NCCCO Load Charts and Capacity Calculations 4 — Questions and Answers
Question 1: When using a load chart with a luffing jib attachment, what additional deduction must be applied to the main boom capacity?
- The weight of the jib and all associated rigging must be subtracted from the rated capacity (Correct answer)
- Only the hook block weight needs to be deducted
- No deduction is needed if the jib is rated separately
- The jib weight is automatically factored into the main boom chart
Correct answer: The weight of the jib and all associated rigging must be subtracted from the rated capacity
The weight of the luffing jib assembly, including pins, rigging, and the jib hook block, must be subtracted from the main boom's rated capacity because the crane must support this weight before lifting any payload.
Question 2: A crane's load chart shows a capacity of 40,000 lbs at a 50-foot radius. If the total rigging weight is 800 lbs and the hook block weighs 600 lbs, what is the maximum net payload that can be lifted?
- 38,600 lbs (Correct answer)
- 39,200 lbs
- 40,000 lbs
- 39,400 lbs
Correct answer: 38,600 lbs
The net payload equals the rated capacity minus all weight below the hook: 40,000 − 800 − 600 = 38,600 lbs.
Question 3: On a telescoping boom crane load chart, why do capacities typically decrease as boom length increases at the same radius?
- Longer booms increase structural stress and reduce stability, lowering the rated capacity (Correct answer)
- Longer booms reduce the counterweight effect
- Load chart ratings are arbitrary and set conservatively
- The hydraulic system loses pressure with extended boom length
Correct answer: Longer booms increase structural stress and reduce stability, lowering the rated capacity
Longer boom sections increase bending moment and reduce structural integrity margins, and the boom's own weight further reduces net lifting capacity at any given radius.
Question 4: A crawler crane load chart lists two sets of capacities: 'over front' and 'over side.' Which is typically greater and why?
- Over front, because the carbody and tracks provide greater tipping resistance in the longitudinal direction (Correct answer)
- Over side, because the track width provides a wider base
- They are always equal by ASME B30.5 requirement
- Over rear, which is always the highest rated direction
Correct answer: Over front, because the carbody and tracks provide greater tipping resistance in the longitudinal direction
Crawler cranes have greater stability lifting over the front due to the longer longitudinal distance between the tipping fulcrum and the crane's center of gravity.
Question 5: What does the term 'structural capacity' mean on a load chart as opposed to 'stability capacity'?
- Structural capacity is limited by the strength of crane components, while stability capacity is limited by the crane's resistance to tipping (Correct answer)
- Structural capacity applies only to the boom, while stability applies to the counterweight
- Structural capacity is always higher than stability capacity at long radii
- They are interchangeable terms on modern load charts
Correct answer: Structural capacity is limited by the strength of crane components, while stability capacity is limited by the crane's resistance to tipping
At short radii, cranes are often structurally limited (component strength governs), while at longer radii, stability (tipping resistance) typically becomes the governing factor.
Question 6: A mobile crane load chart specifies capacities for 'on outriggers, fully extended.' The operator sets outriggers to 75% extension due to site constraints. What must the operator do?
- Use a manufacturer-provided derated chart or formula for intermediate outrigger positions (Correct answer)
- Interpolate linearly between the fully extended and on-rubber capacities
- Reduce all chart capacities by 25% to match the 75% extension
- Proceed using the fully extended chart since outriggers are still deployed
Correct answer: Use a manufacturer-provided derated chart or formula for intermediate outrigger positions
ASME B30.5 requires that operators use manufacturer-provided charts for intermediate outrigger positions; interpolation is not permitted unless specifically authorized by the manufacturer.
Question 7: When determining working radius for a load chart lookup, from where is the radius measured?
- From the centerline of rotation to the center of the vertical hoist line with the load freely suspended (Correct answer)
- From the front of the crane cab to the load
- From the boom foot pin to the load
- From the outrigger pad edge to the load center
Correct answer: From the centerline of rotation to the center of the vertical hoist line with the load freely suspended
Load chart radius is always measured horizontally from the crane's centerline of rotation to the center of the vertical load line with the load hanging freely.
NCCCO Load Charts and Capacity Calculations 4
When using a load chart with a luffing jib attachment, what additional deduction must be applied to the main boom capacity?