What Radius Does to a Crane Capacity and Why It Surprises People

Why this matters

Radius is the one input to a capacity lookup that changes during the lift, and it only ever changes in the direction that costs you. The boom bends down under the load, the load swings out on the way round, and the pick point turns out not to be quite where the plan said. This card follows one lift where those three terms added 3.8 ft to a planned 32 ft radius and moved the pick from 96 percent of chart to 113 percent, and it accounts for every foot of that 3.8. The rule it ends on is not "add a margin". It is that the radius which governs is the largest one the load will ever occupy, and you find it by measuring rather than by planning.

What radius is measured from, and to

Radius is the horizontal distance from the centre of rotation of the machine to the centre of gravity of the load, with the load hanging free and plumb. Two halves of that definition get lost regularly.

The centre of rotation is the swing axis, not the boom foot, not the front of the carrier, and not the outrigger line. On a machine with any tail swing at all those points are feet apart.

The load's centre of gravity is not the boom tip and not the hook. Where the load hangs plumb they line up in plan view, which is why measuring from the ground under the load works. Where the load does not hang plumb, because it is being dragged or pulled sideways, the geometry has already left the chart's assumption and the answer is to stop and re-rig rather than to measure something.

   center of rotation
        |
        |    boom position with no load
        |   .-'''''''.
        |.-'          '-.  boom under load, tip drops
       [ ]                '.
       [ ]                  |  hoist rope hangs plumb
    ===[ ]===               |
    ////////////////////[load]////////////
        |<--- planned radius --->|
        |<------ governing radius ------>|

The chart's radius already has deflection inside it

Before applying any correction, write down what the number you are correcting already contains. Mobile crane charts are published against the loaded radius: the radius the load actually sits at once the boom has taken the weight and bent. That is a re-basing point, not a bonus.

The consequence runs the way most people do not expect. If you compute radius from the boom angle indicator and the boom length, you get the unloaded geometry, which is shorter than the loaded radius. Reading the chart at that unloaded figure hands you a capacity the machine will not have once the load is on. So the boom-angle calculation is a planning tool for choosing where to set up, and a tape measure on the ground with the load just clear of grade is the verification. The two disagree by the amount of deflection, and deflection grows with boom length, with load, and as the boom is lowered.

Why capacity falls at least as fast as the radius grows

Over the part of a chart where stability governs, what stays roughly constant is a moment, not a load, and moment is capacity multiplied by radius. Double the radius and roughly half the capacity is left before you account for anything else.

It is actually a little worse than that, because as the boom comes down to reach further, the boom's own weight acts at a longer arm too, so more of the machine's available resisting moment is spent on the boom rather than the load. You can see it in any chart by multiplying each row out. Using one illustrative table for a single boom length and configuration:

The third column drifts down as you go out, which is the boom's own arm growing. Check it against strict inverse proportionality, which is what a constant moment would give. At 35 ft that would be 18,000 x 30 / 35 = 15,429 lb and the chart publishes 15,200. At 40 ft it would be 18,000 x 30 / 40 = 13,500 lb and the chart publishes 12,900. The chart sits below the constant-moment line at every row and falls further below it the further out you go, which is the boom's own arm growing. Near proportional, slightly worse than proportional, and nothing like the "a few feet will not matter" instinct.

Where in the chart this holds: in the stability-governed rows. High on the chart, at short radius and steep boom, the numbers are frequently limited by boom structure or by hoist reeving instead, and there capacity can be flat across several radius rows. Check which limit governs your cell before you assume the moment relationship applies to it.

The case: a lift planned at 32 ft

A crew set up to place a fabricated frame on a mezzanine. Total suspended weight, load plus block plus rigging, was 14,600 lb, all from certified weights and markings. The configuration gave the illustrative table above. The planned radius, scaled off the site drawing, was 32 ft.

The planner did the lookup correctly. 32 ft falls between published rows, and the rule is to use the next longer radius, so the 35 ft row governed at 15,200 lb.

• Total suspended against the governing row: 14,600 / 15,200 = 96.1 percent

Tight, legal, and signed off. Nobody was casual about it. The plan carried the right rounding rule and the arithmetic closed.

On the day, the load came off the ground and the machine's load moment indicator went into alarm partway through the swing. The operator set the load back down. Someone taped the actual radius with the load re-landed and the crew clear of the swing radius, and the tape read 35.8 ft.

The 3.8 ft, accounted for

Nobody had to guess where it came from, because all three terms are measurable.

Boom deflection under load: 2.1 ft. The 32 ft figure came from the drawing, which is unloaded geometry. With the load just off the ground, the tape read 34.1 ft. That difference is the boom bending, and it is the term the chart already assumes you are including.

Swing-out: 1.2 ft. The load is a pendulum on the hoist rope. Accelerating into the swing leaves it trailing, decelerating out of it sends it leading, and at the far end of the arc it hangs outside the boom tip's own radius until it settles. That extra distance is not on any drawing.

Pick point offset: 0.5 ft. The frame was set down on dunnage half a foot outboard of where the plan put it, which is normal, and which nobody logged.

Sum: 2.1 + 1.2 + 0.5 = 3.8 ft. Planned 32 ft plus 3.8 ft is 35.8 ft.

What that did to the lookup. 35.8 ft is past the 35 ft row, so the next longer published row governs, and that is 40 ft at 12,900 lb.

• Total suspended against the governing row: 14,600 / 12,900 = 113.2 percent

The lift was over the chart by 13 points at the moment the alarm went off. Note what the next-longer-row rule did and did not do: rounding 32 ft up to the 35 ft row bought 3 ft of cushion, and the real overrun was 3.8 ft, so the conservative rounding absorbed most of the error and still ran out. A rounding rule is a margin against small mistakes, not against unmeasured geometry.

One thing the alarm did not do. A load moment indicator is an operational aid, covered under 29 CFR 1926.1416 for construction work, and an aid is a backstop, not a permission. The lift was over the chart before the alarm sounded, and had the aid been out of service under the temporary-alternative-measures provisions of that section, nothing would have told the crew at all.

What actually fixed it

Not a bigger margin. A shorter planned radius, chosen so that all three terms still land inside a better row.

The crane was repositioned to a 26 ft planned radius. The same three terms apply and they do not shrink, so the governing radius becomes 26 + 3.8, which is 29.8 ft, and 29.8 ft takes the 30 ft row at 18,000 lb.

• Total suspended against the governing row: 14,600 / 18,000 = 81.1 percent

Six feet of repositioning moved the lift from 113 percent to 81 percent. That is the leverage this card is about, and it is available at setup and almost nowhere afterward.

What getting this wrong looks like, and how to catch it

The dangerous version of this failure is the one where nothing alarms. A machine with the aid isolated, or a lift a few points over instead of thirteen, gets picked, swung and landed with no cue at all. The tell is not a sound, it is a discrepancy: a taped radius that does not match the planned one, a boom angle indicator reading lower than the plan assumed, or a load that visibly leads or trails through the swing.

So the check is a tape, and it happens twice: once at setup with the load landed, and once with the load just clear of grade. Both readings are taken from outside the swing radius, from a position that is never under the boom or the load, with the operator informed and the swing stopped. If the second reading is more than trivially longer than the first, that difference is your deflection term and it belongs on the plan for every subsequent pick with that configuration.

Two conditions genuinely change the method rather than the numbers. On a machine that is out of level, the geometry above stops describing it and the ground condition becomes the governing question; the soft-ground card in this library owns that. And on a multi-crane lift, each machine's radius changes as the load rotates or tilts between them, so radius is not a single measured number at all and the lift needs an engineered plan.

References

• 29 CFR 1926 Subpart CC, cranes and derricks in construction, including 1926.1417 on operation within rated capacity and 1926.1416 on operational aids and the measures required when one is out of service
• ASME B30.5, mobile and locomotive cranes, in the edition adopted by your employer's program, your contract or your authority having jurisdiction, which is how it binds
• Manufacturer load chart and range diagram for the specific machine and configuration, which owns every capacity and radius figure
• See related: How to Read a Crane Load Chart Without Getting It Wrong; Why a Mobile Crane Is a Different Machine on Soft Ground