Two Radii, One Chart, and a Margin That Is Not There
A 100-ton all-terrain crane is rigged with 42.5 ft of main boom on full outriggers, using the 360° chart. With the load hanging at the pick point, the load line measures 30 ft from the rotation center; the chart at 30 ft reads 43,000 lb. The load is a 34,000 lb steel tank. The hook block weighs 650 lb, the slings and shackles 320 lb, and the chart notes require a 210 lb deduction for the stowed lattice jib. The tank must be set 4 ft further out than the pick point, so the final radius is 34 ft; the chart at 34 ft reads 36,500 lb. The set point is over the side of the carrier.
1. Compute the net capacity at both radii and state the decision.
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Total deductions: 650 (hook block) + 320 (rigging below the hook) + 210 (stowed jib, per notes) = 1,180 lb.
At 30 ft: 43,000 − 1,180 = 41,820 lb net. Load 34,000 lb. Margin 7,820 lb, about 19%. Comfortable.
At 34 ft: 36,500 − 1,180 = 35,320 lb net. Load 34,000 lb. Margin 1,320 lb, about 3.9%.
The 30 ft figure is irrelevant - it describes a moment during the pick, not the governing case. The radius that governs is the greatest radius reached during the entire lift, which is the set point at 34 ft. And even that is optimistic, because the tape was read at the pick point with the boom in a geometry that will change.
A 3.9% margin is not a margin. It is smaller than the radius growth you get from boom deflection under load, from a tagline pulling the load off plumb, or from a slight list in the carrier. The right call is to replan: reduce the set radius, take a chart with more capacity, use a bigger machine, or split the load.
2. Why can the real radius be larger than the tape measurement, and how do you measure correctly?
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Several effects all push the same direction:
- Boom deflection. A loaded boom droops, and the tip moves outward at the same time it drops. The heavier the load relative to the rating, the more real radius you gain that no tape at the pick point shows.
- Out-of-level machine. A small list is a long lever. Roughly 1% of list across a long boom translates to about a foot of tip movement at 100 ft.
- Load not plumb. Dragging the load, a taut tagline, or a pick point offset from the boom tip all convert into side load and added radius.
- Swing and wind. A load does not stop when the swing stops; it lags behind and swings out.
Correct method: the radius is the horizontal distance from the rotation center to the centerline of the load line, measured with the load applied and the boom in the lift configuration, at the worst point of the lift. Confirm it in the air, compare to the chart, and if the measured radius falls off the planned value, stop and re-rate the lift. Never interpolate upward toward a bigger number on the assumption that the chart is conservative.
3. What would change in your calculations if the lift swung through both the side and rear quadrants, or the jib were erected but unused?
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Area of operation. Charts often publish different ratings over the side, over the rear, and for full 360° swing. If the boom passes through more than one quadrant during the lift, the capacity that governs is the lowest rating along that path, not the most favorable column. Here the 360° chart is already the conservative choice; but confirm the chart actually in use says 360° and not "over the rear" in the header, which is a common and expensive misread.
Jib erected but unused. A jib or lattice extension left erected is not a free attachment: the machine must be rated from the jib chart, and the jib's weight and any offset are deducted. Stowed is different from erected - a stowed jib typically carries a fixed deduction in the notes, which is the 210 lb here. Confirm from the notes which situation you are in, because the two produce very different capacities.
Other deductions that are easy to forget: the auxiliary or rooster sheave if the line is reeved through it, lifting devices such as magnets or vacuum lifters, and any below-the-hook hardware. Every pound of it comes off the chart number before the load is compared.