Rigging, Sling Inspection, Hitch Selection, and Load Control
Know the hitch, know the derating
Every sling carries a vertical capacity on its tag; the hitch changes the math. Vertical = 1.00, choker = 0.80 (typical - check the tag), basket = 2.00 only when the legs are vertical. Multi-leg assemblies share load only in theory: T = W ÷ (n × sin θ), where θ is measured from horizontal. The multiplier is 0.5 at 30°, 0.707 at 45°, and 0.866 at 60°. Low sling angles are the most common cause of overloaded slings.
Inspection and removal criteria
Wire rope slings: ten broken wires in one rope lay, five in one strand in one lay, kinking, bird-caging, core protrusion, heavy corrosion, damaged end fittings, or a missing/illegible tag → remove from service. Chain: cracks, stretch, gouges, weld repairs, worn or deformed links. Synthetic web: cuts, burns, UV degradation, chemical damage, broken stitching, or exposed red warning yarn. Hooks: missing latch, throat opening increased more than 15%, wear over 10%, or twist over 10° → tag out.
Load control
Locate the center of gravity before the pick; an off-center load does not split evenly between legs. Use edge protection on sharp corners, spreader bars to keep angles steep, tag lines to stop rotation, and softeners where the sling bears on the load.
Traps
- Using 2 × W for a basket hitch whose legs are sloped.
- Reading the sling angle from vertical instead of horizontal.
- Assuming identical tension in both legs of an asymmetric bridle.
- Forgetting that a knot or a D/d ratio below 25 reduces capacity.
Worked example
Two-leg bridle at 60° from horizontal, 10,000 lb load: 10,000 ÷ (2 × 0.866) = 5,774 lb per leg. At 30°: 10,000 ÷ (2 × 0.5) = 10,000 lb per leg - double.