Three-piece golf ball

Abstract

A golf ball according to the present invention is excellent in carry while holding spin performance and shot feeling.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a three-piece golf ball excellent in shot feeling and spin performance and further improved in carry.

[0003] 2. Description of the Background Art

[0004] Among characteristics of a golf ball, shot feeling, spin performance and a carry are most important. The shot feeling includes an impact applied to a player hitting the ball with a driver and a sound resulting from the impact. The term “spin performance” denotes capability of the golf ball to quickly stop on the green by backspin and capability of causing sidespin for controlling the trajectory to be drawn or faded. A pro golfer or a low handicapper prefers a golf ball having a large spin rate due to such excellent controllability. The term “carry” denotes the distance between the point where the player hits the ball with the driver and the point where the golf ball comes to a standstill. This characteristic is important for both of a beginner and a pro golfer.

[0005] In order to improve these characteristics, the structure of a conventional golf ball, the types or physical properties of the materials for the members forming the golf ball etc. are adjusted for optimizing the characteristics.

[0006] In general, a solid golf ball formed by a solid core coated with a cover is excellent in carry but inferior in spin performance and limited in controllability. On the other hand, the so-called three-piece golf ball formed by winding rubber thread on a center and thereafter coating the same with a cover is excellent in shot feeling and spin performance but limited in carry.

[0007] As to the carry of a golf ball, it is known that the carry relates to the primary natural frequency of the golf ball (FB) and the primary natural frequency of the head of a golf club (FK). If these primary natural frequencies FB and FK are approximate to each other, high resilience is obtained when hitting the golf ball with the golf club due to mechanical impedance matching.

[0008] The primary natural frequency (FB) of a commercially available golf ball is about 600 to 1600 Hz. On the other hand, the primary natural frequency (FK) of a recently mainstreaming club head made of a titanium alloy having excellent carry performance is about 1400 to 1800 Hz approaching that of the golf ball. In order to increase the carry of the golf ball under such circumstances, the primary natural frequency of the golf club (FK) may be reduced or the primary natural frequency of the golf ball (FB) may be increased. If the primary natural frequency of the golf ball (FB) is increased, however, the golf ball is so hardened that the same applies a large impact when hit although the carry is increased in this case. A golf ball having a primary natural frequency (FB) of at least 1000 Hz currently put on the market provides an excellent carry but also presents a large impact and hard shot feeling when the same is hit.

[0009] U.S. Pat. No. 6,123,629 proposes a technique related to a solid golf ball, which aims at improving the carry and the shot feeling. The feature of this technique resides in that, in a golf ball consisting of a solid center and a cover for the solid center, the ratio (CF1/BF1) of the primary natural frequency of the solid center (CF1) to the primary natural frequency of the golf ball (BF1) satisfies the following expression:

0.95≦CF1/BF1≦1.10

[0010] However, the golf ball disclosed in this prior art document is a solid golf ball, which is limited in improvement of shot feeling and spin performance.

[0011] On the other hand, there is proposed a three-piece golf ball presenting an excellent carry while holding spin performance and shot feeling. For example, Japanese Patent Laying-Open No. 7-313630 discloses a three-piece golf ball having a center of 30 to 35 mm in diameter with a quantity of distortion of 1.2 to 2.5 mm from application of an initial load of 10 kg up to application of a final load of 30 kg to the aforementioned center. According to this technique, thermoplastic resin is blended into a rubber composition forming the solid center, deformation under load is reduced by a cross-linking agent and the diameter of the solid center is set in the prescribed range thereby improving the carry while maintaining excellent shot feeling intrinsically provided in the three-piece golf ball.

[0012] U.S. patent Publication No. US2002/0052250A1 discloses such a technique that compressive deformation of a solid center having a diameter of 35 to 38 mm exhibits compressive deformation of at least 0.5 mm and less than 1.5 mm from application of an initial load of 9.8N up to application of a final load of 294N, Shore D hardness (CD) of a cover is 40 to 55 and a prescribed relation holds between a total dimple volume VT (cm3) indicating the total of the volumes of portions enclosed with cavities of dimples and surfaces connecting openings of the dimples with each other and the voltage VG (cm3) of a golf ball assumed to be a spherical body having absolutely no dimples on its surface.

[0013] In each of these conventional three-piece golf balls, however, none of cross-link density of the center, the modulus and the elongation percentage of rubber thread and the like are taken into consideration, to result in a low ratio (FC/FB) between the primary natural frequencies (FC) and the center and the golf ball. Therefore, these three-piece golf balls are not sufficiently satisfiable in shot feeling, spin performance (controllability) and carry in total.

SUMMARY OF THE INVENTION

[0014] The inventor has recognized that spin performance, shot feeling and a carry of a three-piece golf ball can be totally improved by approaching the ratio (FC/FB) of the primary natural frequency of the center (FC) of the three-piece golf ball to the primary natural frequency of the three-piece golf ball (FB) to 1.0. An object of the present invention is to provide a three-piece golf ball wholly excellent in spin performance, shot feeling and carry on the basis of this recognition.

[0015] The inventive three-piece golf ball comprises a solid center, rubber thread wound on the solid center and a cover for the solid center and the rubber thread, while the ratio (FC/FB) of the primary natural frequency of the solid center (FC) to the primary natural frequency of the golf ball (FB) satisfies the following expression:

0.95≦FC/FB≦1.10

[0016] According to the present invention, the primary natural frequency of the solid center (FC) is preferably 750 to 1000 Hz. When deformation (mm) from application of a load of 98N to the solid center up to application of a load of 1274N is set to 2.7 to 4.3, spin performance, shot feeling and a carry can be totally improved in a well-balanced manner.

[0017] 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

[0018] FIG. 1 is a sectional view of a three-piece golf ball according to the present invention; and

[0019] FIG. 2 schematically illustrates a measuring apparatus for primary natural frequencies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] (Ball Structure)

[0021] The present invention is directed to a three-piece golf ball prepared by winding rubber thread on a solid center for forming a rubber thread layer and coating the rubber thread layer with a cover. The solid center may be a single-layer center or a multiple center having at least two layers. The cover is not limited to a single-layer cover either but may be formed by a multiple cover having at least two layers. Referring to FIG. 1, a golf ball 1 according to the present invention comprises a solid center 2, a rubber thread layer 3 formed by winding rubber thread on the solid center 2 and a cover 4 for the rubber thread layer 3. Dimples 5 are formed on the surface of the cover 4 in prescribed arrangement.

[0022] (Definition of Primary Natural Frequency)

[0023] The term “the primary natural frequency of the solid center (FC) of the golf ball” denotes the primary natural frequency of the solid center alone. When the solid center is formed by a multiple center having at least two layers, this term denotes the primary natural frequency of the overall multiple center. The term “the primary natural frequency of the golf ball (FB)” denotes the primary natural frequency of the overall golf ball.

[0024] The outline of a method of measuring the primary natural frequencies of the solid center (FC) and the golf ball (FB) is described with reference to FIG. 2. While FIG. 2 shows a method of measuring the primary natural frequency of the golf ball (FB), the primary natural frequency of the solid center (FC) can be measured by the same method.

[0025] (a) A golf ball G is polished to have a flat circular shape of 10 mm in diameter, and the flat circular portion thereof is fixed onto a support part 17b of a mount 17a of an exciter 17 with an instant adhesive.

[0026] (b) An acceleration pickup 19 is mounted on the lower side of the mount 17a of the exciter 17.

[0027] (c) The vibrational velocity V of the golf ball G is measured through a reflecting tape 20 arranged on the upper surface of the golf ball G, a laser application part 14, a manipulator 12 and a laser Doppler velocimeter 11 while vibrating the golf ball G with the exciter 17. The principle of a well-known laser Doppler vibrometer is employed for this measurement. The reflecting tape 20, prepared by Scotch Light reflecting tape by Sumitomo 3M Ltd., of about 5 mm square is bonded to the golf ball G while directing the reflecting surface toward the laser application part 14.

[0028] (d) A power amplifier 15 amplifies a voltage signal output from the acceleration pickup 19, and an FFT analyzer 13 thereafter incorporates acceleration A. The FFT analyzer 13 also incorporates the measured vibrational velocity V from the laser Doppler velocimeter 11.

[0029] (e) The FFT analyzer 13 obtains a frequency transfer function G(s) from the acceleration A and the velocity V measured in the frequency domain of 100 to 4000 Hz according to the following equation:

G (s)=Fourier transform of output velocity V/Fourier transform of input acceleration A

[0030] (f) A frequency exhibiting the maximum peak value in the frequency range of 100 Hz to 2000 Hz is read from the aforementioned frequency transfer function G(s) as the frequency having the maximum value and regarded as the primary natural frequency. The peak is detected in the frequency range of 100 Hz to 2000 Hz since there is a possibility that a secondary natural frequency is detected if the frequency exceeds 2000 Hz. Referring to FIG. 2, an exciter amplifier 16 for controlling the amplitude of vibration of the exciter 17 has a function of amplifying the voltage signal output from the FFT analyzer 13.

[0031] Table 1 shows the specifications of various measuring apparatuses employed for the aforementioned measurement.

TABLE 1
Measuring Apparatus for FB and FC
Measuring Apparatus Name of Maker and Model
Laser Velocimeter   by DANTEC
                    TRACKER MAIN UNIT TYPE55 N21
Manipulator         by DANTEC
                    60X24
FFT Analyzer        by HEWLETT PACKARD
                    DYNAMIC SIGNAL ANALYZER
                    3562A
Power Amplifier     by PCB PIEZOTRONICS Inc.
                    MODEL 482A18
Exciter Amplifier   by SHIN NIHON SOKUKI K.K.
                    POWER AMPLIFER TYPE 360-B
Exciter             by SHIN NIHON SOKUKI K.K.
                    513-A
Acceleration Pickup by PCB PIEZOTRONICS Inc.
                    MODEL 352B22

[0032] (Optimum Range of Ratio FC/FB)

[0033] In the golf ball according to the present invention, the ratio (FC/FB) of the primary natural frequency FC of the center to the primary natural frequency FB of the overall golf ball is at least 0.95 and not more than 1.10, preferably at least 0.95 and not more than 1.05, more preferably at least 0.95 and not more than 1.02, and particularly at least 0.99 and not more than 1.02. In this range of the ratio FC/FB, the golf ball is most excellent in spin performance, shot feeling and carry in total. This condition can be achieved by adjusting the types of the materials for the center, the type of blending thereof and hardness and the diameter etc. of the center and further adjusting the thickness of the rubber thread layer and the modulus of the rubber thread as well as the material, blending, hardness and the thickness of the cover.

[0034] (Solid Center)

[0035] The primary natural frequency of the solid center (FC) used for the inventive three-piece golf ball is preferably set in the range of 750 to 1000 Hz, preferably in the range of 800 to 950 Hz, more preferably in the range of 800 to 920 Hz and particularly preferably in the range of 805 to 910 Hz. If the primary natural frequency of the solid center is less than 750 Hz, a sound (hereinafter referred to as “impact sound) resulting from an operation of hitting the golf ball with a driver is excessively reduced while the carry is also reduced. If the primary natural frequency exceeds 1000 Hz, the impact sound unpreferably presents hard feeling.

[0036] Deformation HB (mm) from application of a load of 98N to the solid center up to application of a load of 1274N is in the range of 2.7 to 4.3, more preferably in the range of 3.0 to 4.1 and particularly preferably in the range of 3.3 to 4.4. The golf ball is extremely deformed to reduce the carry if the deformation is in excess of 4.3, while shot feeling of the golf ball is unpreferably hardened if the deformation is less than 2.7.

[0037] The diameter of the overall golf ball is defined as at least 1.68 inches (42.67 mm) according to R & D standards. The diameter of a golf ball generally put on the market is in the range of 1.680 inches (42.67 mm) to 1.686 inches (42.82 mm). In order to improve resilience resilience, shot feeling and spin performance of the three-piece golf ball, the diameter of the solid center is preferably 35 to 38 mm. In the case of a multi-piece center having at least two layers, the thickness of each layer of the solid center can be arbitrarily adjusted if the diameter of the overall center is 35 to 38 mm. Resilience of the golf ball is reduced to result in an insufficient carry if the diameter of the solid center is less than 35 mm, while the thickness of the rubber thread layer is insufficient to result in a disadvantage for the resilience if the diameter of the solid center exceeds 38 mm. The primary natural frequency FC is increased when the diameter of the solid center is reduced, while the former is reduced when the latter is increased.

[0038] According to the present invention, a rubber composition for the solid center preferably contains high-cis polybutadiene rubber having at least 40 percent by weight, preferably at least 80 percent by weight of a cis-1,4 structure as a rubber component. A diene-based rubber component such as natural rubber, polyisoprene rubber, styrene-polybutadiene rubber or EPDM may be employed in the range of less than 20 percent by weight of the rubber component. The rubber composition may further contain a rubber component other than the diene-based rubber, a polymer component such as an elastomer or an ionomer other than rubber. The primary natural frequency of the solid center (FC) can be increased as the content of the high-cis polybutadiene rubber in the rubber composition is increased.

[0039] A filler such as a vulcanizing agent, a cross-linking agent, a vulcanization accelerator, an activator, a modifier, a specific gravity adjuster or the like can be blended into the composition of the solid center at need. The cross-linking agent can be prepared from metallic salt of unsaturated carboxylic acid, organic peroxide or the like. The metallic salt of unsaturated carboxylic acid can be prepared from monovalent or divalent metallic salt such as zinc or magnesium salt of α,β-unsaturated carboxylic acid having a carbon number of 3 to 8 such as acrylic acid or methacrylic acid, in particular. Zinc acrylate providing high resilience without much increasing the primary natural frequency (FC) is particularly preferable. The degree of cross-linking of the center is increased as the quantity of the blended unsaturated carboxylic acid is increased, to harden the solid center (increase the primary natural frequency FC). In order to satisfy the requirement of the present invention, therefore, it is preferable to blend unsaturated carboxylic acid by 15 to 45 parts by mass with respect to 100 parts by mass of the base rubber in general. The solid center is excessively hardened (the primary natural frequency FC is excessively increased) to increase the impact in shot if the quantity of the blended unsaturated carboxylic acid exceeds 45 parts by mass, while the solid center is excessively softened (the primary natural frequency FC is excessively reduced) to result in inferior resilience and reduction of the carry if the quantity of the blended unsaturated carboxylic acid is less than 10 parts by mass.

[0040] The organic peroxide can be prepared from dicumyl peroxide or t-butyl peroxide, and dicumyl peroxide is preferable. The quantity of the blended organic peroxide is preferably in the range of 0.5 to 3.0 parts by mass with respect to 100 parts by mass of the base rubber. If the quantity of the organic peroxide is less than 0.5 parts by mass, the solid center is excessively softened (the primary natural frequency FC is excessively reduced) to result in inferior resilience and reduction of the carry. If the quantity of the organic peroxide exceeds 3.0 parts by mass, the solid center is excessively hardened (the primary natural frequency FC is excessively increased) to increase the impact in shot.

[0041] A low or high specific gravity filler can be properly blended as the specific gravity adjuster in order to satisfy the requirement (FC/FB) of the present invention. The low specific gravity filler is blended in order to reduce the weight of the solid center thereby increasing the primary natural frequency (FC). More specifically, the low specific gravity filler can be prepared from zinc oxide, barium sulfide or calcium carbonate, while zinc oxide is particularly preferable. The high specific gravity filler, blended in order to increase the weight of the center thereby reducing the primary natural frequency (FC), is prepared from metallic powder, metal oxide, metal nitride or a mixture thereof having specific gravity of 8 to 20. More specifically, the high specific gravity filler can be prepared from tungsten (specific gravity: 19.3), tungsten carbide (specific gravity: 15.8), molybdenum (specific gravity: 10.2), lead (specific gravity: 11.3), lead oxide (specific gravity: 19.3), nickel (specific gravity: 8.9), copper (specific gravity: 8.9) or a mixture thereof. The low specific gravity filler may be mixed with high specific gravity metallic powder. The rubber composition blended with the aforementioned compound(s) is kneaded in a roll or a kneader and thereafter compressed and vulcanized/molded in a mold thereby providing the solid center.

[0042] The primary natural frequency of the solid center (FC) can be changed by changing not only the material therefor (blending composition of the center) but also preparation conditions therefor. More specifically, the primary natural frequency (FC) can be increased by adjusting the kneading time, Mooney viscosity, the vulcanization temperature and the time thereby increasing hardness. In particular, the primary natural frequency (FC) is increased when hardness of the solid center is so distributed that the surface is hard and the inside is soft.

[0043] (Rubber Thread Layer)

[0044] According to the present invention, the rubber thread composition forming the rubber thread layer is preferably based on low-cis polyisoprene rubber as a rubber component. Further, natural rubber, synthesized high-cis polyisoprene rubber or high-cis polybutadiene rubber can be mixed into low-cis polyisoprene rubber. In addition, a rubber component prepared from latex can also be used. A compounding agent such as a vulcanizing agent, a cross-linking agent, a vulcanization accelerator, an activator, a modifier or the like is properly blended to the rubber thread composition at need.

[0045] In relation to vulcanization characteristics of the rubber thread composition, the ratio of natural rubber or synthetic high-cis polyisoprene rubber to low-cis polyisoprene rubber may be increased in order to increase the modulus. Consequently, the primary natural frequency (FB) is increased. Further, the primary natural frequency (FB) can be increased by increasing the volume ratio of the rubber thread layer to the golf ball.

[0046] The primary natural frequency (FB) of the golf ball can be adjusted through the sectional area of the rubber thread layer or the width or the thickness of the rubber thread such that the primary natural frequency (FB) is reduced when the sectional area of the rubber thread layer is increased. The primary natural frequency (FB) can be set high by increasing the elongation percentage of the rubber thread wound on the solid center.

[0047] Assuming that HC (mm) represents deformation from application of a load of 98N to a three-piece core formed by coating the solid center with the rubber thread up to application of a load of 1274N and HB (mm) represents deformation of the solid center, the difference (HB−HC) (mm) in deformation is preferably in the range of 0.2 to 0.8. If the difference in deformation is greater than 0.8, shot feeling of the golf ball is hardened. Further, tensile force of the rubber thread is generally kept high to reduce durability of the golf ball by thread breakage. If the difference in deformation is less than 0.2, tensile strength of the rubber thread is generally kept low to reduce resilience.

[0048] (Cover)

[0049] The cover of the inventive three-piece golf ball can be prepared from a well-known cover material such as ionomer, balata, polyurethane resin, thermoplastic elastomer, fiber-reinforced resin, metallic powder, blended resin or the like. In particular, ionomer or a mixture of ionomer and thermoplastic resin is preferably employed. The cover is so hardened that the primary natural frequency of the overall golf ball (FB) can be increased as the content of ionomer is increased in the cover composition.

[0050] The aforementioned ionomer is prepared by partially neutralizing carboxylic acid in an ethylene-(meth)acrylic acid copolymer with metallic ions or a mixture thereof. The metallic ions employed for neutralization can be prepared from alkaline metal ions such as Na ions, K ions or Li ions, divalent metallic ions such as Zn ions, Ca ions or Mg ions or trivalent metallic ions such as Al ions or Nd ions. More specifically, the metallic ions can be prepared from Hi-milan by Du Pont-Mitsui Chemicals Co., Ltd., Iotec by Exxon Corporation or the like. The aforementioned balata can be prepared from natural balata, synthetic balata or a mixture thereof, and synthetic balata of transpolyisoprene “TP301” by Kuraray Isoprene Co., Ltd. is commercially available.

[0051] A colorant such as titanium dioxide or a filler such as an ultraviolet absorber, a light stabilizer or a fluorescent whitener can be properly blended to the cover composition in the range satisfying the requirement of the present invention.

[0052] The primary natural frequency of the golf ball (FB) can be increased by increasing hardness and rigidity of the cover. Further, the primary natural frequency (FB) can be increased by increasing the thickness of the cover.

[0053] While the blending composition of the cover composition and the thickness of the cover are so adjusted that the primary natural frequency of the overall golf ball (FB) is preferably in the range of 700 to 1800 Hz, more preferably 750 to 1000 Hz, and particularly preferably 800 to 900 Hz, the thickness of the cover is preferably set to 1.0 to 5.0 mm. The Shore D hardness of the cover is preferably in the range of 40 to 62. The spin rate is increased to result in a disadvantage in the carry if the Shore D hardness is less than 40, while shot feeling is hardened if the Shore D hardness exceeds 62.

[0054] (Method of Preparing Golf Ball)

[0055] The inventive golf ball can be prepared by a well-known method. For example, rubber thread is wound on the solid center at a constant elongation percentage for forming a rubber thread layer. Thereafter a pair of previously prepared half shells for a cover may be applied to the outer side of the rubber thread layer and heated in a mold to be integrally molded, or a cover composition may be injection-molded on the rubber thread layer for preparing the golf ball.

EXAMPLE

[0056] Inventive samples Nos. 1 to 4 and comparative samples Nos. 1 to 4 of golf balls were prepared in the following method and subjected to evaluation of performance:

[0057] (1) Solid Center

[0058] Solid centers were formed by rubber compositions S1 to S7 prepared by blending high-cis polybutadiene (1,4-cis content: 96 %) by JSR Co., Ltd., employed as base rubber, dicumyl peroxide (DCP) employed as a cross-linking agent and zinc acrylate or the like employed as a co-cross-linking agent. Table 2 shows the contents of blending. All solid centers were molded under conditions of 165° C. and 20 minutes.

TABLE 2
Center Composition and Vulcanization Condition (part by mass)
Symbol	S1	S2	S3	S4	S5	S6	S7
Polybutadiene	100	100	100	100	100	100	100
Rubber
(BR18)
Zinc Acrylate	29.5	28.5	28.5	26	28.5	31	26
Zinc Oxide	5	5	5	5	5	5	5
Barium Sulfate	25.5	26	26	27	26	25	27
Dicumyl	0.7	0.7	0.7	0.7	0.7	0.7	0.7
Peroxide
Molding Condition
Temperature (° C.)	165	165	165	165	165	165	165
Time (min.)	20	20	20	20	20	20	20

[0059] (2) Rubber Thread Layer

[0060] Vulcanized rubber thread members (width: 1.5 mm, thickness: 0.5 mm) were wound on the aforementioned solid centers for forming rubber thread layers, thereby preparing thread wound centers having diameters and quantities of deformation shown in Table 4. The vulcanized rubber was prepared by mixing natural rubber and low-cis polyisoprene rubber in a mass ratio 40/60 as base rubber.

[0061] (3) Cover

[0062] Covers were formed by resin compositions prepared by blending 4 parts by mass of titanium dioxide to 100 parts by mass of a polymer component such as ionomer resin. The thickness of each cover was set to 1.6 mm or 2.85 mm. Table 3 shows cover blending compositions C1 to C7 and the employed materials in detail.

TABLE 3
Cover Composition (part by mass)
Symbol	C1	C2	C3	C4	C5	C6	C7
Surlyn 8945(note 1)	30	30	30	30	—	30	—
Surlyn 9945(note 2)	30	30	30	30	—	30	—
Hi-milan 1605(note 3)	—	—	—	—	50	—	50
Hi-milan 1706(note 4)	—	—	—	—	50	—	50
SEPTON HG252(note 5)	30	30	30	30	—	30	—
Epofriend A1010(note 6)	10	10	10	10	—	10	—
Titanium Oxide	4	4	4	4	4	4	4
Surnol LS770(note7)	0.2	0.2	0.2	0.2	0.2	0.2	0.2
(note 1)Surlyn 8945: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin by Du Pont, Shore D hardness: 61
(note 2)Surlyn 9945: zinc ion neutralized ethylene-methacrylic acid copolymer ionomer resin by Du Pont, Shore D hardness: 59
(note 3)Hi-milan 1605: sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin by Du Pont-Mitsui Chemicals Co., Ltd., Shore D hardness: 61
(note 4)Hi-milan 1706: zinc ion neutralized ethylene-methacrylic acid copolymer ionomer resin by Du Pont-Mitsui Chemicals Co., Ltd., Shore D hardness: 60
(note 5)SEPTON HG252: hydrogenated styrene-isoprene-styrene (SIS) block copolymer containing hydroxyl groups at the ends of molecular chains by Kuraray Co., Ltd., styrene content: about 40 mass %, JIS-A hardness: 80
(note 6)Epofriend A1010: styrene-butadiene-styrene (SBS) block copolymer having a polybutadiene block containing epoxy groups by Daicel Chemical Industries, Ltd., containing styrene and butadiene in the mass ratio 40/60, epoxy content: about 1.5 to 1.7 mass %, JIS-A hardness: 70
(note 7)Surnol S770: antioxidant by Sankyo Co., Ltd.

[0063] (4) Method of Preparing Golf Ball

[0064] Each golf ball was formed by coating the rubber thread center prepared in the aforementioned manner with two cover materials molded into half shells from above and from below and press-molding the same in a mold having dimples for the golf ball. The surface of the molding was painted thereby forming a three-piece golf ball having a diameter of 42.75 mm.

[0065] (5) Method of Performance Evaluation of Golf Ball

[0066] Solid centers, rubber thread layers and cover compositions were adjusted for preparing golf balls having different ratios of primary natural frequencies of the solid centers (FC) and the golf balls (FB). Table 4 shows the carries, shot sounds and spin performance of the obtained golf balls.

[0067] The golf balls were evaluated as follows:

[0068] <Carry>

[0069] A metal head driver (XX10#1 by Sumitomo Rubber Industries, Ltd., loft angle: 10°, S shaft) was mounted on a swing robot by Golf Laboratories, Inc. for hitting each golf ball at a club head speed of 45 m/sec. and measuring the distance between the hitting point and the point of fall as the carry. The golf ball was hit 12 times, and the average carry was regarded as the result of each golf ball.

[0070] <Spin Performance>

[0071] A metal head driver (XX10 Sand Wedge by Sumitomo Rubber Industries, Ltd.) was mounted on a swing robot by Golf Laboratories, Inc. for hitting each golf ball at a club head speed of 20 m/sec. and measuring the back spin rate immediately after starting the shot. The golf ball was hit 12 times, and the average quantity was regarded as the result of each golf ball.

[0072] <Shot Feeling>

[0073] (i) Impact Feeling

[0074] Littleness of an impact in the shot was expressed in the average of sensory evaluation by 10 golfers. In the sensory evaluation, the golf ball exhibiting the minimum impact was regarded as full marks of 10 points. The impact of the comparative sample No. 3 was assumed to be 5 points.

[0075] (ii) Sound Favorableness

[0076] The sound (impact sound) of the shot was expressed in the average of sensory evaluation by 10 golfers. In the sensory evaluation, the impact sound felt as the best was regarded as full marks of 10 points. The impact sound of the comparative Example No. 3 was assumed to be 5 points.

[0077] (6) Results of Evaluation

[0078] Golf balls of the inventive examples Nos. 1 to 4, having the ratios FC/FB in the range of 0.95 to 1.10, are totally excellent in carry, shot feeling and spin performance (controllability). The comparative example 1 exhibiting a small ratio FC/FB has a low carry, and is inferior in spin performance and shot feeling. The comparative example 2 exhibiting a large ratio FC/FB is excellent in spin performance, but inferior in carry and shot feeling. The comparative example 3 of a two-piece solid golf ball is excellent in carry, but inferior in spin performance and shot feeling.

[0079] The inventive examples Nos. 2 and 4 and the comparative example No. 2, exhibiting similar primary natural frequencies (FC) of 870 Hz, 845 Hz and 850 Hz respectively, are remarkably different in carry and shot feeling from each other. It is understood that the carry and the shot feeling (impactiveness and impact sound) vary with the primary natural frequency of the center regardless of the primary natural frequency of the golf ball, and it is also understood that the carry and the impactiveness vary with the primary natural frequency of the golf ball regardless of the primary natural frequency of the center by comparing the inventive examples Nos. 2 and 3 with the comparative example No. 1. As to the primary natural frequency of the golf ball, therefore, it is understood that the ratio between the primary natural frequencies of the solid center (FC) and the golf ball (FB) is important.

TABLE 4
Result of Evaluation
	Example	Comparative Example
	1	2	3	4	1	2	3
Center	Diameter (mm)	37	37	37	37	37	37	37
	Primary Natural	910	875	875	805	875	960	805
	Frequency (FC)
	Deformation	3.3	3.5	3.5	4.0	3.5	3.0	4.0
	HB (mm)
Thread	Diameter (mm)	39.5	39.5	39.5	39.5	39.5	39.5	—
wound	Deformation	2.7	2.8	3.3	3.2	3.0	2.9	—
center	HC (mm)
	HB-HC	0.6	0.7	0.2	0.8	0.5	0.1	—
Cover	Thickness (mm)	1.6	1.6	1.6	1.6	1.6	1.6	2.85
	Hardness (Shore D)	51	51	51	51	61	51	61
Golf	Diameter (mm)	42.7	42.7	42.7	42.7	42.7	42.7	42.7
Ball	Primary Natural	900	870	800	845	1060	850	1050
	Frequency (FB)
	Deformation of Ball	2.5	2.6	3.1	3.0	2.6	2.7	2.7
	(mm)
	(FC/FB)	1.01	1.01	1.09	0.95	0.83	1.13	0.77
Spin Rate (rpm)	6950	6900	6850	6700	5200	6900	4600
Carry (m)	222.3	224.0	222.7	223.1	222.2	214.0	223.1
Shot Feeling
Shot Favorableness	7.5	8.8	8.1	9.9	7.2	4.2	5.0
Sound Favorableness	7.1	7.3	6.4	6.4	5.2	3.2	5.0

[0080] In the golf ball according to the present invention, the carry can be improved while holding spin performance and shot feeling equivalent to those of a conventional three-piece ball by setting the ratio (FC/FB) between the primary natural frequencies of the solid center (FC) and the golf ball (FB) in the range of 0.95 to 1.10.

[0081] Further, a more excellent carry and more excellent shot feeling can be attained by adjusting the primary natural frequency of the solid center (FC) in the golf ball according to the present invention.

[0082] 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 spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims

1. A three-piece golf ball comprising a solid center, rubber thread wound on said solid center and a cover for said solid center and said rubber thread, wherein the ratio (FC/FB) of the primary natural frequency of said solid center (FC) to the primary natural frequency of said golf ball (FB) satisfies the following expression:

2. The three-piece golf ball according to claim 1, wherein the primary natural frequency of said solid center (FC) is 750 Hz to 1000 Hz.

3. The three-piece golf ball according to claim 1, wherein deformation (mm) from application of a load of 98N to said solid center up to application of a load of 1274N to said solid center is 2.7 to 4.3.