Boom deflection and loaded radius: meaning and why it matters
Boom deflection is the bending of a boom under load. The tip moves outward and down, so the radius measured with the load hanging is larger than the radius of the same boom with no load. A chart is read at the loaded radius, since that is where the load actually sits.
What happens to the radius
A boom under load acts like a long beam. It bends, and the tip moves outward and downward. The load line hangs from the new tip position, so the horizontal distance from the center of rotation to the load line is larger than the geometry of the unloaded boom would suggest. A longer boom, a heavier load and a lower boom angle all increase the bending.
An instrument that reads boom angle may not show the bend at the tip. Reading the actual loaded radius, with the load suspended, is what the chart calls for.
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Question 1 of the free 10-question check on NCCCO written. No signup.
Operations
Which action creates side loading on a crane boom?
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A worked example on a practice chart
On our invented chart PC-1, take a 60 ft boom. At a 30 ft radius the cell reads 19,500 lb gross. Suppose the boom's unloaded geometry gives 30 ft, but with the load on, the radius measures 32 ft. The chart lists radii of 30 and 35 ft. Our practice charts follow the rule used in the Boom Angle practice tool, which is to use the next longer listed radius; on a real chart, the notes say how to treat a radius between columns. So the cell to read is 35 ft, which is 16,000 lb gross, not 19,500 lb.
| Radius used | Gross capacity | Difference |
|---|---|---|
| 30 ft (unloaded geometry) | 19,500 lb | |
| 35 ft (next listed above 32 ft) | 16,000 lb | 3,500 lb lower |
A two-foot change in radius cost 3,500 lb, and a plan built on the unloaded figure would have been overloaded. The numbers are invented. The pattern applies to real charts.
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What to do about it
- Take radius readings with the load on the hook, not before.
- Check the chart's notes. Some charts say to add an allowance or to use a loaded radius, and the note governs.
- Leave margin when a lift is close to the chart. Deflection grows as the boom lowers and lengthens.
- Do not read a radius from the boom angle alone without allowing for bending. See boom angle.
Common mix-ups
- Treating deflection as a safety margin. It works against you, since it increases radius.
- Using the radius from a drawing. A drawing shows the unloaded position.
- Assuming only long booms matter. Short booms bend less, but the effect is still real.
On the NCCCO written exam
Deflection is named with dynamic and side loading and weather in Core tasks 2.16 to 2.19, in Operations, about 28% of the exam under the 01/25 handbook. A problem may give an unloaded radius and a deflection allowance and ask for the cell to read.
Practice it
The crane radius and boom angle calculator shows the unloaded geometry, and its note reminds you the loaded radius is larger. Read crane radius vs boom length and boom angle alongside it. The crane terms glossary lists the whole set.
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Sources
Show the list
- 29 CFR 1926.1416: Operational aids, OSHA.
- 29 CFR 1926.1417: Operation, OSHA.
- Mobile Crane Operator Candidate Handbook, CCO, Core outline tasks and weights (01/25 revision). Task numbers on this page follow it.
Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.
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