There are many formulas that explain oscillatory motion, and they all define the mathematical “how,” but none solve the mechanics required in preventing the unique problem involving cable dancing and galloping in heavy winds. It’s a problem applicable to all communications utilities utilizing strand and cable. More specifically, because cables lashed to strand are oblong cross-sections installed at different tensions, formulas defining oscillating frequencies miss a few factors that specifically apply to both telecom and coax cables.
Pic and coax cables only dance and gallop when exposed to broadside winds varying +/- 100 degrees. Dangerous low cables, resulting from fatigued broken lashing wire, require immediate attention. Telecom terminal repairs impact service as signal loss affects coax customers. To make matters worse, repair expenditures are directly connected to maintenance budgets and cannot be capitalized.
In the 1950s and 1960s, population in rural areas exceeded the capabilities of existing open wire and older lead cables. Many state-of-the-art newly constructed central offices and the Pic cables serving outside plant were being funded by low-interest loans secured through the Rural Electrification Administration. By 1965, thousands of small, antiquated magneto and common battery exchanges had merged forming many large companies. This article involves one named Contel.
Plant managers overseeing the rural countryside discovered cables in high winds possessed an Achilles Heel – cable dancing and galloping, which surfaced when open wire and lead cables were replaced with lighter Pic cables. Different methods were attempted for solving the issue.
Where right-of-way conditions were favorable, midsections were tied down to stakes. Another method used heavy concrete weights, but weights were not always heavy enough to control cables dancing and galloping in high wind velocities. In addition, shipping costs were prohibitive, hanging was labor-intensive, and they were aesthetically unsightly.
Some system practices involved spiraling cable around strand. This practice applied to new construction and resulted in excessive cost per foot. Other companies were installing pre-forms that spiraled the entire lengths of sections. The primary problems with these solutions is they were too costly and sometimes ineffective.
Using 68 degrees as a rule, cable support strands are tensioned to 1,100 lbs. for 6M, and 1,500 lbs. for 10M. Cables lashed to strand contain no tension. A contributing factor that formulas fail to address is the tension disparity between strand at very high tension and none for Pic and coax cables.
The task of any aerodynamic cable damper is transferring wind energy force to the point where the harmonic frequency begins. That location is called the “un-node of a sine wave.” Winds encountering a damper placed on un-nodes generate sufficient drag, creating a separate flexible position.
As sections of cables are buffeted by varying wind velocities, the proportionally larger cables surface area pivots, twisting on the strand. The potential energy provided by wind is the catalyst helping establish the sections’ harmonic frequency sine wave. Contributing to this developing harmonic frequency is the oblong configuration of strand and lashed cable.
Air traveling across the cable and strand oblong cross-sections causes an uneven flow or vortex. The uneven flow then creates a ventura effect, similar to the wind’s lift on a sailboat sail or airplane wing. The ventura magnifies the wind’s potential energy, which aids the twisting motion.
When an aerodynamic damper is installed in the center of a cable section, its location is on the position where the cable section’s movement begins. That position becomes the highest point, called the “un-node of a building harmonic frequency sine wave.” Wind forces acting against the damper set in motion small frequencies. Emitting from both sides of the damper, these frequency waves resonate off both fixed poles, radiating back toward the damper. Meeting each other from different directions results in the disruption of developing harmonic frequencies.
At the peak of the twisting motion, the section’s induced energy is spent, allowing built in kinetic energy to return the section back to its original position and past. The wind’s continuing velocity encourages the building motion frequency until the lashing wire fatigues, allowing cable to drop, coax cable sheaths to crack, and telecom terminals to lose sheath continuity. Therefore, a wind damper’s effectiveness is proportional to the wind’s strength.
On cable sections up to 170 feet, a single damper placed in the center of a section will disturb the fundamental harmonic frequency from developing. On sections exceeding 170 feet, two un-nodes develop, creating a first overtone harmonic frequency. For these longer sections, it’s necessary to install two dampers located on the un-nodes located one-quarter of the total distance from both poles.
In 1965, an employee working for Contel was involved overseeing cable repair to sections damaged by a three-day storm. That evening, he fabricated a device. Since the problem occurs when wind blows, the solution used the air densities moving mass force causing the problem.
The following morning the storm was still producing varying winds 10–65 mph. Permission was granted for testing on dancing sections. A ladder was placed upwind of the worst dancing section, allowing the ladder to blow back against it. A device was installed, ladder removed, and section monitored. After more than 30 minutes of observation, dancing never started. After removal of the device, dancing resumed within a few minutes. For future study, this sequence was performed and recorded six consecutive times.
Lacking a wind tunnel for endurance testing, two metal brackets were fabricated over the cab of a Contel vehicle. Suspended between the two brackets was a duplication of strand, 50 x 22 pair Pic cable, lashing wire, hose clamps, lashing wire clamp and new device.
During the two years waiting for patent approval, the device was mounted on a Contel vehicle traveling thousands of miles, being subjected to constant varying wind velocities and agitated movement. Other than slight abrasions to cable sheath resulting from S/S hose clamps, periodic inspections found zero component failures.
Because of the important savings, the worker was encouraged by management to apply for a patent granting all rights to market his invention. In 1967, based on results of a patent search, a patent was issued, making the device the first patented aerodynamic cable vibration damper.
A Chicago research company was then hired to investigate the damper’s marketability. Its findings: Unmarketable. Disregarding the report, confident with his invention’s effectiveness, the employee and management continued forward.
After the patent was issued, further testing was performed at Contel’s testing facility in Bakersfield, California. Based on test results, the damper was added to Contel’s plant practices, meeting all requirements for eliminating cable dancing and galloping during extreme wind conditions.
The basis for Bell system approval depended on providing Bell Laboratories the equation for the operation. Approval was granted by providing the equation for simple harmonic motion. A year later, other major telecoms followed suit.
Recently, a new Cable Vibration Damper Patent has been issued. Improved upgrades follow:
- Added spoiler tabs providing greater surface area adding to wind motion’s drag force;
- Eliminated metal hose clamps for sheath compatible black composite UV resistant 400 lb. tensel strength cable straps;
- Changed to light ¼ hard aluminum, allowing faster reaction time to disrupt developing harmonics;
- Lowered profile, eliminating potential contact between coax and telecom cables when sagged together in the same section; and
- Added a single tie-wrap, stabilizing damper for attaching and adjusting cable straps.
About the author: Curtis M. Wright is a retired telecom engineer, with more than four decades of experience with several telecom providers, including Western Counties Telephone, Continental Telephony, Contel, GTE, and most recently, Verizon (News - Alert). He has been issued four telecom-related patents.
Edited by Erik Linask