CA1176092A - Helically wrapped cable - Google Patents

Abstract

ABSTRACT
HELICALLY WRAPPED CABLE
A helica1 layer on a cable core produces a zero torque response by choosing a particular lay angle.
Lightguide cables having a metallic helical armour layer, for example, advantageously utilize the present technique, which reduces the tendency of the cable to kink.

Description

5 1 7~(~9~

(W~C.L. Weinraub Case 1 ~LICALLY WRAPPED CA~LE
This invention relates to cables having a helical element, including coaxial cables, multipair cables, and lightguide ca'bles ~or telecommunications use.
Cables having one or more helical elements are used in a variety of applications, including telecommunications cables, such as coaxial cables, multipair cables, or lightguide cables. A helical wrap can serve as an armour member of such a cable when located near the outer portion of the cable Other helical wrap elements include tapes, plastic ribbons, binders, etc. A
helical wrap of a conductive material may also be used in the transmission of electrical power or electrical information 'Unless steps are taken to prevent i-t, a helical element typically introduces a torque response to a cable, That is, when an axial force is applied to the eable, causing it to elonga-te, there is a tendency of the cable to twist. Depending upon the construction of the cable, this twist may be either in the dir&ction that causes the helical wrap to tighten, or in the dlrection that causes it to unwind, Such a torque response can in some cases-cause a cable to kink when a cable is being unwound from a drum or otherwise handled, as for example durlng installation of the cable. Other handling difficulties can also result from a torque response. The tightening or unwinding of $he 28 helical element can also'adversely affect tl-le performance l l ~6092

- 2 of the cable.
It is Qossible to counteract the torque response of one or more helical layers by providing ~or an opposite torque response in another helicaL layer. For example, one helieal layer wound in one direetion ean be eounter-aeted by an overlying helical layer wound in the opposite direetion. Teehniques for analyzing the torque response of helieally wrapped eables have been developed whieh allow for predieting the lay angle of a counteraeting helical element; see, for example, "Meehanieal Characterization of Cables Containing Helically Wrapped Reinforcing Elements", by T.C. Cannon and M.R. Santana in the Proceedings of the 24th International Wire and Cable Symposium (1975), Cherry Hill, New Jersey.
Aceording to the present invention there is provided a cable eomprising a helieal element surrounding a eylindrieal eore, in ~hieh the lay angle ~ o~ said helieal element is ehosen se that said helieal ~lement produees a substantially zero torque response when said cable is axially stressed.
In accordanee with an aspect of the invention there is provided a cable co-mprising a substantially cylindrical eore, the core having a torque response when a length of the eore is stressed in axial tension, and a helical element surrounding the core with a substantially eonstant lay angle 3, in whieh the lay angle ~ is ehosen in accordanee with the formula = tan l [N -1/2]
~ Rc/Rc, wherein R is the radius of the core where N = ~ / e Le is the length of a segment of the core, ~Lc is the ehange in the length of said segment produced hy the applieation of a first axial stress to the core, and ~Rc 1 1 7609~!
- 2a -is the change in the radius of the core produced by applica~ion of said first axial stress whereby the torque response of the helical-element-surrounded core is substantially the same as the torque response of the core.
The foregoing and other features of the present invention will now be described reference being made to the accompanying drawings, in which:
FIG. 1 shows an experimental setup suitable for determining the parameters used to calculate the helical lay angle.

~ 1 7609 FIG. '~ sllows an optical fiber c~ble having a helical armour layer in accLlrdance with the present invention.
The follol~in~ detaileLd description relates to helically wrapped cable whereby a zero torque response is produced in a single helical wrap. In the present cable design, a single helical wrap contributes zero torque to the cable core on which it is wound. Thus if the cable core has an essentially zero torque response prior to winding the helical element thereon, the resulting cable witll the helical element also obtains an essentia]ly zero torque response in the presence of an applied axial strain.
As shown in the above-noted paper by Cannon and Santana, the strain in a helical element (~s) is related to the axial strain in the cable by the equation (1~:

Ss = Ec(cos2e-N sin2~ lls sin 2~

where ec is the axial strain in the cable, ~is the lay angle of the helical element (the angle formed by the longitudinal axis of the cable core and the axi~ o~ the helical element) 9 ~ iS the cable twist in turns per unit length that results from the axial strain, 119 iS the radial location of the helical element~ alld N i9 t~le radial fitrain per unit of axial strain th~t the ca~le core exper-lences at the helical e`Lement due to the axial straill, and is given by equation (2):
-~IC/R

3 N = ~ ~ (2) where Rc is the radius of tlle core on wllich the helical element is wrapped, nnd Lc is -the len~th of a segment of the core It has ~eell determilled that a bero torque response can be obtained by solving equation (1) as follows By impo~ing a zero t~ist e~l~l conditlon (~ a 0) 38 equation (3) results:

~ 1 76092 S = eC(cos2P-N sin2~) (3 Under tl~e condition of zero torque, s~ tlle strain in the helical element, becomes zero. This occurs when the lay angle~ is chosen according to the following equation:
-1 r~-1/21 (4) ~R
Therefore, by evaluating the values of R c and aL
L c, the lay angle yielding zero torque can be determined, A test setup suitable for determining these values is shown in FIG. 1. The source of stress can be a ratchet pulling winch 11 connected to the cable core through a one-fourth inch stranded steel ro~e 12. A
hook 13 is attached to the rope by a swivel connection9 and attached to the cable core under test 16 by a grip 14, The cable core under test is attached to a load measuring device 18 through a grip 17, A s~itable load measuring device is tlle Data Instruments Inc., Tyco Model JP-2000 load cell, which is in turn attached to a stationary object 19, Startin~ from a zero (or gi~en) stres6 condition, the challge in the len~th of the cable core under test over ~ given gauge length due to an applied stress is determined, The 6auge len6tll is measured to provide the length of the cable core as suspended, including any sag in the cable core, Tlle gauge length provides the value of Lc, while the chan6e in lengtll is the value ofQLc. The 3o diameter of the cable core under test is also measured; for example, by an Instron*trurlsverse straill sensor, ~Sodel G-57-11, The diameter is ~lrst deterlllined under the ~ame stress conditions at wllicll the gau6e length is measured, and then agaill at the same applie(l stress at ~hlcll ~Lc is measured, The ~alue of llc is thell olle-llrllr the initial diameter, an(l ~llc is one-half the chan~e in the diameter under the aforesaid applied stress.
38 To provide for improYecl accuracy and conslste~lcy * Trade mark ! )7609~

in the determirlat,io~l o~ N, the abo~e data is preferably measured over a series ol stress incremellts, and the reslllting values of N aver~ed. A still more desirable ;nethod in many cases is to fit the data resulting from successive stress incren~ents to a regression formula; a least-squares fit metho~l can be usedO If zero strain is assumed at some small ~restress, the formula (5) can be used, where r is the radial strain of the core, L is the axial strain of the core, and K is a constant.
r ~ -N ~L ~ ~ (5 In this formula, each set of data points 15 ~c aLc is plotted or otherwise recorded on a graph having ~r and EL as the coordinate axes. The values of N
and K are then chosen to result in a line having a minimum (least-squares) deviation from the data. A numerical calculation can alternately be used to obtain N and K from the data points, according to known methods.
'rhis technique has been ~uccess~ully ~pplied to a lightguide cable. It is important in lightguide cables to reduce the tendency of the cable to kink during h~ndling.
25 This is facilitated when an armouring layer on the cable obtains a zero torque response. As an illslstrative example of the pre~ent tecllnique, a lightguide cable comprising optical fibers, a~ otherwise descril)ed in U,S, Patent No. 4,2l~1,979, is measured to determine the appropriate 0 lay angle Or a helical armouring layer, EX~IPLE
A li~lltguide cable, as substantiAlly s~own in ~IG, 2, is cun~tructed aocording to the followin~
desoription, The core oI the cable refers to all portions 35 of the cable inslde the helical armour layer (20g, 210), At the center of the core is a sl~ace for liglltguides, whicll may ~e ~aclcaged in the f~rm of rib~ons, Typicallyl each 38 ribbon (201) comprises 12 o~)tical Libers, wi-th fewer shown ~ 176092 .. ~, for clarity. The ri~holls may be t~isted, with one twlst per 46 cm for the ca~lle sho~n. An unsintered pol~rtetrafluoroetllylene (~TI~) tal~e (not shown) is applie~
over the ribbons to act as a thermal barrier. The PTFE tape is about 21 mm wide by 0.08 mm thick and is applied longitudinally with an overlapped seam. A
polyethylene tul)e 202 extruded over the PTFE tape acts as a protective chamber for the rib~on structure. The tube material is a high-density polyetllylene formed in a continuous extrusion, having an inside diameter of 6.35 mm and a thickness of 0.71 mm. A spunbonded polyester tape 203 is applied over the polyethylene tube. The tape is 2,54 cm wide by 0.2 mm thick. It is applied longitudinally with an overlapped seam, The next layer comprises fourteen stainless steel wires 204, each having a diameter of 0,43 mm and being of type 302 stainless steel.
The wires are a~plied so as to co~plete one turn in a longitudinal distance of 25.4 cm. The next layer is a jacket of polyethylene 205 applied over the steel wires.
The ~jacket is a continuous e~trusion of higil-density polyethylene having a wall thicl~ness of 0.69 mm, with the outside diameter being 9.78 mm. A spunbonded polyester tape 206 ls then applied, Wit]l the tape being 2.54 cm wide by 0.2 mm thiclc. The tape is applied in a longitudinal manner, resulting in a gap Or approximately 5.6mm, Fourteen stainless steel wires 07 comprise the next layer and are wound with A lay in t~le opposite directio~ as the preceding wires, completing one turn in 38.4 cm, The wire are type ~02 stainle~s stsel, having a diameter of 0,43 mm, A jaclcet of polyethylene 208 forms t}le next layer, wherein the steel wires are incor?orate~ into the wall $hicknes~ of the polyethylene. The thickness of this Jacket i~ 1~02 mm, with an outside diameter of 12.2 mm. Thl~ cable core ha~
substantially zero torque response.
~5 This cal~le core was tested in tlle experimeJltal setup shown in FIG. 1 to ~eterllline the value of N. A
length of the cable core sufficient to provide a gauge 38 length of ~ppro~imately 297 cm ~117 inc~le~) was suspended ~ ~ 7609 ~

across rollers ~5, as sho~n. The (liameter of the cable was measured at ap~l^o~imately the midpoint, with an Instron transversc strain sensor. A stress was then applied to the cable U~illg the winch to result in an elongation of the cable (~L ) of 0.79 mm (1/32 inch). The transverse strain sensor ~Yas again used to measure the diameter to determine the value o~ aRc. The a~ial stress a~plied by the winch was inerease(l to prodllce an additional elongation of 0.79 mm (1/32 inch), and the measurements repeated. This procedure was accomplished for 24 incremental stress values, producing a total elongation of 1.91 cm (3/4 inch).
bLc ~ c For each increment, the values f L and R were determined. When the complete set of data points was obtained, the vaiues of N and K were determined by a "least squares fit'l method for equation (5). Three separate series of elongations, eacll starting from appro~imately zero stress, were accomplished. The average value of N uas thereby determined to be 0.42. The value of the lay angleB was thereby calculated from equation (4) and determined to be approximately 57 degrees. For the ~bove cable core, this results in 12 ~urns per foot ~0,30 meter) ~or the helical ~rmour layer on the core, A suitable armour layer comprises two helical steel wraps applied at ~ lay angle of 57 degrees. Each wrap has the following characteristics: 16.3 mm width, 0.~27 mm thicknes~, overlapped 5.59 mm on eac}-l si(le. These wraps can be applied in two partially overlap~ed portions 209, 210, a5 shown i~l FIGo 2, in order to ensure no ~aps occur during flexlng of the cable ~lowever, they are still considered to be a single helical element for the purposes Of the present invention ~ecause they have the same lay ~ngle, the same "sense" of the lay (i.e., both have right-hand lay or left-hand lay); and substantially the same distance from the center Or -the core~ l~ather tha~l alternatill~ the overlapped edges as shown in FIG. 2, one of the wraps can 38 be applied so as to cover the other ~rap at both edges.

1 1'~609~

Additiona1 wraps can l)e similarl~ ~rovil~ed and still be considered a single helical elelnellt. ~inally, a jacke-t of high-dellsity polyet~y1ene 211 can ~e extruded over the helical armour layer. The re~ulting cable has a substantially zero torque response.
To determine the torque response of a cable, a simple approximation is to measure the unrestrained twist of the cable wllell stressed. A cable can be hung vertically and 6tressed l)y means of a weight. For the purpose of the present invention, a cable is considered to have substantially ~ero torque response when the cable twists less than 3 turns for a lO0 meter (328 foot) vertically suspended len~th when a l percent axial strain is applied.
This criterion can be scaled for other lengths and other twist~ accordingly; e.g., a twist of less than 0.3 turns for a lO meter length when a l percent axial strain is applied. An alternative measuring method which yields substantially similar information is to measure the torque of the cable when tensioned in a twist restrained condition and divide it by the torsional stiffness of the cabls. For example, the experimental setup of FIG l can be utilized~
with a torque-measuring device inserted in place of the load cell. A suitable torque transducer, manufactured by the Vibrac Corporation, is the ~lodel T~l6Q0 ~tntic transducer. For a given strain, the torque in newton-meters i8 determilled, Next, the torsional stif~ness of the same length of cable is determined according to techniques known in the art The strain rate o~ torque divided by the tor6ional stiffness yield~ a figure of merit having the units turns par meter per unit of strain~ By this measurement procedure, a cable having a figure of merit of less than 3 turns per meter per unit of strain i~
considered to have a sub~tantlally zero torque re~ponse for the purpo~e of the present invention. In ~ome ca~es, a value of less than l turn per meter per unit of ~train c~n be achieved in commercial practice using tlle present technique 38 In modern cal~le manufacturing operations, It Ls I 1 76(~9~
~, tyl)icfllly llo~sible to ol)t~ a helical lay angle within ~1 degree of tlle (lesigll vallle. In the cable of the ahove -EXaml)le, tlliS correspon(~s to ~1/2 turn per foot (0.30 meter) difference from tlle (lesign value of 12 tu~ns per foot (0.30 meter).
lIavin6 thus determined tl~at a substantially torque-free single helical layer can be applied, it can be seen that other techniques can be used to find the proper lay angle. The most straightfo~ard technique is to s-mply vary the lay angle, by providing more or less turns per foot of the helical ele~ent to a co~e, and test the cable torque response~ In this manller, a substantially zero torque response helical layer can be obtained.
Normally, the core on which the torqlle-free helical layer is applied also has substantially zero torque response~
Thus, the resulting cable has substantially zero torque response. However, it is possible to apply a zero torque response helical layer according to the present techni4ue to cores that do not have a substantially zero torque response, The resulting cable, if twist restrained, will then have a torque response that is substantially the same as the core prior to applying the llelical element. Thc value of N ir~ equation (Il) can also be determined by other techniques than the test metllod described, For example~ if ~5 the core is a substantially isotropic incom~ressible ma-teri~l 5 the value of N can be theoretically calculat~d, being 0-5- With non-isotro~ic core material, the abov~
experimental metllod i9 advantageously used to evaluate N, especially when N di~fers from the above theoretical value by greater than 10 percerlt; i.e., ~rhen N is less than C~,4$
or greater than 0.55~ llelical layers other than armour layers can also advantageously be al)plied, For example, tape layers or st~an~ed layers can be applied. All such variations and deviations througll whicll the present invention has advanced the art are considered to be within the scope of the ~resent inventionO

3~

Claims (3)

Claims:

1. A cable comprising a substantially cylindrical core, the core having a torque response when a length of the core is stressed in axial tension, and a helical element surrounding the core with a substantially constant lay angle .theta., in which the lay angle .theta. is chosen in accordance with the formula .theta. = tan -1[N -1/2]
where wherein Rc is the radius of the core Lc is the length of a segment of the core, .DELTA.Lc is the change in the length of said segment produced by the application of a first axial stress to the core, and .DELTA.Rc is the change in the radius of the core produced by application of said first axial stress whereby the torque response of the helical-element surrounded core is substantially the same as the torque response of the core.

2. A cable as claimed in claim 1, in which the lay angle of the helical element is within about ? 1° of the angle .theta. defined by said formula.
3. A cable as claimed in claim 1, in which the difference in the torque response between the helical-element-surrounded core and the core produces at most a 3-turn twist difference between a 100 meter long segment of core stretched by 1% and a 100 meter long segment. of helical-element-surrounded core stretched by 1%.
4. A cable as claimed in claim 2, in which said Lc, .DELTA.Lc, Rc, and .DELTA.RC are determined by steps comprising suspending a segment of the core, measuring the length of said segment or a portion thereof, measuring the diameter of said core, applying a stress to said segment to obtain a strain in said segment, measuring the resulting change in said length, and measuring the resulting change in said diameter.
5. A cable as claimed in claim 4, in which said core is not substantially incompressible, whereby the value of N differs from 0.5 by greater than 10 percent, being less than 0.45 or greater than 0.55.

6. A cable as claimed in claim 4, in which said measuring is accomplished a multiplicity of times for a multiplicity of strains produced by a multiplicity of stress increments, and thereafter using a least-squares fit of the resulting values to obtain the value of N from the equation:
.epsilon.r = -N .epsilon.L + K (5) where .epsilon.r is the radial strain of the core, .epsilon.L is the axial strain of the core, and K is a constant.
7. A cable as claimed in any of claims 1, 2 or 3, in which said cylindrical core produces a substantially zero torque response when axially stressed so that said cable comprising said helical element and said core produces a substantially zero torque response when axially stressed.
8. A cable as claimed in claim 1, 2 or 3, in which said cylindrical core comprises one or more lightguides.

3. A cable as claimed in claim 1, 2 or 3, in which said helical element is a metallic armour element.