Boom Angle and Radius Calculations Flashcards

A load must be set 35 feet from the crane centerline. The boom is 70 feet long. What boom angle is required?

Answer: 60°

cos(angle) = radius/boom length = 35/70 = 0.5, so the boom angle = cos⁻¹(0.5) = 60°.

On a lattice boom crane, the boom angle indicator shows 72°. The operator lowers the load 10 feet. What happens to the radius?

Answer: Radius remains unchanged

Lowering the load vertically on a plumb line does not change the horizontal radius—only boom movement changes radius.

When using a jib on a lattice boom crane, how is the total working radius determined?

Answer: Boom horizontal component plus jib horizontal component

Total working radius equals the horizontal reach of the main boom plus the horizontal reach of the jib from the boom tip.

An operator uses a radius calculator and finds the load will be at 42 feet radius. The load chart shows a maximum of 9,000 lbs at 40 feet and 7,500 lbs at 45 feet. What is the safe load limit?

Answer: 7,500 lbs

When the actual radius falls between chart values, always use the capacity at the next larger (more conservative) radius listed.

A crane is set up on a 2% side slope. How does this affect the working radius on the downhill side?

Answer: Radius increases

On the downhill side, the boom will effectively swing out further than indicated, increasing the actual working radius.

What is the primary reason load charts use radius rather than boom angle as the reference value?

Answer: Radius directly determines the tipping moment regardless of boom configuration

Radius directly determines the overturning moment arm, making it the most safety-relevant variable for crane stability.

A crane operator is asked to pick a load at a 20-foot radius, but the load chart minimum radius is 10 feet. What is the concern?

Answer: The boom angle may exceed the maximum rated angle

At very short radii, the boom angle becomes very steep and may exceed the manufacturer's maximum allowable boom angle.