Load Charts & Capacity

Every number on a load chart traces back to one physical quantity: load radius. Load radius is the horizontal distance from the crane's center of rotation to the center of the load — not the boom length, not the slant distance along the boom. Boom length and boom angle together set the radius. A long boom laid at a low angle produces a large radius; the same boom raised steeply produces a small one. As radius increases, the chart rating drops, often sharply, because the farther out the load is, the greater the overturning moment on the machine.

Radius can also grow during a lift even when the operator does nothing to cause it. Boom deflection under load, dynamic swing-out while slewing, and soft ground can all push the actual radius past what was planned. This is why a competent operator reads the chart for the actual radius under load, not just the starting position.

The capacity figure the chart gives at a given boom length and radius is the gross rated capacity. But gross capacity is not the weight you can pick. To get net capacity — the actual load the hook can lift — you subtract the weight of every load-handling device: hook block, headache ball, slings, shackles, any stowed or erected jib, and auxiliary lines if rigged. All of that hardware weighs something, and the chart does not account for it.

The arithmetic is simple, but the discipline to do it every time is what the exam and the job are testing. A hook block and a set of rigging can together reduce net capacity by several hundred pounds, enough to matter on a near-limit lift. Identify your deductions before you pick, not after.

The range diagram is the geometric companion to the capacity table. It shows how boom length, boom angle, tip height, and load radius relate to each other across the crane's working envelope. You use it to confirm your actual load radius for the current configuration before entering the capacity table. Skip that step and you risk looking up the wrong column.

Outrigger position and working quadrant then determine which table applies. Most cranes have different ratings depending on whether the load is over the front, side, or rear of the machine. The rear is typically the strongest direction because the counterweight is working in your favor; side ratings are usually the most restrictive. When outriggers are not fully extended, ratings drop further. Pick-and-carry — operating on rubber — carries the lowest ratings of all, typically limited to light loads at short radii, usually over the rear.

Not all capacity limits come from the same source. Some are governed by structural strength — the load at which something bends or breaks. Others are governed by stability — the load that would tip the machine. Load charts typically distinguish these two regimes, often by printing stability-governed values in bold or drawing a dividing line across the chart. Neither limit is acceptable to exceed, but understanding which one applies tells you what mode of failure you are approaching.

Most modern cranes include a load moment indicator (LMI or RCI) that warns the operator when the crane approaches its rated capacity. The LMI is a useful tool, but it is not a substitute for reading the chart yourself. Notes, footnotes, and deduction tables in the chart carry conditions the LMI cannot enforce. Operators who understand the chart can also recognize when the LMI reading does not match expectations — a sign that configuration data was entered incorrectly or that conditions differ from what the system assumes.