Rigging, Lifting, and Material Handling
What this domain tests
Rigging questions test load estimation, sling tension at angles, hitch ratings, hardware inspection, and safe lifting practice around cranes and below-the-hook devices.
Ratings and hardware
Working load limit (WLL) already includes the design factor, typically 5:1 for wire rope and synthetic slings and 4:1 for chain. Never treat WLL as breaking strength. Hitch type changes capacity: a choker reduces capacity compared with a vertical hitch, and a basket hitch roughly doubles it only when the legs are vertical. Shackles must be pin-seated and loaded in line, never side-loaded. Hooks need functioning latches. Plate clamps are rated for a thickness range and a pull direction, so check both.
Slings and angles
Sling tension rises quickly as the angle from horizontal drops. For two symmetrical legs, each leg carries (load / 2) divided by the sine of the angle from horizontal. At 60 degrees each leg carries about 58 percent of the load; at 45 degrees about 71 percent; at 30 degrees the full load. Keep included angles at 60 degrees or less from horizontal where possible, and never exceed the sling's rated angle.
Lift planning
Locate the centre of gravity, place the hook above it, and use tag lines. Barricade the area, keep no one under the load, and use one designated signaler. Check the load chart for radius and boom angle, because capacity falls as radius increases. Account for ground bearing pressure under outriggers and mats. Maintain the regulated approach distance from power lines and use a spotter.
Inspection
Check slings before use for broken wires, kinks, bird-caging, heat damage, cuts, UV or chemical damage, missing tags, and distorted hardware. Remove damaged gear from service immediately and tag it.
Worked example
A 4000 kg load is lifted with two symmetrical slings at 45 degrees from horizontal. Each leg tension = (4000 / 2) / sin 45 = 2000 / 0.707 = 2828 kg. Each sling must have a WLL of at least 2.83 t, so select 3.2 t slings and verify that the shackles and master link match. At 30 degrees the same load would put 4000 kg on each leg, exceeding a 3.2 t sling.
Traps
- Confusing WLL with breaking strength.
- Using vertical capacity for a choker hitch.
- Side-loading a shackle.
- Ignoring sling angle on a bridle or spreader.
- Lifting plate with a clamp rated for a different thickness or pull direction.
- Standing under a suspended load to guide it by hand.