Definition
Capacity refers to the maximum load mass, expressed in tonnes, that a crane can safely lift and move in a standardized position. This value represents the safe working load (SWL) under specified conditions and forms the basis for operational planning and load calculations on construction sites.
Determination and Classification
Capacity is determined during design and verified through load testing. For mobile cranes, capacity varies significantly with boom length, boom angle, and radius from the mast center. A typical mobile crane with a 50 m boom may achieve 80 tonnes capacity at 2 m radius but only 5–8 tonnes at maximum radius. Capacity tables in the crane's load chart specify exact values for each configuration.
Tower cranes typically have fixed capacities ranging from 6 to 20 tonnes for standard models, with special heavy-lift versions reaching 100 tonnes or more. Capacity is always measured at a defined radius (usually the trolley's furthest safe position) and specific boom configuration.
Safety Standards and Verification
Capacity is established according to EN 13000 (mobile cranes) and EN 14439 (tower cranes). Load testing is mandatory: proof load testing at 1.25 times the rated capacity confirms structural integrity. The Safe Working Load (SWL) incorporates a safety factor of 4:1 or higher, depending on the lifting mechanism and crane type.
Operators and planners must account for dynamic factors including acceleration, deceleration, wind exposure (wind speed reduces usable capacity, typically by 10–30% above 10 m/s), and cable wear. Actual working capacity may be 20–40% lower than nameplate capacity under real site conditions.
Documentation and Load Charts
Every crane must have a current, legible load chart permanently attached to the operator's cabin or control panel. The chart displays capacity in relation to radius (measured in metres) and boom angle. For example, a 35-tonne mobile crane might show: 35 t at 2 m radius, 18 t at 8 m radius, 8 t at 15 m radius.
Digital capacity management systems are increasingly used on modern cranes, preventing overload by electronically limiting hook movement when calculated load exceeds capacity at the current radius and angle.
Practical Application
Planners use capacity data to select appropriate crane sizes for specific tasks. A load of 25 tonnes at 10 m radius requires a crane with at least 25 t capacity at that radius—not simply a 25-tonne crane. Underestimating the required reach leads to inadequate crane selection; exceeding nameplate capacity voids insurance and creates acute safety hazards including structural failure, tipping, or cable breakage.
