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

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

Answer: (A)

Raising the boom changes the crane’s geometry so the load sits closer to the crane’s base. The radius, which is the horizontal distance from the crane’s rotation point to the load, gets smaller as the boom is raised. With a shorter lever arm, the overturning moment created by the load is reduced, so the crane can safely lift more weight at that configuration. That’s why the rated capacity increases when you raise the boom. Conversely, lowering the boom increases the radius and reduces the allowable load. These relationships are shown in the crane’s load charts, which account for geometry, stability, and other factors like outrigger setup.