Load Charts & Capacity Fundamentals
Rated capacity, gross vs net capacity, load radius, load charts, quadrant of operation, and boom angle effects on lifting capacity.
- Define rated capacity and explain how it differs from maximum load ever lifted
- Calculate net capacity by deducting rigging, hook block, and attachment weights from gross capacity
- Explain the relationship between load radius, boom angle, and crane capacity
- Read a load chart to determine safe lifting capacities at various configurations
- Identify the quadrants of operation and their effect on mobile crane capacity
Lesson 1
Rated Capacity & Load Radius
What Rated Capacity Means
Every crane has a rated capacity - the maximum load it can safely lift under specific conditions as stated by the manufacturer. This is not an estimate, not a historical record, and not an average. It is the manufacturer's certified limit for a given combination of boom length, radius, and configuration.
Never Exceed Rated Capacity
The rated capacity is the absolute maximum load the crane can safely lift under specific conditions. Exceeding it risks catastrophic failure, tipping, or structural collapse.
The rated capacity depends on several variables that change with every lift:
- Boom length - how far the boom extends
- Load radius - horizontal distance from center of rotation to center of load
- Boom angle - the angle of the boom above horizontal
- Crane configuration - outrigger position, counterweight installed, on rubber vs outriggers
Understanding Load Radius
Load radius is the horizontal distance measured from the center of rotation (slewing ring) to the center of the load being lifted. This is one of the most critical measurements in crane operation because it directly determines how much the crane can safely lift.
The fundamental rule every operator must know: as load radius increases, crane capacity decreases. This inverse relationship exists because a longer horizontal distance creates a greater overturning moment. Think of it like holding a weight with your arm extended - the farther from your body, the harder it is to hold.
Why Load Radius Matters More Than Load Weight
Many accidents occur because operators focus on load weight alone without considering the radius. A crane rated at 50 tonnes at 3 metres radius might only handle 10 tonnes at 12 metres. The same crane, the same day, but a completely different safe capacity based solely on where the load hangs relative to the center of rotation.
Rated capacity is the maximum load a crane can safely lift under specific conditions - not the maximum ever lifted or the average daily capacity. As load radius increases, capacity always decreases.