United States Patent: 3553350

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( 16709 of 16709 )

United States Patent	3,553,350
Rawlins	January 5, 1971

SELF-DAMPING CABLE

Abstract

A self-damping conductor structure in which vibration energy is dissipated by the impact of an inner core with an outer mantle, the core being loosely supported within the mantle. Means are provided within the conductor for varying the spacing between the core and mantle at recurring intervals along the length of the conductor. When the core and mantle are suspended with different tension-to-weight ratios, the relative positions of the lines of action of the tension in the core and mantle vary cyclically along the length of the conductor between the supports permitting the core and mantle to contact each other with cyclically varying pressure along the conductor.

Inventors:	Rawlins; Charles B. (Massena, NY)
Assignee:	Aluminum Company of America (Pittsburgh, PA)
Appl. No.:	04/865,306
Filed:	October 10, 1969

Current U.S. Class: 174/130 ; 174/42

Current International Class: H02G 7/00 (20060101); H02G 7/14 (20060101); H01b 005/10 (); H02g 007/14 ()

Field of Search: 174/42,128,130,131,129,28,68,70,70.4

References Cited [Referenced By]

U.S. Patent Documents


3378631	April 1968	Edwards
3445586	May 1969	Edwards et al.

Foreign Patent Documents


762,534	Jan., 1934	FR

Primary Examiner: Askin; Laramie E.

Claims

I claim:

1. A self-damping conductor for suspension from spaced supports, the conductor comprising:

a hollow conductive mantle and a conductive core loosely disposed therein;

means within said conductor varying the relative positions of the lines-of-action of the tension in said mantle and core at spaced intervals along the length of said conductor when said core and mantle are suspended under tension between supports; and

said varying lines of action providing corresponding varying pressures of contact between said mantle and core in a lengthwise direction along said conductor.

2. The conductor described in claim 1 in which the means for varying the relative positions of the lines of action in the mantle and core includes reduced inner diameter portions of the mantle at locations spaced along the length of the conductor.

3. The conductor described in claim 1 in which the means for varying the relative positions of the lines of action in the mantle and core includes spacer means disposed around the core at locations spaced along the length of the conductor.

4. The conductor described in claim 1 in which the core comprises a plurality of helically wound strands of material extending generally in a direction lengthwise of the conductor, and the means in the conductor for varying the relative positions of the lines-of-action of the tensions in the mantle and core comprises an eccentric distribution of tension provided among the plurality of strands.

5. The conductor described in claim 4 in which the eccentric distribution of tension is provided by a group of adjacent strands of the plurality of strands having a stress-strain characteristic different from that of the remaining strands of the plurality.

6. The conductor described in claim 1 in which the core comprises at least two layers of helical wound strands of material with an unequal distribution of tension provided among the strands in both layers, the length of lay of the strands in one layer being different from that of the other layer.

7. A self-damping conductor adapted to be suspended on spaced supports, the conductor comprising:

a hollow conductive mantle and a conductive core loosely disposed therein and having means within the conductor for varying the spacing between the core and the mantle along the length of the conductor at intervals spaced more closely than the supports on which the conductor is to be suspended so that when suspended under tension, the relative lines of the tension in the mantle and core will vary cyclically along the length of the conductor between supports.

8. The conductor described in claim 7 in which the means in the conductor for varying the spacing between the mantle and core comprises a spacer disposed between the mantle and core and extending in a lengthwise direction thereof, said spacer being eccentric in relation to the conductor, the eccentricity of spacer rotating about core with lengthwise position of the spacer.

9. The conductor described in claim 7 in which the means in the conductor for varying the spacing between the mantle and core comprises a sheath of material disposed about the core and between the mantle and core, said sheath having a cyclically varying thickness along the length of the core.

10. The conductor described in claim 7 in which the means in the conductor for varying the spacing between the mantle and core comprises a sheath of material disposed on the core, said sheath having an irregular cross section that rotates about the core with the lengthwise position of the sheath along the core.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to overhead transmission lines, and particularly to an improved self-damping electrical conductor in which a center core is loosely supported within an outer mantle.

As is well known in the art, electrical cables and conductors, such as transmission lines, which are supported between poles or towers, are subject to continuous mechanical vibrations due mainly to the effect of air currents moving across the cable or line wherein the natural period of vibration, or harmonic thereof, of the cable coincides with the period of vibration caused by the air currents. Such vibrations cause a continuous oscillating bending moment at the support of the cable which fatigues the metal thereof at the support with consequent eventual failure of the cable at the support.

A prior art cable structure designed to provide a high order of vibration damping, and one which the present invention is an improvement thereon, consists essentially of a core disposed in a hollow outer mantle, the internal diameter of the mantle being larger than the outer diameter of the core so that an annular clearance exists between them.

In use, the prior art cable or conductor is supported between suitable support structures (i.e., towers or poles) with different tension-to-weight ratios applied to the core and mantle which allows the core and mantle to seek different sags. In this manner, the top or bottom surface of the core is physically pressed against the bottom or top surface of the interior of the mantle depending, of course, on which of the two (cable or mantle) has the higher tension-to-weight ratio. The physical contact of the core and mantle is at one, longitudinally extensive location between two consecutive supports supporting the conductor.

The inequality of tension-to-weight ratios is necessary, according to the teachings of the prior art, to obtain a high order of self-damping within the cable. That is, at a given frequency of cable vibration, the unequal tension-to-weight ratios provide the core and mantle with different vibration wavelengths so that they interfere with each others' vertical movements. In this manner they are caused to impact against each other and thereby dissipate vibration energy by virtue of the impacts.

In the prior art conductor, the pressure at which the core and mantle contact each other due to their unequal tension-to-weight ratios, creates a problem which the present invention overcomes. For small amplitudes of vibration and for low frequency vibrations, i.e., on the order of 5 Hertz, the inertia forces produced by the acceleration of the conductor are not large enough to overcome the force of the contact existing between the core and mantle. The core and mantle, therefore, do not lose contact so that impact between the two is prevented. Thus, in effect, the prior art conductor has a substantial acceleration threshold that must be exceeded before its high order of self-damping comes into play.

Another prior art conductor structure providing vibration damping is shown in U.S. Pat. 3,204,021 issued to H. W. Adams on Aug. 31, 1965. For damping purposes, Adams employs a random distribution of masses, having a substantial weight factor, disposed along the length of the conductor. Such masses add substantially to the overall weight of the conductor, thereby increasing the weight that the conductor and its supporting structure must carry without the attendant advantages of substantially strengthening the conductor or increasing its current carrying capabilities.

BRIEF SUMMARY OF THE INVENTION

The present invention eliminates or substantially reduces the threshold of acceleration by providing varying recurring pressures of contact between the core and mantle along the length thereof by providing one or both of them with a wavy or irregular configuration along the length of the conductor, the wavelengths of the variations being of substantial length for reasons explained hereinafter. These varying contacting pressures have corresponding varying acceleration thresholds, the regions of minimum pressure having minimum threshold values so that only correspondingly small inertia forces and amplitudes of conductor vibration are necessary to initiate relative movement between the core and mantle and thus core-mantle impact. Regions of zero pressure will be activated when the slightest movement or vibration occurs in the conductor. In this manner, i.e., by immediate impact of the core and mantle when the conductor begins to vibrate, the conductor is rendered vibrationless before the metal thereof is worked, and thereby weakened, at the locations of the conductor supports.

The waviness in the core or mantle (or both) providing the varying recurring contacting pressure between the core and mantle may be provided by enforcing a variation in the relative distance between the lines of action of the core and mantle, the lines of action being the loci of the center of effort of the tension that each (the core and mantle) must carry. Enforcing such a variation can be accomplished by interposing a member or material of a predetermined thickness between the core and mantle at spaced apart locations along the length of the conductor between conductor supports, or by providing an unequal distribution of tension among helically wound strands of metal comprising the core or mantle or both. In either case, vibration damping is accomplished with little or no addition of weight to the conductor.

THE DRAWINGS

The invention, along with its objectives and advantages, will be more fully understood from the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic view of an electrical conductor suspended between supporting structures and constructed in accordance with the principles of the present invention;

FIG. 2 is a longitudinal cross-sectional view of a preferred embodiment of the invention;

FIG. 3 is a cross-sectional view taken along lines III-III of FIG. 2;

FIG. 4 is a longitudinal cross-sectional view of another embodiment of the invention;

FIG. 5 is a longitudinal cross-sectional view of yet another embodiment of the invention;

FIG. 6 is a cross-sectional view of another embodiment of the invention;

FIGS. 7a and 7b are cross-sectional views of a core structure illustrating another embodiment of the invention;

FIG. 8 is a side elevation view of the core shown in FIG. 7;

FIGS. 9a through 9e are cross-sectional views of a core showing the changing positions of core forming strands disposed in a helical lay;

FIG. 10 is a diagrammatic cross-sectional view of a self-damping conductor in which certain component distances within the conductor of the invention are illustrated.

PREFERRED EMBODIMENTS OF THE INVENTION

Specifically, FIG. 1 shows diagrammatically a conductor 10 suspended between supporting towers or poles 11 and 12 by suitable supporting hardware not shown. The conductor 10 comprises essentially an outer mantle 14 and an inner core 15 loosely disposed therein to provide a space 16 therebetween. As explained earlier, the core and mantle are strung between supporting structures, such as 11 and 12, with unequal tension-to-weight ratios so that the core and mantle physically contact each other.

In accordance with the invention, the core or mantle (or both) are provided with a wavy or undulating configuration in a direction lengthwise of the conductor and in a recurring manner between the supports 11 and 12 as shown in FIG. 1. In this manner the core and mantle contact each other with a force or pressure that varies in a similarly recurring manner when the core and mantle are strung between the supports with unequal tension-to-weight ratios. The waviness is provided by varying the relative lines of action or the centers of effort of the tension in the core and mantle, and this is accomplished by enforcing cyclical variations in the relative displacement of the core and mantle in a variety of ways as explained hereinafter in reference to the different embodiments of the invention.

If the core 15 and the mantle 14 were suspended with equal tension-to-weight ratios, the core would hang within the mantle independent of the mantle and not touching the mantle. Viewed from the side, the core and the mantle would appear as congruent catenaries without locations of physical contact even with one or both of the members having a wavy configuration. A slight decrease in the tension of the core 15, however, causes it to drop down within the mantle 14 until the core rests upon the bottom inside surface of the mantle as shown in FIG. 1, the contact between the two being the low points of troughs 17 of the core's waves in an embodiment in which the core is the wavy member. Between the low points, the core has peaks 18 which have a clearance of approximately twice the amplitude of the core's wares as measured between the bottom of the core and the bottom of the mantle's inside surface.

If the tension in the core 15 is further reduced, the downward pressure of the core upon the mantle 14 is increased. This increased pressure occurs first of all at the low or trough contact locations 17. As the core's tension is further reduced, the sag in the core between the contact locations increases so that the core-mantle clearance at the wave peaks of the core is reduced, and the regions of contact 17 between the core and mantle are broadened in a direction lengthwise of the conductor.

Eventually, if the tension of the core 15 continues to be decreased, the clearance between the core and the mantle 14 at the original wave peaks 18 will disappear. At the moment of disappearance (and before), the pressure between the core and mantle will be zero at those peak locations. On either side of these locations, the pressure will increase gradually with distance, the pressure reaching a maximum at the original locations of the troughs 17. With this distribution of pressures in the conductor 10, there is essentially no threshold acceleration value that must be exceeded for self-damping action to take place. Regions of zero pressure will be activated into self-damping impact with the slightest degree of conductor movement or vibration. Further, the portions of the core that are free of contact with the mantle are free to resonate during conductor vibration thereby enhancing the impacting action of the core against the mantle.

It should be understood that mere waviness, or mere variation of contacting pressure is not sufficient to overcome the threshold problem. Most metal surfaces possess some measure of roughness so that contact between metal surfaces takes place at a multitude of asperities, the contacting pressure at different asperities being different. Further, helically stranded conductors utilizing the customary round wires present somewhat wavy profiles due to their helical lay. An acceleration threshold exists despite these wavy conditions.

To be effective for self-damping purposes, the frequency of the variations in pressure along the conductor 10 must not be unduly large, i.e., the regions 17 and 18 of high and low (or zero) pressure must be relatively large and thus remote from each other. The reason for this has to do with the relationship between the velocity of conductor 10 vibration as a single unit and the relative velocity of impact between its component parts, namely, the core 15 and the mantle 14. The vibration velocity of the conductor 10 controls the rate at which vibration energy is imparted to the conductor. The relative velocity of impact controls the rate of dissipation of energy due to impact. The objective is to have the rate of energy dissipation exceed the rate at which energy is received from the wind, each of these energies increasing rapidly with its associated velocity. It is therefore desirable that the ratio of the velocity of impact to the velocity of conductor vibration be as large as possible.

The relationship of the distances between high and low contacting pressure locations 17 and 18 between core and mantle and the ability of the core and mantle to impact against each other can be explained as follows: If the vibration of the conductor 10 causes accelerations and inertial forces large enough to separate the core and mantle in regions 18 of low or zero contact pressure, but not in the adjacent regions 17 of higher contact pressure, the magnitude that the separation attains during each vibration cycle will be strongly influenced by the distance dimensions of the higher pressure regions on each side of the low or zero pressure regions, i.e., if the higher pressure regions are too close to the low pressure regions, the stiffness of the core and mantle material and the degree of curvature they tend to assume due to their tensions and masses, do not permit large separations of the core and mantle in the low pressure regions. With small regions and short distances of peak separation, only small impact velocities are achieved. The distances between the locations of high and low contact pressure, therefore, must be relatively large, as mentioned above.

The effective approach to the threshold problem then is to provide the core and mantle or both with a physical waviness with each wave having a substantial wavelength, i.e., a wavelength in the order of feet and perhaps as great as 50 feet, the size of the wavelength for a given design or installation depending upon various conductor parameters and environmental conditions.

When the conductor 10 is strung in the field, vertical components of tension existing along the length of the conductor support the overall weight thereof against the force of gravity. A well-known mathematical derivation leads to the following equation expressing this phenomenon: ##SPC1## In this equation, T = the horizontal component of total tension of the conductor 10;

w = the total weight per unit length of the conductor;

x = is a horizontal component of distance along the cable;

y = the height of the conductor above some horizontal reference plane. The solution of this equation gives y as a function of x, and the shape of this function, when plotted, is that of a catenary. The function thus describes the shape of the curve in which the conductor hangs; however, the function applies precisely and only to the locus of the center-of-effort along which the tension acts in the conductor. This locus usually coincides with the geometric axis of the conductor but that is not always the case as will appear hereinafter.

For typical spans of conductor, the above equation can be approximated, without introducing any material error in the conclusions drawn, by the equation ##SPC2## This equation can also be applied individually to the core 15 and to the mantle 14. When it is applied to the core and mantle, an additional term "p" is included to account for the pressure of contact between them. The equations for the core and mantle thus read as follows: ##SPC3## where Tc and Tm are the horizontal components of the core and mantle tension respectively, and W.sub.c and W.sub.m are their respective weights per unit length. Since the pressure p between them results from the one supporting a portion of the other' weight, a term equal to this amount of weight is added to one equation and subtracted from the other. Here, p is measured in units of force per unit length along the conductor 10.

The fundamental methods for introducing variations in p with distance along the conductor can be discerned from these equations. Considering the equation for the core, for example, for p to vary, T.sub.c' w.sub.c' or d.sup.2 y.sub.c/ dx.sup.2 must vary, or they must vary in some combination. Variations in T.sub.c may practically be introduced by, for example, stringing the conductor initially with the same tension-to-weight ratio in core and mantle, and then releasing the core so that it slips back into the mantle. At the end of the core that was released, the tension will then be zero, while at the opposite end the tension will be significant due to the friction of the core sliding in the mantle. The core and mantle may then be seized together, by compression clamps, at several intermediate points, to prevent the inequality in tensions from dissipating.

Variations in weight (w.sub.c) may be practically introduced by, for example, intermittent application of a heavy covering to the core or to the mantle such as shown in the above mentioned Adams patent.

Providing variations in the contracting pressure p by varying tension (T.sub.c or T.sub.m) or weight (W.sub.c or W.sub.m) are not, however, economical approaches to the problem of reducing the threshold of acceleration necessary to separate the core and mantle for effecting immediate self-damping action. For example, clamping the core and mantle in the field is a laborious, time consuming effort, and thus a costly procedure. Similarly, adding spaced apart weights to a cable is inordinately expensive, since it usually requires interruptions of the stranding operation which is otherwise a continuous process. Further, spaced weights add to the overall weight of the cable, usually without the offsetting advantages of increasing the strength or current carrying capabilities thereof since such weights are only intermittently located along the length of the cable.

Thus, because of the disadvantages and difficulties encountered with varying the tensions or weight parameters along the conductor 10, the present invention is concerned with the variation of the curvature of the core (or mantle) i.e., d.sup.2 y.sub.c/ dx.sup.2 (or d.sup.2 y.sub.m/ dx.sup.2).

The method and means providing these variations in curvature are brought about by enforcing a varying separation of the core and mantle as indicated above in reference to FIG. 1. The actual structures for enforcing these variations are shown in FIGS. 2 through 10, it being borne in mind that the differential equations actually represent the lines of action or the centers of effort of the tensions that the core and mantle must carry.

In FIG. 2, variations in the curvature of the core 15 and the mantle 14 are provided by annular, reduced diameter portions 20 formed in the inside surface of the mantle, and spaced apart a substantial distance in a lengthwise direction of the mantle. Such portions function to raise sections of the core above the bottom surface of the mantle so that the core has peak locations 18 spaced apart from the troughs 17 which touch the mantle as previously described in connection with FIG. 1. In this manner, the core is made wavy, and the relative centers of effort of the tension in the core and mantle are thereby separated by recurring varying distances along the conductor 10 between its supporting structures 11 and 12 (FIG. 1).

The reduced diameter portions 20 may be provided in a variety of ways or with a variety of means. A simple method of providing the reduced diameter portions in a mantle formed of stranded wires would be to bend the innermost layer of strands 22 of the mantle in an inward direction as shown in the cross-sectional view of FIG. 3.

FIG. 4 shows another embodiment of the invention in which the core 15 is provided with spacers 24, the spacers being secured to and around the core at spaced apart locations along the length thereof. The spacers are preferably made of a material having a negligible weight factor, for example, a suitable plastic material. The shape of the spacers may be annular or cylindrical, and they may take the form of annular lumps provided by wrapping a tape around the core, for example.

The spacers 24 serve in the same manner as the reduced diameter portions 20 of the mantle 14, the spacers functioning to space the peak locations 18 of the core from the bottom surface of the mantle, and longitudinally from the troughs 17 in contact with the bottom surface.

Other spacing devices than those described thus far can be employed to space the mantle and core in a recurring varying manner in a lengthwise direction of the conductor 10. For example, the core can be covered with a continuous sheath of material 25 having a varying thickness dimension along the core as seen in FIG. 5. In this FIG., the line of action of the tension in the core is shown by a dashed line congruent with the axis of the core.

FIG. 6 shows, in cross section, an elliptical spacer in the form of a sheath of material 25A disposed about the core 15 in a manner similar to that of FIG. 5. A wavy line of action for the core is provided, however, by twisting the core during the stranding thereof so that the irregular elliptical cross section of the sheath rotates along the length of the core in the mantle. In this manner, the relative positions of the centers of effort of the core and mantle are made to change with the rotating disposition of the elliptical sheath.

Other spacing means may be employed in place of or in combination with the spacers shown in FIGS. 2 to 6 to separate the lines of action of the core and mantle in a recurring varying manner along the length of the core.

In a manner presently to be explained, the core or mantle or both can be made wavy without the use of the spacing members described above. This can be accomplished by causing an unequal and eccentric distribution of tension among helically wound strands of wire 26 (FIGS. 7 to 9) forming the core or mantle. In FIGS. 7 to 9, only the core 15 is shown for purposes of clarity. The unequal distribution of tension may be provided by stranding the core or mantle with groups of slack and taut strands, or by using groups of wires having different stress-strain characteristics, i.e., different temper, alloy or hardness characteristics. (In FIGS. 7 and 9 for purposes of illustration, the higher tensioned strands are indicated with a plus sign). When the core or mantle is tensioned in the field, the relatively slack or soft strands assume less tension than the taut or harder metal strands. This causes the center of effort of the tension in the overall core or mantle to shift away from the strands assuming less tension and towards the strands assuming the greater tension. The line of action of the tension thus moves away from the geometric axis or center of the core or mantle to the higher tension (+) strands. If the strands 26 are applied to the core in a helical lay, the locus of the geometric center traces a helix around the line of action. And since the line of action conforms to a catenary, as explained above, the geometric axis spirals around the catenary resulting in a waviness of the core or mantle as shown in FIG. 8.

It should be noted that the use of unequally tensioned, helically laid strands imparts a waviness in which each wave has a wavelength equal to the length of lay of the strands having the greater tension, a lay being the path of one complete cycle in the helix formed by a helically laid strand. If presently employed lengths of lay are used, this wavelength is too short to adequately longitudinally separate the regions 17 and 18 of high and low pressure existing between the core and the mantle. However, a wavelength of waviness that is greater than the length of lay can be achieved by employing two layers 27 and 28 of helically laid strands 26 with unequally tensioned strands in both layers, as shown in FIG. 7, and with the length of lay of the two layers being slightly different. The waviness of the complete core or mantle would be the superposition of the waviness that would occur with unequally tensioned strands employed only in the inner layer 28 of the two layers (Case 1) and only in the outer layer 27 of the two layers (Case 2). Thus, in either case, the geometric axis of the core or mantle would trace a helix around the line of action of the tension therein, the helix having a wavelength of the lay of the inner layer in Case 1, and of the outer layer in Case 2.

If the inner and outer layers 28 and 27 of the strands 26 had the same length and direction of lay, the superposition of the waviness attendant with each layer would result in augmented or diminished waviness depending respectively upon whether the higher tensioned strands of the two layers are together on the same side of the core as in FIG. 7a, or in opposition as in FIG. 7b. Either situation can be achieved, and the resulting degree of waviness would not change along the length of the core because the relative orientation of the inner and outer groups of higher tensioned strands would not change due to the equal lengths of lay.

Where the lengths of lay are made slightly different, however, the relative orientation of the inner and outer layers 27 and 28 change continually along the length of the core 15 providing regions of augmentation 30 alternating with the regions of diminution 29 of waviness as shown in FIG. 8. The wavelength of waviness is equal to the average of the lengths of lay of the two (inner and outer) layers. The wavelength of the alternation between augmentation and diminution, i.e., the amplitude of the waviness, is greater than the wavelength of waviness, and may be shown by the formula where L.sub.1 and L.sub.2 are the lengths of lay of the inner and outer layers 28 and 27 respectively. The wavelength of the alteration of waviness amplitude may be easily controlled over a broad range by proper selection of L.sub.1 and L.sub.2.

In the above illustration, the directions of lay of the two layers were the same and this resulted in the geometric axis of the core tracing a helical path about the line of action. The directions of lay may, on the other hand, be opposed. Consider the case where this is true and where the lengths of lay are equal. FIG. 9 shows a series of transverse cross sections of a core constructed in this manner, the cross sections being taken at intervals of a quarter of a helical lay length along the core. The series shows how the higher-tension (+) strands move around the geometric axis of the core with distance along the core. The series of views further shows that the direction of movement is opposite for the two layers 27 and 28 resulting in different relative orientations of the two groups of higher tensioned (+) strands at different locations along the core. The change in relative orientation is rapid, executing a complete cycle in one length of lay.

It is clear from FIG. 9 that superposition of Cases 1 and 2, in this instance, results in polarization of the waviness. Whereas Cases 1 and 2 individually show helical waviness, in superposition they augment each other in the vertical direction but tend to cancel each other in the horizontal direction. The cancellation may not be complete, but the difference in the vertical and horizontal amplitudes of waviness identifies the core 15 as having a degree of vertical polarization in its waviness. Had the core been constructed with the inner and outer groups of higher-tension strands adjacent to one another on the sides thereof, rather than at the top and bottom of the core, horizontal polarization would have resulted.

Since the lengths of lay of the two layers 27 and 28 were equal in the above discussion, the direction of polarization will not change with position along the core. If, however, the lengths of lay of the two layers are slightly different, then the plane of polarization will rotate with lengthwise position along the core 15, executing one complete rotation in the distance Such a core, viewed from the side, will present the same profile as the core having both directions of lay the same. It will have the shape illustrated in FIG. 8, having alternate regions 30 and 29 of high- and low-amplitude waviness in the vertical plane.

In use, the regions 30 of high-amplitude waviness will become the regions 17 of high pressure between core and mantle, and the low-amplitude regions 29 will be the regions 18 of low pressure.

The use of groups of higher-tension strands in two layers to obtain waviness variations of a relatively long wavelength, may be made in connection with mantles as well as with cores. Further, a long wavelength in the variation of pressure between core and mantle can be achieved with one of the layers (27 or 28) in the core and the other in the mantle. In addition, what has been accomplished with two layers may also be accomplished with three or more layers to obtain, either the same types of waviness, or types where the amplitudes and polarizations are modulated in more complicated ways. The practical result is the same, however, in that variations in the pressure of contact between core and mantle are brought about.

The above are examples of structures that enforce a variation, in the relative positions of the lines of action of the core and mantle along the conductor 10. In each case, it has been achieved by either interposing a structure of varying dimension between the two lines of action or by providing strands of wire forming the core or mantle with different tension. The function of such structures may be illustrated by dividing the separation of the lines of action into the following component distances.

1. The distance between the core's line of action and the geometric center of the core.

2. The distance between the geometric center of core and the surface participating in contact with the mantle.

3. The distance between the surface of the mantle participating in contact with the core and the geometric center of the mantle.

4. The distance between the geometric center of the mantle and the line of action of the mantle. These component distances are illustrated in the cross-sectional view of FIG. 10, where a is the location of the core's line of action, b is the core's geometric center, c is the point of contact between core and mantle, d is the mantle's geometric center, and e is the location of the mantle's line of action.

The cross section shown in FIG. 10 is illustrative only; in practice, the construction of the core and mantle would be more complex. The geometric centers of core and mantle will, in most cases, be somewhat arbitrary, since axial symmetry will be lacking in many constructions. In these cases, any reference point, fixed with respect to the geometry of the cross section of the member involved (core or mantle) may be employed for present purposes as the geometric center.

A review of FIGS. 1 through 10 shows that any one of these component distances may be varied to achieve a variation in the relative positions of the lines of action of the core 15 and mantle 14. Component 1 was varied in FIGS. 7 and 9; component 2 was varied in FIGS. 2 to 6; components 3 and 4 would be varied by applying the principles of the FIGS. to the mantle.

The basic construction of the present invention is, then, a loose-core conductor 10 in which one or more of the above component distances, separating the lines of action of a core and mantle, is caused to vary along the length of the conductor by varying the structure that fixes the component distance. The distance over which the variation generally repeats itself along the conductor must be large so that substantial separation of the core and mantle can be obtained in the low pressure regions when the conductor begins to vibrate. In this manner high velocity impact between the core and mantle in these low pressure regions can be accomplished to dissipate vibration energy and thereby effect rapid, efficient self-damping action in the conductor.

The self-damping capabilities of the conductor of the invention were demonstrated in tests conducted to compare such capabilities with those of a prior art loose-core conductor, i.e., a conductor without the cyclic variations of contacting pressures between the core and mantle. In the tests, a span of suspended conductor was externally excited or vibrated by the force of a vibration motor, and the rate of decay of vibration was measured when the exciting force was removed. In this type of test, the significant parameter is the logarithmic decrement which is the natural logarithm of the ratio of the amplitudes of two successive cycles of vibration. To achieve satisfactory self-damping in the conductors in question, the logarithmic decrement must exceed .005; when it exceeds this value, the dissipation of energy within the conductor exceeds the rate at which energy can be received from the wind to sustain conductor vibration.

Two series of tests were conducted: one on the prior art conductor, and one on the same conductor provided with spacers 24 of type described above in connection with FIG. 4 of the present disclosure. Logarithmic decrement was measured at various frequencies of vibration, with the conductor under various tensions, and with various divisions of tension between the core and the mantle. The following FIGS. are some examples of the results of the tests.

For a conductor without means for cyclically varying the contacting pressure between the core and mantle, and with conductor tension at 5,000 pounds, of which 1,100 pounds was on the core, and with a frequency of about 15 1/2 Hz, the logarithmic decrement was .016 for free loop amplitude of vibration in excess of .06 inches peak-to-peak. This corresponds to a maximum acceleration of .73 g. For lower vibration amplitudes, and hence accelerations, the logarithmic decrement was .002. Thus, an amplitude of 60 mils is required before self-damping becomes sufficient to prevent a further increase in amplitude.

When the conductor was provided with spacers in a manner similar to FIG. 4, but with all other parameters remaining the same (as above) the logarithmic decrement was .012 for all vibration amplitudes down to a free loop amplitude of .006 inches peak-to-peak which corresponds to a peak acceleration of approximately .08 g. Thus, self-damping action in the present invention is effective at much lower amplitudes of vibration.

At a lower test frequency of about 7 Hz, and at the same tension and division of tension as above, the prior art conductor had a logarithmic decrement of .0143 for vibration amplitudes above .127 inches peak-to-peak, and a decrement of .0023 for amplitudes below this .127 inch level. This corresponds to a maximum acceleration of approximately .32 g. The conductor of the present invention at the 7 Hz vibration frequency had a logarithmic decrement of .0064 for amplitudes down to approximately .012 inches peak-to-peak. Again, the amplitude at which self-damping becomes effective was substantially reduced by the application of the spacers 24.

From the foregoing description, it should now be apparent that the applicant has provided a novel means for substantially increasing the self-damping capabilities of a loose-core conductor. This is accomplished by providing a waviness (of substantial wavelength) in the core or mantle (or both) thereof so that alternate high and low or zero pressure regions of contact exist between the core and the mantle along the conductor in a recurring manner between the structures supporting the conductor. In this manner, the portions of the core and mantle having the low or zero contacting pressures have similarly low thresholds of acceleration thereby providing immediate relative movement of the core and mantle to effect immediate impacting action therebetween.

Though the invention has been described with a certain degree of particularity, changes may be made therein without departing from the spirit and scope thereof.

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Current U.S. Class:	174/130 ; 174/42
Current International Class:	H02G 7/00 (20060101); H02G 7/14 (20060101); H01b 005/10 (); H02g 007/14 ()
Field of Search:	174/42,128,130,131,129,28,68,70,70.4