Rigging, Slinging and Load Estimation
Estimate before you sling
You must know the load's mass and centre of gravity before selecting gear. Sources include the data plate, drawings, weighbridge tickets and calculation. Where mass is unknown, weigh it; do not guess. Estimate the centre of gravity so the hook sits above it and the load hangs level. An off-centre lift shifts load between legs and can overload one sling.
Sling angles and tension
Sling tension rises sharply as the angle from horizontal decreases. The load factor is 1 divided by the sine of the angle from horizontal. At 60 degrees tension equals about 1.15 times the share of load; at 45 degrees about 1.41; at 30 degrees 2.0. Two-leg slings therefore need a wider angle or larger capacity. Also account for the included angle at the hook and any choke or basket configuration, which changes the effective capacity of the sling.
Gear selection and inspection
Know the working load limit (WLL) of wire rope slings, chain slings, synthetic roundslings and shackles, and the tag colour systems used for inspection. Check for cuts, broken wires, corrosion, stretched links, heat damage and missing tags. Protect slings from sharp edges with softeners. Use correct shackle pins, moused or secured, and never side-load a shackle beyond its rated configuration.
Traps
Questions often present a two-leg lift with a 30-degree angle and expect you to double the tension. Another trap is assuming a tag on the load states the true mass. Balance is also tested: a load whose centre of gravity is outside the sling triangle will tilt.
Worked example
A 10 t load on a two-leg sling, each leg at 45 degrees from horizontal. Each leg carries 10 divided by (2 times sine 45), which is about 7.07 t. Choose slings and shackles rated above that, not above 5 t.