How does boom length and radius affect crane capacity?

• A. Capacity increases with radius.
• B. Capacity is independent of radius.
• C. Capacity decreases as radius increases and increases with shorter booms; charts show this relationship.
• D. Longer booms always increase capacity regardless of radius.

Answer: (C)

The main idea is that crane capacity is governed by stability and the load moment. The farther a load is from the base (the radius), the larger the tipping moment the crane must resist. So, as radius increases, the same weight creates a bigger overturning force, and the allowable load must be reduced to keep the crane stable. The boom length works the same way: a longer boom increases the lever arm, raising the overturning moment for a given load, which lowers capacity. Chart data used in crane operations consistently show higher capacities with shorter booms and smaller radii, and lower capacities as these grow. That’s why the correct statement is that capacity decreases with increasing radius and with longer booms, and increases when using shorter booms, assuming the same base conditions and counterweight. The other ideas ignore how stability limits change with reach, so they don’t fit the real-world behavior reflected in the charts.