Why Wire Rope Assemblies Fail

Why do wire rope assemblies fail?

OSHA lists broken wires, kinking, crushing, bird caging, overloading, improper storage and use, and environmental conditions as causes of wire rope sling failure. The same mechanisms apply to cable assemblies in machines, controls, and equipment.

What design factor should you use for a wire rope assembly?

Breaking strength is the point of failure, not a working load. Set the working load limit by dividing the cable’s minimum breaking strength by a design factor, typically between 5:1 and 10:1 depending on risk, application, and environment. Lifting applications may require 5:1. Life-safety and fall-protection systems may require 10:1 or higher.

Breaking strengths for every size and construction are on our wire rope breaking strength chart.

Why do wire rope assemblies break at the end fitting?

Usually because the fitting holds less than the cable, or because it was installed wrong.

The fitting is rated below the cable. A standard ball end on 1/8″ and larger cable holds 60% of cable breaking strength. A hand-swaged fitting holds up to 60%. Apply your design factor to whichever is lower: the cable’s breaking strength or the termination’s holding strength.

The fitting was installed wrong. FAA guidance for aircraft cable flags three installation problems: cable not inserted deep enough (cases with only 1/4″ of cable inside the terminal), cable slipping in the fitting during swaging, and swaging pressure pinching small wires, which leads to premature broken wires near the fitting. Compare fittings in our termination methods guide.

How big should a pulley be for wire rope?

As big as the design allows. Every time a cable bends over a pulley and straightens, the outer wires flex, and over enough cycles they crack. Fatigue research consistently finds that a smaller pulley relative to cable diameter shortens cable life, and higher tension shortens it further.

Industry pulley tables published by wire rope and pulley manufacturers size the pulley tread as a multiple of cable diameter (the D:d ratio). Values vary slightly by source:

The more flexible the construction, the smaller the pulley it tolerates. That’s why 7×19 is usually chosen over 7×7 for cables that run over pulleys. See cable construction.

What pulley problems cause wire rope wear?

FAA inspection guidance links cable damage to pulleys that are rough, have sharp edges or debris in the groove, have bad bearings, are misaligned so the cable rides on the flange, or are the wrong size for the cable. Too much cable tension shows up as pulley wear too.

How does corrosion weaken wire rope?

Corrosion removes load-bearing steel, and the worst of it can be inside the rope where you can’t see it. FAA guidance treats corrosion on the interior strands as failure: the cable must be replaced. It also warns against cleaning cable with steel wool, which embeds dissimilar metal particles and causes more corrosion, or with solvents, which strip internal lubricant.

Material choice is the first defense. Stainless steel is chosen for corrosion resistance. Galvanized cable has marginally higher breaking strength but less corrosion resistance. Compare materials on our wire rope and aircraft cable page.

Can wire rope wear from the inside?

Yes. FAA guidance notes that wire rope running over pulleys wears internally as the inner wires rub against each other, and under some conditions internal wear outpaces surface wear. It isn’t visible unless the strands are separated. For surface wear, the FAA calls for replacement when the outer wires are worn 40 to 50%, which shows up as the individual wires in each strand appearing to blend together.

Can a kinked wire rope be straightened and reused?

No. A kink pulls strands and wires out of position, creating unequal tension and excess wear at that spot. FAA guidance says that even if the kink is straightened and the damage looks slight, the cable can’t give full service and should be replaced. The most common cause is improper unreeling and uncoiling. The same guidance calls for replacing any cable with a popped core or loose strands.

When should coated wire rope be replaced?

For nylon-jacketed cable, FAA guidance says the cable’s usable life is over when the jacket shows cracks or necking down in diameter. A vinyl or nylon coating adds abrasion resistance and longevity, but it doesn’t change the steel cable’s internal strength.

Does heat damage wire rope?

Yes. Any evidence of heat damage is an OSHA removal criterion for wire rope slings. If an assembly sits near welding, torches, or hot equipment, plan for that in the design.

How do you inspect a wire rope cable assembly?

• Focus on the high-fatigue zones. FAA guidance defines these as anywhere the cable runs over, under, or around a pulley, sleeve, or fairlead, anywhere it’s flexed or rubbed, and within 1 foot of a swaged fitting.
• Wipe with a cloth. Run a cloth along those zones. It snags on broken wires.
• Look closely. A broken wire doesn’t always stick out. It can stay in place in the strand and show only as a hairline crack. The FAA recommends a magnifier of 7 power or greater if you suspect one.
• Check the fittings. Look for slippage at the swage reference mark, distortion, and broken strands at the terminal.
• Check the pulleys. Roughness, alignment, bearings, and groove condition.

When should a wire rope assembly be taken out of service?

The threshold depends on the application. Two federal references are widely used:

How do you design a wire rope assembly that won’t fail early?

• Set the design factor first. Decide on 5:1, 10:1, or higher based on the application, then size the cable from the working load.
• Match the termination to the load. If the fitting holds less than 100%, size the assembly from the fitting’s holding strength.
• Size the pulleys. Aim for the desirable D:d ratio for your construction, and use a flexible construction like 7×19 for cables that cycle.
• Pick material for the environment. Stainless for corrosion, galvanized for marginally higher strength where corrosion isn’t a concern, coating for abrasion.
• Plan inspection. Decide who checks the assembly, how often, and against which removal criteria.