Abstract:
A golf club shaft is provided including a metal tip section and a composite butt section. The butt section includes a reduced diameter portion telescopically received within an axial bore of the tip section. An adhesive is disposed between the tip section and the butt section to secure the two together. An insulating layer may be disposed between the tip section and the butt section to prevent galvanic corrosion.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a golf club shaft having improved performance characteristics and more particularly to a two piece golf club shaft combining a metal portion and a composite portion which retains the advantages of both materials while eliminating their disadvantages.  
           [0002]    Control and accuracy in the game of golf is influenced by the torsional stiffness of the shaft. The torsional stiffness of the shaft resists twisting of the club head during the swing and particularly when there is less than perfect contact between the golf ball and club head. Metal golf club shafts, the most popular metal being steel, are used by many golfers. An advantage of steel shafts is their high torsional stiffness which are known in the relevant art as “low torque” shafts.  
           [0003]    Many golf shaft manufacturers offer composite shafts, often referred to as graphite shafts, which are usually made from a composite of graphite or carbon fiber and epoxy resin. Composite shafts can be significantly lighter than metal shafts, but the torsional stiffness of a conventional graphite shaft is less than that of a steel shaft. Graphite shafts are therefore known in the art as ‘higher torque’ than steel shafts.  
           [0004]    In an attempt to torsionally stiffen composite shafts, manufacturers have used different fiber types such as high modulus carbon, aramid and boron fibers. They have also varied the construction by wrapping the fibers at different angles in an attempt to improve torsional stiffness. Most of these changes have increased the cost and often had a negative effect on the playability of the shaft.  
           [0005]    Recent studies have shown that only the tip section of the shaft provides the torsional resistance to prevent club head twisting due to poor ball/head contact. Contact between the club head and ball is a very brief dynamic event and only the tip section of the shaft gets loaded during this time period. The event is effectively over before the full length of the shaft is loaded. It is therefore desirable to construct the tip section of the shaft from metal which has a high torsional stiffness, such as steel, whereas the butt section can be constructed from a composite, such as graphite, with a lower torsional stiffness. In such a manner, the effective torque characteristic of the shaft can be enhanced while maintaining a light weight.  
           [0006]    One attempt to combine the advantages of metal and composite shafts is disclosed in U.S. Pat. No. 4,836,545 to Pompa. However, Pompa does not describe the affect the physical characteristics of the two shaft sections has on shaft performance. The length and weight of the two sections of the Pompa shaft were arbitrarily selected. Testing also revealed that the weight of the metal tip section was extremely heavy as compared to the weight of the composite butt section. By having a tip section that was too heavy, the center of gravity (CG) of the shaft moved towards the tip end and undesirably increased the swing weight of the club.  
           [0007]    Swing weight is a measure of the static moment of the assembled club about a point usually 14″ from the grip end. The absolute weight and balance point (CG position) of head, shaft and grip all affect club swing weight. Although it is common for major club manufacturers to specify head weights to achieve desired club swing weights knowing the specifications of the shaft and grip, this approach is not always possible. Component suppliers typically offer club head weights which conform to industry accepted weight ranges. As such, it is highly desirable to provide shafts that may be used with such heads to achieve popular club swing weights.  
           [0008]    Further, it is undesirable for the head to receive secondary weighting to achieve a desired club swing weight. Secondary weighting is usually introduced at the extreme tip end of the shaft where the shaft is inserted in the hosel of the head. This positioning is not optimal being away from the head center of gravity and thus reduces momentum transfer to the ball for a given swing speed. Less momentum transfer to the ball reduces the distance the ball will travel.  
           [0009]    Pompa also failed to appreciate the difficulty of joining the metal and composite sections of the shaft. The joint must be cosmetically acceptable and strong enough to prevent failure.  
           [0010]    In view of the foregoing, it would be desirable to provide an improved golf club shaft that exhibits the advantages of both metal and composite shafts while eliminating their respective disadvantages.  
         SUMMARY OF THE INVENTION  
         [0011]    It is a primary purpose and principal objective of the present invention to provide a golf club shaft combining the separate advantages of metal and composite shafts into a single, hybrid design.  
           [0012]    It is another objective of the present invention to eliminate the separate disadvantages of metal and composite shafts in a single, hybrid design.  
           [0013]    It is yet another objective of the present invention to provide a method for manufacturing a golf club shaft having a tip section of a metallic material and a butt section of a composite material.  
           [0014]    It is still yet another objective of the present invention to provide a method and device for achieving the above objectives while conforming to the rules of golf as defined by the United States Golf Association.  
           [0015]    According to one embodiment of the present invention, a golf club shaft is provided including a metal tip section and a composite buff section. The cylindrical tip section is formed of a metallic material such as steel. The cylindrical butt section is formed of a composite material such as graphite and includes a reduced diameter portion or plug formed at an end thereof. The plug of the butt section is telescopically received in the end of the tip section such that the end of the tip section overlaps the reduced diameter portion of the butt section. An adhesive, such as epoxy, is disposed between the tip and butt sections to secure the two sections together.  
           [0016]    In another embodiment of the present invention, an insulating layer is disposed between the tip and butt sections to prevent metal to composite contact within the metal/composite joint. By preventing metal to composite contact, the insulating layer reduces or eliminates the potential for galvanic corrosion within the joint. The insulating layer may be formed as a plurality of insulating spacers such as glass beads or a glass layer built into the butt section within the joint.  
           [0017]    The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description taken in conjunction with the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a schematic side elevation view of a golf club shaft incorporating the teachings of the present invention;  
         [0019]    [0019]FIG. 2 is a graph showing the relationship of torsional resistance to metal tip length;  
         [0020]    [0020]FIG. 3 is a graph showing the relationship of torsional resistance to metal and composite tip lengths;  
         [0021]    [0021]FIG. 4 is a graph showing the relationship of shaft weight to tip length and its affect on club swing weight;  
         [0022]    [0022]FIG. 5 is a detailed schematic cross sectional view of a metal/composite joint of the golf club shaft of FIG. 1;  
         [0023]    [0023]FIG. 6 is a graph showing the relationship of the tip bending stiffness of the shaft of the present invention to that of a conventional steel shaft;  
         [0024]    [0024]FIG. 7 is a graph showing the relationship of the tip wall thickness of the shaft of the present invention to that of a conventional steel shaft;  
         [0025]    [0025]FIG. 8 is a graph showing the relationship of the butt wall thickness of the shaft of the present invention to that of a conventional graphite shaft;  
         [0026]    [0026]FIG. 9 is a detailed schematic cross sectional view of a metal/composite joint of the golf club shaft of FIG. 1 according to a second embodiment of the present invention;  
         [0027]    [0027]FIG. 10 is a detailed schematic cross sectional view of a metal/composite joint of the golf club shaft of FIG. 1 according to a third embodiment of the present invention; and  
         [0028]    [0028]FIGS. 11 a  and  11   b  are detailed schematic cross sectional views of a metal/composite joint of the golf club shaft of FIG. 1 according to a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]    Referring to FIG. 1, there is shown a golf club  10  having a grip  12 , a head  14  and a tubular shaft  16 . Although the club  10  is illustrated as a wood, it may also be an iron or a putter. The shaft  16  includes a tip section  18  and a butt section  20 . The tip section  18  is preferably formed of a metallic material such as high strength steel while the butt section  20  is preferably formed of a composite material such as graphite. While the shaft  16  has been illustrated as having a smooth, tapered sidewall  22 , it should be appreciated that a parallel or stepped sidewall could substitute therefore.  
         [0030]    The tip section  18  is secured at a lower end  24  to head  14  by sizing it to fit standard club head hosel sockets. The upper end  26  of tip section  18  is telescopically and slidingly fit over the lower end  28  of the butt section  20 . The physical characteristics of the tip section  18  from head  14  to the joint  30  where it meets the butt section  20 , are designed to provide desired balance of torsional stiffness, bending stiffness (flex), strength, and weight in order to yield the best playability when combined with the composite butt section.  
         [0031]    The relationship between the physical characteristics of the tip  18  and the playability of the shaft  16  is complex and many factors must be taken into consideration.  
         [0032]    1) As the metallic tip section  18  is shortened, the torsional stiffness it provides becomes less significant. As the tip section  18  is lengthened, the weight of the tip section  18  becomes more significant.  
         [0033]    2) It is desirable to retain an industry standard diameter of either 0. 335 or 0.350 inches at the lower end  24  of the tip section  18  to allow fitment of industry standard club heads. However, the diameter can be increased towards the upper end  26  of the tip section  18  to increase both torsional and bending stiffness.  
         [0034]    3) For the same weight tip section  18 , increasing the diameter at the upper end  26  decreases the wall thickness and reduces durability.  
         [0035]    4) To minimize the weight of the tip section  18 , the wall of the tip section  18  can be made thinner. However, as the wall thickness is reduced, the strength and stiffness of the tip section  18  is reduced.  
         [0036]    5) As the diameter and wall thickness of tip section  18  is varied, the bending stiffness (flex) is also changed. If the bending stiffness (flex) is too high or too low the playability and feel of the shaft becomes unacceptable.  
         [0037]    Extensive playability and durability testing has allowed an acceptable geometry range and a preferred geometry to be defined for the tip section  18  of a 46 inch wood shaft weighing between 65 g and 90 g.  
         [0038]    If the length of tip section  18  is less than 6 inches it does not provide sufficient torsional stiffness to improve shot accuracy. FIG. 2 shows how the torque of the club, as measured using a torque test, is reduced as the length of the tip section is increased. FIG. 3 shows the lower torque characteristics of a steel tip compared to a graphite tip.  
         [0039]    If the tip section  18  is greater than 12 inches, the shaft weight is undesirably increased and the center of gravity position of the shaft  16  is moved too far towards the tip end  24 . FIG. 4 shows how the shaft weight increases with tip length. FIG. 4 also shows that with shaft weights for tip lengths between 6 and 12 inches it is possible to achieve club swing weights ranging from D1 to D5 using an industry accepted head weight.  
         [0040]    If the diameter of the upper end  26  of the tip section  18  is increased above 0.415 inches, the bending stiffness becomes undesirably high adversely affecting tip flexibility and providing a low ball trajectory and a harsh feel to the club. Likewise, if the diameter of the upper end  26  of the tip section  18  is reduced below 0.385inches, the bending stiffness is undesirably low providing a high ball trajectory and too soft a feel to the club.  
         [0041]    Durability testing carried out with an air cannon has shown that diameters above 0.415 inches for the upper end  26  of tip section  18  with a length range from 6 to 12 inches and an overall weight of shaft  16  of less than 65 g does not provide a tip section  18  with sufficient durability.  
         [0042]    In the preferred embodiment, a 46 inch wood shaft weighs 75 g and has a tip section  18  with a length of 8 inches and a diameter at the upper end  26  of .4 inches and a diameter of 0.335 or 0.350 inches at the lower end  24 . Such a preferred tip section  18  has a torque of less than 0.6 degrees over the 8 inch length when measured using a torque test. This compares with a torque of greater than 1.5 degrees for the tip section of a typical graphite shaft measured using the same test method.  
         [0043]    Turning now to FIG. 5, the joint  30  of FIG. I is illustrated in greater detail. An important aspect affecting the durability of the shaft  16  is the strength of the joint  30  between the metal tip section  18  and the composite butt section  20 . As can be seen, the tip section  18  is in the form of a hollow metal cylinder and the butt section  20  is formed as a hollow composite cylinder. The butt section  20  includes a reduced diameter cylindrical portion or plug  32  for insertion into the tip section  18 . The reduced diameter portion  32  may be formed during the lay-up of the composite butt section  20  or may be formed by grinding away a pre-selected annular amount of the butt material after initial formation. The reduced diameter portion  32  is dimensioned to ensure a sufficient overlap and durable interconnection with the tip section  18 .  
         [0044]    The metal tip section  18  and composite butt section  20  are joined together with an adhesive, such as epoxy bond  31 . The thickness of the adhesive  31  is carefully controlled and the surface area of the tip section  18  and butt section  20  along the adhesive  31  is sufficient to ensure adequate strength. Bond strength is selected such that the joint  30  does not fail in shear from the torsional loads imposed through generally accepted levels of abuse while playing the game of golf. Limiting the maximum thickness of the adhesive  31  and increasing the surface area of the joint  30  also maintains the highest straightness standard for the assembled shaft  16 .  
         [0045]    Static and dynamic durability testing has shown that bond thickness should be controlled to between 0.003″ and 0.006″. Testing has also shown that for a metal tip section  18  with a diameter at the upper end  26  of between 0.385″ and 0.415″ the composite butt section  20  should be inserted into the metal tip section  18  between about 0.75″ and about 1.5″ to provide an adequate bond area. In the preferred embodiment, a 46 inch shaft driver has a bond thickness of .0045″ and the composite butt section  20  is inserted 1.25″ into the metal tip section  18 . Such geometry has been proven to provide adequate strength and straightness in the assembled shaft  16 .  
         [0046]    The overall bending stiffness of the shaft  16 , which defines the shaft flex, is influenced by the design of the tip section  18 , the butt section  20  and the geometry of the joint  30 . Local stiffness in the joint could be high and the length of the joint  30  must be such to provide sufficient durability while not being excessively stiff.  
         [0047]    Flex ranges for various categories of players with different swing characteristics are generally accepted throughout the industry with those provided by True Temper Dynamic (trademark) shafts often being used as a point of reference. Using the geometry range and overall shaft weights defined above, FIG. 6 shows the bending stiffness of shaft  16  through the joint compares favorably to that of a Dynamic shaft ensuring excellent feel and desirable ball flight. The stiffness of the shaft in FIG. 6 is measured as the tip deflection in a simple cantilever load test.  
         [0048]    [0048]FIGS. 7 and 8 compare the wall thickness along the length of the metal tip section  18  and composite butt section  20  in the preferred embodiment of shaft  16  with the wall thickness found in a popular True Temper Dynamic (trademark) steel shaft and a popular Grafalloy Prolite (trademark) graphite shaft. It will be apparent that the wall thickness in the shaft  16  is very different to that in the available True Temper steel and Grafalloy graphite shafts. This illustrates that the shaft  16  cannot be made by bonding together tip and butt sections cut from commercially available steel and graphite shafts.  
         [0049]    Referring again to FIG. 5, the formation of the reduced diameter portion  32  also defines an edge in the form of a radial wall  34  in the butt section  20 . Although the radial wall  34  is illustrated as extending orthogonally to the reduced diameter portion  32 , the radial wall  34  may also be formed at an acute or obtuse angle relative thereto. The radial wall  34  is preferably dimensioned so as to be equal to or slightly greater than the sum of the thickness of the end  38  of the tip section  18  and the thickness of the adhesive  31  so as to yield a smooth-wall, concentric transition between the tip section  18  and the butt section  20  along the perimeter of the shaft  16  adjacent the joint  30 .  
         [0050]    Turning now to FIG. 9, a second embodiment of the present invention is illustrated. In this embodiment, the components which are the same as those in the previous embodiment are identified with the same reference numeral but increased by 200. The second embodiment differs from the previous embodiment by the insertion of an insulating layer  252  in the form of a plurality of spacers between the tip section  218  and the butt section  220 . The insulating spacers  252  are preferably in the form of beads and are preferably formed of an insulating material such as ceramic or glass. The insulating beads  252  prevent the metal of the tip section  218  from contacting the graphite of the butt section  220  to reduce or eliminate galvanic corrosion within the joint  230 .  
         [0051]    The beads  252  also help control the alignment and separation of the tip section  218  relative to the butt section  220 . In this regard, the diameter of the beads  252  is selected in accordance with the gap  242  so as to provide sufficient space for the adhesive  231  between the beads while also coaxially aligning the tip section  218  with the butt section  220  so as to ensure a smooth perimeter surface along the shaft  216  adjacent the joint  230 . Preferably, the beads  252  are pre-mixed with the adhesive  246  prior to its application within the joint  230 .  
         [0052]    Turning now to FIG. 10, a third embodiment of the present invention is illustrated. In this embodiment, the components which are the same as those in the previous embodiments are identified with the same reference numeral but increased by 300. The third embodiment differs from the previous embodiments by the inclusion of an insulating layer  352  in the form of an overlayer between the tip section  318  and the butt section  320 . The overlayer  352  is preferably in the form of an insulating layer integrally formed along the outboard surface of the reduced diameter portion  332  and is preferably formed of an insulating material such as ceramic or glass to prevent the metal of the tip section  318  from contacting the graphite of the butt section  320  to reduce or eliminate galvanic corrosion within the joint  330 . The layer  352  also helps control the alignment and separation of the tip section  418  relative to the butt section  320 . The layer  352  is preferably formed by forming the butt section  320  with a relatively thick layer of glass at one end thereof prior to the formation of the reduced diameter portion  332 . The reduced diameter portion  332  is then formed by grinding away a pre-selected annular amount of the glass so as to leave the layer  352  as an outboard surface of the reduced diameter portion  332 . In this way, the fabric  352  isolates the remaining composite material of the butt section  320  from the metallic material of the tip section  318 .  
         [0053]    Referring to FIGS. 11  a  and  11   b , in production shafts, a plastic ferrule  500  is incorporated in the joint  430  between the steel tip  418  and the graphite butt section  420 . The ferrule  500 , is made from suitable extruded or injection molded plastic and is used to accommodate any slight geometrical misalignment between the graphite section  420  and steel section  418 . The outside diameter of the ferrule  500  is sized to be slightly larger than the diameter of either the graphite section  410  or steel section  418 . After assembly, excess material can be removed from the plastic ferrule  500  either by buffing on a fine abrasive belt or by wiping with a solvent such as acetone. Removing material from the ferrule  500  in this way can provide a smooth transition on the outside surface between the steel and graphite sections  418  and  420 .  
         [0054]    The cross section of the ferrule  500  can be altered from the rectangular form in FIG. 11  a  to incorporate inwardly sloping faces as shown in FIG. 11  b . Such ferrule geometry can be used to accommodate a small radius in the corner of the machined graphite butt section  420  that can ease the manufacture of the shaft  416 .  
         [0055]    Referring again to FIGS. 1 and 2, the steps for manufacturing the shaft  16  will be described. The hollow cylindrical butt section  20  is formed to a given length by arranging a plurality of layers of a pre-selected composite fibers such as carbon-graghite at different angles relative to one another and bonding them with a resin. The butt section  20  may be formed with parallel, tapered or stepped sidewalls as desired. The reduced diameter portion  32  is formed at one end of the butt section  20  during lay-up or by grinding away a pre-selected annular amount of the material at one end thereof.  
         [0056]    The hollow cylindrical tip section  18  is formed to a given length by drawing a blank of metallic material such as a high strength steel or aluminum through a mandrel. The tip section  18  may be formed with parallel, tapered or stepped sidewalls as desired. The length and weight of the tip section  18  is selected as described above.  
         [0057]    An adhesive is deposited on at least one of the reduced diameter portion  32  and the inside of the tip section  18 . The reduced diameter portion  32  is then telescopically inserted within the tip section  18 . As illustrated in FIGS. 9 and 10, an insulating layer  52  may be inserted between the tip section  18  and the butt section  20  to prevent galvanic corrosion within the joint  30 .  
         [0058]    The foregoing relates to preferred exemplary embodiments of the present invention, it being understood that other embodiments and variants thereof are possible within the scope of the invention, the latter being defined by the appended claims.