Sling Types and Load Distribution Flashcards

A 12,000 lb steel fabrication is rigged using two vertical sling legs of equal length. The center of gravity is located 2 ft from Attachment Point A and 6 ft from Attachment Point B (total span: 8 ft between attachment points). What is the load carried by sling leg A?

Answer: 9,000 lb

When the CG is not centered, each sling leg carries a load proportional to its distance from the OTHER attachment point. Taking moments about Point B: T_A × 8 ft = 12,000 lb × 6 ft → T_A = 9,000 lb. The closer a sling leg is to the CG, the MORE load it bears — the opposite of what many riggers assume. Leg B carries only 3,000 lb (12,000 × 2/8), confirming both legs sum to 12,000 lb.

A two-leg wire rope bridle sling is used to lift a 6,000 lb load. Each leg makes a 30° angle with the horizontal. What is the approximate tension in each sling leg?

Answer: 6,000 lb

Tension per leg = (Total load ÷ 2) ÷ sin(horizontal angle) = 3,000 ÷ sin(30°) = 3,000 ÷ 0.5 = 6,000 lb. At only 30° from horizontal, each sling leg carries the entire load weight — equal to the full suspended weight — because sin(30°) = 0.5. This demonstrates why ASME B30.9 discourages sling angles below 30° from horizontal: shallow angles multiply leg tension dramatically. The distractor 4,243 lb corresponds to the 45° case (3,000 ÷ 0.707).

Under ASME B30.9, when rating a four-leg wire rope bridle sling for a symmetrical overhead lift, what is the maximum number of legs that may be credited for load-sharing purposes?

Answer: 3 legs

ASME B30.9 requires that only 3 legs of a 4-leg bridle sling be credited for load sharing, even during a perfectly symmetrical lift. This accounts for real-world manufacturing tolerances, slight geometric imperfections, and load eccentricities that prevent true equal distribution across all four legs. Crediting all four legs would produce an unconservative rating. The WLL of a 4-leg sling is therefore calculated as: (single-leg WLL × HF for the sling angle) × 3 — not × 4. Proof-testing does not override this design requirement.

A wire rope sling has a standard choker hitch WLL of 3,200 lb, which is based on a choke angle of 120° or greater. The sling is rigged in a choker hitch where the actual choke angle (the angle at the choke point between the sling body and the eye) measures only 60°. Per ASME B30.9, what is the approximate maximum allowable load?

Answer: 2,400 lb

When the choke angle is less than 120°, ASME B30.9 requires an additional reduction to the standard choker WLL. At a choke angle of 60°–89°, an efficiency factor of approximately 0.74 applies: 3,200 lb × 0.74 ≈ 2,368 lb, rounded to approximately 2,400 lb. Many riggers incorrectly use the full choker rating regardless of choke angle. The choke angle narrows when the load diameter is small relative to the sling length at the choke point, causing the sling body to pinch tightly — which reduces the effective load path through the wire rope.

A 1/2" diameter wire rope sling has a catalog vertical hitch WLL of 2,100 lb, which was established at the manufacturer's minimum recommended D/d ratio. A rigger plans to use this sling in a vertical hitch around a 1" diameter shackle pin, yielding a D/d ratio of 2:1. Which action is required before proceeding?

Answer: Apply the manufacturer's D/d derating factor for a 2:1 ratio and reduce the WLL before use

Wire rope sling WLL ratings are established at a minimum recommended D/d ratio (typically 25:1 or higher for the full rating). When the actual D/d ratio is lower — as in this case with D/d = 2:1 — the bending stress through the rope increases significantly, reducing capacity. ASME B30.9 and manufacturer load charts include derating tables for reduced D/d ratios; at D/d = 2:1, the sling may retain only approximately 50% of its catalog WLL. Lubrication and substituting chain do not resolve the underlying bending-efficiency issue for wire rope. The derating table must be consulted and applied.

A three-leg wire rope bridle sling lifts a 9,000 lb symmetrical load. All three legs are attached to a single crane hook and each leg makes an identical 45° angle with the vertical. Assuming the load is equally distributed among all three legs, what is the approximate tension in each sling leg?

Answer: 4,243 lb

Each leg supports an equal share of the vertical load component: 9,000 ÷ 3 = 3,000 lb per leg (vertical component). Since the legs are angled 45° from the vertical, the actual tension in each leg = vertical component ÷ cos(45°) = 3,000 ÷ 0.707 ≈ 4,243 lb. The distractor 3,000 lb is the vertical component only — a common error when riggers forget to account for the angle. The distractor 2,121 lb would result from incorrectly multiplying (rather than dividing) by cos(45°). The horizontal components of all three legs cancel out by symmetry.