Load Moment Indicators and Safety Devices 1 Flashcards

What is the primary function of an anti-two-block (ATB) device, and which crane movements does it typically arrest when activated?

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.

How does operating a crane on an out-of-level surface most directly affect the accuracy of an LMI system?

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.

When an LMI's audible alarm and warning light activate while a lift is in progress, the operator's FIRST required action is to:

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.

In the load moment formula used by LMI systems, the load moment is the product of:

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.

When calculating the total suspended load for comparison against an LMI reading, the operator must include:

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.

If an LMI system becomes inoperative or gives clearly erroneous readings during crane operations, the operator must:

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.