US1873798A - Method of and means for suspending and protecting aerial cables - Google Patents

Description

T.'VARNEY Aug. 23, 1932,.

ITHOD 0F AND MEANS FOR SUSPENDING AND PROT-ECTING AERIAL CABLES 2 sheets-smet 1 Filed Jan. 27, I928 Aug.' 23, 1932. 1 VARNEY 1,873,798

ETHOD '0F AND MEMS FOR SUSPENDING AND PROTECTING AERIAL CABLES Filed Jan. 27, 1928 2 Sheets-Sheet 2 INVENTOR Patented Aug. 23, 1932 UNITED ASTATES PATENT OFFICE THEODORE VARNEY, OF PITTSBURGH, PENNSYLVANIA, ABSIGNOB T0 Alom OOI- PANY 0F AMERICA, 0l' PITTSBURGH, PENNSYLVANIA, A CORPORATION Ol' Im- SYLVANIA METHOD 0F AND MEANS FOB SUSPENDING AND PBOTFHING AERIAL CABLES Application led January 27, 1928. Serial ll'o. 249,984.

This invention relates to the art of suspending cables, .and particularly to the art of suspending electrical power transmission line conductors, wires, or cables employed in such lines or systems.

The present invention contemplates a new and improved method of and means for preventing damage to transmission line conductors, wires, or cables, suspended in the air, due to vibrations set up therein caused by the action of the wind or other disturbances.

When a wire or stranded cable, suspended between substantially horizontal supports is subjected to a transverse wind, it is set into vibration, and, if the velocity of the wind, its direction and uniformity have suitable values, these vibrations will become regular and there will appear throughout the span a certain number of stationary points or nodes k with vibrating loops between.

These vibrating loops occur in a vertical plane and have the shape of sine waves. These loops in the main body of the span do no harm and do not produce any undue stresses in the conductor. However, at the point of support of the span, the vibrating loops sometimes produce destructive effects.

Inall cases of which I am familiar, where broken wires or strands of a conductor have occurred, the breakage has been at or near a point of support or a badly made joint. Since no damage occurs in the loops away from the point of support, it follows that the bending at the middle of the loop is not suiiciently sharp to cause trouble, and it is also to be observed that there is no bending at the nodes in the span. The support, however, being necessarily more rigid than a node in the span, injurious bending sometimes occurs at this point.

An important object of the present invention is to provide means at the point of support of such character that injurious bending will be prevented. I have discovered that if the radius of curvature of the wire or conductor at the point of support is prevented from bein any shorter than the radius of curvature 1n the middle of a loop, such breakage will be prevented, and have, therefore, devised a method of and means for accomplishing this end.

According to the present invention, it is proposed to rovide on the cable or conductor me, at or a jacent to a point of support, or at other recognized critical points, a vibration damper or stiifening means, preferably inoludlng a layer of cylindrical wires, rods, or

other similar elements, each having as great a cross section as may be necessary to the end that the combined moments of inertia of the Cable and 'mass at the point of support will limit the bending of the cable .at thatl point to a curvature having a radius at least equal to or greater than that of the curvature simultaneously existing in the cable at the middle point of a freely vibrating loop in the adjacent span.

The vibration damper should preferablyy include wires or other elements capable of being tightly wound on the conductor, so that j they will not slip longitudinally with respect to the cable as the latter is bent. The wires or other elements entering into the construction y of the vibration damper or stifening device sharply reflected at the ends of the vibration damper; to -provide a new and improved mass or damper structure having a central enlarged portion and tapered end portions; and to provide a structure of the character specified formed of individual wires or similar elements of a form to provide a damper having an enlarged non-twisted central portion and twisted tapering end portions adapted to be applied to the conductor line or cable.

Another important object of the present invention is to provide a structure of such character that it may be readily applied to the conductor or transmission. line in the field.

Other objects of the present invention will be pointed out in or become apparent from a consideration of the. following description taken in connection with the accompanying drawings.

In the said drawings:

Fig. l is a diagrammatic view, showing two adjacent vibrating loops as they occur in the span;

Fig. 2, a diagrammatic view showing an enlarged View of half of a loop at one of the `supports of the span;

Fig. 3, a fragmentary diagrammatic view, particularly showing a transmission line conductor or cable of the charactery employed, with damper means applied thereto;

Fig. 4, a side elevational view, showing my invention applied to an insulator string, showing at one side of the clamp the wires before they are twisted and at the other side of the clamp the finished twisted wires;

Fig. 5, an enlarged View of a portion of the damper and support of Fig. 4, showing the clamp broken away;

Fig. 6, an elevational similar view, showing a completed damper structure applied to a .pin type insulator;

Fig. 7, a detail elevational view, showing an individual wire of the character employed in the construction of the damper; and

Fig. 8, an end elevational view, showing a form of clamp employed at the end of damper structure.

It is known from observation that, when the wind velocity is greater than thirty miles an hour, no destructive vibratory effects occur in transmission line conducto-rs of the sizes used in practice. As the wind velocity decreases, the loop lengths increase, and usually the amplitude increases. The worst conditions of bending at the point of support, and, consequently, the most destructive effects, occur when the loop length is comparatively short, combined with a relatively large amplitude. Sufficient data are available to indicate the shortest loop lengths combined with the maximum amplitude likely to occur in service for any particular size or material of conductor, when installed under a known condition of span length and tension.

It is possible, therefore, to make suitable assumptions for the value of L and A, and

then apply the methods described below for determining the suitable design of the damping device.

Referring particularly to the diagrammatic views, Figs. 1 to 3 inclusive, the letters shown thereon and as referred to in the mathematical equations set forth below, have the following meaning:

L=Loop length=half wave length.

A=Amplitude of complete vibration.

R=Radius of curvature at middle point of loop.

Q=Angle with axis at node vibrating loop.

F=Frequency of vibration in cycles per second.

V=Vel0city of transverse wave in feet per second.

P=Total tension in conductor in pounds (conductor not vibrating).

Z=Arclength of loop.

W'=\Veight per foot of conductor in pounds.

G=Acceleration due to gravity=32-2 feet per second.

K=Moment producing bending at support.

I Moment of inertia of complete cable and damper at support.

Ier-Moment of inertia at steel core of conductor.

Ia= Moment of inertia of aluminum part of conductor.v

Id=Moment of inertia of damper at support.

M=Virtual modulus of elasticity of complete conductor and damper.

M3=Modulus of elasticity of steel.

Ma=Modulus of elasticity of aluminum.

Md=Modulus of elasticity of damper material.

a=Defiector of a single portion of the mass having a lever arm h.

It may be shown by calculus that the length of the arc of' a sine wave, with relation to the straight line between the nodes, is as follows:

The length l for the small values of Q is so nearly equal to L that the tension of the cable may be assumed constant and equal to P. For all vibratory/conditions in electric transmission cables which have been recorded, Q is less than approximately 1/2 degree.

The following expression is generally accepted as correct under these conditions:

PG y -W- 2LF The equation of the curve in Fig. 1 is as follows: Y

point of freely The'following expression is obtained from differential calculus:

The value of R isobtained by calculus, as follows:

When the loop is vibrating freely in the span, away from a support, no bending occurs at (O) and the angle (Q) is produced by the balanced motion of the adjoining loops.

If a support occurs at (O) the angle (Q) becomes zero at the left of O tor at the right of (O) is bent into a curve, alternately concave upward and downward. Although the angle (Q) is small', the bending produces very considerable fibre stresses which, added to the direct tension in the cable, have produced breakage.

In Fig. 2, a circle of radius (R) is drawn tangent to the axis of (X) as the origin (O) At a point (B) a tangent is' drawn to this circle parallel to (OC). This tangent makes an angle (Q) with` the axis of The arc OB subtends an angle at the center equal to (Q) and the angle (BOE) equals The arc length (OB) 4is obtained las follows:

Chord (OB)=arc (OB), very nearly, since (Q) is small, Q

Since (Q is small (X may betaken etpial to (O and substituting this value of Arc (0B) in equation (2) and the conduc-` sion lines.

the bending as follows KA=P BU=P(Y-Y), very nearly.

In order to satisfy these conditions, the following relation must obtain: IM=RK.

In order to satisfy this condition, (I) must usually be greater than the value corres onding to the wire itself. This Dvalue may e obtamed by a reinforcing wrapping or damper, the proportions of which may be determined from the following expression:

RK=IM=IsMs+IaMa+IdMd.

These relations are apparent from Fig. 3, remembering that the deflections a for the several parts of the conductor and damper are the same for the same lever arm h.

, The cable, as illustrated by Fig. 3, is composed of a steel core and an aluminum envelope.V It is to 'be understood, however, that the-cable may consist entirely of any single metalor material or any combination moment at (O) is yrepresented `of materials. Also, the damper may consist of an suitable material.

Re erring now particularlyto Figs. 4 to 8 inclusive, 5 designates a portion of a conductor employed for electric powerftransmis- The particular preferred construction of this conductor is illustrated by Fig. 3, and has heretofore been described. 6 designates generally the yvibration damper. As illustrated, the damper structure or device includes a plurality of wires, rods, or other similar cylindrical elements 7, each of the individual wires being of tapered form, or formed with a central non-tapered middle portion 8 of the desired cross sectional area and gradually tapering end portions 9. These elements 7, of sufficient number and diameter, are superimposed upon the conductor or cable at or adjacent to its point of support to completely surround it, said elements being arranged about the cable or conuctor, parallel with its axis. Then a clamp device 10, of any preferred design, is applied to the central portionof lthe elements and brought into binding engagement therewith. As shown. the clamp is a divided one,

with the parts thereof held together by memsupported by an arm 15 of a transmission 7- tower not shown.

It is my intention to have those portions of the elements 7 located within the clamp device 10, and for a short distance at either end of the said clamp 10 arranged parallel with the axis of the conductor and untwisted. In other words, in order to provide a simple and practical method to accomplish the necessary stifl'ening effect, I apply to the conductor a layer of the said elements 7, each having as great a cross section as may be necessar to the end that the entire cable with its a er of elements 7 at this point will have a su cient moment of inertia to resist bending to the desired extent. I have arranged or applied the said elements 7 to the cable or conductor in such a manner as to provide the necessary increase in moment of inertia and to produce a curvature at the point of support no more harmful than at the middle points of the loops in the adjacent spans.

The moment of inertia of a bundle of wires is equal to the sum of the moments of the individual wires about their respective neutral axes. If the wires are all under tension, the moment of inertia of the bundle may be taken as the sum of the moments of the individual wires about the neutral axis of the bundle.

In a conductor, the wires are usually arranged in a spiral, Vwhich tends to equalize the stresses due to bending. On the other hand, friction between the wires counteracts this tendency to a large extent and at the support where the weight of the span plus the pressure of the clamping bolts increases the friction, it is only safe to assume that the conductor behaves in the manner of a solid member. If the damper can be made to adhere closely to the conductor and not be subject to longitudinal slipping, it will give a maximum amount of resistance to bending. e

This clamping action of the elements 7 on the conductor may be eifected by twisting the wires or elements 7, and particularly the tapered portions 9 thereof. In ractice, a suitable tool is employed for e ecting lthe twisting of the tapered portions of the elements 7.

By the method of applying the elements 7 in the manner above set forth, it will be understood that a vibration damper having an increased mass at its center and provided with tapering twisted end portions will be produced. As the diameter of these elements decreases towards the ends, the pitch angle which the twisted portions make with the axis of the cable increases. This gives a tapering moment of inertia and mass'at each successive transverse section of the conductor. The effect of the damper is not only to increase the radius of the bend at the support, but also to reduce the amplitude, frequency and loop length which stilll further reduces the libre stress due to bending.

It is to be observed that the necessary length of the damper on each side of the support is given by X because, for any distance further away from the support than X', the moment of inertia of the cable itself is suiicient to prevent undue bending.

After the elements 7 have been applied to the conductor in the manner above described,

suitable clamps 7a Vmay be applied to the ends of the damper, and the en s of the elements or wires 7 may be bent thereover, as particularly shown by Fig. 6 of the drawm n the form of Fig. 6, Ihave shown an application of my invention to a pin type insulator made up of a plurality of insulator shells 16, mounted on a supporting pin 17, extending upwardly from a cross arm or other structural element 18 of a tower; In this latter form of my invention, the same idea of having the damper structure larger at the middle and gradually decreasing towards its ends is carried out; also, the feature of having the elements 7 at their middle portions arranged parallel with the axis of the cable or conductor and untwisted, whereas the ends are twisted around the conductor as in the other form. In the form of Fig. 6, 19 designates a means for securing the cable at its point of support to the pin type insulator.

Attention is called to the fact that in both forms of my invention the damper may be readily applied in the field with a minimum of labor and time.

I claim:

1. The method of suspending aerial cables to prevent damage due to vibrations set up in the cable, whlch consists in lapplying to the cable at its point of support a plurality of elements with the central portions thereof disposed parallel with the axis of the cable and untwisted and with the end portions thereof twisted about the cable.

2. The method of suspending aerial cablesv to prevent damage due to vibrations set up in the cable, which consists in applying to the ros to prevent damage from vibrations set up in the cable, which consists in applying to the cable at its point of support a plurality of elements having central non-tapering portions and tapering end portions, securing the central portions in place on the cable, and then twisting the end portions about the cable.

44c. The combination with an aerial cable, of means thereon for damping vibrations therein, said means including a plurality of wires having central non-tapering portions and tapering end portions, said end portions being twisted around a length of the cable to be protected.

5. The method of. protecting an aerial cable from vibrations, which consists in arranging a plurality of tapered elements `upin Lavande len hwise about a relatively short 'length of t e cable at the point to be protected, and

then twisting the ends of the taperednele-y ments about the cable. 6. The method of forming a vibration damper on an aerial cable, which consists in placing a plurality of wires having central non-ta ring portions and tapering end or'- tions a out a relatively short length o the 1 cable parallel with the axis of the cable,

clamping the central portions of the wires to the cable, and then twisting the tapered end portions around the cable.

7. A vibration protector for cables comprising a plurality of individual elements each having a central non-tapering middle portion and tapering twistable end portions,

and clamp means for the central portions.

8. A vibration protector for cables com- W prising a plurality of individual elements each having a central non-tapering middle portion and= tapering twistable end portions, clamp means for the central portion, said tapering end portions characterized in that they may be twisted around the cable, and means for securing ends of the tapered portions to the cable.

9. The method of suspending aerial cables to prevent damage due to vibrations set up in the cable which consists in securing to the cable at its point of support a tapering contacting mass so elongated and proortioned to the length and amplitude of a reely vibrating loop simultaneously existing in an adjacent span, moments ofinertia of the cable and the mass at any section betweenthe point of support and the outer end of the mass will limit the bending of the cable at that section to a curvature substantially equal to the curvature simultaneously existing in the cable at the middle point of the freely vibrating loop. f-

10. The method of suspending aerial cables to prevent damage due to vibrations set up in the cable which consists in securing to the cable at its point of support a taperr ing contacting mass so elongated and proplortioned to the length and amplitude of a eely vibrating loop lsimultaneously existing in an adjacent span, that the combined moments of inertia of the jcable and the mass at an section between the point of support and t e outer end of the mass will limit the bending of the cable at that section to a curvature substantially equal to the curvature simultaneously existing in thecable at the middle point of the freelyl vibrating loop, said mass being formed of individual tapering elements a plied to the cable and intimately connecte therewith by twisting portions of the tapered elements thereon.

11. The method of suspending aerial cables to revent damage due to vibration set e cable, which consists in securing that the combined l to the cable at its pointr of support a tapering contactin mass so elongated and proortioned to t e length and amplitude of a eely vibrating loop simultaneously existing in an adjacent span, as to limit the bending of the cable adjacent the point of'support to a curvature substantially equal to the curvature simultaneously existing in the cable at the middle point of the said loop.

12. The method of suspending aerial cables to revent damage due to vibration set up in t e cable, which consists in securing tothe cable at its point of support a tapering contacting mass so elongated and proportioned to the length and amplitude of a so freely vibrating loop simultaneously existing in an adjacent span, as to limit the bending of the cable adjacent the point of support to a curvature substantially equal to the curvature simultaneously existing in the cable at the middle point of the said loop, said mass comprising a plurality of elonated elements closely wound about the conuctor and presenting portions of varying ross section.

In testimony whereof I afiix my signature.

THEODORE VARNEY.

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