Stability, Load Charts and Lifting Calculations
Stability and capacity questions reward careful reading of the chart and the configuration.
Stability
- Load moment = load × radius. The counter-moment from ballast and counter-jib resists it. Stability is lost when the load moment exceeds the resisting moment, or when the crane is unloaded with the jib downwind (backward stability).
- Out-of-service conditions require the crane to be left in the manufacturer's specified configuration (often jib free to slew, trolley at a stated position) so it weathervanes safely.
Load charts
- Charts are specific to jib length, counter-jib/ballast, height, and whether anchored. Read the chart title before the numbers.
- Capacity reduces with increasing radius; find the radius, read across, apply any deduction for loose gear (sling, shackles, hook block if not included).
- Some charts give gross and some net capacity - a classic trap.
Worked example
A chart gives 2,400 kg at 20 m radius (net). The load weighs 2,150 kg and rigging 180 kg. Total 2,330 kg < 2,400 kg, so it is within capacity - but only if the configuration matches the chart and wind is within limits. If the chart were gross, deduct the hook block and rigging before comparing.
Traps
- Using a chart for a different ballast or jib configuration.
- Ignoring rigging weight.
- Confusing rated capacity with the test load figure.