Birdcaging is structural failure, not wear

Birdcaging means the outer strands leave the core and bulge outward, opening the lay locally so the rope looks like a cage. It is not surface abrasion and not a broken wire: the geometry of the rope has already failed.

The same bulge is sometimes called strand pop-out or a loose strand. The name differs, the nature is the same: the outer strands no longer match the length of the core and are forced outward. Once a bulge appears, running it a while longer is not an option.

An early birdcage is most visible when the rope is slack: the bulge opens before load and closes again under tension, so the rope looks normal. Inspect with no load on the rope, and run a hand along the surface to feel for a local rise or a widening gap between strands.

What happens mechanically

A rope is a balanced helical structure. The outer strands follow a helix and their path is longer than the core; if the core is compressed or the strands are stretched, that balance breaks. When the outer strands become relatively longer with nowhere to go, they bulge outward and form a birdcage.

The reverse case is a strand pulled short or a core pulled long, so the strand sinks into the rope and shows as a loose or sunken strand or an uneven diameter. Either way the internal tension distribution is disturbed, and further loading only enlarges the deformation.

Cause one: torsion

Torsion is the most common cause. When a rope is twisted during pay-off, installation or running, the outer strands are screwed open along the helix, like untwisting a hemp line. A single-part lift of a load free to rotate, a swivel used where it should not be, or pulling the rope from the side of a coil all bring this torsion.

Pay-off method is critical. Lay a coil flat on the ground and pull from the side, and every turn pulled out adds one twist; the correct way is a pay-off stand or turntable that lets the coil rotate, with the rope leaving tangentially and a little back tension kept on it.

Lay direction changes the consequence of torsion. In a lang lay the strands and wires run the same way, which makes the rope more flexible but lets the strands open in the same direction more easily under twist; a regular lay is more stable but is not immune. Whatever the lay, torsion during pay-off must be avoided.

Cause two: shock and sudden release

Shock also causes birdcaging. The rope stretches elastically under a sudden load and snaps back when the load is released; outer strands and core recover at different speeds over different path lengths, they move relative to each other and the lay opens.

Sudden slack, a load run out, hard braking and a load hitting the ground can all trigger it. If the diameter grows locally and the lay looks uneven after a shock, treat it as birdcaging even if the bulge has not fully opened.

In energy terms, the elastic energy stored in a shock rises with the square of the extension. For the same load, if the rope first falls slack for a distance and then comes taut, the stored energy multiplies and the relative movement between strands and core is more violent.

Cause three: spooling and grooves

Poor spooling on the drum causes strand pop-out as well. With too little tension the layers are loose, the upper rope presses into the gaps between turns below, and the crossover flattens the rope and squeezes strands out. A drum groove that does not match the rope diameter and uneven spooling make it worse.

A sheave that is too small or a groove that is too narrow adds compression and bending. The rope is pinched in the groove, the outer strands try to bulge but are held by the groove walls, and the bulge appears after the rope leaves the sheave. This type usually shows on the exit side.

Why it cannot be repaired

Birdcaging cannot be fixed by pushing the strands back. The helix has already deformed plastically and the strand length has changed; pushing it back only makes the outside look flat for a while, while the internal tension distribution stays wrong.

Worse, a gap has opened between strands and core, so water and dirt get in and corrosion starts there; in service the bulge opens again and again and can end in strand failure or rope failure. Standards generally treat birdcaging as grounds for immediate replacement; check the applicable standard and the maker's advice, and for the general logic see the wire rope discard criteria

Some crews cut out the bulged section, re-terminate the rest and keep using it. That is only acceptable where the standard allows it, the remaining length is sufficient and the rest of the rope has been assessed; the new length, terminations and safety factor must be recalculated, not decided on site.

Kinks and loops

A kink is another kind of irreversible damage. The rope first forms a loop, the loop is pulled tight and the rope is forced into a very small radius at the crossing, the wires pass their elastic limit and a permanent bend remains.

A kink shows as a sharp angle, possibly a flattened diameter and a disturbed lay. The wires there have been severely bent and stretched and cannot recover their strength; even if the rope is pulled straight, the damage remains. A kink should be treated as a failure.

Prevention in pay-off, installation and operation

Prevention starts with correct pay-off. Use a stand, turntable or reel support that lets the coil rotate freely and the rope leave tangentially; never pull from the side and never drag the rope on the ground so that loops form. Store it raised off the ground and dry, and keep coils from being crushed, as described in wire rope storage and handling

Keep back tension during installation and spool the rope in even, layer-on-layer turns; do not hammer the rope into a groove. In service, avoid sudden slack and hard stops, and where a load can rotate freely, use a suitable anti-rotation arrangement or a two-part line.

Watch the first few runs after fitting a new rope. Run the full length empty, then run it loaded, and check that the rope sits centred in the grooves, spools evenly and shows no local bulge. Catching a problem early is far cheaper than finding it after hundreds of hours.

What to do when it is found

On finding birdcaging, strand pop-out or a kink, stop the equipment, tag it, record the position and appearance, then check the rest of the rope and the adjacent equipment. Do not try to hammer or pry the strands back, and do not cut out a section and keep using the rope on site unless a competent person has assessed it against the standard. For the inspection rhythm, see wire rope inspection intervals

Before fitting a new rope, find the cause: the pay-off method, drum spooling, sheave diameter and groove, rotating loads and brake settings. If the cause is not removed, the new rope will do the same thing. To match a rope to the duty, go through wire rope quotation