In heavy rigging and crane operations, synthetic roundslings have largely replaced heavy, abrasive wire rope chokers. A roundsling consists of an inner core-made up of hundreds of continuous, untreated polyester or nylon yarns-and an outer woven tubular jacket that protects the core from abrasion and UV light. They are color-coded by capacity, and they are incredibly strong, but they have a fatal, misunderstood weakness: the core cannot be inspected.
The outer jacket is designed to take the wear and tear of the load edges. But the inner core yarns are the actual load-bearing members. If a roundsling is dropped in the mud, dragged across a concrete floor, or crushed between a steel beam and a shackle, the outer jacket might look perfectly intact, perhaps slightly dirty, while the inner core yarns have been severely abraded, cut, or crushed. Because the jacket covers the entire core, a rigger cannot visually inspect the load-bearing fibers. You could be lifting a 10-ton load on a sling that has already lost 50% of its internal strength, and you would never know until it snaps and drops the load.
To combat this hidden failure, manufacturers weave a contrasting "red thread" or "red core" into the inner yarns. The safety rule is simple: "If you see red, the sling is dead." But riggers routinely misunderstand this rule. They think the sling is safe as long as they *don't* see the red thread. This is a dangerous fallacy. The red thread is only exposed if the outer jacket is severely cut or burned. By the time you see the red thread, the core is already heavily damaged.
Furthermore, riggers often try to "repair" a worn jacket by wrapping it in electrical tape or sliding a piece of fire hose over it. This is strictly forbidden. Taping or covering a damaged jacket hides the red thread warning, and it traps heat and moisture against the core, accelerating the degradation. A roundsling must be retired the moment the outer jacket shows any signs of melting, charring, punctures, or abrasive wear that exposes the white core yarns underneath. Never tie knots in a roundsling-the tight bend crushes the internal yarns. And never use a roundsling in a choke hitch that is so tight it bunches the jacket, as this forces all the load onto a tiny fraction of the inner core fibers, guaranteeing a tensile failure under heavy shock loads.
