This invention relates to a multi-piece golf ball comprising an elastic solid core and a resin cover of at least two different hardness layers and exhibiting uniform flight performance.
Known golf balls are generally classified into wound golf balls in which a center is wound with rubber thread and further enclosed with a balata or resin cover, and solid golf balls which, in turn, include one-piece golf balls in the form of a single elastic sphere made entirely of rubber or the like, and multi-piece golf balls in which an elastic core is enclosed with a resin cover consisting of plural layers having different physical properties. These golf balls are provided on the surface with a plurality of dimples for imparting desirable flight characteristics.
The arrangement of dimples on the ball surface is generally determined independent of the ball construction. In the traditional technology employed in the art for arranging dimples uniformly on the entire ball surface and in a high density, the ball""s spherical surface is assumed to be a polyhedral body such as an octahedral, dodecahedral or icosahedral body presenting a corresponding number of polygons, circular-in-plane dimples of two to four types which typically differ in diameter are arranged in each of the polygons, and this grouping of dimples as a unit is distributed over the entire ball surface.
This technology, however, suffers from several problems. In the octahedral or icosahedral arrangement, for example, dimples are arranged in a unit triangle. Due to the technical or economical limitations associated with the manufacture of ball molds, the arrangement of dimples on the sides of a triangle must be avoided. As a result, dimples are arranged only inside the triangle. Sometimes the arrangement density of dimples lacks uniformity between a portion adjacent the side and a central portion of the triangle. Alternatively, the arrangement of dimples at the parting plane of the mold must be avoided. When dimples are distributed over the entire spherical surface using such unit triangles, the resulting arrangement of dimples apparently looks uniform. However, a precise observation revealed that the planar and steric densities of dimple arrangement had fairly large variations.
When the golfer hits a ball with a club, any position on the ball spherical surface has a substantially equal chance of impact. Due to the lack of uniformity of dimple arrangement, there is a possibility that a portion of the ball surface where dimples are distributed in a relatively high or low density be hit with the club or contacted with the club face. As far as the inventor""s precise examination is concerned, the influence of dimples on the club face upon impact (revealing itself as a deviation of the ball in flight) contains a component in the lateral or vertical direction, which is not negligible. Additionally, the dimples also affect the feel and initial velocity of the ball. In golf balls having a cover of two or more layers wherein the surface hardness of the cover outermost layer is lower than the surface hardness of a cover inner layer next to the cover outermost layer, there is a tendency that dimples affect the launch angle, initial velocity and spin of the ball. Among others, a deviation of the launch angle in the lateral direction is likely to affect the ball flight to induce variations. When golf balls bear many dimples having an acute edge angle, such dimples have a more influence.
An object of the invention is to solve the above-discussed problems and to provide a multi-piece solid golf all having uniform flight performance.
The present invention provides a golf ball comprising an elastic core and a resin cover bearing a plurality of dimples and including a cover outer layer having a surface Shore D hardness of at least 50 and a cover inner layer disposed inside the cover outer layer and having a higher surface Shore D hardness than the cover outer layer. The cover defines a ball circumference and an extension thereof, and a phantom ball is given on the assumption that the ball is free of dimples. Provided that a phantom circumference is radially inwardly spaced 0.08 mm from the ball circumference, and as viewed in a radial cross section of a dimple, a tangent A is drawn to the dimple wall at an intersection of the dimple wall surface with the phantom circumference, the edge of the dimple is given by the intersection between the tangent A and the ball circumference or the extension thereof, and a tangent B is drawn to the ball circumference or the extension thereof at the dimple edge, those dimples in which the angle of inclination of the tangent A relative to the tangent B is at least 13xc2x0 account for at least 50% of the entire dimples. Provided that SR is the ratio of the total area of dimples to the entire surface area of the phantom ball, a dimple-free circle having a diameter of 20 mm is drawn on an arbitrary portion of the ball surface, and sr is the ratio of the total area of those dimples residing in the circle to the area of the circle, a ratio sr/SR has a variation in the range of 0.93 to 1.07 depending on the difference of position on the ball surface. Provided that each dimple defines a cavity between the dimple wall surface and the extension of the ball circumference extending over the portion circumscribed by the dimple edge, CV is the ratio of the total cavity volume of the dimples over the entire ball to the volume of the phantom ball, and cv is the ratio of the total cavity volume of those dimples residing in the 20-mm diameter circle to the volume of a dimple-free cone diverging radially outward from the ball center to the 20-mm diameter circle, a ratio cv/CV has a variation in the range of 0.95 to 1.05 depending on the difference of position on the ball surface.
Preferably, when the 20-mm diameter circle drawn on an arbitrary portion of the ball surface is bisected into two semicircles, the difference between the total cavity volume of those dimples residing in one semicircle and the total cavity volume of those dimples residing in the other semicircle is up to 0.88 mm3. Typically, the cover outer layer has a thickness of 0.8 to 2.0 mm, and the cover inner layer has a thickness of 1.0 to 2.0 mm. Most often, the cover outer layer is formed mainly of a thermoplastic or thermosetting polyurethane elastomer, and the cover inner layer is formed mainly of an ionomer resin.
In a preferred embodiment, the cover outer layer has a surface Shore D hardness of at least 52 and the cover inner layer has a surface Shore D hardness of at least 62 and higher than the cover outer layer. The cover defines a ball circumference or an extension thereof, and a phantom ball is given on the assumption that the ball is free of dimples. Provided that a phantom circumference is radially inwardly spaced 0.08 mm from the ball circumference, and as viewed in a radial cross section of a dimple, a tangent A is drawn to the dimple wall at an intersection of the dimple wall surface with the phantom circumference, the edge of the dimple is given by the intersection between the tangent A and the ball circumference or the extension thereof, and a tangent B is drawn to the ball circumference or the extension thereof at the dimple edge, those dimples in which the angle of inclination of the tangent A relative to the tangent B is at least 13xc2x0 account for at least 50% of the entire dimples. Provided that SR is the ratio of the total area of dimples to the entire surface area of the phantom ball, a dimple-free circle having a diameter of 18 mm is drawn on an arbitrary portion of the ball surface, and srxe2x80x2 is the ratio of the total area of those dimples residing in the circle to the area of the circle, a ratio srxe2x80x2/SR has a variation in the range of 0.93 to 1.07 depending on the difference of position on the ball surface. Provided that each dimple defines a cavity between the dimple wall surface and the extension of the ball circumference extending over the portion circumscribed by the dimple edge, CV is the ratio of the total cavity volume of the dimples over the entire ball to the volume of the phantom ball, and cvxe2x80x2 is the ratio of the total cavity volume of those dimples residing in the 18-mm diameter circle to the volume of a dimple-free cone diverging radially outward from the ball center to the 18-mm diameter circle, a ratio cvxe2x80x2/CV has a variation in the range of 0.95 to 1.05 depending on the difference of position on the ball surface.
In a further preferred embodiment, when the 18-mm diameter circle drawn on an arbitrary portion of the ball surface is bisected into two semicircles, the difference between the total cavity volume of those dimples residing in one semicircle and the total cavity volume of those dimples residing in the other semicircle is up to 0.88 mm3.