Two kinds of stretch are not the same

A wire rope stretches under load from two sources: elastic deformation of the steel itself, and geometric rearrangement of the rope structure as it beds in. The first is elastic stretch, recovered on unloading; the second is constructional stretch, only partly recovered. The share each kind takes of total extension varies with construction and load history.

Confusing them causes problems. A pre-tension calculated from elastic stretch alone may not remove constructional stretch, and a travel allowance based on total stretch may be used up gradually under repeated loading. On long ropes and precision machines, that difference shows up directly during installation and commissioning.

On lifting equipment, stretch changes hook height and limit settings; on stays and tensioned structures it changes cable force; on coupled drives, a difference in stretch makes the two sides run out of step. Only by separating the two can the numbers be right.

How to calculate elastic stretch

Elastic stretch is estimated with Hooke's law: load times rope length, divided by metallic area times modulus of elasticity. Load and length are easy; metallic area and modulus are the difficult terms. Keep the units consistent: load in kilonewtons, area in square millimetres and modulus in kilonewtons per square millimetre give a result in millimetres.

The metallic area of a wire rope is not the nominal circle area; it is usually only five or six tenths of it, depending on construction and lay length. The modulus is not the 206 GPa of solid steel either: six-strand ropes commonly sit in the 80 to 110 GPa range, and the looser the construction and lay, the lower the modulus. These values are order-of-magnitude references; published maker data or a site measurement governs the actual product.

Back-calculating from test data is more reliable. Where a pre-tension can be applied with load and extension recorded, the actual modulus of that rope can be derived and used for later calculations.

Where constructional stretch comes from

When a new rope is loaded for the first time, the gaps between strands and between wires are closed, and the rope gets longer. That is constructional stretch. It happens mostly in the first few loadings and then settles, but it never disappears completely.

Its size depends on construction, lay length, core and manufacturing process. Fibre-core ropes usually show more constructional stretch than steel-core ropes, because the core itself compresses. On a pre-stretched rope, part of it has already been released at the factory. Different batches of the same construction can also differ in constructional stretch.

Constructional stretch is usually a small fraction of rope length, but on long ropes, wide spans or duties that need accurate travel, the absolute figure can reach tens of millimetres or more and cannot be ignored.

• The new rope is visibly longer after its first loading.
• Length does not fully return after the load is removed.
• Stretch settles after a few load cycles.

Pre-tensioning and pre-stretching

Pre-tensioning applies a pull higher than the working load, before or after installation, so constructional stretch happens early. Pre-stretching does a similar job at the factory with dedicated equipment. The aim is the same: to settle the rope before it enters service. Longer is not better: holding pre-tension too long can fatigue the termination.

The load, hold time and number of cycles are set by the construction and the project requirement. Too much tension damages wires or terminations; too little leaves constructional stretch in the rope. Critical structures should have pre-tensioning written into the installation method.

Re-measure after pre-tensioning. The rope may spring back a little, and the termination may slip slightly; only a second measurement and adjustment reaches the design cable force. The interval between measurements follows the construction and span; long cables usually need several.

Effects on drum travel and synchronisation

On a drum, rope stretch changes the actual hook position. The greater the lift, the more stretch accumulates, and both the hoist limit and the stopping accuracy must allow for it.

Where several ropes work in parallel or a load hangs from two points, a difference in stretch between the ropes unbalances the load. Control length, construction and batch together when ordering, and where necessary buy and replace ropes as a matched pair. The greater the difference in stretch between two ropes, the sooner the one that takes load first will fatigue.

In long pulling and tensioning systems, elastic stretch sets the stiffness of the system. The longer the rope, the more stretch a given load produces, and the tensioner needs more travel. Temperature change adds further length variation and should be considered in the same calculation.

• The greater the lift, the more stretch accumulates.
• Two lifting points need matched rope stretch.
• Long tensioning runs need enough adjuster travel.
• Temperature change adds further length variation.

Temperature and repeated loading

Steel expands with heat, so a rope gets longer as temperature rises. On a rope one hundred metres long, a ten degree change moves the length by roughly a dozen millimetres. For precision tensioned structures, that figure belongs in the design. The linear expansion coefficient is similar across constructions, but geometric movement of the rope adds to the thermal change.

Under repeated loading, elastic stretch tracks the load in real time while constructional stretch settles. Watching how extension changes with cycle count shows whether constructional stretch has largely been released.

If extension keeps growing without settling, suspect a fault: termination slip, a loosened strand or overload. Stop and inspect rather than tensioning further.

What to specify when ordering

Where stretch matters, state the permitted extension, whether pre-tensioning is required, the pre-tension load and the acceptance method. Writing low stretch alone gives nothing to work with. The acceptance method must be measurable, for example the length or cable force after tensioning.

On construction, single-strand and compacted ropes usually stretch less than a plain six-strand rope, and steel-core ropes less than fibre-core ropes. But low stretch usually means less flexibility, so recheck D/d where the rope bends.

Breaking force and construction data are the inputs for any stretch calculation, and are set out in the wire rope breaking strength guide

Construction and core differences are compared in the wire rope construction guide

One-line rule

Elastic stretch can be calculated and recovers; constructional stretch happens and only partly recovers. Calculate them separately, release constructional stretch with pre-tensioning at installation, and correct by measurement in service. Keeping these figures for the next rope change makes the next judgement quicker.

To screen rope diameter against span and load, use the wire rope selector