Portal Boom Crane Fundamentals, Terminology and Rated Capacity
What a portal boom crane is
A portal boom crane has a portal (gantry-style) base that straddles a roadway, rail line or work area, with a slewing upper structure carrying a boom. This configuration allows loads to pass over the straddled area. Understand the main components: portal legs and bogies, slewing ring and drive, boom (often luffing), hoist drum and rope, counterweight, and the rated capacity indicator/limiter. Terminology questions test the difference between luffing (changing boom angle) and slewing (rotation), and between hoist and travel motions.
Rated capacity and load charts
The rated capacity is the maximum load permitted for a specific configuration, and it varies with radius, boom length, boom angle, counterweight configuration and outrigger/rail condition. The load chart is configuration-specific. Traps: reading capacity at the wrong radius, forgetting to add rigging mass to the load, assuming capacity is constant across the slew arc, and confusing gross load (load plus rigging) with net load. Portal cranes may have reduced capacity at certain slew positions or when travelling, and rail-mounted units may have rail clamp and wind restrictions.
Moment and stability
Capacity is ultimately limited by structural strength and by stability (overturning moment). As radius increases, capacity falls quickly because the load moment rises. Always read the chart for the actual radius measured at the boom tip, not at the load's near edge.
Worked example
If the chart gives 12 t at 10 m radius and 8 t at 14 m, and your load plus rigging is 9 t, you must stay inside the radius where capacity is at least 9 t. Interpolating conservatively, the maximum radius is roughly 12.5 m - verify against the actual chart, never guess.