Patent Publication Number: US-2006009302-A1

Title: Golf club

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to a golf club and more particularly to a combination of a club head having a specific moment of inertia and a club shaft having a specific shaft-torque.  
      Heretofore, in improving the directional stability of the struck balls, it has been considered to be effective to increase the moment of inertia of the club head or to change the bending rigidity and twist rigidity of the club shaft as shown in Japanese patent application publication NOS. 7-112041, 8-187308, 9-38254 and 11-76479. These prior arts, however, failed to provide discussion about the relationship between such parameters of the club head and shaft.  
      In general, in the case of golf clubs targeted at professionals and advanced golfers, the moment Ix of inertia of the head is relatively small, and also the shaft-torque is relatively small. Contrary, in the case of golf clubs targeted at high-handicapped golfers, the moment Ix of inertia and shaft-torque T are relatively large.  
      AS to golf swing, the professionals and advanced golfers can make a body-led swing, and the swing path or swing circle and ball hitting position are stable. And as the head speed is high, the backspin tends to become excessively increased. Therefore, in the club heads targeted at professionals and advanced golfers, it is desirable that the center of gravity of the club head is relatively low in order to avoid an excessive backspin rather than that the moment Ix of inertia is increased on the assumption of miss shots. Further, it is also desirable that the club is sensitive about the golfer&#39;s swing motion, therefore, the moment Ix of inertia and shaft-torque are limited to small values. In the case of the golf clubs targeted at professionals and advanced golfers, the traditional design concepts would appear like that.  
      On the contrary, high-handicapped golfers tend to make a hand swing as the body-led swing is difficult. Especially, the wrist is much more likely to move largely near the impact position. As a result, the swing path and ball hitting position become unstable. Thus, in the case of golf clubs targeted at high-handicapped golfers, on the assumption of miss shots, it is desirable that the club head is increased in the moment Ix of inertia as large as possible. As to the club shaft, on the other hand, it is considered as ideal that the torque is increased so as to cancel the variation of the golf club swing motions and also in order to increase the ball launch angle. In the case of a club shaft having a large torque, its twist can absorb unfavorable wrist motion, and thus the directional stability is not so decreased by the unfavorable wrist motion. During downswing from the top position, the club head is rotated around the shaft center line due to the twist of the club shaft, because the center of gravity of the club head is positioned at a certain distance from the shaft center line. Accordingly, the amount of the rotation becomes larger as the shaft-torque becomes larger. If this rotational motion is large, the ball launch angle becomes increased owing to the loft angle.  
      In general, it is ideal that, during downswing from the top position, the club face is once largely opened, and the opened club face is gradually corrected so as to return the club face to square at impact. However, if the moment Ix of inertia is large, as the club head is hard to rotate, it is difficult to return the club face to square at impact, and the club face is liable to open at impact as shown in  FIG. 6 . Thus, there is a tendency for the high-handicapped golfers to further twist the wrist to make the club face at square, and as a result, the swing is further disturbed and the ball direction becomes unstable.  
     SUMMARY OF THE INVENTION  
      It is therefore, an object of the present invention to provide a golf club easy to use for high-handicapped golfers, in which the returning of the club face at impact and the head movement when missed a shot are improved and the directional stability is improved.  
      According to the present invention a golf club comprises a golf club shaft and a golf club head attached to a tip end of the golf club shaft, wherein a moment Ix of inertia of the golf club head around a vertical axis passing through the center of gravity of the club head is not less than 3800 (g·sq.cm), and a shaft-torque T (degrees) of the golf club shaft and the moment Ix of inertia satisfy the following condition (1): T=&lt;0.0025×Ix−6.0. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a front view of a golf club according to the present invention showing a club head and a club shaft.  
       FIG. 2  is a front view of the club head.  
       FIG. 3  is a top view of the club head.  
       FIG. 4  is a diagram for explaining a method of measuring a shaft-torque.  
       FIG. 5  shows prepreg pieces used to make the club shaft.  
       FIG. 6  is a diagram for explaining an inadequate returning of the club face at impact. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      Embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings.  
      According to the present invention, golf club  1  comprises a club head  2  and a shaft  3  having an end attached to the head and a butt end provided with a grip  4 . The golf club  1  is a wood type club inclusive of number one to five wood. The golf club  1  in this embodiment is a number one wood.  
      The club head  2  comprises a face portion  2   a  whose front face defines a club face F for striking a ball, a crown portion  2   b  intersecting the club face F at the upper edge thereof, a sole portion  2   c  intersecting the club face F at the lower edge thereof, a side portion  2   d  between the crown portion and sole portion which extends from a toe-side edge to a heel-side edge of the club face through the back face of the club head, and a hosel portion  2   e  to be attached to an end of a club shaft  3 .  
      Here, the definitions of the undermentioned terms are provided:  
      “Standard state” of the club head is that the club head is set on a horizontal plane HP so that the shaft center line CL is, within a vertical plane VP, inclined at its lie angle alpha with respect to the horizontal plane HP, and the club face F is inclined at its face angle beta with respect to the vertical plane VP, wherein the face angle beta is, as shown in  FIG. 3 , an angle between a horizontal tangential line N to the sweet spot SS on the club face F the vertical plane VP.  
      “Sweet spot SS” is a point on the club face F which is the intersecting point of a straight line, which is drawn perpendicular to the club face F from the center G of gravity of the club head, with the club face F.  
      “Moment Ix of inertia” is a moment of inertia of a club head alone around the vertical axis VL passing through the center G of gravity of the club head in the standard state. The club head includes a coating painting but excludes a socket for the shaft disposed at the hosel. In this specification, the values measured with a device manufactured by INERTIA DYNAMICS Inc. are shown.  
      “overall length L” of the club is, as shown in  FIG. 1 , defined as a length measured in the standard state from the end  4   e  of the grip  4  to the intersecting point P of the shaft center line CL with the horizontal plane HP.  
      “Shaft-torque” is a twist angle (degrees) of the shaft  3  when, as shown in  FIG. 4 , holders M 1  and M 2  are attached to the shaft  3  at 40 mm and 865 mm, respectively, from the one end  3   a  and a torque of 136.3 N·cm is applied to the holder M 2  on the butt end side while fixing the holder M 1  on the tip end side.  
      In this embodiment, the club head  2  is hollow and made of at least one kind of metal material, e.g. aluminum alloy, titanium, titanium alloy, magnesium alloy, stainless and the like. Aside from such all-metal head, the head may be a hybrid head including a part made of a nonmetallic material such as fiber reinforced resinous material. Further, it may be possible to make the entirety of the head from one or more kinds of nonmetallic materials.  
      The head  2  can be manufactured by assembling a plurality of parts (for example 2 to 4 parts) which are made through appropriate methods. For example, a metal part may be manufactured by casting, forging, press forming or the like. In order to fix one part to another, various methods, e.g. welding, bonding, soldering diffusion bonding, caulking and the like can be used in accordance with the materials of the parts.  
      For example by changing the material thickness in the sole portion  2   c  and/or side portion  2   d , the head  2  is formed to have a moment Ix of inertia in a range of not less than 3800 g·sq.cm, preferably not less than 3850 g·sq.cm, more preferably not less than 4050 g·sq.cm. If the moment Ix is less than 3800 g·sq.cm, the effect to control a movement of the club head, which is caused by a reaction force when missed a shot, becomes insufficient, but rather the problem in the insufficient return motion of the club head during downswing becomes not remarkable.  
      Although it is preferable that the head has a larger moment Ix, if in compliance with a golf club rule the upper limit of the head volume is 470 cc, it will be preferable that the moment Ix is at most 5500 g·sq.cm, more preferably not more than 5000 g·sq.cm, still more preferably not more than 4400 g·sq.cm from realistic views such as production efficiency.  
      If the volume of the club head  2  is too small, it is difficult to increase the moment Ix of inertia. If the volume of the club head  2  is too large, it is difficult to restrain the club head weight from increasing and the club has a tendency to become hard to swing. Thus, the head volume is preferably not less than 300 cc, more preferably not less than 320 cc, still more preferably not less than 350 cc. when compliance with the R&amp;A golf rule, the upper limit is 470 cc as noted above.  
      If the club head is too light, it becomes impossible to obtain the above-mentioned large moment Ix of inertia. If the club head is too heavy, the club has a tendency to become hard to swing. Thus, the weight of the club head is preferably set in the range of not less than 170 grams, more preferably not less than 175 grams, still more preferably not less than 180 grams, but preferably not more than 230 grams, more preferably not more than 220 grams, still more preferably not more than 210 grams.  
      Therefore, the effective control of the head movement when missed a shot and the return motion of the club head during downswing can be achieved  
      In this embodiment, in order to make a club which is easy to swing through, the shaft  3  is made from a fiber reinforced resinous material to have a tubular structure. Further, in view of flexibility of designing and controllability of the shaft torque, the use of a fiber reinforced resinous material is preferable.  
      As shown in  FIG. 4 , the outer diameter of the shaft is decreased from one end  3   b  provided with a grip  4  to the other end  3   a  provided with the head  2 .  
      According to the present invention, the torque T (degrees) of the shaft  3  satisfies the following condition (1) 
 
 T=&lt; 0.0025 ×Ix− 6.0  (1) 
 
 preferably, the following condition (2) 
 
 T=&lt; 0.0025 ×Ix− 6.3  (2) 
 
 more preferably, the following condition (3) 
 
 T&gt;= 0.0025 ×Ix− 8.5  (3) 
 
 wherein 
 
 Ix is the above-mentioned moment Ix of inertia of the head. 
 
      By satisfying these conditions, the twist of the shaft  3  during downswing can be controlled to decrease the rotational motion of the club head  2  during downswing. As a result, the club face F becomes easy to return to its original address position, and the hitting directions become stable.  
      The shaft  3  may be manufactured by various methods, for example: winding or applying prepreg sheets around a mandrel; winding reinforcing filaments around a mandrel; molding the fiber reinforced resinous material by pressurizing the inside and the like.  
      In this embodiment, the winding of prepreg sheets is employed.  
      In the case of the shaft made from prepreg, it is necessary to increase the number of bias prepreg plies in order to decrease the torque. This results in a heavy shaft which is relatively hard to swing. If the torque T is too small, the hit feeling becomes hard and unfavorable. Further, there is a possibility that the hitting directions become unstable because an improper wrist motion during golf swing is sensitively transferred to the club head  2 .  
      Therefore, it is preferable that the torque T is set to be not less than 1.5 degrees, more preferably not less than 2.0 degrees, still more preferably not less than 2.5 degrees.  
      Although the upper limit of the torque T is determined by the above-mentioned conditional expressions (1) and (2), it is absolutely desirable that the upper limit is not more than 7.0 degrees, more preferably not more than 6.5 degrees, still more preferably not more than 6.0 degrees.  
      In the case of a club head having a larger moment Ix of inertia, if a ball hits off the sweet spot, as the club head is hard to rotate, the impact force is directly transmitted to the player&#39;s hands and as a result, the hit feeling becomes hard. When the condition (3) is satisfied, as the lower limit of the shaft torque is changed according to the moment Ix of inertia, the impact force is absorbed by the twist of the club shaft and a good hit feel can be obtained. Thus, both of the directional stability and hit feeling can be achieved.  
      If the overall length L of the club  1  is too short, it is difficult to obtain a long traveling distance or flying distance as the head speed becomes relatively slow. If the overall length L is too long, it becomes difficult that the sweet spot meets the ball and the club becomes awkward.  
      Therefore, the overall length L is preferably set in the range of not less than 43 inches, more preferably not less than 44 inches, still more preferably not less than 45 inches, but preferably not more than 48 inches, more preferably not more than 47 inches.  
      Comparison Tests  
      Wood-type golf clubs (driver) having the specifications shown in Table 1 were made and tested for the stability of hitting direction and the hit feelings.  
      The club heads each have a two-piece structure composed of a head main body formed by casting a titanium alloy 6-4Ti, and a face member formed by press-molding and milling a rolled titanium alloy 6-4Ti, and the face member and head main body were united by TIG welding.  
      The club heads had a volume of 420 cc and a weight of 195 grams in common, but the heads had different thickness distributions as the thickness was changed in the sole portion and side portion to change the moment Ix of inertia.  
      The shafts were each formed by applying prepreg pieces. AS shown in  FIG. 5 , to form bias plies and longitudinal plies, two kinds of prepreg manufactured by Toray Industries, Inc., were used.  
      &lt;Bias Ply&gt;
          Reinforcing fiber: M40J (tensile elastic modulus 377GPa)     Resinous material: Epoxy resin     Resin content: 25%        

      &lt;Longitudinal Plies&gt;
          Reinforcing fiber: M30S (tensile elastic modulus 294 Gpa)     Resinous material: Epoxy resin     Resin content: 25%        

      The shaft-torque can be changed by increasing or decreasing the tensile elastic modulus of the reinforcing fiber, but if the modulus exceeds 450 Gpa, then the strength is decreased and the shaft is liable to be broken. Thus, in order to avoid a drastically change in modulus, the shaft-torque was adjusted by changing the number of bias plies or layers.  
      In the shaft of Ex. 1, the total number of bias plies was two on the butt end side and five on the tip end side. In the others, the torque was changed by decreasing or increasing the number of plies by the same number on both of the butt end side and tip end side. In order that all the shafts had the same flex, the areal weight of the prepreg used as the longitudinal plies was changed.  
      The heads and shafts were assembled, and wood type golf clubs having an overall length of 45 inches were made.  
      Directional Stability Test:  
      Each of ten golfers having handicaps ranging from 20 to 35 struck three-piece balls (commercially available as “Hi-BRID everio” manufactured by SRI Sports, co., Ltd.) ten times with each of the golf clubs. Then, the shortest distance of the stop position of the ball from a straight line drawn from the hitting point to the target point was measured to obtain a hundred measured values (ten times×ten golfers) per a golf club. The results are indicated in Table 1 as the average of a hundred values. The smaller the value, the better the directional stability.  
      Hit Feeling Test:  
      Through the above-mentioned directional stability test, the golfers evaluated the hit feeling of each club based on the following criteria.  
                                                      5: soft and very good   4: fairly soft and good           3: usual   2: slightly hard and not good           1: hard and singularly bad                      
 
      The average values of the ten golfers&#39; evaluations are indicated in Table 1.  
      Swing Feeing Test:  
      Through the above-mentioned directional stability test, the golfers evaluated the swing feeling of each club based on the following criteria.  
                                                      5: very easy   4: easy           3: usual   2: slightly difficult           1: very difficult                      
 
      The average values of the ten golfers&#39; evaluations are indicated in Table 1.  
                       TABLE 1                                      Club                                                                     Ref. 1   Ref. 2   Ex. 1   Ex. 2   Ex. 3   Ex. 4   Ex. 5   Ex. 6   Ex. 7   Ex. 8   Ex. 9   Ex. 10                                                                             Moment Ix of inertia   3850   4400   3850   4100   4400   4100   4100   4100   3960   4300   4250   4350       (g · sq. cm)       Shaft-torque T (degs.)   4.5   5.7   3.3   4   4.7   2.5   2   1.3   2.8   3.7   2.3   2.8       Shaft weight (g)   50   48   58   53   50   63   67   75   61   55   65   61       Directional stability   56.8   51.2   36.7   33.2   31.6   30.8   29.2   26.3   35.3   31.2   29.6   30.3       (distance in yard)       Hit feeling   4.3   4.5   4   4.2   4.4   3.3   2.6   2.1   3.6   3.7   2.5   2.8       Swing feeing   4.3   4.1   3.6   3.9   4.5   3.1   2.7   2.2   3.3   3.8   2.9   3.5                  
 
      From the test results, it was confirmed that the directional stability can be improved.  
      As descried above, in the golf club according to the present invention, as the moment is limited in a specific range, directional stability can be maintained even when missed a shot. Further, by providing a lower limit for the shaft-torque, an incidental unfavorable wrist motion at or near impact becomes hard to transmit to the club head, and the directional stability can be improved. Also hit feeling can be improved.