Load Estimation, Slinging, Lifting Gear and Load Charts
Accurate load estimation is the foundation of every safe lift. The total load includes the payload, the lifting gear (sling, shackles, spreader beam, hook block if required by the chart) and any absorbed water, ice, or loose material. Estimating by eye is a common failure; use markings, drawings, weighbridges or calculation where possible. Slinging then transfers the load safely: know the sling type (chain, wire rope, synthetic web, round sling), its working load limit, its configuration (single leg, two-leg, four-leg, choke, basket, vertical) and the effect of angle. As the angle between legs opens, leg tension rises sharply - a two-leg sling at 90° included angle carries about 0.71 of the load per leg, but at 120° it carries the full load per leg.
Lifting gear selection requires matching the working load limit to the calculated tension, inspecting gear before use (tags, damage, wear, certification), and using correct attachment points. Load charts are specific to configuration: jib length, counterweight, rope falls, boom/trolley position, outrigger or track configuration, and they are derated for wind, for personnel lifting (where permitted) and for special conditions. Read the chart at the actual radius, measured horizontally from the slew centre to the load's centre of gravity - not to the hook.
Traps: reading radius to the hook instead of the load centre; ignoring the weight of the rigging; using a four-leg sling as if all legs share load equally when the load is rigid; using gear without a legible tag; and using a chart for a different jib or counterweight configuration.
Worked example: a 3 t load on a two-leg sling at 90° included angle: each leg tension ≈ 3 / (2 × cos45°) ≈ 2.12 t. Select gear rated above that.