Which operator action will most likely cause a reduction in the capacity of a mobile crane?

• A. Booming up
• B. Booming down
• C. Hoisting while swinging
• D. Rotating with load

Answer: (B)

Crane capacity is defined by the load moment at the base. The moment grows with the horizontal distance from the crane’s base to the load, i.e., the radius, and with how high the load is lifted. Booming down increases that radius, pushing the load farther away and making the overturning moment larger for the same weight. That added moment reduces how much weight you can safely lift, so the capacity drops in this configuration.

Boighting up tends to shorten the radius and can improve or maintain capacity, while hoisting while swinging or rotating with a load introduce dynamic or lateral forces, which can be dangerous but don’t inherently reduce the rated capacity as directly as increasing the lever arm through boomed-down work.