US20090011877A1

US20090011877A1 - Bat for baseball or softball

Info

Publication number: US20090011877A1
Application number: US12/137,694
Authority: US
Inventors: Mototaka Iwata; Takashi Ono; Toshiaki Kida
Original assignee: Mizuno Corp
Current assignee: Mizuno Corp
Priority date: 2007-06-12
Filing date: 2008-06-12
Publication date: 2009-01-08

Abstract

A bat for baseball or softball includes a core member having a depression at a portion to be a ball-hitting portion, and an elastic body attached to the depression and forming the ball-hitting portion. The elastic body is formed of urethane elastomeric foam having specific gravity of at least 0.45 and at most 0.60, JIS C hardness of at least 54 and at most 82, and modulus of elasticity in 300% elongation of at least 3.85 MPa and at most 5.6 MPa.

Description

This nonprovisional application is based on Japanese Patent Application No. 2007-155234 filed on Jun. 12, 2007 and No. 2008-146083 filed on Jun. 3, 2008, with the Japan Patent Office, 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 bat for baseball or softball (hereinafter also simply referred to as a “bat”) and, specifically, to a bat having an elastic body attached to a ball-hitting portion.
2. Description of the Background Art
A bat having an elastic body covering and integrated with a depression of a ball-hitting portion to attain high durability and enabling longer ball flying distance is disclosed, for example, in Japanese Patent Laying-Open No. 2003-19236.
The bat described in the laid-open application above is mainly focused to make smaller the deformation of ball at the time of hitting. Therefore, according to the technical idea described in this reference, it would be desirable that the elastic body attached to the ball-hitting portion has lower hardness.
Simply lowering the hardness of elastic body, however; poses a problem of durability and the like. Therefore, improvement in restitution characteristic attained by the bat described in the reference is limited.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a bat having an elastic body attached to the ball-hitting portion, which ensures sufficient durability while attaining improved restitution characteristic.
The present invention provides a bat for baseball or softball, including: a core member having a depression at a portion to be a ball-hitting portion; and an elastic body attached to the depression and forming the ball-hitting portion; wherein the elastic body is formed of urethane elastomeric foam having specific gravity of about 0.45 to about 0.60, JIS C hardness of about 54 to about 82, and modulus of elasticity in 300% elongation of about 3.85 MPa to about 5.6 MPa. Specifically, the elastic body is formed of urethane elastomeric foam having relatively high hardness and relatively high elasticity.
The elastic body has a surface layer and an inner portion. Preferably, the hardness of the surface layer is higher than that of the inner portion, and difference in hardness between the surface layer and the inner portion is set to be at least 7 and at most 15 in JIS C hardness. Further, a lubricant layer may be provided between the elastic body and the core member.
The elastic body may further have a first characteristic that a hysteresis value is at least 0.15 J and at most 0.42 J, which hysteresis value is obtained by subtracting, from an integrated value of products between load and displacement when compressive load of 800N is applied at a rate of 10 mm/min. to a part of the elastic body detached from the core member, an integrated value of products between the load and displacement when the compressive load is removed at the same rate; and a second characteristic that rebound height of a weight in accordance with ASTM-D2632 is at least 121.2 mm and at most 133.6 mm.
Further, the elastic body may be formed of fine cell urethane elastomeric foam having the cell size of at least 75 μm and at most 115 μm.
The bat has a tip portion and a grip portion. The bottom portion of the depression may have a tip-end-side tapered portion positioned on the side of the tip end, a grip-end-side tapered portion positioned on the side of the grip portion, and a central portion positioned between the tip-end-side tapered portion and the grip-end-side tapered portion. In that case, preferably, outer diameter of the core member forming the central portion is made equal to or smaller than a minimum value of outer diameter of the core member forming the tip-end-side tapered portion and the grip-end-side tapered portion.
In the bat for baseball or softball in accordance with the present invention, the elastic body attached to the portion to be the ball-hitting portion is formed of urethane elastomeric foam having JIS C hardness of about 54 to about 82, and modulus of elasticity of about 3.85 MPa to about 5.6 MPa in 300% elongation and, therefore, sufficient durability and improved restitution characteristic can be attained.
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 partially sectioned plan view of the bat for baseball or softball in accordance with an embodiment of the present invention.

FIG. 2 is a cross-section of the bat for baseball or softball taken along the line II-II of FIG. 1.

FIG. 3 is a schematic illustration of the test apparatus used for restitution test of a model of ball-hitting portion.

FIG. 4 is a schematic illustration of the test apparatus used for restitution test of a bat.

FIG. 5 shows an exemplary shape of the core member.

FIG. 6 is a schematic illustration showing how to make a test piece.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, a bat 1 for baseball or softball in accordance with an embodiment of the present invention will be described with reference to FIGS. 1 to 6.
As shown in FIG. 1, bat 1 in accordance with the present embodiment mainly consists of a core member (base member) 2 having a depression 2 b 1 at a portion to be a ball-hitting portion 2 b, and an elastic body 3.
In the example shown in FIG. 1, core member 2 has a tip portion, depression 2 b 1 at the portion corresponding to the ball-hitting portion, a grip portion, and a tapered portion. The tip portion of core member 2 will be the tip portion 2 a of bat 1, depression 2 b 1 of core member 2 with elastic body 3 attached will be the ball-hitting portion 2 b of bat 1, the tapered portion of core member 2 will be a tapered portion 2 c of bat 1, and the grip portion of core member 2 will be a grip portion 2 d of bat 1. Core member may have a hollow or solid structure.
Tip portion 2 a of bat 1 refers to a portion including and around a tip end (one end) on the side of ball-hitting portion 2 b of bat 1 and, by way of example, a range of about 50 mm from the end on the side of ball-hitting portion 2 b of bat 1 corresponds to the tip portion 2 a. A cap member may be attached to tip portion 2 a.
Ball-hitting portion 2 b of bat 1 is the portion expected to actually hit the ball, and the portion in the range of 200 mm to 350 mm from the grip-side end of tip portion 2 a mentioned above to the side of grip portion 2 d corresponds to the ball-hitting portion 2 b.
FIG. 5 shows an exemplary shape of depression 2 b 1 of core member 2. As shown in FIG. 5, a bottom surface of depression 2 b 1 (outer circumferential surface of core member 2 in depression 2 b 1) has a tip-end-side tapered portion 20 positioned on the side of tip portion (2 a), a grip-end-side tapered portion 22 positioned on the side of grip portion (2 d), and a central portion 21 positioned between tip-end-side tapered portion 20 and grip-end-side tapered portion 22.
In the example shown in FIG. 5, tip-end-side tapered portion 20 has the largest outer diameter on the side of tip portion (2 a), and the outer diameter of tip-end-side tapered portion 20 becomes smaller as it comes closer to central portion 21. The central portion 21 has a constant outer diameter. Grip-end-side tapered portion 22 has the largest outer diameter on the side of grip portion (2 d), and the outer diameter of grip-end-side tapered portion 22 becomes smaller as it comes closer to central portion 21. Specifically, the outer diameter of core member 22 forming the central portion 21 is not larger than the outer diameter of core member 2 forming tip-end-side tapered portion 20 and grip-end-side tapered portion 22.
By providing tip-end-side tapered portion 20 and grip-end-side tapered portion 22 and making the outer diameter of core member 2 forming central portion 21 to be equal to or smaller than the outer diameter of core member 2 forming tip-end-side tapered portion 20 and grip-end-side tapered portion 22, elastic body 3 comes to have sufficient thickness at ball-hitting portion 2 b. This also contributes to improved restitution characteristic of bat 1, as will be described later.
Further, at ball-hitting portion 2 b, a region where elastic body 3 is thick can be increased. Consequently, ball flying distance is not much decreased even when the ball is hit at a portion other than the sweet spot. Particularly, even when the ball is hit at a portion closer to the side of grip portion 2 d from the sweet spot, decrease in ball flying distance can significantly be reduced.
Length L1 of tip-end-side tapered portion 20, length L2 of central portion 21 and length L3 of grip-end-side tapered portion 22 may be set appropriately. By way of example, length L1 may be about 100 to about 110 mm, L2 may be about 30 to about 40 mm and L3 may be about 100 to about 110 mm. In this manner, length L1 of tip-end-side tapered portion 20 and length L3 of grip-end-side tapered portion 22 may be made longer than length L2 of central portion 21.
Tapered portion 2 c of bat 1 is positioned between ball-hitting portion 2 b and grip portion 2 d, and its outer diameter becomes gradually smaller from ball-hitting portion 2 b to grip portion 2 d. By way of example, the portion of about 150 mm to about 450 mm extending from the grip-side end portion of ball-hitting portion 2 b to the side of grip portion 2 d corresponds to the tapered portion 2 c.
Grip portion 2 d of bat 1 is the portion to be gripped by the batter, and the portion extending from the grip-side end of tapered portion 2 c to the other end (grip end) of bat 1 corresponds to the grip portion 2 d. Typically, the grip end is formed to have increased outer diameter. The grip end may be formed integrally with core member 2, or it may be formed separate from core member 2 and then attached to core member 2.
Any material usable for a bat, such as wood, metal or fiber-reinforced plastic may be used for core member 2 described above. In the present embodiment, it is formed of fiber-reinforced plastic.
As shown in FIGS. 1 and 2, elastic body 3 is attached to depression 2 b 1 provided at the portion to be ball-hitting portion 2 b of core member 2 described above. Elastic body 3 may be attached to depression 2 b 1 of ball-hitting portion 2 b by means of an adhesive tape, adhesive or the like. Elastic body 3 typically has a cylindrical shape, though any other shape may be adopted. Though it is preferred to form elastic body 3 by a single material, elastic body 3 may be formed by a plurality of different materials. Further, elastic body 3 may be formed by combining a plurality of parts. Preferably, the inner diameter of elastic body 3 is made smaller than the outer diameter of depression 2 b 1 of core member 2. Consequently, when elastic body 3 is put on depression 2 b 1 of core member 2, tension generates in elastic body 3, whereby positional deviation or slip of elastic body 3 can be prevented, and wrinkles on the surface of elastic body 3 can also be prevented.
In the present embodiment, elastic body 3 attached to depression 2 b 1 of core member 2 is formed of urethane elastomeric foam having “JIS C hardness” of about 54 to about 82 and “modulus of elasticity in 300% elongation” of about 3.85 MPa to about 5.6 MPa (modulus of elasticity in 100% elongation of about 1.84 MPa to about 2.03 MPa). Specifically, both “JIS C hardness” and “modulus of elasticity in 300% elongation” are maintained at relatively high values, so as to realize both good restitution characteristic and high durability. Elongation of elastic body 3 is, preferably, about 290 to about 400.
Here, “modulus of elasticity in 300% elongation” means modulus of elasticity (tensile stress) when elastic body 3 is stretched 300%. Further, “modulus of elasticity in 100% elongation” means modulus of elasticity (tensile stress) when elastic body 3 is stretched 100%. “JIS C hardness” refers to hardness measured in accordance with spring hardness test type C of JIS K 7312, Appendix 2.
It is preferred that elastic body 3 described above also has the following characteristics. Specifically, it preferably has hysteresis value of at least 0.15 J and at most 0.42 J, which hysteresis value is obtained by subtracting, from an integrated value of products between load and displacement (amount of deformation of elastic body 3 at the time of compression) when compressive load of 800N is applied at a rate of 10 mm/min. to a part of elastic body 3 detached from core member 2, an integrated value of products between the load and displacement (amount of deformation of elastic body 3 with the load removed) when the compressive load is removed at the same rate.
Here, “hysteresis” above refers to the difference between the work exerted on the test piece when the load is applied and the work returned from the test piece when the load is removed, on a stress-strain curve. Therefore, the “hysteresis” value can be calculated by (energy value (E1) in load increasing range)−(energy value (E2) in load decreasing range). The energy value is obtained by integrating the values of load×displacement (amount of deformation) during the test.
Next, the method of measuring the hysteresis value will be described. A test piece for measuring the hysteresis value having the size of 25 mm (width)×90 mm (length)×20 mm (thickness) is prepared. The test piece is formed by the following manner.
First, elastic body 3 is detached from core member 2, and a part of the cylindrical elastic body 3 having the circumferential length of L4 is cut out, as shown in FIG. 6. Thus, a curved rectangular piece of elastic body 3 is obtained. Here, the circumferential length L4 is set to about 30 mm. A part of the cut out piece of elastic body 3 is cut to the size of 90 mm×25 mm. Thereafter, thickness of elastic body 3 is adjusted by scraping or the like. In this manner, the test piece having the size of 25 mm (width)×90 mm (length)×20 mm (thickness) is prepared. A test piece having the above-described size may be formed by laminating a plurality of elastic bodies 3.
After the test piece is prepared in the above-described manner, compression test is conducted using Autograph AG-5000G (product name: manufactured by Shimadzu Corporation). Compression rate is 10 mm/s, and load of up to 800N is applied. Thereafter, the load on the test piece is removed by returning at the same rate. Based on the load/displacement curve at this time, the hysteresis value is calculated in accordance with the equation above.
The inventors of the present invention calculated the hysteresis value of a test piece having the above-described size and specific gravity: 0.45, JIS C hardness of surface layer: 69 and modulus of elasticity in 300% elongation: 3.85 MPa (modulus of elasticity in 100% elongation: 1.84 MPa), which hysteresis value was 0.412 J. Similarly, the hysteresis value of a test piece having the above-described size and specific gravity: 0.55, JIS C hardness of surface layer: 78 and modulus of elasticity in 300% elongation: 4.1 MPa (modulus of elasticity in 100% elongation: 1.93 MPa) was calculated, and it was 0.307 J. Further, the hysteresis value of a test piece having the above-described size and specific gravity: 0.6, JIS C hardness of surface layer: 82 and modulus of elasticity in 300% elongation: 5.6 MPa (modulus of elasticity in 100% elongation: 2.03 MPa) was calculated, and it was 0.151 J.
The hysteresis value of a test piece as a conventional example, having the above-described size and specific gravity: 0.35, JIS C hardness of surface layer: 43 and modulus of elasticity in 300% elongation: 1.23 MPa (modulus of elasticity in 100% elongation: 0.54 MPa) was 1.067. Therefore, it is considered that use of elastic body 3 having the characteristics of respective test pieces described above can reduce energy loss before and after hitting the ball, as compared with the conventional example.
Further, it is preferred that elastic body 3 has such a characteristic that rebound height of a weight in accordance with ASTM-D2632 (for example, rebound height of a weight of 27.9 g fallen on a part of elastic body having the thickness of 20 mm detached from core member 2) is at least 121.2 mm and at most 133.6 mm.
The rebound height may be measured by the following drop test (simple rebound test). The test piece has the size of 25 mm (width)×90 mm (length)×20 mm (thickness) and it is prepared by the method described above.
For the test, a tester “Vertical Rebound Resilience Tester” (manufacture by Gotech, part number: GT-7042-V) is used, and the test is performed in accordance with ASTM-D2632 (Standard Test Method for Rubber Property-Resilience by Vertical Rebound). The above-described rebound height can be measured in this manner.
The inventors of the present invention measured the rebound height of test piece having the size mentioned above and specific gravity: 0.45, JIS C hardness of surface layer: 69 and modulus of elasticity in 300% elongation: 3.85 MPa (modulus of elasticity in 100% elongation: 1.84 MPa), and it was 30.3 mm. Similarly, the rebound height of test piece having the size mentioned above and specific gravity: 0.55, JIS C hardness of surface layer: 78 and modulus of elasticity in 300% elongation: 4.1 MPa (modulus of elasticity in 100% elongation: 1.93 MPa) was measured, and it was 31.7 mm. Further, the rebound height of test piece having the size mentioned above and specific gravity: 0.6, JIS C hardness of surface layer: 82 and modulus of elasticity in 300% elongation: 5.6 MPa (modulus of elasticity in 100% elongation: 2.03 MPa) was measured, and it was 33.4 mm.
The rebound height of a conventional example having the size mentioned above and specific gravity: 0.35, JIS C hardness of surface layer: 43 and modulus of elasticity in 300% elongation: 1.23 MPa (modulus of elasticity in 100% elongation: 0.54 MPa) is 11.3. From these results, it is considered that use of elastic body 3 having the characteristic as represented by each of the test pieces realizes improved restitution characteristic as compared with the conventional example. The value of JIS C hardness described above represents a mean value of 5 measurements (5 points).
Specific gravity of elastic body 3 is at least 0.45 and at most 0.60. The reason is as follows. If the specific gravity is smaller than 0.45, larger number of voids would be generated and molding becomes difficult. If the specific gravity is larger than 0.6, overpack occurs, and air generated from the material cannot well escape, resulting in rough surface of elastic body. Further, when the elastic body 3 in the overpack state is covered by a TPU sheet, peeling is likely.
In elastic body 3 used in the present embodiment, JIS C hardness and the modulus of elasticity of elastic body 3 are in proportion to the specific gravity of elastic body 3. When the specific gravity of elastic body 3 is set to be at least 0.45 and at most 0.60, JIS C hardness of elastic body 3 becomes at least 54 and at most 82, and modulus of elasticity in 300% elongation becomes at least 3.85 MPa and at most 5.6 MPa (modulus of elasticity in 100% elongation becomes at least 1.84 MPa and at most 2.03 MPa).
Prior to the present invention, various research and developments have been made mainly focused on lowering the hardness of elastic body for a bat having the elastic body attached to the ball-hitting portion, as described in Japanese Patent Laying-Open No. 2003-19236. If the hardness of elastic body is decreased, durability of the elastic body degrades and, in addition, manufacturing of the elastic body becomes difficult. Therefore, further improvement in restitution characteristic has been considered difficult.
In the course of various trials to improve durability of the elastic body, the inventors of the present invention found that in some cases, durability could be improved and as a result, restitution characteristic could also be improved if the hardness of an elastic body was set in a relatively high range. Specifically, it has been found that rather than simply lowering the hardness of elastic body to reduce deformation of the ball at the time of hitting, setting the hardness of elastic body to a relatively high range is effective to fully utilize the elasticity of the elastic body and to curb deformation of the ball at the time of hitting and, by setting the hardness of elastic body to a relatively high range, restitution characteristic can also be improved. The above-described effects can be particularly well achieved when fine cell urethane elastomeric foam having the cell size of at least 75 μm and at most 115 μm is used as the elastic body 3.
The inventors conducted the following test to verify the effect described above. The results will be described.
First, five different urethane elastomeric foam samples (samples A to E: elastic body 3) having specific gravity of 0.45, JIS C hardness at front and rear surface layers of 70 to 74, JIS C hardness of the layer between the surfaces of 54 to 60, and modulus of elasticity in 300% elongation of 3.85 MPa (modulus of elasticity in 100% elongation of 1.84 MPa) and different thicknesses were prepared and coefficient of restitution after the samples were hit by a ball was measured. The results are as shown in Table 1.

	TABLE 1

	JIS C Hardness		Speed of

	Thickness	Outer	Inner	Specific	Test	incidence	Restitution
Type	(mm)	circumference	circumference	gravity	times	(km/h)	coefficient

Iron plate	—	—	—	—	4	147	0.415
Sample A	10	70-74	54-60	0.45	3	147	0.445
Sample B	12.5			0.45	5	147	0.463
Sample C	15			0.45	5	147	0.490
Sample D	20			0.45	4	147	0.517
Sample E	30			0.45	3	147	0.599

The device and method used for the test will be described with reference to FIG. 3. As shown in FIG. 3, the test device includes a pitching machine 8, a high-speed video camera 6 and an image analyzer (not shown). Ball-hitting portion models 22 as a hit target of a ball 5, one formed of iron plate only, and others formed of iron plate with urethane elastomeric foam adhered thereon, were provided. Ball 5 was fed from pitching machine 8 to impact on fixed ball-hitting portion model 9. High-speed video camera 6 recorded the impact from the side. Then, restitution coefficient was calculated from the speed of ball 5 before and after the impact.
Samples A to E used as ball-hitting portion models 9 are square sheets of 15 cm on a side, having thicknesses of 10 mm, 12.5 mm, 15 mm, 20 mm and 30 mm, respectively. In the test, a rubber ball was used, and the restitution test was performed by causing the rubber ball hit each ball-hitting portion model 9 at the incident speed of 147 km/h.
Next, the method of calculating coefficient of restitution e will be described. The coefficient of restitution e is calculated by using Equation 1 below. Characters used in the equation are also described.
e=V·BALL(OUT)/V·BALL(IN)
e: coefficient of restitution
V·BALL(OUT): barycentric velocity after hitting (m/s)
V·BALL(IN): barycentric velocity before hitting (m/s).
The barycentric velocity of the ball in the equation can be obtained by analyzing the image recorded by the high-speed video camera using the image analyzer.
As can be seen from test results shown in Table 1, coefficients of restitution of samples A to E are much better than that of the iron plate. Further, it can be seen that the coefficient of restitution improves as the thickness increases from 10 mm to 30 mm.
Similar measurements of coefficient of restitution were made on samples F, G and H having different specific gravities (0.45 to 0.6). The results are as shown in Table 2.

TABLE 2

				Speed of
	Thickness	Specific	Test	incidence	Restitution
Type	(mm)	gravity	times	(km/h)	coefficient

Iron plate	—	—	3	161	0.413
Sample F	11	0.45	4	158	0.447
Sample G	11	0.55	4	157	0.430
Sample H	11	0.60	4	162	0.421

If we compare coefficients of restitution of respective samples (elastic bodies) in Table 2, it can be understood that one having smaller specific gravity exhibits superior restitution characteristic. From the results shown in Tables 1 and 2, it is considered that when specific gravity of elastic body 3 is set to about at least 0.45 and about at most 0.60, elastic body 3 having superior restitution characteristic is obtained and, if an elastic body 3 having lower specific gravity (for example, about 0.45 to about 0.55) is used, the restitution characteristic can more effectively be improved.
Sample F having specific gravity of 0.45 had JIS C hardness at front and rear surface layers of 69, JIS C hardness of the layer between the surfaces of 54, and modulus of elasticity in 300% elongation of 3.85 MPa (modulus of elasticity in 100% elongation of 1.84 MPa). Sample G having specific gravity of 0.55 had JIS C hardness at surface layers of 78, inside JIS C hardness of 65, and modulus of elasticity in 300% elongation of 4.1 MPa (modulus of elasticity in 100% elongation of 1.93 MPa). Sample H having specific gravity of 0.60 had JIS C hardness at surface layers of 82, inside JIS C hardness of 75, and modulus of elasticity in 300% elongation of 5.6 MPa (modulus of elasticity in 100% elongation of 2.03 MPa).
From the test results above, it is considered that by forming the elastic body 3 using urethane elastomeric foam having JIS C hardness of about 54 to about 82 and modulus of elasticity in 300% elongation of about 3.85 to about 5.6 MPa (modulus of elasticity in 100% elongation of about 1.84 MPa to about 2.03 MPa), a bat having superior restitution characteristic can be provided. Further, the samples described above all have the relatively high hardness of about 50 or higher in JIS C hardness and, therefore, durability is also improved.
As to the JIS C hardness at the outer and inner circumferential portions of elastic body 3, it is preferred that the surface layers on the inner and outer circumferences (skin layer) have JIS C hardness of about 69 to about 82, and the inner portion between the surface layers has JIS C hardness of about 54 to about 75.
Considering durability, it is possible to form elastic body 3 of urethane elastomeric foam having JIS C hardness of about 65 to about 82, and modulus of elasticity in 300% elongation of about 4.10 to about 5.6 MPa (modulus of elasticity in 100% elongation of about 1.93 MPa to about 2.03 MPa).
As shown in FIG. 2, elastic body 3 has surface layers (skin layers) 3 a on inner and outer circumferences, and an inner portion 3 b between the surface layers 3 a. It is preferred to make higher the hardness of surface layer 3 a (hardness of outer circumference) than that of inner portion 3 b, and to make the difference between the surface layer 3 a and inner portion 3 b in JIS C hardness to be about 7 to about 15. More preferably, the difference is made about 13 to about 15. Large difference in hardness as such realizes both good restitution characteristic and good durability.
In FIG. 2, for convenience of description, borders between the surface layers 3 a on the inner and outer circumferences and the inner portion 3 b are denoted by dotted lines. Actually, however, there is no clear border between the surface and inner portions.
Further, by setting the hardness and the like of elastic body 3 relatively high, it becomes possible to provide a lubricant layer 10 between elastic body 3 and core member 2. This facilitates attachment/detachment of elastic body 3, and elastic body 3 can be exchanged easily. A layer of silicone-based releasing agent, for example, may be used as lubricant layer 10. It is noted, however, that any layer other than the layer of silicone-based releasing agent may be used, provided that the layer has the lubricating function. In the present embodiment, lubricant layer 10 may be formed locally, or it may be formed entirely on the surface of depression 2 b 1 of core member 2.

Example 1

Next, Examples 1 and 2 of bat 1 in accordance with the present invention will be described with reference to Table 3 below. Table 3 shows specifications of the bats in accordance with Examples 1 and 2 as well as Comparative Examples 1 and 2. Further, Table 3 also shows coefficients of restitution obtained by rebound test on respective bats.

TABLE 3

	Overall		Center of		Center of
	Length	Mass	gravity	Inertia moment	impact	Restitution
Type	(mm)	(g)	(mm)	(kg · cm · s²)	(mm)	Coefficient

Example 1	840	715	511	1.70	691	0.534
Example 2	840	720	515	1.70	681	0.546
Comparative	840	712	513	1.71	694	0.520
Example 1
Comparative	840	708	512	1.69	692	0.487
Example 2

As the core member 2 of bats as Examples 1 and 2 of Table 3, fiber reinforced plastics (FRP) is used. Elastic body 3 attached to the bat of Example 1 is formed of urethane elastomeric foam having thickness of 10 mm, specific gravity of 0.45, JIS C hardness at outer circumferential surface layer of 69, JIS C hardness of the layer between the inner and outer circumferential surfaces of 54, and modulus of elasticity in 300% elongation of 3.85 MPa (modulus of elasticity in 100% elongation of 1.84 MPa). Elastic body 3 attached to the bat of Example 2 is formed of urethane elastomeric foam having thickness of 12.5 mm, specific gravity of 0.45, JIS C hardness at outer circumferential surface layer of 69, JIS C hardness of the layer between the inner and outer circumferential surfaces of 54, and modulus of elasticity in 300% elongation of 3.85 MPa (modulus of elasticity in 100% elongation of 1.84 MPa). Elastic body 3 attached to the bat of Comparative Example 1 is the one used for an existing model, formed of foamed polyurethane having thickness of 12.5 mm, specific gravity of 0.35, JIS C hardness at surface layer of 43, inner JIS C hardness between the surfaces of 38, and modulus of elasticity in 100% elongation of 0.54. The bat of Comparative Example 2 is a metal bat of 7064 aluminum alloy.
The method of rebound test conducted on the bats shown in Table 3 will be described with reference to FIG. 4.
As shown in FIG. 4, the experiment device includes a pitching machine 8, a high-speed video camera 6, an image analyzer (not shown) and a bat stand 7.
In the present test, a ball (rubber ball) 5 fed from pitching machine 8 is caused to hit the ball-hitting portion of bat 1 placed on bat stand 7 at 130 km/h. High-speed video camera 6 recorded the impact from above, and restitution coefficient was calculated from the speed of ball 5 before and after the impact and the speed of bat 1. The coefficient of restitution e is calculated by the following equation.
e=−{VBL(OUT)−(VBT(PAL)+a·VBT(ROT))}/VBL(IN)

- e: coefficient of restitution
- V BL(OUT): barycentric velocity of ball after impact (m/s)
- V BL(PAL): translational velocity of bat after impact (m/s)
- a: distance from center of gravity of bat to ball impact position (grip side being positive)
- V BT(ROT): angular velocity around center of gravity of bat after impact (rad/s)
- V BL(IN): barycentric velocity of ball before impact (m/s)

It can be seen from Table 3 that restitution characteristics of the bats in accordance with Examples 1 and 2 are superior to those of Comparative Examples 1 and 2. Specifically, using the bat in accordance with the present examples, restitution characteristics can be improved than the conventional products including existing models.

Example 2

Next, using Table 4 below, Examples 3 and 4 of bat 1 in accordance with the present invention will be described. Table 4 shows specifications of the bats in accordance with Examples 3 and 4 of the present invention as well as Comparative Example 3. Further, Table 3 also shows coefficients of restitution obtained by rebound test on respective bats.

TABLE 4

	Overall		Center of	Inertia	Center of	Restitution
	Length	Mass	gravity	moment	impact	Coefficient
Type	(mm)	(g)	(mm)	(kg · cm · s²)	(mm)	(rubber soft)

Example 3	827	720	523	1.73	680	0.604
Example 4	827	728	522	1.74	679	0.604
Comparative	840	722	482	1.59	690	0.516
Example 3

As the core member 2 of bats as Examples 3 and 4 of Table 4, fiber reinforced plastics is used. Further, elastic body 3 attached to the bat of Example 3 is of the same type as Example 1, and elastic body 3 attached to the bat of Example 4 is formed of urethane elastomeric foam having thickness of 15 mm, specific gravity of 0.45, JIS C hardness at outer circumferential surface layer of 69, and modulus of elasticity in 300% elongation of 3.85 MPa (modulus of elasticity in 100% elongation of 1.84 MPa). The bat of Comparative Example 3 is formed of 7050 aluminum alloy, of which thickness at the ball-hitting portion is 1.7 mm.
It can be seen from Table 4 that restitution characteristics of bats in accordance with Examples 3 and 4 are superior to that of Comparative Example 3.
Table 5 below shows results of physical property test of three different elastic bodies 3 (elastic bodies I, II and III) having the thickness of 20 mm. Table 5 also shows coefficients of restitution when the test of FIG. 3 was conducted with the elastic bodies 3 are adhered on iron plates. Further, results of physical test and coefficients of restitution of Comparative Examples 4 to 9 are also shown. Comparative Example 4 corresponds to an existing model formed of soft polyurethane material, Comparative Example 5 is formed of urethane A, Comparative Example 6 is formed of urethane B, Comparative Example 7 is formed of urethane C, Comparative Example 8 is formed of NR (natural rubber), and Comparative Example 9 is formed of NBR (Nitnile Butadiene Rubber).

TABLE 5

		Compression	Drop test rebound	Surface				Rubber ball
	Thickness	test	height (corrected	C	Specific	100%	300%	restitution
Type	(mm)	hysteresis (J)	value: mm)	hardness	gravity	modulus	modulus	coefficient

Elastic body I	20	0.412	121.2	69	0.45	1.84	3.85	0.517
Elastic body II	20	0.307	126.8	78	0.55	1.93	4.10	0.498
Elastic body III	20	0.151	133.6	82	0.60	2.03	5.60	0.485
Comparative	20	1.067	45.2	43	0.35	0.54	1.23	0.48
Example 4
Comparative	20	1.062	84.0	61	0.40	1.04	2.08	0.455
Example 5
Comparative	20	1.335	71.6	52	0.36	0.89	1.94	0.464
example 6
Comparative	20	0.553	84.8	60	0.40	1.11	2.39	0.452
example 7
Comparative	20	0.093	80.4	74	1.52	2.24	No	0.423
example 8							stretch
Comparative
	20	0.263	76.0	59	1.24	1.19	4.04	0.429
example 9

It can be seen from Table 5 that restitution characteristics of elastic bodies I to III are superior to those of Comparative Examples.
Further, inventors of the present invention also conducted durability test of actual hitting, on the bat as an example of the present invention and on a bat of existing model using polyurethane material as the elastic body. In the test, a rubber ball fed at the speed of 100 to 100 km/h was hit at the swing speed of 100 to 120 km/h, and the number of ball hits until the ball hitting portion was broken was counted. As a result, it was found that the bat in accordance with the present embodiment had durability 1.7 times higher than the bat of existing model.
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 bat for baseball or softball, comprising:

a core member having a depression at a portion to be a ball-hitting portion; and

an elastic body attached to said depression and forming said ball-hitting portion; wherein

said elastic body is formed of urethane elastomeric foam having specific gravity of at least 0.45 and at most 0.60, JIS C hardness of at least 54 and at most 82, and modulus of elasticity in 300% elongation of at least 3.85 MPa and at most 5.6 MPa.

2. The bat for baseball or softball according to claim 1, wherein

said elastic body has a surface layer and an inner portion; and

hardness of said surface layer is higher than that of said inner portion, and difference in hardness between said surface layer and said inner portion is set to be at least 7 and at most 15 in JIS C hardness.

3. The bat for baseball or softball according to claim 1, comprising

a lubricant layer provided between said elastic body and said core member.

4. The bat for baseball or softball according to claim 1, wherein

said elastic body has

a first characteristic that a hysteresis value is at least 0.15 J and at most 0.42 J, which hysteresis value is obtained by subtracting, from an integrated value of products between load and displacement when compressive load of 800N is applied at a rate of 10 mm/min. to a part of said elastic body detached from said core member, an integrated value of products between the load and displacement when the compressive load is removed at the same rate; and

a second characteristic that rebound height of a weight in accordance with ASTM-D2632 is at least 121.2 mm and at most 133.6 mm.

5. The bat for baseball or softball according to claim 1, wherein

said elastic body is formed of fine cell urethane elastomeric foam having the cell size of at least 75 μm and at most 115 μm.

6. The bat for baseball or softball according to claim 1, comprising

a tip portion and a grip portion; wherein

a bottom portion of said depression has a tip-end-side tapered portion positioned on the side of said tip end, a grip-end-side tapered portion positioned on the side of said grip portion, and a central portion positioned between said tip-end-side tapered portion and said grip-end-side tapered portion; and

outer diameter of said core member forming said central portion is made equal to or smaller than a minimum value of outer diameter of said core member forming said tip-end-side tapered portion and said grip-end-side tapered portion.