Shock & Vibration Isolation Glossary

Every term used in the AWRI wire rope isolator selector and in Adonitech engineering reports, defined in plain English with the formula behind it and a worked number. Jump to a term or read straight through — the sequence follows the order these quantities appear in a real selection.

Wire rope isolator (WRI)

An all-metal shock and vibration isolator formed from stranded stainless-steel cable looped between two retainer bars. Energy is dissipated by the strands rubbing and sliding against each other as the cable flexes, producing high damping without rubber, oil or fluid. Also called a cable mount, wire rope mount or helical isolator.

Why engineers choose them: operation from −100 °C to +260 °C, immunity to salt spray, oil, ozone and UV, no ageing or creep, non-linear softening stiffness that protects against both small vibration and large shock, and a service life measured in decades.

Kv and Ks — vibration and shock stiffness (N/mm)

Wire rope isolators are deliberately non-linear: they get softer as they deflect. One stiffness number cannot describe them, so catalogues publish two:

• Kv — vibration stiffness. The small-deflection rate. Use it for natural frequency, transmissibility and any steady-state vibration calculation.
• Ks — shock stiffness. The average rate over the full stroke. Use it for shock response and shock deflection.

Ks is typically 40–70% of Kv. Using Kv for a shock calculation seriously under-predicts stroke and is a common source of bottoming-out failures.

Natural frequency (fn)

The frequency at which the isolator-and-payload system naturally oscillates — the single most important number in isolation.

With k in N/mm and the supported load W in newtons, the practical form is:

Worked number: W = 159 N on an isolator with Kv = 94 N/mm gives fn = 12.1 Hz.

Isolation begins only above √2 × fn. Below that the isolator amplifies. Above it, transmissibility falls at roughly 12 dB per octave.

Transmissibility (T)

The ratio of output motion to input motion at a given frequency — below 1 means isolation, above 1 means amplification.

Damping ratio (ζ) and quality factor (Q)

Damping ratio ζ measures how quickly oscillation dies away, as a fraction of critical damping. Wire rope isolators achieve ζ ≈ 0.15 — far above a steel coil spring (0.005) and comparable to good elastomers, but without their temperature and ageing limits.

Q is the amplification at resonance in the classical form. High damping is what keeps the resonant peak near 3.5 instead of 20+ — critical when a qualification sweep dwells at your natural frequency.

Fragility (g)

The maximum acceleration the protected equipment can survive without damage or loss of function — supplied by the equipment manufacturer or derived from its own qualification testing. It is the governing acceptance criterion: every transmitted shock and vibration response must stay below it. Typical values: 10 g for optics and camera payloads, 15–20 g for ruggedised electronics, 20–30 g for robust mechanical assemblies.

Static load, design load and utilization

Adonitech applies a 2× design rule — the design load (2 × W) must not exceed the catalogue rated load. Utilization is W divided by rated load, expressed as a percentage; the sweet spot is 30–70%, which keeps natural frequency low while preserving shock stroke.

CG load factor

The ratio between the most heavily loaded isolator and the average, caused by an offset centre of gravity. A value of 1.0 means the CG is central; 1.10 means the worst-loaded mount carries 10% more than the average. Ignoring this is a classic cause of premature failure at one corner.

Miles' equation — random vibration response

Random vibration has no single frequency, so response is estimated from the power spectral density at the natural frequency:

Worked number: fn = 12.1 Hz, Q = 3.33, PSD at 12.1 Hz = 0.006 g²/Hz gives 0.62 g rms. Design practice compares the 3σ value (1.9 g here) against fragility, since a Gaussian random signal exceeds 3σ only 0.3% of the time.

PSD and Grms

Power spectral density (PSD), in g²/Hz, describes how random vibration energy is distributed across frequency. Grms is the overall root-mean-square acceleration — the square root of the total area under the PSD curve. A profile with +3 dB/octave roll-on from 20 Hz, a 0.04 g²/Hz plateau from 80–350 Hz and −3 dB/octave roll-off to 2000 Hz gives 6.06 Grms.

Half-sine shock pulse and ΔV

The classical test pulse: a peak acceleration a sustained for a duration τ, shaped as half a sine wave. The velocity change captures its damage potential:

Worked number: 50 g for 11 ms gives ΔV = 3.44 m/s. Two pulses with equal ΔV do similar damage even at different peak g, which is why ΔV rather than peak g drives isolator stroke.

SDOF time-history and maximax

Single-degree-of-freedom (SDOF) analysis models the isolated equipment as one mass on one spring with one damper. Adonitech integrates the equation of motion numerically (Runge-Kutta) through the shock pulse and reads two results:

• Maximax transmitted acceleration — the peak absolute acceleration reaching the equipment, compared against fragility.
• Peak relative deflection — the stroke, compared against rated deflection.

This is more accurate than closed-form approximations because it captures the full response including the post-pulse ringdown.

Stroke and stroke safety factor

Stroke is the peak deflection of the isolator during shock. The stroke safety factor is:

If stroke exceeds rated deflection the isolator bottoms out and transmitted shock spikes far above the calculated value. Equally important: the equipment needs that much clear sway space around it.

Attenuation and isolation efficiency

A 50 g input transmitted at 13.8 g represents 72% attenuation. For vibration the equivalent term is isolation efficiency, equal to (1 − T) × 100.

Load axes — compression, 45° roll and shear/roll

The three published load directions of a wire rope isolator. Compression loads the cable loops face-on and gives the highest rating; shear/roll loads them sideways and typically rates under half of compression; 45° roll sits between the two and gives balanced three-axis behaviour. The mounting orientation determines which axis carries gravity — see the mounting guide.

Stabilizers

Additional isolators fitted at the back or top of tall equipment, carrying no static load but adding stiffness in their own axes to control rocking. A 300 kg cabinet on four base mounts plus two rear stabilizers is a typical arrangement — the stabilizer axes contribute to the horizontal system stiffness and resist the overturning moment.

Sway space

The clear volume that must remain around isolated equipment so it can move freely during shock without striking adjacent structure. Sized from the calculated peak stroke plus a margin. Forgetting sway space is one of the most common installation errors — a perfectly sized isolator is useless if the equipment hits a bulkhead at 40 mm.

TCSS and environmental terms

TCSS — temperature cycling and salt spray, an environmental qualification requirement for naval and outdoor equipment. All-metal wire rope isolators pass inherently, having no elastomer to perish, harden or crack.

Roll and pitch (ship motion)

Quasi-static inclinations of a vessel at sea, commonly specified as roll ±12° and pitch ±2°. They produce a lateral force of m·g·sinθ on each isolator and shift load between mounts:

where h_CG is the centre-of-gravity height and x the mount offset from the centre. The worst-loaded isolator must still sit inside its rated capacity.

AWRI and ACWRI series

AWRI is the Adonitech standard wire rope isolator range — 12 cable families spanning roughly 47 N to 30,270 N rated compression load, with three published load axes per model. ACWRI is the compact series for light payloads such as small electronics, instruments, sensors and cameras. Both ranges — 114 models in total — are evaluated together by the selector.