What Does Bird Caging Mean?
Bird caging is a slang term used in the rigging and lifting industry to describe a severe form of wire rope distortion that can occur when the rope becomes twisted or its load is suddenly released. It causes the individual strands of the cable unravel and expand, giving it an appearance similar to a bird cage.
Bird caging is observed most commonly in rotation-resistant wire rope. It can be very hazardous and a sign that the cable should be immediately removed and replaced.
Bird caging is also known as lantern deformation or basket deformation.
Safeopedia Explains Bird Caging
Wire ropes are composed of multiple strands. These strands can experience bird caging due to torsional vibration or sudden release of tension, as well as several other causes including:
- Mismatch between rope diameter and drum groove dimensions
- Incorrect installation of rope in the crane reeving, which can lead to the ropes crossing over and coming into contact with each other
- Using wire ropes with the wrong sheave grooves for the rope diameter (the correct sheave groove tolerance is equal to the wire rope's nominal diameter plus 7.5%)
- Incorrect shortening (both ends of the wire rope must be secured properly during shortening, otherwise it can cause the layers to move against each other, resulting in an imbalance)
- Bad crane design or improper modifications
- Manufacturing defect (although rare, it is nevertheless possible)
Bird caging (source: WorkSafeNB)
Wire Rope Safety and Inspection
A rope with a bird cage deformation is no longer safe and must be discarded.
OSHA standard 1926.1413 provides instructions regarding wire rope inspection. It requires a competent person to conduct a visual inspection for any apparent deficiencies before each shift during which the equipment will be used. These deficiencies include bird caging, kinking, unstranding, crushing, and steel core protrusion between outer strands.
Another example of bird caging (source: DrillSafe)
OSHA also requires periodic inspection of wire rope slings at intervals no greater than 12 months. It is advisable to inspect wire rope slings yearly for normal use, monthly to quarterly for severe use, and as recommended by a qualified person for special and infrequent use.