Rubber covered cable

A flexible, high strength steel cable having a continuous, flexible outer covering of vulcanized rubber thereon is disclosed. The rubber coating makes the cable easier to handle and use, and reduces flaring of the wires when the cable is cut. There is less tendency for the cable to slip when wrapped around objects to be secured and/or lifted, and marring of such objects by the cable is reduced. In addition to these advantages, the coating on the cable protects the steel substrate from corrosion during weathering, exposure to sea water, or contact with corrosive chemicals. Vulcanization of the rubber coating is carried out at high pressure to provide it with a high Durometer hardness and other desirable properties.

BACKGROUND OF THE INVENTION 
The present invention pertains to improvements in flexible, high tensile 
strength steel cables which are used, inter alia, for securing, lifting, 
towing or pulling objects at conditions under which a cable must withstand 
a heavy physical load. More specifically, the present invention pertains 
to flexible, high tensile strength steel cables having a protective outer 
covering thereon and to methods of producing such cables. 
Cables made of stranded steel wire are relied upon in many commercial and 
industrial applications for securing, towing or lifting of heavy objects, 
or for otherwise repeatedly or continuously applying motive force to an 
object where extreme reliability of force transfer is essential, e.g. 
cables required for moving aircraft control surfaces, and for brakes, 
wenching, lifting and towing. 
Steel cables are extensively used for the aforementioned purposes, and 
although no satisfactory substitute for such cables has yet been devised, 
they nonetheless have certain disadvantages and objectionable 
characteristics. Individual wires or strands of the cable can flare 
outward when the cable is cut, thus creating a hazard to workmen. 
Furthermore, since the cable is made of steel which is relatively hard and 
tough, it has a tendency to slip across the surface of an object around 
which it is wrapped, or from which the object is slung, such as a capstan 
or a heavy object to be lifted or towed. Contrariwise, if the object being 
secured or pulled by the cable is made of a softer material, the cable 
tends to scuff or scrape the surface of the object, and hence mars its 
surface. 
Another problem that is encountered is that the wires in the strands of the 
cable become worn and eventually break as a result of the cable repeatedly 
rubbing against itself or, more seriously, being dragged over sharp or 
abrasive objects. In an attempt to protect the cable, not only against 
cutting and abrasion of the wires but also from corrosion, it must be 
periodically coated with grease or a heavy oil. This provides protection 
which is frequently inadequate, and which at best is only temporary, not 
to mention the difficulty of handling and the mess that it causes. 
The primary object of the present invention is, therefore, to provide a 
flexible, high tensile strength cable comprising an improvement whereby 
the previously mentioned disadvantages and objectionable features are 
either eliminated or alleviated. 
Another object is to provide a flexible, high tensile strength steel cable 
having a flexible, continuous coating of a vulcanized rubber thereon. 
Still another object is to provide a flexible, high tensile strength steel 
cable having a reduced tendency to slip on surfaces of an object around 
which the cable is wrapped, and which is less inclined to blemish the 
surfaces. 
Yet another object is to provide a flexible, high tensile strength steel 
cable having a coating thereon whereby contact of the wires of the cable 
against one another and with sharp or abrasive objects is prevented. 
Even another object is to provide a flexible, high tensile strength steel 
cable with a coating thereon which protects the wires of the cable from 
rust and corrosion. 
Another object is to provide a flexible, high tensile strength steel cable 
with a coating thereon which prevents flare of the wires of the strands 
when the cable is cut. 
Still another object is to provide a process for producing a flexible, high 
tensile strength, steel cable having a protective coating of vulcanized 
rubber thereon. 
Other objects and advantages of the present invention will become apparent 
from the following description and the appended claims. 
SUMMARY OF THE INVENTION 
The present invention is a flexible, high tensile strength steel cable 
having a continuous outer coating of flexible, vulcanized rubber bonded 
thereto. The cable can be produced by first applying a vulcanizable rubber 
compound to the outer surface of the steel cable and then exposing the 
cable with the rubber compound thereon to selected time, temperature and 
pressure conditions to effect curing of the rubber compound and bonding 
thereof to the steel wires of the cable.

DESCRIPTION OF PREFERRED AND ALTERNATIVE EMBODIMENTS 
In preferred embodiments of the present invention the flexible, high 
tensile strength steel cable comprises a plurality of elongated, twisted 
strands which are made up from fine steel wires, e.g. 3 to 19 wires per 
strand. The wires can be stainless steel, but more commonly will be high 
carbon steel. It is preferred that the wires of the strands be brass 
plated. 
The compounding recipe and curing conditions employed for providing the 
flexible, vulcanized rubber coating on the cable is subject to 
considerable variation depending upon the combination of physical 
properties and chemical resistance sought for the vulcanizate, and in 
preferred embodiments of the invention the vulcanized polymer coating will 
have tensile, modulus and elongation properties which are relatively high, 
along with a high hardness and a low brittle point. It is preferred that 
the rubber compound, when vulcanized, have a tensile strength of at least 
about 2500 psi, that the 300% modulus be at least about 1000 psi, and that 
the elongation thereof be at least about 475%. It is also preferred that 
the Shore A Durometer hardness of the vulcanized rubber be at least about 
65 .+-.5, and that the brittle point thereof be at least about -65.degree. 
F, or lower. It is also advantageous that the acetone extract of the 
rubber compound not exceed about 20%. A Government Grade Off-The-Road 
rubber compound is a preferred polymer compound for coating the steel 
cables of this invention. One recipe for an off-the-road tire rubber 
compound is shown in Table 1. 
TABLE-1 
______________________________________ 
Off-The-Road Rubber 
Recipe 
Ingredient Parts by Weight 
______________________________________ 
Natural Rubber, Liberian Pale 
Crepe 100 
Carbon Black, ISAF-LM (N-231) 
50 
Zinc Oxide 5 
Stearic Acid 3 
Age Rite Resin D 2.0 
Sulfur 1.8 
Santocure NS .8 
______________________________________ 
Note: Where preferred, from about 10-15 parts by weight of precipitated 
silica pigment can be substituted for about 10-15 parts of the carbon 
black, and other or different amounts of ingredients can be used in the 
recipe when such is preferable and practical. 
It is advantageous that the polymer compound used for coating the cable 
contain carbon black as a reinforcing pigment, preferably in loadings of 
at least 30 parts by weight for each 100 parts by weight of rubber, for 
imparting high strength and abrasion resistance to the vulcanized cable 
coating. Where preferred, a rubber-to-metal bonding agent can be emplaced 
between the steel cable and the rubber compound in order to enhance the 
bonding strength between the two. 
It will be appreciated that the rubber compound used for coating the cable 
is formulated and cured to provide physical and chemical properties that 
are different from those required for coating electrical cables made of 
copper, aluminum, etc., where the electrical conductivity of the coating 
must be very low. Wire insulation compounds are typically made of 
thermoplastic resins such as polyvinyl chloride, polyethylene, and 
polytetrafluoroethylene, or oxidation resistant rubbers such as butyl, 
EPD, EPDM, blends of such rubbers, and blends thereof with polyolefins 
such as polyehtylene or EVA. Mineral fillers having low electrical 
conductivity, such as clays and fine silicas, have been used in wire 
insulation compounds for further enhancing the physical and chemical 
properties thereof. Regardless, the polyolefins, PVC and PTFE resins have 
provided coatings, which though quite tough and resistant to chemical 
attack, are slippery in comparison to flexible rubber and do not bond well 
to the electrically conductive wire of the cable. On the other hand, 
rubber compounds filled with mineral fillers are frequently too soft, can 
age quickly if exposed to the weather, and have tensile strength, modulus 
and elongation values which are far too low for stress-strain and abrasive 
conditions that will be encountered when applied as a coating on a steel 
cable that is used for pulling, lifting or securing of heavy objects. It 
will therefore be understood that formulation and curing of rubber 
compounds used for coating cables in the present invention required 
consideration of the physical and chemical conditions to which the cable 
is exposed when placed in use. The vulcanized coating should be somewhat 
deformable to help overcome both slipping of the cable and also marring of 
an object around which it is wrapped, but should nonetheless be quite 
hard, to the extent that flexible rubbers can be, to prevent tearing and 
rapid wearing of the coating during normal conditions of use. 
Advantageously, therefore, the cable coating can be at least one-eighth of 
an inch thick, and preferably thicker, e.g. 3/16 1/4, 1/2 inch thick, or 
even thicker depending on the diameter of the steel cable that is covered. 
The term "rubber compound" as used herein is intended to mean compounds 
wherein the base polymer is one or more rubbers, or wherein a major 
portion of one or more rubbers is in mixture with a minor portion of one 
or more nonrubber polymers. Various rubbers, or blends of a rubber with 
another compatible rubber or thermoplastic resin can thus be used as the 
rubber of the coating compound, exemplary rubbers being butadiene-styrene, 
polybutadiene, EPR, EPDM, butyl, chlorobutyl, neoprene, polyisoprene, and 
natural. Thermoplastic resins which can be blended with the rubbers, to 
provide a rubber compound that is predominately rubber, include 
polyethylene and EVA resins. Good chemical and aging resistance, along 
with high tensile strength and modulus which provide toughness, can 
generally be expected from butadiene-styrene, polybutadiene, polyisoprene, 
and natural rubbers, as well as from blends of such rubbers, when they are 
compatible with each other. Butyl, chlorogutyl, EPR, EPDM, neoprene 
rubbers and compatible blends thereof generally provide higher resistance 
to heat and/or chemical attack. 
The use of a rubber reinforcing carbon black, such as an FEF, HAF, ISAF or 
SAF grade for instance, enhances strength and toughness of the vulcanized 
rubber compound and greatly increases its resistance to aging, tearing and 
abrasion. Therefore, the type and amount of carbon black incorporated into 
the rubber can vary in accordance with the type of rubber and the 
properties desired of the vulcanizate. A content of at least about 30 
parts by weight of carbon black per 100 parts of rubber is generally 
preferred; more preferably about 30 to about 70 parts by weight of carbon 
black per 100 parts of rubber. 
Preferred rubbers, which can be used with these proportions of carbon 
black, are natural rubber, butadiene-styrene rubber, polybutadiene rubber, 
blends thereof, and especially natural rubber. 
It should be pointed out that in accordance with the present invention a 
rubber compound which contains carbon black is applied to the steel of the 
cable as a coating without any form of insulating layer between the steel 
and the coating or on the outside of the coating. It is not practical to 
use rubber compounds containing 30 or more parts of carbon black for each 
100 parts of rubber when coating electrical cables for the purpose of 
insulating and protecting them, since carbon black renders the vulcanized 
rubber far more electrically conductive than the mineral fillers used in 
wire insulating compounds. It will therefore be emphasized once again that 
the rubber compound employed in the present invention for coating 
flexible, high tensile strength steel cables are different from those used 
for electrically insulating wires and cables, and they are intended 
instead for protecting cables which must withstand high physical loads 
during conditions of use that are entirely different from those normally 
encountered by electrical cables. 
As was previously indicated, a tire rubber compound can be used as a 
coating for the cable since it can provide a degree of toughness, bonding, 
abrasion resistance and corrosion resistance that is desired for 
protecting the cable against physical and chemical damage. 
In producing the presently disclosed coated cable, a vulcanizable rubber 
compound can first be applied to the steel cable, the rubber compound can 
then be vulcanized, and a steel cable having a continuous, flexible, 
vulcanized rubber coating bonded thereto can thereafter be recovered by 
winding on a reel. In such a process, the cable having the vulcanizable 
rubber thereon can be simultaneously heated and compressed along its 
length for vulcanization of the coating compound and to strengthen the 
bond between the polymer and the metal of the cable. To advantage, the 
coating of rubber compound can be compressed during vulcanization to a 
pressure of at least about 700 psi, and it is preferred that the thickness 
of the rubber coating on the cable be permanently reduced by the 
compression and curing thereof so that the produced cable has a diameter 
significantly smaller than prior to the compression and heating thereof, 
e.g. a diameter of at least 10% less. It is preferred that the pressure 
and temperature applied to the cable during curing of the rubber coating, 
and the time to which the cable is exposed thereto, be selected to provide 
a cable with a cured coating thereon having the previously stated 
preferred values of Shore A Durometer hardness, tensile strength, modulus, 
and elongation. 
Application of the vulcanizable rubber to the cable and the curing thereof 
can be carried out in two successive steps. The rubber can be applied to 
the steel cable by pulling it through the extrusion orifice of a polymer 
extruder while simultaneously extruding the unvulcanized rubber compound 
from the orifice onto the surface of the steel cable. Subsequently, the 
rubber compound thus applied to the cable can be cured thereon in any 
suitable fashion, but there is advantage in pulling the coated cable 
through an elongated tubular die which is heated and which has an inside 
diameter significantly smaller than the diameter of the coated cable prior 
to being pulled through the die. Accordingly, the cable can be compressed 
and heated both for curing of the rubber coating and reducing the 
thickness thereof, as previously described, all for the purpose of 
providing a cable coating of desired strength, hardness, and resistance to 
abrasion and chemical attack. 
If the bonding strength between the metal and the rubber compound need be 
greater than is effected by means of applying and curing the rubber 
compound while in direct contact with the wire of the cable, a 
rubber-to-metal bonding agent can be applied to the wire prior to 
application of the polymer compound thereto. Various bonding agents can be 
employed, one suitable version being an adhesive marketed under the 
tradename Chemlok 220 by the Hughson Chemical Company, and which is 
described as being organic polymers and dispersed fillers in a xylene and 
perchloroethylene solvent system. Where preferred, a primer such as 
Chemlok 205 can be applied to the steel wire ahead of the adhesive, and 
this material is described as being a mixture of polymers, organic 
compounds and mineral fillers in a methyl isobutyl ketone and Cellosolve 
solvent system. Also, as was previously indicated, the bonding of the 
polymer compound to the steel cable is improved when the wires of the 
strands of the cable have previously been coated with brass. 
The invention will be further described with reference to the drawings, a 
preferred processing technique for producing the cable being illustrated 
therein. Although reference will be made to specific materials and 
conditions, it will nonetheless be understood that other materials and 
processing conditions can also be used. 
In FIG. 1, a flexible, high strength steel cable 1 is unwound from a reel 2 
by pulling the cable from the other end by means of a wench represented at 
3. The cable is guided through the processing apparatus by a series of 
pulleys 4. As the cable leaves the reel 2, it first enters a vat 5 which 
contains a degreasing liquid, represented at 6, which strips grease and/or 
oil from the cable. Preferred degreasing liquids are organic solvents, 
although detergent liquids can be used, and trichloroethylene is a 
preferred organic solvent. Upon leaving the vat 5, the cable passes into a 
first drying tunnel, represented at 7, for vaporization and removal of the 
degreasing liquid therefrom. From the dryer 7, the cable passes through a 
first set of soft rollers 8, onto which a primer is fed from a primer 
reservoir 9, and the cable is thereby coated with primer prior to being 
passed into a second tunnel dryer 10 for vaporization and removal of the 
solvent of the primer. Upon leaving the second dryer 10, the cable passes 
through a second set of soft rollers 11 onto which a rubber-to-metal 
adhesive is fed from adhesive reservoir 12. The cable is coated with 
adhesive by rollers 11 and it is then conveyed into a third drying tunnel 
13 for vaporization and removal of solvent of the adhesive. Upon leaving 
the dryer 13, the cable is drawn into and through a polymer extruder, 
represented at 14. The extruder comprises a barrel 18 into which a 
vulcanizable rubber compound is charged through a feed inlet (not shown) 
and is compressed therein by means of a piston 15 that is driven downward 
with a hydraulic cylinder 16. The barrel of the extruder is heated by 
means of an electric or steam heating coil 17 to effect softening, but not 
curing, of the vulcanizable rubber compound contained in the barrel. Below 
the piston 15, the extruder barrel has a sealing inlet 19 for cable 1, and 
this inlet is axially aligned with a circular extrusion orifice 20 through 
which the cable is drawn by wench 3 while the vulcanizable rubber compound 
is simultaneously being extruded from the orifice onto the surface of the 
cable. The cable is centrally aligned in the circular orifice 20 while it 
is being pulled through it, so that the cable thus becomes uniformly 
coated with the rubber compound. The thickness of the deposited coating is 
determined by the inside diameter of the orifice, i.e. use of a larger 
diameter provides a thicker coating, and vice versa. 
Upon leaving the extruder orifice, the coated cable is sprayed with a 
silicone oil by jets 21 and is then drawn into an elongated tubular die, 
represented at 22, which is shown in greater detail in FIG. 2. The 
elongated heating tube 23 of the die has a flare 24 at the inlet end 25 
thereof, and the internal diameter of the tube is larger at the inlet end 
than at the outlet end 26. An electric heating coil 27 surrounds the tube 
22, and a protective metal cover 28 surrounds both the tube 22, and the 
coil 27. A layer of thermal insulation can be applied to the outside of 
cover 28 when the use of such is preferred. 
The cable 1 leaving orifice 20 has a coating 29 thereon of unvulcanized 
rubber compound, and the thickness of the coating is dependent upon the 
outside diameter of the cable 1 and the inside diameter of orifice 20. In 
any case, the diameter of cable 1 and the coating 29, combined, is smaller 
than the largest diameter of the flare 24, thus funneling the coated cable 
into the elongated tube 23. The inlet end 25 of the tube has an inside 
diameter approximately the same or slightly smaller than the coated cable, 
but the inside diameter of the tube converges along its length, thereby 
causing the coating 29 to become compressed as the cable is drawn through 
the tube by wench 3. Also, as the cable 1 and coating 29 are drawn through 
the tube 23, the coating is heated by transfer of heat from heater coil 27 
through the tube. The coating 29 is thus simultaneously heated and 
compressed to effect vulcanization and size reduction thereof during 
transit through the tube. 
Conditions of time, temperature, and pressure to which the coated cable is 
subjected during transit through the tubular die is subject to variation 
depending on the thickness and composition of coating 20, e.g. thicker 
coatings require longer curing times, and curing time will further depend 
upon the type of rubber and the vulcanizers, accelerators, etc., 
incorporated into the rubber compound. Compression of the coating during 
the curing and drawing thereof within tube 1 is preferred for producing a 
dense, hard, and yet flexible cable coating. Accordingly, pressures of at 
least about 700 psi can be used to advantage, and as a consequence, the 
diameter of the produced coated cable leaving the outlet end 26 of the 
tube 23 is significantly less than prior to the compression and heating of 
the cable for vulcanization of the coating 29. To produce a coating of 
high hardness, the diameter of the coated cable is thus reduced by at 
least about 10%. As an example, a 1/4 inch cable having a 5/16 inch 
coating of polymer thereon can be reduced from a diameter of 7/8 inch to 
3/4 inch during compression and heating of the cable in the tubular die 
22, thus reducing the diameter of the cable by about 14% and the thickness 
of the coating by 20%. 
Referring to FIG. 3, the cable having a vulcanized coating thereon 30 is 
drawn out of tubular die 23 by the wench 3 and is wound on the drum 31 of 
a cable reel having cable retainer plates 32 at the end of the drum. The 
cable reel is rotated for winding of coated cable 30 thereon by means of a 
gearmotor 33. The entire wench assembly 3 is mounted on tracks 34 and is 
reciprocated back and forth at a controlled, variable speed by means of 
another gearmotor 35 and crank assembly 36 to effect a smooth, uniform 
winding of the cable on the drum. When the cable 30 has been wound on the 
cable reel to the outer periphery of plates 32, the reel is removed from 
the wench for storage and is replaced by an empty reel. 
As shown in FIGS. 4 and 5, a cable of the present invention comprises a 
flexible, high tensile strength steel cable 1, and a continuous, flexible 
outer covering 37 of vulcanized polymer compound. In the illustrated case, 
the cable 1 comprises a central strand surrounded by six other strands, 
and with each strand being made up from a plurality of fine wires. It 
should also be noted that no separate coating exists between the rubber 
coating 37 and the cable, or on the outside of the coating 37. The use of 
a very thin primer and/or adhesive coating on the cable notwithstanding, 
the cable is thus protected by means of the single coating 37. 
EXAMPLE 
Using an apparatus arrangement substantially in accordance with that shown 
in the drawings, a 1/4 inch diameter 7 .times. 19 steel cable was coated 
in accordance with the present invention. The wire of the strands of the 
cable were high carbon steel and were plated with brass. A Government 
Grade Off-The-Road tire rubber was used to coat the cable. 
Bare cable was drawn from reel 2 and passed into vat 4, which contained 
trichloroethylene solvent, for degreasing the cable. The first dryer 7 was 
maintained at a temperature of about 160.degree. F for vaporization and 
removal of the trichloroethylene from the cable. Chemlok 205 primer was 
applied to the degreased and dried cable by rollers 8, and the solvent of 
the primer was evaporated and removed therefrom by maintaining the second 
dryer 10 at about 160.degree. F. The primed cable passed through rollers 
11 and was thus coated with Chemlok 220 adhesive, and the solvent of the 
adhesive was vaporized and removed by means of the third dryer 13 at 
temperatures of about 160.degree. F. A layer of primer and a layer of 
adhesive, each about 3 to 4 mils thicks, were thus applied to the cable 
before it was drawn through the extruder 14 for application of the rubber 
coating. The unvulcanized rubber compound was heated and thus softened in 
the barrel 18 of the extruder prior to being extruded through the 7/8 inch 
orifice 20 thereof. The diameter of the cable with the polymer coating 
thereon was thus 7/8 inch, and entered tubular die 22 at that diameter and 
exited therefrom at a diameter of 3/4 inch. 
The tubular die had a heated length of 10 feet, and the rubber coating of 
the cable was heated in tube 23 to a temperature of 
300.degree.-350.degree. F as the coated cable was drawn through the tube 
at the rate of 1 foot per minute. The rubber coating was vulcanized during 
transit through the tubular die, and the resulting produced cable having a 
continuous, uniform, flexible coating of cured rubber bonded thereto was 
wound on a cable reel by wench 3 as it discharged from the die. The 
pressure exerted on the cable during transit through die 22 was about 750 
psi. 
The process just described is continuous, and it will be understood that by 
use of a polymer extruder having multiple orifices and a series of tubular 
dies 23 that several cables could be coated simultaneously. It will also 
be appreciated that the tubular dies 23 can be made longer so that the 
coated cable can be pulled through and vulcanized at a faster rate without 
sacrificing curing time needed to effect sufficient vulcanization. 
A coated cable product and process has now been described whereby the 
previously stated objects can be accomplished. Problems associated with 
the knotting, fraying, and slipping of flexible, high tensile strength 
steel cable can thus be avoided, while also providing a cable which does 
not have to be periodically greased, yet which is resistant to cutting and 
abrasion of the wires and to corrosive attack by such substances as fresh 
water, sea water, acids and alkalies. 
Even though the invention has been described with reference to specific 
polymers, compounding ingredients, materials, conditions, dimensions, 
applications, and the like, it will nonetheless be understood that even 
other embodiments will become apparent which are within the spirit and 
scope of the invention defined in the following claims.