A Stockbridge damper is a tuned mass damper

Tuned mass damper

A tuned mass damper, also known as an active mass damper or harmonic absorber, is a device mounted in structures to reduce the amplitude of mechanical vibrations. Their application can prevent discomfort, damage, or outright structural failure...

 used to suppress wind-induced vibrations on taut cables, such as overhead power line

Overhead power line

An overhead power line is an electric power transmission line suspended by towers or utility poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric energy...

s. The dumbbell

Dumbbell

The dumbbell, a type of free weight, is a piece of equipment used in weight training. It can be used individually or in pairs .-History:...

-shaped device consists of two masses at the ends of a short length of cable or flexible rod, which is clamped at its middle to the main cable. The damper is designed to dissipate the energy of oscillations in the main cable to an acceptable level. Its distinctive shape gives it the nickname "dog-bone damper".

Wind-induced oscillation

Wind can generate three major modes of oscillation in suspended cables:

• gallop has an amplitude measured in metres and a frequency range of 0.08 to 3 Hz;
• aeolian vibration (sometimes termed flutter) has an amplitude of millimetres to centimetres and a frequency of 3 to 150 Hz;
• wake
  Wake
  A wake is the region of recirculating flow immediately behind a moving or stationary solid body, caused by the flow of surrounding fluid around the body.-Fluid dynamics:...
  
  -induced vibration has an amplitude of centimetres and a frequency of 0.15 to 10 Hz

The Stockbridge damper targets oscillations due to aeolian vibration; it is less effective outside this amplitude and frequency range. Aeolian vibration occurs in the vertical plane and is caused by alternating shedding of vortices

Vortex shedding

Vortex shedding is an unsteady flow that takes place in special flow velocities . In this flow, vortices are created at the back of the body and detach periodically from either side of the body. See Von Kármán vortex street.Vortex shedding is caused when a fluid flows past a blunt object...

 on the leeward side of the cable. A steady but moderate wind can induce a standing wave

Standing wave

In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...

 pattern on the line consisting of several wavelengths per span. Aeolian vibration causes damaging stress fatigue

Fatigue (material)

'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...

 to the cable and represents the principal cause of failure of conductor strands. The ends of a power line span, where it is clamped to the transmission towers

Electricity pylon

A transmission tower is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes...

, are at most risk. The effect becomes more pronounced with increased cable tension, as its natural self-damping

Damping ratio

[[Image:Damped spring.gif|right|frame|Underdamped [[spring–mass system]] with ζ 1 , and is referred to as overdamped.*Underdamped:If s is a complex number, then the solution is a decaying exponential combined with an oscillatory portion that looks like \exp...

 is reduced.

Description

The Stockbridge damper was invented in the 1920s by George H. Stockbridge, who was an engineer for Southern California Edison

Southern California Edison

Southern California Edison , the largest subsidiary of Edison International , is the primary electricity supply company for much of Southern California, USA. It provides 14 million people with electricity...

. Stockbridge obtained US patent 1675391 on 3 July 1928 for a "vibration damper". His patent described three means of damping vibrations on lines: a sack of metal punchings tied to the line; a short length of cable clamped parallel to the main cable; and a short (30 in, 75 cm) cable with a concrete mass fixed at each end. It would be this last device that developed into the widely used Stockbridge damper.

Vibrations in the main cable were passed down through the clamp and into the shorter damper, or "messenger", cable. This would flex and cause the symmetrically-placed concrete blocks at its ends to oscillate. Careful choice of the mass of the blocks, and the stiffness and length of the damper cable would match the mechanical impedance

Mechanical impedance

Mechanical impedance is a measure of how much a structure resists motion when subjected to a given force. It relates forces with velocities acting on a mechanical system. The mechanical impedance of a point on a structure is the ratio of the force applied at a point to the resulting velocity at...

 of the damper to that of the line, and greatly attenuate oscillation of the main cable. Since Stockbridge dampers were economical, effective and easy to install, they became used routinely on overhead lines. Live-line working

Live-line working

In electrical engineering, live-line working is the maintenance of electrical equipment, often operating at high voltage, while the equipment is energised. The first techniques for live-line working were developed in the early years of the 20th century, and both equipment and work methods were...

 using hot stick

Hot stick

In the electric power distribution industry, a hot stick is an insulated pole, usually made of fiberglass, used by electric utility workers when engaged on live-line working on energized high-voltage electric power lines, to protect them from electric shock...

 tools meant it was possible to retrofit dampers to lines while energised.

Modern designs

Modern designs use metal bell-shaped weights rather than Stockbridge's concrete blocks. The bell is hollow and the damper cable is fixed internally to the distal end, which permits relative motion between the cable and damping weights. To provide for greater freedom of motion, the weights may be partially slotted in the vertical plane, allowing the cable to travel outside the confines of the bell. In some installations, the weights are unequal, allowing damping over a greater frequency range. More complex designs use weights with asymmetric mass distribution, which enables the damper to oscillate in several different frequency modes.

The most vulnerable section of the cable is where it is clamped to the end of an insulator string, so dampers are typically installed at the nearest anti-nodes (points of maximum oscillation) either side of the clamp. There are thus normally two dampers per span, though more can be installed if necessary on longer spans.

Overhead transmission lines form a catenary

Catenary

In physics and geometry, the catenary is the curve that an idealised hanging chain or cable assumes when supported at its ends and acted on only by its own weight. The curve is the graph of the hyperbolic cosine function, and has a U-like shape, superficially similar in appearance to a parabola...

 for which vibration is predominately in the vertical plane. When more than one plane of vibration is anticipated, Stockbridge dampers may be mounted at right angles to each other. This is common when the cable runs in a vertical or off-horizontal plane, for example in cable-stayed bridge

Cable-stayed bridge

A cable-stayed bridge is a bridge that consists of one or more columns , with cables supporting the bridge deck....

s or radio mast guy-wire

Guy-wire

A guy-wire or guy-rope, also known as simply a guy, is a tensioned cable designed to add stability to structures . One end of the cable is attached to the structure, and the other is anchored to the ground at a distance from the structure's base...

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External links

• Vibrationdata article
• "Cable clingers" New Scientist Q&A