Crane Types, Configurations and Load Charts
Reading the chart correctly
A load chart is a set of limits for a specific configuration. Before reading a capacity you must know: the crane model; the boom length and configuration (telescopic, lattice, fly jib); the counterweight; whether the crane is on outriggers, crawler tracks or tyres; the outrigger spread; and the radius. Capacity is then read at the intersection of radius and boom length, and reduced for factors such as duty, number of falls and wind. The classic trap is reading capacity at the wrong radius, or using the "on tyres" chart when the crane is on outriggers, or vice versa.
Radius and geometry
Radius is measured from the slew centre to the centre of gravity of the load, horizontally, with the boom at the working angle. Adding a jib, increasing boom length, or booming out increases radius and reduces capacity. The chart's capacity already includes the hook block and lifting attachments in many cases, but you must check the notes; if not, deduct their mass. Never interpolate between chart values unless the chart permits it; use the lower value.
Structural versus stability limits
Capacity may be limited by structural strength (boom, pins) or by tipping (stability). Near the tipping line, capacity drops steeply with radius. This is why small radius increases can cause large capacity losses. Charts often show a bold line separating structural from stability zones.
Worked rule of thumb
If your required radius is 18 m and the chart gives 4.2 t at 18 m and 3.6 t at 20 m, use 4.2 t at 18 m only if 18 m is a listed radius; if not, use the next longer listed radius. Always round radius up and capacity down.