U.S. patent number 3,887,761 [Application Number 05/333,470] was granted by the patent office on 1975-06-03 for tape wrapped conductor.
This patent grant is currently assigned to W. L. Gore & Associates, Inc.. Invention is credited to Wilbert L. Gore.
United States Patent |
3,887,761 |
Gore |
June 3, 1975 |
Tape wrapped conductor
Abstract
This invention involves products, apparatus and processes for so
controlling the feed rate of a film being spirally wrapped around
an electrical conductor that the elongation of the film is
precisely controlled and consequently the thickness of the
overlapping layers of insulation is very uniform.
Inventors: |
Gore; Wilbert L. (Newark,
DE) |
Assignee: |
W. L. Gore & Associates,
Inc. (Newark, DE)
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Family
ID: |
26988746 |
Appl.
No.: |
05/333,470 |
Filed: |
February 20, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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666007 |
Sep 7, 1967 |
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Current U.S.
Class: |
174/110R; 156/53;
174/110PM; 174/110FC; 174/110N |
Current CPC
Class: |
H01B
13/085 (20130101) |
Current International
Class: |
H01B
13/06 (20060101); H01B 13/08 (20060101); H01b
007/02 () |
Field of
Search: |
;174/11R,11FC,11N,11PM,12SR,12R,121R ;156/53 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Birks, F. B., Modern Dielectric Materials, Hetwood & Co.,
London, 1960, pp. 117-119..
|
Primary Examiner: Goldberg; E. A.
Attorney, Agent or Firm: Uebler; Ernest A. Mortenson; C.
Walter
Parent Case Text
This is a division of application Ser. No. 666,007, filed Sept. 7,
1967, and now abandoned and is also copending with application Ser.
No. 73,769, filed Sept. 21, 1970 now U.S. Pat. No. 3,756,004 issued
Sept. 4, 1973, which is a continuation-in-part of application Ser.
No. 666,007 now abandoned.
Claims
I claim:
1. An insulated electrical conductor of continuous length having
exceptionally uniform outside diameter, said insulated conductor
comprising:
a. a conductor of continuous length;
b. a synthetic polymeric tape spirally wrapped about said
conductor;
c. said tape having the physical property that at strains above
about 5% the tape exhibits a small slope in its characteristic
stress-strain curve;
d. said tape having been constantly elongated during the wrapping
process; and
e. said tape having a substantially constant width along the length
of said conductor
f. said insulated conductor having a uniform outside diameter.
2. An insulated conductor in accordance with claim 1 in which said
polymeric tape is composed of a tetrafluoroethylene polymer.
3. An insulated conductor in accordance with claim 2 in which said
tape is polytetrafluoroethylene.
4. An insulated conductor in accordance with claim 2 in which said
tape is a copolymer of tetrafluoroethylene and
hexafluoropropylene.
5. An insulated conductor in accordance with claim 2 in which said
tape is a copolymer of tetrafluoroethylene and ethylene.
6. An insulated conductor in accordance with claim 1 in which said
tape is poly(chlorotrifluoroethylene).
7. An insulated conductor in accordance with claim 1 in which said
tape is poly(ethyleneterephthalate).
8. An insulated electrical conductor in accordance with claim 1 in
which said tape is a polyester.
9. An insulated conductor in accordance with claim 1 in which said
tape is a polyimide.
10. An insulated electrical conductor in accordance with claim 1 in
which said tape is polyvinylfluoride.
Description
Electrical conductors are usually insulated with dielectric
material by extrusion processes where the dielectric material is a
thermoplastic which can be melted and then formed around the
conductor. In these extrusion processes it is difficult to form the
dielectric around the conductor so that dimensions are perfectly
constant. The outside diameter of the insulation is likely to be
variable and the conductor not perfectly centered in the
insulation. Also, there are a number of materials having very
desirable properties which are not thermoplastic and cannot be
extruded to form electrical insulation around the conductors. Yet a
number of non-extrudable materials having very desirable properties
for electrical insulation are available in thin films. These
materials are used by wrapping them around a metal conductor. In
this process the film material is slit to a narrow width and then
wrapped spirally around the conductor with overlap in the wrapping
so that two or more layers of the film cover the insulated wire.
The overlapping layers are usually sealed together by a subsequent
heat treatment or other procedure to bind the overlapping film
together and make the covering moisture-proof.
In the conventional tape-wrapping process, great precision is
required in correlating the rate of movement of the conductor
relative to the rotational rate of the wrapping process. Also, a
great deal of attention has to be given to providing uniform
back-tension on the tape as it is wrapped around the conductor with
overlapping layers of the tape. It is necessary that the tape be
stretched to draw down snugly against the conductor as well as pass
around the larger diameter of the overlap. The constant
back-tension on the tape is designed to stretch the tape so that it
pulls down snugly against the conductor but not to stretch it so
much that thin spots are produced in the tapes so that an
inadequate insulation thickness is provided.
In commonly used automatic equipment for interconnecting electric
wire for computers and the like and making connections at the
terminals, a high degree of uniformity is necessary in the outside
diameter of the insulated conductors. This is necessary because the
clamps, strippers, and other tools used in the automatic
termination equipment require a high degree of precision in the
clamping force and in the location of the conductor being attached
to the terminal points. Variation in diameters and non-centering of
conductors cause costly loss of time and materials in these
operations.
Thus, an objective of this invention is the provision of apparatus
and methods which overcome the short-comings of the prior art.
Another aim is the provision of apparatus and methods for providing
uniform elongation of the tape during the wrapping process. A
further goal is the provision of means for controlling the
elongation of the tape. A still further object is the provision of
a device which allows the control of the elongation of the tape
independently of the back tension and which allows the production
of insulated wire products that have uniform outside diameters.
These and other aims will appear hereinafter.
The objectives of this invention are accomplished by uniformly
elongating the tape during the wrapping process using the apparatus
of this invention. The products are tape-wrapped conductors having
a new kind of uniformity in their outside diameters not possessed
hithertofore by tape wrapped conductors.
One reason that the prior art products are non-uniform in diameters
stems from the fact that in most tape wrapping devices the spool of
tape is rotated around the conductor and the back-tension is
provided by a rotating clutch mechanism which operates on the spool
holding the tape. As the tape is used the spool diameter
diminishes, and the torque arm becomes shorter, giving an increased
tension during the use of a roll of tape. Periodic adjustments are
made on the clutch tension by the operator of these tape wrappers,
but these tension adjustments are made in steps and not in a
continuous fashion. Also, in spite of the great attention that has
been given to the refinement of the design of the tape payout
clutch, variability in the tension is to some degree inherent in
all of them.
It would seem then that the way to better diameter control is by
perfecting a clutch mechanism to provide more uniform tension.
However, in the usual tape-wrapping device, the clutch spins with
the wrapping head and is inaccessible when the machine is running.
In order to make the clutch accessible while the machine was
running, a machine was developed in which the wire is not only fed
axially but is spun or rotated at the same time. This spinning wire
tape wrapper affords the examination of the clutch while the
machine is running and affords the use of more sophisticated clutch
designs than would be possible with the usual tape-wrapping clutch
machine. However, even when an improved clutch is operating well
and providing uniform back tension, the outside diameter of the
wire varies more than can be accounted for by the variations in
tape width and thickness. It then occurred to me that I might avoid
objectionable diameter variation by controlling the feed rate of
the tape to the wire independently of the tension of the tape. This
was accomplished by feeding the tape through pinched feed rolls,
which are mechanically linked to the wire feed mechanism.
Surprisingly, wire produced with this device is more uniform in
diameter than wire produced with even the most sophisticated clutch
mechanisms that were tried.
This invention will be further understood by reference to the
description, examples and drawings below which are not limitative
but are given for illustrative purposes only and the drawings of
which are as follows:
FIG. 1 is a side view of the apparatus of this invention;
FIG. 1A is a schematic diagram showing one embodiment of this
invention relating to rotating the tape about a non-rotating
core.
FIG. 2 is taken on line 2--2 of FIG. 1;
FIG. 3 is taken on line 3--3 of FIG. 1;
FIG. 4 is an enlarged view of the tape feeding to the advancing
conductor, and a slotted tube support through which the tape passes
just prior to meeting the conductor, and showing certain
mathematical symbols used below, FIG. 4A being a cross-sectional
view of said support.
FIG. 5 is a stress/strain curve showing an area of strain suitable
for tape-wrapping conductors and defining certain terms used in
mathematical considerations given below;
FIG. 6 shows tensile/elongation curves for certain films or tapes;
and
FIG. 7 shows similar curves for two films or tapes widely used in
tape-wrapping conductors.
Shown in FIG. 1 is a tape-wrapping device in which the wire 101 is
rotated and the tape 34 is fed to the spinning wire from a payout
35. The wire is taken off spool 10 over pullies 11, 12 and 13 which
are supported by any suitable conventional means (not shown for
convenience) so that they rotate with tube 14, and strung through
the hollow tube 14 through capstan 15 and through another set of
pullies, 16, 17 and 18 which are supported by any suitable
conventional means (not shown for convenience) so that they rotate
with tube 26, to the take-up spool 19. Tension is maintained on the
wire by a magnetic brake 20 at the wire payout end and also a
similar brake 21 at the wire take-up end. The tension between the
two spools 10 and 19 is so balanced by these magnetic brakes that
the wire capstan 15 controls the advancement of the conductor
through the device, as described below.
The rotating unit is made up of two sections 22 and 23, one (22)
for payout of bare wire and the other (23) for take-up of insulated
wire. These are rotated simultaneously from a single drive shaft
24. Spool 10 is mounted on and is free to rotate with respect to
tube 14 which is driven by jack shaft 24 which is in turn driven by
motor 25. Shaft 24 also drives hollow tube 26 in section 23 through
chain drives 24A. Spool 19 is mounted and rotates on tube 26. Thus,
spools 10 and 19 are being rotated on their respective shafts by
the advancing wire and each is subjected to respective brakes 20
and 21 in a controlled manner.
A power take-off from spinning tube 14 drives a set of pinch
rollers 27 through which the tape 34 is fed. The propulsion speed
of the pinch rollers can be adjusted by adjustable speed drive 28
so that the rate of movement of the tape relative to that of the
wire is perfectly controlled. Since the wire capstan drive 15 and
the pinch rollers 27 are driven from the same drive shaft 24, the
rates of wire advance per revolution, and of tape feed per
revolution are integrated so that they are independent of
rotational rate of the apparatus. Therefore, the elongation of the
tape 34 between the pinch rollers 27 and the wire is held at a
uniform level established through the adjustable speed drive 28.
For a given conductor to be produced, the speed of drive 28 is set
to produce the tape elongation needed to get the desired thickness.
Adjustments needed to get the desired rate of movement of the given
conductor per revolution are made through change gears 29 as
described below with reference to FIG. 2. With the rate of wire
advance, rate of wire revolution, and rate of tape feed per
revolution fixed and in balance, the tape is uniformly elongated
throughout the given run.
The power take-off for the tape feeding comprises timing belt and
pulley assembly 30, shown in FIG. 1 and this is connected to
adjustable drive 28 which in turn is connected to and activates
timing belt pulley assembly 31. Assembly 31 rotates the drive or
pinch rolls assembly 27 which is shown in FIG. 3 as rollers 32 and
33. The insulating tape 34 is pulled off spool 35. As can be seen
in FIGS. 1 and 4 tape 35 has a smaller width and/or thickness at
the exit end of roll assembly 27 than it does at the entrance end.
This reduction in cross-sectional area is due to the stretching of
the tape. Payout spool 35 is freely rotatable and no substantial
tension is applied on tape 34 as it travels to the pinch roller
assembly 27.
Shown in FIGS. 4 and 4A but for convenience omitted in FIG. 1 is a
stationary guide or support 36 which is fixedly mounted on mount
37. This support comprises a concave element 38 which has in it
longitudinal slot 39 through which the elongated tape 40 coming
from assembly 27 passes and is then wrapped around the rotating and
longitudinally moving wire 101. The support 36 is located so that
wire 101 rides on its inner bottom surface and any pulling of the
wire 101 toward the pinch roll assembly 27 due to the tension on
the tape is offset, the positioning being shown in FIG. 4A.
In order to advance the conductor 101 through the device, capstan
assembly 15 is provided, as mentioned above and as shown in FIG. 1
and, in more detail, in FIG. 2. Assembly 15 rotates with tube 26 in
section 23 as shown or with tube 14 in section 22. At the end of
tube 26 nearest tube 14 is a slot 41 (FIG. 1) which provides access
to capstan members 42 and 43. Auxiliary rollers or snubbers 44 and
45 are also contained within tube 26. Rollers 42 and 43 are driven
by gear chain 29 and rotate in the direction shown by the arrows in
FIG. 2 to advance conductor 101 to the left. In order to impart
rotation to the initial gear 46 in chain 29, stationary gear 47 is
provided. This is fixed by mounting 48 and it has a hole in it
through which tube 26 is positioned and is rotating. As tube 26
rotates, it carries gear chain 29 and hence gear 46 with it. Since
the teeth of gear 46 are in meshing contact with the teeth of fixed
gear 47, gear 46 rotates as long as it circles around fixed gear
47. Conductor 101 cannot slip on rollers 42 and 43. The forward
pull of take-up spool 19 is such that the frictional contact of
rolls 42 and 43 with the wire 101 is sufficient to give a positive
movement to the wire when the rollers are rotated. Hence, wire 101
is moved through tube 26 to take-up spool 19.
Conductor 101 is allowed to come off spool 10 at the take-off end
without any traversing mechanism. However, at the take-up end,
spool 19 is equipped to traverse. It and its controlling brake 21
traverse as a unit by a traverse level wind unit, not shown, for
convenience. Thus, the insulated conductor is laid down smoothly on
spool 19 without piling up.
In the spinning wire device just described there is considerable
operational advantage in maintaining the tape payout mechanism in a
stationary position. However, the principle of the elongation
control can be adapted to a device in which the tape roll is
rotated around a conductor which moves along its axis but does not
rotate. The apparatus, process and the products of this alternate
embodiment are also within my invention.
EXAMPLE I
This experiment demonstrates the constant tension process and
products therefrom are described.
A magnetic brake was coupled to the pinch feed rolls of the
spinning wire tape-wrapper in place of the power take-off 30 and
adjustable drive 28. An AWG 30 wire 0.010 inch in diameter was
placed on the payout spool 10 and strung through the wirefeed
capstan 15 and onto the take-up spool 19. A MYLAR biaxially
oriented polyethyleneterephthalate film 0.001 inch thick (coated
with adhesive to a total nominal thickness of 0.0015 inch) slit to
7/32 inch width, was placed on the tape payout 35, passed through
the pinch rolls 27 and attached around the wire (MYLAR being a
trademark of E. I. duPont de Nemours & Co., Inc.). The wire was
then rotated with the capstan adjusted so that the movement per
revolution of the wire along its axis gave a 75 percent overlap,
thus giving four layers of material over the wire. The magnetic
brake coupled to the pinch rollers 27 was adjusted to exactly 3.0
lbs. tension on the tape. This was just sufficient to draw it down
snugly against the bare wire 101 the section section at the point
of application where there was no overlap. About 1,000 feet of wire
was insulated with the four overlaps of film and the tension on the
tapes through the pinch rolls again checked. It was still exactly
3.0 lbs. The outside diameter of the wire was carefully measured at
intervals of approximately 20 feet along the length. The frequency
distribution of these measurements is given in column A of Table I
below. The product does not have the uniformity of the products of
this invention.
EXAMPLE II
This example demonstrates the exceptional results obtained by this
invention.
The magnetic brake used in Example I was removed from the pinch
rolls, and the power take-off 30 and adjustable speed drive 28 were
attached as shown in FIG. 1 so that the rotation of the pinch rolls
27 was coupled to or integrated with the rotation of the wire. The
same spool of wire and roll of tape used in Example I were left on
the tape-wrapping machine. The wire was again rotated and the
adjustable drive 28 was adjusted so that the elongation of the tape
was uniform and just sufficient to draw the tape snugly against the
bare wire at the point of application. Approximately 1,000 feet of
wire were insulated with four overlaps of the tape. The outside
diameter of the wire was carefully measured at intervals of
approximately 20 feet along the length of the wire. The frequency
distribution of these measurements is shown in column B of Table
I.
TABLE I ______________________________________ FREQUENCY
DISTRIBUTIONS ______________________________________ AWG 30 Wire
(conductor O.D. = 0.010") Example I Example II Column A Column B
Tension Control Elongation Control
______________________________________ N N .0230 - .0234 in. 1 0
.0225 - .0229 in. 4 0 .0220 - .0224 in. 10 2 .0215 - .0219 in. 12
10 .0210 - .0214 in. 11 27 .0205 - .0209 in. 13 7 .0200 - .0204 in.
7 1 .0195 - .0199 in. 5 0 .0190 - .0194 in. 2 0 65 47 x = .0212" x
= .0215 " s.d. = .00093" s.d. = .00039" ##EQU1##
The standard deviation (s.d.) of outside diameter measurements for
the wire produced in this example by the constant elongation
process of this invention was 0.00039 inch. This is much lower than
the 0.00093 inch standard deviation of the measurements made on the
outside diameter of wire produced by the constant tension process
of Example I. The "F" ratio of the two variances is 5.7, greatly
exceeding the 2.0 F ratio expected at a probability of 0.01. The
insulated conductor produced by this invention is far superior in
uniformity of diameter.
EXAMPLE III
This example illustrates the results which are obtained when using
the conventional controlled tension process which does not employ a
rotating wire.
The same spool of wire used in the two previous examples and a roll
of coated MYLAR tape slit to the same width from the same large
roll were then placed on a conventional tape-wrapper where the tape
spool is rotated around the wire. About 1,000 feet of insulated
wire was produced, making an initial adjustment on the tape-feed
clutch to bring the tape snugly against the bare wire, and then
running the machine smoothly at a uniform rate with no further
adjustments. A tensionmeter reading of the back-tension of the tape
gave a value of 3.1 lbs. at the beginning of the run and 3.3 lbs.
at the end of the run.
Fifty-two outside diameter readings made about every 20 feet along
the length of wire gave an average diameter of 0.0199 inch with a
standard deviation of .00121. A series of thickness measurements
were made on the residual tape on the roll by stacking 10 layers of
the tape and measuring the thickness of the stack. These varied
from 0.0152 to 0.0156 inch. Measurements made of a single thickness
of tape gave a range from 0.0014 to 0.0017 inch.
An analysis indicates that the diameter variations are
substantially greater than can be accounted for by the variations
in tape thickness.
EXAMPLE IV
This example compares the elongation effects on the insulating tape
by constant-tension wrapping versus the constant-elongation
wrapping by this invention.
A length of coated MYLAR 0.0015 inch thick, 7/32 inch wide was
marked on one edge with an ink dot every 0.1 inch along the length.
Part of this material was wrapped on AWG 30 wire using the
constant-tension wrapper as in Example I and part was wrapped on
the constant elongation tape-wrapper of this invention as in
Example II, the wire being rotated in each instance. The spacing of
the dots was then measured on the resultant wrapped wires.
This spacing was very uniform at about 0.115 inch on the insulated
wire produced by the elongation-control device of this invention.
However, the spacing of the dots varied from about 0.105 inch at
thick areas of the insulated wire from the constant tension
tape-wrapper to about 0.130 inch at thin areas of the insulated
wire. The tape was unwrapped from a thin area of the constant
tension product where the outside diameter was down to 0.0195 inch
and also from a thick area of the product where the outside
diameter was 0.0225 inch. The width of the tape was 0.155 inch
where it was removed from the thin section and the width was 0.210
inch where it was removed from the thick section, a very wide
variance. Tape was also removed from the constant elongation
tape-wrapped product of this invention, and measurements made at
various sections along this tape. At all points the width was
uniform between 0.190 inch and 0.195 inch.
Therefore, the control of elongation of the MYLAR tape during
tape-wrapping maintains a constant tape width so that exactly four
wraps (and 75 percent overlap) can be held, whereas the constant
tension process produces variable stretch of the tape, causing it
to be narrow in some spots, wide in others with consequent
variation in the overlap and in the outside diameter of the
insulated wire. Uniform width of tape, it appears, can be
maintained with MYLAR only when the elongation of the tape is held
constant.
With the results of the above examples in mind and upon reflection,
it can be seen that the stress-strain characteristics of films
impose a limit on the uniformity of insulated wire that can be
produced with a controlled tension tape feed. The difficulty is
that the stress-strain relationship of some films rises rather
quickly to a maximum strain as the material is stretched and then
further strain produces little or no increase in tension. Put
another way, beyond the yield point the elongation of many films is
extremely sensitive to variations in stress. During wrapping, not
all variations in the elongation are caused by variations in clutch
tension. Small variations in the width and thickness of tape also
cause variations in the amount of elongation. In addition, the
stress-strain characteristics of the film change along the length
of the tape due to changes in temperature, balance of orientation,
etc., so that even a tape that is perfectly uniform in width, and
perfectly uniform in thickness, and is wrapped with perfectly
uniform tension, may produce a wire with substantial variations in
diameter. Because real films are not perfect, they are subject to
all of these variations. The controlled elongation device of this
invention minimizes, or completely eliminates, the effect of these
variables on the diameter of the insulated wire.
It turns out that controlled tension wrapping and controlled
elongation wrapping can be compared by the well-known mathematical
method for analysis of small errors. The analysis determines the
variation of wire diameter that is caused by small changes in the
cross-sectional area of the tape, A.sub.O, by small changes in the
strength of the tape, which is characterized by b in the
stress-strain relationship, and by small changes in the ratio of
stress and strain, which is characterized by "K", the slope of
stress-strain relationship. The general mathematical statement of
the variation of wire diameter, D, with respect to small changes of
the variables A.sub.o, b, and K is: ##EQU2##
Nomenclature is listed below. These symbols are further defined in
FIGS. 4 and 5.
A = cross-sectional area of tape (length.sup.2)
b = intercept on stress axis of linear approximation of
stress-strain relationship (force/unit area)
D = diameter of insulated wire (length)
d = diameter of conductor (length) or differential operator
depending on context
F = tension on tape during wrapping (force)
K = slope of stress-strain curve (force/unit area)
t = thickness of tape (length)
w = width of tape (length)
s = speed (length/unit time)
.rho. = density of insulating material (mass/unit volume)
.delta. = strain (length stretched/original length of tape)
.theta. = wrap angle of tape
subscripts:
o = condition when .delta. = 0 (original unstretched condition)
.delta. = condition when = .delta.=.delta.
T = refers to tape
c = refers to the conductor
Refer to FIG. 4 for the following development. Under steady
conditions, the mass of insulation passing station 300 per unit
time must equal the mass of insulation passing station 100 per unit
time, i.e., the mass of insulation on the wire must be equal to the
mass of tape applied from station 200 ##EQU3## or by rearranging,
##EQU4##
Also, from the definition of strain, .delta., we have the following
relationship ##EQU5## and from geometry it can be seen that
##EQU6##
If equations (3) and (4) are combined to eliminate
S.sub.T.sub..delta., the ratio S.sub.T.sub..delta. /S.sub.To can be
solved for and substituted into equation (2). The resulting
equation is ##EQU7##
Equation (5) can be used for calculations involving strain since
.delta. appears explicitly. For calculations involving tension, F,
equation (5) can be made explicit in F from the stress-strain
relation of the tape.
The general linearized form of the stress-strain relationship is
(see FIG. 5) ##EQU8##
If one solves for .delta. in equation (6) and substitutes the
results into equation (5), one obtains the following ##EQU9##
Usually .rho..sub..delta.=.rho..sub.o , i.e., straining the tape
does not appreciably affect its density, and equation (7) then
becomes ##EQU10##
The partial derivatives in equation (1) can be evaluated from
equation (8) for the case of constant elongation, .delta.=constant
##EQU11##
Similarly, the derivatives in equation (1) can be evaluated from
equation (8) for the case of constant tension, F = constant
##EQU12##
The stress-strain relationships for several commercially available
films are plotted in FIG. 6. These plots provide the necessary
numerical data for Table II. The values for all films are evaluated
in the area of .delta.=0.1, which is generally found to be the
minimum amount of stretch that is needed for wrapping small
conductors. Each of the variables (A.sub.o, b, K) will be examined
separately for a specific case in the following numerical examples
to determine their effect on the diameter.
TABLE II
__________________________________________________________________________
Material* K(psi) b(psi) F/KA.sub.o
__________________________________________________________________________
(1) "MYLAR" (DuPont polyester) 13,500 13,700 1.115 (2)
"CELANAR"](Celanese polyester) 7,420 14,000 1.95 (3) ICI polyester
("MELINEX") 21,000 14,000 0.728 (4) "TEDLAR" (DuPont polyvinyl-
diene fluoride) 63,000 10,000 0.200 (5) Polysulfone (Union Carbide)
0 7,200 .infin. (6) Unsintered polytetrafluoro- ethylene 7,500 0
0.100 (7) FEP "TEFLON" (DuPont) 1.5 1,900 1.330 (8) "KAPTON"
polyimide (DuPont) 32,400 14,000 0.525
__________________________________________________________________________
*The quoted items are believed to be trademarked as indicated by
the respective companies.
EXAMPLE V
Variations of Diameter Caused by Variations in Tape Thickness: A
Comparison of Controlled Tension vs. Controlled Elongation Wrapping
for MYLAR
Under consideration is a construction for which d = 0.010 inch, D =
0.0195 inch, and .theta. = 15.degree.. This construction is
currently being supplied for use in an automatic terminating
device. The tape used is 7/32 inch wide and is 0.00157 inch thick
(about 0.001 inch MYLAR plus 0.00057 inch adhesive).
In determining the effect on D of small variations of tape
thickness or cross sectional area, the incremental variation of
area, .DELTA.A., is substituted for the infinitesimal variation,
dA., which appears in equation (1). Similar substitutions are made
in examples VI and VII for the infinitesimal variations "db" and
"dk."
Consider a variation in the thickness of the MYLAR of .+-.0.0001
inch. The total variation of tape thickness is .DELTA.t.=0.0002
inch, and the variation of the cross-sectional area of the tape is
.DELTA.A.=(7/32).times.(0.0002)= 4.38 .times. 10.sup..sup.-5
in.sup.2.
From Table II the following values are obtained for MYLAR:
##EQU13##
By substituting these values into equation (12), we obtain the
value for the partial differential in the first term of equation
(1), (.differential.D/.differential.A.sub.o).sub.F.sub.=constant =
62.0. The variation in diameter that results from controlled
tension wrapping then is found to be ##EQU14##
Similarly from equation (9) one obtains the variation in diameter
that results from controlled elongation wrapping: ##EQU15##
Thus, the variation in diameter of the wire wrapped with controlled
elongation is only about half as great as with controlled tension.
It should be noted that MYLAR is not the most sensitive film to
variations of this sort. For example, both the FEP-"TEFLON" and
"CELANAR" polyester show greater differences.
EXAMPLE VI
Variations of Diameter Caused by Variations in Film Strength: A
Comparison of Controlled Tension vs. Controlled Elongation Wrapping
for MYLAR
We will now consider how changes in film strength affect the
diameter. The same wire construction is considered as in the
previous example: i.e. d = 0.010 inch, D = 0.0195 inch, .theta. =
15.degree., t.sub.o = 0.00157 inch, w.sub.o = 7/32 inch, F/KA.sub.o
= 1.115, b/k = 1015.
Consider a 10% change in "b" (which corresponds, for example, to
about a 10.degree.C. change in film temperature for MYLAR).
.DELTA.b = 0.1 (13,700) = 1,370
By substituting these values in equation (13), to evaluate the
second term of equation (1), one obtains the variation in diameter
for controlled tension wrapping, which is ##EQU16##
For controlled elongation wrapping there is no effect on the
diameter as can be seen from equation (10).
.DELTA.D.sub.(.sub..delta. .sub.= constant) = 0
EXAMPLE VII
Variations of Diameter Caused by Variations in Degree and Balance
of Film Orientation: A Comparison of Controlled Tension vs.
Controlled Elongation Wrapping for MYLAR
Many filsm, such as MYLAR, owe their good physical properties to
biaxial orientation of the film, which is accomplished by
stretching the film both transversely and longitudinally during
manufacture of the film. The slope, K, of the stress-strain curve
is greatly affected by the balance of transverse and longitudinal
properties established by this process.
Apparently, the biaxial orientation of film is not an easy process
for the manufacturer to control. For example, some published
stress-strain curves for MYLAR (DuPont Bulletin M-2C) show that K
may become zero or even negative in the region 0.07 < .delta.
< 0.18. Under these conditions such large variations in diameter
occur with controlled tension wrapping that the method used to
analyze small errors no longer applies.
Considering here a variation in K of only .+-.10%, then .DELTA.K =
0.2 (13,500) = 2,700 psi. By substituting these values into
equation (14), the third term of equation (1) then gives the
variation in diameter for controlled tension wrapping, which is
##EQU17##
For controlled elongation wrapping there is no effect on the
diameter as can be seen from equation (11):
.DELTA.D .sub.(.sub..delta..sub.=constant) = 0
While this invention has been described with reference to certain
films, one skilled in the art will easily recognize that the
principles of this invention are applicable to many more polymeric
films that exhibit small slopes in their stress-strain curves at
strains near about 10% and preferably in the area of strain from
about 5% to about 50%. Those films or tapes mentioned herein are
representative of such tapes and of tapes that are presently in
commercial use. The polymeric tapes may be made of such polymers as
polytetrafluoroethylene or copolymers of tetrafluoroethylene with
other unsaturates as ethylene and fluorinated propylenes such as
hexafluoropropylene as well as poly(chlorotrifluoroethylene),
poly(ethyleneterephthalate) and other polyesters, various
polyimides, polyvinyl fluoride, among others. Also, one skilled in
the art will appreciate that the tapes from such polymers may be of
different thicknesses than those given in the examples. Generally,
tapes more than 0.005 inch thick are not used in spirally wrapping
small conductors though they may be if desired and this invention
is applicable to such thicker tapes. The unwrapped conductors used
are uniform in diameter along their lengths generally varying only
about .+-. 0.0002 inch. Conductors may be employed that have larger
or smaller diameters than the 0.010 inch wire used in the
examples.
Further, one skilled in the art may wish to employ mechanical
equivalents for the various mechanical elements in the apparatus of
this invention, and this may be done without departing from the
principles of this invention. For example, instead of driving the
means for advancing the conductor, the means for advancing the tape
and the means for effecting relative rotation from the same source,
one could drive these separately using synchronous motors and by
making the adjustments described herein one could effect the means
for controlling the elongation to elongate it uniformly as it is
being passed to the advancing conductor and as it is being spirally
wound on said conductor. One may use other braking means besides
the magnetic brakes used on the wire payout and take-up, and if
desired the amount of braking may be changed during a given run
though normally this is not necessary. While a driven capstan or
other equivalents can be used for advancing the tape, generally the
pinch roller assembly is used. In the embodiment of this invention
in which the wire is rotated any means that applies the required
friction as an advancing force to said conductor, such as the
described capstan assembly, may be used. In the given capstan
assembly the rollers are usually grooved though they need not be.
Thus, to obtain the spirally wrapped products of this invention,
one selects a tape which can be stretched and with an apparatus of
this invention and while applying the principles of this invention,
he elongates the tape uniformly at a chosen value within the
preferred area of strain as he effects the spirally wrapping on the
chosen conductor to produce the desired number of overlaps. The
uniform elongation effected as the spiral wrapping is being
produced leads to insulated conductors having exceptionally uniform
outside diameters.
While the invention has been disclosed herein in connection with
certain embodiments and certain structural and procedural details,
it is clear that changes, modifications or equivalents can be used
by those skilled in the art; accordingly, such changes within the
principles of the invention are intended to be included within the
scope of the claims below.
* * * * *