NCCCO Anti-Two-Block Devices and Safety Systems 2 — Questions and Answers
Question 1: What is a two-block condition on a crane and why is it dangerous?
- A condition where two loads are lifted simultaneously, doubling the stress
- A condition where the load block or hook assembly contacts the boom tip sheaves, which can snap the hoist rope and drop the load (Correct answer)
- A condition where the crane is blocked from slewing in two directions
- A condition where two brakes are applied simultaneously
Correct 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.
Two-blocking (block-to-block) is a critical crane hazard that occurs when the hook block, load block, or overhaul ball travels so far up the hoist that it contacts the boom tip sheave assembly. When two-blocking occurs, the wire rope is subject to an abrupt increase in tension — all the hydraulic or mechanical hoist force is concentrated in a very short rope section. The rope will likely snap, causing uncontrolled load drop, and the structural members of the boom tip are also at risk of damage. Two-blocking can occur during rapid raising, inattention, or when operating at high boom angles where the geometry brings the load close to the tip naturally.
Question 2: What is the function of an anti-two-block (ATB) device on a crane?
- It prevents the crane from lifting loads heavier than its rated capacity
- It detects when the hook block approaches the boom tip and automatically stops the hoist to prevent two-blocking (Correct answer)
- It locks the boom in position to prevent uncontrolled lowering
- It prevents two cranes from performing tandem lifts without authorization
Correct 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 anti-two-block (ATB) device typically consists of a weighted switch suspended from the boom tip or jib tip by a chain or wire. As the hook block rises and approaches the tip, it contacts the weighted switch, which activates a limit switch and sends a signal to the crane's control system. This signal stops further hoist-up movement and may also stop boom-lowering that would reduce two-block clearance. Some ATB systems also sound an alarm before stopping the hoist to alert the operator. ASME B30.5 requires ATB devices on all hydraulic boom cranes, and OSHA 1926.1415 requires them to be inspected before each shift.
Question 3: An operator performing a pre-shift inspection finds that the anti-two-block device's warning weight is missing. What should the operator do?
- Proceed with the lift but keep a close watch on the hook block position
- Remove the crane from service until the ATB device is repaired or replaced (Correct answer)
- Use the crane only for lifts below 50% capacity where two-blocking is less likely
- Tie a rope to the hook block as an alternative warning system
Correct 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.
OSHA 29 CFR 1926.1415(a)(1)(iii) requires anti-two-block devices to function properly, and ASME B30.5 prohibits operation of a crane with a defective two-blocking prevention device. If the ATB weight, chain, switch, or associated wiring is missing or damaged, the crane must be taken out of service and tagged with an out-of-service tag. Improvised solutions are not acceptable substitutes for the manufacturer-approved ATB device. The deficiency must be reported to maintenance and documented in the equipment inspection record. The crane may not return to service until the ATB system is restored to full function and tested.
Question 4: Which of the following crane operations is most likely to cause a two-block condition if the operator is not vigilant?
- Lowering the boom angle while the hook is at ground level
- Raising the boom angle while the hook is near the top of its travel with a short rope length deployed (Correct answer)
- Slewing the crane with a load near maximum radius
- Extending the outriggers while a load is suspended
Correct 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.
Two-blocking most commonly occurs when the operator raises the boom angle while the hook block is near the top of its travel, or when the operator hoists up while at a high boom angle where the geometry limits vertical hook travel distance. The risk is greatest when operating at high boom angles, using a short length of wire rope, the operator is distracted, or a fly jib reduces the working height available. ATB devices protect against all these scenarios, but operators must also develop awareness of block-to-tip clearance as part of safe crane operation.
Question 5: What is the purpose of the dead wraps requirement on a crane hoist drum?
- To ensure the ATB device has enough rope to trigger before the block reaches the tip
- To maintain anchor friction on the drum so the rope does not slip off when the hook is at its lowest point (Correct answer)
- To provide extra rope for emergency lowering beyond normal travel
- To prevent the hoist drum from rotating backward under load
Correct 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.
When the hoist rope is lowered to its maximum depth, some wraps of rope must remain on the drum to maintain the anchor point tension. ASME B30.5 requires a minimum of two dead wraps to remain on the drum at the hook's lowest point. These wraps maintain friction between the rope and the drum clamp, preventing the rope end from being pulled off the drum. Loss of the dead wraps allows the rope anchor to become the load-carrying point, which is not designed for sustained loading and can result in catastrophic failure. Operators must know the minimum drum wrap requirement for their specific crane and stop lowering when that limit is reached.
Question 6: Under ASME B30.5, how often must anti-two-block devices be tested?
- Once per year during the annual inspection
- Before each shift in which the crane will be used (Correct answer)
- Every 500 operating hours
- Only after maintenance or repair
Correct 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.
The pre-shift inspection requirement for ATB devices reflects their critical safety role. OSHA 29 CFR 1926.1415(a) requires inspection of all safety devices before each shift. For ATB devices, this means physically verifying that the weight is in place, the switch activates when the weight is lifted (simulating the block contacting it), and the control system responds by stopping hoist-up movement. This test takes only seconds but confirms the entire system is functional. Any failure means the crane must not be used until the defect is corrected. Records of pre-shift inspections should be maintained per the site's safety plan.
NCCCO Anti-Two-Block Devices and Safety Systems 2
What is a two-block condition on a crane and why is it dangerous?