Golf club with compound shaft

Abstract: A golf club shaft (21) comprising an at least partially hollow outer shaft (22) having a butt end (2) and a head end (3), and an inner shaft (28) located within said outer shaft (22) and spaced radially therefrom, said shafts (22, 28) abutting one another at two locations (29, 30), the first location (29) being closer to said butt end (2) than the second, wherein the shafts (22, 28) are substantially securely fastened together at said first location (29) and are arranged at said second location (30) such that said outer shaft (22) can pivot about the inner shaft (28).

Description: This invention relates to an improved golf club shaft. The shaft of a conventional golf club having a head bends into a so-called primary vibration mode in mechanics when it is being swung. The shape is characterized by a simple half-wave curve as shown in the dotted line curve in Fig. 1. The primary vibration mode is slow for the shaft to complete the trajectory to drive the ball. An experienced golfer has to time his swing properly so that the head will hit the ball at the tee and follows the swing until the head ceases contacting the ball, thereby giving the head the longest possible time in accelerating the ball. The present applicant's US-A-5588920 and EP-A-754477 disclose a golf club having a partially hollow outer shaft and an inner shaft located partially within the outer shaft. The inner and outer shafts abut one another at two points. In one embodiment, both ends of the outer shaft are simply supported by the inner shaft. In another embodiment, the inner and outer shafts are rigidly secured together at one end. US-A-2124534 discloses a golf club having a compound shaft, the shaft comprising an inner shaft covered along part of its length by an outside metal sleeve. The inner shaft and outside sleeve abut one another at two points, being securely fixed together at one point and arranged at a second point so as to be mutually pivotable. An object of the invention is to change the conventional primary vibration mode into a high vibration mode which would produce less bending of the golf club shaft, and achieve full acceleration at the earliest moment, hence the head speed could be the maximum when it completes the trajectory. A second object is to enable the golfer to control the movement of the head through the creation of an intermediate pivot point along the length of the shaft, which can manipulate the head more effectively. These objects are achieved by having a golf club with a compound shaft in accordance with the present invention. According to the invention, there is provided a golf club shaft comprising an at least partially hollow outer shaft, said outer shaft having a butt end and a head end, and an inner shaft located wholly within said outer shaft and spaced radially therefrom, said shafts abutting one another at two locations, the first location being closer to the butt end than the second, wherein the shafts are substantially securely fastened together at said first location and are arranged at said second location such that the outer shaft can pivot about the inner shaft. The pivot arrangement hereinabove described is achieved by having the inner shaft pivoting, in a simple support manner, against the outer shaft at a specific point. The result of this simple support contact is the creation of an inflection point, as defined below. If the inner shaft transmits also bending moment at that point, even a small frictional force, the inflection point will be pushed towards the butt end and becomes less effective. By 'inflection point' herein is meant a point where an upwardly curved portion of the shaft meets a downwardly curved portion of the shaft when the shaft bends. Due to the changeover in bending curvature at such a point, the shaft has no internal bending moment and no curvature at that point; only an internal lateral force is acting. A simple support is defined as a joining of two members in which one may turn relative to the other about the center of the joint, substantially free from rotational interference, but they do not move away from the center of the joint. Mechanically, it means the joint can transmit force but can not transmit bending moment between the joining members when one turns around the other. Examples of means of simple support include: ball and socket type, universal joint, a journal turns in a bearing, etc. A layer of cushion may be adapted in at least a part of the space between contacting surfaces. To produce a pivoting arrangement, it is required that the first location ( ie that closest to the butt end) is joined to the butt end in substantially a fixed manner. This can be achieved by, for example, welding, bolting, glueing, or fastening the contacting surfaces by friction by tight fit, forced fit, shrink fit, etc. Material for shafts may be similar to prior art golf club shaft, including metals, synthetics, and reinforced fiber composites, etc. A shaft may be composed of different materials. Preferably, clearance is provided between the outer and inner shafts by the outer shaft having an elongated bulged section with a diameter larger than the diameter of the handle inside the grip. More preferably, the second end of the inner shaft at the pivot point is located at or around the end of this bulge closest to the head end (hereinafter also referred to as the 'bulge end'). The bulged section is preferred to have its largest diameter at a point (hereinafter referred to as the 'peak point') located about one third of the distance from the bulge end to the butt end. A preferred embodiment of the simple support means of the compound shaft is having the second end of the inner shaft directly wedged inside the bulge end of the section of the outer shaft, forming a joint with a turning center, wherein their contacting surfaces are adequately rounded with sharp edges removed and friction minimized so that the outer shaft can turn around the second end of the inner shaft substantially free from rotational resistance. The diameter of the butt end of the outer shaft is preferred to be equal to or less than the diameter of the outer shaft which is between the butt end and the peak point. This includes the particular embodiment of a constant diameter handle, or even a reverse tapered handle whereby the diameter of the grip is generally increasing from the butt end towards the head end along the length of the shaft. The diameter at the peak point is preferred to be at least equal to any other diameter along the length of the shaft, including the handle portion. The peak point is preferred to be located outside that portion of the handle which is under a grip. The diameter at this point is preferred to be at least 18 mm. The bulge end is preferred to be at least about 30 cm from the butt end, or not less than 25% of the length of the compound shaft therefrom. The first end of the inner shaft (ie that fixed to the outer shaft) is preferred to be located less than 15 cm from the butt end 2. The invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 shows the bending of a conventional golf club shaft having the primary vibration mode; Fig. 2 shows a golf club shaft according to a preferred embodiment of the invention; Fig. 3 shows the deflected center line of the outer shaft of the Fig. 2 compound shaft as being bent during swing of the club; Fig. 4 shows bending moment diagrams of the primary vibration mode and the higher vibration mode; Fig. 5 shows a free diagram of force wherein the end force P and Q maintains balance with the hand force for the partial structure cut at the inflection point 31; and Fig. 6 shows a free diagram of force wherein the end force P plays seesaw with the head end load W, and the pivot force Q is controlled by the hand. Fig. 1 shows a prior art golf club shaft 1 having a butt end 2, head end 3 and a grip 4. The dotted line 6 shows the shaft bent under the end load W. The bending curve is known as a primary vibration mode characterized by a half-wave simple harmonic deflection curve. Fig. 2 shows a preferred compound shaft 21 of the invention, comprising a grip 4 covering the handle portion of an outer shaft 22, including a butt end 2, and a section 5 having an elongated bulge. The section is formed with a hollow portion 32, which begins from a separation end 29, reaches its largest diameter at 27, and ends at a bulge end 30. A head section 26 of the shaft 21 connects the end 30 to the head end 3 to which the golf club head is attached. Wholly inside the outer shaft is an inner shaft 28, having a first end 23 which co-locates with the end 29 of the outer shaft. The end portion is fixed to the outer shaft at the butt end 2. Its second end 25 engages the outer shaft at its bulge end 30 so that the outer shaft 22 can pivot about the inner shaft 28. There is a clearance region of length 24, from end 29 to 30, containing opening 20 which separates at least the outer shaft from the interiorized inner shaft when the outer shaft is not bending. Soft cushion material, not shown here, may optionally be provided in the opening. A conventional shaft has the largest diameter (15 to 18 mm) at the butt end 2, tapering down to 9 to 12 mm at the head end 3. The total shaft length is about 114 cm. The grip 4 covers the handle portion for at least 22 cm. The bulged section 5 has its greatest diameter at the peak point 27, larger than the diameter of the handle portion in the grip; and the peak 27 is about one third of the length 24 to the bulge end 30. Relationships have been studied between openings in the clearance length required to maintain surface separation versus bending load of the head. Since there are numerous critical parameters affecting the results, only a general, statistical impression, can be provided to guide the design of the invention shaft. In general, better performance results are obtained for the elongated bulge design by the following embodiments which are preferred, but the invention is not necessarily limited thereto. Fig. 3 shows the center line of the outer shaft of a preferred embodiment of the invention. The deflection curve is known as a high vibration mode. The shaft returns to the original straight position in less time than the primary vibration mode with the same head force. In the Fig. 3 curve, the inner shaft is stiff enough to lift the outer shaft up substantially, create an inflection point 31 and reduces the average curvature of the outer shaft from the separation end 29 to the bulge end 30. The same head force W of the conventional shaft is produced in the compound shaft with less bending of the shaft, having less deflection of the head, and hits the ball in less time. The bending moment diagram a-b-c of Fig. 4 is the moment in the outer shaft if there is no inner shaft, ie that for the conventional shaft which is shown in Fig. 1. Due to the upward contact force Q from the stiff inner shaft at its end 25, shown in Fig. 6, which counteracts with the bending force from the end load W, the bending moment is reduced in the portion of the shaft from end 30 up to the end 29. The corresponding moment diagram of the outer shaft is changed to the figure of a-d-f-e-c in Fig. 4. The double-crossed area d-b-g-f is the moment removed from the outer shaft by having the inner shaft pivoting, in a simple support manner, against the outer shaft at point 30 according to the invention. After the end force increases from zero to the maximum W, and the player continues to swing with the same force W, the shaft cannot be bent further than what has already been bent. Therefore, the driving force W from now on is entirely devoted to accelerate the head at the 'full' acceleration. Since the whole swing takes only a few seconds to accomplish, it is crucial that the shaft should be swung to reach the full head force W in less time, so that the full acceleration can begin as early in the swing as possible. Then the head speed when the new shaft separates with the ball should be faster than the conventional shaft because the head of the invention shaft had been fully accelerated at a much earlier point in time. This is the advantage of the Fig. 2 invention club over the Fig. 1 conventional club. Fig. 5 and Fig. 6 relate to control improvement. In mechanics, a 'free body' diagram is often used to study the movement of a part of a complicated mechanism by virtually isolating that part from the rest of the structure and the surrounding forces around the virtual boundary are identified. Applied to the invention shaft, the 'free body' is the partial outer shaft 22, cut from the handle at the inflection point 31, shown in Fig. 6. At the inflection point 31 in Fig. 5, the handle applies a shear force P. The inner shaft applies a lateral contact force Q at the end 30. To maintain balance, there is the end force W. Looking at Fig. 6, a keen observer will discover that the outer shaft behaves as a seesaw balanced by the two end forces P and W and pivoted at an intermediate fulcrum 25. It seems reasonable to infer that the inner shaft shown in the dotted line in Fig. 6 could have controlled the movement of the fulcrum 30. Since the fulcrum is controlled by the hand shown in Fig. 5, and the end 25 is much closer to the head end 3 than is the grip 4, it is reasonable to infer that control of the golf club head ought to be improved by the invention shaft. The contact at the end 30 between the outer and the inner shaft which is drawn as a classical ball and socket joint in Fig. 2 is meant only to convey the concept of a simple support, not to be taken literally as the only viable design of the invention. A conventional sized outer shaft may have difficulty in achieving the objects of the present invention. The prior art does not devise a sufficient clearance from the end 29 to end 30 between the surfaces of the co-axial shafts, as this would have some difficulty in achieving a high vibration mode with the characteristic moment distribution in the outer shaft as shown in Fig. 4. However, with a stiff but small inner shaft, a compound shaft as described which has an outer shaft similar to the conventional shaft in appearance is not to be excluded from the invention. The reason for the rapid reduction of the outer shaft diameter is based on a concern that the outer shaft is too stiff. Taking 19 mm as a nominal diameter of the outer shaft and with nominal wall thickness, say of 0.6 mm, the moment of inertia of the cross section is about 1 470 mm 4 . A conventional shaft has a diameter at the handle of about 16 mm; a wall thickness of about 0.9 mm and a cross sectional moment of inertia about 1 220 mm 4 . It is clear that most of the outer shaft in the clearance region 24 is much stiffer than the conventional shaft in that corresponding length. What finally made the outer shaft in that region becoming really too stiff is that the bending moment there has been drastically reduced as a result of having the contact force Q from the inner shaft canceling the head force W in the length from 31 to 30 as shown in Fig. 6. According to line d to e of Fig. 4, the bending applied to the outer shaft in that length is linearly reduced to almost zero at the butt end. The consequence is: if not designed properly, the outer shaft is invariably too inflexible to create the all-important inflection point 31 and the high frequency mode during swing. This observation agreed with the results of actual play tests. The present invention provides a remedy in the form of a short diameter transition zone 42 in the outer shaft as shown in Fig. 2. Analysis indicates the largest deflection point 27, for the outer shaft 22 fixed at 29 and pivoted at 30 by the second end 25 of a stiff inner shaft, would occur at one-third of the length 24 measured from the bulge end 30. Assuming a preferred length of the inner shaft 28 as 45 cm, a third of 45 cm is 15 cm, which is the length 40 where open space between the two co-axial shafts is mostly needed to avoid the inner shaft interfering with bending of the outer shaft when the head hits the ball. Naturally, the outer shaft should have a large diameter in that region, then taper towards point 30 and beyond. After crossing the point 30, the outer shaft should reduce its diameter as quickly as possible to reach the size of the conventional shaft in order to become more flexible as soon as possible. Assuming point 37 as the point closest to point 30, and 41 the distance to 30, whereat the diameter of the outer shaft first becomes equal in size to the conventional shaft at the same distance from the head end 3. The length 42 in Fig. 2, is the diameter transition zone of the outer shaft which includes the bulge end 30. After the transition zone, the oversized diameter has been reduced to 'normal' and the remaining shaft is about the same in size and in flexibility as a conventional shaft; from hereon up to the head end 3, the shaft may be hollow or partially solid. Tests and analysis have been conducted to determine the geometry of the diameter reduction zone. The specification should be tight but tolerable in practice for mass production, wherein the laminated fiber cloth used to fabricate the outer shaft is able to be smoothly wrapped over a sharply tapered forming mandrel for quick reduction of diameter yet without causing wrinkles or fold-overs during the process. It is a difficult process because the cloth is not stretchable and fiber orientation in which its strength depends is not to be altered in the process. Two critical parameters are to be studied: one is how rapid can be the diameter reduction in the zone 42, and how short it can be made. The design guide is as follows: the diameter transition zone of the outer shaft is not more than about 23 cm in length which may include the bulge point 30 anywhere within that length; after passing through the zone, from the butt end towards the head end, the reduction in outer shaft diameter is at least 29%. For example, if measured in shaft length not more than 23 cm, which includes the bulge end within, yields the extreme shaft diameters as 18 mm and 12 mm respectively, the diameter reduction is 33%. With this simplistic design criteria, the risk of having an outer shaft which is simply too big for a length too long, which if not controlled will render the invention useless, will be minimised. Finally, it is to be noted that in the deflected center line of the outer shaft as shown in Fig. 3, the outer and the inner shafts are in contact only at points 29 and 30 which are the two ends of the inner shaft. Here it is assumed that the bending of the outer shaft upon impact is moderate and clearance remains open. If the impact is greater, or the geometry of the shafts is different, there may be more contact points between the two shafts within the length 24 and the deflection curve of the outer shaft will have more bends. It is noted that the embodiments describe a single inflection point bending of a golf club with the compound shaft design. More complicated bending of the outer shaft, effected by more intermediate contact points made possible by different dimensional design of the shafts and the clearance space in between, may lead to different performance benefits of the golf club. These variations derived from the basic form of manipulating the frequency modes of vibration of the shaft as illustrated in Figures 2 and 3 are deemed to be within the scope of the present invention.

Claims: A golf club shaft (21) comprising an at least partially hollow outer shaft (22) having a butt end (2) and a head end (3), and an inner shaft (28) located within said outer shaft (22) and spaced radially therefrom, said shafts (22, 28) abutting one another at two locations (29, 30), the first location (29) being closer to said butt end (2) than the second, the shafts (22, 28) being substantially securely fastened together at said first location (29) and arranged at said second location (30) such that said outer shaft (22) can pivot about the inner shaft (28), characterised in that the inner shaft (28) is located wholly within the interior of the outer shaft (22). The shaft of claim 1, wherein the outer shaft (22) further includes a section (5) formed with a hollow portion (32) extending to a bulge end (30) and a head section (26); and the inner shaft (28) is arranged at least partially within said hollow portion of said section (5) and being generally parallel thereto; said inner shaft being spaced interiorly from said outer shaft to form a clearance region (24) which includes said hollow portion (32); and open space (20) is provided in the clearance region (24) between the inner surface of the outer shaft and the inner shaft at least when the outer shaft is not bending because of an impact of the golf club with a ball. The shaft of claim 2, wherein said section (5) is an elongated bulged section, having a diameter which is equal to or greater than the diameter at the butt end (2), the diameter of said bulged section reaching a maximum at a peak point (27). The shaft of claim 2 or claim 3, wherein the second end (25) of the inner shaft (28) joins the bulge end (30) of the outer shaft in an approximate simple support manner, wherein the second end (25) and the bulge end (30) can rotate relative to each other substantially free from rotational interference but cannot move away from the center of the joint. The shaft of claim 2, wherein the second end (25) of the inner shaft is about at the same location as the bulge end (30) of the outer shaft. The shaft of claim 4, wherein said simple support between the second end (25) of the inner shaft (28) and the bulge end (30) of the outer shaft (22) is achieved by having the second end of the inner shaft directly wedged inside the bulge end (30) of said elongated bulge section (5) of the outer shaft, forming a joint with a turning center, wherein their contacting surfaces are adequately rounded and minimized with friction, so that the outer shaft can turn around the second end of the inner shaft, substantially free from resistance, but cannot move away from the center of the joint. The shaft of claim 4, wherein one or more layers of cushion material is adapted in at least a part of the space between said second end (25) of the inner shaft and said bulge end (30) of the outer shaft. The shaft of claim 3, wherein the maximum diameter (27) of the elongated bulged section (5) of the outer shaft (22) is at least equal to the maximum diameter anywhere else in the outer shaft. The shaft of claim 2, wherein a diameter reduction near the bulge end (30) of the outer shaft is such that when the maximum and the minimum diameters are measured, across a span of 23 cm which includes the bulge end (30), the reduction in diameter is not less than about 29%. The shaft of claim 2, wherein the bulge end (30) of the outer shaft is at least about 30 cm from the butt end (2), or its distance from the butt end (2) is not less than 25% of the length of the shaft. The shaft of claim 2, wherein the diameter of the butt end (2) of the outer shaft (22) is approximately equal to or less than the diameter of the outer shaft anywhere between the butt end (2) and the maximum diameter (27) of the elongated bulged section. A golf club comprising a head and a shaft as defined in any of the preceding claims. A golf club comprising a head and a shaft (21) as defined in claim 1, wherein the outer shaft (22) has a hollow portion (32) extending to a bulge end (30); the outer shaft further including a butt end (2), a head section (26) and a head end (3) in which said head is installed; the inner shaft (28) is arranged at least partially within said hollow portion (32) of said outer shaft and is generally parallel therewith; said inner shaft (28) being spaced interiorly from said outer shaft (22) within said hollow portion (32) and including a first end (23) and a second end (25), the latter being closer to said head section (26) than is said first end (23); the shaft further comprising connection means arranged for connecting said inner shaft at both of said ends (23, 25) thereof to said outer shaft, wherein during bending of said shaft due to said head impacting with a ball, the outer shaft (22) can rotate about said second end (25) of said inner shaft (28) substantially free of interference from the inner shaft, and the bending moment which bends the outer shaft is being affected by said connection means so that the outer shaft (22) bends in a high vibration mode. The club of claim 13, wherein said connection means includes a substantially secure connection of said first end (23) of said inner shaft (28) to said outer shaft (22) adjacent said butt end (2) and a substantially simple support connection of said second end (25) of inner shaft to said outer shaft. The club of claim 14, wherein said simple support connection is selected from a universal joint and a ball and socket joint.

Inventors: SOONG TSAI CHEN, SOONG, TSAI CHEN
Applicants: SOONG TSAI C, SOONG, TSAI CHEN
Application year: 1998
Main classification: A63B53
IPC codes: A63B53