Rigging, Sling Angles, and Below-the-Hook Devices
Sling angle is the whole game
A sling's tension depends on the angle between the leg and the horizontal. For two legs, tension per leg = load ÷ (2 × sin θ). At 90° each leg carries half the load; at 60° each carries 0.577 × load; at 45°, 0.707 × load; at 30°, a full load per leg. Never use load ÷ number of legs unless the legs are vertical. A choker hitch typically reduces a sling's capacity to about 80% of its vertical rating; a basket hitch can double capacity only with vertical legs and an adequate D/d ratio. Check the tag and ASME B30.9.
Hardware and below-the-hook devices
Shackles must be rated, pinned, and loaded along the intended axis; never substitute a bolt for a shackle pin. Hooks need a functioning latch, no tip loading, no side loading, and no cracks or excessive throat opening. Spreader beams and lifting beams are rated for vertical loading and a stated spread; do not alter them. Lifting magnets and vacuum lifters need a controlled landing zone and a plan for power loss.
Worked example
A 10,000 lb load is lifted with a two-leg bridle at 60° from horizontal. Tension per leg = 10,000 ÷ (2 × 0.866) = 5,774 lb. If each leg is choked, the required vertical sling rating is 5,774 ÷ 0.80 = 7,218 lb. A 7,000 lb sling is therefore not adequate even though the load is only 10,000 lb.
Traps
The classic error is dividing by two without the sine factor. Others: measuring the angle from vertical, forgetting the choker derate, assuming a basket always doubles capacity, tying knots in web slings, and using unmarked hardware.