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3,061,667 TELEGRAPH
POLE LINE INSULATOR
William F. Markley, Ridgewood, and James L. Slater, Glen Rock, N.J., assignors to The Western Union Telegraph Company, New York, N.Y., a corporation of New York Filed
Feb.10, 1960, Ser. No.13,676 9
Claims. Cl. 174-211) This
invention relates generally to electrical insulators, and more
particularly to pole line insulators for use with high frequency open
wire telegraph and telephone carrier service. The invention herein
represents an improvement over insulators of the character disclosed and
claimed in U.S. patents to Smith and Wheeler, No. 2,218,497, issued
October 15, 1940, and No. 2,304,483, issued December 8, 1942. The signaling currents employed in communication circuits, and
particularly high frequency carrier circuits, are small in amplitude,
and a considerable portion of each signal pulse transmitted is required
for operating the receiving apparatus, such as relays in telegraph and
telephone circuits. Repeaters
are employed at convenient locations, but in certain instances the
repeater points are necessarily spaced considerable distances apart and
the signals reaching the receiving apparatus may become considerably
attenuated. Since the
margin of operation on communication circuits under favorable conditions
is relatively small, any substantial loss of signaling current due to
line leakage or other causes seriously interferes with the normal
operation of the system. Moreover,
communication circuits frequently are disposed along highways and
railroad right of ways in connection with other signaling and power
lines, and are subject to interference therefrom, and a considerable
portion of the signaling current is required to override the transient
or interfering currents set up therein because of such adjacent
signaling and power lines, and this further reduces the operating margin
of the communication signals. In insulators of the character disclosed herein there are two
electrical resistance effects that have a bearing on the electrical
performance of the circuit. One
of these effects is determined from the dissipation factor of the
insulator material itself and is substantially constant in all kinds of
weather. The other effect on the attenuation of the circuit,
particularly in the higher frequency ranges, is caused by films which
form on the surfaces of the insulators, due to dust, soluble salts, fine
metallic particles, acid-forming films of various concentrations,
sulphur fumes, and oily films and the like.
The resistance of such films is, in effect, in series with the
capacitance of the insulator, and the effect of these films on
attenuation is limited by the capacitance of the insulator; with low
capacitance the leakage current through the films is reduced with
corresponding reduction in line attenuation losses. Thus, the films tend to have a marked effect on the
attenuation of the circuit so that insulators of prior types would
exhibit a wide change in resistance values with varying weather
conditions, and particularly between dry and wet conditions. These
effects ordinarily required continual checking and adjustment of
amplifier equipment in the circuits and sometimes required much closer
spacing of amplifiers in order to maintain adequate transmission
efficiency under adverse weather conditions.
Since the deleterious effects of the resistance films is limited
by the capacitance of the insulator, the lower the capacitance the
smaller will be the effect of the resistance films and hence result in
smaller variations respectively in periods of dry and wet weather
conditions. Pole line insulators form ideal targets for malicious persons who
throw stones and missiles or shoot at them, and hence it is important
that an insulator have non-breakable
characteristics since if an insulator is broken it enables dirt or other
foreign matter carried by the elements to collect in the cracks or
broken portion and this forms a conducting leakage path over the surface
of the insulator, particularly in rainy or humid weather, and if a
portion of the skirt or petticoat of the insulator is broken away this
reduces the length of the leakage path of the insulator with the result
that the insulator loses its desired insulation value.
Moreover, when insulators are cracked or broken, there is a
tendency for them to be pulled or to fly off the pins on which they are
supported, and the line wire carried by the loose insulator may ground
on the cross-arm or swing into an adjacent wire, thus either grounding
or short-circuiting the communication circuit or causing the same to be
crossed with an adjacent circuit. A further important consideration is to cause the surface of the
insulator to have non-wetting characteristics thereby to prevent surface
leakage during periods of rain, fog and high humidity. Insulators in accordance with the foregoing patents were
substantially composed of a soft vulcanized rubber compound including a
waxlike substance compounded therewith to cause the surface of the
insulators to exhibit high interfacial tension with water to produce the
desired non-wetting surface characteristics, and operated satisfactorily
in service where the carrier frequencies were in general below 30
kilocycles. More recently,
with the increased demand for communication facilities, equipment has
been developed to provide a marked increase in the operating efficiency
of open wire carrier circuits at frequencies up to and exceeding 100
kilocycles. In this higher frequency range, the electrical
characteristics of the pole line insulators play a highly important part
in maintaining circuit efficiency, and the electrical characteristics of
the insulators of the foregoing patents proved to be inadequate in this
higher range. Insulators are also subject to other effects; for example, in hot
humid regions there is a tendency for molds or other fungus to develop
on the exposed surfaces of the insulators and this appreciably reduces
the electrical insulating properties of the insulators.
Preferably, the composition of the instant insulator includes a
suitable fungicide to obviate this difficulty. An object of the instant invention is to provide an insulator of
the character disclosed, which exhibits improved elasticity and
resiliency, particularly to the wire groove portion, to absorb sustained
vibration and to prevent the formation of bends of abrasions in the line
wire ordinarily caused by the stress imposed thereon by the tie wire and
by swinging of the wire spans in service, and thus obviate the
likelihood of breakage of the wire due to vibration and to such bends or
abrasions. A further object of the invention is to provide a non-breakable
pole line insulator which has higher non-wetting surface
characteristics, and which causes the line to have extremely low
attenuation losses at carrier frequencies up to and exceeding 100
kilocycles. A further object is an insulator of the character disclosed and
including a compound that inhibits the growth of fungus on the exposed
surfaces of the insulator. Other objects and advantages will appear from the following
detailed description taken in connection with the accompanying drawings
in which: FIG. 1 is a view in elevation of a communication type insulator
compounded in accordance with the instant invention; FIG. 2 is a longitudinal section of FIG. 1 showing the manner in
which the insulator may be mounted on a metal supporting pin; FIG. .3 shows a form of the insulator in accordance with
the invention, adapted to be mounted on a wooden cob or wooden pin; FIG. 4 shows a form of insulator in accordance with the
invention, adapted for mounting on a line wire spacer bracket or a
transposition bracket; and FIG. 5 is a longitudinal section of FIG. 4. Referring to FIGS. 1 and 2, there is shown a communication type
insulator compounded in accordance with the instant invention, the
insulator having a crown portion 10, a wire groove portion 11 for
receiving the line wire w and tie wire t, a reentrant skirt or petticoat
portion 12, and as seen in FIG. 2 a pin hole 13 for receiving the
threaded portion 14a of a metal insulator supporting pin 14 which is
secured, by a reduced portion 14b thereof, to the crossarm of a pole
line in known manner. As shown in FIG. 3, the insulator may be made so that its pin
hole is sufficiently large to accommodate the conventional wooden pin or
cob 16, so that it is unnecessary to change the pins on an existing
circuit in order to replace the insulator therein with insulators in
accordance with the present invention.
Due to the resiliency and elasticity of the insulator the
threaded portion 17 thereof grips the threads is of the cob and prevents
loosening and unscrewing of the insulator in service. FIGS. 4 and 5 show an insulator in accordance with the present
invention adapted for mounting on a line wire spacer bracket or a
transposition bracket. The
line wire groove in the insulator is indicated at 20, and the insulator
has a pin hole 21 for receiving the pin of the spacer bracket or
transposition bracket. A flanged portion 22 increases the length of the
leakage path over one end of the insulator; the other end has a
petticoat portion for a similar purpose. The losses in signaling current occasioned by line insulators are
in general due to three causes: First, the leakage from the line
conductor, including its tie wire, over the outer surface of the
insulator and under its skirt or petticoat to the supporting insulator
pin; second, the leakage from the line conductor and tie wire through
the material of the insulator to the insulator pin; and third, and most
important when dealing with high frequency carrier current, the
dielectric and dissipation losses introduced by the insulator. In accordance with the instant
invention, the composition of the
material of which the insulator is composed includes one or more
suitable elastomeric rubber compounds, either natural or synthetic, for
example, natural rubber, isobutylene diolefin copolymer (commonly
referred to as a butyl rubber), buna-S or SBR (butadiene styrene
copolymer) and the like; one or more suitable polymers or copolymers,
preferably hydrocarbon polymers, for example, polyethylene, polystyrene,
polypropylene, polybutylene and the like; and including other substances
for reinforcing, filling, coloring, softening, vulcanizing, and as
age-resistors and accelerators, and preferably a fungicide, some of such
other compounding substances including silica, clay, carbon, zinc oxide,
wax, amine antioxidants, carbon black, tetramethyl thiuram disulphide,
benzothiazol disulphide, sulphur and calcium aluminum silicate. The polymers and copolymers above mentioned and particularly the
hydrocarbon polymers, have very low dielectric constants, and hence, in
combination with the elastomeric hydrocarbon and the other compounding
substances assist in reducing the dielectric constant of the insulator,
and hence the capacitance of the insulator is very low.
The elastomeric component of the formulation is sufficient to
give the insulator the necessary elasticity and resilience to prevent
deformation and support a line wire in proper position, and also prevent
the occurrence of bends and abrasions in the line wire.
The polymers or copolymers cause the insulator to have the
desired increased high surface tension with water and consequently
greatly improved non-wetting characteristics as
compared with the previous rubber insulators referred to. An extremely important characteristic of the instant insulators
lies in the fact that after exposure to the elements for a considerable
period of time, the resiliency and elasticity of their formulations
shows an increase in these properties of from 42% to 62% over the
compounds of the insulators disclosed in the above-mentioned patents.
Furthermore, formulations composed of the ingredients referred to herein
have shown practically no change in the elasticity and resiliency after
a long period of exposure to the elements, whereas the formulations
referred to in the previous patents show a decrease in elasticity and
resiliency of approximately 40% during the same period of exposure, as
shown by the following table of measurements:
The
amount of the elastomeric rubber compound relative to the amount of the
polymers, such as the hydrocarbon polymers, may vary within considerable
limits depending upon the particular polymer employed, for example, from
a minimum of 20% to a maximum of 50% by weight of the anal homogeneous
mixture comprising the insulator. Polystyrene, polypropylene and polybutylene when compounded in
whole or in part with buna-S or SBR or the butyl component in accordance
with the instant invention will be found to exhibit the same advantages
from the standpoint of elasticity and resiliency, as well as providing
the equivalent electrical characteristics, and this is likewise true
when natural rubber is employed as the elastomeric component in the
formulation. The polymers
also assist in giving the insulator surface the desired high interfacial
tension with water to improve the non-wetting characteristics. The amount of the hydrocarbon polymer relative to the amount of the elastomesic component may vary within considerable limits, depending upon the particular kinds of compounds employed, for example, from a minimum of 5% to a maximum of approximately 25% by weight of the anal homogeneous mixture comprising the insulator, depending upon the type and amount |
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of the elastomeric rubber employed in the mixture. If the
amount of the hydrocarbon polymer is too high or if its molecular weight
is too low the insulator will he too plastic; on the other hand if the
amount of the hydrocarbon polymer is too low there is a detrimental
effect on the non-wetting characteristics.
If a polymer such as polyethylene is
employed, it may be a
polyethylene of a suitable molecular weight or a mixture of
polyethylenes of different molecular weights to give the desired average
molecular weight. The molecular weight and/or the amount of the hydrocarbon polymer relative to the composition of the completed insulator may readily be determined empirically and should be such that a Shore Durometer hardness "C" scale reading of the resultant mixture, when cured, lies in the range of from approximately 75 to approximately 95. The plasticizers, fillers and accelerators are embodied in the final mixture in relatively small quantities and are used primarily for the purpose of maintaining in field service the desired stiffness and hardness of the insulator
in accordance with the values of hardness above mentioned. The
wax employed to cause the insulator surface to maintain the desired high
interfacial tension with water in service and thus result in maintaining
its non-wetting characteristics may comprise hydrocarbon waxes, including
paraffin, ceresin, ozokerite, and amorphous types such as wax tailings,
mineral beeswax, and the like, vegetable waxes such as carnauba, montan,
and others, and animal waxes such as stearin, stearic acid, beeswax,
spermaceti, and the like. The
proportions of such wax substances may vary within relatively wide limits;
satisfactory results have been obtained by using two and one quarter parts
by weight of a wax such as the paraffin waxes. A sufficient amount of wax
must be used to cause the surface of the insulator to maintain the desired
high interfacial tension with water, without employing such amount of the
substance as deleteriously affects the physical and electrical properties
of the insulator. The desired proportion of the wax or waxes relative to
the insulator compound may well be determined empirically in each
instance. When
polyethylenes are employed as the polymer it may be found desirable to use
a mixture of polyethylenes some of which have a low molecular weight and
others of which have a higher molecular weight, thereby preventing the
polyethylene constituents from being either too soft or too hard and
causing the insulator compound to be either too soft or too hard. The
polymers or copolymers, such as the polyethylenes, by reason of their waxy
nature, add to the effectiveness of the wax component in causing the
surface of the insulator to have high interfacial tension with water and
thus produce better non-wetting characteristics than is obtained with the
wax component alone. The
carbon black employed to resist sun aging should be kept at a low level
consistent with the property of resistance to sun aging. The amount of the
carbon black has been found to be extremely important in maintaining the
desired low dielectric constant, low dissipation factor and low
capacitance, which are extremely necessary characteristics in an insulator
for use with carrier circuits operating at frequencies in excess of 30
kilocycles. In contrast to the insulators disclosed in the prior patents
referred to herein, in which the amount of carbon was present up to 10% by
weight of the formulation it has been found that the amount of the carbon
black in the instant insulators should be of the order of only about 0.5%
to 1.5% by weight of the formulation in order to prevent adverse effects
on the electrical characteristics of the insulators.
An example of carbon black is a semi-reinforcing black, referred to
in the trade as SRF black, made by the furnace process.
The various ingredients comprised in the formulation are thoroughly
mixed so as to form a homogeneous mixture which is then formed into the
final insulator, either by extrusion or various molding processes, and
heated during or after these processes a sufficient length of time to
vulcanize and cure the mixture. An
illustrative specific formulation is as follows, in which the parts of the
various constituents are specified by weight:
Parts Styrene
butadiene rubber - - - 24 Polyethylenes
- - - - - - - - - - 20 Paraflin wax (to give non-
wetting characteristics)
- - - 2.25 Amine antioxidants
(age
resistors) - - - - - - - - - 1.50 Plasticizer (to provide
adequate moldability) - - - - 3 Carbon black SRF type
(to
resist sun aging) - - - - - - 1 Tetramethyl thiuram
disulphide
(fungicide) - - - - 0.25 Benzothiazol
disulphide (accelerator) - - - - - - - - - - - 0.10 Sulphur
(vulcanizing agent) - - 0.15 Calcium aluminum silicate
(filler)
- - - - - - - - - - - - - - 46 Zinc oxide (filler) - - - - - - - - 1.75
_______ 100.00
As
is well known in the art certain of the ingredients may serve a dual
purpose, for example, the zinc oxide provides reinforcement and
acceleration. Comparative
tests that have been conducted on the aforementioned elastomeric compounds
show that the dielectric constant of the compounds of which the instant
insulator is composed has a value at one megacycle of from 3.3 to 3.8 as
compared to a value of 5.9 for the compounds of the insulators of the
Patents Nos. 2,218,497 and 2,304,483. Thus, from the standpoint of
dielectric constant the new compounds are at least twice as good as those
employed in the above mentioned patents. In regard to typical soda-lime
insulator glass which is a type that is commonly specified for glass
insulators for circuits in both the higher as well as the lower frequency
ranges, and established as national standard by the American Society for
Testing materials covered by their Specifications D-879-49, show a value
of 7.5 for the dielectric constant at one megacycle. In
the case of dissipation factor, the compounds of the instant insulator at
one megacycle show a value of 0.5 to 1.0 as compared with 3.9 for the
compositions of the fore-going patents, and in regard to this
characteristic the instant insulators are four to eight times superior to
the prior type. In
regard to relative capacitance at frequencies above 30 kilocycles,
formulations for the insulators compounded in accordance with the instant
invention are approximately twice as good as formulations employed; in the
insulators of the aforesaid patents. As
hereinbefore set forth, outdoor weathering tests have demonstrated that
the foregoing electrical characteristics of the instant insulator are not
appreciably altered as a result of action by the sun and weather, and thus
the insulator not only has improved sustained resiliency and elasticity
but also possesses the necessary physical and weathering characteristics
that must be attained to make the insulator suitable for open wire high
frequency carrier circuits. While
there are shown and described herein certain preferred substances and
embodiments which have proven adequate, various other substitutes and
equivalents will doubtless be suggested to those versed in the art without
departing from the invention, and the invention is therefore not limited
except as indicated by the scope of the appended claims. We
claim: 1. A non-breakable insulator for supporting a line conductor, adapted to be mounted on an insulator supporting member and to maintain high insulation values at frequencies up to and exceeding 100 kilocycles over long periods of time and under adverse weather conditions, said insulator having a wire groove portion for receiving said line conductor, the body of said insulator being substantially composed of a heat-cured homogeneous mixture of at least one elastomeric rubber compound having a low dielectric constant, at least one hydrocarbon polymer having a low dielectric constant, a wax compound, and fillers and pigments including a sun-resistant agent, said elastomeric rubber compound being present in an amount sufficient to impart to the insulator the necessary elasticity and resiliency to prevent deformation and support said line conductor in predetermined fixed position with respect to said insulator supporting member and said hydrocarbon polymer being present in an amount sufficient to enhance the flexibility of the insulator to an extent to prevent bending and abrasion of the line wire at the wire groove and to enhance the non-wetting surface characteristics of the insulator, said wax compound being present in an amount sufficient to cause the surface of the insulator to maintain high interfacial tension with water and provide non-wetting surface characteristics over long periods of service without detrimentally affecting the foregoing physical and electrical characteristics of the insulator. 2.
An insulator according to claim 1, in which said homogeneous mixture
contains a sufficient amount of a fungicide to inhibit the growth of molds
or other fungus on the exposed surfaces of the insulator. 3.
A non-breakable insulator for supporting a line conductor, adapted to be
mounted on an insulator supporting member and to maintain high insulation
values at frequencies up to and exceeding 100 kilocycles over long periods
of time and under adverse weather conditions, said insulator having a wire
groove portion for receiving said line conductor, the body of said
insulator being substantially composed of a heat-cured homogeneous mixture
of at least one elastomeric rubber compound having a low dielectric
constant, at least one hydrocarbon polymer having a low dielectric
constant, a wax compound, carbon black, fillers and pigments, said
elastomeric rubber compound being present in an amount sufficient to
impart to the insulator the necessary elasticity and resiliency to prevent
deformation and support said line conductor in predetermined fixed
position with respect to said insulator supporting member and said
hydrocarbon polymer being present in an amount sufficient to enhance the
flexibility of the insulator to an extent to prevent bending and abrasion
of the line wire at the wire groove and to enhance the non-wetting surface
characteristics of the insulator, said wax compound being present in an
amount sufficient to cause the surface of the insulator to maintain high
interfacial tension with water and provide non-wetting surface
characteristics over long periods of service without detrimentally
affecting the foregoing physical and electrical characteristics of the
insulator, the amount of said carbon black being sufficiently small to
prevent undesired increase in the electrical capacitance of the insulator
but sufficient in amount to maintain adequate resistance of the insulator
to sun aging. 4.
An insulator according to claim 3, in which the amount of said carbon
black is in the range of approximately 0.5% to 1.5% by weight of said
homogeneous mixture. 5. A non-breakable insulator for supporting a line conductor, adapted to
be mounted on an insulator supporting member and to maintain high
insulation values at frequencies up to and exceeding 100 kilocycles over
long periods of time and under adverse weather conditions, said insulator
having a wire groove portion for receiving said line conductor, the body
of said insulator being substantially composed of a heat-cured homogeneous
mixture of at least one elastomeric rubber compound of the class
consisting of natural rubber, isobutylene dioletin copolymer and butadiene
styrene copolymer, at
least one hydrocarbon polymer of the class consisting of polyethylene,
polystyrene, polypropylene and polybutylene, a wax compound, and fillers
and pigments including a sun-resistant agent, said elastomeric rubber
compound being present in an amount sufficient to impart to the insulator
the necessary elasticity and resiliency to prevent deformation and support
said line conductor in predetermined fixed position with respect to said
insulator supporting member and said hydrocarbon polymer being present in
an amount sufficient to enhance the flexibility of the insulator to an
extent to prevent bending and abrasion of the line wire at the wire groove
and to enhance the non-wetting surface characteristics of the insulator,
said wax compound being present in an amount sufficient to cause the
surface of the insulator to maintain high interfacial tension with water
and provide non-wetting surface characteristics over long periods of
service without detrimentally affecting the foregoing physical and
electrical characteristics of the insulator. 6.
An insulator according to claim 5, in which the amount of said elastomeric
rubber compound is in the range of approximately 20% to approximately 50%
by weight of said homogeneous mixture. 7.
An insulator according to claim 5, in which the amount of said elastomeric
rubber compound is approximately 24% by weight of said homogeneous
mixture. 8.
An insulator according to claim 5, in which the amount of said hydrocarbon
polymer is in the range of approximately 5% to approximately 25% by weight
of said homogeneous mixture. 9.
An insulator according to claim 5, in which the amount of said hydrocarbon
polymer is approximately 20% by weight of said homogeneous mixture. References
Cited in the file of this patent: UNITED
STATES PATENTS 2,146,596
Schwartz et al. - Feb. 7,
1939 2,304,483
Smith et al. - - - Dec. 8, 1942 2,512,459
Hamilton - - - - June 20, 1950 FOREIGN
PATENTS
508,338 Canada
- - - - - - Dec. 21, 1954
800,128
Great Britain - - Aug. 20, 1958 OTHER
REFERENCES Text: Longton, Blacks and Pitches, published 1925 by D. Van Nostrand Company of New York, page 62 relied on. Publication: Crafton et al., "A New Dielectric For Cables," Modern Plastics, July 1944, pp.90-93.
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