Rigging, Hoisting, and Load Handling
Load weight and centre of gravity
Every lift starts with a weight you can defend. Steel is 7850 kg/m³, so a 1 m² plate 25 mm thick is 0.025 m³ × 7850 = 196 kg. The centre of gravity must sit under the hook, or legs take unequal shares and the load lands crooked.
Sling tension
- Two-leg bridle, equal legs, angle θ measured from vertical: tension per leg = (W ÷ number of legs) ÷ cos θ.
- Choker hitches derate the sling; basket hitches roughly double capacity when vertical; the horizontal sling angle governs everything.
- Shackle WLL, hooks with working latches, no shop-made fittings, taglines on any load that will rotate.
Load charts and the real radius
The chart capacity is not the lifted capacity. Deduct the hook block, slings, spreader beam and any stowed attachment; measure radius from the slew centre to the load's centre of gravity; confirm the outrigger spread and ground bearing match the chart you are reading.
Worked example: bridle tension
An 8000 kg load on two legs. At 30° from vertical: tension = (8000 ÷ 2) ÷ cos 30° = 4000 ÷ 0.866 = 4619 kg per leg. At 60° from vertical: 4000 ÷ 0.5 = 8000 kg per leg, equal to the whole load in each leg. This is why low sling angles need engineered approval.
Inspection and rejection
Know removal criteria for wire rope (broken wires per lay, diameter loss, kinking, heat damage), synthetic slings (cuts, UV damage, worn cover, missing tag) and chain (stretch, gouges, cracked links). No tag means out of service.
Traps
- Using the angle from horizontal in a formula that wants the angle from vertical.
- Assuming equal leg loading when the centre of gravity is offset.
- Shock loading: a snatch off the floor multiplies tension far beyond the static value.
- Turning a load without a second line and a plan for the shift in centre of gravity.