|
(11) | EP 0 788 817 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||
| (54) | Sports racket with odd number of longitudinal strings |
| (57) A sports racket (20) having a frame (10) comprising a top portion (1), two opposed
sides (2), a throat (3), a shank portion (4), a handle (5), and a string network which
has longitudinal string segments (7) and cross string segments (9), the frame (10)
comprising an odd number of longitudinal strings and a longitudinal central string
(21) coinciding with the central longitudinal axis (8) of the racket whereby the string
segments, their anchoring with the frame thereabouts, arrangement of accessories and
attachments in the frame, such as grommet strips and bumper guard, are positioned
so as to be an exact mirror image with respect to the central longitudinal axis of
the frame, thus enhancing the balance, power and handling of the racket due to the
symmetrically balanced arrangements. |
BACKGROUND OF THE INVENTION
[0001] The present invention is related to an innovative stringing and frame construction associated with stringing of sports rackets, such as tennis, racquetball, badminton squash, etc.
[0002] The conventional string network comprises a system of longitudinal string segments approximately parallel to the axis of the frame, having an even number of string segments, and a cross string system approximately perpendicular to the longitudinal string system. For tennis racket, the structure of the frame is symmetrical to the axis of the racket, there is customary 16 to 20 longitudinal string segments, evenly distributed on both sides of the longitudinal axis which is itself void of string. A conventional racket is shown in Fig. 1 where the frame 10 comprising the head portion 1, side frame 2, a throat 3 opposite to the top, shanks 4, and the handle 5. The string network 6 comprises a longitudinal string system 7 approximately parallel to the axis of the frame 8, and a cross string system 9. For most racket design, strings are anchored around the frame by means of string holes drilled at the center plane of the frame which is shown as plane M in Figures 4 and 5. Since a string develops a reactive force component perpendicular to the center plane M of the string network to return a ball only when the string is depressed deep enough to produce a significant inclination angle with respect to the central plane. For strings such as 12 which is near the boundary of the frame, the normal deflection is always small and the string network in boundary zones is often weak in tension and is not effective.
[0003] A feature of the invention is shown in the tennis racket 20 of Fig. 2. where there is a longitudinal string segment 21 at the center line 8 of the frame and the rest of the longitudinal string segments are distributed and connected to each other as a mirror image on both sides of the central longitudinal string 21. The cross string segments which are interwoven up and down with the corresponding longitudinal string segments, are also arranged as a mirror image with respect to said center line. Therefore, the string pattern and the stress on the frame is truly symmetric and balanced about the central axis of the racket. It is to be noted that this inventive departure from the traditional stringing pattern of sports racket improves the performance of the racket but makes it difficult to be strung properly. The difficulty in stringing is that for many years, the consensus of the racket design is that there has been no racket which has a longitudinal string at the center line of the frame, and as a consequence, nearly all existing commercial stringing machines have mechanical difficulties to string rackets which have a string at the center line of the frame. This fact indicated that the present invention is not obvious and has not been expected.
FIGURES
DESCRIPTION OF SOME PREFERRED EMBODIMENTS
[0011] The present invention applies to general sports racket having a string net work. The tennis racket is shown as an example to illustrate the invention.
[0012] Figure 1 shows a prior art racket frame 10, with a string network. Fig. 2 shows a feature of the invention where the frame 20 has a longitudinal string segment 21 coinciding with the longitudinal center line 8 of the frame, comprising an odd number of longitudinal string segments symmetrically distributed as a mirror image at opposite sides of the central string. Segment is defined as a portion of a string end to end within the boundary of a frame. Ends of segments are connected through loops such as 24, 25, to become a continuous length of a piece of string. Loops are the anchors of string segments on the frame. Cross string segment is supported by respective longitudinal strings. A merit of the mirror imaged pattern can be seen from Figures 1 and 2 in which a ball 17 hits the center line of the racket. In Fig. 1 conventional string system, the ball will be in contact with at most four longitudinal strings. The inner pair, 15 and 14, and the outer pair, 13 and 16, are interwoven non-symmetrically with respect to the center line. Their resultant is inclined to the plane of the frame. Therefore, a ball hits the exact center line of the frame will not bounce back along the exact normal direction of the frame. The playing public had long tolerated this inaccuracy as a fact inherited from the equipment. Better players learned to compensate for the error.
[0013] However, when the same ball 17 hits the center line string 21 of the Fig. 2 frame, it will be in contact with three longitudinal string segments 21, 22 and 23, or, if the hit is hard, two more strings farther to the outboard sides could be involved. The resultant force, including the forces from the cross string segments, is exactly normal to the central plane of the frame because of the mirror imaged string arrangement. This improvement is important to seasoned players. They can be confident that the ball will be returned exactly along the perpendicular line of the frame of the racket if the hit is exactly at the center line of the racket.
[0014] Another merit of the invention is shown in Fig. 3 and its view along the cut section 4-4 shown in Fig. 4. It is suggested here that for a type of string pattern in which a multiples of strings are anchored to the frame at points some distance away from the central plane M of the frame, the resulting weaving pattern will be exactly symmetric, including the slanted portion of the length of the string segment near the boundary of the frame, with respect to the central axis. This not only improves the stress distribution, reduces frame distortions along the two lateral sides, but also makes handling of the racket easier, because then the structure and the string network is exactly balanced about the center line of the frame. Ends of the segments may wrap around the frame for anchor instead of through an elevated opening in the frame. In Figures 6 and 7, more will be said about frame distortion.
[0015] In Fig. 3, the end point 41 of the central longitudinal string 21 anchors at the upper half L of the frame, and the end 42 of the adjacent string segment is at the opposite half H of the frame. Anchor points in the same region, such as 41 and 43, are arranged along a line 44 approximately symmetric to plane M as shown in Fig. 4 with respect to end points at the opposite region. Similarly end points of cross string segments may be located away from the central plane of the frame. The line 44 of end point trajectory may be a curved line, a straight line, or a composite line comprising line elements and arc elements.
[0016] Fig. 5 shows the cross section 6-6 of Fig. 3. It shows the cross string segment 61 anchors at side 2 of the frame. This cross string weaves up and down through longitudinal strings with all odd numbering strings, 21, 63, etc., on lower half L and even numbering string segments like 62, on the upper half H. The cross string adjacent to 61 is 64 which is inclined oppositely at an angle with respect to the M plane. This inclination tightens the string and increases the string tension when a ball hits. With the string segment symmetrically wraps over the frame at the sides, control is improved. Fig. 5 shows graphically the so-called mirror image arrangement. If the player rolls the racket about the handle 180 degrees, the string pattern remains symmetric to the center line 8. In a conventional racket, the sloping of the opposite ends is anti-symmetric: if the left end points upward, the right end points downward. This is not a desirable feature.
[0017] Fig. 6 shows a string pattern of the Fig. 2 frame in which a single piece of string 81 strings the whole longitudinal string system. The segments within the frame's boundary is a mirror image with respect to the axis 8, however, the whole string pattern, including the loops 66, 67, are not symmetric to the axis 8. As a mater of fact, if one piece of string is used to complete the longitudinal string system from end a to end b, the frame will be strongly twisted by the diverging force couple 82 and 83. Fig. 7 shows that the same frame in Fig. 2 is strung by two pieces of string: the first piece 84 and the second piece 85. They are joined at a fork 86 near the axis of the frame, so that there are two identical string branches, separated at 86 and extended outwardly towards the outboard sides to form two symmetric longitudinal string systems. This is the exact, mirror image pattern this invention prefers. The second end d of 84 is anchored at the frame. Or, it may be tied to an adjacent string. Of course, the fork 86 may be located at the head portion instead of at the throat. Or, a string may be made specifically for the invention and the two pieces are threaded together at 86 to form a fork instead of being joined by a tie as shown. Other ways are possible to combine two pieces into a single one, All these practical arrangements will be named as a fork device in the specification. For the Fig. 7 arrangement, the two end force 87 and 88 are parallel forces and are pulling towards the same axial direction. This is a significant improvement over the Fig. 6 one piece string arrangement. It is to be noted the fork device enables the loops such as 68, 69 to be a mirror image to the axis 8.
[0018] A truly mirror imaged stringing pattern improves greatly vibration and control. Without the centrally placed string and an odd number of symmetrical, mirror imaged longitudinal string pattern, and a two-piece, fork device of the longitudinal string arrangement, these improvements can not be achieved.
1. A sports racket, comprising a handle (5) and a frame (10) having a longitudinal axis
(8) which is the central axis of the racket, a top part (1), two lateral sides (2),
and a throat (3), defining a general ball-hitting area, said frame having a central
plane (M) which is the plane dividing equally the thickness of the frame, and a plane
of symmetry (N) perpendicular to said central plane and containing the central axis,
a plurality of cross string segments (9), generally at an angle with the longitudinal string segments, each having two ends anchored separately at the two sides (2) of the frame; said longitudinal and cross string segments being interwoven into an approximate planar string network in said central plane, characterised in that it includes:
an odd numbered plurality of longitudinal string segments (7) generally parallel to said longitudinal axis (8) of the frame, extending from said top part (1) of said frame (10) towards the handle (5), each having two ends (42,43), anchored at the top part and the opposite side of the frame, said odd number of string segments comprising a central string segment (21) substantially coinciding with said central axis of the racket; so as to form two opposing arrays of string segments;
said pluralities of string segments forming a plurality of string loops (24) to anchor said string segments to the frame, wherein said pluralities of string segments and said string loops being arranged symmetrically about the plane of symmetry (N).
2. A sports racket as claimed in Claim 1 wherein the opposing arrays of longitudinal
string segments comprise at least two pieces of string (84,85) which bifurcate from
the central axis (8).
3. A sports racket as claimed in claim 2 wherein said two pieces of string may include
string segments and loops for said cross string system.
4. A sports racket as claimed in Claim 3 wherein three pieces of strings are used to
provide said string segments and loops for the racket.
5. A sports racket as claimed in Claim 1 wherein a proportion of said ends (41,42,43)
of said pluralities of string segments are located at different distances measured
normal from said central plane (M).
6. A sports racket as claimed in Claim 5 wherein a proportion of said ends (41,42,43)
may be substantially in the central plane (M).
7. A sports racket as claimed in either of Claims 5 or 6 wherein said ends of adjacent
cross string segments are positioned alternately in opposite directions from said
central plane of the frame (M).
8. The sports racket of Claim 7 wherein said ends of adjacent cross string segments are
positioned along a substantially straight line (44) parallel to the central plane
of the frame (M).
9. The sports racket of Claim 6 wherein said ends of said pluralities of string segments
wrap over the frame for support instead of passing through openings made inside the
frame.