Tennis Racket

Abstract

A racket construction having a central dampening core sandwiched between skins of high strength material, the skins serving as the racket faces. In the bow portion of the racket at least one web, having higher strength characteristics than the core, extends normal to the skins. Layers of elastomeric material are utilized between the skins and the core to assist in laminating the core, skins and web into a unitary structure.

Parent Case Text

This is a continuation-in-part of application Ser. No. 11,547, filed Feb. 16, 1970, now abandoned.

Description

The present invention relates to an improved tennis racket which is capable of being tailored in its physical characteristics in order to meet the demands of a wide variety of players.

For many years conventional tennis rackets have been formed using wood as the basic material for the racket. However, wood rackets have a number of shortcomings. For example, in order to have a racket of adequate strength, there are limitations as to how light the racket can be, and even the strongest wood rackets are subject to breakage in use. It is also well known that fibers of wood deteriorate and loosen when flexed repeatedly. Accordingly, a wood racket loses its "life" relatively quickly. Of course, wood is also subject to warping. A still further wear factor affecting a wood racket is its susceptibility to the adverse effects of abrasion when the racket engages the playing surface.

The physical characteristics required for good play also challenge the versatility of wood rackets. Two important parameters in tennis racket design are longitudinal bending rigidity and torsional rigidity. With a wood racket these parameters are mutually dependent. If a designer established a particular bending rigidity, he must, in general, accept the degree of torsional rigidity which results. The designer effectively has no means to establish the torsional rigidity of the racket independently of the bending rigidity.

Another important parameter of racket design is "liveliness." For some purposes maximum resilience will give better performance. For other purposes internal "damping" may be desired to soften the action of the racket. With a wood racket, it is impossible to pre-select the degree of damping independent of other mechanical properties.

In an attempt to overcome the deficiencies of the wood racket, a variety of constructions have been developed utilizing metal. For example, several rackets employ only metal as the structural material in the bow and throat portions of the racket, whereas other arrangements use metal to cover a dampening core, such as wood. While such constructions reduce the breakage and wear disadvantages found in conventional wood rackets, they still retain shortcomings affecting the play characteristics of the racket. For example, in order to provide high torsional stiffness desirable in a racket, the resultant construction is stiff in its longitudinal bending property. This causes the user's arm to be exposed to a high shock load which not only is uncomfortable, but which also leads to the well-known "tennis elbow."

Another problem which metal rackets are faced with is the difficulty in properly stringing the racket without exposing the strings to sharp edges which cut the strings.

The present invention overcomes the deficiencies of the known racket constructions just discussed. More particularly, in a preferred embodiment of the invention, a syntactic foam core, generally formed in the shape of a racket, is located between metallic skins. Layers of elastomeric material are placed between the skins and the core. In the bow portion of the racket a pair of webs, having higher strength characteristics than the syntactic foam, extend normal to the skins at the inner and outer peripheries of the bow. The entire structure is laminated into a unitary structure. The bow of the resultant racket is drilled and strung in the usual manner, the webs serving to prevent the core from splitting as a result of the stringing operation. The metallic skins provide the racket with the desired torsional stiffness. Inasmuch as the longitudinal bending rigidity of the racket is a function of skin thickness and separation of the skins due to the thickness of the core, the bending stiffness may be established to a considerable degree substantially independent of torsional rigidity. The core itself also serves to absorb shock energy as the racket strikes the ball.

The invention will be described in further detail by reference to the accompanying drawings, wherein:

FIG. 1 is a plan view of a first embodiment of a racket constructed in accordance with the invention;

FIG. 2 is an enlarged plan view partially in section illustrating the bow and throat portions of the racket prior to the stringing operation;

FIG. 3 is an enlarged fragmented view in section taken along line 3--3 of FIG. l;

FIG. 4 is an enlarged sectional view taken along line 4--4 of FIG. l, the string being shown in dash lines for convenience of illustration;

FIG. 5 is an enlarged fragmented view of a portion of the bow shown in section in FIG. 2 with stringing added;

FIG. 6 is a plan view of a preferred embodiment of a racket constructed in accordance with the invention; and

FIG. 7 is an enlarged sectional view taken along line 7--7 of FIG. 6, the string being shown in dash lines for convenience of illustration.

Referring now to the drawings, the invention will be described in detail.

In FIG. 1 a tennis racket is illustrated, the racket comprising a handle 10, a throat portion 12 and a bow 14. The throat is provided with a cutout area 16 to increase the torsional stiffness of the racket without at the same time proportionally increasing the longitudinal stiffness as would be the case if there were no cutout. The cutout area also reduces the wind resistance to the racket as it is swung. In addition, the utilization of the cutout 16 contributes to lowering the overall weight of the racket. The bow 14 serves to support conventional strings, or gut, 18.

FIG. 2 illustrates the core of the racket. More particularly, the core 20 is a lightweight syntactic foam which is formed to the general contours of the racket with a cutout 22 provided in the core in the lower throat and in the handle to lighten the racket. The syntactic core consists of a high compressive strength resin, such as epoxy, filled with microbubbles of glass or phenolic material. Such a core has high shear, flexural, tensile and impact properties. A more complete discussion of this core material can be found in an article entitled "Syntactic Foam" appearing in the September, 1967 issue of MODERN PLASTICS, page 215. To further increase the physical properties of the core, loose, chopped or milled fibers are added to the resin before the foam is cured. A first web 24 bounds the core 20 about its outer periphery, and a second web 25 is located along the inner periphery of bow 14. Both webs have higher strength characteristics than core 20 for reasons which will be discussed hereinafter.

FIG. 3 illustrates the complete racket assembly in section. The exterior surfaces of the racket comprise spaced metallic skins 26 and 28 which are preferably formed of a lightweight material, such as aluminum or magnesium, having high strength characteristics. Conventional handle pieces 34 are secured to the outer surfaces of skins 26 and 28 in the usual manner.

Now that the basic structure of the racket has been outlined, more detail will be presented. In fabricating the racket, the first skin 26, stamped to the contour of the racket, is placed in a mold. Skin 26 is primed on its upper surface with a hardenable resin such as epoxy. Webs 24 and 25 are then positioned on the skin 26 so that their major dimensions extend normal to the plane of skin 26. The space between the webs 24 and 25 is next filled with uncured syntactic foam loaded with loose, chopped or milled fibers. This step is followed by laying the stamped skin 28 in the mold. The lower surface of skin 28 is primed in the same manner as skin 26. The mold is then closed, and heat and pressure are applied causing the resins to cure and unitizing the assembly. Following this step the structure is removed from the mold, and the stringing operation is performed, as will be described hereinafter.

Typically a tennis racket made in accordance with the foregoing method comprises aluminum skins 26 and 28 having thicknesses of approximately 20 mils. The thickness of webs 24 and 25 is a function of the material used. In the preferred embodiment the webs are cloth-backed polyethylene approximately 50 mils thick. The cloth backing facilitates the adherence of the webs to core 20 during the curing step. A practical formulation for the core 20 is as follows:

epoxy resin 100 PBW hardener 50 PBW reactive diluent 5 PBW chopped fiberglass (1/4" lengths) 20 PBW phenolic or glass microbubbles 35 PBW pigment 5 PBW

of course, it should be appreciated that the foregoing formulation is for illustrative purposes only and other proportions could be used consistent with the desired physical characteristics of the resultant construction.

To complete the racket, a stringing operation is performed. As can best been seen in FIGS. 4 and 5, this comprises drilling holes 36 through the core 20 and webs 24 and 25. In these Figures, the cloth backing for the webs is shown as 24a and 25a, respectively. During this drilling operation, the presence of webs 24 and 25 assumes considerable importance inasmuch as the webs reinforce the core 20 to prevent the core from splitting along the stringing plane. A groove 40 is provided on the outer periphery of the bow 14 in order to provide a recess for the string portions positioned on the outside of the bow. Such a recess protects these string portions from contacting the playing surface. After holes 36 and groove 40 are formed, the racket is strung in the usual manner. Again, the tensile strength imparted by webs 24 and 25 to core 20 prevents the core from splitting along the plane of the strings as the core is exposed to the stress of the stringing operation. Another important characteristic of the core construction comes into play at this point. More particularly, the compressive strength of the cured syntactic foam core 20 is such that as string 18 is pulled under tension into groove 40 (FIG. 5), localized crushing of the core occurs at the sharp edge 42 of the core. This results in the bluting of edge 42 to insure that the edge does not cut the string. The compressive strength of the web material also permits such localized crushing wherever the strings engage the webs.

The racket is completed by the usual attachment of the handle pieces 34.

The embodiment illustrated in FIGS. 6 and 7 is identical to that just described except that thin layers 44 and 46 of elastomeric material, such as rubber, polyurethane or the like, are interposed between the core 20 and skins 26 and 28. These layers may fully separate the skins and the core, but preferably layers 44 and 46 are arranged to only partially separate these elements at the outer periphery of the racket, as shown in FIGS. 6 and 7. The utilization of layers 44 and 46 improves the lamination of the structure. This is particularly desirable in the vicinity of the end of the racket bow which is frequently subjected to considerable stress by being struck against the surface of the tennis court. Partial elastomeric layers are preferred in order to minimize the shear deflection damping which such layers contribute to the racket.

The constructions just described afford a number of advantages which make the resultant tennis rackets marked improvements over known devices. The most important of these is the fact that the torsional rigidity and longitudinal bending rigidity can be established substantially independently of one another. The desirability of such design capability can be appreciated in view of the following discussion.

High torsional stiffness in a tennis racket is necessary in order to properly control the height of a shot and to obtain the feeling that each shot is a "crisp" one even though the ball is hit off-center.

If a racket is of low torsional stiffness and the ball is not hit at the center of the racket, the torsional deflection of the racket, as it is swung in a substantially horizontal plane, causes the ball to be returned too low or too high. With the present racket, the high tensile modulus of the metallic skins 26 and 28, the open throat area, the proper geometry of the composite sandwich, and the high shear modulus of the core 20 all combine to supply the racket with the high torsional stiffness required to obtain "crisp" shots which are properly directed. The skin thickness is the principal contributor to the amount of torsional rigidity obtained.

With conventional rackets, high torsional stiffness usually results in stiff longitudinal bending characteristics. Consequently, when a ball is hit, considerable shock is imparted to the player's s, arm. This can cause discomfort and even injury. It is desirable to provide longitudinal flexibility to a racket to reduce shock load. Also, longitudinal flexibility increases the power a player can put into his shot. However, if the bending stiffness of the racket is too low, one loses control of the direction of a shot when the racket is swung in a substantially horizontal plane. Therefore, it can be seen that it would be advantageous to provide a selected degree of longitudinal flexibility without sacrificing torsional rigidity. This is accomplished in the present racket by varying the skin and/or core thicknesses. It has been found that the longitudinal bending characteristic of a racket is substantially proportional to the first power of skin thickness and to the square of the core thickness.

From the foregoing, it is apparent that the present constructions permit rackets to be fabricated with high torsional rigidity and different longitudinal bending characteristics. In fact by appropriately adjusting the skin and/or core thicknesses along the length of a racket, localized control of the longitudinal bending characteristics can be attained. Thus, the invention permits rackets to be tailored to the preferences of a wide variety of players.

It has been stated previously that longitudinal flexibility increases the shock absorbing properties of a racket. It should also be noted that the core 20 serves as a dampener of shock. This is due to the internal resilience of the Yet, yet, such core material has sufficiently high shear and tensile strength so as to permit the desired torsional and longitudinal bending properties to be established and to support the strings of the racket.

The embodiments of the invention which have been heretofore discussed utilize metal for the skins 26 and 28. However, it is not intended that the invention be restricted to the use of such material. Rather, it has been found that other materials such as synthetics, may be utilized so long as they have a minimum yield strength of 50,000 psi and a tensile modulus of at least 1,000,000 psi. The webs 24 and 25 in the embodiments described are formed of polyethylene. However, as in the case of skins 26 and 28, other materials can be used. For use as a web, the material employed should have a compressive strength of approximately 4,500 to 20,000 psi and a tensile modulus less than approximately 1,000,000 psi. The use of such material provides the advantage of reinforcing the core so that it will not split as a result of the stringing operation. Yet this type of web material only minimally affects the torsional rigidity and longitudinal flexing characteristics of the racket. Substitutes may also be made for the syntactic foam used in core 20. An example is a urethane foam. Materials having a Rockwell hardness of L 50 to L 100 (ASTM D 785-65) are suitable for use as a core.

Certain other modifications may also be made within the spirit of the invention. For example, partial or full skins may be utilized to reinforce skins 26 and 28 to control the physical properties as hereinbefore discussed, and these additional skins may be isolated from the primary skins by energy absorbing layers, such as visco-elastic rubber, to provide shear deflection damping. Also, additional layers of material may be used in conjunction with webs 24 and/or 25 for protective purposes or to otherwise improve the physical or decorative characteristics of the racket.

The structures disclosed herein are examples of tennis rackets in which the inventive features of this disclosure may be utilized. However, the principles employed equally apply to rackets for other games such as squash, badminton, etc.

Claims

What is claimed is:

1. A tennis racket frame consisting essentially of a flat, single piece, unitary, and light weight plastic foam material central core having the contour of a finished tennis racket frame and including an elongated handle, an integral continuously oval head part, and an integral intermediate throat part integrally interconnecting the handle and oval part, a pair of plastic material reinforcing webs bounding said core contour along the inner and outer peripheral edges thereof, said web reinforced core being sandwiched between a pair of similarly shaped, single piece, unitary, and flat aluminum plates, said core, webs, and plates being bonded together by means including a thin layer of elastomeric material between said web reinforced core and plates, said oval part comprising the stringing plane of said frame, said aluminum plates being parallel to said stringing plane and having a contour similar to the contour of said web reinforced core and coextensive therewith, and said reinforcing webs being perpendicular to said stringing plane, the throat part of said frame having an elongated and generally triangular shape which is aligned with the lengthwise axis of said frame and being merged with the handle and oval part along a gradual curvature, a concentric elongated generally triangular shaped cutout formed in said throat part through said plates and core, and a cross section through said frame at the handle, throat and oval parts being rectangular in shape and being bounded on opposite exterior sides thereof by said plates and webs with said core enclosed therein, the thickness of said plates and webs being thin relative to the thickness of said core, and a hand grip cover for the butt end of said frame.

2. In a tennis racket frame as in claim 1, a hollow formed in said core at the butt end of said frame, said core material comprising a syntactic epoxy loaded with short lengths of fiberglass, said web material comprising cloth backed polyethylene, said plates having a thickness of the order of 20 mils, the webs having a thickness of the order of 50 mils, and stringing holes formed through the core and webs of the oval part for stringing said frame.