Rigging and load handling
Domain 2: Rigging and load handling
This domain tests sling selection, working load limits, sling-angle effects, hitch types, centre of gravity, hardware inspection, and control of the load. Questions often give a load weight and sling geometry and ask for tension or the correct sling.
Core rules
- Distinguish working load limit (WLL) from breaking strength. WLL already includes the design factor; never use breaking strength as a working limit. Typical design factors are 4:1 to 5:1, but use the sling tag and manufacturer's data.
- Sling angle matters. For a symmetric two-leg bridle, tension per leg is T = W / (2 sin theta), where theta is the angle from horizontal. As theta decreases, tension rises sharply.
- Hitch capacity: vertical hitch is the baseline; choker and basket hitches have specific efficiencies from the sling manufacturer. Do not assume a choker equals a vertical rating.
- Centre of gravity: the hook must be over the CoG. Off-centre lifts tilt the load, shift load between legs, and can cause shock loading. Use a spreader or adjust pick points.
- Hardware: hooks need working latches; no side loading of hooks; shackles rated and pinned; no makeshift hardware. Protect slings from sharp edges and heat.
- Control: use tag lines, keep people out from under the load, start and stop smoothly, and never drag or side-pull a load.
Worked example
A 4,000 kg load is lifted with a two-leg bridle at 45 degrees from horizontal. Add 300 kg for slings and hardware: W = 4,300 kg. T = 4,300 / (2 x sin 45) = 4,300 / 1.414 = 3,041 kg per leg. A 3.5 t WLL sling is acceptable at 45 degrees. If the same lift were rigged at 30 degrees, T = 4,300 / (2 x 0.5) = 4,300 kg per leg, which exceeds 3.5 t. The fix is a longer sling, a spreader, or higher-rated slings, not a slow lift.
Traps that cost marks
- Using the vertical WLL for an angled or choker hitch.
- Forgetting the weight of slings, shackles, and the hook block in the gross load.
- Judging sling angle from the hook instead of from horizontal.
- Ignoring CoG and letting the load tilt.
- Shock loading by rapid hoist or sudden stops.