U.S. patent number 3,745,756 [Application Number 05/225,151] was granted by the patent office on 1973-07-17 for construction of tennis strings.
This patent grant is currently assigned to Ashaway Line & Twine Mfg. Co.. Invention is credited to Julian T. Crandall.
United States Patent |
3,745,756 |
Crandall |
July 17, 1973 |
CONSTRUCTION OF TENNIS STRINGS
Abstract
A string for use in connection with athletic rackets, such as
tennis, badminton, squash and the like comprising a thermoplastic
core made from a plurality of twisted thermoplastic strands, said
core having a coating of plastic material covering and integrated
therewith, a thermoplastic filament wrap extending spirally around
said coated core, the pitch of said wrap being such that
substantial spacing exists between adjacent convolutions, and a
further coating of plastic material covering and integrated with
said coated and wrapped core.
Inventors: |
Crandall; Julian T. (Washington
County, Ashaway, RI) |
Assignee: |
Ashaway Line & Twine Mfg.
Co. (Ashaway, RI)
|
Family
ID: |
22843742 |
Appl.
No.: |
05/225,151 |
Filed: |
February 7, 1972 |
Current U.S.
Class: |
57/234; 57/7 |
Current CPC
Class: |
D02G
3/38 (20130101); D02G 3/444 (20130101); A63B
51/02 (20130101); D02G 3/404 (20130101); D07B
1/02 (20130101); D07B 1/165 (20130101); D07B
1/162 (20130101); D07B 2201/2088 (20130101); D07B
2201/2087 (20130101); A63B 51/026 (20200801); D07B
2201/2083 (20130101) |
Current International
Class: |
D02G
3/40 (20060101); D02G 3/44 (20060101); A63B
51/00 (20060101); A63B 51/02 (20060101); D02G
3/22 (20060101); D07B 1/02 (20060101); D07B
1/00 (20060101); D02g 003/38 (); D02g 003/36 ();
D02g 003/22 () |
Field of
Search: |
;57/139,140,144,149,153,156,157,160,162,163,164 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Watkins; Donald E.
Claims
What is claimed is:
1. An integrated string having a core comprising a plurality of
groups of thermoplastic strands, each group comprising a plurality
of strands twisted together in a first direction, said groups being
twisted together to form said core, a coating of plastic material
covering and integrated with said core, a thermoplastic filament
wrap extending spirally around said coated core, the pitch of said
wrap being such that substantial spacing exists between adjacent
convolutions, and a further coating of plastic material covering
and integrated with said coated and wrapped core.
2. In the string of claim 1, a second thermoplastic filament wrap
extending around said further coating, said second wrap being of
substantially the same pitch as said first wrap, and an outer
coating of plastic material coverning and integrated with said
second wrap and said string.
3. In the string of claim 2, the pitch of said thermoplastic
filament wraps being substantially the same as the pitch at which
said groups are twisted together, but in the opposite hand.
4. In the string of claim 1, each of said groups having a coating
of plastic material thereover, said coated groups then being
twisted together to form said core.
5. In the string of claim 3, said strands being twisted together at
approximately three to seven turns per inch to form said groups,
said groups being twisted together at approximately 1.5 to 3.5
turns per inch to form said core.
6. The method of manufacturing an integrated string comprising the
following steps:
A. twisting a plurality of the thermoplastic strands together to
form a group,
B. twisting a plurality of said groups together to form a core,
C. dipping said core into a thermoplastic formulation.
D. passing said coated core through a drying chamber,
E. stretching the integrated string under heat;
F. repeating steps C and D supra,
G. spirally wrapping a thermoplastic strand around said coated
string with the adjacent convolutions of said strand substantially
spaced from each other,
H. repeating steps C, D and E; and
I. repeating steps C and D at a substantial time interval after the
completion of step H.
7. The method of claim 6 further characterized in that between
steps G and H steps C, D and G are again performed.
8. In the method of claim 7, all of said strands and said formation
being nylon.
9. In the method of claim 7, said thermoplastic strands being
wrapped in the opposite hand from the twist of said groups, but at
substantially the same pitch.
10. In the method of claim 9, said strands being twisted together
at approximately three to seven turns per inch to form said groups,
said groups being twisted together at approximately 1.5 to 3.5
turns per inch to form said core.
11. In the method of claim 6, said groups each being coated with a
plastic material prior to performing step B.
12. In the method of claim 6, the time interval between steps H and
I being approximately 24 hours.
13. In the method of claim 6, the string being elongated
approximately 15 percent during the performance of Step E.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
It has been found that a highly durable and effective string for
tennis rackets and the like may be formed by providing a core of
twisted thermoplastic strands, coating said core with a plastic
formulation, providing a wrap around said coated core with a
thermoplastic filament, and then providing another similar coating
over the wrap. Strings of this general construction have been
heretofore known in the art, reference being made, for example, to
applicant's prior U.S. Pat. Nos. 2,649,833 dated Aug. 24, 1953; No.
2,712,263 dated July 5, 1955; No. 2,735,258 dated Feb. 21, 1956;
and No. 2,861,417 dated Nov. 25, 1958. In all of these prior string
constructions, the wrap around the core of the string was either of
a closely braided sheath, or a spiral wrap with the adjacent
convolutions of the wrap in abutting engagement with each other.
Although strings of this type have generally proven satisfactory
for the intended purpose, it has been found that the braided sheath
or the closely wound spiral wrap acts as a shield during the final
coating operation of the string that impedes and resists complete
integration of the outer coating with the inner core. Furthermore,
the braided sheath or closely wound spiral wrap imparts more
rigidity to the string than is actually necessary, which excessive
rigidity makes stringing of a racket a rather difficult chore.
On the other hand, where the braided sheath or the spiral wrap is
eliminated altogether, the string is so limp and lacking in
rigidity as to likewise make stringing of a racket an unnecessarily
difficult chore. Also, of course, the complete elimination of the
braided sheath or the spiral wrap tends to materially weaken the
string.
It is therefore an object of the present invention to provide a
string of the character described having a core of twisted
thermoplastic filamentous strands, which core is then coated in a
plastic formulation, the coated core then having a thermoplastic
strand wrapped spirally therearound, but with adjacent convolutions
of the spiral substantially spaced from each other. The wrapped
core is then once again coated; and for best results, a second
similar wrap is applied before final coating of the completed
string.
It has been found that a string construction in accordance with the
present invention is possessed of sufficient rigidity and strength
as a result of the aforesaid spiral wrap, so as to facilitate
stringing of a racket, while at the same time not making the string
overly rigid. In other words, the spaced spiral wrap which
constitutes an important feature of the present invention has been
found to impart the desired degree of rigidity to the string so as
to facilitate and not impede subsequent stringing of a racket. In
addition, due to the spacing between adjacent convolutions of the
spiral wrap, more complete and uniform integration of the coated
string is achieved, since the spacing between adjacent convolutions
permits relatively free flow of the outer coating to the inner core
of the string.
Other objects, features and advantages of the invention shall
become apparent as the description thereof proceeds when considered
in connection with the accompanying illustrative drawings.
DESCRIPTION OF THE DRAWINGS
In the drawings which illustrate the best mode presently
contemplated in carrying out the present invention:
FIG. 1 is a fragmentary perspective view showing one of the groups
of twisted strands that form a part of the core of my improved
string;
FIG. 2 shows a plurality of the groups of FIG. 1 twisted together
to form the string core;
FIG. 3 shows the core of FIG. 2 with a plastic coating
therearound;
FIG. 4 shows the coated core of FIG. 3 after it has been
stretched;
FIG. 5 shows the stretched core of FIG. 4 with an additional
plastic coating formed therearound;
FIG. 6 shows the coated core of FIG. 5 with a filamentous strand
spirally wrapped therearound;
FIG. 7 shows the wrapped core of FIG. 6 with a further plastic
coating therearound;
FIG. 8 shows the string of FIG. 7 with a second spiral wrap
therearound;
FIG. 9 shows the string of FIG. 8 once again coated;
FIG. 10 shows the string of FIG. 9 after it has been heat
stretched; and
FIG. 11 shows the string of FIG. 10 with the final coating applied
thereto.
DESCRIPTION OF THE INVENTION
Referring now to the drawings, and more specifically to FIG. 1,
there is shown at 10 a group comprising a plurality of
thermoplastic filamentous strands 12 gently twisted together; and
specifically, the preferred twist being somewhere between 3 and 7
turns per inch. Any number of the strands 12 may be twisted
together to form the group 10, although, in practice, three strands
has been found to give good results. The strands 12 may be of any
suitable thermoplastic material, although nylon filaments of 840
denier have proven highly satisfactory.
Referring now to FIG. 2, a plurality of groups 10 have been twisted
together to form the core 14; and in this case, a total of three
such groups have been twisted, thus resulting in a total of nine
individual strands in the core; although, a greater or lesser
number of strands may be used in the formation of the core with
satisfactory results. Preferably the groups 10 are twisted together
at 1.5 to 3.5 turns per inch. The next step is to coat the core 14
with a plastic coating 16, as shown in FIG. 3, it being noted that
the coating 16 penetrates into and integrates with the core 14. For
purposes of illustration, in FIG. 3, and succeeding figures, the
core strands are shown as being separate and distinct from the
various coatings; but it will be understood that in actual
practice, said strands will substantially integrate with the
surrounding coating.
Although it has been found that many synthetic resins and solutions
may be used for the coating 16, and the subsequent coating steps to
be later described, it has been found that a nylon base solution of
the following formulation provides extremely satisfactory results:
A nylon solution, preferably comprising by weight approximaterly
14.4 percent nylon; 61.1 percent Methanol; 4.9 percent
tetrahydrofurfyl alcohol; and 19.6 percent water. The above
proportions are not critical but have been found to be desirable
for providing a nylon solution of suitable consistency.
It will be understood that in applying the plastic coating 16 to
the core 14, the core is passed through a tank containing the
aforesaid formulation, the tank temperature preferably being in the
range of 120.degree.F and then the coated string is passed through
a drying tower which is maintained at a temperature of
approximately 220.degree.F. It has been found that rather than
applying a single heavier layer, several thin layers are
preferable; and to this end core 14 is passed through the coating
tank and drying tower a total of 10 times; although, obviously, the
precise number of trips may be somewhat varied.
In order to better integrate the string 16 after the coating has
been applied, the coated string is heat stretched, whereby the
string is somewhat reduced in diameter and is more completely
integrated, as the core and coating are softened under the heat and
pressure to more effectively lock together. More specifically, the
heat stretched string, shown at 18 in FIG. 4, is preferably
stretched under approximately 30 pounds tension at a temperature of
approximately 450.degree.F. The percentage stretch of the string
under these conditions is approximately 15 percent. It will be
understood that the heat stretching of the string reduces the
elongation of the string so as to more closely match the elongation
of the filamentous strands 12.
The next step is to once again coat the string, using the same
procedure and formulation as heretofore described, and at this
stage of the operation the further coating 20 (FIG. 5) is formed
preferably by passing the string through the coating tank and the
drying tower twice.
Referring now to FIG. 6, it will be seen that a thermoplastic
filamentous strand 22, preferably 30-denier nylon, is spirally
wrapped around the coated string of FIG. 5, the pitch of said wrap
being substantially the same as the pitch at which the groups 10
are twisted together, i.e., approximately 1.3 to 3.5 turns per
inch, but in the opposite hand. This opposite wrapping of the
strand 22 tends to equalize the elongation of the core, so that
both the core and wrap will have approximately the same limit of
elongation, thus maximizing the strength of the integrated string.
It is important to note that the pitch of the strand 22 is such
that adjacent convolutions are substantially spaced from each
other, thus stiffening and strengthening the string to the desired
degree, without imparting undue rigidity to the string. Also, the
substantial spacing between adjacent convolutions of the strand 22
permit better integration of the string when subsequent coats are
applied, and in FIG. 7 it will be seen that a further coating 24 of
the aforesaid plastic formulation has been applied, and preferably
the coating 24 is achieved by passing the string through the
coating tank and drying tower six additional times.
For best results, although not absolutely essential to a successful
end product, the string of FIG. 7 is provided with a second spiral
wrap 26, in the same hand and pitch as that of the aforesaid wrap
24, although preferably staggered with respect thereto. The second
wrap also preferably comprises a 30-denier nylon filamentous
strand. After the wrap 26 has been applied, the string is passed
through the coating tank and drying tower six additional times to
form the further coating 28, after which the coated string is once
again stretched at a temperature of approximately 450.degree.F.,
but this time preferably at a tension of approximately 45 pounds,
to impart a further elongation of approximately 15 percent to the
string. The heat-stretched string 30, shown in FIG. 10, is then
given a final coating, as shown at 32 in FIG. 11. More
specifically, after the heat stretching operation, the string goes
through the coating tank and drying tower four times and then is
allowed to sit for approximately 24 hours at room temperature to
allow the solvents in the plastic formulation to evaporated. After
this time interval, the string is once again passed through the
coating tank and drying tower four times, whereupon cavities in the
string caused by evaporation of the solvents are filled in to
provide a substantially smooth and uninterrupted string
surface.
In some cases, it has been found desirable, although not absolutely
essential, to coat the groups 10 by passing them through the
aforesaid formulation prior to twisting the groups together to form
the core 14.
While there is shown and described herein certain specific
structure emobdying the invention, it will be manifest to those
skilled in the art that various modifications and rearrangements of
the parts may be made without departing from the spirit and scope
of the underlying inventive concept and that the same is not
limited to the particular forms herein shown and described except
insofar as indicated by the scope of the appended claims.
* * * * *