NCCCO Principles of Crane Stability and Structural Integrity 2 — Questions and Answers
Question 1: What is the 'tipping axis' of a mobile crane on outriggers?
- The center of the crane
- The line connecting the outrigger pad closest to the load and the adjacent outrigger pad (Correct answer)
- The rear axle of the crane
- The boom pivot point
Correct answer: The line connecting the outrigger pad closest to the load and the adjacent outrigger pad
The tipping axis is the line between the two outrigger pads on the load side. If the load moment (load × radius) exceeds the resisting moment about this axis, the crane will tip.
Question 2: What factor has the greatest effect on crane stability?
- The color of the crane
- The operating radius — the horizontal distance from the center of rotation to the load (Correct answer)
- The height of the boom
- The number of boom sections extended
Correct answer: The operating radius — the horizontal distance from the center of rotation to the load
Operating radius is the single greatest factor because capacity decreases rapidly as radius increases. A small increase in radius can cause a large decrease in available capacity.
Question 3: What happens to crane stability when the crane is not level?
- Nothing — outriggers compensate for unlevel conditions
- The crane becomes less stable because the effective tipping axis shifts, and the load effectively moves to a greater radius (Correct answer)
- Stability increases on a slope
- Only the operator's comfort is affected
Correct answer: The crane becomes less stable because the effective tipping axis shifts, and the load effectively moves to a greater radius
An unlevel crane shifts the center of gravity away from center, effectively increasing the radius on the downhill side and reducing the resisting moment. Load charts assume a level crane.
Question 4: What is 'structural failure' in a crane and how does it differ from a stability failure?
- They are the same thing
- Structural failure is when a component (boom, pin, wire rope) breaks; stability failure is when the crane tips over (Correct answer)
- Structural failure only happens to old cranes
- Stability failure only happens in high winds
Correct answer: Structural failure is when a component (boom, pin, wire rope) breaks; stability failure is when the crane tips over
Structural failure involves component breakage (boom buckling, pin shearing, rope breaking). Stability failure involves the crane tipping over. Both are prevented by staying within load chart ratings.
Question 5: How does the weight of the boom itself affect crane stability?
- Boom weight has no effect on stability
- The boom's weight creates a tipping moment that increases with boom length and angle, reducing available capacity for the load (Correct answer)
- Boom weight improves stability by acting as counterweight
- Boom weight only matters during travel
Correct answer: The boom's weight creates a tipping moment that increases with boom length and angle, reducing available capacity for the load
The boom itself has weight that creates a moment about the tipping axis. Longer booms weigh more and create a greater tipping moment, which is why capacity decreases with longer boom configurations.
Question 6: Why are outriggers critical for crane stability?
- They prevent the tires from getting flat
- They provide a wider support base and transfer loads directly to the ground, dramatically increasing the crane's stability and rated capacity (Correct answer)
- They are only used for appearance
- They protect the crane from theft
Correct answer: They provide a wider support base and transfer loads directly to the ground, dramatically increasing the crane's stability and rated capacity
Outriggers widen the crane's footprint far beyond the chassis width, creating a much larger stability triangle/rectangle. Load chart capacities for outrigger-deployed operations are significantly higher than on-rubber.
NCCCO Principles of Crane Stability and Structural Integrity 2
What is the 'tipping axis' of a mobile crane on outriggers?