The short answer: every new wire rope gets longer in two distinct ways. Constructional stretch is permanent — the strands and core bed down under the first real loads and the rope grows by roughly 0.2–1% of its length depending on construction. Elastic stretch is recoverable — the rope behaves like a very long spring, and you can calculate it with the formula e = (P × L) / (A × E). Neither is a defect, but both will move your hook, sink your kelly bar and eat your levelling margins if you do not plan for them. After twenty years of putting ropes onto cranes, rigs and hoists, I can tell you that stretch is one of the least-discussed reasons a new installation feels wrong in its first weeks. Here is how to predict it and what to do about it.
Why a New Rope Gets Longer: The Three Phases
A freshly closed rope is not the tight, dense column of steel it appears to be. Its strands sit slightly proud of their final positions, the core is not fully compressed, and every wire is still finding its seat. Put the rope to work and three things happen, roughly in this order:
- Phase 1 — constructional (permanent) stretch. Load cycles squeeze the wires tighter into the strands, the strands tighter around the core, and the core itself compresses. The rope permanently lengthens. Most of it arrives in the first days of hard work; on lightly loaded ropes it can drag on for months.
- Phase 2 — elastic stretch. Once the construction has bedded in, the rope stretches in proportion to load like a spring, and recovers when the load comes off. This behaviour never goes away — and it should never go away, because it is the rope’s shock-absorbing capacity.
- Phase 3 — slow secondary growth. Over months and years, wire bedding under wear, metal loss and the gradual exhaustion of lubrication let the rope grow a little more. This is normal ageing, tracked by inspection rather than by formula.
Each phase needs a different response: Phase 1 you anticipate and then take up, Phase 2 you calculate and design around, and Phase 3 you monitor against the ISO 4309 discard criteria.
Constructional Stretch: The Permanent Part
Constructional stretch is the rope manufacturing looseness working itself out. Its size depends mostly on four things: the core (a fibre core compresses far more than an independent steel core), the lay length (longer lays stretch less), whether the rope is compacted (denser strands — much less to bed down), and the magnitude of early loads. Heavy bending and high working loads push the stretch out early; light duty spreads it over months, which is exactly what elevator ropes do.
| Construction | Typical constructional stretch (% of length) | Notes |
|---|---|---|
| 6×7 class, fibre core | 0.35–0.50% | Very stable strands, compressible core |
| 6×19 class, fibre core | 0.40–0.60% | Common on older utility winches |
| 6×19 class, IWRC | 0.25–0.40% | Steel core roughly halves the core compression |
| 6×36 class, fibre core | 0.60–0.90% | More wires, more bedding |
| 6×36 class, IWRC | 0.25–0.50% | The standard crane and winch rope today |
| 8×19 class, fibre core (elevator) | 0.50–0.80% | Pre-stretched versions roughly halve this |
| 18×7 / 35×7 rotation-resistant | 0.30–0.60% | Multi-layer counter-laid construction |
| Compacted 6×K36WS + IWRC/EPIWRC | 0.15–0.25% | Compaction removes most of the looseness at source |
| Spiral strand (1×19) | 0.03–0.10% | No strands or core to seat — nearly rigid |
Treat these as planning values, not guarantees — the exact figure depends on the maker, the lay length and the load profile, so ask for the manufacturer’s data with the rope certificate. One useful option: a pre-stretched rope leaves the factory with most of the constructional stretch already removed, at roughly half the residual stretch of the same construction off the machine. More on that below.
Elastic Stretch: The Part You Can Calculate
Once the rope is bedded in, it obeys Hooke’s law like any other spring:
e = (P × L) / (A × E)
where e is the elastic stretch, P the load on the rope, L the loaded rope length, A the rope’s steel cross-sectional area and E the modulus of elasticity of the rope as a structure. The catch is that E for a wire rope is a construction constant, not a material constant. The steel in the wires itself has a modulus of about 196,000 N/mm², but because the wires are helical and cross-pressed against each other, the rope as a whole stretches several times more. That is also why the rope’s modulus depends on the load range: measure a rope between 0 and 20% of its minimum breaking force and you get a lower E than between 20 and 65%. For precise work, ask the manufacturer for a modulus test on the actual rope; for planning, the table below is what the industry works with.
| Construction | Typical rope modulus E (N/mm²) | Comment |
|---|---|---|
| 6×36 class, fibre core | 45,000–55,000 | Softest common construction |
| 6×36 class, IWRC | 60,000–90,000 | Rises with load range; low end suits light-duty planning |
| Compacted 6×K36WS, IWRC | 65,000–80,000 | Denser construction is also stiffer |
| 8×19 elevator, fibre core | 40,000–50,000 | Designed for smooth traction, not stiffness |
| 35×7 rotation-resistant | 65,000–70,000 | Reasonably stiff for a multi-strand rope |
| 18×7 rotation-resistant | 40,000–45,000 | Noticeably springy — plan hook travel accordingly |
| Spiral strand 1×19 | 105,000–130,000 | Approaches solid-bar behaviour |
There is one bookkeeping trap worth clearing up, because it makes handbook numbers look contradictory. You can run the calculation two equivalent ways: with the metallic area (about 0.6 × πd²/4 for a 6-strand IWRC rope) and a correspondingly higher modulus, or with the nominal area πd²/4 and an apparent (lower) modulus. Both routes give the same stretch. If a supplier’s modulus looks suspiciously low, check which area it is meant to be paired with before you conclude the rope is stretchy.
A Worked Example: 28 mm Rope on a 200 m Hoist
Take a 28 mm 6×36WS+IWRC rope in 1960 grade (MBL about 570 kN) on a winch with 200 m of rope working at the 5:1 design factor, so about 114 kN on the line. Using the nominal-area convention with an apparent modulus of 60,000 N/mm²:
e = (114,000 × 200,000) / (616 × 60,000) ≈ 617 mm
About 0.3% of the working length, on top of the constructional stretch of 0.25–0.50% — another 500 to 1,000 mm. Add it up and a new 200 m rope can easily grow more than a metre in its first weeks of service. That is why rope length margins, dead wraps on the drum and follow-up re-tensioning are not optional extras; they are how the numbers above get absorbed without drama. For drum-side geometry, see our sheave and drum guide.
Multi-Part Reeving Divides the Spring
One consolation for crane people: with n parts of line, each rope part carries only P/n, so the elastic stretch per part — and therefore the hook travel you see — divides by roughly n. A four-fall overhead crane hook settles about a quarter as much as a single-part winch under the same load. Constructional stretch does not get the same discount, because it is a percentage of each part’s length regardless of load, but in practice multi-fall machines feel much tighter than single-fall ones for exactly this reason.
Three More Things People Call Stretch
Beyond the two main mechanisms, three other effects change a rope’s working length, and each has a different fix:
- Thermal expansion. Steel grows about 0.0000125 per °C. A 200 m rope seeing a 20°C temperature swing lengthens by 0.0000125 × 200 × 20 = 50 mm. Trivial on a crane, but real on long guy ropes, ski-lift haul ropes and deep shaft installations where it returns every season.
- Rotation-induced elongation. Let a loaded rope end spin — usually because someone fitted a swivel, or the rope was allowed to rotate during installation — and the lay opens, permanently lengthening the rope. This is one more reason swivels are banned on rotation-resistant ropes like 35WXK7 and on Lang’s lay rope, and why ropes must be installed off a reel under controlled tension.
- Wear-driven growth. As wires wear and metal cross-section is lost over the years, the rope very gradually gets longer and thinner. It is normal, but it compounds the other effects and it is the same metal loss your diameter caliper is measuring for the discard criteria.
What Stretch Means on Real Machines
| Machine | How stretch shows up | What to do |
|---|---|---|
| Tower crane (single-fall hoist, 18×K7 / 35×K7) | Hook drift and slower load spotting in the first weeks; hoist limit settings drift | Re-check limit switches and rope tension after break-in; plan the take-up |
| Rotary drilling rig (35WXK7 main line) | Kelly bar and rotary feed slow down; the string seems to lose hoist speed | Take up at the drum anchor; re-check after the first hundred metres drilled |
| Overhead / gantry crane (multi-fall) | Hook level and travel drift; less than single-fall because falls share the load | Re-tension and re-level; check dead-end anchors |
| Elevator (8×19 ropes) | Counterweight runby shrinks, car levelling drifts, ropes settle unevenly | Equalise rope tensions, then shorten at the terminations — stretch cannot be tensioned away |
| Boom pendants, guys, structural stays | Any permanent growth changes geometry and sag | Order pre-stretched, measured-under-tension assemblies only |
| Long marine lines (towing, mooring, warps) | A few percent of hundreds of metres is metres of scope change | Add length margin, mark and re-measure the scope after bedding in |
The elevator row deserves emphasis because it is where stretch is managed most formally: a levelling fault in week three of new ropes is not a rope defect, it is the constructional stretch arriving exactly on schedule. Our elevator rope guide covers the replacement routine, and the tower-crane habits are in the tower crane guide.
Pre-Stretching: When It Is Worth Ordering
Pre-stretching is the factory practice of cycling a finished rope to a prescribed tension — always below the elastic limit — until it shows no further permanent growth. What it buys you:
- Roughly half the residual constructional stretch of the same rope off the machine, and a much shorter bedding-in period.
- Accurate length under tension. With the construction settled, the rope can be measured and marked at a defined measuring tension, which is how socketed assemblies, boom pendants and guy ropes are built to tight tolerances.
- Uniform modulus along the rope, which matters for long structural applications such as suspension and stayed structures.
When is it worth the cost? For any measured assembly, for long hanging ropes where centimetres matter, for structural strand and guys, and for elevator and ropeway installations that want a short, predictable settling curve. For ordinary running ropes on cranes and winches, in-service break-in does the same job at no extra cost — you just have to plan the follow-up visit. Two cautions: pre-stretching never removes elastic stretch (nothing does), and repeated coiling and uncoiling lets a little constructional stretch creep back until the rope is loaded again.
Taking Stretch Out in the Field
On the machine, the routine that manages stretch is simple and cheap compared with the problems it prevents:
- Break the rope in deliberately. Run the first shifts at moderate loads rather than full rated capacity, and avoid shock loading, so the bedding happens under controlled conditions. The full procedure is in our installation and maintenance guide.
- Mark before you trust. Mark the rope at the drum anchor and at each termination on the day of installation. Any later movement reads directly off those marks.
- Re-tension after bedding in. On a hard-working crane or rig, re-check tension within the first week or two of service, then again after the first month. On multi-rope sets (elevators, friction winders), equalise the tensions — a rope carrying more than its share stretches more than its share, which is how vibration and uneven wear start.
- Shorten, do not stretch back. Once the take-up range is used up, recover the length at the terminations — wedge sockets make this quick — rather than fighting the rope with tension. Constructional stretch is real steel movement; it cannot be tensioned away.
- Write it down. Record dates, marks and adjustments in the rope log, so the next growth spurt is interpreted against history rather than guesswork. The same log is your evidence trail for the service life decisions later.
When Growing Length Is a Warning, Not Normal Behaviour
Most growth is boring and predictable, which is exactly why the exceptions matter. Rapid or asymmetric lengthening is one of the cheapest early warnings a rope gives you — if you are measuring for it.
| Symptom | Most likely cause | Action |
|---|---|---|
| Growth concentrated in the first days or weeks of a new rope | Normal constructional bedding | No rope action; re-tension and reset limits as planned |
| Sudden growth mid-life, often with diameter loss or a dull, flat feel | Broken wire pack, damaged or dried-out core | Full ISO 4309 inspection now; expect replacement |
| Growth with birdcaging near a termination | Wedge or socket slip, wrong wedge size | Cut back and re-terminate; check the mating parts |
| One rope of a set drifting longer than the others | Tension imbalance or sheave condition differences | Re-equalise tensions; inspect sheaves and grooves |
| Progressive growth with visible lay opening or spinning | Swivel fitted or free rotation during service | Remove the swivel; inspect for lay distortion |
| Slow steady growth over years alongside wear | Normal ageing and metal loss | Track against discard criteria in the rope log |
Aulone is a professional manufacturer and supplier of steel wire ropes, wire rope slings, synthetic ropes and rigging hardware for lifting, marine, mining, construction and industrial applications. We supply conventional, compacted and pre-stretched ropes with over 20 years in the industry, 200,000 tons of annual capacity, EN 12385-4 manufacturing under ISO 9001 and BV, ABS, DNV, RMRS and CE certification. Common constructions are in stock for fast shipment, custom assemblies are measured under tension, and third-party testing — including modulus and elongation tests — is available on request. Write to me at info@wireropes.net with your machine, rope construction and working length, and I will help you put a number on the stretch before the rope goes to work.
Frequently Asked Questions
How much will a new wire rope stretch?
Expect permanent constructional stretch of roughly 0.2–1% of length depending on construction (about 0.25–0.5% for a typical 6×36 IWRC crane rope), plus elastic stretch calculated from e = (P × L) / (A × E). On a 200 m working length that can total more than a metre, so plan take-up and drum margins accordingly.
How do I calculate wire rope elastic stretch?
Use e = (P × L) / (A × E): load in newtons, loaded length in millimetres, steel cross-section in mm² and the rope modulus in N/mm². You can pair the metallic area with a higher modulus or the nominal circumscribed area with an apparent modulus — both give the same answer. For a precise figure, ask the manufacturer for a modulus test on the actual rope.
Does pre-stretching eliminate wire rope stretch?
No — it removes most of the constructional component, typically leaving about half the residual stretch of a non-pre-stretched rope, and it makes measured lengths accurate under tension. Elastic stretch always remains, because it is the rope behaving as a spring under load.
Is wire rope stretch a defect?
No. Constructional and elastic stretch are inherent, designed-for behaviour. The defect is unmanaged stretch — skipped re-tensioning, drifted limit switches, unequal rope tensions. What is a genuine warning is sudden or asymmetric growth mid-life, which points to internal damage and calls for an immediate inspection.
Why is my new rope longer than the length I ordered?
Cut lengths carry a manufacturing tolerance, and ropes are measured unloaded — a rope measured under tension will read differently. If you need a precise hanging length, order it as a pre-stretched, measured assembly, and add the expected constructional stretch to any field-measured length before cutting.
How long does constructional stretch take to settle?
It is front-loaded. On hard-working cranes and rigs most of it works out in the first days to weeks of service; on lightly loaded ropes such as elevator installations it can take months. The trigger for taking it out is measured drift in the system — hook drift, runby change, levelling faults — not the calendar.
