NCCER Crane Load Charts & Rigging Principles 4 — Questions and Answers
Question 1: What is the primary reason crane manufacturers provide separate load charts for different counterweight configurations?
- Different counterweights affect fuel consumption
- Counterweight mass directly affects the crane's stability and therefore its rated lifting capacity (Correct answer)
- Counterweights change the boom angle rating
- Different counterweights require different wire rope grades
Correct answer: Counterweight mass directly affects the crane's stability and therefore its rated lifting capacity
Counterweight provides the righting moment that balances the lifted load; more counterweight generally increases stability and allows higher rated capacities at greater radii.
Question 2: When a load must be rigged using a spreader bar, what is the primary advantage over a standard multi-leg sling bridle?
- Spreader bars are lighter than slings
- A spreader bar eliminates inward compressive forces on the load from inclined sling legs (Correct answer)
- Spreader bars always allow higher lift capacities
- Spreader bars reduce the number of slings required
Correct answer: A spreader bar eliminates inward compressive forces on the load from inclined sling legs
A spreader bar keeps sling legs vertical, eliminating the horizontal compressive force that inclined sling legs would otherwise exert on the sides of a load.
Question 3: On a lattice boom crawler crane load chart, why are capacities typically higher over the rear than over the side?
- The rear counterweight provides greater stability when lifting over the rear (Correct answer)
- The boom can extend further over the rear
- Track tension is higher at the rear
- The hoist drum is located at the rear
Correct answer: The rear counterweight provides greater stability when lifting over the rear
The counterweight is mounted at the rear of the crane, providing the greatest stabilizing moment when the boom swings over the rear, resulting in higher rated capacities in that direction.
Question 4: What is the correct formula to determine the horizontal leg tension in a two-leg bridle sling where the sling angle from horizontal is 30 degrees?
- Load ÷ (2 × sin 30°) (Correct answer)
- Load × cos 30°
- Load ÷ (2 × cos 30°)
- Load × sin 30°
Correct answer: Load ÷ (2 × sin 30°)
Tension in each leg = Load ÷ (number of legs × sin θ), where θ is the angle from horizontal, so T = Load ÷ (2 × sin 30°).
Question 5: A load chart shows crane capacity drops significantly between 40-ft and 50-ft radii. This drop most likely indicates a transition from:
- Wire rope type to chain rigging
- Stability-limited capacity to structurally-limited capacity
- Fully extended outriggers to retracted outriggers
- Structural capacity to stability-limited capacity (Correct answer)
Correct answer: Structural capacity to stability-limited capacity
At shorter radii, capacity is often limited by structural strength of components; as radius increases, the load moment can exceed the crane's stability margin, making stability the governing limit.
Question 6: When attaching a shackle to a sling eye, which condition requires immediate rejection of the shackle?
- The shackle pin is threaded and moused with wire
- The shackle bow is cross-loaded perpendicular to its intended plane (Correct answer)
- The shackle is rated at exactly the sling's WLL
- The shackle is a screw-pin type
Correct answer: The shackle bow is cross-loaded perpendicular to its intended plane
Shackles are designed to be loaded along their bow's plane; cross-loading (side-loading) can reduce the shackle's capacity by up to 50% and requires the shackle to be rejected or properly realigned.
Question 7: What is the maximum permissible drum diameter-to-rope diameter (D/d) ratio effect, and why does it matter for wire rope life?
- Higher D/d ratios reduce rope fatigue life by increasing bending stress
- Lower D/d ratios increase rope fatigue life
- D/d ratio has no effect on rope life if WLL is not exceeded
- Higher D/d ratios increase rope fatigue life by reducing bending stress (Correct answer)
Correct answer: Higher D/d ratios increase rope fatigue life by reducing bending stress
A larger drum diameter relative to rope diameter (higher D/d) reduces the bending stress on individual wires as they wrap the drum, extending the rope's fatigue life.
NCCER Crane Load Charts & Rigging Principles 4
What is the primary reason crane manufacturers provide separate load charts for different counterweight configurations?