Patent Publication Number: US-8992339-B2

Title: Golf club head

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a golf club head capable of changing a depth of the center of gravity without substantially changing a distance of the center of gravity. 
     Performances of golf club heads change greatly by change of a location of the center of gravity. For example, in case of a golf club head that a depth of the center of gravity which is a distance between the center of gravity of a club head and a sweet spot of a club face, is small, it is easy to change the direction of the club head and, therefore, it is easy to manipulate the direction of hitting a golf ball freely. On the other hand, in case of a golf club head having a large depth of the center of gravity, a straight flight property of a hit ball is high because the direction of the club head is hard to change even at mis-shot. 
     In recent years, there are proposed golf club heads having a good so-called custom fitting that the depth of the center of gravity can be freely changed in accordance with a condition of a golfer or a course layout. For example, as shown in  FIG. 10A , this type of a golf club head “a” is composed of a head body “b” having a concave portion or hole “c” which is opened at a back face, a weight “d” which is inserted into the hole “c”, and a buffer “e” for adjusting the position of the weight inserted. Thus, a depth “f” of the center of gravity (=distance between a center of gravity “g” and a sweet spot “ss”) can be changed to a position that a golfer desires by exchanging the inserting positions between the weight “d” and the buffer “e” as shown in  FIGS. 10A and 10B . A similar golf club head is disclosed in JP 2004-159680 A. 
     This golf club head “a” can change the depth “f” of the center of gravity to a depth “f1” or to a depth “f2” by the position interchange. However, simultaneously, a distance “h” of the center of gravity which is the shortest distance from an axial center line “j” of a shaft-inserting hole “i” to the center of gravity “g” of the head “a”, also changes from a distance “h1” to a distance “h2”. Thus, the moment of inertia of the golf club head about an axis of a shaft also changes with the position interchange. If the distance “h” of the center of gravity is decreased, the moment of inertia of the golf club head about the shaft axis decreases, so the face tends to turn over the address position at the time of impact and hooking of the ball is easy to occur. On the other hand, if the distance “h” of the center of gravity is increased, the moment of inertia of the golf club head about the shaft axis increases, so the face tends to be difficult to return back to the address position at the time of impact and a slice of the ball is apt to occur. Thus, the golf club head “a” as mentioned above has a problem that performances of the golf club greatly alter. 
     It is an object of the present invention to provide a golf club head, particularly a wood-type golf club head, which is capable of changing a depth of the center of gravity without substantially changing a distance of the center of gravity. 
     This and other objects of the present invention will become apparent from the description hereinafter. 
     SUMMARY OF THE INVENTION 
     The present invention has achieved the above-mentioned object by interchanging locations of weight members which have different specific gravities from each other and which are to be fixed to a hollow head body of a golf club head. 
     Thus, in accordance with the present invention, there is provided a golf club head having a face for hitting a golf ball, a shaft-inserting hole to attach a shaft and a hollow interior, said golf club head comprising a hollow head body and a gravity center adjuster fixed to the head body, said gravity center adjuster comprising a first weight member and a second weight member having a lower specific gravity than the first weight member, said first and second weight members being provided in said head body so that the locations thereof are exchangeable with each other, whereby a depth of the center of gravity which is a distance from the center of gravity of the head to a sweet spot of the face can be changed without substantially changing a distance of the center of gravity which is the shortest distance from an axial center line of the shaft-inserting hole to the center of gravity of the head. 
     The gravity center adjuster is fixed in the hollow interior of the head. 
     It is preferable that the center of gravity of the first weight member moves substantially on the circumference of a circle whose center is an axial center line of the shaft-inserting hole, or on a tangent to a circle whose center is an axial center line of the shaft-inserting hole, by the location exchange between the first and second weight members. 
     In an embodiment of the present invention, the hollow head body includes a tubular member having an insertion slot opened at an outer surface of the head, a bottom and a hollow part or hole for inserting the gravity center adjuster, the hollow part extending from the insertion slot into the head&#39;s hollow interior and terminating at the bottom; and a cover attached to the insertion slot for preventing the gravity center adjuster from falling out of the hollow part. 
     The tubular member may be in a right tubular form wherein the hollow part extends straight, or in a curved tubular form wherein the hollow part preferably extends substantially on the circumference of a circle whose center is an axial center line of the shaft-inserting hole. 
     The tubular member may be in any form, e.g., a cylindrical form or a polyhedral form. The tubular member has a wall facing the hollow interior of the head, and the wall may be provided with an opening extending in an axial direction of the hollow part. Further, an inner surface of the wall may be in any form, e.g., a cylindrical form or a polyhedral form. 
     The gravity center adjuster may further include an elastic member having a lower specific gravity and a lower elasticity than those of the first and second weight members. The elastic member is disposed in the hollow part of the tubular member in a state of being compressed in an axial direction of the tubular member. Preferably, the elastic member is disposed in contact with the bottom of the tubular member. 
     It is preferable that the first and second weight members have a different length from each other. It is more preferable that a length L2 of the first weight member having a larger specific gravity which is measured along an axial direction of the hollow part is smaller than a length L1 of the second weight member having a smaller specific gravity which is measured along an axial direction of the hollow part. 
     The tubular member may be fixed to a sole portion of the head which provides a bottom of the head, for example, through a supporting member by welding, or by integral molding by means of casting. 
     The tubular member may be formed from a material having a smaller specific gravity than the sole portion and may be fixed to the sole portion by adhesion or mating. 
     The golf club head according to the present invention can easily change the depth of the center of gravity without substantially changing the distance of the center of gravity by location exchange between the first and second weight members. Since such a golf club head can adjust only the gravity center depth independently of the gravity center distance, it is possible to easily alter the golf club, for example, to a club which is easy to freely manipulate the direction of hitting a golf ball by decreasing the gravity center depth, or to a club which is easy to stabilize the direction of a ball hit by increasing the gravity center depth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a golf club head in the standard state according to an embodiment of the present invention; 
         FIG. 2  is a side view of the club head of  FIG. 1  viewed from the heel side; 
         FIG. 3  is a back view of the club head of  FIG. 1 ; 
         FIG. 4  is a partially cutaway perspective view of the club head of  FIG. 1 ; 
         FIG. 5A  is a cross sectional view of the club head when it has a larger depth of the center of gravity, and  FIG. 5B  is a cross sectional view of the club head when it has a smaller depth of the center of gravity; 
         FIG. 6  is a cross sectional view showing another embodiment of the present invention; 
         FIG. 7A  is a perspective view showing an example of the second weight member,  FIG. 7B  is a perspective view showing another example of the second weight member, and  FIG. 7C  is a perspective view showing still another example of the second weight member; 
         FIGS. 8A and 8B  are cross sectional views of golf club heads according to another embodiment of the present invention; 
         FIG. 9  is a cross sectional view of a golf club head according to still another embodiment of the present invention; and 
         FIGS. 10A and 10B  are perspective views of a golf club head for illustrating prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will be explained below with reference to the accompanying drawings. 
       FIGS. 1 to 3  show a golf club head  1  in a standard state according to an embodiment of the present invention. The term “standard state” as used herein denotes the state that the club head  1  is placed on a horizontal plane HP with keeping prescribed lie angle α and loft angle β (shown in  FIG. 2 ). The golf club head  1  shown in the drawings is placed in the standard state unless otherwise noted. 
     The golf club head  1  in this embodiment comprises a head body  1   a  having a hollow structure, and a gravity center adjuster  14  fixed to the head body  1   a , as shown in a partially cutaway view of  FIG. 4 . 
     The head body  1   a  includes a face portion  3  having a face  2  for hitting a golf ball on its front side, a crown portion  4  which extends from an upper edge  2   a  of the face  2  and forming the upper surface of the head  1 , a sole portion  5  which extends from an lower edge  2   b  of the face  2  and forming the bottom surface of the head  1 , a side portion  6  which extends between the crown portion  4  and the sole portion  5  to connect them from a toe side edge  2   c  of the face  2  to a heel side edge  2   d  of the face  2  through a back face BF of the head  1 , and a hosel portion  7  which is disposed on a heel side of the crown portion  4  for inserting a shaft and which has a shaft inserting hole  7   a  to attach a shaft (not shown). The head body  1   a  has a hollow interior U. Since the axial center line of the shaft inserting hole  7   a  substantially agrees with the axial center line CL of the shaft when the shaft is inserted into the hole  7   a , it is used as the axial center line CL of the shaft when no shaft is attached to the club head  1 . 
     Preferably, the head body  1   a  is formed to provide wood-type golf club heads such as driver (#1) and fairway woods. 
     The head body  1   a  in this embodiment is made of a metallic material. The metallic material is not particularly limited. Examples of the metallic material are, for instance, an aluminum alloy, a titanium alloy, a stainless steel, and other metallic materials known to use in the production of golf club heads. The head body  1   a  can be produced by known methods, for example, by joining a plurality of members or pieces in a known manner, e.g., welding, adhesion, swaging, or brazing. Each member or piece is formed by various molding methods, e.g., casting, forging and pressing. 
     The head body  1   a  in this embodiment has a two piece structure and is produced from a face plate  1   b  constituting a main part of the face portion  3 , and an integrally formed head base body  1   c  to which the face plate  1   b  is welded. However, the present invention is not particularly limited to such an embodiment. 
     From the viewpoints of large moment of inertia and improvement in flight directionality and swing balance, it is preferable that the club head  1  of the present invention has a head volume of at least 380 cm 3 , especially at least 400 cm 3 , and it has a head volume of at most 470 cm 3 , especially at most 460 cm 3 . From the same viewpoints as above, it is also preferable that the total weight of the club head  1  is at least 175 g, especially at least 180 g, and it is at most 220 g, especially at most 215 g. 
     As shown in  FIGS. 1 to 4 , the head body  1   a  includes a tubular member  10  having an insertion slot  12  opened at an outer surface  1 A of the head, a bottom  11   b  and a hollow part or hole  11  extending from the insertion slot  12  into the head&#39;s hollow interior U and terminating at the bottom  11   b ; and a cover  13  attached to the insertion slot  12 . 
     The tubular member  10  is in an approximately cylindrical shape extending straight in its longitudinal direction. It is fixed to head body  1   a , for example, to an inner surface of the sole portion  5  through a supporting member P. The tubular member  10  can be provided in the head body  1   a  by various methods so long as it does not come off by impact received at the time of hitting a ball or the like. For example, the tubular member  10  may be integrally formed with the head body  1   a  by casting. The tubular member  10  in this embodiment is disposed so that in the standard state, the height measured from the horizontal plane HP gradually increases from the inserting slot  12  to the bottom  11   b.    
     Further, as shown in  FIG. 5A , a front end  10   a  of the tubular member  10  is located in the hollow interior U of the head body  1   a  without contacting a rear surface of the face  2 . A rear end of the tubular member  10  extends toward a heel side of and a back face BF side of the side portion  6  and is opened at an outer surface of the side portion  6 . Thus, the hollow part  11  inside the tubular member  10  extends from the insertion slot  12  opened at the head&#39;s outer surface  1 A on the heel side of the side portion  6  toward a toe side of the face  2 . 
     The hollow part  11  is a space in the form of an approximately circular column. The gravity center adjuster  14  is inserted into this space, i.e., hollow part  11  of the tubular member  10 . The tubular member  10  has a smooth inner surface facing the hollow part  11 , provided that a female screw portion  11   n  for attaching the cover  13  may be formed in the inner surface within an area having an axial constant distance Li from the insertion slot  12 . 
     The cover  13  has an approximately disc-like shape as shown in  FIG. 4 , but the shape is not limited thereto. A male screw portion  13   a  to be fitted to the female screw portion  11   n  of the tubular member  10  is formed in an outer peripheral surface  13   n  of the cover  13 . The gravity center adjuster  14  can be prevented from getting out of the tubular member  10  by screwing the cover  13  onto the tubular member  10 . It is desirable to form a groove on a head&#39;s outer surface side of the cover  13  so that the cover  13  is easily detachable by a tool such as a screw driver or a special tool. 
     The gravity center adjuster  14  in this embodiment comprises a first weight member  15  having a higher specific gravity and a second weight member  16  having a lower specific gravity than the first weight member  15 , and an elastic member  17  having a lower specific gravity and a lower elasticity than those of the first and second weight members  15  and  16 . 
     The first and second weight members  15  and  16  are disposed in the head body  1   a  so that the locations thereof are exchangeable with each other in the hollow part  11  of the tubular member  10 . Thus, the golf club head  1  of the present invention is constituted so that a depth GL of the center of gravity which is a distance from the center of gravity G of the head  1  to a sweet spot SS of the face  2  can be changed without substantially changing a distance GK of the center of gravity which is the shortest distance from an axial center line CL of the shaft-inserting hole  7   a  to the center of gravity G of the head  1 , by the location exchange between the first weight member  15  and the second weight member  16 . 
     An embodiment of such a constitution for realizing the change of the gravity center depth GL without substantial change of the gravity center distance GK is shown in  FIGS. 5A and 5B . By the way,  FIGS. 5A ,  5 B,  6 ,  8 A,  8 B and  9  show cross sectional views along the line A-A of  FIG. 1  wherein the head  1  is cut at a plane passing through the head&#39;s center of gravity G and being parallel to the horizontal plane HP in a state that the shaft-inserting hole  7   a  is set up vertically with respect to the horizontal plane HP.  FIG. 5A  shows a state that the first weight member  15  is disposed in the hollow part  11  on its back face side BF and the second weight member  16  is disposed in the hollow part  11  on its face  2  side.  FIG. 5B  shows a state that location exchange has been made between the first weight member  15  and the second weight member  16  from the state shown in  FIG. 5A . As apparent from  FIGS. 5A and 5B , the center of gravity G of the head  1  shifts from a point Ga to a point Gb by the change of the location of the weight member  15 . With this change, the depth GL of the center of gravity of the head  1  decreases from GL1 to GL2. Further, the distance GK of the center of gravity changes from GK1 to GK2, but no substantial change is found between the distance GK1 before the location exchange and the distance GK2 after the location exchange. For convenience of explanation, the center of gravity Ga of the head  1  before the location exchange is also shown in  FIG. 5B . 
     The expression “without substantial change” or the like means that a change of a degree exerting no influence on hitting by a golfer is tolerated. As a result of inventor&#39;s investigation by actual hitting tests, it has been found that an influence on the gravity center depth GL with respect to a directionality of a hit ball is small when an amount of change (GK2−GK1) in the gravity center distance GK by the location exchange between the first weight member  15  and the second weight member  16  is 1.0 mm or less. Therefore, when the amount of change (GK2−GK1) is from 0 to 1.0 mm, the gravity center distance GK is regarded as substantially unchanged. 
     Another embodiment of the constitution for realizing the change of the gravity center depth is shown in  FIG. 6 . 
     In the embodiments shown in  FIGS. 5A ,  5 B and  6 , it is preferable that the center of gravity G 15  of the first weight member  15  moves substantially on a tangent line “m” to a circle Ra having a radius “ra” whose center is an axial center line CL of the shaft-inserting hole  7   a  by the location exchange between the first and second weight members. In the embodiment shown in  FIG. 6 , the center of gravity G 15  of the first weight member  15  moves from G 15   a  to G 15   b . Since the center of gravity Gm of the club head  1  except the first weight member  15  is limited to a specific location depending on the shape of the club head  1 , the center of gravity G of the head  1  to which the first weight member  15  is added is located at a position Ga or Gb allocating a straight line “n” connecting the center of gravity G 15  of the first weight member  15  and the center of gravity Gm of the club head  1  except the first weight member  15  in a weight ratio of the both members, i.e., the first weight member  15  and the club head  1  except the first weight member  15 . Symbol “na” denotes the line “n” before the location exchange, and symbol “nb” denotes the line “n” after the location exchange. When the location for disposing the first weight member  15  is set as shown in this embodiment in view of the relationship mentioned above, the center of gravity G of the head  1  moves from Ga to Gb on a circumference Rb of a circle having a radius “rb” (corresponding to the gravity center distance GK) whose center is an axial center line CL of the shaft-inserting hole  7   a . Thus, the golf club head  1  of the present invention can change the depth GL of the center of gravity without substantially changing the distance GK of the center of gravity. In  FIG. 6  are shown two first weight members  15  for convenience sake, since the states before and after the location exchange of the first weight member  15  are shown in a single drawing. 
     As shown in  FIG. 6 , it is more preferable that the centers of gravity G 15   a  and G 15   b  of the first weight member  15  before and after the location exchange are located symmetrically with respect to a contact point Za between the tangent line “m” and the circle Ra. In such a preferable embodiment, the centers of gravity Ga and Gb of the club head  1  before and after the location exchange are more easy to locate on the circumference Rb of the circle having its center at the axial center line CL of the shaft-inserting hole  7   a.    
     The gravity center distance GK of the head  1  increases with increase of the radius “r” of the circle R, so the moment of inertia about the axial center line CL increases. Therefore, in that case, there is a tendency that the face is hard to return to a state in address when hitting a ball and a slice is easy to occur. On the other hand, if the radius “r” of the circle R is too small, there is a tendency that the face is easy to return excessively over the address state when hitting a ball and a hook is easy to occur. In view of such points, it is preferable that the radius “r” of the circle Ra is at least 36 mm, especially at least 38 mm, and it is at most 43 mm, especially at most 41 mm. 
     From the viewpoint of securing the volume and rigidity of the club head, it is preferable that a specific gravity “ph” of the head body  1   a  is at least 4.30, especially at least 4.35, and is at most 4.45, especially at most 4.43. 
     As understood from  FIGS. 5 and 6 , the first weight member  15  contributes largely to the change in the center of gravity G of the head. Therefore, it is preferable that a length L2 in the axial direction of the member  15  is relatively small, and a length L1 in the axial direction of the second weight member  16  is relatively large, whereby the gravity center position of the first weight member  15  can be moved largely, so the gravity center depth GL can be changed more largely without substantially changing the gravity center distance GK of the head  1 . 
     It is particularly preferable that a ratio L1/L2 is at least 2.0, especially at least 3.0, since a large difference (change) in gravity center depth, e.g., a difference of 3 mm or more, can be obtained. On the other hand, if the ratio L1/L2 is too large, the center of gravity is hard to shift even if location exchange is conducted between the first and second weight members. Therefore, it is preferable that the ratio L1/L2 is at most 10, especially at most 8. 
     An axial length L of the hollow part  11  is preferably at least mm, more preferably at least 65 mm, and is preferably at most 100 mm, more preferably at most 90 mm. To sufficiently secure the length L of the hollow part  11  is effective for obtaining a large amount of adjustment for the gravity center depth of the head  1 . The length L of the hollow part  11  denotes a length from the bottom  11   b  of the tubular member  10  to a bottom end of the cover  13  inserted into the insertion slot  12 , that is to say, an effective length capable of holding the gravity center adjuster  14 , of the tubular member  10  except the female screw portion  11   n.    
     The axial length L2 of the first weight member  15  is not particularly limited, but if the length L2 is too large, the amount of change in the gravity center depth is small even if the location exchange is conducted with the second weight member  16 . Further, if the length L2 is too small, the weight of the first weight member  15  excessively decreases, so a large amount of change in gravity center depth is hard to be obtained. From such points of view, it is preferable that the axial length L2 of the first weight member  15  is at least 7 mm, especially at least 9 mm, and is at most 25 mm, especially at most 23 mm. 
     Similarly, if the specific gravity “ρo” and/or weight Wo of the first weight member  15  are too small, there is a possibility that the position of the center of gravity of the head  1  cannot be greatly changed even if the position of the member  15  is changed. Further, if they are too large, a swing balance tends to deteriorate since the weight of the club head  1  itself excessively increases. From such points of view, it is preferable that the specific gravity “ρo” of the first weight member  15  is at least 6.0, especially at least 7.0, and is at most 18.0, especially at most 16.0. Similarly, it is preferable that the weight Wo of the first weight member  15  is at least 8.0 g, especially at least 9.5 g, and is at most 20.0 g, especially at most 18.0 g. 
     Examples of the metallic material of such first weight member  15  are, for instance, stainless steel, tungsten, tungsten alloy, copper alloy, nickel alloy, and combinations of two or more of these metals. 
     From the viewpoint of securing a sufficient amount of movement of the first weight member  15  when exchanging the positions, the length L1 of the second weight member  16  is preferably at least 30 mm, more preferably at least 35 mm. On the other hand, if the length L1 is too large, the length L2 of the first weight member  15  is excessively decreased since the hollow part  11  has a limited length. Therefore, the length L1 of the second weight member  16  is preferably at most 70 mm, more preferably at most 65 mm. 
     The smaller the specific gravity “ρk” and weight Wk of the second weight member  16  are, the better since a larger weight can be allocated to the first weight member  15 . However, it is also important to have a sufficient strength and an adequate rigidity. From such points of view, it is preferable that the specific gravity “ρk” of the second weight member  16  is at least 0.9 and at most 1.7. Similarly, it is preferable that the weight Wk of the second weight member  16  is at least 2.5 g and at most 4.5 g. 
     Thus, the second weight member  16  is preferably made of a lightweight material having an adequate rigidity. Examples of such material are, for instance, polymeric materials such as polyethylene (PE), polyamide (nylon resins), polyurethane (PU), fluorine-containing resin (e.g., Teflon™), and other resin materials. 
     If the hardness of the second weight member  16  is too small, the member  16  easily causes plastic deformation when the cover  13  is attached and, therefore, there is a possibility that the location of the member  16  is not stable. From such a point of view, it is preferable that the second weight member  16  have a Shore D hardness of at least 60, especially at least 65. As to the upper limit thereof, it is preferable that the second weight member  16  have a Shore D hardness of about 95 or less, especially about 90 or less. 
     The shape of the second weight member  16  is not particularly limited. For example, a columnar shape as shown in  FIG. 7A  is suitable, wherein a plurality of grooves  16   g  extending in the axial direction are formed in an outer surface of a columnar body. In this embodiment, the columnar weight member  16  has an approximately crisscross section. The second weight member  16  having such a shape is preferred in enabling use of a large first weight member  15  while securing more weight margin since such a second weight member  16  has a small volume and a light weight. Further, the second weight member  16  may be formed into a hollow body having a hollow portion “k” inside it, as shown in  FIG. 7B . Such a weight member can also effectively realize a weight reduction. 
     In order to secure an adequate rigidity while achieving weight reduction of the second weight member  16 , it is preferable that a ratio Sk/Sb of a sectional area Sk of the second weight member  16  to a sectional area Sb of the hollow part  11  is at least 0.45, especially at least 0.5, and is at most 0.8, especially at most 0.75. 
     Further, as shown in  FIG. 7C , the second weight member  16  may be evenly or unevenly divided into two pieces. Since such a second weight member  16  enables to dispose the first weight member  15  at three places, it is easy to adjust the location of the gravity center depth GL. 
     As shown in  FIG. 4 , the gravity center adjuster  14  in this embodiment has, in a free state prior to inserting the hollow part  11 , a total length (L1+L2+L3) of the length L2 of the first weight member  15 , the length L1 of the second weight member  16 , and a length L3 of an elastic member  17 , and the total length (L1+L2+L3) in the free state is longer than the length L of the hollow part  11 . Thus, a compression force generates in the gravity center adjuster  14  when the adjuster  14  is inserted into the hollow part  11  and the cover  13  is attached to the insertion slot  12 , whereby the elastic member  17  having the highest elasticity among three members undergoes compression deformation most greatly in the axial direction in an elastic region, and by a reaction force thereto, the first and second weight members  15  and  16  are firmly, positionally fixed in the hollow part  11 . Thus, the club head  1  in this embodiment can surely prevent a positional shift of the first and/or second weight members  15  and  16  which may occur by impact of hitting a ball, while securing a large gravity center adjustment mentioned above. 
     In order to more stably fix the gravity center adjuster  14 , it is preferable that a ratio L3′/L3 of a length L3′ after the compression of the elastic member  17  to the length L3 before the compression is regulated in a predetermined range. That is to say, in case that the ratio L3′/L3 is too small, it is considered that the rigidity of the elastic member  17  is very small and, therefore, there is a possibility that the locations of the first and second weight members are not stabilized, so the location of the center of gravity of the head  1  changes on its own. If the ratio L3′/L3 is too large, a sufficient reaction force is hard to receive from the elastic member  17 , so the location of the center of gravity of the head  1  may change on its own. From such points of view, it is preferable that the ratio L3′/L3 is at least 0.40, especially at least 0.43, more especially at least 0.45, and it is at most 0.70, especially at most 0.67, more especially at most 0.65. 
     From the viewpoint of stably fixing the gravity center adjuster  14  (first and second weight members  15  and  16 ), the axial length L3 of the elastic member  17  is preferably at least 3 mm, more preferably at least 3.8 mm. On the other hand, if the length L3 is too large, the length of the first and second weight members is decreased. Therefore, the length L3 is preferably at most 7.5 mm, more preferably at most 6.7 mm. In particular, it is preferable that the length L3 of the elastic member  17  is smaller than the length L2 of the first weight member  15 . 
     Since the elastic member  17  is compressed in its axial direction, it is preferable that in the state prior to the compression, the elastic member  17  has a cross section area smaller than that of the hollow part  11  so as to form a space between the elastic member  17  and the inner surface of the tubular member  10 . Thus, the elastic member  17  can expand in a radial direction by a compression force received when attaching the cover  13  to the insertion slot  12 , so it can be easily compressed. Therefore, the elastic member  17  having such a size can more firmly fix the first and second weight members  15  and  16  within the hollow part  11 . 
     The material of the elastic member  17  is not particularly limited so long as it undergoes elastic deformation by a compression force generated by attachment of the cover  13 . Examples of such an elastic material are, for instance, a cured rubber wherein a rubber such as NBR or IR is vulcanized by a vulcanizing agent, a silicone rubber, a thermoplastic elastomer comprising a soft segment and a hard segment such as a styrene-based thermoplastic elastomer or a urethane-based thermoplastic elastomer, a thermoplastic elastomer such as nylon, a polymer alloy wherein at least two kinds of polymers are blended or chemically bonded. 
     The smaller the elastic member  17  is, the better in allocating a larger weight to the first weight member  15 . Thus, in order to have a sufficient strength and an adequate rigidity, the elastic member  17  is preferred to have a specific gravity pd of at least 0.85 and at most 1.80. Similarly, it is also preferable that the elastic member  17  has a weight Wd of at least 0.3 g and at most 1.0 g. Further, it is preferable that the elastic member  17  has a Shore A hardness Hd of at least 35, especially at least 45, and a Shore A hardness Hd of at most 75, especially at most 67. 
     A length L4 of the cover  13  along the axial direction of the hollow part  11  is not particularly limited. However, if the length L4 is too large, a large change in the center of gravity tends to be obtained with difficulty since the length of the gravity center adjuster  14  becomes relatively small. On the other hand, if the length L4 is too small, the cover  13  is hard to be attached to the female screw portion  11   n . From such points of view, it is preferable that the length L4 of the cover  13  is at least 4 mm, especially at least 4.7 mm, and is at most 10 mm, especially at most 9.3 mm. Further, even if the axial length or the like of the female screw portion  11   n  is previously determined, the gravity center adjuster  14  to be inserted into the hollow part  11  is not excessively compressed by the cover  13 . 
     The cover  13  receives a reaction force to a compression force generating in the gravity center adjuster  14 . Therefore, the cover  13  is preferred to have a sufficient strength. It is preferable that the cover  13  has a specific gravity ρc of at least 4.0, especially at least 4.4, and at most 8.5, especially at most 8.1. It is also preferable that the cover  13  has a weight We of at least 1.5 g, especially at least 1.7 g, and at most 3.5 g, especially at most 3.2 g. 
     Preferably, at least one opening  19 , especially a plurality of openings  19 , are provided in a wall  10   h  facing the head&#39;s hollow portion U of the tubular member  10 , whereby a weight increase of the head  1  caused by disposing the tubular member  10  can be minimized. 
     The shape of the opening  19  is not particularly limited. The opening  19  in this embodiment has a horizontally long shape extending in the axial direction of the hollow part  11 , e.g., rectangular shape. The axial length Lo of the opening  19  is preferably about 0.30 to about 0.75 times, more preferably about 0.38 to about 0.67 times, the length L of the hollow part  11  of the tubular member  10 . 
     Another embodiment of the present invention is shown in  FIGS. 8A and 8B . In this embodiment, the center of gravity G 15  of the first weight member  15  moves on a circumference Rc of a circle having its center substantially at the axial center line CL of the shaft-inserting hole  7   a  by the location exchange. In this embodiment, too, the depth GL of the center of gravity of the head can be changed without substantially changing the distance GK of the center of gravity of the head by exchanging the locations of the first and second weight members  15  and  16 . 
     The expression “substantially at the axial center line CL” comprehends not only the movement of the center of gravity G 15  of the first weight member  15  on the circumference Rc of the circle having its center at the axial center line CL, but also cases where the center of gravity G 15  of the first weight member  15  moves on a circumference Rc1 of a circle having its center at a location away from the axial center line CL by a distance W, as shown in  FIG. 8B . From the viewpoint of influence on the gravity center distance with respect to the directionality of a hit ball, it is preferable that the distance W is 3 mm or less. 
     It is preferable in practicing the embodiment as shown in  FIG. 8A  that when viewed from above in the vertical state that the shaft-inserting hole  7   a  is set up vertically with respect to the horizontal plane HP, the axial center line CN of the tubular member  10  is curved in a circular arc form so as to extend on the circumference Rc of a circle having a radius “rc” whose center is the axial center line CL of the shaft-inserting hole  7   a . In such a golf club head  1 , since the center of gravity G 15  of the first weight member  15  easily moves on the circumference Rc, the center of gravity G of the head  1  also easily moves from Gc to Gd on a circumference Rd of a circle having its center at the axial center line CL of the shaft-inserting hole  7   a . The radius “rc” of the circumference Rc is preferably at least 36 mm, more preferably at least 38 mm, and is preferably at most 43 mm, more preferably at most 41 mm. 
     Still another embodiment of the present invention is shown in  FIG. 9 . The tubular member  10  in this embodiment comprises two tubular members  10   a  and  10   b  each extending in the same direction as a tangent line “me” to a circle Re having its center at the axial center line CL and a tangent line “mf” to a circle Rf having its center at the axial center line CL and having a larger radius than the circle Re. Such a club head  1  can achieve a larger depth or smaller depth of the head&#39;s center of gravity without further changing the gravity center distance, since an arrangement pattern of the gravity center adjuster  14  can be changed in each of the tubular members  10   a  and  10   b.    
     While preferable embodiments of the present invention have been described with reference to the drawings, it goes without saying that the present invention is not limited to only such embodiments and various changes and modifications may be made. For example, although a wood-type golf club head has been exemplified in the above embodiments, the present invention is of course applicable to various types of golf club heads, e.g., wood-type, putter-type, and utility-type golf club heads. 
     The present invention is more specifically described and explained by means of the following examples. It is to be understood that the present invention is not limited to these examples. 
     Examples 1 to 8 and Comparative Examples 1 to 4 
     Wood-type golf club heads having a base structure of the head body shown in  FIGS. 1 to 4  were prepared according to the specifications shown in Table 1. 
     The respective club heads were prepared by plasma-welding a face plate and a head base body. The face plate was prepared by press molding of a Ti-6Al-4V alloy (specific gravity 4.42), and the head base body was prepared by lost-wax precision casting of the Ti-6Al-4V alloy. Main specifications of the heads are as follows: 
     Head weight: 195 g 
     Head volume: 460 cm 3    
     Loft angle: 10.5° 
     Lie angle: 58.0° 
     A first weight member made of a stainless steel (specific gravity 7.8) and a second weight member made of a polyethylene (specific gravity 0.94) were used as a gravity center adjuster, and they were formed into a columnar shape. The depth of the center of gravity of the heads was changed by using weight members having different lengths L1 and L2. The weight members used in Comparative Examples have the same weight as the corresponding weight members used in Examples, but have a different sectional area (different outer diameter) from the corresponding weight members used in the Examples. Similarly, the head bodies per se have the same weight and the same location of the center of gravity, but the thickness of the wall and the diameter of the tubular members were changed. The cover was formed from a stainless steel and the elastic member was formed from a silicone rubber, and those having common weights were used in both the Examples and the Comparative Examples. 
     The first weight member and the cover were prepared by machining using an NC cutting machine, and the second weight member and the elastic member were prepared by injection molding. 
     An “amount of change in gravity center depth” and an “amount of change in gravity center distance” were measured as a difference in gravity center depth and a difference in gravity center distance after exchanging the locations of the first and second weight members inserted into the hollow part of the tubular member. 
     Same shafts (SV-3003J, flex SR, made by SRI Sports Limited) were attached to all club heads to be tested to give wood-type gold clubs. Each of ten right-hitting golfers having a handicap of 8 to 15 struck 10 golf balls with each club. The test was made for the heads wherein the depth of the center of gravity was made small by the location exchange of the weight members, and for the heads wherein the depth of the center of gravity is made large. 
     When golf balls were struck by the heads having a small depth of the center of gravity, estimation was made as follows:
     A (excellent): golf club by which 9 or more balls could be struck freely in a desired direction   B (good): golf club by which 7 or 8 balls could be struck freely in a desired direction   C: golf club by which 5 or 6 balls could be struck freely in a desired direction   D: golf club by which 3 or 4 balls could be struck freely in a desired direction   E (bad): golf club by which at most two balls could be struck freely in a desired direction   

     When gold balls were struck by the heads having a large depth of the center of gravity, estimation was made as follows:
     A (excellent): golf club by which 9 or more balls could be struck straight   B (good): golf club by which 7 or 8 balls could be struck straight   C: golf club by which 5 or 6 balls could be struck straight   D: golf club by which 3 or 4 balls could be struck straight   E (bad): golf club by which at most two balls could be struck straight   

     A better result was adopted for each golfer, and a rating for which the largest number of golfers had estimated was adopted as a test result. 
     The test results are shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                 Com. 
                 Com. 
                 Com. 
                 Com. 
                   
                   
               
               
                   
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 1 
                 Ex. 2 
               
               
                   
               
               
                 Drawing showing structure of club head 
                 FIG. 10 
                 FIG. 5 
                 FIG. 5 
                 FIG. 5 
                 FIG. 5 
                 FIG. 5 
               
               
                 Amount of change in gravity center depth (mm) 
                  6 
                 8 
                 8 
                 8 
                 2 
                 5 
               
               
                 Amount of change in gravity center distance (mm) 
                 10 
                 2 
                 5 
                 8 
                 0 
                 0 
               
               
                 Radius of circumference on which the center 
                 — 
                 34 
                 34 
                 34 
                 34 
                 34 
               
               
                 of gravity of first weight member moves (mm) 
               
               
                 Hitting test 
                 E 
                 D 
                 D 
                 E 
                 C 
                 B 
               
               
                   
               
               
                   
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
                 Ex. 6 
                 Ex. 7 
                 Ex. 8 
               
               
                   
               
               
                 Drawing showing structure of club head 
                 FIG. 5 
                 FIG. 5 
                 FIG. 5 
                 FIG. 5 
                 FIG. 8A 
                 FIG. 9 
               
               
                 Amount of change in gravity center depth (mm) 
                 8 
                 5 
                 8 
                 8 
                 8 
                 8 
               
               
                 Amount of change in gravity center distance (mm) 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Radius of circumference on which the center 
                 34 
                 36 
                 36 
                 43 
                 40 
                 40 
               
               
                 of gravity of first weight member moves (mm) 
               
               
                 Hitting test 
                 B 
                 B 
                 A 
                 A 
                 A 
                 B 
               
               
                   
               
            
           
         
       
     
     From the results shown in Table 1, it is confirmed that the golf club heads of the Examples according to the present invention have received a high evaluation as a golf club head capable of changing the depth of the center of gravity.