Derrick crane operation, load charts and safe working limits
Reading the load chart
A derrick load chart gives capacity for a given radius, boom length and configuration. Radius is measured horizontally from the mast/slew centre to the load line, and it changes as the boom luffs and the load swings. Capacity falls as radius increases. Deduct the rigging mass. Traps: reading the chart at the wrong radius; using a chart for a different configuration; assuming capacity is constant around the slew; and ignoring wind area of the load.
Operating principles
Plan the lift: know the path, the landing point, the escape routes and the signal plan. Lift, swing and lower smoothly - no shock loading, no dragging, no side pulls. Keep the load as low as practical and control swing with tag lines. Never exceed the chart; if the load is near capacity, verify mass and radius before lifting.
Wind and environment
Wind acts on the load's area and on the boom. Know the manufacturer's wind limit; stop when it is reached or when the load becomes uncontrollable.
Worked example
Boom at a radius of 12 m gives 5 t capacity. The load is 4.6 t plus 0.5 t rigging = 5.1 t gross. That exceeds capacity - reduce radius, use a different configuration, or reduce the load.
Near-capacity lifts
Treat any lift above about 75–80% of chart capacity as a planned critical lift with a documented procedure.