How a Wire Rope Sling Fails and What You See First

Why this matters

A wire rope sling is the one thing in a rigging kit that tells you it is dying before it dies. It is made of many small wires, they do not all fail at once, and the ones that go first push out where you can see them. That warning is the whole reason wire rope survives in a trade that has lighter and cheaper alternatives, and a shop that does not read it has paid for the warning and thrown it away.

The trade knows the broken-wire counts. What it usually does not know is that the count and the pattern are answering two different questions. The count is a removal trigger: cross it and the sling comes off the rack, no judgement involved. The pattern is a diagnosis: it tells you what is eating the sling, which is the only thing that stops the replacement going the same way. This card is about running both at once.

Inspect with the sling off the load and hanging slack, never on a rope under tension. Run a rag along the body rather than a bare hand, because a rag snags on a broken wire and a hand gets punctured by one, and wear gloves for the same reason.

A rope lay, and why the count is per lay

Every published broken-wire criterion is stated per rope lay, so the count means nothing until you know what a lay is. Pick one strand, mark where it sits on the surface, and follow it along the rope until that same strand returns to the same position. That distance is one lay: the length in which a strand makes one complete turn around the core.

For common constructions a lay runs roughly six to eight times the rope diameter, but measure it rather than assume it. If you must estimate, estimate it long. A window measured longer than the real lay counts more wires in it and errs toward removing the sling, which is the direction you want to be wrong in.

The criterion is per lay because a broken wire does not stay useless. A short distance either side of the break, friction and the surrounding wires pick that wire's load back up, so its loss is local rather than spread down the rope. That is exactly why a rope's condition is assessed in a window rather than as a total count over its length, and why five breaks in one place matter more than fifteen scattered over twenty feet.

The gate

A wire rope sling is removed from service at ten randomly distributed broken wires in one rope lay, or five broken wires in one strand in one rope lay. That criterion sits in the wire rope sling paragraph of 29 CFR 1910.184 for general industry work and in 29 CFR 1926.251 for construction work; name which Part covers the job before quoting it, because a field-service shop can be under either depending on the site.

Two things about that gate. It is a floor, not a permission: a competent or qualified person may remove a sling for less, and the other criteria below stand on their own regardless of the wire count. And it is not a strength calculation. Nothing about crossing or not crossing it tells you what capacity remains.

Two slings, same count, opposite calls

Two slings came off the same job, same body diameter, same set, both inspected on the bench with the slings slack, gloved, using a rag drawn along the body.

Sling A. Within the worst single lay: 6 broken wires, spread across 4 different strands, no more than 2 in any one strand, all of them crown breaks on the outer surface, and similar small counts appear in several lays along the length.

• Against the distributed trigger: 6 is fewer than 10. Not met.
• Against the per-strand trigger: 2 is fewer than 5. Not met.
• Call: stays in service, subject to the rest of the criteria and to the competent person's judgement.
• Diagnosis: breaks spread evenly across strands and along the length are ordinary fatigue. The sling is aging, not being injured by something specific. What that earns is a trend line in the periodic inspection record, so the next count is compared to this one rather than to zero.

Sling B. Within the worst single lay: 6 broken wires, all of them in one strand, all clustered at the throat of the eye where the sling bears on hardware.

• Against the distributed trigger: 6 is fewer than 10. Not met.
• Against the per-strand trigger: 6 is more than 5. Met.
• Call: removed from service. Tag it out and physically get it off the rack, because a sling set aside for a second look is a sling that gets used.
• Diagnosis: breaks concentrated in one strand at one location are a bearing signature, not an age signature. Something at that point is bending or crushing the rope harder than the sling can take, and the usual culprit is an undersized pin. The D over d card in this group carries that relationship.

Same six wires. Opposite calls, and, more usefully, opposite follow-up. Sling A generates a record. Sling B generates a question about every other sling on that job, because they all went over the same hardware.

Why the two thresholds are different numbers

Ten distributed and five in one strand look arbitrary until you put them on the same rope.

Take a construction whose strands carry around nine wires in their outer layer, which is common in the 6 by 19 class, and confirm the actual construction from the sling's documentation rather than from this sentence. Ten breaks distributed across six strands averages about 1.7 per strand, which is roughly 19 percent of one strand's outer layer if they land evenly. Five breaks concentrated in one strand is over half that strand's outer layer gone at a single point, with the other five strands untouched.

So the two triggers are not two versions of the same number. One describes a rope that is generally tired. The other describes a rope with a hole in it. The second condition is reached with half as many wires because a rope's load has to find its way past that point through the strands that are left, and a strand that has lost most of its outer wires at one place cannot hand its share off gracefully.

The damage the count never sees

Broken wires are the visible failure mode, which is why they get all the attention, and several of the worst conditions never show up in a count at all. Each of these stands on its own as a removal condition under the same paragraphs.

• Valley breaks. A wire that breaks down in the valley between two strands, rather than at the crown, usually broke because it was being worked against the core or a neighbouring strand from the inside. Valley breaks mean the damage started internally, so what you can see is a fraction of what is there. Treat a valley break as far more serious than the same count of crown breaks.
• Kinking, crushing, birdcaging or any distortion of the rope structure. A kink is permanent. The rope's geometry is the source of its strength, and once the strands have moved relative to the core they do not go back, no matter how straight the rope looks afterwards.
• Severe localised abrasion or scraping. Flattened crowns over a length reduce the wire cross-section without breaking anything.
• Heat damage. Heat drives off the lubricant, and on a fibre-core rope it melts the core out from the middle. The outside can look nearly untouched.
• Corrosion. Surface corrosion is a warning. Internal corrosion, in the valleys and against the core, is the failure, and it is not visible. A rope that has been in a wet, salted or chemically exposed environment is a routing question for the qualified person your employer designated rather than a count.
• End attachments. Cracked, deformed, worn or corroded fittings, a hook that has opened or twisted, a damaged splice or ferrule. The termination fails as readily as the body and gets inspected as carefully.

What flips the answer

• The sling is rotation-resistant construction. Those ropes have their own, stricter removal criteria and are less tolerant of broken wires than the general figures above. Route to the manufacturer's documentation for the specific product.
• The damage is at a splice or within one rope diameter of a termination. Criteria there are tighter than in the body, and the manufacturer's documentation governs.
• The sling is a matched set and one leg fails. Replacing one leg of a set changes the leg lengths and therefore the angles and the shares, so a mismatched set is a new geometry rather than a repair. Sets get replaced as sets.
• It is not wire rope. None of this transfers. Synthetic slings do not give this warning, which is the whole reason their inspection is a different discipline, and the synthetic inspection card in this group covers it.

How to verify you got this right

Confirm the window before you trust the count. Measure a lay on the actual sling and mark both ends of it, then count inside those marks. A count taken over an unmarked stretch of rope is not comparable to a published criterion, and estimating the lay short is the flattering error because it counts fewer wires.

Then confirm you counted per strand as well as in total. The pair of triggers exists precisely so that a low total cannot hide a crippled strand, and a rigger who only totals will pass Sling B above every time.

Check the location of every break you found, and write it down rather than just the number. A count without a location is half an inspection: it can remove a sling but it cannot stop the next one going the same way. If the breaks cluster at a bearing point, measure the pin or saddle that point sits on before another sling goes over it.

Last, if the sling failed, make the removal physical. Cut the eyes or the body so it cannot be rigged, and take it out of the area. A tag on a rack is a note; a cut sling is a decision.

References

• 29 CFR 1910.184, slings, for general industry work, and 29 CFR 1926.251, rigging equipment for material handling, for construction work, including the wire rope removal-from-service criteria and the daily inspection duty. Establish which Part covers the job before quoting a number from either.
• ASME B30.9, slings, in the edition your authority having jurisdiction, contract or employer programme has adopted, for inspection intervals and documented periodic inspection.
• The sling manufacturer's documentation for the specific construction, which owns the criteria for rotation-resistant rope, splices and end fittings.
• See related: What the D Over d Ratio Does to a Sling; How to Inspect a Synthetic Sling and What Removes It From Service.