What happens to the radius and rated capacity when a crane's boom is lowered?

• A. Radius increases and rated capacity decreases
• B. Radius decreases and rated capacity increases
• C. Radius increases and rated capacity stays the same
• D. Radius stays the same and rated capacity changes

Answer: (A)

Lowering the boom changes the geometry so the load sits farther from the crane’s base. As the boom angle flattens toward the ground, the horizontal reach (the radius) increases. At the same time, the crane’s capacity is determined by a stability chart that links how much load you can lift to how far out the load is. When the radius grows, the resisting moment about the base increases, so the safe lifting capacity decreases. So the correct idea is that the radius increases and the rated capacity decreases. The other scenarios would require the reach to stay the same or the capacity to rise, which doesn’t happen when you lower the boom.