What is an anti-two-block device on a crane?

An anti-two-block device is a mandatory safety system on cranes designed to accommodate one. It prevents the two-blocking condition by automatically cutting hoist power before the hook block contacts the boom tip sheave assembly. Without this device functioning, two-blocking depends entirely on the operator noticing the approach of the hook block in time to stop hoist travel, which is particularly difficult when the block is traveling in a blind zone near the boom tip or when the operator is distracted by a complex lift.

How Anti-Two-Block Devices Work

The most common anti-two-block device design uses a weight suspended from the hook block by a chain or cable. The weight hangs freely from the hook block during normal operation. As the hook block rises toward the boom tip, the weight contacts the anti-two-block switch housing mounted at the boom tip and is pushed upward by the approach of the block. This upward push activates a limit switch that cuts power to the hoist, stopping the hook block before it contacts the sheave assembly. The device is designed so that the limit switch activates with enough clearance remaining to stop the hoist before physical contact occurs, accounting for the hoist coasting distance after power cutoff.

Inspection as Part of the Pre-Shift Check

OSHA 1926.1412(e) requires that safety devices, including anti-two-block devices, be checked during the pre-shift inspection. The pre-shift inspection of the anti-two-block device should verify that the weight is present and freely suspended, that the chain or cable connecting the weight to the hook block is intact and of the correct length, that the limit switch housing at the boom tip is undamaged and correctly positioned, and that the limit switch activates and cuts hoist power when the weight is manually lifted to simulate the approach of the hook block. If the switch does not cut hoist power when activated, the device is nonfunctional and the crane must be removed from service under 1926.1412(a) until the device is repaired.

Anti-Two-Block Devices on Telescoping Boom Cranes

On telescoping boom cranes, the two-blocking hazard is particularly acute because the block clearance at full boom extension can be just a few feet. The anti-two-block device must function correctly at all boom extension lengths. On multi-section telescoping booms, the device is typically mounted on the fly section or the end section, and the limit switch wiring must accommodate the full range of boom extension without the wiring becoming taut or disconnected. During the pre-shift inspection, the operator should verify that the wiring or cable to the boom tip limit switch is intact and properly routed for the current boom configuration before hoisting begins.