Center of gravity of a load: meaning and why it matters

What it does

A load hangs from a hook the way a plumb bob hangs from a string. It will turn and tilt until its center of gravity lies directly below the point of support. If you lift an uneven block with the hook above its middle, the block hangs level only if the middle and the center of gravity are the same place. If they are not, it tilts until they line up.

For a uniform bar, plate or block, the center of gravity is at the geometric center. A pipe with a heavy valve on one end, a machine with a motor on one side or a beam with a plate welded near one end has its center of gravity shifted toward the heavy part.

How it splits the load

With two lift points, the load divides in inverse proportion to the distance from the center of gravity. Take a 10,000 lb load with lift points 10 ft apart and the center of gravity 4 ft from point A and 6 ft from point B. Point A, closer to the center of gravity, carries 6/10 of the load, 6,000 lb. Point B carries 4,000 lb. A rigging plan that assumes 5,000 lb at each point understates point A by 1,000 lb.

That matters for slings, shackles and the lifting lugs on the load itself, all of which must carry their real share. Combine it with the sling angle factor to get the true tension in each leg.

Setting a pick

• Find or estimate the center of gravity before the lift. Drawings, labels and the load's weight data help. For simple shapes, the load weight calculator gives weight, and the center is at the middle of a uniform shape.
• Put the hook over it. Choose attachment points and leg lengths so the hook ends up above the center of gravity.
• Attach above it. Lift points above the center of gravity make the load stable. Points below it let the load flip.
• If you do not know where it is, lift slowly a short distance, watch which way it tilts and set it down to adjust.

Common mix-ups

• Center of gravity is not always the middle. It is the middle only for evenly built shapes.
• Equal slings do not mean equal loads. An off-center load loads the near leg more.
• A level load is not proof the pick is right. The lift points may still carry unequal shares.
• Lift points below the center of gravity. The load can roll over once it is off the ground.

On the NCCCO written exam

Center of gravity belongs to Technical Knowledge, about 23% of Core under the 01/25 handbook, with rigging devices named in task 3.5. Questions describe an uneven load and ask where the hook should be, which leg takes more, or why a load tilted on lift-off.

Practice it

Work weights with the load weight calculator. Pair this page with the sling angle factor and choker vs basket hitch. 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 1926.251: Rigging equipment for material handling, OSHA.
• Hoisting and Rigging Fundamentals, TR244C Rev. 5, U.S. Department of Energy, sling loading.
• Mobile Crane Operator Candidate Handbook, CCO, Core outline tasks and weights (01/25 revision). Task numbers on this page follow it.