Load Estimation, Slinging and Lifting Calculations
Estimating load mass
You must know the mass before you lift. Sources, in order of reliability: marked mass, manufacturer data, weighbridge or load cell, then calculated volume times density. Estimation by eye is a last resort and must be conservative. Traps include confusing mass with volume, forgetting that a container's marked mass may be its tare or its maximum gross, and underestimating wet or contaminated loads.
Slinging and sling angles
Sling tension rises as the angle from horizontal decreases. The load in each leg of a two-leg sling is W/(2 × sin θ), where θ is the angle to the horizontal. At 60° each leg carries about 0.58W; at 30° each leg carries W. Always use rated slings with current tags, protect against sharp edges, and match the sling configuration to the load's centre of gravity. Traps: using the included angle instead of the angle to horizontal, ignoring the effect of asymmetric pick points, and forgetting that a basket hitch can double capacity while a choker reduces it.
Gross load and the chart
Add slings, spreader beams and any lifting attachments to the load mass to get gross load, then compare with rated capacity at the actual radius.
Worked example
A 4 t load on a two-leg sling at 45° to horizontal: each leg tension = 4/(2 × sin 45°) ≈ 2.83 t. If each sling is rated 3 t in that configuration, it is acceptable but close; check the tag and configuration carefully.