Anti-Two-Block Devices and Safety Systems Flashcards
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A crane is equipped with a load moment indicator (LMI) and an anti-two-block (ATB) device that both trigger simultaneously during a lift. Which condition requires the operator's immediate first response?
Answer: The ATB signal — stop hoist-up motion immediately to prevent rope failure
The ATB condition requires immediate cessation of hoist-up motion because continuing will snap the wire rope and drop the load. This is an immediate catastrophic failure mode. The LMI overload, while serious, typically allows controlled lowering of the load.
What is a secondary anti-two-block device and when is it required?
Answer: An ATB device installed on a jib or fly section when the main boom ATB cannot detect the jib tip position
When a jib or fly section is attached to the main boom, the main boom ATB device cannot detect proximity of the load block to the jib tip. A separate ATB device must be installed on the jib tip for that configuration.
How does a hoist limit switch differ from an anti-two-block device?
Answer: A hoist limit switch stops the hoist at a preset position, while an ATB device reacts to actual physical contact between the block and the weighted switch at the boom tip
A hoist limit switch is a fixed mechanical stop set to prevent the hook from traveling above a certain preset height, while an ATB device dynamically senses when the rising block physically contacts the weighted actuator hanging from the boom tip.
What action should the operator take immediately upon hearing the anti-two-block alarm?
Answer: Stop hoist-up motion and either lower the load or raise the boom to create clearance
When the ATB alarm sounds, the operator must immediately stop hoisting up. The appropriate corrective action is to lower the load (hoist down) or raise the boom angle to create clearance between the hook block and the boom tip.
What is the relationship between the number of rope parts in a hoist line (reeving) and the likelihood of a two-block condition?
Answer: More rope parts require the hook block to travel a shorter distance to two-block for the same drum rotation, increasing risk at high boom angles
With more rope parts, the hook block rises more slowly per drum rotation, but the total hook travel is still limited by the boom tip geometry. At high boom angles with multiple-part reeving, the hook block can be positioned close to the tip without the operator realizing it.
According to OSHA 1926.1416, what is the operator's responsibility regarding safety devices before each shift?
Answer: Operators must visually inspect all safety devices and verify they are functioning before beginning work each shift
OSHA 1926.1416 assigns the operator responsibility for conducting a visual inspection of all safety devices (including ATB, LMI, and limit switches) and verifying their function before each shift begins.