What is two-blocking and how can it be prevented during a lift?

• A. When two blocks of the same material collide; prevent by using steel blocks
• B. When the hoist lines loop around the drum twice; prevent by winding carefully
• C. Two-blocking occurs when the hook block contacts the boom tip or other equipment; prevent by ensuring adequate slack in the line, using limit switches, using a spotter, and not relying on the operator alone
• D. When two cranes operate in tandem; prevent by using only one crane

Answer: (C)

Two-blocking happens when the hook block comes into contact with the boom tip or another fixed point, effectively pinching the hoist line between blocks. This can occur if the line is pulled up too far or there isn’t enough slack, so the block can’t move freely and the line can dig into itself or against the tip. The danger is real: it can damage or fray the rope, damage blocks or the drum, and in a worst-case scenario lead to a sudden load shift or crane instability.

Preventing it comes down to giving the system proper safeguards. Keeping adequate slack in the line ensures the hook block isn’t pressed into the boom tip as you lift. Using limit switches that automatically stop hoisting before contact is made provides a hard safety boundary. Having a spotter watch the path and guide the lift adds an extra check beyond what the operator can see from the controls. And, importantly, don’t rely on the operator alone to prevent this hazard; independent oversight helps catch any misjudgments about clearance during the lift.