Load Weight, Center of Gravity, and Sling Tension
Core Concepts
Before any lift, the rigger must know the load's weight and its center of gravity (CG). Weight can be found from shipping papers, nameplates, or calculated from volume and material density. The CG is the point where the load would balance; if the CG is not directly below the hook, the load will tilt. Riggers must identify the CG and rig so the hook is over it, or account for the offset.
Sling Tension and Angles
Sling tension depends on the load weight and the angle of the sling legs. For a symmetrical two-leg lift, each leg carries half the load divided by the sine of the angle from horizontal-so flatter slings carry far more tension. A common rule: at 30° from horizontal, each leg carries the full load; at 60°, about 0.577 of the load; at 90°, half the load. Asymmetric lifts and off-center CGs shift more load to one leg, and the rigger must calculate or conservatively estimate that share.
Traps and a Rule of Thumb
Traps: using the load's total weight as if it were shared equally when the CG is off-center; forgetting to add the weight of the rigging itself; and confusing the angle from horizontal with the angle between legs. Another trap is assuming a four-leg sling shares load equally-in practice, only two legs may bear the load unless the lift is rigid and level.
Rule of thumb: flatter is heavier. Keep sling angles steep (closer to vertical) whenever possible, and always verify the CG before choosing pick points.