Load moment: meaning and why it matters

The idea

Try holding a bucket at arm's length and then close to your chest. The bucket weighs the same, but at arm's length it is much harder to hold. The difference is the moment, the weight times the distance from your body. A crane faces the same thing, with the tipping axis in place of your shoulder.

A 10,000 lb load 20 ft from the pivot has a moment of 200,000 ft-lb. The same load at 40 ft has 400,000 ft-lb. Doubling the distance doubled the moment, and a crane has to resist that moment with its own weight and counterweight acting on the other side of the pivot.

Where it shows up on a chart

Capacity falls as radius grows because the load's moment grows with it. On stability-limited cells the crane can resist only so much moment, so a longer arm means a smaller load. See tipping axis.

Do not turn this into a formula for reading charts. On our invented chart PC-1, an 80 ft boom is rated 18,000 lb at 30 ft and 8,500 lb at 50 ft. Multiplying out gives 540,000 ft-lb and 425,000 ft-lb, which are not equal. Chart radius is measured from the center of rotation, not from the tipping axis, and the crane's limits are not purely about tipping. A chart is a rating, not a calculation, so you read the cell.

Moment indicators

A load moment indicator or limiter on a crane works from the load and radius to compare the lift against the chart. See rated capacity indicator vs limiter.

Common mix-ups

• Distance is horizontal. Use radius, not boom length or line length.
• Moment is not capacity. It is a measure of how hard the load is turning the crane.
• Units multiply. Pounds times feet give foot-pounds. Do not add the two.
• Equal weights at different radii are not equal demands. The far one asks more of the crane.

On the NCCCO written exam

The idea behind moment sits in Technical Knowledge, which is about 23% of Core under the 01/25 handbook, and explains the radius axis of every chart in Load Charts, about 27%. Know that increasing radius, with the same load, increases the turning effect, and what that does to capacity.

Practice it

The crane percent of capacity calculator shows how loaded a crane is against the chart figure. Then read tipping axis and boom angle. The crane terms glossary lists the whole set.

Know the terms? Take the free 10-question check

Take the free 10-question check. It is scored by the Core outline domains and needs no signup, so you see whether site, operations, technical knowledge or load charts costs you the most points. Boom Angle's course then gives you a question bank with each answer key checked by a second solve, timed practice exams, and a study plan built from your exam date.

Sources

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• 29 CFR 1926.1433: Design, construction and testing, OSHA.
• 29 CFR 1926.1416: Operational aids, OSHA.
• Mobile Crane Operator Candidate Handbook, CCO, Core outline tasks and weights (01/25 revision). Task numbers on this page follow it.