NCCCO - National Commission for the Certification of Crane Operators Sling Types and Load Distribution 1 — Questions and Answers
Question 1: A two-leg bridle sling is used to lift a 10,000 lb load, with each sling leg inclined 45° from vertical. What is the approximate tension in each leg?
- 5,000 lb — each leg carries exactly half the total load
- 7,071 lb — the incline increases tension by a factor of 1/cos(45°) (Correct answer)
- 6,667 lb — the load is divided by the sine of the included angle
- 8,660 lb — tension is calculated using the tangent of the sling angle
Correct answer: 7,071 lb — the incline increases tension by a factor of 1/cos(45°)
At a 45° angle from vertical, the tension in each leg equals (load ÷ 2) ÷ cos(45°) = 5,000 ÷ 0.707 ≈ 7,071 lb. Departing from vertical always increases leg tension above the simple half-load value.
Question 2: Under ASME B30.9, what is the rated load capacity of a wire rope sling used in a choker hitch expressed as a percentage of its vertical hitch capacity?
- 100% — the rated capacity is the same regardless of hitch type
- 80% — choker hitches reduce capacity due to the sharp bend
- 75% — the standard choker derating factor for wire rope slings (Correct answer)
- 50% — wrapping the rope around the load halves its rated capacity
Correct answer: 75% — the standard choker derating factor for wire rope slings
ASME B30.9 rates wire rope slings in a choker hitch at 75% of vertical hitch capacity. The sharp bend at the choke point stresses the rope, reducing effective strength compared to a straight vertical pull.
Question 3: When inspecting a synthetic web sling, which of the following conditions requires immediate removal from service?
- Surface abrasion that has worn through 10% of the outer yarn bundles (Correct answer)
- A faded but still-legible capacity tag
- Slight stiffening from routine exposure to low-humidity air
- Minor oil contamination on the outer cover that has been wiped clean
Correct answer: Surface abrasion that has worn through 10% of the outer yarn bundles
ASME B30.9 requires removal from service when abrasion, cuts, or wear penetrate or significantly reduce the load-bearing yarns. Wearing through any portion of the outer yarn bundles signals structural compromise. Faded tags and mild surface soiling do not meet the removal threshold on their own.
Question 4: What does the designation 'Grade 80' (G80) stamped on an alloy steel chain sling indicate?
- The chain has a working load limit of 80 tons
- The minimum tensile strength class of the alloy steel, which governs the chain's rated capacity (Correct answer)
- The chain contains 80% iron and may not be used for overhead lifting
- The chain passed an 80,000-cycle fatigue test
Correct answer: The minimum tensile strength class of the alloy steel, which governs the chain's rated capacity
Grade 80 is a tensile strength classification defined in ASME B30.9 and NACM standards. It designates the minimum tensile strength of the alloy steel used, which directly determines the chain's working load limit. Higher grade numbers (e.g., G100, G120) indicate stronger alloys with higher rated capacities.
Question 5: A rigger uses a two-leg sling of equal leg lengths to lift a load whose center of gravity is offset significantly toward one end. What is the most likely result?
- Both legs share the load equally because the sling legs are the same length
- The leg over the heavier end carries a greater share of the load, potentially exceeding its rated capacity (Correct answer)
- The load automatically re-centers as the crane lifts, equalizing tension in both legs
- The load will not move until the sling legs are adjusted to different lengths
Correct answer: The leg over the heavier end carries a greater share of the load, potentially exceeding its rated capacity
Load distribution between sling legs depends on horizontal distance from each attachment point to the center of gravity, not on equal leg lengths. The leg positioned over or closer to the heavier side carries proportionally more load. Rigging without accounting for CG offset can overload one leg.
Question 6: Which sling hitch configuration provides the highest rated load capacity for a single wire rope sling of a given size and length?
- Vertical hitch — the rope is used in direct tension without any bends
- Choker hitch — wrapping around the load adds friction that increases capacity
- Basket hitch with the two legs vertical and parallel — the load is shared across two runs of rope (Correct answer)
- Inverted choker hitch — reversing the choke direction doubles rated load
Correct answer: Basket hitch with the two legs vertical and parallel — the load is shared across two runs of rope
A basket hitch passes the sling under the load and attaches both eyes to the hook, creating two rope segments supporting the load. When the legs are vertical, this effectively doubles the rated vertical hitch capacity. Choker hitches derate to 75% and inverted choke is not a recognized configuration.
NCCCO - National Commission for the Certification of Crane Operators Sling Types and Load Distribution 1
A two-leg bridle sling is used to lift a 10,000 lb load, with each sling leg inclined 45° from vertical.
What is the approximate tension in each leg?