What is load radius and how does it affect crane capacity?

Load radius is the fundamental input to load chart reading. Every rated capacity value in a crane's load chart is keyed to a specific load radius at a specific configuration. The load radius defines how far the load is from the crane's tipping fulcrum, and the farther the load is from the fulcrum, the greater the tipping moment for a given load weight. Because tipping moment increases linearly with radius, rated capacity decreases rapidly as radius increases, particularly at longer radii where tipping limits dominate over structural limits.

Measuring Load Radius in the Field

In planning, the load radius can be calculated from the boom length and boom angle using trigonometry. In the field, operators may use the load chart's boom angle and length columns as reference, or on cranes equipped with LMI systems, the radius may be displayed directly from sensor inputs. The most reliable method for confirming the actual load radius on a critical lift is a direct physical measurement from the crane's centerline of rotation to the point where the load will be picked and where it will be set. This measurement should be made before the crane is positioned and again once the crane is set up, because the assumed radius during planning and the actual radius after setup may differ due to site conditions, obstacles, or changes in the pick point location.

Why Radius Errors Are Serious

Using a rated capacity value from the wrong radius column is one of the most common errors in crane operation. A one-foot error in radius at long boom lengths can result in using a rated capacity that is hundreds or thousands of pounds higher than the actual rated capacity at the true radius. Because load charts are stepped tables (the next row may represent 5-foot or 10-foot radius increments), an operator who rounds down to the nearest row when the actual radius falls between rows is operating at a capacity the chart does not certify. The conservative practice, and the requirement under ASME B30.5, is to always use the rated capacity for the row at or beyond the actual radius, not the row below it.

Radius Changes During the Lift

The load radius is not necessarily fixed throughout a lift. As the boom is raised or lowered during a lift, the radius changes. As the crane swings from one position to another, the radius to the load pick point changes if the swing covers different distances. On crawler cranes that travel with the boom extended but without a load, the boom angle may change on graded surfaces, which changes the radius. The operator must monitor radius throughout the lift and, if the radius increases beyond the planned value, must compare the actual load against the rated capacity at the new radius before allowing the increase to proceed.