Free NCCCO Principles of Crane Stability and Structural Integrity Questions and Answers — Questions and Answers
Question 1: On a mobile crane load chart, capacities listed above a bold line or marked with an asterisk (*) are typically based on what?
- The crane's stability against tipping.
- The crane's structural integrity. (Correct answer)
- The wire rope's breaking strength.
- The engine's hydraulic power.
Correct answer: The crane's structural integrity.
Load charts are divided into two primary domains. Capacities above the bold line (or marked with an asterisk) are limited by the structural strength of the crane's components, like the boom. Capacities below the line are limited by the crane's stability, or its resistance to tipping.
Question 2: How does increasing the load radius affect a crane's rated capacity?
- It increases the capacity.
- It has no effect on the capacity.
- It decreases the capacity. (Correct answer)
- It only affects capacity when the boom is short.
Correct answer: It decreases the capacity.
The load radius is the horizontal distance from the crane's center of rotation to the center of the load. Increasing this distance increases the leverage, or moment, that the load exerts on the crane. To maintain stability, the weight of the load must be decreased as the radius increases.
Question 3: What is the 'tipping axis' or 'fulcrum' of a crawler crane?
- The center pin of the crane.
- The edge of the crawler tracks. (Correct answer)
- The top of the counterweight.
- The boom hinge pins.
Correct answer: The edge of the crawler tracks.
The tipping axis is the line about which a crane will tip over if it becomes unstable. For a crawler crane, this tipping point is typically the idler or sprocket at the end of the crawler tracks on the side of the lift.
Question 4: A crane's stability is greatest when its center of gravity is:
- As high as possible.
- Directly over the tipping axis.
- As low as possible and near the center of rotation. (Correct answer)
- As far from the load as possible.
Correct answer: As low as possible and near the center of rotation.
A lower center of gravity increases an object's stability. For a crane, keeping the combined center of gravity (including the crane's own weight, counterweight, and load) as low as possible and as close to the center of rotation as possible maximizes its stability and resistance to tipping.
Question 5: What is the most critical factor for ensuring maximum stability when setting up a crane on outriggers?
- Ensuring the tires are inflated to the correct pressure.
- Using the largest possible hook block.
- Making sure the outriggers are on firm, level ground. (Correct answer)
- Parking the crane facing downhill.
Correct answer: Making sure the outriggers are on firm, level ground.
While all setup steps are important, the foundation is the most critical. The crane must be perfectly level on ground that is firm enough to support the immense pressures exerted by the outrigger pads. Failure of the ground beneath an outrigger is a common cause of crane tipping accidents.
Question 6: When a crane is 'on rubber' (not on outriggers), its capacity is generally much lower because:
- The tires might go flat.
- The tipping axis is narrower and the suspension allows movement. (Correct answer)
- The engine provides less power when stationary.
- The brakes are not strong enough to hold the load.
Correct answer: The tipping axis is narrower and the suspension allows movement.
When operating on tires ('on rubber'), the tipping axis is the outer edge of the tires. This creates a much narrower and less stable base compared to when outriggers are fully extended. The suspension and tire flex also reduce stability, necessitating significantly lower capacity ratings.
Question 7: Backward stability of a crane is most at risk when:
- Lifting a heavy load at a short radius.
- Traveling with a load.
- Using a long boom at a high angle with no load. (Correct answer)
- Swinging the load over the side.
Correct answer: Using a long boom at a high angle with no load.
Backward stability is a concern when the crane is set up with a long, heavy boom at a very high angle, especially with no load on the hook. In this configuration, the weight of the boom and counterweight can create a moment that causes the crane to tip over backward, particularly on a slope.
Free NCCCO Principles of Crane Stability and Structural Integrity Questions and Answers
On a mobile crane load chart, capacities listed above a bold line or marked with an asterisk (*) are typically based on what?