When a Crane's Wire Rope Must Be Replaced

A crane's wire rope is a consumable — and it's also something lives depend on.

In this business, cranes are the one machine we never cut corners on. When an excavator breaks, it sits there. When a crane breaks, it hits people.

In November 2016, a customer skipped a rope replacement to save 1,800 yuan. Three months later the rope snapped. This article is about his loss, and about the standards for replacing rope.

The accident

The customer was in Lagos, running a 25-ton truck crane bought from us; the rope had been in service for two years. The site used it daily — lifting formwork, lifting rebar.

When he bought the crane we gave him a maintenance schedule. The rope replacement item — he had checked it off, read it, and never taken it seriously.

When we shipped parts in November, we reminded him as a matter of routine: the rope is due, there are already plenty of broken wires on the outer layer. A new rope is 1,800; with freight, 2,200.

He said he'd look at it later — the site was busy, couldn't afford downtime, and the crane earned more per day than a rope cost.

In February 2017, while lifting a generator set, the rope snapped. Three meters up, one sharp crack. The generator crashed down and smashed its base — a $14,000 loss. Mercifully nobody was hurt, and the boom checked out undeformed.

He called me. His first sentence: the 1,800-yuan rope — I saved it. His second: send me two new ropes.

Three days after the accident he sent me photos of the broken rope. Half the break was old, long-broken wires; half was fresh. The old half should have been seen long ago.

The site shut down for four days of safety inspection, and non-destructive testing on the boom cost another two thousand dollars. Add it all up and it buys eight new ropes.

Broken wire count: the easiest check

The first retirement criterion is broken wires. If broken wires within one lay length exceed ten percent of the total wire count, the rope must go. For a six-strand rope, more than ten broken wires in one lay length means it's done.

How to check: put on gloves, wrap the rope in cotton waste, and run it slowly along. Broken wires snag the cotton — count them and you're done.

How long is one lay length? It's the distance for one strand to wrap once around the rope — roughly six to seven times the rope diameter. If unsure, measure a twenty-centimeter section and err on the strict side.

Broken wires clustered in one spot are more dangerous than scattered ones. Clustering means that strand took a hit — pinched or crushed by something.

Don't only check what's visible. The rope section wound on the drum wears fastest — pay out the rope and check all the way to the end. Many ropes snap on the drum section.

Diameter, corrosion, deformation

Second, check diameter. Measure with a caliper: if it's down seven percent from nominal, retire the rope. Shrinkage means internal wear or a ruined core — invisible from outside, hollow inside.

Measure across the crowns of the strands, not the valleys. Take two readings at the same cross-section, rotated, and use the larger one.

Third, corrosion. Surface rust that wipes off is trivial — just grease it. But if rust has crept between the wires and the core has gone dry and brittle, replace it. Pry the strands open and look inside. Red rust inside — don't hesitate.

Fourth, deformation. Kinks, flattening, birdcaging, strands pushing out — any one of these is immediate retirement, no negotiation. A deformed rope no longer carries load evenly.

On a seaside site I once saw a one-year-old rope that gleamed on the outside and was solid red rust inside. The owner didn't believe it — he said it looked perfectly fine.

Sockets and rope ends matter too

Cracks at the poured socket, or clips that slip out of position — the whole rope is scrapped. Many accidents aren't rope breaks; they're rope ends pulling loose.

While checking rope, check the drum and sheaves too. A sheave with worn-sharp grooves chews up a new rope — three months and it's ruined.

How often to replace

There's no fixed interval — it depends on duty. Three reference numbers:

A site crane working every day: replace yearly. Intermittent use: two to two and a half years. Seaside and salt-spray environments: cut the interval in half — salt eats rope faster than wear does.

Every time you replace a rope, write the date and hours with a paint marker on the rope end — who changed it, when — visible at a glance next time. After your second replacement you'll know the true interval for your own duty cycle, more accurate than any manual.

There is no such thing as repairing wire rope. Splicing a broken rope, welding a worn one — that's gambling with lives. Rope gets replaced, never repaired.

Not sure if it's due? Run through the four criteria above; if any single one is exceeded, replace. An 1,800-yuan rope versus $14,000 of cargo — no comparison needed.

Two small things about replacing rope

Wind new rope onto the drum under tension. Wind it slack and the upper layers bite into the lower ones — half the rope life gone. Best done by two people: one paying out, one holding tension.

For the first fifty lifts after replacement, halve the load and let the strands seat themselves. Squeaking in the first few lifts is normal — the strands are finding their positions.

Don't throw the old rope away. Cut a section and keep it next to the new rope for comparison. Once you can see the difference, you've learned half of it.

That Lagos customer now orders rope on schedule every February, two at a time. He says it's a date he can remember — the month the rope snapped.

The numbers in this article — ten percent broken wires, seven percent diameter loss — are the retirement criteria in the Chinese national standard. If you can't remember them all, remember these two.

Before publishing this, I showed it to him. He said: write it, leave my name out, let people know which is more expensive — 1,800 or 14,000.

Rope isn't expensive. Downtime is expensive. Accidents are the most expensive of all.