engineering

Stockbridge damper

Stockbridge damper is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Stockbridge damper rather than just read about it. In short: A Stockbridge damper is a tuned mass damper used to suppress wind-induced vibrations on slender structures such as overhead power lines, long cantilevered signs and cable-stayed bridges. The dumbbell-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.

Key takeaways

Stockbridge damper belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Stockbridge damper to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Stockbridge damper from memory before moving on to harder problems.

Reference excerpt

A Stockbridge damper is a tuned mass damper used to suppress wind-induced vibrations on slender structures such as overhead power lines, long cantilevered signs and cable-stayed bridges. The dumbbell-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.

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-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 on the leeward side of the cable. A steady but moderate wind can induce a standing wave pattern on the line consisting of several wavelengths per span. Aeolian vibration causes damaging stress fatigue 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, are at most risk. The effect becomes more pronounced with increased cable tension, as its natural self-damping is reduced.

Description

The Stockbridge damper was invented in the 1920s by George H. Stockbridge, who was an engineer for Southern California Edison. 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. This last device 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 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 using hot stick 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. Some more complex designs use weights with asymmetric mass distribution, which enables the damper to oscillate in several different frequency modes and ranges.

Another modern design is the Dogbone invented by Philip Dulhunty in 1976. It is so called due to its configuration, a larger metal sphere attached to the end of the damper, with a smaller sphere projecting sideways from it, resembling a dog's bone. The damper offsets the weights sideways in order to introduce a third degree of freedom, twisting the damper cable in addition to bending it up and down. Additional intra-strand friction was created in the damper cable, dissipating significantly more energy. 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 displacement) 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 for which vibration is predominantly 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 bridges or radio mast guy-wires.

See also Conductor gallop Tacoma Narrows Bridge - destruction once thought to be a case of resonance

References

External links

Vibrationdata article "Cable clingers" New Scientist Q&A

Illustrations

Worked examples

Example 1 — a first encounter with Stockbridge damper

Start with the simplest possible case. Write down what Stockbridge damper claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Stockbridge damper before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Stockbridge damper ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Stockbridge damper

In research: Stockbridge damper appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Stockbridge damper in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Stockbridge damper is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aerodynamics, Electric power systems components, Weights, so understanding it makes those chapters shorter.
In everyday life: Look for Stockbridge damper outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Stockbridge damper in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Stockbridge damper means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Stockbridge damper out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Stockbridge damper in simple terms?

Why does Stockbridge damper matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Stockbridge damper?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Stockbridge damper.