Abstract:
A golf ball has a large number of dimples (A to D) on a surface thereof. A surface area occupation ratio Y of the dimples is 0.80 to 0.90. A mean occupation ratio y to be a value obtained by dividing the surface area occupation ratio Y by the total number of the dimples is 0.00220 or more. A total contour length X of the dimples and the surface area occupation ratio Y satisfy a relationship indicated by an expression (I): 
       X≧3882 * Y+1495   (I) 
     The number of dimples having a contour length x of 10.5 mm or more to a total number of the dimples is 91% or more.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a golf ball and more particularly to an improvement in a dimple pattern of the golf ball.  
           [0003]    2. Description of the Related Art  
           [0004]    A golf ball has approximately200to 550dimples on a surface thereof. The role of the dimples resides in one aspect that such dimples disturb an air stream around the golf ball during the flight to accelerate the transition of a turbulent flow at a boundary layer, thereby causing a turbulent flow separation (which will be hereinafter referred to as a “dimple effect”). The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backward so that a drag coefficient (Cd) is reduced, resulting in an increase in the flight distance of the golf ball. Moreover, the acceleration of the transition of the turbulent flow increases a differentia between upper and lower separating points of the golf ball which is caused by a back spin. Consequently, a lift acting on the golf ball is increased.  
           [0005]    There have been proposed various golf balls having improved dimple patterns in order to enhance a flight performance. For example, Japanese Patent Publication No. Sho 58-50744 (U.S. Pat. No. 5,080,367) has disclosed a golf ball in which dimples are densely provided such that a pitch between the dimples is 1.62 mm or less if possible. Japanese Laid-Open Patent Publication No. Sho 62-192181 (U.S. Pat. No. 4,813,677) has disclosed a golf ball in which dimples are densely provided so as not to form a new dimple having an area which is equal to or larger than a mean area in a land portion other than the dimples. Japanese Laid-Open Patent Publication No. Hei 4-347177 (U.S. Pat. No. 5,292,132) has disclosed a golf ball in which dimples are provided very densely such that the number of land portions in which a rectangle having a predetermined dimension can be drawn is 40 or less.  
           [0006]    All the golf balls disclosed in the known publications have dimples provided densely, in other words, the surface area occupation ratio of the dimple is increased. Those skilled in the art have recognized that the surface area occupation ratio is one of important elements to influence a dimple effect.  
           [0007]    However, the surface area occupation ratio is not the only index to guess the dimple effect. In respect of other respects in addition to the surface area occupation ratio, there is room for investigating an improvement intended for a further enhancement in a flight performance.  
         SUMMARY OF THE INVENTION  
         [0008]    The present inventor has taken note of a mean occupation ratio to be a mean value of an area ratio occupied by respective dimples in addition to a surface area occupation ratio as an important element to influence the dimple effect. The present inventor has studied the existing golf balls that have the same surface area occupation ratio to find that a golf ball that has a higher mean occupation ratio would be more excellent in a flight performance. By setting the relationship between the surface area occupation ratio and the mean occupation ratio to a range which cannot be obtained by the existing golf ball, the flight performance could be enhanced.  
           [0009]    The present invention provides a golf ball having a large number of dimples on a surface thereof. A surface area occupation ratio Y of the dimples is 0.80 to 0.90. A mean occupation ratio y to be a value obtained by dividing the surface area occupation ratio Y by a total number of the dimples is 0.00220 or more.  
           [0010]    A golf ball in which the surface area occupation ratio Y satisfies the range and the mean occupation ratio y satisfies the range includes a large number of dimples having comparatively large areas. The reason why the flight performance of the golf ball is excellent is not clear in detail. It is guessed that the dimple pattern contributes to a reduction in a drag coefficient (Cd), particularly, a reduction in the drag coefficient (Cd) in a high-speed region immediately after hitting.  
           [0011]    It is preferable that a total contour length X (a sum of a contour length x of the dimple) and the surface area occupation ratio Y should satisfy a relationship indicated by an expression (I): 
             X≦ 3882* Y+ 1495  (I) 
           [0012]    The golf ball includes a-dimple pattern having a smaller total contour length X as compared with the surface area occupation ratio Y. Such a golf ball presents a more excellent flight performance.  
           [0013]    It is preferable that a ratio of the number of dimples having a contour length x of 10.5 mm or more to a total number of the dimples should be 91% or more. The golf ball presents a particularly excellent flight performance. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a plan view showing a golf ball according to an embodiment of the present invention,  
         [0015]    [0015]FIG. 2 is a front view showing the golf ball in FIG. 1,  
         [0016]    [0016]FIG. 3 is a typical enlarged sectional view showing a part of the golf ball in FIG. 1,  
         [0017]    [0017]FIG. 4 is a plan view showing a golf ball according to an example 2 of the present invention,  
         [0018]    [0018]FIG. 5 is a front view showing the golf ball in FIG. 4,  
         [0019]    [0019]FIG. 6 is a plan view showing a golf ball according to an example 3 of the present invention,  
         [0020]    [0020]FIG. 7 is a front view showing the golf ball in FIG. 6,  
         [0021]    [0021]FIG. 8 is a plan view showing a golf ball according to a comparative example 1 according to the present invention,  
         [0022]    [0022]FIG. 9 is a front view showing the golf ball in FIG. 8,  
         [0023]    [0023]FIG. 10 is a plan view showing a golf ball according to a comparative example 2 of the present invention,  
         [0024]    [0024]FIG. 11 is a front view showing the golf ball in FIG. 10,  
         [0025]    [0025]FIG. 12 is a plan view showing a golf ball according to a comparative example 3 of the present invention, and  
         [0026]    [0026]FIG. 13 is a front view showing the golf ball in FIG. 12. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    The present invention will be described below in detail based on a preferred embodiment with reference to the drawings.  
         [0028]    A golf ball shown in FIGS. 1 and 2 usually has a diameter of approximately 42.67 mm to 43.00 mm. The golf ball includes an A dimple having a circular plane shape and a diameter of 4.5 mm, a B dimple having a circular plane shape and a diameter of 4.1 mm, a C dimple having a circular plane shape and a diameter of 3.6 mm, and a D dimple having a circular plane shape and a diameter of 2.8 mm. The number of the A dimples is 130, that of the B dimples is 150, that of the C dimples is 60 and that of the D dimples is 32. The total number of the dimples of the golf ball is 372.  
         [0029]    [0029]FIG. 3 is a typical enlarged sectional view showing a part of the golf ball in FIG. 1. FIG. 3 shows a section taken along the deepest portion of the dimple. In FIG. 3, the diameter of the dimple is shown in an arrow d. The diameter d represents a distance between both contacts with a common tangential line drawn on both ends of the dimple. A dimple volume represents the volume of a portion surrounded by the virtual sphere of the golf ball (which is a sphere on the assumption that the dimple is not present and is shown in a two-dotted line of FIG. 3) and the surface of the dimple.  
         [0030]    The area of the dimple represents the area of a region surrounded by the contour of the dimple (that is, the area of a plane shape) when the center of the golf ball is seen at infinity. In the case of the circular dimple, an area s is calculated by the following equation. 
         s=(d/2) 2  *π 
         [0031]    In the golf ball shown in FIG. 1, the A dimple has an area s of 15.9 mm2, the B dimple has an area s of 13.2 mm 2 , the C dimple has an area s of 10.2 mm 2 , and the D dimple has an area s of 6.2 mm 2 . Accordingly, total area S of the dimple areas is 4855.7 mm 2 . The total area S is divided by the surface area of the virtual sphere so that a surface area occupation ratio Y is calculated. In the golf ball, the surface area occupation ratio Y is 0.848. The surface area occupation ratio Y implies an area ratio at which all the dimples occupy the spherical surface of a virtual sphere. By dividing the surface area occupation ratio Y by the total number of the dimples, a mean occupation ratio y is calculated. In the golf ball, the mean occupation ratio y is 0.00228. The mean occupation ratio y implies an area ratio at which dimple having a mean area occupy the spherical surface of the virtual sphere.  
         [0032]    In the case in which a designer is to design a dimple pattern having a high surface area occupation ratio Y, he (she) can use means for increasing the number of the dimples and means for increasing the diameter d of the dimple. When the designer mainly employs the means for increasing the diameter d to achieve the surface area occupation ratio Y, a golf ball having a mean occupation ratio y of 0.00220 or more is obtained.  
         [0033]    If the mean occupation ratio y is less than 0.00220, a drag coefficient (Cd) might be increased in a region in which a flight speed is high, resulting in an insufficient flight distance of the golf ball. In this respect, the mean occupation ratio y is more preferably 0.00225 or higher, further preferably 0.00230 or higher, and particularly preferably 0.00250 or more. The golf ball having an extremely high mean occupation ratio y is hard to design in respect of the maintenance of the original features that the golf ball is an almost sphere. Therefore, an ordinary golf ball has a mean occupation ratio y of 0.00300 or less.  
         [0034]    It is preferable that the surface area occupation ratio Y should be 0.80 to 0.90. If the surface area occupation ratio Y is less than the range, the lift of the golf ball might become insufficient during a flight. In this respect, the surface area occupation ratio Y is more preferably 0.81 or more and particularly preferably 0.83 or more. If the surface area occupation ratio Y exceeds the range, a trajectory might be too high. In this respect, it is particularly preferable that the surface area occupation ratio Y should be 0.87 or less.  
         [0035]    In the golf ball shown in FIG. 1, the A dimple has a contour length x of 14.1 mm, the B dimple has a contour length x of 12.9 mm, the C dimple has a contour length x of 11.3 mm and the D dimple has a contour length x of 8.8 mm. In the golf ball, a total contour length X to be the sum of the contour lengths x is 4730.0 mm.  
         [0036]    A contour length x of the dimple represents a length measured along the contour of the dimple. For example, in the case in which the dimple has a triangular plane shape, the total length of three sides is represented by the contour length x. Since these sides are present on a spherical surface, they are not straight lines but circular arcs in a strict sense. The length of the circular arc is set to be the length of the side. In the case of a circular dimple, the contour length x is calculated by the following equation. 
         x=d * π 
         [0037]    It is preferable that the surface area occupation ratio Y and the total contour length X should satisfy the relationship in the following expression (I). 
           X≦ 3882 *  Y+ 1495  (I) 
         [0038]    The golf ball has a smaller total contour length X as compared with the surface area occupation ratio Y. The golf ball has a small drag coefficient (Cd) during a flight and an excellent flight performance. As long as the present inventor knows, there has not been a golf ball satisfying the expression (I).  
         [0039]    In respect of a reduction in the drag coefficient (Cd), it is more preferable that the total contour length X and the surface area occupation ratio Y should satisfy the following expression (II), further preferably the following expression (III), and particularly preferably the following expression (IV). 
           X≦ 3882 *  Y+ 1445  (II) 
           X≦ 3882 *  Y+ 1335  (III) 
           X≦ 3882 *  Y+ 1085  (IV) 
         [0040]    In order to maintain the original feature that the golf ball is an almost sphere, the total contour length X and the surface area occupation ratio Y are to satisfy the relationship in the following expression (V). 
         X≦3882 *  Y+ 95  (V) 
         [0041]    The total contour length X is properly determined based on the relationship with the surface area occupation ratio Y within the range to satisfy the expression (I), and is usually 2800 mm to 5000 mm, particularly, 3100 mm to 4700 mm.  
         [0042]    In respect of a reduction in the drag coefficient (Cd), the number of the dimples having a contour length x of 10.5 mm or more is preferably 91% of the total number of the dimples or more, and particularly preferably 95% or more. The ratio is ideally 100%.  
         [0043]    While the size of each dimple is not particularly restricted, the circular dimple usually has a diameter d of 2.0 mm to 8.0 mm, and particularly 3.0 mm to 7.0 mm. It is possible to form a dimple of a simple kind or plural kinds. A non-circular dimple (a dimple having no circular plane shape) may be formed in place of the circular dimple or together with the circular dimple.  
         [0044]    The total volume of dimples is preferably 300 mm 3  to 700 mm 3 . If the total volume is less than the range, a trajectory might be too high. From this viewpoint, it is particularly preferable that the total volume should be 350 mm 3  or more. If the total volume exceeds the range, the trajectory might be dropped. From this viewpoint, it is particularly preferable that the total volume should be 600 mm 3  or less.  
         [0045]    The total number of the dimples is preferably 200 to 500. If the total number is less than the range, it might be hard to cause the golf ball to take the shape of an almost sphere while maintaining a predetermined surface area occupation ratio Y (in other words, the smoothness of the surface of the golf ball might be damaged). From this viewpoint, it is particularly preferable that the total number should be 250 or more. If the total number exceeds the range, the mean occupation ratio y might be reduced. From this viewpoint, it is particularly preferable that the total number should be 400 or less.  
         [0046]    The structure of the golf ball is not particularly restricted and a so-called wound golf ball or a solid golf ball (a one-piece golf ball, a two-pieces golf ball, a three-pieces golf ball or the like) may be used. Moreover, a material is not particularly restricted and a well-known material can be used.  
       EXAMPLES  
     Example 1  
       [0047]    A core formed of a solid rubber was put in a mold and an ionomer resin composition was subjected to injection molding to form a cover around the core. The surface of the cover was coated so that a golf ball according to an example 1 which has a dimple pattern shown in a plan view of FIG. 1 and a front view of FIG. 2 was obtained. The golf ball had an outside diameter of approximately 42.70 mm, a weight of approximately 45.4 g, a compression of approximately 93 (by an ATTI compression tester produced by Atti Engineering Co., Ltd.) and a total dimple volume of approximately 500 mm d.  
         [0048]    The golf ball includes 130 A dimples having a circular plane shape and a diameter of 4.5 mm, 150 B dimples having a circular plane shape and a diameter of 4.1 mm, 60 C dimples having a circular plane shape and a diameter of 3.6 mm, and 32 D dimples having a circular plane shape and a diameter of 2.8 mm. In the golf ball, the total number of the dimples is 372, a total contour length X is 4730.0 mm, a surface area occupation ratio Y is 0.848 and a mean occupation ratio y is 0.00228.  
       Example 2  
       [0049]    A golf ball according to an example 2 which has a dimple pattern shown in a plan view of FIG. 4 and a front view of FIG. 5 was obtained in the same manner as in the example 1 except that the mold was changed. The golf ball includes 170 A dimples having a circular plane shape and a diameter of 4.4 mm, 120 B dimples having a circular plane shape and a diameter of 4.1 mm, 60 C dimples having a circular plane shape and a diameter of 3.5 mm, and 12 D dimples having a circular plane shape and a diameter of 2.4 mm. In the golf ball, the total number of the dimples is 362, a total contour length X is 4645.8 mm, a surface area occupation ratio Y is 0.838, and a mean occupation ratio y is 0.00232.  
       Example 3  
       [0050]    A golf ball according to an example 3 which has a dimple pattern shown in a plan view of FIG. 6 and a front view of FIG. 7 was obtained in the same manner as in the example 1 except that the mold was changed. The golf ball includes 72 A dimples having a circular plane shape and a diameter of 5.9 mm, 24 B dimples having a circular plane shape and a diameter of 4.5 mm, 88 C dimples having a circular plane shape and a diameter of 3.9 mm, 112 D dimples having a circular plane shape and a diameter of 3.6 mm, and 24 E dimples having a circular plane shape and a diameter of 2.8 mm. In the golf ball, the total number of the dimples is 320, a total contour length X is 4229.8 mm, a surface area occupation ratio Y is 0.819, and a mean occupation ratio y is 0.00256.  
       Comparative Example 1  
       [0051]    A golf ball according to a comparative example 1 which has a dimple pattern shown in a plan view of FIG. 8 and a front view of FIG. 9 was obtained in the same manner as in the example 1 except that the mold was changed. The golf ball includes 30 A dimples having a circular plane shape and a diameter of 4.3 mm, 130 B dimples having a circular plane shape and a diameter of 4.0 mm, 180 C dimples having a circular plane shape and a diameter of 3.7 mm, 60 D dimples having a circular plane shape and a diameter of 3.4 mm, and 32 E dimples having a circular plane shape and a diameter of 2.8 mm. In the golf ball, the total number of the dimples is 432, a total contour length X is 5053.6 mm, a surface area occupation ratio Y is 0.829, and a mean occupation ratio y is 0.00192. The dimple pattern according to the comparative example 1 has been disclosed as an example 1 in the Japanese Laid-Open Patent Publication No. Hei 4-347177 (U.S. Pat. No. 5,292,132) described above.  
       Comparative Example 2  
       [0052]    A golf ball according to a comparative example 2 which has a dimple pattern shown in a plan view of FIG. 10 and a front view of FIG. 11 was obtained in the same manner as in the example 1 except that the mold was changed. The golf ball includes 30 A dimples having a circular plane shape and a diameter of 4.3 mm, 130 B dimples having a circular plane shape and a diameter of 4.0 mm, 180 C dimples having a circular plane shape and a diameter of 3.7 mm, 60 D dimples having a circular plane shape and a diameter of 3.4 mm, and 20 E dimples having a circular plane shape and a diameter of 2.8 mm. In the golf ball, the total number of the dimples is 420, a total contour length X is 4948.0 mm, a surface area occupation ratio Y is 0.816, and a mean occupation ratio y is 0.00194. The dimple pattern according to the comparative example 2 has been disclosed as an example 2 in the Japanese Laid-Open Patent Publication No. Hei 4-347177 (U.S. Pat. No. 5,292,132) described above.  
       Comparative Example 3  
       [0053]    A golf ball according to a comparative example 3 which has a dimple pattern shown in a plan view of FIG. 12 and a front view of FIG. 13 was obtained in the same manner as in the example 1 except that the mold was changed. The golf ball includes 132 A dimples having a circular plane shape and a diameter of 4.4 mm, 60 B dimples having a circular plane shape and a diameter of 4.2mm, 60C dimples having a circular plane shape and a diameter of 3.5 mm, and 60 D dimples having a circular plane shape and a diameter of 3.3 mm. In the golf ball, the total number of the dimples is 312, a total contour length X is 3898.1 mm, a surface area occupation ratio Y is 0.686, and a mean occupation ratio y is 0.00220. The dimple pattern according to the comparative example 3 has been disclosed as an example 1 in the Japanese Laid-Open Patent Publication No. Sho 62-192181 (U.S. Pat. No. 4,813,677) described above.  
       Flight Distance Test  
       [0054]    20 golf balls according to each of the examples and the comparative examples were prepared and were maintained at 23° C. On the other hand, a driver comprising a metal head (trade name of “XXIOW#1” produced by Sumitomo Rubber Industries, Ltd., loft: 8 degrees, shaft hardness: X) was attached to a swing machine (produced by Golf Lab Co., Ltd.). Machine conditions were set to have a head speed of 50 m/sec, a back spin amount of approximately 2000 rpm obtained immediately after hitting and a launch angle of approximately 10 degrees, and the golf ball was hit and a flight distance (a distance between a launch point and a stationary point) was measured. The following Tables 1 and 2 show the mean value of the results of measurement for the 20 golf balls.  
                                                                                                                                     TABLE 1                           Result of evaluation of golf ball according to example            Dimple       Total                        Contour               contour       Surface area   Mean                   Diameter   length               length   Total area   occupation   occupation       Flight           d   x       Ratio   Total   X   S   ratio   ratio   Plan view   distance       Kind   (mm)   (mm)   Number   (%)   number   (mm)   (mm 2 )   Y   y   Front view   (m)                    Example 1                                                   A   4.5   14.1   130   34.9   372   4730.0   4855.7   0.848   0.00228       253.1       B   4.1   12.9   150   40.3                               C   3.6   11.3   60   16.1       D   2.8   8.8   32   8.6       Example 2       A   4.4   13.8   170   47.0   362   4645.8   4800.8   0.838   0.00232       254.2       B   4.1   12.9   120   33.1                               C   3.5   11.0   60   16.6       D   2.4   7.5   12   3.3       Example 3       A   5.9   18.5   72   22.5   320   4229.8   4689.2   0.819   0.00256       256.3       B   4.5   14.1   24   7.5                               C   3.9   12.3   88   27.5       D   3.6   11.3   112   35.0       E   2.8   8.8   24   7.5                  
 
         [0055]    [0055]                                                                                                                                     TABLE 2                           Result of evaluation of golf ball according to comparative example            Dimple       Total                        Contour               contour           Mean                   Diameter   length               length   Total area   Surface area   occupation       Flight           d   x       Ratio   Total   X   S   occupation   ratio   Plan view   distance       Kind   (mm)   (mm)   Number   (%)   number   (mm)   (mm 2 )   Y   y   Front View   (m)                    Comparative                                                   example 1       A   4.3   13.5   30   6.9   432   5053.6   4746.5   0.829   0.00192       248.2       B   4.0   12.6   130   30.1                               C   3.7   11.6   180   41.7       D   3.4   10.7   60   13.9       E   2.8   8.8   32   7.4       Comparative       example 2       A   4.3   13.5   30   7.1   420   4948.0   4672.6   0.816   0.00194    FIG. 10   247.3       B   4.0   12.6   130   31.0                        FIG. 11       C   3.7   11.6   180   42.9       D   3.4   10.7   60   14.3       E   2.8   8.8   20   4.8       Comparative       example 3       A   4.4   13.8132   42.3   312   3898.1   3928.8   0.686   0.00220    FIG. 12   242.3       B   4.2   13.2   60   19.2                        FIG. 13       C   3.5   11.0   60   19.2       D   3.3   10.4   60   19.2                    
         [0056]    As is apparent from the Tables 1 and 2, the golf balls according to the examples 1 to 3 have greater flight distances than those of the golf balls according to the comparative examples 1 to 3. From the results of evaluation, the advantage of the present invention is apparent.  
         [0057]    The above description is only illustrative and can be variously changed without departing from the scope of the present invention.