Vibration Dampers

Abstract

A vibration damper including a wire supported by a clamp, said vibration damper comprising an elongated damping member secured to inhibit longitudinal movement along the wire and having rigidity and character to damp substantial amounts of vibrational energy. The weight per foot of the damping member is in the range of from about 20 percent to about 125 percent of the weight per foot of the wire. The minimum length of the damping member is approximately 3 feet. At least a portion of the damping member is in contact with the wire and the damping member is free to move relative to the wire whereby impact between the wire and portion of the damping member dissipates said vibrational energy.

Description

This invention relates to vibration dampers and more particularly to impact vibration dampers for overhead wires such as transmission line conductors, ground wires, guy wires or the like.

Overhead wires are subject to a number of different processes from which vibration can develop. The two types of greatest importance are referred to as aeolian vibration and galloping or "dancing" wires.

Aeolian vibration occurs in relatively light winds usually from one to about 15 miles per hour and results from eddies which form on the lee side of the wire. When the frequency of the eddies coincides with one of the many natural frequencies of the wire, the forces arising from the eddies cause vibration to occur. This type of vibration is usually present for 50 percent of the time and normally if permitted to occur without adequate control results in mechanical failure of the wire, sometimes within a very short period of time.

Galloping of wires or conductors usually, although not always, occurs during or after an icing storm. Ice forms on the windward side of the conductor and the resulting airfoil gives rise to aerodynamic lift and drag forces which cause the conductor to vibrate. The winds giving rise to the phenomenon normally range from about 10 to 35 miles per hour. The frequency of vibration is usually in the range of 0.25 to 0.5 cycles per second with amplitudes up to about 30 feet peak to peak. This compares with 3 to 150 cycles per second and a peak to peak amplitude equal to the conductor diameter for aeolian vibration. The principal difficulty arising from galloping is that the conductors constituting the transmission line collide with each other or move sufficiently close to each other to cause short circuits between them. This results in outages and the line becomes inoperative, sometimes for many hours.

This invention is directed to the control of these two types of vibration but is also applicable to other types referred to but not described herein. It involves the use of loose members on or about the wire. For example, these may take the form of tubes surrounding or rods on top of the wire. It has been found that many materials, e.g. metal, wood, plastic and elastomers are effective for this purpose. An important consideration is that the tube or rod be free to move relatively to the wire so that under conditions of vibration collision or impact occurs between the damper and wire. In the case of a tube surrounding the wire clearance between the inside wall of the tube and the wire need only be a few thousandths of an inch, although for practical purposes a minimum radial clearance of say 0.030 inches is used. The impacts cause energy in the moving parts to be given up in the form of heat and result in a greatly increased capacity for dissipating vibration energy as compared with that inherent in the wire alone. This principle, which is employed in the impact damper, effectively controls the vibration to workable and safe levels. It is particularly effective for small wires and for large wires for controlling vibration at the upper end of the vibration frequency spectrum where conventional dampers tend to be marginally effective and are subject to fatigue failure themselves.

The damper in accordance with this invention provides a simple yet efficient, low cost damper which is effective in controlling aeolian vibration from about 8 cycles per second throughout the frequency range encountered in aeolian vibration.

The present invention further provides a damper which is easily installed and which requires a minimum of engineering time as the location of the damper in the span has little if any effect on its performance. It is therefore simply a matter of selecting the size and quantity of dampers for the particular conductor span.

Accordingly the present invention provides a vibration damper for an overhead transmission line or the like including a wire supported by a clamp, the vibration damper comprising an elongated member at least a portion of which rests on the wire and is free to move relative to the wire whereby impact between the wire and a portion of the member dissipates vibration energy.

In the accompanying drawings:

FIG. 1 shows vibration characteristics of an undamped conductor and conductors equipped with a conventional damper and various impact dampers in accordance with this invention.

FIG. 2 shows the vibration characteristics of a conductor equipped with an impact damper in accordance with this invention comprising 2, 3, 4, 6 and 12 -foot lengths of aluminum rod.

FIG. 3 shows graphically the minimum length of free impact damper in feet for satisfactory vibration control plotted against conductor weight in pounds per foot.

FIG. 4 shows graphically maximum bending amplitude plotted against frequency of 3/8-inch ground cable provided with a conventional damper as compared with two 4 -foot lengths of tubing type impact damper in accordance with this invention. The bending amplitude was measured by the method defined in the I.E.E.E. Task Force paper No. 31 CP 65--156 dated January 31, 1965 entitled Standardization of Conductor Vibration Measurements.

FIG. 5 is a perspective view of a portion of a suspended conductor provided with a vibration damper in accordance with this invention.

FIG. 6 is a perspective view of a portion of a suspended conductor provided with an alternate form of vibration damper.

FIG. 7 is a sectional side elevational view of a portion of the vibration damper of FIG. 6.

FIG. 8 is a sectional view taken along the line 8-8 of FIG. 6.

FIG. 9 shows an alternative clamping arrangement for use with the vibration damper of FIG. 6.

FIGS. 10, 11, 12 and 13 are perspective views of alternate forms of the invention.

Referring now in detail to the drawings, FIGS. 1 to 4 show typical performance characteristics of impact dampers. FIG. 1 shows the results of tests on an indoor laboratory span of a medium size power conductor of about 1 inch in diameter. An 80 -foot span is excited by a constant force over a wide range of frequencies. The resulting conductor loop velocity for an undampered conductor is compared with the same conductor fitted with conventional dampers and loose-fitting rods and tubes. The graph demonstrates the remarkable ability of the impact damper to control vibration over a wide range of frequencies. It will be seen that the particular conventional damper used for this comparison is relatively ineffective in the 40 cycle per second range which is in the conductor vibration frequency spectrum that occurs on operating lines. FIG. 2 demonstrates the effect of various lengths of impact dampers showing that for this particular size of damper there is little advantage in using lengths greater than 6 feet.

There is also an optimum weight of damper, i.e. weight per foot above which little apparently is gained in damping efficiently by varying the weight per foot of the damper. This weight per foot of the damper appears to be in the range of about 10 percent to about 200 percent of the weight per foot of the conductor and is preferably in the range of about 30 percent to 125 percent of the weight of the conductor or wire. FIG. 3 shows the minimum length of free damper versus conductor weight for damper to conductor a weight ratio of 1 to 3. Increasing the length of damper over some optimum length for the range of frequencies normally encountered with aeolian vibration does not appear to provide additional vibration control.

FIG. 4 shows field results using the impact type damper which demonstrates the relative effectiveness of a single conventional damper compared with two 4-foot lengths of rubber tube on a 3/8-inch diameter steel cable. This illustrates the remarkable damping efficiency of the impact damper at high frequencies and indicates that it has fully satisfactory vibration control over the whole frequency spectrum. Similar results have been obtained for impact dampers allowed to work their way into the middle of the span. The vibration damping effectiveness of the impact damper is independent of conductor tension and temperature. It is necessary though for the material to be capable of withstanding the operating conditions. For example, materials are available for temperatures in excess of 300.degree. F. at which conductors are operated. These materials also have brittle points well below 60.degree. F.

In FIG. 5 one form of an impact vibration damper in accordance with this invention is indicated generally by the numeral 10. The damper 10 is shown in use on a conductor 12 which is suspended from a supporting tower (not shown) by a conventional clamp 14.

The damper 10 comprises an elongated member in the form of a length of tube or rod 15. The tube or rod 15 may be of such materials as steel, aluminum or semirigid plastic and a wooden dowel may also be used. The impact damper 10 is secured to the conductor 12 by the clamp 14 or by any convenient means. It will be noted however that at least a portion of the vibration damper 10 rests on the conductor. This can be accomplished by having one end of the elongated member 15 secured in the clamp so that the free end of the member 15 will be resting loosely on the conductor 12. Alternatively a double length of tube or rod 15 may be used and when clamped adjacent its midportion provides a damper 10 at each side of the clamp 14.

In FIG. 6 an alternate form of a vibration damper in accordance with this invention is shown generally at 20. The damper 20 is installed on the conductor 12 adjacent the clamp 14 described with reference to FIG. 5.

The damper 20 comprises a length of tubing 22 provided with a slit 23 extending throughout its length to permit installation of the damper 20 on an existing conductor. It will be appreciated however that the slit 23 in the sidewall of the tube would not be necessary if the tubing 22 were to be inserted over the conductor 12 before it is suspended from the supporting towers.

The damper 20 preferably comprises a synthetic plastic material such as for example ethylene propylene terpolymer or polyethylene. It is desirable to provide a semiconducting material to minimize deterioration caused by an electrical phenomenon known as tracking. Ethylene Propylene Terpolymer was selected as it combines all the desirable features of maximum damping, resistance to its environment, conductivity and uniformity of performance over a wide atmospheric temperature range in an economical material. The use of the semiconducting rubber and careful dimensional control minimizes radio or television interference. An annular clamp 24 may be provided to keep the slit 23 from opening after the tubing 22 has been installed on the conductor 12. The end of the tubing 22 adjacent the suspension clamp 14 is preferably provided with a clamp 25 if it is desirable to locate the damper 20 for easy accessibility from the tower. It is noted however that the damper 20 may be located anywhere on the conductor 12 and be free to move axially therealong while performing the desired function.

The clamp 25 shown more clearly in FIG. 7 comprises two substantially hemispherical portions 30 and 31 secured together by bolts 32 as shown in FIG. 6. The clamp 25 is adapted to grip the conductor 12 as well as the associated end of the tubing 22. The clamp 25 is so shaped as not to be a source of corona discharge.

It will also be appreciated that it is desirable to have a clamping system designed to provide minimum contact pressure thus reducing compression set of the elastomer which would otherwise result in loss of the clamping function.

In manufacturing the tube-type damper 20, it is very important to provide a uniform internal diameter throughout, otherwise the damper will not have the necessary loose fit on the conductor. To avoid distortion extruded tubes of large cross section should be cured on mandrels. Furthermore the damper must be straight and therefore it is necessary to take precautions to prevent any permanent set or deformation resulting from handling, shipping or storing. It has been found that packing the tube-type dampers tightly in crates or packaging individual dampers in tubular containers alleviates this problem.

In FIG. 9 an alternative clamping arrangement is shown wherein a split sleeve 40 is inserted between the conductor 12 and the damper 20 to increase the effectiveness of a clamp 24', similar to clamp 24, provided on this portion of the damper 20. The sleeve 40 and clamp 24' perform the same function as the clamp 30 described above with reference to FIG. 6.

Alternative forms of impact dampers in accordance with this invention are generally indicated at 10a, 10b, 10c and 10d in FIGS. 10, 11, 12 and 13 respectively.

In FIG. 10 the damper 10a comprises a member 15a having a semiannular cross section. The damper 10b in FIG. 11 comprises a member 15b having a varying cross section of alternate rod and slit tube configuration. The damper 10c in FIG. 12 comprises a member 15c of uniform cross section so shaped as to make contact at one or more points. It will be appreciated that dampers 10a and 10b provide for dissipation of heat generated in the conductor. The damper 10 of FIG. 6 may also be provided with apertures to dissipate heat. Alternatively as shown in FIG. 13, a member 15d is provided which is in the form of a tube having alternating portions of two different diameters.

The impact dampers described have the advantages of simplicity, ease of manufacture and economy and open up the possibility of economically increasing conductor operating mechanical tension while maintaining vibration at acceptable levels. Hitherto this has been inhibited due to the difficulty of controlling the vibration problem and because of the high cost of conventional vibration dampers. The tube type damper also has been found to have a substantial measure of control over galloping conductors. Firstly, the tube completely covers the conductor and thus reduces the tendency of ice to lock the damper to the conductor. This allows the damper to move over most of its length relative to the conductor thereby permitting the impact principle to be used in the presence of ice. As shown more clearly in FIG. 8 the damper 20 is installed with the slit 23 at the underside of the conductor to make it more difficult for water to enter the damper 20. This arrangement thus minimizes the danger of ice locking the damper 20 to the conductor 12. Furthermore, by distributing the dampers along the conductor to cover between 10 and 20 percent of its length, a substantial measure of control of galloping is obtained. Observation on a 5-mile line of four circuits of 1.6 in. dia., conductor supported by a common steel structure, two of which were fitted with distributed impact dampers, showed that the critical velocity to induce galloping has been increased from about 8 miles per hour to about 25 miles per hour and that above this velocity the amplitude was reduced generally by about 50 percent compared with the undamped conductors. The principle used here is to inhibit the development of a relatively high frequency wave which would initiate galloping by travelling up and down the conductor span picking up energy from the wind. If unchecked this wave gradually builds up and eventually locks into one of the first four vibration modes of the conductor, usually the first or the second causing short circuiting. This vibration may also cause destruction of the transmission line or supporting structures.

Although the term conductor is used herebefore however, the vibration damper of this invention is applicable to any overhead wires such as for example guy wires.

Claims

I claim:

1. A vibration damper including a wire supported by a clamp, said vibration damper comprising:

a. an elongated tube surrounding said wire and secured to inhibit longitudinal movement along said wire and having rigidity and character to damp substantial amounts of vibrational energy;

b. the weight per foot of said tube being in the range of from about 20 percent to about 125 percent of the weight per foot of said wire;

c. at least a portion of said tube being in contact with said wire; and

d. said tube being free to move relative to said wire whereby impact between said wire and a portion of said tube dissipates said vibrational energy.

2. A vibration damper as claimed in claim 1 wherein a clamp is provided to prevent longitudinal movement of said tube relative to said wire.

3. A vibration damper as claimed in claim 2 wherein said clamp comprises a pair of substantially hemispherical members for gripping said tube and said wire.

4. A vibration damper as claimed in claim 2 wherein a split sleeve is inserted between said tube and said clamp.

5. A vibration damper as claimed in claim 1 wherein said tube comprises semiconducting rubber.

6. A vibration damper as claimed in claim 1 wherein a slit is provided along the length of a wall of said tube.

7. A vibration damper as claimed in claim 6 wherein at least one clamp is provided to prevent the slit from opening when the tube has been installed on the wire.