Rigging, Slinging, Load Estimation and Lifting Gear
Load estimation
Estimating mass is a core skill. Use known data (drawings, markings, weighbridge), then calculate from volume and density for regular shapes. Steel is roughly 7,850 kg per cubic metre; concrete about 2,400 kg per cubic metre; water 1,000 kg per cubic metre. Always add for contents, packaging, ice, water or debris. If the mass is unknown and cannot be calculated, do not lift - get it weighed.
Slinging and gear selection
Select gear rated for the load and the configuration. A two-leg sling at 60 degrees included angle shares load differently from one at 120 degrees; the included angle changes the tension in each leg. Use the manufacturer's rating tables, check for tag and colour coding, inspect for wear, cuts, broken wires and damaged shackles, and ensure hooks have functioning latches. Protect slings from sharp edges and heat. The centre of gravity must be under the hook; otherwise the load tilts and the sling tensions become unpredictable.
Traps
Common traps: assuming a two-leg sling doubles capacity regardless of angle; using a sling without a readable tag; ignoring the effect of an off-centre centre of gravity; and forgetting that lifting points on the load may be weaker than the gear. Another trap is using a shackle pin that can unscrew under vibration - use a mouse or correct pin type.
Worked rule of thumb
For a symmetric two-leg sling, each leg carries half the load divided by the cosine of half the included angle. As the angle widens, leg tension rises sharply. Keep included angles within the gear's rated configuration and never exceed the tag.