An object of the present invention is to provide a golf ball traveling a great distance on driver shots and on iron shots. The present invention provides a golf ball comprising a spherical core including an inner layer and an outer layer, and a cover, wherein a difference (H X+1 −H X−1 ) between a hardness (H X+1 ) at a point outwardly away in a radial direction from a boundary between the inner layer and the outer layer of the spherical core by 1 mm and a hardness (H X−1 ) at a point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm is 0 or more in Shore C hardness, a surface hardness (H X+Y ) of the spherical core is more than 65 in Shore C hardness, an angle α of a hardness gradient of the inner layer is 0° or more, a difference (α−β) between the angle α and an angle β of a hardness gradient of the outer layer is 0° or more, a thickness of the cover is 3 mm or less, a difference (Hc−H x+y ) between a hardness (Hc) of the cover and the surface hardness (H X+Y ) of the spherical core is 2 or more in Shore C hardness, and the cover has a highest hardness among the constituent members of the golf ball.
1. A golf ball comprising a spherical core and a cover positioned outside the spherical core, wherein
the spherical core includes an inner layer and an outer layer,
a difference (H X+1 −H X−1 ) between a hardness (H X+1 ) at a point outwardly away in a radial direction from a boundary between the inner layer and the outer layer of the spherical core by 1 mm and a hardness (H X−1 ) at a point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm is 0 or more in Shore C hardness,
a surface hardness (H X+Y ) of the spherical core is more than 65 in Shore C hardness,
an angle α of a hardness gradient of the inner layer calculated by a formula (1) is 0° or more,
a difference (α−β) between the angle α and an angle β of a hardness gradient of the outer layer calculated by a formula (2) is 0° or more,
a thickness of the cover is 3 mm or less,
a difference (Hc−H x+y ) between a hardness (Hc) of the cover and the surface hardness (H X+Y ) of the spherical core is 2 or more in Shore C hardness, and
the cover has a highest hardness among the constituent members of the golf ball,
α=(180/π)× a tan [{ H x−1 −Ho }/( X− 1)] (1)
β=(180/π)× a tan [{ H X+Y −H x+1 }/( Y− 1)] (2)
where X represents a radius (mm) of the inner layer, Y represents a thickness (mm) of the outer layer, Ho represents a center hardness (Shore C) of the spherical core, H X−1 represents the hardness (Shore C) at the point inwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm, H X+1 represents the hardness (Shore C) at the point outwardly away in the radial direction from the boundary between the inner layer and the outer layer of the spherical core by 1 mm, and H X+Y represents the surface hardness (Shore C) of the spherical core.
2. The golf ball according to claim 1 , wherein the center hardness (Ho) of the spherical core is less than 65 in Shore C hardness.
3. The golf ball according to claim 1 , wherein the angle β ranges from −20° to +20°.
4. The golf ball according to claim 1 , wherein the hardness (Hc) of the cover is 75 or more in Shore C hardness.
5. The golf ball according to claim 1 , wherein the hardness (H X−1 ) ranges from 63 to 82 in Shore C hardness.
6. The golf ball according to claim 1 , wherein the hardness (H X+1 ) ranges from 70 to 90 in Shore C hardness.
7. The golf ball according to claim 1 , wherein a difference (H X−1 −Ho) between the hardness (H X−1 ) and the center hardness (Ho) of the spherical core ranges from 4 to 27 in Shore C hardness.
8. The golf ball according to claim 1 , wherein a difference (H X+Y −H X+1 ) between the surface hardness (H X+Y ) of the spherical core and the hardness (H X+1 ) ranges from −7 to 10 in Shore C hardness.
9. The golf ball according to claim 1 , wherein a difference (H X+Y −Ho) between the surface hardness (H X+Y ) and the center hardness (Ho) of the spherical core ranges from 14 to 35 in Shore C hardness.
10. The golf ball according to claim 1 , wherein the radius (X) of the inner layer ranges from 7 mm to 16 mm.
11. The golf ball according to claim 1 , wherein the thickness (Y) of the outer layer ranges from 3 mm to 12 mm.
12. The golf ball according to claim 1 , wherein a ratio (Y/X) of the thickness (Y) of the outer layer to the radius (X) of the inner layer ranges from 0.2 to 2.0.
13. The golf ball according to claim 1 , wherein the golf ball further comprises an intermediate layer positioned between the spherical core and the cover.
14. The golf ball according to claim 13 , wherein a hardness (Hm) of the intermediate layer ranges from 60 to 98 in Shore C hardness.
15. The golf ball according to claim 13 , wherein a thickness (Tm) of the intermediate layer ranges from 0.5 to 2.0 mm.
16. The golf ball according to claim 13 , wherein the intermediate layer is formed from a resin composition including an ionomer resin and a thermoplastic elastomer containing a styrene block as a resin component.
17. The golf ball according to claim 16 , wherein a mass ratio (ionomer resin/thermoplastic elastomer containing a styrene block) of the ionomer resin to the thermoplastic elastomer containing a styrene block is 1.3 or more.
18. The golf ball according to claim 1 , wherein the spherical core is formed from a rubber composition.
19. The golf ball according to claim 1 , wherein the cover is formed from a resin composition including an ionomer resin as a resin component.