US20120244970A1

US20120244970A1 - Baseball

Info

Publication number: US20120244970A1
Application number: US13/425,812
Authority: US
Inventors: Kazuhiro Kume; Takashi Ono; Yohei Yamashita
Original assignee: Mizuno Corp
Current assignee: Mizuno Corp
Priority date: 2011-03-23
Filing date: 2012-03-21
Publication date: 2012-09-27
Also published as: JP2012200270A

Abstract

A baseball includes an inner core, and an outer layer covering the outer circumferential surface of the inner core. The inner core has a surface hardness of not less than 60 and not more than 80 in Asker C hardness, and has an elastic modulus of not less than 0.6 MPa and not more than 1.0 MPa. In this way, a baseball can be obtained which can achieve a trajectory comparable to that of a ball for hardball after hitting and which can achieve an impact force comparable to a ball for rubber-ball baseball.

Description

This non-provisional application is based on Japanese Patent Application No. 2011-64474 filed with the Japan Patent Office on Mar. 23, 2011, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a baseball, and particularly to a solid baseball.
2. Description of the Background Art
As for a baseball, it is desired to improve safety by decreasing the impact force when the baseball hits against a human body. For example, in Little League, the hardness and the restitution coefficient of a ball for hardball are defined to improve safety when younger children play baseball. According to the rules of Little League, the hardness (compression hardness) of the baseball is defined such that a load when the baseball is compressed by 6.35 mm is less than 45 pounds (lbs) (200.17 N). In addition, the restitution coefficient of the baseball when the baseball hits against an iron plate at a speed of 26.82 m/s is defined to be 0.45 to 0.55.
An usual ball for hardball is configured by spherically winding a wool yarn on a rubber core, further winding a cotton yarn thereon to make a surface smooth, and putting a cow leather thereon and sewing up the leather with a sewing thread. It should be noted that a ball for hardball having a structure different from that of this usual ball for hardball is proposed. Japanese Patent Laying-Open No. 2002-210043, for example, proposes a ball for hardball configured by wrapping a rubber core in an intermediate core made of urethane foam.
In addition to the ball for hardball, a ball for rubber-ball baseball is used as the baseball. The impact force of the ball for rubber-ball baseball is smaller than that of the ball for hardball. Hence, the ball for rubber-ball baseball provides higher safety than the ball for hardball.
The impact force of the ball for rubber-ball baseball is reduced as compared with that of the ball for hardball, thereby achieving high safety. However, a behavior of the ball for rubber-ball baseball is greatly different from that of the ball for hardball after hitting against a bat and a ground.
Further, the ball for hardball has a compression hardness greater than that of the ball for rubber-ball baseball. Hence, when a user of the ball for rubber-ball baseball holds the ball for hardball, he/she feels that the ball for hardball is harder than the ball for rubber-ball baseball. This may make the user of the ball for rubber-ball baseball feel uneasy.
The ball for hardball disclosed in the above publication is formed such that a feeling when the ball hits is almost the same as that of the usual ball for hardball, although the rubber core is wrapped in the intermediate core made of urethane foam. Thus, the ball for hardball disclosed in the above publication has the impact force comparable to that of the usual ball for hardball. Therefore, in the ball for hardball disclosed in the above publication, it is not assumed to further reduce the impact force as compared with the usual ball for hardball in order to improve safety.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoing problem, and has its object to provide a baseball that achieves an impact force comparable to that of a ball for rubber-ball baseball, that achieves a behavior comparable to that of the ball for hardball after hitting, and that achieves a compression hardness comparable to that of the ball for rubber-ball baseball.
As a result of diligent study by the present inventors, it has been found that when hitting, the ball for rubber-ball baseball and the ball for hardball are deformed in different degrees, with the result that their behaviors after hitting will greatly differ from each other. Based on this finding, the present inventors have found that by forming a baseball to be solid and adjusting physical properties of an inner core thereof, the baseball achieves an impact force comparable to the ball for rubber-ball baseball, achieves a behavior comparable to that of the ball for hardball after hitting, and achieves a compression hardness comparable to the ball for rubber-ball baseball.
The present inventors have found that the surface hardness (Asker C hardness) of the inner core has an influence over the behavior of the baseball after hitting. Specifically, when the surface hardness (Asker C hardness) of the inner core is low, the baseball is greatly deformed when hitting, with the result that the behavior of the baseball becomes greatly different from that of the ball for hardball after hitting. To address this, the present inventors have found that by adjusting the surface hardness (Asker C hardness) of the inner core, a behavior comparable to that of the ball for hardball after hitting can be achieved.
The present inventors have found that the surface hardness (Asker C hardness) of the inner core is correlated with the compression hardness. The ball for rubber-ball baseball has a compression hardness of less than 30 lbs. Hence, when a user of the ball for rubber-ball baseball holds a baseball having a compression hardness of 30 lbs or greater, he/she will feel that the baseball is harder than the ball for rubber-ball baseball. This may make the user of the ball for rubber-ball baseball feel uneasy. In order to render the compression hardness thereof comparable to that of the ball for rubber-ball baseball, the surface hardness (Asker C hardness) of the inner core needs to be 80 or smaller. Hence, with the surface hardness (Asker C hardness) of the inner core being 80 or smaller, it has been found that a compression hardness comparable to that of the ball for rubber-ball baseball can be achieved. In the baseball of the present invention, the compression hardness comparable to that of the ball for rubber-ball baseball can restrain the user from feeling uneasy.
On the other hand, if the surface hardness (Asker C hardness) of the inner core is smaller than 60, the leather of the outer layer cannot be sewed up to obtain a round baseball. In this case, the baseball does not function properly as a baseball. Hence, the surface hardness (Asker C hardness) of the inner core is set at 60 or greater.
The present inventors have found that the elastic modulus of the inner core has an influence over the impact force. Further, the present inventors have found that the elastic modulus of the inner core is correlated with the impact force. The inventors have found that when the elastic modulus of the inner core is 1 MPa or smaller, an impact force comparable to the ball for rubber-ball baseball, i.e., an impact force of 80 G or smaller can be achieved.
Further, when the elastic modulus of the inner core is low, the baseball will be slow in recovering from the deformation. This makes it impossible to attain a behavior comparable to that of the ball for hardball after hitting. The inventors have found that when the elastic modulus of the inner core is equal to or greater than 0.6 MPa, the baseball can be quickly recovered from the deformation.
Thus, the present inventors have conceived that by adjusting the surface hardness (Asker C hardness) and the elastic modulus of the inner core, an impact force comparable to that of the ball for rubber-ball baseball can be achieved, a behavior comparable to that of the ball for hardball after hitting can be achieved, and a compression hardness comparable to that of the ball for rubber-ball baseball can be achieved.
Namely, a baseball of the present invention includes: an inner core; and an outer layer covering an outer circumferential surface of the inner core. The inner core has a surface hardness of not less than 60 and not more than 80 in Asker C hardness, and has an elastic modulus of not less than 0.6 MPa and not more than 1.0 MPa. In this way, the baseball of the present invention achieves an impact force comparable to that of the ball for rubber-ball baseball, achieves a behavior comparable to that of the ball for hardball after hitting, and achieves a compression hardness comparable to that of the ball for rubber-ball baseball.
Preferably in the baseball, the inner core has a loss coefficient (tan δ) of not less than 0.15 and not more than 0.31.
The present inventors have found that the loss coefficient (tan δ) of the inner core has an influence over a trajectory (flying manner) of the baseball after hitting. The loss coefficient (tan δ) herein is a ratio between a loss elastic modulus, which is an imaginary part of a complex elastic modulus, and a storage elastic modulus, which is a real part of the complex elastic modulus. The complex elastic modulus is a difference between dynamic stress and dynamic strain when sinusoidal vibrations are provided to a viscoelastic material.
Further, the present inventors have found that the loss coefficient (tan δ) of the inner core is correlated with the restitution coefficient. Further, it has been found that when the restitution coefficient of the inner core has an influence over the trajectory (flying manner) of the baseball after hitting. When the loss coefficient (tan δ) of the inner core is 0.31 or smaller, the restitution coefficient of the inner core is 0.5 or greater, which is comparable to that of the ball for hardball. Accordingly, a trajectory (flying manner) comparable to that of the ball for hardball after hitting can be achieved.
On the other hand, when the restitution coefficient of the inner core is too high, the trajectory (flying manner) thereof after hitting will differ from that of the ball for hardball. Specifically, when the inner core has a loss coefficient (tan δ) of less than 0.15, the restitution coefficient of the inner core becomes high, with the result that the trajectory (flying manner) thereof after hitting differs from that of the ball for hardball.
Thus, by adapting the loss coefficient (tan δ) of the inner core to be not less than 0.15 and not more than 0.31, the restitution coefficient of the inner core of the baseball can be comparable to that of the ball for hardball. In this way, the trajectory (flying manner) thereof comparable to that for the ball for hardball can be achieved after hitting.
Preferably in the baseball, an impact force when the baseball hits against a dummy at a speed of 26.82 m/s is 80 G or smaller. In this way, an impact force comparable to that of the ball for rubber-ball baseball can be achieved. Accordingly, safety comparable to the ball for rubber-ball baseball can be obtained. A soft material such as natural leather, artificial leather, synthetic leather, cloth, and knitted material is generally used in the outer layer of the baseball. Therefore, even a baseball configured by attaching the outer layer to the inner core can achieve the impact force comparable to that of the ball for rubber-ball baseball.
Preferably in the baseball, a restitution coefficient when the baseball hits against an iron plate at a speed of 26.82 m/s is not less than 0.50 and not more than 0.55. Accordingly, the baseball conforms to the rules of Little League, thereby providing a baseball satisfying the rules of Little League. Further, as the restitution coefficient thereof is higher, a trajectory (flying manner) closer to that of the ball for hardball can be achieved after hitting. Hence, in addition to the compliance with the rules of Little League, a trajectory (flying manner) close to that of the ball for hardball can be achieved after hitting.
Preferably in the baseball, a load when compressing an outer diameter of the baseball by 6.35 mm is 16 lbs or more and is less than 30 lbs (133.4466N). Accordingly, the baseball conforms to the rules of Little League, thereby providing a baseball satisfying the rules of Little League.
Another baseball of the present invention includes: an inner core; a thread-wound layer obtained by winding a yarn to cover an outer circumferential surface of the inner core; and an outer layer covering an outer circumferential surface of the thread-wound layer. The inner core has a surface hardness of not less than 71 and not more than 75 in Asker C hardness, has an elastic modulus of not less than 0.9 MPa and not more than 1.0 MPa, and has a loss coefficient (tan δ) of not less than 0.25 and not more than 0.27. The inner core is made of urethane foam. The inner core has an outer diameter of 70.7 mm. The outer layer has an outer diameter of 73.2 mm.
Preferably, the inner core has a surface hardness of 73 in Asker C hardness, has an elastic modulus of 0.95 MPa, and has a loss coefficient (tan δ) of 0.26.
In this way, the present inventors have found that an impact force comparable to that of the ball for rubber-ball baseball can be achieved, a behavior comparable to that of the ball for hardball can be achieved after hitting, and a compression hardness comparable to that of the ball for rubber-ball baseball can be achieved. Thus, the another baseball of the present invention achieves an impact force comparable to that of the ball for rubber-ball baseball, achieves a behavior comparable to that of the ball for hardball after hitting, and achieves a compression hardness comparable to that of the ball for rubber-ball baseball.
As described above, each of the baseballs of the present invention achieves an impact force comparable to that of the ball for rubber-ball baseball, achieves a behavior comparable to that of the ball for hardball after hitting, and achieves a compression hardness comparable to that of the ball for rubber-ball baseball.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a baseball according to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view of a modification of the baseball according to the embodiment of the present invention.

FIG. 3 shows a relationship between impact force and compression hardness in Comparative Examples and Examples in the example.

FIG. 4 shows a relationship between restitution coefficient and compression hardness in Comparative Examples and Examples in the example.

FIG. 5 shows a relationship between impact force and compression hardness in Comparative Examples in the example.

FIG. 6 shows a relationship between restitution coefficient and compression hardness in Comparative Examples in the example.

FIG. 7 shows a relationship between compression hardness and surface hardness in Comparative Examples and Examples in the example.

FIG. 8 shows a relationship between restitution coefficient and surface hardness in Comparative Examples and Examples in the example.

FIG. 9 shows a relationship between impact force and elastic modulus in Comparative Examples and Examples in the example.

FIG. 10 shows a relationship between impact force and surface hardness in Comparative Examples and Examples in the example.

FIG. 11 shows a relationship between restitution coefficient and tan δ in Comparative Examples and Examples in the example.

FIG. 12 shows a state of Comparative Example A in the example when hitting.

FIG. 13 shows a state of Comparative Example A in the example when having restituted after the hitting.

FIG. 14 shows a state of Comparative Example I in the example when hitting.

FIG. 15 shows a state of Comparative Example I in the example when having restituted after the hitting.

FIG. 16 shows a state of Example C in the example when hitting.

FIG. 17 shows a state of Example C in the example when having restituted after the hitting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes one embodiment of the present invention with reference to figures.
Explained first is a configuration of a baseball of the embodiment of the present invention.
Referring to FIG. 1, a baseball 1 of the embodiment of the present invention mainly has an inner core 2, and an outer layer 3. Inner core 2 is placed in a central portion of baseball 1. An outer circumferential surface of inner core 2 is covered with outer layer 3. Inner core 2 is made of urethane foam. Outer layer 3 mainly has, for example, a leather and a sewing thread for sewing up this leather. In this case, outer layer 3 is configured by putting a leather over the outer circumferential surface of inner core 2, and sewing up this leather.
The surface hardness of inner core 2 is not less than 60 and not more than 80 in Asker C hardness. Inner core 2 has an elastic modulus of not less than 0.6 MPa and not more than 1.0 MPa.
Further, inner core 2 may have a loss coefficient (tan δ) of not less than 0.15 and not more than 0.31.
Further, the impact force of baseball 1 may be 80 G or smaller when inner core 2 hits against a dummy at a speed of 26.82 m/s.
Further, the restitution coefficient of baseball 1 may be not less than 0.50 and not more than 0.55 when baseball 1 hits against an iron plate at a speed of 26.82 m/s. It should be noted that a value of the restitution coefficient of baseball 1 decreases slightly due to outer layer 3. Specifically, a value of the restitution coefficient decreases by a value within the range of 0.01 to 0.02, e.g., by approximately 0.015. Therefore, the restitution coefficient of inner core 2 of baseball 1 may be not less than 0.515 and not more than 0.570 when inner core 2 of baseball 1 hits against the iron plate at a speed of 26.82 m/s.
Further, baseball 1 may have a load of 16 lbs or more and less than 30 lbs when the outer diameter of baseball 1 is compressed by 6.35 mm.
Referring to FIG. 2, baseball 1 according to a modification of the embodiment of the present invention may have a thread-wound layer 4 configured by winding a thread to cover the outer circumferential surface of inner core 2. Thread-wound layer 4 is configured by winding, for example, a cotton yarn to cover the outer circumferential surface of inner core 2 to make a surface thereof smooth. It should be noted that when thread-wound layer 4 is wound, inner core 2 becomes a little smaller due to tension when the thread is wound, and thus, the outer diameter after the thread is wound is almost the same as the outer diameter of inner core 2 when no thread is wound. Thread-wound layer 4 has an outer circumferential surface covered with outer layer 3.
In baseball 1 of the modification of the embodiment of the present invention, inner core 2 has a surface hardness of not less than 71 and not more than 75 in Asker C hardness, has an elastic modulus of not less than 0.9 MPa and not more than 1.0 MPa, and has a loss coefficient (tan δ) of not less than 0.25 and not more than 0.27. Inner core 2 is made of urethane foam. Inner core 2 has an outer diameter of 70.7 mm Outer layer 3 has an outer diameter of 73.2 mm.
Further, it is preferable that inner core 2 has a surface hardness of 73 in Asker C hardness, has an elastic modulus of 0.95 MPa, and has a loss coefficient (tan δ) of 0.26.
It should be noted that the outer diameter of outer layer 3 corresponds to the outer diameter of baseball 1. Further, each of the dimensions such as the outer diameter of inner core 2, i.e., 70.7 mm and the outer diameter of outer layer 3, i.e., 73.2 mm, has a dimensional tolerance of ±0.2 mm. In the following, each dimension has a dimension tolerance, similarly. Further, baseball 1 configured by affixing a leather to inner core 2 as outer layer 3 and sewing up the leather with a sewing thread has an outer circumference of, for example, 230 mm.
The following describes the material of the inner core of the baseball of the embodiment of the present invention more in detail. An exemplary, usable material therefor is a rubber, a resin, an elastomer, or a foam of a mixture thereof. Examples of the rubber usable therefor includes: a butadiene rubber, a styrene butadiene rubber, a chloroprene rubber, an isoprene rubber, an ethylene propylene rubber, an ethylene propylene diene terpolymer rubber, a silicone rubber, an urethane rubber, a natural rubber, and foams thereof. Examples of the resin usable therefor include: an urethane-based resin; a styrene-based resin; olefin-based resins including polyethylene, polypropylene, and ethylene vinyl copolymer; a polyester-based resin; a polyamide-based resin; a vinyl chloride-based resin; an ionomer resin; and foams thereof. Examples of the elastomer usable therefor include: an urethane-based elastomer; styrene-based elastomers including styrene-butadiene-styrene, styrene-isoprene-styrene, or a substance obtained by adding water to the styrene-butadiene-styrene or the styrene-isoprene-styrene; an olefin-based elastomer composed of polyethylene or polypropylene and a diene-based rubber or the like; a polyester-based elastomer; a polyamide-based elastomer; a chlorine-based elastomer; a fluorine-based elastomer; a silicone-based elastomer; and foams thereof.
The following describes a method for manufacturing the baseball of the embodiment of the present invention.
In the case of producing the inner core using a foam of polyurethane, a predetermined amount of a liquid having a mixture of polyol, isocyanate, a catalyst, a hardening agent, and a foaming agent is introduced into a metal mold to form the inner core. Then, the liquid thus introduced is left for a predetermined period of time until the polyol and the isocyanate react with each other to form into polyurethane. With complete reaction thereof, the shape of the inner core is formed. Thereafter, the metal mold is opened to take out the inner core therefrom.
Next, a description will be given to functions and effects of the baseball according to the embodiment of the present invention.
As a result of diligent study of the present inventors, it has been found that when the ball for rubber-ball baseball and the ball for hardball hit, their respective degrees of deformation are different from each other to result in a great difference between their behaviors after hitting. This point will be now further described.
The ball for rubber-ball baseball does not have a core and is formed to be hollow. In contrast, the ball for hardball is formed to be solid. Because the ball for rubber-ball baseball is thus formed to be hollow, the ball for rubber-ball baseball is dramatically deformed when hitting against a bat and a ground. Meanwhile, because the ball for hardball is formed to be solid, the ball for hardball is less deformed when hitting, as compared with the deformation of the ball for rubber-ball baseball when hitting. Thus, the ball for rubber-ball baseball and the ball for hardball are different from each other in terms of the degree of deformation when hitting. This results in a great difference between the behavior of the ball for rubber-ball baseball and the behavior of the ball for hardball after hitting.
Based on this finding, the present inventors have found that by forming baseball 1 to be solid and adjusting physical properties of inner core 2, baseball 1 achieves an impact force comparable to that of the ball for rubber-ball baseball, achieves a behavior comparable to that of the ball for hardball after hitting, and achieves a compression hardness comparable to that of the ball for rubber-ball baseball.
The present inventors have found that the surface hardness (Asker C hardness) of inner core 2 has an influence over the behavior of baseball 1 after hitting. Specifically, when the surface hardness (Asker C hardness) of inner core 2 is low, baseball 1 is greatly deformed when hitting, with the result that the behavior of baseball 1 becomes greatly different from that of the ball for hardball after hitting. To address this, the present inventors have found that by adjusting the surface hardness (Asker C hardness) of inner core 2, a behavior comparable to that of the ball for hardball after hitting can be achieved.
The present inventors have found that the surface hardness (Asker C hardness) of inner core 2 is correlated with the compression hardness thereof. The ball for rubber-ball baseball has a compression hardness of less than 30 lbs. Hence, when a user of the ball for rubber-ball baseball holds baseball 1 having a compression hardness of 30 lbs or greater, he/she will feel that baseball 1 is harder than the ball for rubber-ball baseball. This may make the user of the ball for rubber-ball baseball feel uneasy. In order to render the compression hardness thereof comparable to that of the ball for rubber-ball baseball, the surface hardness (Asker C hardness) of inner core 2 needs to be 80 or smaller. Hence, with the surface hardness (Asker C hardness) of inner core 2 being 80 or smaller, it has been found that a compression hardness comparable to that of the ball for rubber-ball baseball can be achieved. In baseball 1 of the embodiment of the present invention, the compression hardness comparable to that of the ball for rubber-ball baseball can restrain the user from feeling uneasy.
On the other hand, if the surface hardness (Asker C hardness) of inner core 2 is smaller than 60, the leather of outer layer 3 cannot be sewed up to obtain a round baseball 1. In this case, baseball 1 does not function properly as a baseball. Hence, the surface hardness (Asker C hardness) of inner core 2 is set at 60 or greater.
The present inventors have found that the elastic modulus of inner core 2 has an influence over the impact force. Further, the present inventors have found that the elastic modulus of inner core 2 is correlated with the impact force. The inventors have found that when the elastic modulus of inner core 2 is 1 MPa or smaller, an impact force comparable to the ball for rubber-ball baseball, i.e., an impact force of 80 G or smaller can be obtained.
Further, when the elastic modulus of inner core 2 is low, the baseball will be slow in recovering from the deformation. This makes it impossible to achieve a behavior comparable to that of the ball for hardball after hitting. The inventors have found that when the elastic modulus of inner core 2 is equal to or greater than 0.6 MPa, the baseball can be quickly recovered from the deformation.
Thus, the present inventors have conceived that by adjusting the surface hardness (Asker C hardness) and the elastic modulus of inner core 2, an impact force comparable to that of the ball for rubber-ball baseball can be achieved, a behavior comparable to that of the ball for hardball after hitting can be achieved, and a compression hardness comparable to that of the ball for rubber-ball baseball can be achieved.
Specifically, baseball 1 of the embodiment of the present invention includes inner core 2, and outer layer 3 covering the outer circumferential surface of inner core 2. Inner core 2 has a surface hardness of not less than 60 and not more than 80 in Asker C hardness, and has an elastic modulus of not less than 0.6 MPa and not more than 1.0 MPa. In this way, baseball 1 of the embodiment of the present invention achieves an impact force comparable to that of the ball for rubber-ball baseball, achieves a behavior comparable to that of the ball for hardball after hitting, and achieves a compression hardness comparable to that of the ball for rubber-ball baseball.
In baseball 1 of the embodiment of the present invention, inner core 2 may have a loss coefficient (tan δ) of not less than 0.15 and not more than 0.31.
The present inventors have found that the loss coefficient (tan δ) of inner core 2 has an influence over a trajectory (flying manner) of baseball 1 after hitting. Further, the present inventors have found that the loss coefficient (tan δ) of inner core 2 is correlated with the restitution coefficient thereof. Further, it has been found that when the restitution coefficient of inner core 2 has an influence over the trajectory (flying manner) of baseball 1 after hitting. When the loss coefficient (tan δ) of inner core 2 is 0.31 or smaller, the restitution coefficient of inner core 2 is 0.5 or greater, which is comparable to that of the ball for hardball. Accordingly, a trajectory (flying manner) comparable to that for the ball for hardball after hitting can be achieved.
On the other hand, when the restitution coefficient of inner core 2 is too high, the trajectory (flying manner) thereof after hitting will differ from that of the ball for hardball. Specifically, when inner core 2 has a loss coefficient (tan δ) of less than 0.15, the restitution coefficient of inner core 2 becomes high, with the result that the trajectory (flying manner) thereof after hitting differs from that for the ball for hardball.
Thus, by adapting the loss coefficient (tan δ) of inner core 2 to be not less than 0.15 and not more than 0.31, the restitution coefficient of inner core 2 of baseball 1 can be comparable to that of the ball for hardball. In this way, the trajectory (flying manner) thereof comparable to that of the ball for hardball can be achieved after hitting.
The impact force of the baseball according to the embodiment of the present invention when baseball 2 hits against a dummy at a speed of 26.82 m/s may be 80 G or smaller. In this way, the impact force comparable to that of the ball for rubber-ball baseball can be achieved. Accordingly, safety comparable to that for the ball for rubber-ball baseball can be achieved. A soft material such as natural leather, artificial leather, synthetic leather, cloth, and knitted material is generally used in outer layer 3 of baseball 1. Therefore, even a baseball 1 configured by attaching outer layer 3 to inner core 2 can achieve the impact force comparable to that of the ball for rubber-ball baseball.
The restitution coefficient of the baseball according to the embodiment of the present invention may be not less than 0.50 and not more than 0.55 when baseball 1 hits against the iron plate at a speed of 26.82 m/s. Accordingly, baseball 1 conforms to the rules of Little League, thereby providing a baseball 1 satisfying the rules of Little League. Further, as the restitution coefficient thereof is higher, a trajectory (flying manner) closer to that of the ball for hardball can be achieved after hitting. Hence, in addition to the compliance with the rules of Little League, a trajectory (flying manner) close to that of the ball for hardball can be achieved after hitting.
The baseball of the embodiment of the present invention may have a load of not less than 16 lbs and less than 30 lbs (133.4466N) when the outer diameter of baseball 1 is compressed by 6.35 mm. Accordingly, baseball 1 conforms to the rules of Little League, thereby providing a baseball satisfying the rules of Little League.
The baseball of the modification of the embodiment of the present invention includes: inner core 2; thread-wound layer 4 obtained by winding a yarn to cover the outer circumferential surface of the inner core; and outer layer 3 covering the outer circumferential surface of thread-wound layer 4. Inner core 2 has a surface hardness of not less than 71 and not more than 75 in Asker C hardness, has an elastic modulus of not less than 0.9 MPa and not more than 1.0 MPa, and has a loss coefficient (tan δ) of not less than 0.25 and not more than 0.27. Inner core 2 is made of urethane foam. Inner core 2 has an outer diameter of 70.7 mm. Outer layer 3 has an outer diameter of 73.2 mm.
Preferably, inner core 2 has a surface hardness of 73 in Asker C hardness, has an elastic modulus of 0.95 MPa, and has a loss coefficient (tan δ) of 0.26.
In this way, the present inventors have found that an impact force comparable to that of the ball for rubber-ball baseball can be achieved, a behavior comparable to that of the ball for hardball can be achieved after hitting, and a compression hardness comparable to that of the ball for rubber-ball baseball can be achieved. Thus, baseball 1 of the modification of the embodiment of the present invention achieves an impact force comparable to that of the ball for rubber-ball baseball, achieves a behavior comparable to that of the ball for hardball after hitting, and achieves a compression hardness comparable to that of the ball for rubber-ball baseball.

Example

An example of the present invention will be described hereinafter. It should be noted that the same or corresponding portions as above are given the same reference characters and may not be described repeatedly.
Referring to Table 1, FIG. 3, and FIG. 4, Comparative Examples A, B are comparative examples for the present invention. Examples C, D are examples of the present invention. A vertical axis in FIG. 3 indicates the magnitude of impact force (G) and a horizontal axis therein indicates the magnitude of compression hardness (lbs). A vertical axis in FIG. 4 indicates the magnitude of restitution coefficient and a horizontal axis therein indicates the magnitude of compression hardness (lbs).

TABLE 1

Compression	Restitution	Impact Force
Hardness (lbs)	Coefficient	(G)

Comparative	23.1	0.450	72.1
Example A
Comparative	26.5	0.455	75.5
Example B
Example C	27.5	0.519	73.0
Example D	17.1	0.509	71.0

Comparative Examples A, B corresponds to the balls for rubber-ball baseball. Examples C, D correspond to the baseballs of the present invention. Each of Examples C, D has a structure including inner core 2 and outer layer 3 covering the outer circumferential surface of inner core 2 as shown in FIG. 1. Inner core 2 of each of Examples C, D is made of urethane foam. Respective physical properties of urethane foams in Examples C, D are different from each other. Inner core 2 of each of Examples C, D has an outer diameter of 70.7 mm Outer layer 3 of each of Examples C, D has an outer diameter of 73.2 mm.
Each item in Table 1 will be described. The compression hardness (lbs) refers to the load when the outer diameter of the baseball is compressed by 6.35 mm. The restitution coefficient refers to the restitution coefficient when the baseball hits against the iron plate at a speed of 26.82 m/s. The impact force (G) refers to the impact force when the baseball hits at a speed of 26.82 m/s. As to these items, the same is applied in each table and each figure in the following.
The compression hardness (lbs) was measured by using AG-5000D manufactured by Shimadzu Corporation as a measuring instrument, in accordance with a test method based on ASTM (American Society for Testing and Materials) F 1888 “Test Method for Compression-Displacement of Baseballs and Softballs 1.”
The restitution coefficient was measured by using a light gate as a measuring instrument, in accordance with a test method based on ASTM F 1887 “Standard Test Method for Measuring the Coefficient of restitution (COR) of Baseballs and Softballs.” The light gate is a measuring instrument for calculating speed by sensing the passage of a ball through a box from which light is emitted. The restitution coefficient is a value obtained by dividing the speed of the ball after hitting against an iron plate by the speed of the ball before hitting against the iron plate.
The impact force (G) was measured by using a testing machine based on “Approval Standard and Standard Confirmation Method for Baseball Helmets” by the Consumer Product Safety Association to measure acceleration when a ball hits against a dummy head by an accelerometer attached to the dummy head.
Referring to Table 1, FIG. 3, and FIG. 4, a value of the compression hardness (lbs) in Example C was comparable to those of Comparative Examples A, B. Further, a value of compression hardness (lbs) in Example D was smaller than those of Comparative Examples A, B.
Values of the restitution coefficient in Examples C, D were larger than those in Comparative Examples A, B. Values of the impact force (G) in Examples C, D were close to each other as compared with those in Comparative Examples A, B. In each of Examples C, D, the restitution coefficient was 0.509 to 0.519. Further, in each of Examples C, D, the impact force (G) was 71.0 to 73.0.
Referring to Table 2, FIGS. 5 and 6, Comparative Examples E to H in addition to Comparative Examples A, B are comparative examples for the present invention. A vertical axis in FIG. 5 indicates the magnitude of impact force (G) and a horizontal axis therein indicates the magnitude of compression hardness (lbs). A vertical axis in FIG. 6 indicates the magnitude of restitution coefficient and a horizontal axis therein indicates the magnitude of compression hardness (lbs). Each of Comparative Examples E to H has a structure including inner core 2 and outer layer 3 covering the outer circumferential surface of inner core 2 as shown in FIG. 1. In Comparative Examples E to H, the inner core is made of urethane foam. Respective physical properties of urethane foams in Comparative Examples E to F are different from each other. Inner core 2 of each of Comparative Examples E to H has an outer diameter of 70.7 mm Outer layer 3 of each of Comparative Examples E to H has an outer diameter of 73.2 mm.

TABLE 2

Compression
Hardness	Restitution	Impact Force
(lbs)	Coefficient	(G)

Comparative	23.1	0.450	72.1
Example A
Comparative	26.5	0.455	75.5
Example B
Comparative	21.6	0.423	109.6
Example E
Comparative	25.6	0.444	123.2
Example F
Comparative	35.3	0.453	124.5
Example G
Comparative	38.3	0.480	127.3
Example H

Values of the compression hardness (lbs) in Comparative Examples E, F were comparable to those in Comparative Examples A, B, and values of the compression hardness (lbs) in Comparative Examples G, H were larger than those in Comparative Examples E, F. In addition, it was found that a value of the impact force (G) did not change easily as compared with the compression hardness (lbs) in Comparative Examples E to H. More specifically, it was found that a rate of decrease in the impact force (G) was smaller than a rate of decrease in the compression hardness (lbs). Values of the restitution coefficient in Comparative Examples E to H were comparable to those in Comparative Examples A and B. Values of the impact force (G) in Comparative Examples E to H were much larger than those in Comparative Examples A and B.
From Table 1 and Table 2, it was found that in the structure of baseball 1 shown in FIG. 1, the compression hardness (lbs), the restitution coefficient, and the impact force (G) were greatly fluctuated depending on the physical properties of the urethane foam, which was a material of inner core 2. It was found that in each of Comparative Examples E to H, the impact force was dramatically larger than those of Comparative Examples A, B, each of which corresponded to the ball for rubber-ball baseball.
Next, properties of the material used for inner core 2 were specified.
First, in order to obtain the properties of the material used for inner core 2, CAE (Computer Aided Engineering) analysis was carried out using an SS curve based on drop impact and a viscoelasticity value obtained by a viscoelasticity test.
In the CAE analysis, an Ogden coefficient (elasticity) and a relaxation function (viscosity) were used as parameters to be input. A weight-drop test and a dynamic viscoelasticity test were used to calculate the parameters. The elasticity was measured by the weight-drop test, and the viscosity was measured by the dynamic viscoelasticity test.
The weight-drop test was conducted using a buffer impact tester CST-180 manufactured by Yoshida Seiki Co., Ltd. A test method is as follows. First, a sample having a thickness of 20 mm was prepared. Next, a weight having an outer diameter of 45 mm and a certain weight was dropped onto the sample from a certain height to measure a displacement-acceleration curve using an accelerometer. Then, a stress-strain curve was calculated from the displacement-acceleration curve. Coefficients μ and α of a strain energy function were calculated from this stress-strain curve based on an equation (1). “μ” refers to a shear elastic modulus and “α” refers to an exponent. In addition, “λ” in equation (1) refers to an extension ratio, “K” refers to a volume elasticity coefficient and “J” refers to a volume change rate.
$\begin{matrix} W = \sum_{A = 1}^{3} \sum_{i = 1}^{n} 2 \frac{μ_{i}}{α_{i}} (λ_{A}^{α i} - 1) + \frac{K}{2} {(J - 1)}^{2} & (1) \end{matrix}$
Elastic modulus E was calculated using μ and α based on an equation (2). More specifically, an initial elastic modulus in the stress-strain curve was calculated.
$\begin{matrix} E = \frac{3}{2} \sum_{n = 1}^{N} α_{n} μ & (2) \end{matrix}$
The dynamic viscoelasticity test was conducted using Rheogel-E4000 manufactured by UBM. A test method is as follows. Stress was measured from strain of sinusoidal vibration. In addition, temperature characteristics and frequency characteristics were measured by measuring a phase difference between input strain and response stress.
Then, a complex elastic modulus was measured from an amplitude ratio and a phase difference between a drive unit and a response unit at 20° C. when forced vibration was produced at frequencies of 1, 2, 4, 8, and 16 Hz in a frequency-temperature dependence mode and the temperature was raised at 2° C./min From a result of this measurement, a coefficient of the relaxation function was calculated based on an equation (3) using a curve fit program manufactured by Mechanical Design Co. In equation (3), g represents a relaxation function, γ represents a relaxation shear elastic modulus, and t represents a relaxation time.
$\begin{matrix} {g (t)}_{PAM} = \sum_{i = 1}^{M} γ_{i} e^{- \frac{t}{τ_{i}}} & (3) \end{matrix}$
The complex elastic modulus will now be described. First, as shown in an equation (4), the elastic modulus is a ratio between stress σ and strain ε (Hooke's law). The complex elastic modulus is a dynamic value of the material properties considering energy lost as heat at the time of deformation and recovery. As shown in an equation (5), complex elastic modulus E* is a sum of storage elastic modulus E′ and loss elastic modulus E″.
$\begin{matrix} E^{*} = \frac{σ}{ɛ} & (4) \\ E^{*} = E^{'} +  E^{″} & (5) \end{matrix}$
Referring to Table 3, Comparative Examples A, B are the same as those in Table 1 and Table 2. Comparative Examples I to L, O, Q, and S to V are comparative examples for the present invention. Examples M to N, P, and R are examples of the present invention. The restitution coefficient is a value obtained by actually measuring the restitution coefficient when the baseball hits against the iron plate at a speed of 26.82 m/s. The impact force (G) is an actually measured value thereof when the baseball hits at a speed of 26.82 m/s. The elastic modulus is a value of elastic modulus E described above. Further, tan δ (loss coefficient) is a ratio of storage elastic modulus E′ and loss elastic modulus E″ of complex elastic modulus E*. The surface hardness (Asker C) refers to a hardness measured using a spring type hardness tester (Asker C type) for an expanded rubber as defined in SRIS0101 (standard specification of Society of Rubber Industry, Japan). The compression hardness is a value measured in the same manner as described above.

TABLE 3

		Elas-
Restitution	Im-	tic		Surface	Compres-
Coefficient	pact	Mod-		Hard-	sion
(Actually	Force	ulus		ness	Hardness
Measured)	(G)	(MPa)	tanδ	Asker C	(lbs)

Comparative	0.450	72.1	—	—	—	23.1
Example A
Comparative	0.455	75.5	—	—	—	26.5
Example B
Comparative	0.440	63.0	0.30	0.500	50	11.1
Example I
Comparative	0.410	68.0	0.45	0.650	56	13.8
Example J
Comparative	0.651	86.6	1.10	0.100	73	27.5
Example K
Comparative	0.594	105.7	3.31	0.170	87	45.6
Example L
Example M	0.405	76.1	0.90	0.722	71	17.7
Example C	0.519	73.0	0.95	0.260	74	27.5
Example D	0.509	71.0	0.60	0.299	62	17.1
Example N	0.462	75.9	0.66	0.380	60	16.0
Comparative	0.576	81.0	1.27	0.180	72	21.5
Example O
Example P	0.610	80.0	1.00	0.140	80	30.0
Comparative	0.600	82.0	1.10	0.150	78	29.0
Example Q
Example R	0.500	75.0	0.80	0.310	67	19.5
Comparative	0.420	90.0	1.50	0.600	68	21.6
Example S
Comparative	0.530	95.0	1.92	0.250	72	25.6
Example T
Comparative	0.450	105.0	2.90	0.400	82	35.3
Example U
Comparative	0.480	100.0	2.43	0.350	85	38.3
Example V

Referring to FIG. 7, in each of Comparative Examples I to L, O, Q, and S to V as well as Examples M to N, P, and R, it was found that the surface hardness (Asker C) was correlated with the compression hardness (lbs). A vertical axis in FIG. 7 indicates the magnitude of compression hardness (lbs) and a horizontal axis therein indicates the magnitude of surface hardness (Asker C). It was found that when the surface hardness (Asker C) was 80 or smaller, the compression hardness of inner core 2 became 30 lbs or smaller.
Referring to FIG. 8, it was found that in each of Comparative Example I to L, O, Q, and S to V as well as Examples M to N, P, and R, there was no correlation between the surface hardness (Asker C) and the restitution coefficient. A vertical axis in FIG. 8 indicates the magnitude of restitution coefficient, and a horizontal axis therein indicates the magnitude of surface hardness (Asker C).
Referring to FIG. 9, in each of Comparative Examples I to L, O, Q, and S to V as well as Examples M to N, P, and R, it was found that the elastic modulus (MPa) was correlated with the impact force (G). A vertical axis in FIG. 9 indicates the magnitude of impact force (G) and a horizontal axis therein indicates the magnitude of elastic modulus (MPa). It was found that when the elastic modulus (MPa) was 1.0 MPa or smaller, the impact force (G) became 80 G or smaller.
Referring to FIG. 10, in each of Comparative Examples I to L, O, Q, and S to V as well as Examples M to N, P, and R, it was found that there was no correlation between the surface hardness (Asker C) and the impact force (G). A vertical axis in FIG. 10 indicates the magnitude of impact force (G), and a horizontal axis therein indicates the magnitude of surface hardness (Asker C).
Referring to FIG. 11, in Comparative Examples I to L, O, Q, and S to V as well as Examples M to N, P, and R, it was found that the loss coefficient (tan δ) was correlated with the restitution coefficient. A vertical axis in FIG. 11 indicates the magnitude of restitution coefficient and a horizontal axis therein indicates the magnitude of loss coefficient (tan δ). It was found that when the loss coefficient (tan δ) was 0.31 or smaller, the restitution coefficient became 0.5 or greater. Accordingly, it was found that when the loss coefficient (tan δ) was 0.31 or smaller, the restitution coefficient became 0.5 or greater, which was comparable to that of the ball for hardball.
Further, it was found that when the loss coefficient (tan δ) was less than 0.15, the restitution coefficient exceeded 0.6. When the restitution coefficient became higher, a trajectory after hitting would differ from that of the ball for hardball. Hence, it was found that when the loss coefficient (tan δ) was less than 0.15, the restitution coefficient became high, with the result that the trajectory after hitting would differ from that of the ball for hardball.
The following describes deformation taking place when hitting and restitution taking place after hitting, with reference to Comparative Example A, Comparative Example I, and Example C.
Referring to FIG. 12, when ball 11 for rubber-ball baseball in Comparative Example A hit against a dummy 10, ball 11 for rubber-ball baseball in Comparative Example A was greatly deformed. In other words, it was found that when hitting, ball 11 for rubber-ball baseball in Comparative Example A was greatly deformed.
Referring to FIG. 13, when ball 11 for rubber-ball baseball in Comparative Example A started to restitute after hitting against dummy 10, ball 11 for rubber-ball baseball in Comparative Example A was still considerably deformed. In other words, it was found that ball 11 for rubber-ball baseball in Comparative Example A was not immediately recovered to its round state after hitting.
Referring to FIG. 14, when baseball 1 of Comparative Example I hit against dummy 10, baseball 1 of Comparative Example I was also deformed. However, the deformation of baseball 1 of Comparative Example I was less than the deformation of ball 11 for rubber-ball baseball in Comparative Example A. In other words, it was found that baseball 1 of Comparative Example I was less deformed than ball 11 for rubber-ball baseball in Comparative Example A when hitting. This is presumably because baseball 1 of Comparative Example I was formed to be solid and therefore was less deformed when hitting.
Referring to FIG. 15, when baseball 1 of Comparative Example I started to restitute after hitting against dummy 10, baseball 1 of Comparative Example I was also deformed. In other words, it was found that baseball 1 of Comparative Example I was not immediately recovered to its round state after hitting.
However, baseball 1 of Comparative Example I was closer to the round state as compared with ball 11 for rubber-ball baseball in Comparative Example A. This is presumably because baseball 1 of Comparative Example I was less deformed than ball 11 for rubber-ball baseball in Comparative Example A and therefore readily restituted after hitting.
Referring to FIG. 16, when baseball 1 of Example C hit against dummy 10, baseball 1 of Example C was also deformed. However, the deformation of baseball 1 of Example C was less than the deformation of baseball 1 of Comparative Example I. In other words, it was found that baseball 1 of Example C was less deformed than baseball 1 of Comparative Example I when hitting.
This is presumably due to the following reasons: baseball 1 of Example C had a surface hardness (Asker C) larger than that of baseball 1 of Comparative Example I and was therefore less deformed when hitting; and baseball 1 of Example C had a compression hardness larger than that of baseball 1 of Comparative Example I and was therefore less deformed when hitting.
Referring to FIG. 17, when baseball 1 of Example C started to restitute after hitting against dummy 10, baseball 1 of Example C was hardly deformed. In other words, it was found that baseball 1 of Example C was recovered to its round state immediately after hitting. This is presumably because baseball 1 of Example I was less deformed than baseball 1 of Comparative Example I and therefore readily restituted after hitting.
Thus, it was found that although baseball 1 of Comparative Example I and baseball 1 of Example C were both formed to be solid, degrees of deformation when hitting were different, which led to different restitutions after hitting. It is considered that because the restitutions after hitting were thus different, their trajectories after hitting became different from each other.
Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being interpreted by the terms of the appended claims.

Claims

1. A baseball comprising:

an inner core; and

an outer layer covering an outer circumferential surface of said inner core,

said inner core having a surface hardness of not less than 60 and not more than 80 in Asker C hardness, and having an elastic modulus of not less than 0.6 MPa and not more than 1.0 MPa.

2. The baseball according to claim 1, wherein said inner core has a loss coefficient (tan δ) of not less than 0.15 and not more than 0.31.

3. The baseball according to claim 1, wherein an impact force when said baseball hits against a dummy at a speed of 26.82 m/s is 80 G or smaller.

4. The baseball according to claim 1, wherein a restitution coefficient when said baseball hits against an iron plate at a speed of 26.82 m/s is not less than 0.50 and not more than 0.55.

5. The baseball according to claim 1, wherein a load when compressing an outer diameter of said baseball by 6.35 mm is 16 lbs or more and is less than 30 lbs.

6. A baseball comprising:

an inner core;

a thread-wound layer obtained by winding a yarn to cover an outer circumferential surface of said inner core; and

an outer layer covering an outer circumferential surface of said thread-wound layer,

said inner core having a surface hardness of not less than 71 and not more than 75 in Asker C hardness, having an elastic modulus of not less than 0.9 MPa and not more than 1.0 MPa, and having a loss coefficient (tan δ) of not less than 0.25 and not more than 0.27,

said inner core being made of urethane foam,

said inner core having an outer diameter of 70.7 mm, and

said outer layer having an outer diameter of 73.2 mm.