NCCCO - National Commission for the Certification of Crane Operators Load Moment Indicators and Safety Devices 1 — Questions and Answers
Question 1: What is the primary function of an anti-two-block (ATB) device, and which crane movements does it typically arrest when activated?
- It prevents the load from swinging freely by engaging the swing brake when the load exceeds 90% capacity
- It prevents the hook block from contacting the boom tip sheaves by automatically stopping hoist-up and boom-down movements (Correct answer)
- It prevents crane travel while a load is suspended by locking the drive motors
- It prevents the outriggers from retracting during a lift by sensing downward pressure on the pads
Correct answer: It prevents the hook block from contacting the boom tip sheaves by automatically stopping hoist-up and boom-down movements
An ATB device uses a weight-and-switch assembly hung near the boom tip. When the rising hook block lifts the ATB weight, the circuit triggers and stops hoist-up and boom-down movements — the two motions that would drive the block further into the sheaves and cause catastrophic structural damage.
Question 2: How does operating a crane on an out-of-level surface most directly affect the accuracy of an LMI system?
- It causes the LMI display screen to flicker, making readings unreliable
- It shifts the crane's center of gravity, increasing actual radius and causing the LMI to underestimate the load moment (Correct answer)
- It trips the anti-two-block circuit prematurely because the boom angle sensor references level ground
- It overloads the slew ring sensors, causing the LMI to shut down automatically
Correct answer: It shifts the crane's center of gravity, increasing actual radius and causing the LMI to underestimate the load moment
Load moment is calculated as weight × horizontal radius. When a crane is not level, the true horizontal radius to the load increases beyond what a perfectly level setup would show, meaning the actual load moment is greater than the LMI may indicate — a direct safety hazard.
Question 3: When an LMI's audible alarm and warning light activate while a lift is in progress, the operator's FIRST required action is to:
- Reduce hoist speed to creep mode and continue the lift while monitoring the display
- Stop all crane motion immediately and assess the situation before any further movement (Correct answer)
- Lower the load directly to the ground without any lateral movement
- Radio the rigger to attach an additional sling to redistribute the load weight
Correct answer: Stop all crane motion immediately and assess the situation before any further movement
Stopping all crane motion is the mandatory first response to an LMI alarm. Any continued movement — even lowering — can change the load moment unpredictably. The operator must assess what triggered the alarm before deciding on a safe next action.
Question 4: In the load moment formula used by LMI systems, the load moment is the product of:
- The gross load weight divided by the boom length in feet
- The gross load weight multiplied by the horizontal radius from the crane centerline to the load (Correct answer)
- The boom angle in degrees multiplied by the rated line pull
- The hoist line tension multiplied by the number of parts of line
Correct answer: The gross load weight multiplied by the horizontal radius from the crane centerline to the load
Load moment = weight × horizontal radius. This is why radius is the dominant variable in crane stability calculations — doubling the radius doubles the overturning moment even if the load weight stays the same, which is why rated capacity decreases as radius increases on all load charts.
Question 5: When calculating the total suspended load for comparison against an LMI reading, the operator must include:
- Only the weight of the cargo being lifted, as the crane's hook block is pre-zeroed by the manufacturer
- The cargo weight plus the combined weight of the hook block, ball, shackles, slings, and all other below-the-hook hardware (Correct answer)
- The cargo weight plus the hook block weight only, since slings are considered part of the crane's rigging system
- The cargo weight multiplied by a dynamic factor of 1.25 to account for hook and rigging hardware
Correct answer: The cargo weight plus the combined weight of the hook block, ball, shackles, slings, and all other below-the-hook hardware
Rated capacity must not be exceeded by the total suspended load, which includes every item hanging from the boom tip: hook block, swivel, shackles, wire rope slings, spreader bars, and the load itself. Omitting rigging weight is a common cause of unintentional overloading.
Question 6: If an LMI system becomes inoperative or gives clearly erroneous readings during crane operations, the operator must:
- Continue operations at no more than 75% of the rated capacity shown on the load chart, using personal judgment
- Cease operations until the LMI is repaired, or until a qualified person authorizes continuation using manual load chart procedures with appropriate precautions (Correct answer)
- Continue operations only for lifts in which the load weight is known exactly from certified scale tickets
- Notify the employer in writing and then continue normal operations until the end of the shift
Correct answer: Cease operations until the LMI is repaired, or until a qualified person authorizes continuation using manual load chart procedures with appropriate precautions
A malfunctioning LMI removes a critical layer of protection. ASME B30.5 and OSHA 1926.1416 require that defective safety devices be repaired before use, or that a qualified person determine and document that operations can safely continue using alternative means such as manual load chart calculations.
NCCCO - National Commission for the Certification of Crane Operators Load Moment Indicators and Safety Devices 1
What is the primary function of an anti-two-block (ATB) device, and which crane movements does it typically arrest when activated?