This invention relates to measuring and orienting a golf club shaft. More particularly, this invention relates to a method and apparatus for automatically and reliably identifying the location of a planar oscillation plane, and particularly the principal planar oscillation plane, of a golf club shaft and for aligning that planar oscillation plane in a desired orientation, as well as for determining parameters of the golf club shaft, such as roundness, stiffness and straightness, that characterize golf club performance.
When a golfer swings a golf club, the shaft of the golf club bends or twists, especially during the downswing. The direction the shaft bends or twists is dependent on how the golfer loads or accelerates the club, but the bending or twisting direction and magnitude also are dependent on the stiffness of the shaft. If a shaft is soft, it will bend or twist more during a given downswing than if it is stiff. Additionally if a shaft exhibits different transverse stiffness in different planes—i.e., the stiffness, roundness and straightness of the shaft are not symmetric—the shaft will bend or twist differently depending upon in which plane (direction) it is loaded.
Immediately prior to the impact of the head of a golf club with a golf ball, the shaft of the golf club goes through significant vibratory movements in both the toe up/down direction (plane perpendicular to the hit direction) and in the lead/lag direction (plane parallel to the hit direction). Research has shown the shaft of a golf club vibrates up and down in the toe up/down direction immediately prior to impact with the golf ball. This up and down movement, known as “vertical deviation oscillation,” “vertical deflection oscillation” or “droop oscillation,” can be as large as ±1.5 inch (±3.8 cm). Inconsistent bending or twisting makes it more difficult for the golfer to reproduce the downswing shaft bending or twisting from club to club, thereby resulting in less consistent impact repeatability within the set. Because any inconsistent bending or twisting due to asymmetric shaft behavior immediately prior to impact is substantially impossible for the golfer to correct with his or her swing, any reduction in the aforementioned oscillation immediately prior to impact will help the golfer improve his or her impact repeatability, thereby enhancing performance. This is true for golfers of all skill levels.
In addition, a golf club, immediately prior to impact, “springs” forward in the direction of the shot. This is commonly referred to as the “kick” of the shaft. If it is possible to analyze and orient a shaft in a way that the kick direction of vibration is stable, this shaft position would improve the golfer's ability to repeat the impact position with the ball. In other words the shaft would have less of a tendency to “bob” up and down immediately prior to impact thereby improving impact repeatability.
Inconsistent bending or twisting contributes to movements of the club head that would not be present if the shaft had been perfectly symmetric. Golf club shaft manufacturers attempt to build shafts with symmetric stiffness to minimize inconsistent bending or twisting during the swing, but as a result of manufacturing limitations it is difficult to build a perfectly symmetric golf club shaft. Specifically, it is well known that, as a result of irregularities or variations in materials or manufacturing processes, golf club shafts have a preferred angular orientation. For example, it is sometimes said that a golf club shaft has a “spine” whose orientation may be significant. (See, e.g., U.S. Pat. Nos. 4,958,834 and 5,040,279, which are hereby incorporated by reference in their entireties.) Therefore, substantially all golf club shafts exhibit some degree of asymmetry which results in some degree of inconsistent bending or twisting during the swing.
The asymmetry of golf club shafts can result from nonsymmetrical cross sections (shafts whose cross sections are not round or whose wall thicknesses are not uniform), shafts that are not straight, or shafts whose material properties vary around the circumference of the shaft cross section. Because it is substantially impossible to build a perfectly symmetric golf club shaft and the objective is to minimize inconsistencies from club to club in a golf club set and from set to set within a brand, it makes sense, if possible, to analyze each golf club shaft in a set of golf clubs to understand its asymmetric bending or twisting behavior and construct the golf clubs in the set to maximize consistency from club to club within a set and from set to set within a brand.
It has been recognized—e.g., in above-incorporated U.S. Pat. No. 5,040,279—that although substantially all golf club shafts exhibit some degree of asymmetry, substantially every golf club shaft exhibits at least one orientation in which, when the shaft is clamped at its proximal, or handle, end and displaced at the tip, the resultant vibration of the shaft will remain substantially planar. That is, the shaft will remain substantially in a single plane and the tip of the shaft will vibrate back and forth substantially along a line.
It is also recognized in above-incorporated U.S. Pat. No. 4,958,834 that the construction of all golf clubs within a set with their respective planar oscillation planes (“POPs”) oriented in the same angular direction relative to their respective club faces will exhibit less inconsistency in shaft bending or twisting during the downswing than a set that has been haphazardly or randomly constructed. In particular, a set of golf clubs normally will function best if the respective preferred angular orientations of the respective golf club shafts are aligned in the “hit direction”—i.e., substantially perpendicularly to the respective golf club faces.
However, heretofore there has not been any convenient automated way to determine with consistency the parameters of a golf club shaft that would allow manufacturers or others to predict the performance of a golf club shaft. And while copending, commonly-assigned U.S. patent application Ser. No. 09/494,525, filed Feb. 1, 2000, which is hereby incorporated by reference in its entirety, showed a method and apparatus for determining the preferred angular orientation of a golf club shaft, that method and apparatus were partly manual, and relied on an iterative technique which, in identifying a planar oscillation plane, could identify a planar oscillation plane other than the principal planar oscillation plane. It would be desirable to be able to provide a method and apparatus for quickly and reliably determining the preferred angular orientation of a golf club shaft. It also would be desirable to be able to provide a method and apparatus for using the determination of the preferred angular orientation to automatically assemble golf clubs with each respective golf club shaft consistently aligned relative to the respective club face. It would further be desirable to be able to determine parameters of a golf club shaft to allow prediction of golf club performance.