A wire rope isolator (also called a cable mount, wire rope mount or helical isolator) is an all-metal shock and vibration isolator built from stranded stainless-steel cable looped between two retainer bars. Because the cable flexes, slides and rubs against itself under load, it delivers high damping (ζ ≈ 0.15) with no rubber, no oil and no ageing — which is why it dominates naval, defence, aerospace and heavy transport applications.
This guide walks through the exact procedure Adonitech engineers use to size an AWRI or ACWRI isolator, the same chain of calculations built into our free online wire rope isolator selector.
The short version
Work out the load each isolator carries → check it against the rated load with a safety factor → calculate the natural frequency and confirm it is well below the disturbing frequency → run the shock case and check both transmitted g and stroke → verify the mounting orientation uses the correct load axis. Every step is automated in the selector tool.
Step 1 — Define the problem before touching a catalogue
Selection fails most often because the input data was never pinned down. Before opening any catalogue, establish:
- Equipment mass (kg) — the full supported mass including brackets and cabling.
- Centre of gravity — height above the mounting plane and any horizontal offset. An offset CG means one isolator carries more than its share.
- Number and layout of isolators — minimum three for a stable platform; four is normal.
- Mounting orientation — floor, wall/bulkhead, ceiling, 45° or with stabilizers. This decides which isolator axis carries gravity.
- The disturbance — vibration profile (sine bands, random PSD) and shock pulse (peak g, duration).
- Fragility limit (g) — the maximum acceleration your equipment survives. This is the acceptance criterion everything is judged against.
- Space envelope — available height and sway clearance. Isolators must be free to move.
Step 2 — Static load per isolator and the 2× design rule
Start with the load each mount carries under gravity:
Where M is the supported mass in kg, g = 9.81 m/s², N is the number of load-carrying isolators, and the CG factor accounts for an offset centre of gravity (1.0 for a central CG, typically 1.05–1.20 when offset).
Adonitech engineering practice is then to apply a 2× design rule: the design load must not exceed the catalogue rated load for the chosen axis.
This margin covers dynamic load transfer, manufacturing tolerance and the load shift that occurs when a ship rolls or a vehicle corners.
Step 3 — Aim for the right utilization
Utilization is the static load per isolator divided by its rated load. It is the single most useful number for judging whether a selection is sensible:
| Utilization | What it means | Verdict |
|---|---|---|
| Below 20% | Isolator far too stiff for the load — natural frequency high, little isolation | Oversized |
| 30–70% | Low natural frequency with shock stroke still in reserve | Ideal window |
| 70–100% | Excellent isolation but limited shock margin — check stroke carefully | Acceptable with checks |
| Above 100% | Overloaded — permanent set and cable damage | Reject |
Counter-intuitively, a softer isolator loaded further up its range usually isolates better than a stiff one running at 10% utilization. The limit is shock stroke, not static strength.
Step 4 — Natural frequency: the heart of the selection
The isolator and the payload form a spring-mass system with a natural frequency:
Isolation only begins above √2 × fn. Below that the isolator actually amplifies the input, peaking at resonance. So the rule is simple: place fn well below the lowest frequency you need to isolate — a ratio of 3:1 or better gives roughly 90% isolation.
Kv or Ks — use the right stiffness
Wire rope isolators are deliberately non-linear. Catalogues give two rates: Kv, the small-deflection vibration stiffness (use this for natural frequency and transmissibility) and Ks, the average large-deflection shock stiffness (use this for shock stroke and shock response). Using Kv for a shock calculation will badly under-predict stroke.
Step 5 — Check the vibration response
Transmissibility T is the ratio of output to input motion at a given frequency:
At resonance with ζ = 0.15, T peaks near 3.5 — the isolator amplifies by about 3.5×. High wire-rope damping is precisely what keeps this peak low; a lightly damped steel spring would peak above 20×. Above √2·fn, T drops steeply and isolation takes over.
For random vibration, response is estimated at resonance with Miles' equation:
Design practice is to compare the 3σ value (three times the rms response) against the fragility limit.
Step 6 — The shock case, and why stroke usually decides
Shock is normally specified as a half-sine pulse: a peak acceleration and a duration (for example 50 g for 11 ms). The energy content is captured by the velocity change:
Adonitech solves the full single-degree-of-freedom equation numerically through the pulse and reads two results: the maximax transmitted acceleration (must stay below fragility) and the peak stroke — how far the isolator actually deflects.
Stroke is the usual limiting factor
A softer isolator transmits less g but moves further. Push too soft and the isolator bottoms out, at which point the transmitted shock spikes far beyond the calculated value. Always confirm stroke safety factor = rated deflection / peak stroke ≥ 1, and confirm the equipment has that much clear sway space around it.
Step 7 — Match the load axis to the mounting
Every wire rope isolator has three published load directions, and the mounting orientation decides which one carries gravity:
| Mounting | Gravity axis | Typical use |
|---|---|---|
| Floor / base mounted | Compression | Cabinets, gensets, deck equipment |
| Wall / bulkhead mounted | Shear / Roll | Bulkhead-hung electronics |
| 45° mounting | 45° Roll | Transport frames needing balanced 3-axis response |
| Ceiling / suspended | Tension (use compression data, confirm with factory) | Overhead assemblies |
| Floor + stabilizers | Compression, with stabilizer axes adding stiffness | Tall cabinets prone to rocking |
Reading compression data for a wall-mounted isolator is one of the most common and most expensive selection errors — shear ratings are typically less than half the compression rating. See the applications and mounting guide for detail.
A worked example — 65 kg naval open-deck director
An electro-optical director weighing 65 kg is floor mounted on four isolators on an open deck. Requirements: JSS 55555 vibration, BR 3021 Grade 2 shock (50 g vertical / 22 g horizontal, 11 ms), fragility 20 g.
- Static load per isolator: W = 65 × 9.81 / 4 = 159 N; design load = 319 N.
- Selecting AWRI-95-80 (rated 592 N compression): utilization 27%, static safety factor 1.86.
- Natural frequency from Kv = 94 N/mm gives fn = 12.1 Hz, comfortably below the 23–33 Hz band energy.
- Numerical SDOF shock solution: 13.8 g transmitted (below the 20 g fragility) with 58.5 mm stroke against 68 mm rated — stroke safety factor 1.16.
- Verdict: PASS on all axes, with 72% shock attenuation.
The online selector performs this entire chain across all 114 AWRI and ACWRI models in about a second and ranks the candidates for you.
Seven mistakes that ruin isolator selections
- Choosing on load capacity alone. An isolator that merely holds the weight may isolate nothing. Frequency governs.
- Ignoring the CG offset. One corner quietly carries 30% more than the average and fails first.
- Using Kv for shock. Under-predicts stroke dramatically; use Ks.
- Forgetting sway space. The calculation passes, then the equipment strikes a bulkhead at 40 mm of stroke.
- Reading the wrong axis. Compression figures applied to a wall mount — usually a factor-of-two error.
- Over-stiffening "to be safe". Raises fn into the excitation band and amplifies the very vibration you meant to remove.
- Ignoring the resonance dwell. Qualification tests sweep through fn; the amplified response there must still sit below fragility.
Frequently asked questions
How many wire rope isolators should I use?
Minimum three for stability; four is standard and simplifies load sharing. More isolators reduce the load each carries, which raises the system natural frequency — so adding mounts is not automatically better for isolation. Tall equipment often uses four base mounts plus two stabilizers to control rocking.
What natural frequency should I target?
Aim for fn at or below one-third of the lowest frequency you must isolate. For naval and transport equipment this typically lands between 8 and 15 Hz. Going lower improves isolation but increases static deflection and shock stroke.
Can wire rope isolators be used outdoors or at sea?
Yes — that is their strongest advantage. All-metal construction (stainless cable and lugs) resists salt spray, oil, UV and temperature from −100 °C to +260 °C with no ageing, unlike elastomeric mounts. Specify SS 316 lugs with SS 302/304 cable for marine duty.
What is the difference between AWRI and ACWRI?
AWRI is the standard Adonitech range spanning roughly 47 N to 30,270 N across 12 cable families. ACWRI is the compact series for light payloads — small electronics, instruments and sensors — with ratings from about 1 N upwards. Both are evaluated together in the selector.
Do I need to qualify the isolator or the whole assembly?
Standards qualify the equipment as installed, so the isolator, mounting plate and fasteners are tested as a system. The calculations here predict that behaviour and size the mount correctly; first-article testing then confirms it.
Ready to select?
Open the free AWRI wire rope isolator selector — enter your mass, standard and mounting, and get a ranked list with a full engineering report you can send to your customer. Or install the free Android app to work offline on site.