Rigging, Slinging and Load Estimation
Know the mass before you lift
Every lift starts with confirming the load's mass and centre of gravity. Use marked mass, shipping documents or calculation from volume and density; never guess. Unknown mass is a stop-work condition. The centre of gravity determines how the load hangs and where the hook must sit. An offset centre of gravity causes load tilt and unequal leg tensions, which can overload one leg even when total capacity looks fine.
Sling angles and leg tension
The included angle between legs matters enormously. As legs approach horizontal, tension rises sharply. The multiplier for a two-leg sling is 1/(2 sin θ) where θ is the angle to horizontal. At 60 degrees, each leg carries about 0.577 of the load; at 30 degrees, about 1.0 of the load per leg. Most gear is rated for a maximum included angle, commonly 120 degrees, and the WLL is reduced for wider angles. Always check the tag and the angle.
Gear selection and inspection
Match gear to the load: shackles, slings, spreader beams, softeners at sharp edges. Check WLL tags, damage, wear and certification. The trap is using a sling at its vertical WLL when it is choked or at an angle; choked capacity is lower than vertical. Also remember that a basket hitch doubles capacity only if legs are vertical and the load is balanced.
Worked example
A 10 t load on a 2-leg sling at 45 degrees to horizontal: leg tension = (10/2)/sin45 ≈ 7.07 t per leg. Select slings with WLL at least 7.07 t each, and verify shackles and master link match.