Golf ball

Abstract

At all points Pa in a zone A extending over a distance range from equal to or greater than 1 mm to less than 5 mm from the central point of a core 4, a mathematical formula, Ha 2 −Ha 1 <5, is satisfied. At any of points Pb in a zone B extending over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point of the core 4 , a mathematical formula, Hb 2 −Hb 1 ≧5, is satisfied. Ha 1 and Ha 2 indicate JIS-C hardnesses at points Pa 1 and Pa 2 located radially inward and outward of each point Pa at a distance of 1 mm from the point Pa. Hb 1 and Hb 2 indicate JIS-C hardnesses at points Pb 1 and Pb 2 located radially inward and outward of the point Pb at a distance of 1 mm from the point Pb.

1 . A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, wherein
 the core comprises a center and an envelope layer positioned outside the center,   a ratio of a volume of the core to a volume of a phantom sphere of the golf ball is equal to or greater than 76%,   a JIS-C hardness Ho of the cover is less than JIS-C hardness Ho at a central point of the core,   at all points Pa included in a zone A that extends over a distance range from equal to or greater than 1 mm to less than 5 mm from a central point of the core, the following mathematical formula (I) is satisfied,   at any of points Pb included in a zone B that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point of the core, the following mathematical formula (II) is satisfied,
     Ha 2 −Ha 1<5   (I),
 
     Hb 2 −Hb 1≧5   (II),
 
   in the mathematical formula (I), Hal indicates a JIS-C hardness at a point Pa 1  that is located radially inward of each point Pa at a distance of 1 mm from the point Pa, and Ha 2  indicates a JIS-C hardness at a point Pa 2  that is located radially outward of the point Pa at a distance of 1 mm from the point Pa, and   in the mathematical formula (II), Hb 1  indicates a JIS-C hardness at a point Pb 1  that is located radially inward of the point Pb at a distance of 1 mm from the point Pb, and Hb 2  indicates a JIS-C hardness at a point Pb 2  that is located radially outward of the point Pb at a distance of 1 mm from the point Pb.   
     
     
         2 . The golf ball according to  claim 1 , wherein the JIS-C hardness Hc of the cover is equal to or less than 65. 
     
     
         3 . The golf ball according to  claim 1 , wherein a thickness of the cover is equal to or less than 0.8 mm. 
     
     
         4 . The golf ball according to  claim 1 , wherein a JIS-C hardness Hm of the mid layer is equal to or greater than 90. 
     
     
         5 . The golf ball according to  claim 1 , wherein a thickness of the mid layer is equal to or less than 1.5 mm. 
     
     
         6 . The golf ball according to  claim 1 , wherein
 the cover is formed from a resin composition, and   a principal component of a base material of the resin composition is a thermoplastic polyurethane.   
     
     
         7 . The golf ball according to  claim 6 , wherein
 a shear loss elastic modulus G″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., is equal to or less than 1.95×10 7  Pa, and   a ratio (E″/G″) of a tensile loss elastic modulus E″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., to the shear loss elastic modulus G″ is equal to or greater than 1.76.   
     
     
         8 . The golf ball according to  claim 7 , wherein the tensile loss elastic modulus E″ is equal to or greater 2.00×10 7  Pa. 
     
     
         9 . The golf ball according to  claim 6 , wherein a polyol component of the thermoplastic polyurethane is polytetramethylene ether glycol having a number average molecular weight of 1500 or less. 
     
     
         10 . The golf ball according to  claim 1 , wherein a difference between a JIS-C hardness He at a surface of the core and the hardness Hb 2  is equal to or greater than 10. 
     
     
         11 . The golf ball according to  claim 1 , wherein a difference between a JIS-C hardness He at a surface of the core and the hardness Ho is equal to or less than 40. 
     
     
         12 . The golf ball according to  claim 1 , wherein the hardness Ho is equal to or greater than 40 but equal to or less than 80. 
     
     
         13 . The golf ball according to  claim 1 , wherein a JIS-C hardness He at a surface of the core is equal to or greater than 75 but equal to or less than 95. 
     
     
         14 . The golf ball according to  claim 1 , wherein a difference between a JIS-C hardness He at a surface of the core and a JIS-C hardness Hi at an innermost portion of the envelope layer is equal to or greater than 10 but equal to or less than 25. 
     
     
         15 . The golf ball according to  claim 1 , wherein a thickness of the envelope layer is equal to or greater than 8 mm but equal to or less than 18 mm. 
     
     
         16 . The golf ball according to  claim 1 , wherein a diameter of the center is equal to or greater than 10 mm but equal to or less than 20 mm. 
     
     
         17 . The golf ball according to  claim 1 , wherein
 a JIS-C hardness He at a surface of the core is greater than a JIS-C hardness at a surface of the center, and   the hardness Hm of the mid layer is greater than the hardness He.   
     
     
         18 . The golf ball according to  claim 1 , wherein a difference between the hardness Ho and the hardness Hc is equal to or greater than 3.