Crane types, configurations, load charts and rated capacities
You must know the crane you operate. Slewing mobile cranes include truck-mounted, rough-terrain, all-terrain and crawler types. Each has a carrier and an upper structure with boom, jib, counterweight, outriggers or crawlers, and a slew ring. Configuration changes capacity dramatically: boom length, jib fitted, counterweight, outrigger spread, tyre pressures, pick-and-carry mode and slew restrictions all matter.
The load chart is the controlling document. It gives rated capacity for a given radius, boom length and configuration, and that capacity already includes deductions for hook block, slings and any jib or auxiliary head. You must determine the actual radius at the point of lift, not the radius at the start of the slew. Working radius is measured horizontally from the slew centre to the centre of gravity of the load. Capacity decreases as radius increases and as boom length increases.
Traps: reading the chart for the wrong configuration, using the maximum radius instead of the working radius, forgetting to deduct rigging and hook block mass, and ignoring the note that the chart assumes level firm ground and specific outrigger extension. Structural limits and stability limits both apply, and the lower governs.
Rule of thumb: gross load equals load plus all lifting gear below the hook plus the hook block. Worked example: chart capacity at 12 m radius is 18 t. Load is 14 t, slings 0.4 t, hook block 0.6 t. Gross is 15 t, so the lift is within capacity but you must confirm the configuration notes and ground conditions before proceeding.