Anti-Two-Block Devices and Safety Systems Flashcards

What is a two-block condition on a crane and why is it dangerous?

Answer: A condition where the load block or hook assembly contacts the boom tip sheaves, which can snap the hoist rope and drop the load

Two-blocking occurs when the load block travels up and contacts the boom tip sheave assembly. The continued hoist pressure places extreme force on the wire rope, which can snap and drop the load.

What is the function of an anti-two-block (ATB) device on a crane?

Answer: It detects when the hook block approaches the boom tip and automatically stops the hoist to prevent two-blocking

An anti-two-block device uses a weight suspended from the boom tip with a limit switch; when the rising block contacts the weight, it triggers the switch and stops the hoist drum to prevent the block from contacting the boom tip sheaves.

An operator performing a pre-shift inspection finds that the anti-two-block device's warning weight is missing. What should the operator do?

Answer: Remove the crane from service until the ATB device is repaired or replaced

A missing or defective anti-two-block device requires the crane to be removed from service until the device is repaired or replaced. Using the crane without a functional ATB is a violation of ASME B30.5 and OSHA standards.

Which of the following crane operations is most likely to cause a two-block condition if the operator is not vigilant?

Answer: Raising the boom angle while the hook is near the top of its travel with a short rope length deployed

Raising the boom angle while the hook is already high reduces the distance between the boom tip and the hook block, which can cause two-blocking. This combination of boom up and hook near the top of travel is the most common two-block scenario.

What is the purpose of the dead wraps requirement on a crane hoist drum?

Answer: To maintain anchor friction on the drum so the rope does not slip off when the hook is at its lowest point

Dead wraps (typically two to three wraps of wire rope retained on the drum at all times) maintain enough friction between the rope and drum to prevent the rope from slipping off the anchor point when the hook is at its lowest position.

Under ASME B30.5, how often must anti-two-block devices be tested?

Answer: Before each shift in which the crane will be used

ASME B30.5 and OSHA 1926.1415 require anti-two-block devices to be checked before each shift. The operator must verify the device activates and stops hoist-up motion at the proper point before beginning work.