NCCCO Boom Angle and Radius Calculations 4 â Questions and Answers
Question 1: A load must be set 35 feet from the crane centerline. The boom is 70 feet long. What boom angle is required?
- 30°
- 60° (Correct answer)
- 45°
- 15°
Correct answer: 60°
cos(angle) = radius/boom length = 35/70 = 0.5, so the boom angle = cosâ»Âč(0.5) = 60°.
Question 2: On a lattice boom crane, the boom angle indicator shows 72°. The operator lowers the load 10 feet. What happens to the radius?
- Radius decreases by 10 feet
- Radius remains unchanged (Correct answer)
- Radius increases slightly as load swings out
- Radius decreases by the cosine of 72°
Correct answer: Radius remains unchanged
Lowering the load vertically on a plumb line does not change the horizontal radiusâonly boom movement changes radius.
Question 3: When using a jib on a lattice boom crane, how is the total working radius determined?
- Boom radius only, jib is ignored
- Jib radius only
- Boom horizontal component plus jib horizontal component (Correct answer)
- Boom length plus jib length
Correct 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.
Question 4: 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?
- 9,000 lbs
- 8,400 lbs
- 7,500 lbs (Correct answer)
- 8,700 lbs
Correct answer: 7,500 lbs
When the actual radius falls between chart values, always use the capacity at the next larger (more conservative) radius listed.
Question 5: A crane is set up on a 2% side slope. How does this affect the working radius on the downhill side?
- Radius decreases
- Radius is unaffected
- Radius increases (Correct answer)
- Radius is doubled
Correct answer: Radius increases
On the downhill side, the boom will effectively swing out further than indicated, increasing the actual working radius.
Question 6: What is the primary reason load charts use radius rather than boom angle as the reference value?
- Radius is easier to measure with a tape
- Radius directly determines the tipping moment regardless of boom configuration (Correct answer)
- Boom angles vary by manufacturer
- OSHA requires radius measurements
Correct 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.
Question 7: 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?
- The load will be too heavy
- There is no concernâshorter radius is always safer
- The boom angle may exceed the maximum rated angle (Correct answer)
- The load line will be too short
Correct 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.
NCCCO Boom Angle and Radius Calculations 4
A load must be set 35 feet from the crane centerline.
The boom is 70 feet long.
What boom angle is required?