GB2203953A

GB2203953A - Racket frames

Info

Publication number: GB2203953A
Application number: GB08804334A
Authority: GB
Inventors: Masanori Takatsuka
Original assignee: Yamaha Corp
Current assignee: Yamaha Corp
Priority date: 1987-04-28
Filing date: 1988-02-24
Publication date: 1988-11-02
Anticipated expiration: 2008-02-24
Also published as: FR2614541A1; GB8804334D0; FR2614541B1; JPH0562553B2; GB2203953B; DE3807540A1; JPS63270068A

Description

2203953 AN IMPROVED RACKET FRAME The present invention relates to an

improved racket frame, and more particularly, but not exclusively, relates to improvements in the rebound or repulsion characteristics of a racket frame used, for example, in a tennis racket.

In general, a racket includes an elongated grip for being held by the hand and a substantially oval racket frame connected to the grip and holding strings or stringing under tension. The racket may be made of a wide variety of materials such as wood, metal and fibre reinforced plastics (FRP). The total weight of a racket frame is usually in the range from. 260 to 370g.

When a ball is struck by the strings of a racket frame, the ball and the racket frame perform different modes of vibration quite independently from each other. With the conventional dimensions of rackets, a racket frame is designed to have an eigen or natural frequency in a range from 90 to - 140 Hz. Due to this low range of the natural frequency of the racket frame, the mode of vibration of the racket frame does not match that of the ball struck by the racket.

More specifically, as later described in more detail in reference to the accompanying draWings, it has been confirmed through experimental tests that the direction of vibration of a racket frame is opposite to the advancing direction of a ball struck by.the racket frame at the moment of release of the ball, in particular when the ball is struck at a section of the stringing off the sweet spot. Thus, the vibration of the racket frame acts as a negative factor in propelling the ball at release. In addition, the vibration energy of the racket frame and the ball cannot be effectively utilized for accelerating the ball. Thus, the conventional range of the natural frequency of a racket frame much degrades its rebound or repulsion characteristics.

Embodiments of the present invention more effectively utilize the vibration energy of a racket frame and a ball to propel the ball on release from the racket frame.

Th e inventors have recognised that there is a close V - 2- VA relationship between the natural frequency of a racket frame on striking a ball and the recovery in elastic deformation energy of the ball, and that the rebound or repulsion characteristics of the racket frame are greatly affected by this relationship.

In accordance with the basic concept of the present invention, a racket frame is constructed so that its primary natural frequency is in the range from 180 to 300 Hz.

The invention will now be described by way of non-limiting example, reference being made to the accompanying drawings in which:- Figure 1 is a graph showing the relationship between the weight and the primary natural frequency of an embodiment of racket frame in accordance with the present invention; Figures 2A to 2L show in sequence the vibratory behaviour of an embodiment of racket frame of the present invention when striking a ball at its top side section off the sweet spot; Figures 3A to 3L show in sequence the vibratory behaviour of the same racket frame when striking a ball at a grip side section off the sweet spot; Figure 4 shows an example of a system for measuring the primary natural frequency of a racket; Figure 5 shows an example of a system for Measuring, in sequence, the vibratory behaviour of a racket frame when striking a ball; Figures 6A to 61---show, in sequence, the vibratory behaviour of a conventional racket frame when striking a ball at its top side section off the sweet spot, and Figures 7A to 71---show the same behaviour of the same racket frame when striking a ball at its grip side section off the sweet spot... ' An embodiment of racket frame in accordance with this invention is constructed so that its primary natural frequency lies in the range from 180 to 300 Hz and the weight of the racket frame lies in the range from 260 to 370g. The preferred frequency range for the primary natural frequency is from 220 Hz to 260 Hz. More specifically, the preferreo range of the primary natural freqyency vs. the weight of the racket frame is shown graphically in Figure 1. In the broadest sense, the preferred range of the primary natural frequency falls in an area defined by lines connecting four points A(260, 240), B(260, 300), C(370, 280) and D(370, 180). To obtain ideal rebound or repulsion characteristics, however, the preferable range should fall in an area defined by lines connecting four points A'(260, 240), B'(260, 260), C'(370, 240) and D'(370, 220).

Such a primary natural frequency of a racket frame is achieved through adjustment in material, configuration and manner of shaping of the racket frame. Adjustment May also be applied to the transverse cross sectional profile of the racket frame, the elastic modulus distribution of the material and/or the weight distribution throughout the entire racket frame. However, using high elastic materials such as high elastic carbon fibres and boron fibres easily found on the market, it may be difficult to construct the racket frame in accordance with the present invention without any change in the transverse cross sectional profile and the configuration which are conventionally employed in the field of art.

In the Case of one eX2mple of the construction resulting such a specified primary natural frequency, a racket frame includes an FRP shell and a. circumferential groove is formed in the outer peripheral surface of the racket frame. When necessary, a like circumferential groove may be formed in the inner peripheral surface too. In the area of 170mm from the centre of the face in the longitudinal direction of the oval shape, the racket frame is largest in thickness, and the average thickness of the racket frame in the same area is 22mm or larger. For example, in the section of the racket frame holding the stringing, the racket frame is constructed so that its average secondary moment of area is in the range from 6,000 to 20,000mm4 and -high elastic carbon fibres or boron fibres are used in the section.

An arrangement such as shown in Figure 5 is used for measuring the modes of vibration of a racket frame and a ball at the moment of release. A clamp 5 is pivoted to a horizontal fixed shaft 4 and the grip 3 of the racket 1 is held by the clamp 5 to allow the racket to be swung in a vertical plane. In separate tests a ball B is ejected, for example by a batting machine, towards the frame 2 of the racket in directions to strike at sections 2a and 2b respectively of the frame 2 off the sweet spot of the stringing. The modes of vibration of the racket frame 2 and the ball B are photographically recorded in succession at 1/2000 sec. intervals.

The result of the measurement with an embodiment of racket frame in accordance with the present invention is shown in Figures 2A to 2L and 3A to 3L whereas the result with a conventional r2cket frame is shown in Figures 6A to 6L and 7A to 7J, respectively. For easier comparison, the position of the arrangement in Figure 5 is turned counterclockwise through 901 in the illustrations.

In the case of the conventional racket frame, the frame 2 moves, more exactly flexes, in the opposite direction to the advancing direction of the ball B at the moment of release, as best seen in Figures 61 and Q as well as in Figures 71 and 7J.

In the case of the racket frame in accordance with the pr"esent invention, the frame 2 moves, more exactly flexes, in a direction same as the advancing direction of the ball 8 at the moment of release as best seen in Figures 21 and 2J as well as in Figures 31 and 3J. That is, the mode of vibration of the racket frame at release well matches that of the ball struck by the frame.

In the case of the above-described embodiment, the specified range of the primary natural frequency is given by adjustment in thickness distribution of the racket frame. In addition to this expedient, the FRP shell May include higher elastic reinforcing fibres.

One method for measuring the primary natural frequency 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 frame. From this point of view, the racket 1 without stringing is suspended from a suppor.t by a pair of rubber cords 6 each connected to a point on the frame which corresponds to a node of primary natural vibration. For generation of the vibration, the point P of the head of the frame is struck by, for example, a plastic hammer. An acceleration meter is attached for measurement to the frame at the point P on the face opposite to the struck face.

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Claims

1. A racket frame having a primary natural frequency in the range from 180 to 300 Hz and a weight in the range from 260 to 370g.

2. A racket frame as claimed in claim 1 wherein said primary natural frequency falls in an area defined by straight lines connecting four points (260, 200), (260, 300), (370, 280) and (370, 180) when said weight is taken on the abscissa and said primary natural frequency is taken on the ordinate of a graph.

3. A racket frame as claimed in claim 1 wherein said primary natural frequency is in the range from 220 to 260 Hz.

4. A racket frame as claimed in claim 3 wherein said primary natural frequency falls in an area defined by straight lines connecting four points (260, 240), (260, 260), (370, 240) and (370, 220) when said weight is taken on the abscissa and said primary natural frequency is taken on the ordinate of a graph.

5. A racket frame as claimed in any one of the preceding claims wherein said racket frame has an average secondary moment of area in a range from 6,000 to 20,00Omm4 in its string holding section.

6. A racket frame as claimed in any one of the preceding claims, wherein the thickness of said racket frame is largest in the area about 170mm from the centre of the face in the longitudinal direction of its ovalshape.

7. A racket frame as claimed in any one of the preceding claims wherein the average thickness of said racket frame is 22mm or larger-in 1 P 11 the area about 170mm from the centre of the face in the longitudinal direction of _its oval shape.

8. - A racket frame substantially as hereinbefo re described with reference to, and as illustrated in, any of the accompanying drawings.

9. Any and all novel features and combinations and subcombinations thereof substantially as herein disclosed.

H r33, London WCIR 4TP. Further copies may be obtainedfmia The Patent OM08, published 198a at The Patent Office, State House, 66171 E[igh 01b0 1/87.

Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St MarY CrAY, Kent- Con, Calub

Amendments to the claims have been filed as follows CLAIMS 1. A racket frame having a primary natural frequency in the range from 180 to 300 Hz and a weight in the range from 260 to 370g.

2. A racket frame as claimed in claim 1 wherein said primary natural frequency falls in an area defined by straight lines connecting four points (260, 200), (260, 300), (370, 280) and (370,180) when said weight is taken on the abscissa and said primary natural frequency is taken on the ordinate of a graph.

4. A racket frame as claimed in claim 3 wherein said primary natural frequency falls in an area defined by straight lines connecting four points (260, 200), (260, 260), (370, 240) and (370, 220) when said weight is taken on the abscissa and said primary natural frequency is taken an the ordinate of a graph.

5. A rapket frame as claimed in any one of the preceding claims wherein said racket frame has an average secondary moment of area in a range from 6,000 to 20,00Omm4 in its string holding section.

7. A racket frame as claimed in any one of the preceding claims wherein the average thickness of said racket frame is 22mm or larger in .Cthe area about 170mm from the centre of the face in the longitudinal direction of its oval shape.

8. A racket frame substantially as hereinbefore described with reference to, and as illustrated in, any of the accompanying drawings.