Practice golf ball

Abstract

In a practice golf ball having a core and a cover encasing the core, the cover has a material hardness, expressed as the Shore D hardness, of from 30 to 57. The core has an initial velocity CV (m/s) and the ball has an initial velocity BV 1 (m/s) such that the value CV−BV 1 satisfies the condition (CV−BV 1 )=0.7×(cover thickness)−b, where 0.1≦b≦1.0. The ball has a soft feel on impact like that of a one-piece golf ball and thus is able to reduce strain on the player's body, in addition to which it has much better durability to cracking than commonly used game balls, good retention of appearance even with long-term use, and a distance that is controlled so as to be appropriate for a driving range.

1 . A practice golf ball comprising a core and a cover encasing the core, wherein
 the cover has a material hardness, expressed as the Shore D hardness, of from 30 to 57; and   the core has an initial velocity CV (m/s) and the ball has an initial velocity BV 1  (m/s) such that the value CV−BV 1  satisfies the condition
   ( CV−BV 1)=0.7×(cover thickness)− b,  
 
   
       where 0.1≦b≦1.0. 
     
     
         2 . The practice golf ball of  claim 1 , wherein the ball initial velocity BV 1  is not more than 76 m/s. 
     
     
         3 . The practice golf ball of  claim 1 , wherein the cover has a thickness of from 0.3 mm to 1.9 mm and is formed primarily of polyurethane. 
     
     
         4 . The practice golf ball of  claim 1 , wherein the core is formed of a rubber composition comprising a base rubber and, as compounding ingredients: a co-crosslinking agent, a crosslinking initiator and a metal oxide, said co-crosslinking agent being methacrylic acid. 
     
     
         5 . The practice golf ball of  claim 1  wherein, when the core and the ball are each compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), letting deflection by the core be CH and deflection by the ball be BH 1 , the ball deflection BH 1  is from 2.0 mm to 7.0 mm, and the ratio CH/BH 1  is from 0.95 to 1.1. 
     
     
         6 . The practice golf ball of  claim 1 , wherein the core, when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), has a compression CH of from 2.0 to 7.0 mm; the core has a hardness profile in which, letting A be the JIS-C hardness at a surface of the core, B be the JIS-C hardness at a position 2 mm inside the core surface, C be the JIS-C hardness at a position 5 mm inside the core surface, D be the JIS-C hardness at a position 10 mm inside the core surface, E be the JIS-C hardness at a position 15 mm inside the core surface, and F be the JIS-C hardness at a center of the core: A is from 60 to 88, B is from 54 to 83, C is from 56 to 85, D is from 54 to 80, E is from 51 to 75, and F is from 48 to 72; the relative hardness conditions A>B<C≧D>E>Fare satisfied; the value A−F is not more than 19; the core is formed in such a way that A has the highest value among A to F; the value A−C is from 0 to 8; and the core has a specific gravity of from 1.05 to 1.2. 
     
     
         7 . The practice golf ball of  claim 1 , wherein the ball has formed on a surface thereof a plurality of dimples, each dimple having a spatial volume below a flat plane circumscribed by an edge of the dimple, and the sum of the dimple spatial volumes, expressed as a percentage (VR) of the volume of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, being from 0.8% to 1.7%. 
     
     
         8 . The practice golf ball of  claim 1 , wherein the ball has formed on a surface thereof a plurality of dimples which satisfy conditions (1) and (2) below:
 (1) the dimples have a peripheral edge provided with a roundness represented by a radius of curvature R of from 0.5 mm to 2.5 mm; and   (2) the ratio ER of a collective number of dimples RA having a radius of curvature R to diameter D ratio (R/D) of at least 20%, divided by a total number of dimples N on the surface of the ball, is from 15% to 95%.   
     
     
         9 . The practice golf ball of  claim 8  which further satisfies condition (3) below:
 (3) the ball has thereon a plurality of dimple types of differing diameter, and the ratio DER of a combined number of dimples DE obtained by adding together dimples having an own diameter and an own radius of curvature larger than or equal to a radius of curvature of dimples of larger diameter than said own diameter plus dimples of a type having a largest diameter, divided by the total number of dimples N on the surface of the ball, is at least 80%. 
 
     
     
         10 . The practice golf ball of  claim 9  which further satisfies conditions (4) to (6) below:
 (4) the number of dimple types of differing diameter is 3 or more; 
 (5) the total number of dimples N is not more than 80; and 
 (6) the surface coverage SR of the dimples, which is the sum of individual dimple surface areas, each defined by a flat plane circumscribed by an edge of the dimple, expressed as a percentage of the surface area of a hypothetical sphere representing the ball were the ball to have no dimples on the surface thereof, is from 60% to 74%. 
 
     
     
         11 . The practice golf ball of  claim 4 , wherein the compounding ingredients are included in respective amounts of from 10 to 40 parts by weight of methacrylic acid as the co-crosslinking agent, from 15 to 30 parts by weight of zinc oxide as the metal oxide, and from 0.3 to 5.0 parts by weight of the crosslinking initiator, per 100 parts by weight of the base rubber.