U.S. patent number 3,856,603 [Application Number 05/359,815] was granted by the patent office on 1974-12-24 for method of manufacturing game rackets.
This patent grant is currently assigned to General Dynamics Corporation. Invention is credited to Richard E. Bender, William E. Schaefer.
United States Patent |
3,856,603 |
Schaefer , et al. |
December 24, 1974 |
METHOD OF MANUFACTURING GAME RACKETS
Abstract
A method of manufacturing a game racket having improved
stiffness and uniformity is disclosed. The racket comprises a shell
formed from a molded synthetic resin with a high strength fiber
reinforced plastic facing laminated to each face of the shell. The
shell is internally recessed in a truss-like pattern to provide
lightness with strength. The handle of the racket includes a pair
of plastic pallets secured to the handle portion of the shell and
covered with a covering material, such as a thin leather strip
winding. The finished racket is finally strung in a conventional
manner and is then ready for use.
Inventors: |
Schaefer; William E. (San
Diego, CA), Bender; Richard E. (San Diego, CA) |
Assignee: |
General Dynamics Corporation
(San Diego, CA)
|
Family
ID: |
26934069 |
Appl.
No.: |
05/359,815 |
Filed: |
May 14, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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241176 |
Apr 5, 1972 |
3840230 |
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Current U.S.
Class: |
156/245; 264/250;
156/293; 264/259; 473/547; 473/549 |
Current CPC
Class: |
A63B
49/02 (20130101); A63B 49/10 (20130101); A63B
60/10 (20151001); A63B 49/08 (20130101); A63B
60/08 (20151001); A63B 2049/0205 (20130101); A63B
60/06 (20151001) |
Current International
Class: |
A63B
49/08 (20060101); A63B 49/02 (20060101); B29g
001/00 (); B29g 007/00 () |
Field of
Search: |
;156/242,245,292,293
;264/241,250,251,254,259,328 ;273/73C,73F,73J |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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208,945 |
|
Jul 1957 |
|
AU |
|
132,698 |
|
Sep 1919 |
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GB |
|
1,310,470 |
|
Dec 1962 |
|
FR |
|
1,512,401 |
|
Jan 1968 |
|
FR |
|
Primary Examiner: Van Horn; Charles E.
Assistant Examiner: Weston; Caleb
Attorney, Agent or Firm: Duncan; John R.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a division of U.S. Patent Application Ser. No.
241,176, filed Apr. 5, 1972, now U.S. Pat. No. 3,840,230.
Claims
We claim:
1. A method of manufacturing a game racket which comprises the
steps of:
molding a racket shell having a head frame and a handle extension
connected to the frame by a throat portion, said shell being
substantially planar with two substantially parallel faces, from a
material comprising a synthetic resin having flexural modulus of
from about 3 .times. 10.sup.5 to about 17 .times. 10.sup.5 psi and
a notched Izod impact measurement of from about 2 to 15 ft-lb/in.,
said shell being formed with a plurality of spaced pockets
extending inwardly from the faces;
forming substantially planar facing members having the general
shape of said shell from a material comprising high-strength fibers
in a synthetic resin matrix, said fibers having a Young's modulus
of from about 20 .times. 10.sup.6 to about 80 .times. 10.sup.6 psi
and a density of from about 0.05 to about 0.07 lb/in..sup.3 ;
laminating a facing member to each of said shell faces to cover
said inwardly extending pockets; and
securing pallet members to the handle extension of said shell to
form an easily gripped handle.
2. The method according to claim 1 wherein the walls between said
spaced pockets in said shell are formed with thickened transverse
portions, and transverse stringing holes are drilled through said
thickened portions.
3. The method according to claim 1 wherein said racket shell is
molded from a synthetic resin selected from the group consisting of
polycarbonate and polyphenylene oxide resins; said resin comprising
up to about 40 weight percent chopped glass fibers.
4. The method according to claim 1 wherein said facing members
include a surface layer of fiberglass cloth.
5. The method according to claim 4 wherein said facing members are
formed by arranging graphite fiber tow pre-impregnated with an
epoxy resin in a pre-compaction mold; encasing said tow in
fiberglass cloth; placing the resulting composite in a compression
mold; and subjecting said composite to heat and pressure for a
period sufficient to cure said epoxy resin.
6. A method of manufacturing a tennis racket which comprises the
steps of:
injection molding a racket shell from a synthetic resin selected
from the group consisting of polycarbonate and polyphenylene oxide
resins having a flexural modulus of from about 3 .times. 10.sup.5
to about 17 .times. 10.sup.5 psi and a notched Izod impact measure
of from about 2 to about 15 ft-lb/in.; said resin containing up to
about 40 weight percent chopped glass fibers; said shell formed
with recessed faces in a truss-like configuration in which an array
of pockets extends inwardly from the shell faces alternately from
opposite faces;
compression molding facing members comprising high strength
graphite fibers in an epoxy resin matrix; said fibers having a
Young's modulus of from about 20 .times. 10.sup.6 to about 80
.times. 10.sup.6 psi and a density of from about 0.05 to about 0.07
lb/in.sup.3 ; said facings having a surface layer of fiberglass
cloth;
drilling transverse stringing holes through said shell, said holes
passing through webs between adjacent pairs of said pockets;
and
bonding a facing member to each face of said shell with an epoxy
adhesive; said facing members covering said truss-like
recesses.
7. The method according to claim 6 wherein said facing members are
formed by arranging graphite fiber tow pre-impregnated with an
epoxy resin in a pre-compaction mold; encasing said tow in
fiberglass cloth; placing the resulting composite in a compression
mold; and subjecting said composite to heat and pressure for a
period sufficient to cure said epoxy resin.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to tennis rackets or the like and,
more specifically, to a racket formed from reinforced plastic
materials.
Strung rackets for use in tennis and similar games have long been
made from wood and strung with gut or nylon strings. Manufacture of
high quality rackets is a complex process. Despite great care in
manufacture, the playing characteristics of wood rackets vary
greatly due to natural variations in the wood used and
manufacturing process variables.
Playing characteristics of wood rackets also vary with changes in
temperature and humidity and with age of the racket, which changes
may also cause the head to warp due to varying string tension.
Recently, steel and aluminum rackets have been developed in order
to obtain greater uniformity. However, it has been found that these
metal rackets do not provide the same playing qualities as wood
rackets, since sufficient stiffness cannot be provided without
excessive weight. Also, metal rackets frequently have short useful
lives, due to cracking apparently caused by metal fatigue or stress
concentrations.
Rackets are also being manufactured from fiberglass reinforced
plastics. These rackets, however, generally are overly flexible and
do not provide the desired stiffness for equivalent weight. Also,
they lack uniformity and are expensive due to the number of manual
manufacturing steps.
No matter which material has been used, there have always been
problems in providing sufficient strength in the racket head to
resist stresses during stringing of the racket. As stringing
progresses, severe stress concentrations result in portions not yet
supported by strings. As stringing continues, the locations of
stress concentrations shift. The racket frame may severely warp or
break due to these stresses, if the frame lacks the strength and
stiffness to resist the stresses induced by the very taut strings.
Generally, attempts to strengthen the frame by increasing its
cross-sectional area have not been successful due to the resulting
weight increase and balance shift to the head.
Thus, there is a continuing need for rackets of improved strength,
stiffness and playing characteristics.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide a racket
of improved strength and stiffness.
Another object of this invention is to provide a racket having
improved resistance to stringing stresses.
Still another object of this invention is to provide a simple and
consistent method of manufacturing rackets.
The above objects, and others, are accomplished in accordance with
this invention by a racket for tennis or the like which basically
comprises a shell formed from a molded synthetic resin with a high
strength fiber reinforced plastic facing laminated to each face of
the shell. The shell has a truss-like internal structure which is
covered by the facing. This internal structure provides high
strength with low weight and supports the strings in a desirable
manner. Preferably, the shell is formed by injection molding, since
this allows rapid production of a highly uniform product. The
racket is completed by the addition of a pair of pallets to the
handle portion of the shell, after which a surface layer of thin
leather strips or the like may be wound around the handle portion
to provide a desired gripping surface.
The racket may have any desired shape. In general, an elliptical or
round frame is preferred. The method of the present invention is
adaptable to a wide variety of frame shapes.
The racket shell may be formed from any suitable resin having the
required properties. The shell resin should have high stiffness,
good impact resistance, predictable and consistent mold shrinkage,
good dimensional stability and high fatigue resistance. For best
results, the material should have a flexural modulus in the range
of from about 3 .times. 10.sup.5 to about 17 to 10.sup.5 psi and an
Izod impact measurement of from about 2 to about 15 ft-lb./in.
(notched bar). Suitable resins include polycarbonate and
polyphenylene oxide resins, typically available from General
Electric Co. under the trademarks Lexan and Noryl,
respectively.
The shell resin may contain fillers or additives to improve desired
properties. For example, up to 40 volume per cent randomly chopped
glass fibers, typically having lengths of from about 0.1 to 0.25
inch may be added to improve strength properties.
The weight of the shell is determined largely by shell thickness,
the internal truss pattern and the resin/filler combination
selected. Where it is desired to market a line of similar rackets
having different weights, varying shell thickness to vary the
racket weight has been found to be convenient and effective.
Alternatively, the thickness of the truss-like internal webs may be
varied. If desired, the internal openings of the shell may be
filled with a foamed synthetic resin. In a typical racket of the
sort described herein, a reduction of the thickness of the shell in
the plane of the racket of 0.05 inch provides approximately one
ounce difference in shell weight.
The facing layers may comprise any suitable high strength fiber in
a synthetic resin matrix. Best results are obtained with fibers
having Young's modulus of from about 20 .times. 10.sup.6 to about
80 .times. 10.sup.6 psi and density of from about 0.05 to about
0.07 lb/in.sup.3. Therefore, fibers having these properties are
preferred. Especially suitable fibers include high strength
graphite fibers, such as those available from Union Carbide
Corporation under the trademark "Thornel" and high strength organic
fibers such as those available from E. I. duPont de Nemours &
Co. under the trademark PRD-41.
The high-strength fibers may be imbedded in any suitable synthetic
resin. The resin selected should have high impact resistance, good
dimensional stability and good peel strength. In general, best
results are obtained with epoxy polyimide and phenolic resins.
Preferably, the high-strength fibers are positioned in a
substantially parallel arrangement around the racket face and down
the handle. In order to provide a uniform, smooth surface and to
increase the transverse strength of the facing, it is preferred
that a surface layer of a resin impregnated woven fiber be formed
over the facing. This over-layer may surround the parallel-fiber
main portion, or may merely cover both planar surfaces of the
facing. Any suitable fibers may be used in the overlayer. While
high-strength fibers, such as high-strength graphite fibers, are
preferred, other fibers, such as organic or glass fibers, may be
used, if desired.
The handle pallets may be formed from any suitable material, such
as wood or plastic. Acrylonitrile-butadiene-styrene (ABS) polymers
are preferred, since they are inexpensive, easily injection molded,
and are sufficiently flexible to give a desirable handle "feel." If
desired, any suitable covering material, such as leather, fabric or
plastic sheet or strip material, may be added over the racket
handle to improve the grip characteristics. Typically, the
circumference of the racket grip (circumference of the pallets plus
the leather or other covering) ranges from about 4 to 5 inches.
Preferably, eight different pallet molding are prepared, ranging
from 4 to 5 inches in circumference in 1/8-inch increments, so that
the racket can be sized to meet any player's preference.
BRIEF DESCRIPTION OF THE DRAWING
Further details of the invention will be understood upon reference
to the drawing, which illustrates a preferred embodiment of the
racket of this invention.
In the drawing:
FIG. 1 is a perspective view of a complete racket;
FIG. 2 is an exploded perspective view illustrating the major
racket components;
FIG. 3 is a plan view of the face of the racket;
FIG. 4 is a side elevation view of the racket shown in FIG. 3.
FIG. 5 is an enlarged detail view showing the base of the racket
taken on line 5--5 in FIG. 3;
FIG. 6 is an enlarged section view through the racket handle, taken
on line 6--6 in FIG. 3;
FIG. 7 is an enlarged section view through the racket throat and
frame, taken on line 7--7 in FIG. 3;
FIG. 8 is an enlarged transverse section view through the racket
frame, taken on line 8--8 in FIG. 3;
FIG. 9 is a longitudinal detail view of a portion of the frame,
taken on line 9--9 in FIG. 3;
FIG. 10 is an enlarged section view through the racket frame, taken
on line 10--10 in FIG. 9;
FIG. 11 is a plan view of the facing;
FIG. 12 is a side view of the facing;
FIG. 13 is an enlarged section view through the facing frame, taken
on line 13--13 in FIG. 11;
FIG. 14 is a plan view of a handle pallet;
FIG. 15 is a section view of the pallet taken on line 15--15 in
FIG. 14; and
FIG. 16 is an enlarged section view through the pallet, taken on
line 16--16 in FIG. 14.
DETAILED DESCRIPTION OF THE DRAWING
An overall view of a complete, strung, tennis racket is shown in
FIG. 1. The racket basically consists of a shell 10 having a fiber
reinforced plastic facing 12 on each face. The racket frame or head
portion 14 is strung with a taut gut or nylon string 16 which
passes through a plurality of transverse holes 18 in frame 14. A
pair of pallets 20 are secured to the sides of handle portion 22 of
shell 10 and wrapped with a thin covering 24 to provide a
comfortable grip area.
The relationship of the major racket components can more clearly be
seen in the exploded view of FIG. 2. Shell 10 is preferably
recessed at 26 on each face to receive facing 12, giving a smoothly
contoured outer surface to the completed racket. Behind recesses 26
shell 10 is pocketed in a truss-like pattern to provide maximum
strength with minimum weight. These truss structures are hidden by
facing 12 in the completed racket.
Facing 12 extends entirely around frame 14 to provide stiffness and
strength to resist impact against a ball in play, and also to
strengthen frame 14 against varying stresses in the plane of frame
14 during stringing. Facing extends from frame 14 well down handle
22, adding strength to the racket throat portion 28, which is often
weak in rackets made from other materials. Facing 12 also adds
desired stiffness to handle 22. Stiffness of the different portions
of the racket can easily be varied by varying the cross-sectional
area of the facing 12 at different locations to give the racket the
desired playing characteristics.
A groove 30 is provided in the outer surface of frame 14 so that
the string is recessed as it passes between holes 18. This protects
the string against abrasion, should the racket strike the court
playing surface during use.
Pallets 20 substantially surround the ends of handle 22. Pallets 20
are securely held in place by sides 32 which extend around the
sides of handle 22 and by pins 34 on pallets 20 which enter holes
36 in handle 22. The bases of pallets 20 are preferably recessed at
38 so that the manufacturer's trademark or other emblem may be
emplaced there.
Facing 12 may be secured to shell 10 in any suitable manner.
Preferably, adhesive bonding, such as with an epoxy adhesive, is
used. While pallets 20 are held in place by the interlocking effect
of sides 32 and pins 34 together with the leather wrapping, they
may also be adhesively bonded to shell 10, if desired.
When assembled as illustrated in FIG. 1, the resulting racket has
outstanding strength, stiffness and playing characteristics. These
characteristics result in large measure from the configuration and
composition of the component parts, which are illustrated in detail
in the remaining figures.
The top of shell 10 is shown in FIG. 3 and the side of the shell in
FIG. 4. This is a single unitary member which is preferably formed
from a polycarbonate resin in a single injection molding step. The
truss-like pattern of pockets in shell 10 is arranged so as to
permit convenient production in a simple mold.
As best seen in FIGS. 3, 5, and 6, the handle portion 22 can be
thought of as a solid polycarbonate member with a plurality of
triangular recesses or pockets 40 extending nearly through the
handle alternately from the upper and lower surfaces thereof.
Pockets extending upwardly in FIG. 3 are illustrated by broken
lines 42. This produces a very strong truss-like structure with
very light weight, since the webs between adjacent pockets 40 and
hidden pockets 42 are thin, and the base 44 of each pocket is thin,
as seen in FIG. 6.
As seen in FIGS. 3 and 5, the base of handle 22 is closed by an end
wall 48. Also, the sides of handle 22 are grooved in a continuation
of groove 30 which surrounds frame 14 to protect the racket strings
against abrasion should the playing surface be struck during play.
Groove 30 serves to reduce the weight of handle 22 while retaining
strength and stiffness. The upper and lower surfaces of handle 22
include a depression or recess 48 sized to receive the handle
portion of facing 12. When bonded in place, facing 12 covers and
hides the truss arrangement produced by the array of pockets 40 and
42.
As seen in the lower portion of FIG. 3, a depression or hole 36 is
formed to receive a locating pin on pallets 20, as further
described below. Moving up the handle as seen in FIG. 3, the
pattern of pockets changes in throat area 28 to a pattern around
frame 14 which accommodates stringing holes. As shown in section in
FIG. 7, which illustrates the transition between throat 28 and
frame 14, pockets 52 in the upper face of frame 14 and pockets 54
in the lower face of frame 14 alternate around the frame to produce
a truss-like web between adjacent pockets. Web 55 is the final
handle web before the frame pocket pattern develops. A transverse
thickened portion 56 is formed in each inter-pocket web through
which a stringing hole 58 may be formed. The stringing holes 58 may
either be produced during the molding operation by removable pins
positioned in the mold, or may be drilled after molding of the
shell. This arrangement in which stringing holes 58 penetrate
through solid portions of shell 14 is highly desirable both from a
strength standpoint and because only two apertures need be deburred
or smoothed to prevent abrading the string. If the holes penetrated
through a pocketed area, there would be four surface apertures to
be smoothed.
A transverse section through shell 14 is seen in FIG. 8. The frame
faces are recessed, with upstanding edges 60, so that when facing
12 is bonded in place the outer facing surface will blend smoothly
into edge 60. As described below, the facing is grooved in the
frame area, producing a broad U-shaped cross-section. A ridge 62 is
provided around frame 14 to fit precisely within the facing groove,
assuring excellent bonding between facing 12 and shell 14. As
discussed above, a string-protecting groove 30 is provided in the
outer edge of frame 14. Also, a groove 64 of generally semicircular
cross-section is provided in the inner edge of frame 14 to reduce
weight while retaining maximum strength.
FIGS. 9 and 10 illustrate a pair of raised areas 66 within groove
30 adjacent to racket throat 28. In the groove area, the angle
between the string as it lies within groove 30 between string holes
58 and the string as it lies with holes 58 in this area is an acute
angle. This sharp angle as the string changes direction upon
entering holes 58 has been found to damage the string. Raised areas
66, properly positioned within groove 30, relieve these severe
localized stresses.
Details of the facing members 12 can be seen in FIGS. 11 and 12.
The facing generally consists of a frame portion 68 and a handle
portion 70. Handle portion 70 may have a constant cross-section, or
may taper in thickness and/or width to vary the stiffness
characteristics of the completed racket handle. As seen in section
in FIG. 13, frame portion 68 has a broad U-shaped cross-section,
with raised edges 72 bounding a central groove 74. Ordinarily the
thickness of the facing material in the area of groove 74 is about
one-half the thickness of the facing in the handle portion 70. It
has been found that the raised edges 72 add greatly to the strength
of the facing and racket frame 14 without adding appreciably to the
weight of the racket. Typically, the facing may be about 0.070 to
0.090 inch thick in the handle 70 portion, with raised edges 72 and
groove 74 having thicknesses in the range of about 0.110 to 0.130
inch and about 0.025 to 0.055 inch, respectively. Handle 70
typically may have a width in the range of from about 0.90 to 1.10
inches. The width of the facing in frame portion 68 may typically
range from about 0.45 to about 0.65 inch.
Strength and stiffness in the frame area are especially important,
since forces in the plane of frame 14 must be resisted during
stringing, and ball impact forces in a direction substantially
perpendicular to the plane of frame 14 must be resisted during
play. As discussed above, groove 74 and raised edges 72 mesh with
ridge 62 on shell 10 when facing 12 is bonded to shell 10.
Facing 12 consists primarily of graphite fibers in a resin matrix.
Preferably, a thin surface layer 69 comprising resin impregnated
fiberglass cloth is applied to the inner, outer or all surfaces of
facing 12 in order to improve the transverse strength
characteristics of the facing, as discussed above. The inner
fiberglass surface layer 69 extends down handle portion 70 only as
far as line 71 (FIG. 12) because the increased transverse strength
is not required in the handle portion 70 of facing 12.
Details of the handle pallets 20 are shown in FIGS. 14, 15, and 16.
FIG. 14 shows the side of pallet 20 which fits against shell handle
22. The outer surface configuration of pallet 20 can be seen in
FIG. 2. When installed, pallets 20 substantially surround handle
portion 22. Ribs 80 add strength and rigidity to the pallets and
rest on the surface of handle 22. Rib 82 includes an outwardly
extending pin 34 which engages hole 36 in handle 22 to locate the
pallet in the desired position. If desired, pallets 20 may be
adhesively bonded to handle 22, though this is not always
necessary. After pallets 20 are fitted in place, a winding of thin
leather or the like is applied to give a desired gripping surface,
as shown in FIG. 1. The racket is then ready for stringing and
use.
While the method of manufacturing these rackets has been described
in general terms, a preferred embodiment of this method is provided
in the following example.
EXAMPLE
A mold is prepared for the racket shell 10 in the configuration
described above and shown in the drawing. An injection molding
material comprising "Lexan" polycarbonate resin, available from
General Electric, filled with 10 percent chopped glass fibers is
prepared and injected into the mold. After completion of the
molding operation, 0.136-inch diameter holes 58 are drilled through
thickened areas 56 to receive the racket strings. Although typical
strings are only about 0.05-inch in diameter, the larger sized hole
is used in order to accommodate double strings either as part of
the basic stringing arrangement or to allow for repairs.
A pair of facing members 12 is prepared by compression molding.
Material for the facings is a Type A graphite filament
preimpregnated with an epoxy resin, available from the Fiberite
Company under the X505 designation. This material is in the form of
continuous tows about 57 inches long. Successive tows are laid up
in a pre-compaction die and encased in a thin layer (about 0.005
inch thick) of fiberglass cloth. The layup is then placed in the
facing forming die, compacted at about 50 psi and cured at about
250.degree.F for about 30 minutes. After minor cleanup and removal
of flash, the facings are ready for installation on the shell. The
fiberglass casing becomes an integral part of the facing and
provides cross-tension strength to the laminate.
The surfaces of the shell and facing are prepared for bonding with
a light sandblast and a liquid Freon (a fluorocarbon liquid
available from E. I. duPont de Nemours & Co.) wipe just prior
to bonding to insure a good "tooth" and a clean interface. The
facings are bonded to the shell with an epoxy resin, Epoxy 934,
available from the Hysol Chemical Co. About 10 psi mechanical
pressure, is applied during bonding. Excess adhesive which may
squeeze out is removed immediately.
After the facing adhesive is cured, the racket is inspected,
tested, any desired decals are applied and a urethane resin finish
coat is applied. The coating is dried at about 120.degree.F for
about 8 hours to drive off all volatiles.
A pair of pallets 20 as shown in the drawing are injection molded
from Cycolac resin, an acrylonitrile-butadiene-styrene resin
available from the Morbon Chemical Division of the Borg-Warner
Corporation. The pallets are bonded to the racket with conventional
contact cement. The pallets are coated with a grip adhesive, Inco
No. 155 from the intercoastal Corp. The leather grip material is
installed over the pallets, after which the grip adhesive is
activated by applying a solvent through the leather covering.
Finally, any desired labels or identification symbols are applied
and the racket is strung in a conventional manner. The racket is
found to have excellent playing characteristics and durability.
While certain specific materials, arrangements and conditions are
specified in the above description of a preferred embodiment, these
may be varied or other materials or steps added where suitable,
with similar results as described above.
Other modifications and ramifications of the invention will become
apparent to those skilled in the art upon reading the present
disclosure. These are intended to be included within the scope of
this invention, as defined in the appended claims.
* * * * *