GOLF BALL

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2016-250588 filed in Japan on Dec. 26, 2016, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a golf ball having numerous dimples formed on the surface thereof. More particularly, the invention relates to a golf ball whose aerodynamic performance has been enhanced by optimizing the cross-sectional shape of the dimples formed on the ball surface.

BACKGROUND ART

To increase the distance traveled by a golf ball, it is important both to increase the rebound of the ball and also to reduce the air resistance during flight by means of dimples formed on the ball surface and thus improve the aerodynamic performance. This fact is generally well known, which is why, for example, many golfers use golf balls on which have been formed numerous dimples that are circularly arcuate in cross-section as shown in FIG. 5. In order to further enhance the aerodynamic performance of the ball, various disclosures concerning the dimple shape and the method of configuring the dimples have hitherto been made in, for example, JP-A H11-57065, JP-A 2005-342407, JP-A 2006-149929, JP-A 2006-158778, JP-A 2006-187476, JP-A 2006-187485 and JP-A 2008-93481.

U.S. Pat. Nos. 8,888,613 and 8,974,320 describe golf balls in which, when deciding on the cross-sectional shape of a dimple, a distinctive cross-sectional curve shape is obtained by dividing the interior of the dimple into a plurality of specific regions and quantifying the dimple interior in such a way that the average depth in each region satisfies a specific relationship. However, even such art does not sufficiently improve the distance of the golf ball, and so there remains room for improvement in the aerodynamic performance and flight performance of the ball. Nor have there existed many golf balls with numerous dimples thereon of the above distinctive cross-sectional shapes.

Developing golf balls which enable more golfers to obtain a satisfactory flight performance is important for expanding the golfer base, and further improvement in the aerodynamic performance of the ball is essential for achieving a better flight performance.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball which is able to further increase the aerodynamic performance of the ball and enhance the flight performance.

As a result of extensive investigations, the inventors have discovered that, when deciding on the cross-sectional shape of a dimple, by letting a straight line that passes through any one point on the edge of the dimple and the foot of a perpendicular drawn from a deepest point of the dimple to an imaginary plane defined by the peripheral edge of the dimple serve as a reference line, dividing the reference line from the dimple edge at 0% (origin) to the foot at 100% into 20% segments, and setting within a fixed range the change in dimple depth ΔH in dimple regions that are divided into 20% each, the dimple cross-sectional shape stabilizes the dimple effect during flight of the ball, which is effective for enhancing the aerodynamic performance.

That is, improving the aerodynamic performance of the ball is essential for achieving a. better flight performance. In this invention, the cross-sectional shape of the dimples is optimized even further than in the prior art, thereby reducing the variability in flight and improving the aerodynamic performance. Moreover, the percentage change in depth at given positions in the dimple is held within a fixed range, which stabilizes the dimple effect and enables the aerodynamic performance to be improved.

Accordingly, the invention provides a golf ball having numerous dimples formed on a surface thereof, wherein the ball has arranged thereon at least one dimple with a cross-sectional shape that is described by a curved line or by a combination of a straight line and a curved line and is specified by steps (i) to (iv) below:

(i) letting the foot of a perpendicular drawn from a deepest point of the dimple to an imaginary plane defined by a peripheral edge of the dimple be the dimple center and a straight line that passes through the dimple center and any one point on the edge of the dimple be the reference line;

(ii) dividing a segment of the reference line from the dimple edge to the dimple center into at least 100 points and computing the distance ratio for each point when the distance from the dimple edge to the dimple center is set to 100%;

(iii) computing the dimple depth ratio at every 20% from 0 to 100% of the distance from the dimple edge to the dimple center; and

(iv) at the depth ratios in dimple regions 20 to 100% of the distance from the dimple edge to the dimple center, determining the change in depth ΔH every 20% of this distance and designing a dimple cross-sectional shape such that the change ΔH is at least 6% and not more than 24% in all regions corresponding to from 20 to 100% of this distance.

The total number of dimples on the surface of the ball is from 250 to 380.

In a preferred embodiment of the golf ball of the invention, in dimples having the specified cross-sectional shape, the change ΔH in dimple depth reaches a maximum at 20% of the distance from the dimple edge to the dimple center.

In another preferred embodiment of the invention, dimples having the specified cross-sectional shape account for at least 60% of the total number of dimples.

In yet another preferred embodiment, the dimples formed on the ball surface are of at least two types of differing diameter and/or depth.

In a further preferred embodiment, the curved line describing the cross-sectional shape of the dimple includes two or more points of inflection.

ADVANTAGEOUS EFFECTS OF THE INVENTION

The golf ball of the invention has at least one dimple of a distinctive cross-sectional shape which stabilizes the dimple effect during flight of the ball, enabling the aerodynamic performance to be further enhanced.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 2 is a graph showing the relationship between the dimple cross-section and regions established at the interior of the dimple.

FIG. 3A is a plan view showing the outward appearance of a golf ball according to another embodiment of the invention, and FIG. 3B is an enlarged cross-sectional view of one of the dimples formed on the surface of the golf ball shown in FIG. 3A.

FIG. 4A is a plan view showing the outward appearance of a golf ball on which conventional double dimples have been formed, and FIG. 4B is an enlarged cross-sectional view of one of the dimples formed on the surface of the golf ball shown in FIG. 4A.

FIG. 5A is a plan view showing the outward appearance of a golf ball on which conventional dimples that are circularly arcuate in cross-section have been formed, and FIG. 5B is an enlarged cross-sectional view of one of the dimples formed on the surface of the golf ball shown in FIG. 5A.

FIG. 6 is a schematic cross-sectional view showing an example of the structure of the inventive golf ball.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the foregoing diagrams.

FIG. 1A is a plan view showing the outward appearance of a golf ball according to one embodiment of the invention, and FIG. 1B is an enlarged cross-sectional view of one of the dimples formed on the surface of the golf ball shown in FIG. 1A. In these diagrams, the symbol D represents a dimple, E represents an edge of the dimple, P represents a deepest point of the dimple, the straight line L is a reference line which passes through the dimple edge E and a center O of the dimple, and the dashed line represents an imaginary spherical surface. The foot of a perpendicular drawn from the deepest point P of the dimple to an imaginary plane defined by a peripheral edge of the dimple D coincides with the dimple center O. The dimple edge E is the boundary between the dimple D and regions (lands) on the ball surface where dimples D are not formed, and corresponds to points where the imaginary spherical surface is tangent to the ball surface (the same applies below). The dimple D shown in FIG. 1 is a circular dimple as seen in a plan view; the center O of the dimple in a plan view coincides with the deepest point P.

The cross-sectional shape of the dimple shown in FIG. 1B, which is not drawn to scale, relies on a proper understanding of the invention. The same applies also to the cross-sectional shapes of the dimples shown subsequently in FIGS. 2, 3B, 4B and 5B.

In the invention, it is critical for the cross-sectional shape of the dimple I to satisfy the following conditions.

First, as condition (i), let the foot of a perpendicular drawn from a deepest point P of the dimple to an imaginary plane defined by a peripheral edge of the dimple be the dimple center O, and let a straight line that passes through the dimple center O and any one point on the edge E of the dimple be the reference line L.

Next, as condition (ii), divide a segment of the reference line L from the dimple edge E to the dimple center O into at least 100 points. Then compute the distance ratio for each point when the distance from the dimple edge to the dimple center is set to 100%. That is, referring to FIG. 2, the dashed lines in the chart are dividing lines represented along the dimple depth. The dimple edge E is the origin, which is the 0% position on the reference line, and the dimple center O is the 100% position with respect to segment EO on the reference line.

Next, as condition (iii), compute the dimple depth ratio at every 20% from 0 to 100% of the distance from the dimple edge E to the dimple center O. In this case, the dimple center O is at the deepest part P of the dimple and has a depth H (mm). Letting this be 100% of the depth, the dimple depth ratio at each distance is determined. Also, the dimple depth ratio at the dimple edge E becomes 0%.

Next, as condition (iv), at the depth ratios in dimple regions 20 to 100% of the distance from the dimple edge E to the dimple center O, determine the change in depth ΔH every 20% of the distance and design a dimple cross-sectional shape such that the change ΔH is at least 6% and not more than 24% in all regions corresponding to from 20 to 100% of the distance.

In this invention, by quantifying the cross-sectional shape of the dimple in this way, that is, by setting the change in dimple depth ΔH to at least 6% and not more than 24%. and thereby optimizing the dimple cross-sectional shape, the flight variability decreases, enhancing the aerodynamic performance of the ball. This change ΔH is preferably from 8 to 22%, and more preferably from 10 to 20%.

Also, to further increase the advantageous effects of the invention, in dimples having the specified cross-sectional shape, it is preferable for the change in dimple depth ΔH to reach a maximum at 20% of the distance from the dimple edge to the dimple center. Also, the inclusion of two or more points of inflection on the curved line describing the specified cross-sectional shape of the dimple is preferable in terms of increasing the advantageous effects of the invention.

Dimples that are circular as seen in the plan view are depicted in FIG. 1 by way of illustration, although the dimple shape (plan-view shape) is not limited to a circular shape. Dimples of other shapes, such as polygonal, teardrop or elliptical dimples, may be suitably selected. Even with dimples of non-circular shape, it is possible to set the cross-sectional shape by a method similar to that indicated above. In the example shown in FIG. 1, the center O and the deepest point P of the dimple coincide. However, the deepest point P does not necessarily have to coincide with the center O of the dimple. Even when the center O and the deepest point P of the dimple D do not coincide, this does not pose any particular problem; the change in dimple depth ΔH within dimple regions that are divided into 20% each can be determined in the same way as described above.

The cross-sectional shape of the dimple D is illustrated in FIG. 1 by a shape composed primarily of a gently curved line and including straight lines in portions thereof, but is not limited thereto so long as it is a curve within the scope of the invention that is described by a curved line or by a combination of a straight line and a curved line.

The diameter of the dimple D (in polygonal dimples, the diagonal length), although not particularly limited, may be set to preferably at least 1.5 mm, and more preferably at least 2.0 mm. There is no particular upper limit, although the dimple diameter is preferably set to not more than 7.0 mm, and more preferably not more than 6.0 mm.

The dimple D has a depth H at the deepest point P thereof which, although not particularly limited, may be set to preferably from 0.05 to 0.5 mm, and more preferably from 0.1 to 0.4 mm.

The method of configuring the dimples is not particularly limited, although preferred use may be made of a method which uses a geometrically configured pattern in the form of a regular polyhedron such as a regular octahedron, a regular dodecahedron or a regular icosahedron, or a method that involves configuring the dimples with rotational symmetry about the poles of the ball, such as three-fold symmetry, four-fold symmetry, five-fold symmetry or six-fold symmetry.

The total number of dimples formed on the ball surface is set to at least 250, preferably at least 275, and more preferably at least 300. The upper limit in the number of dimples is set to not more than 380, preferably not more than 370, and more preferably not more than 360.

In this invention, the dimples formed on the surface of the ball include at least one dimple having the above-described cross-sectional shape, such dimples accounting for a portion of all the dimples. Accordingly, in this invention, it is not necessary for all the dimples formed on the ball surface to be dimples having the above-described cross-sectional shape, it being possible to intersperse conventional dimples. In such a case, the dimples having the above-described cross-sectional shape account for a proportion of the total number of dimples formed on the ball surface which, although not particularly limited, may be set to 20% or more, preferably 50% or more, more preferably 60% or more, even more preferably 80% or more, and most preferably 100%.

In the invention, although not particularly limited, it is recommended that preferably at least two types, and more preferably at least three types, of dimples of mutually differing diameter and/or depth be formed. When conventional dimples which do not satisfy the above-described conditions are included, if such dimples include ones of mutually differing diameter and/or depth, they shall be regarded here as differing types of dimples.

The proportion SR (%) of the total surface area of the imaginary spherical surface of the ball that is circumscribed by the edges of the above dimples, sometimes referred to as the “dimple coverage ratio,” is generally at least 70%, and preferably at least 80%. At an SR value outside of this range, a suitable trajectory may not be obtained, which may result in a decreased distance.

The sum of the volumes of the individual dimple spaces formed below a flat plane circumscribed by the edge of each dimple, expressed as a ratio VR (dimple spatial occupancy) with respect to the volume of an imaginary sphere were the ball surface assumed to have no dimples thereon, although not particularly limited, may be set to generally at least 0.7%, preferably at least 0.75%, and more preferably at least 0.8%, with the upper limit being preferably not more than 1.5%, more preferably not more than 1.45%, and even more preferably not more than 1.4%. By setting the dimple spatial occupancy VR in the above range, when the ball is struck with a distance club such as a driver, the shot can be prevented from rising too steeply or from dropping without gaining enough height.

To fabricate a mold for molding the above golf ball, a technique may be employed in which 3D CAD/CAM is used to directly cut the entire surface shape three-dimensionally into a master mold from which the golf ball mold is subsequently made by pattern reversal, or to directly cut three-dimensionally the cavity (inside walls) of the golf ball mold.

As with conventional golf balls, various types of coatings, such as white enamel coatings, epoxy coatings and clear coatings, may be applied to the ball surface. In such cases, to avoid marring the cross-sectional shape of the dimples, it is desirable to evenly and uniformly coat the surface.

The golf ball of the invention is not particularly limited with regard to the internal construction. That is, the present art may be applied to any type of golf ball, including solid golf balls such as one-piece golf balls, two-piece golf balls, and multi-piece golf balls having a construction of three or more layers, and also wound golf balls. Referring to FIG. 6, the use of a multi-piece solid golf ball Gin which an intermediate layer 2 composed of one or more layers is formed between a solid core 1 and a cover 3 is especially preferred. In FIG. 6, the symbol D represents a dimple.

In the golf ball G shown in FIG. 6, the solid core 1 is preferably formed primarily of polybutadiene. Also, the solid core 1 has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, may be set to preferably at least 2.0 mm and preferably not more than 6.0 mm.

The materials used in the intermediate layer 2 and the cover 3 are not particularly limited. Preferred use may be made of, for example, known ionomer resins, thermoplastic elastomers and thermoset elastomers. Exemplary thermoplastic elastomers include polyester, polyamide, polyurethane, olefin and styrene-type thermoplastic elastomers.

The material hardness of the intermediate layer, although not particularly limited, may be set to a Shore D hardness of generally at least 30 and generally not more than 75.

The material hardness of the cover, although not particularly limited, may be set to a Shore D hardness of generally at least 30 and generally not more than 75.

The above material hardnesses are values obtained by using a molding press to mold the material into sheets having a thickness of 2 mm, stacking the molded sheets to a thickness of at least 6 mm, and measuring the hardness in accordance with ASTM D2240 with a type D durometer.

The thickness of the intermediate layer and the thickness of the cover, although not particularly limited, are each preferably set in the range of 0.3 to 3.0 mm. Ball parameters such as the weight and diameter may be suitably set in accordance with the Rules of Golf

As described above, the golf ball of the invention, by having dimples of a characteristic cross-sectional shape formed on the surface thereof, has a reduced air resistance during flight. This improves the aerodynamic performance of the ball, enabling a higher trajectory to be achieved. As a result, the distance traveled by the ball can be further increased.

EXAMPLES

Working Examples and Comparative Examples are given below by way of illustration, although the invention is not limited by the following Examples.

Working Examples 1 and 2, and Comparative Examples 1 and 2

Golf balls having the dimples shown in Working Example 1 (FIG. 1), Working Example 2 (FIG. 3), Comparative Example 1 (FIG. 4, conventional double dimples) and Comparative Example 2 (FIG. 5, dimples that are circularly arcuate in cross-section) formed on the hall surface were manufactured, and the flight properties were compared. Four types of dimples were used in each of these Examples. Details on the dimples are shown in Tables 1 to 4.

The depth of each dimple from the reference line L to the inside wall of the dimple was measured at 100 equally spaced points along the reference line L from the dimple edge E to the dimple center O. The results are presented in Tables 1 to 4.

Next, the percent change in dimple depth ΔH every 20% of the distance along the reference line L from the dimple edge E was determined. The results are presented in Tables 1 to 4.

With regard to the interior construction of the golf balls in these Examples, as shown in FIG. 6, the ball had a three-layer construction composed of a core 1, an intermediate layer 2 and a cover 3. Details on each of these layers are given below.

Core

A rubber composition containing 80 parts by weight of polybutadiene A (available from JSR Corporation under the product name BR51), 20 parts by weight of polybutadiene B (available from JSR Corporation under the product name BR11), 28.5 parts by weight of zinc acrylate, 1.2 parts by weight of a mixture of 1,1-di(t-butylperoxy)cyclohexane and silica (available from NOF Corporation under the product name Perhexa C-40), 4 parts by weight of zinc oxide, 19.1 parts by weight of Barium Sulfate 300 (from Sakai Chemical Co., Ltd.), 0.1 part by weight of an antioxidant (available from Ouchi Shinko Chemical Industry Co., Ltd. under the product name Nocrac NS-6) and 0.1 part by weight of the zinc salt of pentachlorothiophenol was prepared. The resulting rubber composition was molded and vulcanized in a core mold at vulcanization conditions of 155° C. and 13 minutes, thereby producing solid cores having a diameter of 37.7 mm. The resulting solid cores had a deflection, as measured following compression under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), of 3.6 mm.

Intermediate Layer and Cover

Using the intermediate layer material described below, an intermediate layer having a thickness of 1.7 mm was formed by an injection molding process over the cores obtained as described above. Next, using the cover material described below, a cover having a thickness of 0.8 nun was formed by an injection molding process, thereby producing three-piece solid golf balls having a diameter of 42.7 mm and a weight of 45.4 g. Dimples were formed on the ball surface at the same time as the cover was molded. The intermediate layer material was a resin composition obtained by blending Himilan® 1605, Himilan® 1557, Himilan® 1706 (all ionomer resins available from DuPont-Mitsui Polychemicals Co., Ltd.) and trimethylolpropane in the weight ratio 50/15/35/1.1. The cover material was a resin composition obtained by blending Pandex T-8295 (a polyurethane thermoplastic elastomer available from DIC Bayer Polymer, Ltd.), titanium oxide, Sanwax 161P (a polyethylene wax available from Sanyo Chemical industries, Ltd.) and an isocyanate compound (4,4′-diphenylmetharie diisocyanate) in the weight ratio 100/3.5/1/7.5. The intermediate layer material and the cover material had Shore D material hardnesses of respectively 62 and 47.

Performance Test

A driver (W#1) was set on a swing robot, and the height at the top of the trajectory (highest point attained) as well as the carry and the total distance traveled by the ball when struck were measured. The striking conditions were set as follows: initial ball velocity, about 65 m/s; launch angle, about 10° ; initial backspin, about 3,000 rpm. The club used was a TourStage X-Drive 701 (loft, 9°) manufactured by Bridgestone Sports Co., Ltd. The measured results are shown in Tables 1 to 4.

It is apparent from the results in Tables 1 to 4 that, owing to differences in the cross-sectional shapes of the dimples in the Working Examples and the Comparative Examples, although the heights of the trajectories were the same, the balls in Working Examples 1 and 2 had longer carry distances than the golf balls in Comparative Examples 1 and 2. As a result, the golf balls in Working Examples 1 and 2 traveled significantly longer distances than the golf balls in Comparative Examples 1 and 2.

Japanese Patent Application No. 2016-250588 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

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