame profile  8   a ,  8   b  or  13 . 1 ,  13 . 2 , said spacing layers being brought together or located parallel to each other in the shaft area and in particular below the connecting element  16 .  
         [0027]     In alternative embodiments, the spacing layers  11 ,  12  can also be provided in different sections of the hollow frame profile  8  of the racquet frame  6 ; in particular, the spacing layers  11 ,  12  can extend either individually or in pairs over the connecting areas  5 ′- 5 ″ between the throat  5  and the racquet frame  2 . Also, the alternating integration of the spacing layers  11 ,  12  in sections of the upper and/or lower wall sections  8 . 1 ,  8 . 2  or  13 . 1 ,  13 . 2  of the tennis racquet  1  is possible.  
         [0028]      FIG. 5  shows by way of example a simplified depiction of the hollow frame profile  8  in various stages of production during the manufacture of the hollow frame profile  8  according to the blow molding process. In a first production step (a) the prepreg layers  10  are oriented carefully according to the load and then placed by winding on a core (not depicted in  FIG. 5 ) covered with a hose. This forms at least one first prepreg layer  10 . 1  for achieving the upper and lower as well as left and right wall sections  8 . 1  through  8 . 4 .  
         [0029]     In a second production step (b), one first and second spacing layer  11 ,  12  made of the material  11 . 1 ,  12 . 1  described above are placed on the at least first prepreg layer  10 . 1  in the upper and lower wall area and surrounded with at least one second prepreg winding or layer  10 . 2 . The spacing layers  11 ,  12  of the hollow frame profile  8  that are already surrounded by a plurality of further prepreg layers  10  are shown in a further production step (c) in  FIG. 5 .  
         [0030]     Afterwards, the prepreg layers  10  including the spacing layers  11 ,  12  are pressed to the hollow frame profile  8  according to  FIG. 3  under application of heat. The application of heat causes the resin provided in the prepreg layers  10  to liquefy, bonding the synthetic layers and the spacing layers  11 ,  12  embedded therein, which then are compressed to a stiff hollow frame profile  8 ,  8   a ,  8   b  in the pressing process. The hollow frame profile  8 ,  8   a ,  8   b  created by the heating and pressing process is shown in  FIG. 5  in a fourth production step (d).  
         [0031]     The invention was described above based on exemplary embodiments. It goes without saying that numerous modifications and alterations are possible without abandoning the underlying inventive idea upon which the invention is based.  
       REFERENCE NUMBERS  
       [0000]    
       
           1  tennis racquet  
           2  tension frame  
           3  stringing  
           4  strings  
           4 ′ string length  
           5  throat  
           5 ′,  5 ″ connecting area  
           6  shaft  
           7  grip  
           8  hollow frame profile  
           8 . 1  upper wall or wall section  
           8 . 2  lower wall or wall section  
           8 . 3  left wall or wall section  
           8 . 4  right wall or wall section  
           9  hollow space of hollow frame profile  
           10  prepreg layers  
           10 . 1  first prepreg layers or windings  
           10 . 2  second prepreg layers or windings  
           11  first spacing layer  
           11 . 1  material of first spacing layer  
           12  second spacing layer  
           12 . 1  material of second spacing layer  
           13  hollow frame profile of shaft  
           13 . 1  upper wall or wall section  
           13 . 2  lower wall or wall section  
           13 . 3  left wall or wall section  
           13 . 4  right wall or wall section  
           14  first connecting section  
           15  second connecting section  
           16  connecting piece  
           17  hollow space  
          W wall thickness  
          L length of hollow frame profile of the throat