GOLF BALL

Abstract

Provided is a golf ball that is suitable for golf players who can achieve a sufficiently high trajectory due to initial elements and is also suitable for golf players who cannot achieve a sufficiently high trajectory due to initial elements. The golf ball has a core, a cover, and a paint layer. The golf ball has a variable dimple on a surface thereof. The cover has a hole. A space is formed by the hole. The cover and the paint layer are not joined at a location directly below the dimple. A space is formed between the cover and the paint layer. The volume of the space varies due to a centrifugal force caused by backspin.

Claims

1. A golf ball comprising a plurality of dimples on a surface thereof, wherein the dimples include one or more variable dimples, and each of the variable dimples has a structure that makes a shape thereof during flight different from a shape thereof at rest.

2. The golf ball according to claim 1, wherein transition from the shape at rest to the shape during flight is achieved by a centrifugal force resulting from spin of the golf ball.

3. The golf ball according to claim 1, wherein a volume V2 of the variable dimple with the shape during flight is different from a volume V1 of the variable dimple with the shape at rest.

4. The golf ball according to claim 3, wherein the volume V2 with the shape during flight is smaller than the volume V1 with the shape at rest.

5. The golf ball according to claim 3, wherein the golf ball includes a first variable dimple in which the volume V2 with the shape during flight is smaller than the volume V1 with the shape at rest, and a second variable dimple in which the volume V2 with the shape during flight is larger than the volume V1 with the shape at rest.

6. The golf ball according to claim 3, wherein the golf ball includes the variable dimple for which an absolute value of a volume change rate Pv calculated by the following mathematical formula is not less than 5%,
Pv=(V2V1)/V1.Math.100 where V1 represents a volume of the dimple with the shape at rest, and V2 represents a volume of the dimple with the shape during flight when a rate of the spin is 2500 rpm.

7. The golf ball according to claim 1, wherein the golf ball has a main part and a membrane covering the main part, a surface of the membrane has a plurality of dimples including the variable dimple and a land that is a part other than the dimples, and a distance between the membrane and the main part at a location directly below the variable dimple can vary.

8. The golf ball according to claim 7, wherein the golf ball has a fluid between the membrane and the main part at the location directly below the variable dimple, and the distance between the membrane and the main part can vary due to expansion or contraction of the fluid.

9. The golf ball according to claim 7, wherein the golf ball has a fluid between the membrane and the main part at the location directly below the variable dimple, and the distance between the membrane and the main part can vary due to increase or decrease of the fluid.

10. The golf ball according to claim 9, wherein the golf ball has a first variable dimple directly below which the golf ball has the fluid between the membrane and the main part, a second variable dimple directly below which the golf ball has the fluid between the membrane and the main part, and a passage that allows the fluid present directly below the second variable dimple to move to a location directly below the first variable dimple.

11. The golf ball according to claim 1, wherein a ratio of a number of the variable dimples to a total number of the dimples is not less than 50%.

12. The golf ball according to claim 1, wherein the golf ball has a core, a cover positioned outside the core, and a paint layer positioned outside the cover, and wherein the variable dimple comprises a space between the cover and the paint layer.

13. The golf ball according to claim 12, wherein the cover has a hole, and the space is formed by the hole.

14. The golf ball according to claim 1, wherein the golf ball comprises: a first variable dimple that decreases in volume during flight, and a second variable dimple that increases in volume during flight.

15. The golf ball according to claim 14, wherein the golf ball has a passage connecting the first variable dimple and the second variable dimple, and a fluid moves between the first variable dimple and the second variable dimple through the passage during flight.

16. The golf ball according to claim 15, wherein the fluid is a non-compressible liquid.

17. The golf ball according to claim 1, wherein the variable dimple has a volume change rate Pv calculated by the formula Pv=(V2V1)/V1100, where V1 is the volume at rest and V2 is the volume during flight at 2500 rpm, and an absolute value of the volume change rate Pv is at least 10%.

18. The golf ball according to claim 1, wherein at least 80% of the dimples are variable dimples.

19. A golf ball comprising: a spherical body having a core and a cover; and a plurality of dimples on a surface of the spherical body, wherein the dimples include variable dimples having a structure that allows the depth of the variable dimples to change in response to centrifugal force during spin of the golf ball.

20. A golf ball comprising: a golf ball body having a core and a cover; a membrane covering the golf ball body, the membrane having dimples and lands on a surface thereof; and fluid positioned between the membrane and the golf ball body at locations corresponding to the dimples, wherein a distance between the membrane and the golf ball body varies during flight due to centrifugal force acting on the fluid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 is a partially cutaway cross-sectional view showing a golf ball according to an embodiment;

[0028] FIG. 2 is a cross-sectional view showing a part of the golf ball at rest in an enlarged manner;

[0029] FIG. 3 is a cross-sectional view showing the part of the golf ball in FIG. 2 in a further enlarged manner;

[0030] FIG. 4 is a cross-sectional view showing the part of the golf ball during flight in an enlarged manner;

[0031] FIG. 5 is a cross-sectional view showing the part of the golf ball in FIG. 4 in a further enlarged manner;

[0032] FIG. 6 is a cross-sectional view showing another part of the golf ball in FIG. 5 in an enlarged manner;

[0033] FIG. 7 is a cross-sectional view showing the part of the golf ball during flight in an enlarged manner;

[0034] FIG. 8 is a cross-sectional view showing the part of the golf ball in FIG. 7 in a further enlarged manner;

[0035] FIG. 9 is a cross-sectional view showing a golf ball according to another embodiment;

[0036] FIG. 10 is a cross-sectional view of a part of the golf ball at rest in an enlarged manner;

[0037] FIG. 11 is a cross-sectional view showing the part of the golf ball in FIG. 10 in a further enlarged manner;

[0038] FIG. 12 is a cross-sectional view showing another part of the golf ball in FIG. 11 in an enlarged manner;

[0039] FIG. 13 is a cross-sectional view showing the part of the golf ball during flight in an enlarged manner;

[0040] FIG. 14 is a cross-sectional view showing another part of the golf ball in FIG. 13 in an enlarged manner;

[0041] FIG. 15 is a cross-sectional view showing a part of a golf ball according to Sample B1;

[0042] FIG. 16 is a front view showing the golf ball according to Sample B1 together with a laser displacement meter;

[0043] FIG. 17 is a graph showing a measurement result of the golf ball according to Sample B1;

[0044] FIG. 18 is a graph showing a measurement result of the golf ball according to Sample B1;

[0045] FIG. 19 is a graph showing a measurement result of the golf ball according to Sample B1;

[0046] FIG. 20 is a graph showing a measurement result of a golf ball according to Sample B2;

[0047] FIG. 21 is a graph showing a measurement result of the golf ball according to Sample B2; and

[0048] FIG. 22 is a perspective view showing a golf ball used in an experiment for the golf ball in FIG. 1.

DETAILED DESCRIPTION

[0049] Hereinafter, preferred embodiments will be described in detail with appropriate reference to the drawings.

Preliminary Experiment

[0050] FIG. 22 is a perspective view of a golf ball 2 for an experiment conducted by the present inventors. In FIG. 22, reference character AX indicates the rotation axis of backspin of the golf ball 2. An arrow X represents the direction of the rotation axis AX. FIG. 22 shows a north pole NP, a south pole SP, an equator EQ, a 30 north latitude line NL, and a 30 south latitude line SL when the rotation axis AX is considered to be the axis of the earth. The rotation axis AX passes through the north pole NP and the south pole SP.

[0051] Of the surface of the golf ball 2, the region between the latitude line NL and the latitude line SL is referred to as equator vicinity region. Of the surface of the golf ball 2, the regions other than the equator vicinity region are referred to as pole vicinity regions. The golf ball 2 has one equator vicinity region and two pole vicinity regions. Although not shown in FIG. 22, a plurality of dimples are arranged in the equator vicinity region, and a plurality of dimples are also arranged in each pole vicinity region.

[0052] The rotation axis AX is determined according to a hitting point upon collision of the golf ball 2 with a golf club. The equator vicinity region and the pole vicinity regions are determined according to this rotation axis AX. In other words, in one golf ball 2, the position of the equator vicinity region is indefinite, and the position of each pole vicinity region is also indefinite.

[0053] The distance from the rotation axis AX to each dimple present in the equator vicinity region is large. The circumferential speed during spin of each dimple present in the equator vicinity region is high. The distance from the rotation axis AX to each dimple present in each pole vicinity region is small. The circumferential speed during spin of each dimple present in each pole vicinity region is low.

[0054] The present inventors prepared three types of golf ball samples. The lift force coefficients of these samples were measured in accordance with the ITR standards established by the United States Golf Association (USGA). The total volume VS (mm.sup.3) of all dimples present in the equator vicinity region, the total volume VP (mm.sup.3) of all dimples present in the two pole vicinity regions, a lift force coefficient CL1 under a first condition, and a lift force coefficient CL2 under a second condition, of each sample, were as follows. The volumes of the dimples were adjusted by changing the depths of the dimples.

TABLE-US-00001 VS VP CL1 CL2 Sample A1 170 168 0.1690 0.2319 Sample A2 170 188 0.1565 0.2222 Sample A3 190 168 0.1747 0.2482

First Condition

[0055] Ball speed: 40 m/s [0056] Spin rate: 1770 rpm

Second Condition

[0057] Ball speed: 40 m/s [0058] Spin rate: 2830 rpm

[0059] As is obvious from a comparison of Samples A1 and A2, the golf ball in which the volume VP of the dimples in the pole vicinity regions (dimples having small distances from the rotation axis AX) was large had smaller lift force coefficients. As is obvious from a comparison of Samples A1 and A3, the golf ball in which the volume VS of the dimples in the equator vicinity region (dimples having large distances from the rotation axis AX) was large had larger lift force coefficients. Based on these findings, the present inventors have completed a golf ball with which a large flight distance can be obtained regardless of initial elements.

First Embodiment

[0060] FIG. 1 shows a golf ball 4 according to an embodiment. The golf ball 4 shown in FIG. 1 has a spherical core 6 and a cover 8 positioned outside the core 6. The golf ball 4 has a large number of dimples 10 on the surface thereof. Of the surface of the golf ball 4, a part other than the dimples 10 is a land 12. The golf ball 4 has a paint layer and a mark layer on the external side of the cover 8, but these layers are not shown in FIG. 1. The golf ball 4 may have one or more mid layers between the core 6 and the cover 8.

[0061] The golf ball 4 preferably has a diameter of not less than 40 mm and not greater than 45 mm. From the viewpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably not less than 42.67 mm. From the viewpoint of suppression of air resistance, the diameter is more preferably not greater than 44 mm and particularly preferably not greater than 42.80 mm.

[0062] The golf ball 4 preferably has a mass of not less than 40 g and not greater than 50 g. From the viewpoint of attainment of great inertia, the mass is more preferably not less than 44 g and particularly preferably not less than 45.00 g. From the viewpoint of conformity to the rules established by the USGA, the mass is particularly preferably not greater than 45.93 g.

[Core]

[0063] The core 6 is formed by crosslinking a rubber composition. Examples of the base rubber of the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. Two or more rubbers may be used in combination. From the viewpoint of resilience performance, polybutadienes are preferable, and high-cis polybutadienes are particularly preferable.

[0064] The rubber composition of the core 6 includes a co-crosslinking agent. Preferable co-crosslinking agents from the viewpoint of resilience performance are zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. The rubber composition preferably includes an organic peroxide together with a co-crosslinking agent. Examples of preferable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy) hexane, and di-t-butyl peroxide.

[0065] The rubber composition of the core 6 may include additives such as a filler, sulfur, a vulcanization accelerator, a sulfur compound, an anti-aging agent, a coloring agent, a plasticizer, and a dispersant. The rubber composition may include a carboxylic acid or a carboxylate. The rubber composition may include synthetic resin powder or crosslinked rubber powder.

[0066] The core 6 has a diameter of preferably not less than 30.0 mm, more preferably not less than 37.0 mm, and particularly preferably not less than 38.0 mm. The diameter of the core 6 is preferably not greater than 42.0 mm, more preferably not greater than 41.5 mm, and particularly preferably not greater than 41.0 mm. The core 6 may have two or more layers. The core 6 may have a rib on the surface thereof. The core 6 may be hollow.

[Cover]

[0067] The cover 8 is formed from a resin composition. A preferable base polymer for the resin composition is an ionomer resin. Examples of preferable ionomer resins include binary copolymers formed with an -olefin and an ,-unsaturated carboxylic acid having 3 to 8 carbon atoms. Examples of other preferable ionomer resins include ternary copolymers formed with: an -olefin; an ,-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an ,-unsaturated carboxylate ester having 2 to 22 carbon atoms. For the binary copolymers and the ternary copolymers, preferable -olefins are ethylene and propylene, and preferable ,-unsaturated carboxylic acids are acrylic acid and methacrylic acid. In the binary copolymers and the ternary copolymers, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for neutralization include sodium ions, potassium ions, lithium ions, zinc ions, calcium ions, magnesium ions, aluminum ions, and neodymium ions.

[0068] The resin composition of the cover 8 may include another polymer instead of or together with an ionomer resin. Examples of the other polymer include polyurethanes, polystyrenes, polyamides, polyesters, and polyolefins. The resin composition may include two or more polymers.

[0069] The resin composition of the cover 8 may include a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, etc. For the purpose of specific gravity adjustment, the resin composition may include powder of a metal with a high specific gravity such as tungsten and molybdenum.

[0070] The cover 8 has a thickness of preferably not less than 0.3 mm, more preferably not less than 1.0 mm, and particularly preferably not less than 1.5 mm. The thickness of the cover 8 is preferably not greater than 2.5 mm, more preferably not greater than 2.2 mm, and particularly preferably not greater than 2.0 mm. The cover 8 has a specific gravity of preferably not less than 0.90 and not greater than 1.10. The cover 8 may have two or more layers.

[Dimples]

[0071] FIG. 2 shows a cross-section of the golf ball 4 along a plane passing through the central point of the dimple 10 and the central point of the golf ball 4. FIG. 2 shows the golf ball 4 in a rest state. FIG. 2 shows the core 6, the cover 8, and the paint layer 14. The core 6 and the cover 8 form a main part 16 of the golf ball 4. The paint layer 14 covers the main part 16.

[0072] In FIG. 2, the up-down direction is the depth direction of the dimple 10. In FIG. 2, a chain double-dashed line 18 indicates a phantom sphere. The surface of the phantom sphere 18 is the surface of the golf ball 4 when it is postulated that no dimple 10 exists. The diameter of the phantom sphere 18 is the same as the diameter of the golf ball 4. The dimple 10 is recessed from the surface of the phantom sphere 18. The land 12 coincides with the surface of the phantom sphere 18.

[0073] As shown in FIG. 1, in the present embodiment, the contour shape of each dimple 10 is a circle. In FIG. 2, an arrow Dm indicates the diameter of the dimple 10. The diameter Dm is the distance between two tangent points Ed appearing on a tangent line Tg that is drawn tangent to the opposite ends of the dimple 10. Each tangent point Ed is also the edge of the dimple 10. The edge Ed defines the contour of the dimple 10.

[0074] In FIG. 2, a double headed arrow Dp indicates the depth of the dimple 10. The depth Dp is the distance between the deepest part of the dimple 10 and the surface of the phantom sphere 18.

[0075] The total number of dimples 10 is preferably not less than 250 and not greater than 450. The total number is more preferably not less than 270 and particularly preferably not less than 280. The total number is more preferably not greater than 432 and particularly preferably not greater than 380.

[0076] In the present specification, the volume of the dimple means the volume of a space surrounded by a plane including the tangent line Tg and perpendicular to the depth direction and the dimple 10. The total volume V of all dimples 10 is preferably not less than 250 mm.sup.3 and not greater than 450 mm.sup.3. With the golf ball 4 in which the total volume V is not less than 250 mm.sup.3, rising of the golf ball 4 during flight is suppressed. From this viewpoint, the total volume V is more preferably not less than 280 mm.sup.3 and particularly preferably not less than 300 mm.sup.3. With the golf ball 4 in which the total volume V is not greater than 450 mm.sup.3, dropping of the golf ball 4 during flight is suppressed. From this viewpoint, the total volume Vis more preferably not greater than 400 mm.sup.3 and particularly preferably not greater than 380 mm.sup.3.

[0077] In the case where the contour shape of the dimple 10 is not a circle, the volume of a portion surrounded by the surface of the phantom sphere 18 and the surface of the dimple 10 is used as the volume of the dimple 10 instead. In this case, the total volume Vis preferably not less than 450 mm.sup.3 and not greater than 750 mm.sup.3. With the golf ball 4 in which the total volume V is not less than 450 mm.sup.3, rising of the golf ball 4 during flight is suppressed. From this viewpoint, the total volume V is more preferably not less than 480 mm.sup.3 and particularly preferably not less than 500 mm.sup.3. With the golf ball 4 in which the total volume V is not greater than 750 mm.sup.3, dropping of the golf ball 4 during flight is suppressed. From this viewpoint, the total volume V is more preferably not greater than 700 mm.sup.3 and particularly preferably not greater than 670 mm.sup.3.

[0078] As shown in FIG. 3, the cover 8 has a hole 20. A space S1 is formed by the hole 20. In the space S1, a fluid is accommodated. In other words, the golf ball 4 has a fluid between the main part 16 and the paint layer 14. The fluid may be a gas or may be a liquid. The fluid is preferably a compressible fluid. In the present embodiment, the fluid is particularly preferably a gas. The fluid is typically air. In the present embodiment, the pressure of this air is substantially the same as the atmospheric pressure.

[0079] The paint layer 14 covers the cover 8. The paint layer 14 is in contact with the cover 8 at locations other than the hole 20. Therefore, except for the location directly above the hole 20, the distance between the cover 8 and the paint layer 14 is zero. At the location directly below the dimple 10, the paint layer 14 is not joined to the cover 8. At the location directly below the land 12 (see FIG. 2), the paint layer 14 is joined to the cover 8. By applying a release material to the surface of the dimple 10 prior to the formation of the paint layer 14, the paint layer 14 can be prevented from being joined to the cover 8 at the location directly below the dimple 10.

[0080] FIGS. 4 and 5 show a dimple 10E of the golf ball 4 during flight. The golf ball 4 is flying with backspin. In FIGS. 4 and 5, an arrow X represents the direction of the rotation axis AX. The dimple 10E is located in the equator vicinity region. Therefore, the distance from the rotation axis AX to the dimple 10E is large. The circumferential speed of the dimple 10E during spin is high. A large centrifugal force acts on the dimple 10E. As described above, the paint layer 14 is not joined to the cover 8 at the location directly below the dimple 10E. Due to the centrifugal force, the paint layer 14 is separated from the cover 8. The distance between the cover 8 and the paint layer 14 is larger than zero. In addition to the space S1, a space S2 is generated between the paint layer 14 and the cover 8. In other words, the air, which is a fluid, expands. The pressure of this air is lower than atmospheric pressure.

[0081] In FIG. 5, an arrow Dp1E indicates the depth of the dimple 10E. As is obvious from a comparison of FIGS. 3 and 5, the depth Dp1E during flight is smaller than the depth Dp at rest. In other words, the volume of the dimple 10E during flight is smaller than the volume of the dimple 10 at rest. The shape of the dimple 10E during flight is different from the shape thereof at rest. In the present specification, a dimple 10 having a structure that makes the shape thereof during flight different from the shape thereof at rest is referred to as variable dimple.

[0082] FIG. 6 shows a dimple 10P of the golf ball 4 during flight. The dimple 10P is located in a pole vicinity region. If the dimple 10P is located at the north pole NP or the south pole SP, the distance from the rotation axis AX to the dimple 10P is zero. If the dimple 10P is located at a position other than the north pole NP and the south pole SP, the distance from the rotation axis AX to the dimple 10P is small. The circumferential speed of the dimple 10P during spin is zero or low. No centrifugal force acts on the dimple 10P, or a relatively small centrifugal force acts on the dimple 10P. The dimple 10P is a variable dimple 10, but since the centrifugal force is small, the paint layer 14 is in contact with the cover 8.

[0083] In FIG. 6, an arrow Dp1P indicates the depth of the dimple 10P. As is obvious from a comparison of FIGS. 3 and 6, the depth Dp1P during flight is the same as the depth Dp at rest. In other words, the volume of the dimple 10P during flight is the same as the volume thereof at rest. When a small centrifugal force acts on the dimple 10P, the depth Dp1P during flight may be slightly smaller than the depth Dp at rest.

[0084] FIGS. 7 and 8 show the dimple 10E of the golf ball 4 during flight. The golf ball 4 is flying with backspin. The rotation axis of this backspin coincides with the rotation axis in the state shown in FIGS. 4 and 5. The rate of the backspin in the state shown in FIGS. 7 and 8 is higher than that in the state shown in FIGS. 4 and 5. Therefore, a very large centrifugal force acts on the dimple 10E. Due to the centrifugal force, the paint layer 14 is largely separated from the cover 8. In addition to the space S1, a space S3 is generated between the paint layer 14 and the cover 8. Since the centrifugal force is very large, the space S3 is larger than the space S2 (see FIG. 5).

[0085] In FIG. 8, an arrow Dp2E indicates the depth of the dimple 10E. As is obvious from a comparison of FIGS. 5 and 8, the depth Dp2E is smaller than the depth Dp1E. In other words, the volume of the dimple 10E during flight with backspin having a high rate is smaller than the volume of the dimple 10E during flight with backspin having a low rate.

[0086] Even when the golf ball 4 flies with backspin having a high rate, the centrifugal force acting on the dimple 10P (see FIG. 6) in the pole vicinity region is zero or small. Therefore, the depth of the dimple 10P is the same as the depth Dp at rest. In other words, the volume of the dimple 10P during flight is the same as the volume thereof at rest. The depth during flight may be slightly smaller than the depth Dp at rest.

[0087] The initial spin rate of the golf ball 4 that is hit with a golf club by a powerful player is generally higher than that when the golf ball 4 is hit with the same golf club by a less powerful player. In the golf ball 4 after being hit by a powerful player, a large centrifugal force acts on the dimple 10E in the equator vicinity region. The volume of the dimple 10E during flight is smaller than the volume thereof at rest. Meanwhile, the centrifugal force acting on the dimple 10P in the pole vicinity region is small. The volume of the dimple 10E during flight is the same or slightly smaller than the volume thereof at rest. In the golf ball 4, the lift force during flight can be reduced. The trajectory height of the golf ball 4 is appropriate. With the golf ball 4, a large flight distance can be obtained.

[0088] The initial spin rate of the golf ball 4 that is hit with a golf club by a less powerful player is generally lower than that when the golf ball 4 is hit with the same golf club by a powerful player. In the golf ball 4 after being hit by a less powerful player, the centrifugal force acting on the dimple 10E in the equator vicinity region is small. The volume of the dimple 10E during flight is not excessively small compared to the volume thereof at rest. In the golf ball 4, a sufficient lift force is generated. The trajectory height of the golf ball 4 is appropriate. With the golf ball 4, a large flight distance can be obtained.

[0089] The golf ball 4 is suitable for golf players who can achieve a sufficiently high trajectory due to initial elements. The golf ball 4 is also suitable for golf players who cannot achieve a sufficiently high trajectory due to initial elements. The golf ball 4 has excellent versatility.

[0090] During flight, spin decreases. As the spin decreases, the fluid gradually contracts. As the fluid contracts, the volume of the dimple 10E increases. When the golf ball 4 comes to rest after landing, the dimple 10E has the shape shown in FIGS. 2 and 3.

[0091] A volume change rate Pv of the variable dimple 10 is calculated by the following mathematical formula.

Pv=(V2V1)/V1.Math.100

[0092] In this mathematical formula, V1 represents the volume of the dimple 10 with the shape at rest, and V2 represents the volume of the dimple 10 with the shape during flight. In the measurement of V2, the rate of spin is 2500 rpm. In the measurement of V2, the dimple 10 to be measured is located on the equator EQ. From the viewpoint of versatility, the absolute value of the volume change rate Pv is preferably not less than 5%, more preferably not less than 8%, and particularly preferably not less than 10%. The upper limit of the absolute value of the volume change rate Pv for the golf ball 4 that can be put into practical use is 100%.

[0093] From the viewpoint of versatility, the ratio of the number of variable dimples 10 to the total number of dimples 10 is preferably not less than 50%, more preferably not less than 70%, and particularly preferably not less than 80%. This ratio may be 100%.

[0094] In golf competitions, players are not allowed to touch golf balls except on the teeing ground and the green. Players cannot select the rotation axis AX of backspin for shots other than the tee shot. From the viewpoint of suppressing dependence of a trajectory on the position of the rotation axis AX, it is preferable that the variable dimples 10 are uniformly arranged on the surface of the golf ball 4.

[Symmetry]

[0095] A typical golf ball is formed using a mold having upper and lower mold halves. When the upper mold half is mated with the lower mold half, a parting line is formed at the boundary therebetween. There are molds in which the parting line is circular. There are also molds in which the parting line is zigzag. Of the surface of the golf ball, a part corresponding to the parting line is referred to as seam. There are golf balls with a circular seam. There are also golf balls with a zigzag seam.

[0096] The USGA has a standard for a symmetry test for the flight performance of golf balls. In this test, the flight performances during PH rotation and during POP rotation of the golf ball 4 are compared. The rotation axis AX during PH rotation is a straight line connecting a point corresponding to the top of the upper mold half and a point corresponding to the bottom of the lower mold half on the surface of the golf ball 4. The rotation axis AX during POP rotation is a straight line orthogonal to the rotation axis AX during PH rotation. In the surface of the golf ball 4, the seam and the vicinity thereof are a unique region. The trajectory height during PH rotation tends to be lower than the trajectory height during POP rotation due to the influence of the seam. The seam can impair the symmetry of the golf ball 4.

[0097] From the viewpoint of improving symmetry, a golf ball 4 in which non-variable dimples 10 are mainly present in the region of the seam and the vicinity thereof and variable dimples 10 are mainly present in other regions, is preferable. In the PH rotation of this golf ball 4, non-variable dimples 10 are mainly present in regions where the distance from the rotation axis AX is large. Even when a centrifugal force acts on each of the non-variable dimples 10, the volumes of the non-variable dimples 10 do not decrease. These non-variable dimples 10 generate a sufficient lift force during PH rotation. In the POP rotation of the golf ball 4, variable dimples 10 and non-variable dimples 10 are present in the regions where the distance from the rotation axis AX is large. When a centrifugal force acts on each of the variable dimples 10, the volumes of the variable dimples 10 decrease. These variable dimples 10 reduce the lift force in POP rotation. These variable dimples 10 make the trajectory height during POP rotation closer to the trajectory height during PH rotation. The golf ball 4 can conform to the standards of the USGA. The variation of the trajectory of the golf ball 4 depending on a hitting point is small.

[Relationship with Golf Club]

[0098] The loft angle of a short iron is large. When the golf ball 4 is hit with this short iron, the golf ball 4 flies with a high spin rate. This spin suppresses the run (roll) after landing. The high spin rate is advantageous for a player who wishes to stop the golf ball 4 at the target location.

[0099] When the golf ball 4 is hit with a short iron, a large centrifugal force acts on the variable dimple 10E due to the spin rate. Therefore, the volume of the variable dimple 10E is relatively small. In the golf ball 4, the lift force is relatively small despite the high spin rate. The trajectory height of the golf ball 4 can be reduced. The trajectory of the golf ball 4 is less influenced by wind.

[0100] The loft angle of a driver (W #1) is small. When the golf ball 4 is hit with this driver, the golf ball 4 flies with a low spin rate. This spin rate can contribute to a large flight distance.

[0101] When the golf ball 4 is hit with a driver, the spin rate is low, and thus the centrifugal force acting on the variable dimple 10E is relatively small. Therefore, the volume of the variable dimple 10E is relatively large. In the golf ball 4, a large lift force is generated upon a shot with a driver despite the low spin rate. With the golf ball 4, a high trajectory is achieved upon a shot with a driver, so that a large flight distance is achieved.

[0102] This golf ball 4 can achieve: [0103] (1) a trajectory upon a shot with a short iron being less influenced by wind; [0104] (2) run upon a shot with a short iron being suppressed; and [0105] (3) a large flight distance being obtained upon a shot with a driver.
[Relationship with Trajectory Regions]

[0106] The golf ball 4 ascends from a hitting point to the top of a trajectory. In the present specification, this region is referred to as ascending region. The golf ball 4 descends from the top to a landing point. In the present specification, this region is referred to as descending region. Until landing after the golf ball 4 is hit, the spin of the golf ball 4 decreases. The spin rate in the ascending region is generally high, and the spin rate in the descending region is generally low. In the ascending region, the centrifugal force acting on the dimple 10E is large, and in the descending region, the centrifugal force acting on the dimple 10E is small. The volume of the variable dimple 10E in the ascending region is relatively small. The volume of the variable dimple 10E in the descending region is relatively large.

[0107] The direction of the lift force is perpendicular to the traveling direction of the golf ball 4. The lift force in the ascending region includes a vertically upward component and a horizontally backward component. Since the volume of the variable dimple 10E in the ascending region is small, the lift force acting on the golf ball 4 is not excessively large. In the golf ball 4, in the ascending region, the horizontally backward component is not excessively large. The golf ball 4 has excellent flight performance.

[0108] The lift force in the descending region includes a vertically upward component and a horizontally forward component. Since the volume of the variable dimple 10E in the descending region is large, a sufficient lift force acts on the golf ball 4. In the golf ball 4, the horizontally forward component in the descending region is sufficiently large. The golf ball 4 has excellent flight performance.

Second Embodiment

[0109] FIG. 9 shows a golf ball 22 according to another embodiment. The golf ball 22 has a spherical core 24 and a cover 26 positioned outside the core 24. The core 24 and the cover 26 constitute a main part 30. The golf ball 22 has a paint layer and a mark layer on the external side of the cover 26, but these layers are not shown in the drawing. The core 24 may have two or more layers. The cover 26 may have two or more layers. The golf ball 22 may have one or more mid layers between the core 24 and the cover 26. The diameter, the mass, and the material of the golf ball 22 are the same as those of the golf ball 4 shown in FIG. 1.

[0110] The golf ball 22 has a large number of dimples 32 on the surface thereof. Of the surface of the golf ball 22, a part other than the dimples 32 is a land 34. The number of dimples 32 is the same as the number of dimples 10 in the golf ball 4 shown in FIG. 1. The diameter, the depth, and the volume of each dimple 32 are the same as those of the dimple 10 shown in FIG. 2. These dimples 32 include a first dimple 32E and a second dimple 32P. In FIG. 9, an arrow X indicates the direction of the rotation axis of backspin. When the golf ball 22 flies with spin, the first dimple 32E belongs to the equator vicinity region, and the second dimple 32P belongs to a pole vicinity region.

[0111] As shown in FIG. 9, the golf ball 22 has a passage 36. The passage 36 penetrates the core 24 and penetrates the cover 26 at two locations. The golf ball 22 has a plurality of passages 36. In FIG. 9, one passage 36 is shown, and the other passages 36 are not shown. The passage 36 shown in FIG. 9 passes through the central point of the golf ball 22. The golf ball 22 may have a passage 36 that does not pass through the central point thereof.

[0112] FIGS. 10 and 11 show the vicinity of the first dimple 32E of the golf ball 22 in a rest state. These drawings also show the paint layer 38. The golf ball 22 has a first chamber 40E at the location directly below the first dimple 32E. The first chamber 40E is sandwiched between the cover 26 and the paint layer 38. The first chamber 40E is connected to the passage 36. A fluid 42 is accommodated in the first chamber 40E and the passage 36. The fluid 42 is preferably a non-compressible fluid. In the present embodiment, the fluid 42 is particularly preferably a liquid having a low viscosity and a large specific gravity. The paint layer 38 is joined to the cover 26 at the location directly below the land 34.

[0113] FIG. 12 shows the vicinity of the second dimple 32P of the golf ball 22 in a rest state. The golf ball 22 has a second chamber 40P at the location directly below the second dimple 32P. The second chamber 40P is sandwiched between the cover 26 and the paint layer 38. The second chamber 40P is connected to the passage 36. The passage 36 connects the first chamber 40E and the second chamber 40P. The same fluid 42 as in the first chamber 40E is accommodated in the second chamber 40P.

[0114] FIG. 13 shows the first dimple 32E of the golf ball 22 during flight. The golf ball 22 is flying with backspin. The distance from the rotation axis AX to the first dimple 32E is large, and thus the circumferential speed of the first dimple 32E during spin is high. A large centrifugal force acts on the first dimple 32E. As is obvious from a comparison of FIGS. 11 and 13, the paint layer 38 is separated from the cover 26 due to the centrifugal force. Accordingly, the fluid 42 flows into the first chamber 40E. The amount of the fluid 42 in the first chamber 40E in FIG. 13 is larger than that in FIG. 11.

[0115] In FIG. 13, an arrow DpE indicates the depth of the first dimple 32E. As is obvious from a comparison of FIGS. 11 and 13, the depth DpE during flight is smaller than the depth Dp at rest. In other words, the volume of the first dimple 32E during flight is smaller than the volume of the first dimple 32E at rest. The first dimple 32E is referred to as variable dimple.

[0116] The amount of the fluid 42 flowing into the first chamber 40E varies in accordance with the magnitude of the centrifugal force acting on the first dimple 32E. In other words, the amount of the fluid 42 flowing into the first chamber 40E varies in accordance with the spin rate of the golf ball 22. The amount of the fluid 42 flowing into the first chamber 40E when the spin rate is high is larger than that when the spin rate is low. Therefore, the depth DpE of the first dimple 32E when the spin rate is high is smaller than that when the spin rate is low. The volume of the first dimple 32E when the spin rate is high is smaller than that when the spin rate is low.

[0117] FIG. 14 shows the second dimple 32P of the golf ball 22 during flight. The distance from the rotation axis AX to the second dimple 32P is small, and thus the circumferential speed of the second dimple 32P during spin is low. The centrifugal force acting on the second dimple 32P is small. As described above, the fluid 42 moves toward the first chamber 40E. As a result of this movement, the fluid 42 flows out from the second chamber 40P. The amount of the fluid 42 in the second chamber 40P decreases. In the example of FIG. 14, all of the fluid 42 in the second chamber 40P has moved to the passage 36 or the first chamber 40E. Therefore, the paint layer 38 is in contact with the cover 26. A small amount of the fluid 42 may be present in the second chamber 40P.

[0118] In FIG. 14, an arrow DpP indicates the depth of the second dimple 32P. As is obvious from a comparison of FIGS. 12 and 14, the depth DpP during flight is larger than the depth Dp at rest. In other words, the volume of the second dimple 32P during flight is larger than the volume of the second dimple 32P at rest. The second dimple 32P is referred to as variable dimple.

[0119] The amount of the fluid 42 flowing out from the second chamber 40P varies in accordance with the amount of the fluid 42 flowing into the first chamber 40E. In other words, the amount of the fluid 42 flowing out from the second chamber 40P varies in accordance with the spin rate of the golf ball 22. The amount of the fluid 42 flowing out from the second chamber 40P when the spin rate is high is larger than that when the spin rate is low. Therefore, the depth DpP of the second dimple 32P when the spin rate is high is larger than that when the spin rate is low. The volume of the second dimple 32P when the spin rate is high is larger than that when the spin rate is low.

[0120] The initial spin rate of the golf ball 22 that is hit with a golf club by a powerful player is generally higher than that when the golf ball 22 is hit with the same golf club by a less powerful player. In the golf ball 22 after being hit by a powerful player, a large centrifugal force acts on the first dimple 32E. The volume of the first dimple 32E during flight is smaller than the volume thereof at rest. Meanwhile, the centrifugal force acting on the second dimple 32P is small. The volume of the second dimple 32P during flight is larger than the volume thereof at rest. In the golf ball 22, the lift force during flight can be reduced. The trajectory height of the golf ball 22 is appropriate. With the golf ball 22, a large flight distance can be obtained.

[0121] The initial spin rate of the golf ball 22 that is hit with a golf club by a less powerful player is generally lower than that when the golf ball 22 is hit with the same golf club by a powerful player. In the golf ball 22 after being hit by a less powerful player, the centrifugal force acting on the first dimple 32E is small. In the golf ball 22 after being hit, the amount of the fluid 42 flowing into the first chamber 40E is small. The volume of the first dimple 32E during flight is not excessively small compared to the volume thereof at rest. Since the amount of the fluid 42 flowing out from the second chamber 40P is small, the volume of the second dimple 32P during flight is not excessively large compared to the volume thereof at rest. In the golf ball 22, a sufficient lift force is generated. The trajectory height of the golf ball 22 is appropriate. With the golf ball 22, a large flight distance can be obtained.

[0122] The golf ball 22 is suitable for golf players who can achieve a sufficiently high trajectory due to initial elements. The golf ball 22 is also suitable for golf players who cannot achieve a sufficiently high trajectory due to initial elements. The golf ball 22 has excellent versatility.

[0123] During flight, spin decreases. As the spin decreases, the fluid 42 flows out from the first dimple 32E and flows into the second dimple 32P. When the golf ball 22 comes to rest after landing, the first dimple 32E has the shape shown in FIG. 11, and the second dimple 32P has the shape shown in FIG. 12.

[0124] In the golf ball 22 as well, the absolute value of the volume change rate Pv of a variable dimple 32 is preferably not less than 5%, more preferably not less than 8%, and particularly preferably not less than 10%. The upper limit of the absolute value of the volume change rate Pv for the golf ball 22 that can be put into practical use is 100%.

[0125] The golf ball 22 has a plurality of variable dimples 32 in addition to the first dimple 32E and the second dimple 32P. The golf ball 22 has a chamber 40 at a location directly below each variable dimple 32. This chamber 40 is connected to the passage 36. The golf ball 22 may have variable dimples 32 connected to the passages 36 and variable dimples not connected to the passages 36. As the variable dimples not connected to the passages 36, the dimple 10 shown in FIG. 3 is illustrated.

[0126] From the viewpoint of versatility, the ratio of the number of variable dimples 32 to the total number of dimples 32 is preferably not less than 50%, more preferably not less than 70%, and particularly preferably not less than 80%. This ratio may be 100%. It is preferable that the variable dimples 32 are uniformly arranged on the surface of the golf ball 22.

Experiment

[Sample B1]

[0127] A golf ball having no paint layer was prepared. This golf ball had 338 dimples. One recess was formed in the core and the cover of this golf ball using a tool, as shown in FIG. 15. This recess had a diameter of 2.5 mm and a depth of about 2 mm. The center position of this recess coincided with the center position of one dimple. Meanwhile, a disk-shaped weight was prepared. This weight had a diameter of 1.0 mm and a thickness of 0.2 mm. Furthermore, a laminate was prepared. The laminate includes a film having a diameter of 4.0 mm and a thickness of 30 m, and an adhesive layer. The weight was attached to the adhesive layer. The laminate was attached to the dimple. This attachment was achieved using an adhesive of the adhesive layer. The center position of the weight coincided with the center position of the dimple. The laminate adhered to a portion of the surface of the dimple other than the recess. In the golf ball, a space surrounded by the recess and the laminate was present. Air was present in this space. In this dimple, the distance between the laminate and the bottom of the recess could vary due to deformation of the laminate. In other words, this dimple was a variable dimple. The number of variable dimples in this golf ball was one. Each of the remaining dimples was not a variable dimple. Hereinafter, dimples that are not variable dimples are referred to as normal dimples.

[0128] The golf ball was set in a device shown in FIG. 16. This device had two chucks. The golf ball was sandwiched between these chucks. These chucks rotated about a line indicated by AX in FIG. 16 as an axis. As the chucks rotated, the golf ball also rotated. A laser was emitted from a laser displacement meter (LK-H022 & LK-G5000 of KEYENCE CORPORATION) toward the golf ball. The displacement of the surface of the golf ball (change in the distance from the laser displacement meter to the surface of the golf ball) was measured by reflected light of this laser.

[0129] A measurement result when the rotation speed of the golf ball was 500 rpm is shown in a graph in FIG. 17. In this graph, the horizontal axis represents the rotation angle of the golf ball, and the vertical axis represents height (i.e., displacement). In this graph, the displacement in the variable dimple and the displacement in a normal dimple adjacent to the variable dimple are measured continuously. In this graph, an arrow D1 indicates the depth of the variable dimple, and an arrow D2 indicates the depth of the normal dimple.

[0130] Similarly, the displacement was measured when the rotation speed of the golf ball was 1000 rpm, 3000 rpm, and 5000 rpm. This result is shown in a graph in FIG. 18. In this graph, the vertical axis represents dimple depth, and the horizontal axis represents rotation speed. In this graph, the depth of the variable dimple is shown by a solid line, and the depth of the normal dimple is shown by a dashed line. In the variable dimple, the dimple depth decreased as the rotation speed increased. On the other hand, in the normal dimple, the dimple depth did not depend on the rotation speed. It is inferred that in the variable dimple, since the centrifugal force generated by rotation acts on the laminate and the weight, the laminate and the weight move outward in the radial direction of the golf ball, resulting in a smaller dimple depth. From the graph in FIG. 18, it can be seen that the difference between the dimple depth when the rotation speed was 500 rpm and the dimple depth when the rotation speed was 5000 rpm is almost 50 m. The difference of 50 m can significantly influence the aerodynamic characteristics of the dimple.

[0131] FIG. 19 is a graph showing the dimple depth of the variable dimple. In this graph, the vertical axis represents the dimple depth, and the horizontal axis represents the square of the rotation speed. The four points plotted on this graph generally lie on one straight line. In other words, the square of the rotation speed and the dimple depth have a relationship represented by a linear expression. Due to this, it is presumed that the change in the dimple depth of the variable dimple depends on centrifugal force. This is because the centrifugal force is proportional to the square of the rotation speed.

[Sample B2]

[0132] A golf ball was obtained in the same manner as Sample B1, except that the diameter of the recess was set to 2.0 mm and the diameter of the laminate was set to 3.0 mm. This golf ball was evaluated by the same method as for the golf ball of Sample B1. The results are shown in graphs in FIGS. 20 and 21. In the graph in FIG. 20, the depth of the variable dimple is shown by a solid line, and the depth of the normal dimple is shown by a dashed line. In the variable dimple, the dimple depth decreased as the rotation speed increased. On the other hand, in the normal dimple, the dimple depth did not depend on the rotation speed. It is inferred that in the variable dimple, since the centrifugal force generated by rotation acts on the laminate and the weight, the laminate and the weight move outward in the radial direction of the golf ball, resulting in a smaller dimple depth. As is obvious from the graph in FIG. 21, the square of the rotation speed and the dimple depth of the variable dimple have a relationship represented by a linear expression. Due to this, it is presumed that the change in the dimple depth of the variable dimple depends on centrifugal force. This is because the centrifugal force is proportional to the square of the rotation speed.

[0133] As is obvious from a comparison between FIG. 19 and FIG. 21, the dependence of the depth of the variable dimple on the rotation speed is greater in Sample B1 than in Sample B2. This is inferred to be because the diameter of the recess in Sample B1 is larger than that in Sample B2.

[0134] From the above experimental results, it was found that it is possible to realize a golf ball having a dimple having a structure that makes a shape thereof during flight different from a shape thereof at rest. The dependence of the depth of the variable dimple on the rotation speed can be adjusted by [0135] (1) the mass of the weight, [0136] (2) the thickness of the film, [0137] (3) the material and the physical properties of the film, and [0138] (4) the size of the recess.

Disclosure Items

[0139] Each of the following items is a disclosure of a preferred embodiment.

Item 1

[0140] A golf ball comprising a plurality of dimples on a surface thereof, wherein the dimples include one or more variable dimples, and each of the variable dimples has a structure that makes a shape thereof during flight different from a shape thereof at rest.

Item 2

[0141] The golf ball according to Item 1, wherein transition from the shape at rest to the shape during flight is achieved by a centrifugal force resulting from spin of the golf ball.

Item 3

[0142] The golf ball according to Item 1 or 2, wherein a volume V2 of the variable dimple with the shape during flight is different from a volume V1 of the variable dimple with the shape at rest.

Item 4

[0143] The golf ball according to Item 3, wherein the volume V2 with the shape during flight is smaller than the volume V1 with the shape at rest.

Item 5

[0144] The golf ball according to Item 3, wherein the golf ball includes [0145] a first variable dimple in which the volume V2 with the shape during flight is smaller than the volume V1 with the shape at rest, and [0146] a second variable dimple in which the volume V2 with the shape during flight is larger than the volume V1 with the shape at rest.

Item 6

[0147] The golf ball according to any one of Items 3 to 5, wherein [0148] the golf ball includes the variable dimple for which an absolute value of a volume change rate Pv calculated by the following mathematical formula is not less than 5%,

Pv=(V2V1)/V1.Math.100

where V1 represents a volume of the dimple with the shape at rest, and V2 represents a volume of the dimple with the shape during flight when a rate of the spin is 2500 rpm.

Item 7

[0149] The golf ball according to any one of Items 1 to 6, wherein [0150] the golf ball has a main part and a membrane covering the main part, [0151] a surface of the membrane has a plurality of dimples including the variable dimple and a land that is a part other than the dimples, and [0152] a distance between the membrane and the main part at a location directly below the variable dimple can vary.

Item 8

[0153] The golf ball according to Item 7, wherein [0154] the golf ball has a fluid between the membrane and the main part at the location directly below the variable dimple, and [0155] the distance between the membrane and the main part can vary due to expansion or contraction of the fluid.

Item 9

[0156] The golf ball according to Item 7, wherein [0157] the golf ball has a fluid between the membrane and the main part at the location directly below the variable dimple, and [0158] the distance between the membrane and the main part can vary due to increase or decrease of the fluid.

Item 10

[0159] The golf ball according to Item 9, wherein the golf ball has [0160] a first variable dimple directly below which the golf ball has the fluid between the membrane and the main part, [0161] a second variable dimple directly below which the golf ball has the fluid between the membrane and the main part, and [0162] a passage that allows the fluid present directly below the second variable dimple to move to a location directly below the first variable dimple.

Item 11

[0163] The golf ball according to any one of Items 1 to 10, wherein a ratio of a number of the variable dimples to a total number of the dimples is not less than 50%.

[0164] The golf ball described above is suitable for shots with various golf clubs.