Abstract:
A hollow tennis ball which comprises a hollow core of elastic material which encloses an interior space, and a layer of substantially felt-like material which is applied to the outer surface of the hollow core. The overall weight of the hollow tennis ball is between 30 and 50 grams. The pressure in the interior space is from 0 to 0.4 kg/cm 2  higher than atmospheric pressure. The thickness of the hollow core is between 2.2 and 3.0 mm. The forward deformation of the hollow tennis ball is from 10.0 to less than 11.5 mm under loading force of 3.5 and 18 pounds. The return deformation is from 13.5 to less than 14.0 mm after it has deformed to 2.54 cm under a loading force of 18 pounds.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of application Ser. No. 190,560 filed on May 5, 1988, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to tennis balls, and more particularly to tennis balls having a small weight and rebound so that even children of lower age having little experience, adult beginners and adults who lack physical strength and athletic ability, can play tennis without difficulty corresponding to physical strength and skill thereof. 
     Tennis balls have a standard of ITF (International Tennis Federation), and only those complying with the standard are used in regular games. Usually, general tennis players also use such tennis balls as complying with the above-mentioned standard. Though some tennis balls out of the standard are on the market as practice balls, these balls are not largely out of the standard. That is, there are not much different from the balls complying with the standard and those not complying with the standard in weight, rebound, and the like. The present ITF standards are shown hereinafter for references sake. 
     
         ______________________________________Weight               56.7 to 58.5 gOuter diameter       6.35 to 6.67 cmForward deformation  0.56 to 0.74 cmReturn deformation   0.89 to 1.08 cmRebound              135 to 147 cm______________________________________ 
    
     Children of tender age, from 4 to 12 years old, particularly those who play tennis for the first time or have little experience adult beginners and adults who lack physical strength and athletic ability find great difficulty in playing tennis with the above-mentioned balls, since they are too heavy, they bounce too much, they are too hard, they fly too much, particularly their speed becomes too high after bounce for such players. Namely, the tennis balls complying with the standard of ITF made for adults are not suitable for the children of tender age, since they are immature in physical strength, athletic ability and physical constitution. As a result, children are slow in progress for tennis as compared with other sports, and it brings to them a concept that tennis is a dull or uninteresting sport. Then their interesting move from tennis to other sports such as baseball and soccer. Moreover, there is a problem that the children are injured when they continuously play tennis with the above-mentioned balls which overstress their wrists, shoulders and waists and the like. These problems apply the above-mentioned adult beginners and the like. 
     In order to solve the above-mentioned problems, there is proposed a big ball having about 3 to 11% larger diameter than that of the ITF standard ball which enables players to play leisurely (Japanese Unexamined Patent Publication No. 48385/1986). Players can play tennis leisurely by this type of tennis balls indeed since ball speed becomes slow not only in flight but also after bounce. However, the balls have such disadvantages as they are heavy since the weights are from 54 to 58 g, as the rebound is high since it is about 146 cm, and as the internal pressure of the hollow core is high since it is 0.8 to 1.0 kg/cm 2  higher than the atmospheric pressure. As a result, it is very difficult for children of tender age to play tennis with the big balls as well as ITF standard balls, since children feel they are heavy and they bounce high. Namely, there are no substantial differences between the big balls and ITF standard balls for children of tender age and for adult beginners. The above-mentioned problems are, therefore, not solved by the big balls. 
     It is an object of the present invention to provide a tennis ball which corresponds to physical constitution, physical strength and athletic ability of players whereby they can play tennis easily, their progress in tennis is accelerated and their amusement in tennis is increased. In the tennis ball of the present invention, the physical properties of the ball such as weight and internal pressure thereof are selected mainly suitable for children of tender age, from about 4 to 12 years old. 
     SUMMARY OF THE INVENTION 
     A tennis ball of the present invention comprises a hollow core made of a rubber composition, a felt covering the hollow core, wherein a tennis ball of the present invention is characterized in that the weight of the ball is from 30 to 50 g, the internal pressure of the hollow core is 0 to 0.4 kg/cm 2  higher than the atmospheric pressure, the thickness of the hollow core is from 2.2 to 3.0 mm, the forward deformation of said ball is from 10.0 to less than 11.5 mm under loading force of 3.5 and 18 pounds and the return deformation is from 13.5 to less than 14.0 mm after it has been deformed to 2.54 cm under a loading force of 18 pounds; the hardness of the rubber of the hollow core is from 55 to 70 based on JIS A standard (Japanese Industrial Standard) which is similar to a Shore A hardness scale, and rebound is from 90 to 120  cm, when dropped, from a height of 254 cm, onto a concrete base. 
     As described above, it is an object of the present invention is to provide a tennis ball which enables those of tender age, beginners, those of weak physical strength and those of inferior athletic ability, to make progress in tennis with ease by lessening impact force and by reducing ball speed, particularly ball speed after rebound. 
     In order to realize the above-mentioned object, in the present invention, the weight, the internal pressure, the thickness and the hardness of the rubber of the hollow core are limited in a specific range respectively, so that the impact force and the ball speed are lowered. 
    
    
     BRIEF EXPLANATION OF THE DRAWING 
     FIG. 1 shows a schematic sectional view of an embodiment of a tennis ball of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawing for more complete understanding of the present invention, there is shown a tennis ball. 
     FIG. 1 shows a schematic sectional view of a tennis ball A of an embodiment of the present invention. In the drawing, numeral 2 indicates a hollow core, numeral 3 indicates a rubber layer of the hollow core, and numeral 1 indicates a felt covering the hollow core. 
     In making a tennis ball A of the present invention, there is no limitation for rubber composition composing the hollow core 2 provided that it satisfies the weight and the rebound mentioned hereinafter. Namely, material selected from the group consisting of nitrile rubber, synthetic and natural isoprene rubber, butadiene rubber, EPM, EPDM, butyl rubber and styrene-butadiene rubber can be used for rubber material. A mixture of two or more selected among them can also be used as rubber material. 
     A tennis ball A of the present invention is characterized in that the weight, the internal pressure and the thickness and the hardness of the rubber of the hollow core and rebound are limited in a specific range corresponding to player&#39;s physical strength, athletic ability and the like. 
     Hereinafter is a detailed explanation of a ball of the present invention. 
     A tennis ball of the present invention is designed for children of tender age, from about 4 to 12 years old. In designing the tennis ball of the present invention, players having the weakest physical strength and poorest athletic ability among tennis players are taken into consideration. Accordingly, the weight of the ball is lessened, the thickness of the hollow core is thinned, the hardness of the rubber is lowered, the internal pressure of the hollow core is zero or a little higher than the atmospheric pressure, so that the rebound, ball speed and impact force at the time of striking are lowered for them to handle the ball very easily. More particularly, in the tennis of the present invention, the weight is from 30 to 50 g, the internal pressure of the hollow core is 0 to 0.4 kg/cm 2  higher than the atmospheric pressure, the thickness of the hollow core is from 2.2 to 3.0 mm, the forward deformation of said ball is from 10.0 to less than 11.5 mm under loading force of 3.5 and 18 pounds and the return deformation is from 13.5 to less than 14.0 mm after it has been deformed to 2.54 cm under a loading force of 18 pounds; the hardness of the rubber of the hollow core is from 55 to 70 based on JIS A standard (Japanese Industrial Standard), and rebound is from 90 to 120 cm when dropped, from a height of 254 cm, onto a concrete base. Children from 4 to 12 years old, who have weak physical strength, make rapid progress in handling balls when they start playing tennis with the ball of the present invention. 
     When the weight of a ball exceeds 50 g , players feel it heavy on striking the same. On the other hand, when the weight of a ball is less than 30 g, the flight line of the ball becomes unstable and the ball loses its controllability. When the internal pressure of the hollow core exceeds 0.4 kg/cm 2 , there are such disadvantages as the rebound becoming higher, the ball flying too much, and the ball speed after rebound being felt fast. When the thickness of the hollow core, i.e. the thickness of the rubber layer t is less than 2.2 mm, there are such disadvantages as the rebound being low, players feel the ball as very soft at the time of striking it, and the flight property being poor. On the other hand, when the thickness exceeds 3.0 mm, there are such disadvantages as players feel the ball as very hard and heavy, and flying too much. The hardness of the rubber is from 55 to 70 based on JIS A standard (Japanese Industrial Standard), i.e. a value measured by JIS A (Japanese Industrial Standard) type hardness meter is from 55 to 70. When the hardness is less than 55, there are such disadvantages as a ball becoming very soft, it does not sufficiently fly, and the rebound being low. On the other hand, when the hardness exceeds 70, there are such disadvantages as a ball becoming too hard, the ball speed becoming high and flying too much. In addition, when rebound is less than 90 cm, the rebound is too low, and when rebound exceeds 120 cm, the rebound is too high. In both cases, rebound is not preferable for playing since players feel some difficulty at the time of striking it. 
     It is preferable that the forward deformation is from 10.0 to 11.5 mm. When the forward deformation is less than 10.0 mm, there are such disadvantages as a ball becoming too hard, player being able to return the ball under his control, it flying too much and the ball speed after rebound becoming high. On the other hand, when the forward deformation exceeds 11.5 mm, there are such disadvantages as a ball becoming very soft, the rebound height becoming low and not being able to sufficiently fly. 
     Further, it is preferable that the return deformation is from 13.5 to 14.0 mm. When the return deformation is less than 13.5 mm, there are such disadvantages as players feeling a ball as hard, and flying too much. On the other hand, when the return deformation exceeds 14.0 mm, there are such disadvantage as players feeling a ball as soft and not being able to sufficiently fly. 
     In the above explanation, it is mentioned that the tennis ball A of the present invention is designed for players who have inferior physical strength and athletic ability. The application of the tennis ball of the present invention, however, is not limited to the players as mentioned above. Players who have long experience in tennis playing and play well, enjoy playing tennis leisurely with the ball of the present invention since the impact force is weak and the ball speed is low. 
     Fundamentally, the tennis ball of the present invention is designed to display its characteristics when it is used in a tennis court of standard size with a net of standard height. According to circumstances, however, it can be used in a small sized tennis court or with a net of lower height. 
     The tennis ball of the present invention can be made by the same manufacturing process as that of the standard tennis ball. 
     Next there is explained a tennis ball of the present invention based on Examples and Comparative Examples. The scope of the present invention is not, however, limited to the examples. 
     EXAMPLES 1 to 3 
     Composition were prepared according to Table 1 (Table 1 is shown by PHR (Per Hundred Rubber)), thereafter the hollow cores were covered with felts to obtain tennis balls. 
     
                                           TABLE 1__________________________________________________________________________  Natural       High styrene              Zinc                 Stearic                     Wood                         Hydrou  rubber       SBR *1 oxide                 acid                     flour                         silica *2                              Sulfur                                  DM DPG__________________________________________________________________________Example 1  100  --     7.0                 1.0 --  16   4.2 2  1Example 2  100  --     8.0                 1.0 --  16   4.0 2  1Example 3  100  --     3.0                 1.0 --  16   4.0 2  1ComparativeExample -1  100  --     11.0                 1.0 --  16   3.0 2  12      100  --     14.0                 1.0 --  16   3.4 2  13      100  --     8.0                 1.0 --  16   3.4 2  1__________________________________________________________________________ *1 Nippol 2007J (made by Nippon Zeon Co., Ltd.) *2 Hydrou silica made by Nippon Silica Kogyo Kabushiki Kaisha 
    
     Rubber composition were prepared by kneading a rubber and ingredients other than sulfur and an accelerator in a Banbury mixer, and then kneading the mixture with sulfur and the accelerator on rolls. 
     The compositions were formed into sheets, and extruded by an extruder in the form a rod. After cutting the rods into pieces suited for a mold for preparing half-shells, the pieces were placed in the molds and compression-molded at 160° C. for 2 minutes to produce half-shells. A pair of the half-shells were placed in a mold for preparing ball cores and compression-molded at 150° C. for 12 minutes to produce tennis ball cores. 
     The results of measurement of physical properties of the obtained tennis balls are shown in Table 2. 
     The physical properties shown in Table 2 were measured as follows: 
     Forward deformation 
     The measurement was carried out with a Stevens machine. 
     A tennis ball was subsequently compressed about 2.54 cm in three direction at right angles to each other. This procedure was repeated 3 times. That is to say, the ball was compressed 9 times total. After the above preliminary compression, the deformation was measured in 2 hours according to the following manner. The ball was compressed with a load of 3.5 pounds (1.575 kg) and the deformation was measured, and the ball was then compressed with a load of 18 pounds (8.165 kg) and the deformation was measured. The forward deformation is expressed by the difference between the deformation by a load of 3.5 pounds and the deformation by a load of 18 pounds. 
     Return deformation 
     After measuring the deformation in the above forward deformation test, the ball was compressed so that the deformation was 2.54 cm. Then the compression was reduced to a load of 18 pounds, and the deformation was measured. 
     Bounce 
     A tennis ball was dropped from a height of 254 cm onto a concrete base, and the bounce of the ball was measured. 
     COMPARATIVE EXAMPLES 1 to 3 
     The physical properties of Comparative Examples 1 to 3 are shown in Table 2, wherein Comparative Example 1 to 3 were prepared in accordance with Prinz et al (U.S. Pat. No. 4,739,989). 
     Composition of Comparative Examples 1 to 3 are shown in Table 1. 
     
                                           TABLE 2__________________________________________________________________________           Internal      Outer           pressure                Thickness                      Hardness Forward                                      Return  Weight      diameter           of core                of core                      of rubber                           Bounce                               deformation                                      deformation  (g) (mm) (kg/cm.sup.2)                (mm)  (JIS A)                           (cm)                               (mm)   (mm)__________________________________________________________________________Example1      40  65.0 0.2  2.5   68   105 10.8   14.02      45  65.3 0.3  2.8   68   115 10.1   13.53      35  65.4 0.4  2.7   67   110 10.3   13.7ComparativeExample1      40  64.8 0.0  2.0   58    90 20     272      45  65.0 0.2  2.0   62   100 18     253      35  65.1 0.4  1.8   61   105 15     20__________________________________________________________________________ 
    
     Experiment 
     Children of tender age played tennis with balls of Examples 1 to 3 and Comparative Examples 1 to 3 in order to estimate strike feeling, flight, and bounce height. 
     Results 
     The estimation of applicability for children of tender age about strike feeling, flight, and bounce height for Examples 1 to 3 and Comparative Examples 1 to 3 is shown in Tables 3 to 5. In Tables 3 to 5, indication of marks are as follows: 
     O: excellent 
     Δ: passable 
     X: bad 
     
                                           TABLE 3__________________________________________________________________________Strike feeling                   Comparative                          Comparative                                 ComparativeSubject No. Example 1       Example 2             Example 3                   Example 1                          Example 2                                 Example 3__________________________________________________________________________ 1    O     O     O     X      X      Δ 2    Δ       O     O     X      X      Δ 3    O     O     O     X      X      Δ 4    O     O     O     X      Δ                                 Δ 5    O     O     O     X      X      Δ 6    Δ       O     O     X      Δ                                 Δ 7    O     O     O     X      X      Δ 8    O     O     O     X      X      Δ 9    Δ       O     O     X      X      Δ10    Δ       O     O     X      X      Δ11    O     O     O     X      Δ                                 Δ12    O     O     O     X      X      Δ13    O     O     O     X      X      Δ14    O     O     O     X      X      Δ15    Δ       O     O     X      X      Δ16    O     O     O     X      X      Δ17    O     O     O     X      X      Δ18    Δ       O     O     X      X      Δ19    O     O     O     X      X      Δ20    O     O     O     X      X      Δ21    O     O     O     X      X      Δ22    O     O     O     X      X      Δ23    Δ       O     O     X      Δ                                 Δ24    O     O     O     X      X      Δ25    O     O     O     X      X      Δ26    O     O     O     X      X      Δ27    Δ       O     O     X      Δ                                 Δ28    O     O     O     X      X      Δ29    O     O     O     X      Δ                                 Δ30    O     O     O     X      X      Δ__________________________________________________________________________ 
    
     
                                           TABLE 4__________________________________________________________________________Flight                   Comparative                          Comparative                                 ComparativeSubject No. Example 1       Example 2             Example 3                   Example 1                          Example 2                                 Example 3__________________________________________________________________________ 1    O     O     O     X      X      Δ 2    O     O     O     X      X      Δ 3    O     O     O     X      X      Δ 4    O     O     O     X      X      X 5    Δ       O     O     X      X      X 6    O     O     O     X      Δ                                 Δ 7    O     O     O     X      X      X 8    Δ       O     O     X      X      X 9    O     O     O     X      X      X10    O     O     O     X      X      X11    O     O     O     X      X      Δ12    O     O     O     X      X      Δ13    O     O     O     X      Δ                                 Δ14    O     O     O     X      X      Δ15    O     O     O     X      X      Δ16    O     O     O     X      X      X17    O     O     O     X      X      Δ18    O     O     O     X      X      Δ19    O     O     O     X      Δ                                 Δ20    O     O     O     X      Δ                                 X21    O     O     O     X      X      X22    O     O     O     X      X      X23    O     O     O     X      X      Δ24    O     O     O     X      X      Δ25    Δ       O     O     X      X      X26    O     O     O     X      Δ                                 X27    O     O     O     X      X      X28    O     O     O     X      X      X29    Δ       O     O     X      X      X30    O     O     O     X      Δ                                 X__________________________________________________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________Bounce height                   Comparative                          Comparative                                 ComparativeSubject No. Example 1       Example 2             Example 3                   Example 1                          Example 2                                 Example 3__________________________________________________________________________ 1    O     O     O     X      X      Δ 2    O     O     O     X      X      Δ 3    O     O     O     X      Δ                                 X 4    O     O     O     X      X      X 5    Δ       O     O     X      X      Δ 6    O     O     O     X      X      X 7    O     O     O     X      X      X 8    O     O     O     X      Δ                                 Δ 9    O     O     O     X      X      Δ10    O     O     O     X      X      X11    O     O     O     X      X      X12    O     O     O     X      X      X13    O     O     O     X      Δ                                 X14    O     O     O     X      X      Δ15    O     O     O     X      X      X16    Δ       O     O     X      X      Δ17    Δ       O     O     X      X      X18    O     O     O     X      X      X19    O     O     O     X      Δ                                 X20    O     O     O     X      X      Δ21    O     O     O     X      X      Δ22    Δ       O     O     X      X      Δ23    O     O     O     X      X      X24    O     O     O     X      X      Δ25    O     O     O     X      X      X26    O     O     O     X      X      X27    Δ       O     O     X      X      X28    O     O     O     X      X      Δ29    O     O     O     X      X      X30    O     O     O     X      Δ                                 X__________________________________________________________________________ 
    
     From Tables 3 to 5, it is understandable that the tennis balls of Examples according to the present invention are suitable for children of tender age and adult beginners since the rebound of them is low and strike feeling is good. 
     According to the present invention, as described hereinabove, there can be realized a tennis ball having suitable physical properties, i.e. rebound, speed, and hardness for not only children of tender age who are inferior to adult in physical constitution and athletic abilities, but also adult beginners and adults who have weak physical strength and inferior athletic ability. Consequently, the tennis ball of present invention can hasten their progress in tennis playing and can increase their interesting for tennis. 
     The effect is improved when they use a light and short racket for children in playing. 
     While the invention has been particularly shown and described in reference to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the spirit and scope of the invention.