Golf balls generally include a spherical outer surface with a plurality of dimples formed thereon. Conventional dimples are depressions on the golf balls' surface that reduce drag and increase lift. Drag is the air resistance that opposes the golf ball's flight direction. As the ball travels through the air, the air that surrounds the ball has different velocities thus, different pressures. The air exerts maximum pressure at a stagnation point on the front of the ball. The air then flows around the surface of the ball with an increased velocity and reduced pressure. At some separation point, the air separates from the surface of the ball and generates a large turbulent flow area behind the ball. This flow area, which is called the wake, has low pressure. The difference between the high pressure in front of the ball and the low pressure behind the ball slows the ball down. This is the primary source of drag for golf balls.
The dimples on the golf ball cause a thin boundary layer of air adjacent to the ball's outer surface to flow in a turbulent manner. Thus, the thin boundary layer is called a turbulent boundary layer. The turbulence energizes the boundary layer and helps move the separation point further backward, so that the layer stays attached further along the ball's outer surface. As a result, there is a reduction in the area of the wake, an increase in the pressure behind the ball, and a substantial reduction in drag. It is the circumference of each dimple, where the dimple wall drops away from the outer surface of the ball, which actually creates the turbulence in the boundary layer.
Lift is an upward force on the ball that is created by a difference in pressure between the top of the ball and the bottom of the ball. This difference in pressure is created by a warp in the airflow that results from the ball's backspin. Due to the backspin, the top of the ball moves with the airflow, which delays the air separation point to a location further backward. Conversely, the bottom of the ball moves against the airflow, which moves the separation point forward. This asymmetrical separation creates an arch in the flow pattern that requires the air that flows over the top of the ball to move faster than the air that flows along the bottom of the ball. As a result, the air above the ball is at a lower pressure than the air underneath the ball. This pressure difference results in the overall force, called lift, which is exerted upwardly on the ball. The circumference of each dimple is important in optimizing this flow phenomenon, as well.
In order to optimize ball performance, it is desirable to have a large number of dimples, hence a large amount of dimple circumference, evenly distributed around the ball. In arranging the dimples, an attempt is made to minimize the space between dimples, because such space does not contribute to the aerodynamic performance of the ball. In practical terms, this usually translates into 300 to 500 circular conventional dimples on the surface of a conventional golf ball.
When compared to conventional size dimples, theoretically, an increased number of small dimples will create greater aerodynamic performance by increasing the total dimple circumference. An example of a golf ball with small dimples is discussed in U.S. Pat. No. 4,991,852, which discloses a golf ball having 812 concave hexagonal dimples. However, in reality small dimples are not as effective in decreasing drag and increasing lift. This results at least in part from the susceptibility of small dimples to paint flooding. Paint flooding occurs when the paint coat on the golf ball fills the small dimples, and consequently decreases the dimple's aerodynamic effectiveness. On the other hand, a smaller number of large dimples also begin to lose effectiveness. This results from the circumference of one large dimple being less than that of a group of smaller dimples.
Conventional dimples are typically circular depressions and are formed where a dimple wall slopes away from the outer surface of the ball forming the depression. Typically, these depressions have circular perimeters on the ball surface and have spherical or substantially spherical depressions. It has been demonstrated that dimples comprising spherical or substantially spherical depressions exhibit superior aerodynamic performance than dimples comprising non-spherical depressions. However, the circular perimeters of conventional dimples to a large extent limit the maximum dimple density attainable, due to the irregular shape of the spaces between the circular dimples on the ball surface.
To minimize the spaces between the dimples on the ball surface, polygonal dimples have been proposed. Polygonal dimples have been disclosed in U.S. Pat. Nos. 2,002,726, 6,290,615 B1, 5,338,039, 5,174,578, 4,090,716, 4,869,512, and 4,830,378, among others. None of these references, however, discloses dimples with spherical or substantially spherical depressions. With the exception of the '726 reference, which describes square dimples with a complex concave depression having varying radii, these references disclose polygonal dimples having depressions formed of planar surfaces, i.e., surfaces formed by polygons joined along vertices. It has also been demonstrated that dimples with polyhedron depressions do not perform as well aerodynamically as dimples with spherical or substantially spherical depressions.
Hence, there remains a need in the art for a golf ball that exhibits superior aerodynamic performance and maximum dimple density.