Rigging, slinging, load estimation and centre of gravity
Rigging transfers the load safely from pick to placement. You must understand sling types and their limits: chain, wire rope, synthetic webbing and roundslings, each with tags, working load limits and inspection criteria. Know the effect of sling angle. As the angle between legs widens, tension rises sharply. Two-leg slings at 60 degrees from horizontal carry about 0.58 of the load per leg, at 45 degrees about 0.71, and at 30 degrees a full 1.0 of the load per leg.
Load estimation is a core skill. Use known mass, volume times density, or manufacturer data. Never guess a heavy or unfamiliar load. Centre of gravity determines how the load hangs and how it behaves during slew. An offset centre of gravity causes tilt, load shift and unpredictable swing. Identify the centre of gravity, rig above it, and use taglines to control rotation.
Traps: forgetting the included angle versus the angle to horizontal, using damaged or unmarked gear, double-counting sling legs beyond two when the load is rigid, and ignoring edge protection or sharp edges cutting webbing. Another trap is shock loading from a slack take-up or a load dropping onto the slings.
Rule of thumb: keep sling angles at 60 degrees or steeper to horizontal and use edge protection always. Worked example: a 6 t load on a two-leg sling at 45 degrees to horizontal gives about 4.24 t per leg, so 5 t rated slings are adequate only if the angle is maintained; if the angle drops to 30 degrees each leg sees 6 t and the slings are overloaded.