How a Chain Sling Fails Differently From Wire Rope

Why this matters

A chain sling and a wire rope sling of the same rated capacity will hold the same load on a good day. They do not tell you the same thing on a bad one. Chain deforms before it parts, so its damage is a dimension you can measure with calipers and a tape. Wire rope loses individual wires while the survivors pick up their share, so its damage is a count you have to take per unit length. Bring the chain method to a wire rope sling and you will pass a sling that is well into its decline; bring the wire rope method to a chain sling and you will stare at a stretched link and see nothing wrong with it.

Two materials, two different warnings

Alloy steel chain is a ductile assembly of identical links. Overload it and the links yield before anything breaks, which means the sling gets measurably longer and each stressed link gets measurably thinner at its bearing points. That is the warning, and it is generous: the sling is telling you in inches that it went somewhere it should not have.

Wire rope is a bundle of many small wires laid into strands and the strands laid around a core. It has no single yield point you can see. Wires break one at a time from fatigue, abrasion and corrosion, and each break transfers that wire's share onto its neighbours, so the rate of breaking accelerates. The warning is real but it is statistical: you are counting how far a population has degraded, not measuring how far one part has moved.

Write the mechanism and the symptom as one pair before you inspect. Chain: plastic deformation, so look for a changed dimension. Wire rope: progressive wire fracture, so look for a changed count. The instrument follows from that, and so does what a clean result is worth.

What the tag already contains

Both tags publish a rated capacity, and both ratings already have a design factor inside them. The design factor is the ratio of the sling's nominal breaking strength to the capacity printed on the tag, and under ASME B30.9, Slings, in whichever edition your jurisdiction, your contract or your employer's programme has adopted, it is not the same number for both materials: alloy steel chain slings carry a design factor of 4, wire rope slings carry 5. That difference is not an accident of tradition. Chain gets the lower factor partly because it warns you dimensionally before it fails, and wire rope gets the higher one partly because it does not.

Two consequences follow, and both get missed. First, a rated capacity is a statement about ONE loading condition: a straight vertical pull, single leg, on a sling in as-new condition. Every hitch other than vertical, and every angle other than straight up, is a departure from the condition the number was published under, and the departure comes off the capacity or gets added to the tension. Second, the design factor was computed for a STATIC pull. It is not a shock allowance and it is not spare capacity you are entitled to spend.

Chain: the damage is a dimension

Three measurements, all taken with the sling on the ground, unloaded, off the hook, with the load landed and blocked so nothing can settle onto your hands.

Overall reach against the tag. The tag states the reach. Measure it. Chain that reads longer than its tag has yielded, and yielding is not a wear allowance, it is evidence of an overload. Any measurable elongation past the tag is a removal.

Link body at the bearing points. Each link wears where it seats against its neighbours, on the inside of the crown. Caliper the link body there and compare it against the chain manufacturer's minimum allowable body dimension, which is tabulated by chain size in the manufacturer's own literature and is the value that owns this decision. 29 CFR 1910.184(e) for general industry sets the same idea as a maximum allowable wear at any point of the link, tabulated by size.

Link geometry. A link that is stretched, bent, twisted, gouged, nicked or cracked is out, with no measurement and no discussion. So is any evidence of heat damage or weld repair, and so is any sling whose durable identification tag is missing, because without the tag you no longer know the grade, the size, the rated capacity or the reach.

Grade is part of this. Only alloy grades intended for overhead lifting, commonly the Grade 80 and Grade 100 designations, belong in a lifting sling. Proof coil and high test chain, the Grade 30 and Grade 43 designations sold for tie-down and utility work, are not overhead lifting chain and no inspection result makes them acceptable.

Wire rope: the damage is a population

The unit of measurement is the rope lay: the length along the rope over which one strand makes one complete revolution around the core. Find it by picking one strand and marking where it returns to the same clock position. Everything is counted inside that length.

For wire rope SLINGS, 29 CFR 1910.184(f) in general industry sets removal at ten randomly distributed broken wires in one rope lay, or five broken wires in one strand in one rope lay. That criterion is for slings. The criterion for a crane's running rope is a different and tighter one and it lives in 29 CFR 1926 Subpart CC for construction cranes, so do not carry one number across to the other.

Wear cut-resistant gloves for this and never run a bare hand along a wire rope to find broken wires. A broken wire stands proud with a fractured end and it will go through skin.

The count is necessary and it is not sufficient. Remove the sling regardless of count for kinking, crushing, birdcaging, unstranding, corrosion pitting between the wires, heat damage, and any damage or distortion at the end attachment, because each of those is a loss the surface count cannot see.

D over d: the derate chain barely feels

Bend a wire rope around something and the outer wires have to travel a longer path than the inner wires. The tighter the bend, the more unevenly the load distributes across the wires, and the less of the rope's strength you get. The governing ratio is D over d: D is the diameter of whatever the rope bends around, d is the rope's own diameter, and a wire rope sling's published rating assumes a generous ratio. As D over d falls toward one, efficiency falls sharply, and by the time the rope is bent around something its own diameter you are into the region where roughly half the strength has gone. The rope manufacturer's D over d efficiency table owns the actual factor for your rope construction.

Chain does not have this problem in the same form, because a chain link is already designed to bear against a link of its own size. What chain has instead is a corner problem: a link loaded across a sharp edge takes a bending load the link was never rated for, and the fix is the same as for rope, which is to soften the corner with proper edge protection rather than to derate and hope.

One gate, two slings: the pick that snatched

The gate is a single question: this sling was on a pick where the operator hoisted through slack and the load came off the deck abruptly. Does it stay in service?

The pick, with the angle convention stated, because half the trade quotes it the other way: all angles here are measured from horizontal. A two-leg bridle, hook centred over the load's centre of gravity, legs at 60 degrees from horizontal.

Take an illustrative load of 6,000 lb, with the hook centred so each leg's vertical share is 3,000 lb.

• Rated capacity, chain leg, as tagged: single leg, vertical hitch, say 4,300 lb.
• Hitch correction: this leg is a straight vertical hitch, so the factor is 1.00 and the capacity stays 4,300 lb. Were it choked, the tag's choker rating would replace this figure, not adjust it.
• Angle correction, applied to the TENSION rather than the capacity: leg tension is the leg's vertical share divided by the sine of the angle from horizontal. Sine of 60 degrees is 0.866, so 3,000 / 0.866 = 3,464 lb, carried as 3,470 lb because rounding a tension upward is the conservative direction.
• Result: 3,470 lb of leg tension against 4,300 lb of rated capacity, about 81 percent. Statically acceptable.

Then the snatch happened, and here is where the design factor discussion pays. The 4:1 on that chain was computed for a static pull. Hoisting through slack applies a dynamic load whose multiplier nobody on that job measured. 29 CFR 1910.184(c) prohibits shock loading outright in general industry, and 29 CFR 1926.251 is the construction home of the same duty. So the number you would need in order to clear the sling arithmetically does not exist, and that is the finding.

Chain leg, run through the gate. Tag reach 6 ft 0 in. Measured reach 6 ft 0-3/8 in, which is 0.5 percent longer. Chain is ductile and it has moved. Out of service, tagged, and physically separated from the rigging stock so nobody re-hangs it. The chain answered the question.

Wire rope leg, run through the same gate. Visual: nothing obvious. Count per rope lay: four randomly distributed broken wires in the worst lay, two of them in a single strand. Both counts sit under the 1910.184(f) thresholds of ten and five. Under the count criterion alone, this sling stays in service.

It should not. The count criterion detects progressive fatigue, and the event this sling went through was an overload. An overload can crush the core, break the lay pattern internally and leave the outer wires looking untouched, and the count has no visibility into any of that. The criterion that condemned the chain does not exist for the rope, and the criterion that would clear the rope does not detect what the event did.

So the resolution is not arithmetic. A shock-loaded wire rope sling with an unquantified overload goes to a qualified person and, where the event is severe or the sling is critical, back to the sling manufacturer, and it stays out of service until one of them clears it. Where the load's weight and the dynamic multiplier are both unknown, that is the finding rather than a gap to guess past.

How to verify you got this right

Pull one sling of each type from your own rack and try to state, before you touch either, which instrument decides it. If you reach for the tape on the wire rope or the count on the chain, you have the two mental models crossed.

Then check three things that catch most of what goes wrong. The chain slings in stock all still carry legible identification with size, grade, rated capacity and reach, which 29 CFR 1910.184(e) requires and which a missing tag ends. Your periodic inspection is on a schedule rather than on a memory, at intervals no greater than 12 months for chain slings under 1910.184(e), with a record, on top of the each-shift inspection before use. And the removal decisions you made this year are traceable to a measured value or a count, not to a judgement that the sling looked fine.

References

• 29 CFR 1910.184, Slings (general industry): safe operating practices at (c) including the shock loading prohibition, alloy steel chain slings at (e), wire rope slings at (f) including the broken-wire count per rope lay
• 29 CFR 1926.251, Rigging equipment for material handling: the construction counterpart, including inspection before use on each shift
• 29 CFR 1926 Subpart CC, Cranes and derricks in construction: running rope inspection criteria, which differ from sling criteria
• ASME B30.9, Slings, in the edition adopted by your jurisdiction, your contract or your employer's programme: design factors by sling material and the basis of published ratings
• Sling and chain manufacturer's literature: minimum allowable link body dimension by chain size, choker and basket ratings, and the D over d efficiency table