Load Charts, Capacity, and Structural/Limit Considerations
Rated load is a system number
The crane's rated load is the lowest of the hoist, trolley, bridge, and runway ratings and is stated for a specific reeving and configuration. On a main/auxiliary crane the two hoists have separate capacities and are not additive. Before lifting, subtract everything hanging below the hook: hook block where applicable, spreader beam, magnet, slings, and rigging.
Reading the chart
Cranes with variable ratings publish them by reeving, hoist, and sometimes trolley position or duty class. Find the row matching how the crane is currently configured, then the column for your load. Duty class matters: a crane rated for light service is not intended for continuous heavy cycling.
Structural and limit considerations
Overhead cranes are usually limited by structure rather than stability, so watch for side loading, shock loading, wind on large sail-area loads, and off-center lifts - each can exceed the design assumptions without exceeding the number printed on the chart. Limit devices (hoist upper limit, trolley and bridge travel limits) protect the machine; they add no capacity and must not be used as operating stops.
Worked example
Rated capacity 10 tons (20,000 lb). A 500 lb spreader beam plus 200 lb of slings plus a 9,000 lb load totals 9,700 lb, which is acceptable. Substitute a 12,000 lb load and the total is 12,700 lb, which exceeds 10 tons - the answer is a larger crane, not a slower lift.
Traps: ignoring rigging weight, adding main and auxiliary capacities together, and treating a test load as usable capacity.