Short answer
The rated capacity of an overhead crane is calculated from the maximum load actually handled, plus the weight of lifting accessories (spreader beams, clamps, slings) and a margin for growth. It is never derived from the average load.
Start from the maximum load, not the average
A workshop that handles 3-tonne parts on average but receives an 8-tonne machine once a month needs a 10-tonne crane. The crane is sized for the worst case, not the common one.
Add lifting accessories
Spreader beams, coil clamps, magnets and slings can weigh several hundred kilos. Their weight adds to the payload to define the hoist's rated capacity.
Choose the FEM class
FEM standards classify mechanisms by number of cycles and load spectrum. A maintenance crane (M3, M4) and a foundry crane (M6, M7) of the same capacity have neither the same hoist nor the same service life. INDURA defines the class with you from cycles per day.
Plan for growth
An overhead crane lasts twenty to thirty years. If the activity may move to heavier parts, a 20 to 30% margin at the start costs less than reinforcing the runways later.
Check the supporting structure
Crane capacity must be compatible with the allowable reactions on columns and runways. For an existing building, a structural calculation is essential before fixing the final capacity.
