Why visual inspection is not enough

Wire rope usually fails from the inside first. The outer wires can look intact while strands inside have broken wires, the core has corroded or dried out, and strand-to-strand contact has worn. None of this is visible on the outside of the rope, and hand feeling will not find it either.

Visual inspection does reveal broken wires, reduced diameter, corrosion, deformation and terminal faults, and these are important signals. But by the time they show on the surface, damage has usually been developing for some time. For equipment that lifts people, heavy loads or runs continuously, visual inspection alone cannot establish how much margin remains inside.

Electromagnetic testing does not replace visual inspection; it covers the part visual inspection cannot see. It measures continuously along the whole rope and turns changes in metallic area and local faults into curves that can be located and compared, so replacement decisions rest on records.

How magnetic flux leakage works

Wire rope is ferromagnetic. A test head uses permanent magnets or an excitation coil to set up an axial field and magnetise the rope close to saturation. When the rope is sound, most of the flux travels inside it along the wires and very little leaks outside.

Where broken wires, corrosion pits or a clear loss of section occur, the magnetic resistance rises, part of the flux is forced outside the rope and a leakage field forms. Sensors held against the rope convert that leakage field into an electrical signal; this is the physical basis of the magnetic flux leakage (MFL) method.

The size and shape of the signal depend on the depth, length and position of the defect, and on test speed, magnetisation level and the distance between sensor and rope surface. The same defect can therefore read differently under different conditions, and signal amplitude must not be treated directly as a number of broken wires.

A second signal comes from the main flux. It reflects the total amount of metal in the tested length and is usually called the loss of metallic area (LMA) channel. LMA is sensitive to gradual section loss from wear and corrosion, while the LF channel is sensitive to local faults such as broken wires. Read both together; one channel alone invites misjudgement.

What the instrument can and cannot measure

Electromagnetic testing shows how metallic area changes along the rope, marks the position and relative size of local faults, and allows the same position to be compared on later surveys. Instruments with position encoding can locate a fault to the metre for follow-up on site.

It cannot tell you the condition of the core directly. Drying, breaking and internal corrosion of a fibre core are very hard to resolve from leakage signals, and steel core defects are masked by the outer strands. The grip inside a terminal is likewise invisible to routine testing.

Diameter, lay length, broken-wire distribution and deformation still need gauges and eyes. An electromagnetic signal does not replace a caliper, nor the cleaning and examination of the rope surface. For non-ferromagnetic rope, such as some stainless steel ropes, conventional leakage testing does not apply; confirm the instrument and head are suitable first.

When electromagnetic testing pays off

Not every rope needs it. Short fixed stays, cheap ropes that are replaced on a fixed schedule, and duties where failure has limited consequences are usually better served by visual inspection and planned replacement. Testing costs a shutdown, a specialist and equipment, and that cost has to be justified.

It pays off in several situations: the rope is long and replacement is expensive; downtime is costly; the rope carries people or high-value property; much of the rope runs in closed conduits or hard-to-reach places; there is a history of internal broken wires, corrosion or early failure; or the customer, regulator or insurer requires test records.

A baseline survey on a new rope is valuable. Test once shortly after installation, record the initial signal along each section, and compare every later survey with that baseline. Absolute values are affected by instrument and duty; trends are far less affected and far more reliable.

Reading the results

Start with the overall shape of the LMA curve. Is section loss gradual or a sudden step? Gradual loss usually points to wear and corrosion; a step often appears at a change of rope section, near a terminal, or where the rope was previously damaged.

Then look at the peaks in the LF channel. An isolated spike may be a single broken wire; a dense cluster is more likely corrosion pitting, inter-strand wear or local crushing. Every peak worth noting should be checked on the rope itself, with the position and nature confirmed visually.

Judgement must rest on a stated basis. Discard criteria are usually expressed as a percentage of the original nominal breaking force or metallic area; the actual limit depends on the applicable standard, the equipment type, the duty and the rope construction. Reading the remaining section together with the breaking force margin is more meaningful than a single percentage.

To convert section loss into load capacity, refer to the wire rope breaking strength data

Practical points: speed, calibration and surface

Keep the test speed steady. Speed changes affect the amplitude and filtering of leakage signals, and most instruments specify a recommended range; outside it the signal is distorted. Mark start, stop and acceleration sections separately and do not read them together with the steady-speed data.

Calibration cannot be skipped. Calibrate with a reference rope section or test piece containing known defects, and record gain, threshold and sensor clearance. Run a function check before each survey and confirm both channels respond normally.

Surface condition directly affects the result. Heavy rust, grease, coating and ice alter the magnetic circuit and keep the head from sitting close to the rope. Clean before testing, but do not use methods that change the diameter or surface. Keep the rope centred in the head; eccentricity adds signal fluctuation.

Construction, diameter and nominal data can be compared in the steel wire rope data tables

Records and re-inspection

Records must allow the survey to be repeated. Note instrument model, head specification, test speed, calibration method, rope section identification, date, ambient conditions and the acceptance standard used. Archive the raw curve files; a one-line verdict is not a record.

Set the re-inspection interval by duty. Frequent bending, corrosive atmosphere, heavy load, high temperature or dust all call for shorter intervals. After a baseline, the first few surveys can be closer together, then relaxed on experience once the signals are stable.

Trend matters more than a single point. If the signal at one position rises survey after survey, plan replacement even before the limit is reached. A new peak that appears suddenly, or a large change at the same section between two surveys, should be re-checked at once to rule out instrument, calibration and surface effects.

One-line rule

Eyes read the surface, electromagnetics read the inside, and only the two together make a complete inspection. For long, expensive or hard-to-replace ropes, build a baseline and judge by trend; for short, cheap, easily replaced ropes, spend the testing budget where it matters more.

To fix diameter and construction by duty, start with the wire rope selector