Why the angle matters
A vertical sling carries its share of the load and nothing more. Tip it toward horizontal and it starts pulling sideways as well as up, so the tension climbs. At 60° from horizontal each leg carries about 1.15 times its share. At 45° it's 1.41 times, and at 30° it's 2 times. Below 30° the numbers run away fast, which is why ASME B30.9 doesn't allow those angles without a qualified person's approval.
An example
A 10,000 lb skid on a two-leg bridle, with 10 ft slings and 7 ft of height from the hook to the lifting lugs: sin(angle) = 7 ÷ 10, so the angle is about 44°. Each leg carries 5,000 ÷ 0.7, about 7,143 lb. A sling rated 6,000 lb in a vertical hitch isn't enough for that pick.
What it doesn't cover
- An off-center load, where the leg nearer the center of gravity takes more
- Shock loading from a jerky start or stop
- Choker and basket hitch reductions, which come from the sling's tag
- Rigging hardware, the crane's chart and below-the-hook devices
Rigging requirements are in 29 CFR 1926.251. A critical lift still needs a written lift plan.