Rigging and Lifting
Load, geometry, capacity
Start with weight: calculate volume × density (steel ≈ 490 lb/ft³ or 0.283 lb/in³) or read it from the drawing. Locate the center of gravity - an off-center pick tilts and shock-loads one leg. The working load limit comes from the tag, and design factors per ASME B30.9 are chain 4:1, wire rope 5:1, and synthetic web 5:1.
Hitches derate capacity. A choker is roughly 75–80% of a single vertical leg; a basket hitch gives about twice the capacity only when both legs are truly vertical; a two-leg sling no longer doubles capacity once the angle drops.
The angle math
Tension per leg = (W ÷ number of legs) ÷ sin θ, where θ is the angle from horizontal. For two legs: at 60° each carries 0.577 W, at 45° 0.707 W, at 30° a full 1.0 W, and at 15° nearly 2 W. Questions often quote the included angle between the legs - convert it before calculating.
Hardware and practice
Respect D/d ratio for wire rope over pins and sheaves, use edge protection, tag lines, and hook latches, never side-load a hook, use spreader bars for wide loads, compute ground bearing pressure under outriggers, and use standard hand signals with one designated signal person.
Traps
Using the included angle in place of the angle from horizontal, ignoring shackle and hook ratings, shock loading, and exceeding the load chart at radius.
Rule of thumb
Keep sling legs at 60° from horizontal or steeper; below 45° capacity falls away quickly.