Load Estimation and Sling Tension Calculations
Estimating Weight Before You Lift
You must estimate load mass before rigging. Use known data first - mass stamped on the item, delivery dockets, manufacturer data - then calculate from volume and density for regular shapes. Know densities: steel about 7850 kg/m³, concrete about 2400 kg/m³, aluminium about 2700 kg/m³, water 1000 kg/m³, timber roughly 500–800 kg/m³. For a rectangular solid, mass = length × width × height × density. Add the weight of rigging gear itself, and allow for any attachments, contents, ice or water.
Sling Tension and Angle Factors
For a two-leg sling with equal legs at angle θ from the horizontal, each leg tension = (W/2) ÷ sin θ. Equivalently, total vertical share divided by the sine of the angle. The included angle between legs matters: as legs flatten, tension rises sharply. Common multipliers per leg (for a symmetrical two-leg lift) relative to half the load: 30° = 1.0 (vertical legs), 60° = 1.15, 90° = 1.41, 120° = 2.0. The classic trap: a 120° included angle doubles the tension in each leg compared with a vertical lift.
Traps in Questions
Questions often give the included angle between legs when the formula needs the angle to horizontal - included angle of 90° means 45° to horizontal. Another trap is forgetting to add rigging weight, or assuming a four-leg sling shares load evenly (in practice only two or three legs may bear load unless the geometry is rigid).
Worked Example
Load 4 t, two equal legs at 60° to horizontal. Each leg tension = (4/2) ÷ sin 60° = 2 ÷ 0.866 = 2.31 t. If the legs were vertical, each would carry 2 t - flatter legs increase tension.