Anti-Two-Block Devices and Safety Systems Flashcards

A crane is operating with a 6-part line reeving configuration. An operator argues that the slow hook travel speed in this setup gives extra reaction time before two-blocking, reducing reliance on the ATB device. Which statement best describes the actual risk profile of this configuration?

Answer: While hook travel speed is reduced by the mechanical advantage ratio, that same ratio amplifies the destructive force applied to boom tip components if two-blocking occurs, making the ATB device more critical, not less

Mechanical advantage works in both directions simultaneously: it reduces hook travel speed but amplifies the force applied at the boom tip when two-blocking occurs. In a 6-part line, the force transmitted to boom tip sheaves can approach 6× the hoist line pull. This means a two-block event in a multi-part line configuration causes far greater structural damage per unit of rope travel than a single-part line — making the ATB device more critical, not less.

A mobile crane is configured with a main boom, a fixed swingaway jib extension, and two hoists: a main hoist at the boom tip and an auxiliary hoist routed through the jib tip. Which statement correctly describes the ATB device requirements under OSHA 1926 Subpart CC and ASME B30.5?

Answer: Each hoist line that is capable of two-blocking requires its own dedicated ATB device, regardless of where each line terminates

Both ASME B30.5 and OSHA Subpart CC require that every individual hoist line capable of two-blocking have its own ATB device. A device at the main boom tip provides no protection for a line routed through a jib extension — the jib tip has its own point of two-block contact. The requirement is per line/hoist path, not per crane, because each line can independently two-block at a different physical location.

During a lift, an anti-two-block device activates and cuts power to the hoist's upward travel. After the ATB trips, what is the correct immediate sequence of operator actions before resuming hoisting?

Answer: Lower the hook block away from the boom tip using the lowering function, then visually inspect the ATB pendant, limit switch, boom tip sheaves, and load line before resuming

When an ATB device activates, the hook block has reached or nearly reached the boom tip — a condition that can damage sheaves, wire rope, and structural members. The correct response is to immediately lower the hook block away from the boom tip (the lowering function is not interrupted by the ATB), then inspect the ATB pendant, limit switch, boom tip sheaves, and rope for damage before resuming upward hoisting. Simply resetting the override and re-hoisting without lowering and inspecting risks continuing into a two-block condition.

ATB device specifications commonly call for a minimum weight for the pendant suspended below the boom tip sheave. What is the primary engineering reason for specifying this minimum weight?

Answer: To provide enough mass so that contact with the rising hook block generates reliable mechanical force to actuate the limit switch every time

The minimum pendant weight ensures that when the rising hook block or headache ball contacts the pendant, sufficient downward force remains to reliably trip the limit switch mechanism. A pendant that is too light may be deflected, lifted, or pushed aside by the hook block contact without generating enough force to actuate the switch — resulting in a missed activation and a true two-block event. The weight specification is fundamentally about guaranteed switch actuation reliability.

A crane operator notices that the anti-two-block device activates repeatedly during normal hoisting operations, well before the hook block is anywhere near the boom tip. Inspection shows the load line is properly reeved and the hook block is at mid-air. What is the MOST likely root cause?

Answer: The ATB pendant weight has detached or is missing, causing the limit switch arm to trigger from boom vibration or minor rope movement alone

The ATB pendant weight holds the limit switch arm in its normal 'safe' (deactivated) position via gravity. If the pendant weight has fallen off or detached — a common field failure — the switch arm loses its positional reference and can be triggered by boom vibration, rope movement, or wind. This produces repeated false activations even when the hook block is far from the boom tip. A missing pendant weight is an immediate out-of-service condition; the device cannot function correctly without it.

Under ASME B30.5, how frequently must an anti-two-block device be functionally tested to confirm proper operation?

Answer: Prior to each shift or use period in which the equipment will perform hoisting operations

ASME B30.5 requires that safety devices — including anti-two-block devices — be tested prior to each shift in which the equipment is used for hoisting. A pre-shift functional test (typically by slowly raising the hook block to contact the ATB pendant and confirming hoist motion stops) is the only way to catch a failed device before a lift begins. Monthly or quarterly intervals would leave a non-functional ATB device undetected across multiple shifts and lifts.