GB2216018A

GB2216018A - Racket frame

Info

Publication number: GB2216018A
Application number: GB8903813A
Authority: GB
Inventors: Tetsuyuki Awano; Masanori Takatsuka
Original assignee: Yamaha Corp
Current assignee: Yamaha Corp
Priority date: 1988-02-19
Filing date: 1989-02-20
Publication date: 1989-10-04
Anticipated expiration: 2009-02-20
Also published as: JPH01212579A; GB2216018B; GB8903813D0; JP2625825B2

Abstract

A racket frame for ball games such as tennis. The head (2) and shaft (4) preferably made of a hollow FRP material, have a uniform thickness along the longitudinal direction of the racket frame in a range from 23 to 28mm with a natural frequency in a range from 160 to 260 Hz. The specified natural frequency makes the vibratory performance of the racket frame very close to that of a ball just after striking so that the vibration energy of the racket frame can be efficiently utilized to accelerate the balls. <IMAGE>

Description

A RACKET FRAME The present invention relates to an improved racket frame, and more particularly relates to improvements in the ball-striking characteristics of such a frame when used for ball games such as tennis.

Racket frames of this type generally comprise a head connected to a grip via a shaft including a throat, the head having a substantially oval shape defining a face formed by a latticework of strings held in tension.

The head is made up of a crown and shoulders. When a ball is struck by a racket of this kind, the ball and the racket perform different modes of vibration soon after the ball contacts the face of the racket.

In the case of most conventional racket frames, the thickness of the frame (i.e. the dimension of the racket as measured in a perpendicular direction to the face) is uniform over its entire length and about 18nm or smaller. This small frame thickness attenuates the abrupt decay of vibration after a ball has been hit.

In addition, the racket frame is usually designed to have a natural frequency in a range from 90 to 140 Hz and such a low frequency causes a big difference in vibratory performance between the ball and the racket frame and, as a consequence, the vibration energy of the racket cannot be efficiently utilized to accelerate the rebounding ball.

In an attempt to overcome these disadvantages, a new racket frame is proposed in US Patent No. 4664380. In one embodiment of this prior proposal, the thickness of the racket frame increases gradually from the joint of the shaft with the grip, reaches its largest value at about the middle of the length of the head and decreases gradually towards the head end. By varying the thickness in this way along the longitudinal direction of the racket frame, it is intended that the natural frequency of the racket frame should be closer to the excitation frequency of balls. However, a problem of this prior proposal is that vibration of the racket frame decays abruptly when a ball is struck by a section of the face close to the head end due to the reduced thickness of the frame.Such abrupt attenuation in vibration results in a low flight speed of the ball and poor accuracy in striking. Thus, the ball cannot fly a long distance in an intended direction.

It is the object of the present invention to provide a racket frame which achieves a high flight speed for the ball as well as improved accuracy in striking even when the ball is struck near the head end of the racket frame.

In accordance with the basic aspect of the present invention, a racket frame is provided having a uniform thickness in the longitudinal direction in a range from 23 to 28 mm with a natural frequency in a range from 160 to 260 Hz.

A preferred embodiment of the invention will now be described, with reference to the accompanying drawings, in which Figure 1 is a plan view of a racket frame of the invention, Figure 2 is a side view of the frame of Figure 1, Figures 3A to 3C are transverse cross-sectional views taken along lines A-A, B-B and C-C in Figure 2, and Figure 4 shows a system for measuring the primary natural frequency of a racket.

Figures 1 to 3C show a racket frame 1 of the invention which is made of an elongated fibre reinforced plastic (FRP) material and is of a hollow construction. The racket frame 1 further includes a head 2 including a face G, a shaft 4 connected to the head 2 via a throat 3, and a grip 5 connected to the shaft 4 at its grip attachment point 4a. The head 2 has a thickness ta, the throat 3 has a thickness tb and the shaft 4 excluding the throat 3 has a thickness tc. These thicknesses ta to tc are all chosen so as to fall in the range 23 to 28mm, more preferably from 23 to 26mm.

As shown in Figures 3A to 3C, the head 2 has a wall thickness sa, the throat 3 has a wall thickness sb and the shaft 4 has a wall thickness sc. The wall thickness sb of the throat 3 is designed to be larger than those sa, sc of the head 2 and the shaft 4 so that the natural frequency of the racket frame 1 should be in a range from 160 to 260 Hz, more preferably from 180 to 240 Hz. Such wall thickness adjustment can be carried out as desired by properly selecting the number of resin prepregnated sheets which are to be superimposed with each other in formation of the hollow FRP material. For example, 5 to 8 sheets are used for the grip 5 and the shaft 4 excluding the throat 3 or head 2, and 12 to 20 sheets are used for the throat 3.

Again, as shown in Figures 3A to 3C, the head 2 has a width wa, the throat 3 has a width wb and the shaft 4 has a width wc. These widths wa to wc should preferably be in a range from 9 to 15mm. Further, the ratio between the wall thickness sb of the throat 3 and the wall thickness sa of the head 2 should preferably be in a range from 2 to 4, more preferably from 2 to 3.

The widths and wall thicknesses should be chosen in combination with each other so that the moment of inertia of area (1) on neutral axis Z of the entire racket frame 1 should be 2 to 5 times larger than those of conventional racket frames.

In accordance with the present invention, the various dimensions and the natural frequency of the racket frame are specified to the above-described values for the following reasons. The excitation characteristics of a racket frame are greatly influenced by the natural frequency of the racket frame which is greatly influenced by a product of the Young's modulus (E) of the material of the frame and the moment of inertia of area (I). As is well known, the moment of inertia of area (I) of a mass is fixed by the thickness, width and height of the mass. The thickness refers to the size of the mass as measured in a direction normal to its neutral axis and the width refers to the size of the mass as measured in a direction parallel to its neutral axis. The wall thickness is also concerned when the mass is hollow.In the case of the present invention, the neutral axis Z runs in a direction parallel to the face defined by the head of the racket frame and the moment of inertia of area (I) is fixed with respect to such a neutral axis Z.

The following relationships exist between the Young's modulus (E), the moment of inertia of area (I) and the natural frequency (F) of a racket frame.

where: K: Spring constant M: Weight In one preferred embodiment of the present invention, the throat 3 and the other parts may be made of materials of differing Young's moduli. For example, the throat 3 is made of a high grade type CFRP (carbon fibre reinforced plastics) having a Young's modulus of 500 GPa whereas the other parts are made of a normal grade type CFRP having a Young's modulus of 220 GPa.

One example of the system for measuring the primary natural frequency of the racket is shown in Figure 4.

For correct measurement, the racket frame has to be held in a manner to exclude factors affecting the natural frequency of the racket frame. From this point of view, a racket frame 1 without strings is held to a support by a pair of rubber cords 6 each connected to a point on the racket frame 1 which corresponds to the knot of primary natural vibration. For the generation of vibrations, the head end 2a of the racket frame 1 is struck by, for example, a plastic hammer. An acceleration meter is attached for measurement to the racket frame 1 at the head end 2a on the face opposite to the struck face.

As an alternative, the racket frame 1 may be suspended from the support by a single rubber cord connected to a point which corresponds to the knot of primary natural vibration.

According to one aspect of the present invention, the natural frequency (F) of a racket frame is set to a value in a range from 160 to 260 Hz. With such a specified range, the natural frequency (F) of the racket frame is very close to the excitation frequency of the balls to be struck by the face of the racket and, as a consequence, the position resumed by the face on striking a ball well matches the position of the ball at release from the face. Thus, the vibratory performance of the racket frame is well transmitted to the ball so that the vibration energy of the frame is very efficiently utilized for acceleration of the ball by repulsion. In addition, the ball can fly accurately in the direction intended by the player.

According to a preferred aspect of the present invention, the racket frame has a uniform thickness in its longitudinal direction. In particular the head end 2a of the racket is much thicker than that of a conventional racket. As a result, vibration of the racket frame decays gradually even when a ball is struck by a section of the face close to the head end 2a. Such attenuation in vibration results in a high flying speed for the ball and good accuracy in striking. Thus, the ball can go a long distance in an intended direction.

In accordance with another preferred feature of the present invention, the wall thickness and/or Young's modulus (E) of the throat are designed larger than those of the head and the shaft for natural frequency adjustment. So, the natural frequency of the racket frame can be easily set to a value in a specified range without varying its thickness along the longitudinal direction thereof.

Claims

1. A racket frame comprising a head, a shaft including a throat and a grip, said head, shaft and grip having a uniform thickness along the longitudinal direction of said racket fame in a range from 23 to 28 mm with a natural frequency in a range from 160 to 260 Hz.

2. A racket frame as claimed in claim 1 wherein the head and shaft are made of an elongated FRP material of a hollow construction.

3. A racket frame as claimed in claim 1 or 2 wherein the product of the Young's modulus of the material and the moment of inertia of the area of said throat is larger than that of the grip, head and shaft excluding said throat.

4. A racket frame as claimed in claim 1 or 2 wherein the moment of inertia of area (I) of said throat is larger than that of the grip, head and shaft excluding said throat.

5. A racket frame as claimed in claim 1 or 2 wherein the Young's modulus (E) of said throat is larger than that of the grip, head and shaft excluding said throat.

6. A racket frame as claimed in claim 1 or 2 wherein the width of the head and shaft including said throat is in the range from 9 to 15mm.

7. A racket frame as claimed in claim 2 wherein the wall thickness of the throat is larger than that of the head and shaft excluding the throat.

8. A racket frame as claimed in claim 7 wherein the ratio of the wall thickness of the throat with respect to that of the head is in a range from 2 to 4.

9. A racket frame substantially as herein described with reference to Figures 1, 2, 3A, 3B, 3C of the accompanying drawings.

10. A method of measuring the primary natural frequency of a racket substantially as herein described with reference to Figure 4 of the accompanying drawings.