How to Read a Crane Load Chart Without Fooling Yourself

What the numbers in the boxes actually mean

The figures in the main capacity table are gross rated capacities. They are the most the crane may lift at that boom length and radius, and they include everything hanging below the boom tip, not just the load. Before you compare a chart value to a load weight, you have to take off the hook block and rigging (more on that below).

Each capacity is limited by one of two things: the strength of the boom and other structural parts, or the crane’s resistance to tipping. Charts mark the difference, often with shading, a bold line or an asterisk. National Crane charts, for example, identify shaded areas as structurally limited, and Manitowoc crawler charts state that capacities do not exceed 75% of minimum tipping loads, with structurally limited values flagged by an asterisk. The practical lesson: in the structural zone, the crane can fail without any warning that it is getting light, so you never rely on the feel of the machine there.

Federal rules make the chart part of the job. Under 29 CFR 1926.1417(c), rated capacities, operating instructions and the operator’s manual must be readily available in the cab, and if an electronic chart fails and becomes inaccessible, the operator must stop until it is available again.

Radius and boom length: find the right cell

Load radius is the horizontal distance from the crane’s center of rotation to the center of the hook (the vertical line through the load), not from the outrigger or the edge of the carrier. Measure it, or read it from the rated capacity indicator, and remember that the radius grows once the load leaves the ground because the boom deflects. A pick that starts at 38 feet can end up past 40.

Boom length is the actual extended length, read from the indicator or calculated from the telescoping sequence on a hydraulic, or the assembled length on a lattice boom. Line up the boom length column with the radius row to find the gross capacity.

When your radius or length falls between listed values, most chart notes tell you not to interpolate. The conservative habit is to use the next longer radius and the next longer boom length, which gives the lower capacity. Also check the boom angle column: many charts list a minimum boom angle for a given length, and below it the capacity is zero no matter what the radius looks like.

Quadrants, outriggers and counterweight

The same crane can have very different capacities depending on where the boom points relative to the carrier. Charts divide the swing into areas of operation, usually over the rear, over the side and over the front, plus a 360° column. Some machines publish a separate, lower over-front rating, and some prohibit lifting over the front entirely. The chart’s quadrant diagram shows where each area begins and ends, often referenced to outrigger beam centerlines.

Then match the configuration. Most hydraulic cranes have separate tables for fully extended, intermediate and retracted outrigger spreads, and for different counterweight packages, and a separate on-rubber chart if the crane may lift on tires at all. If the crane is set up with mid-span outriggers on one side because of a curb or a trench, you use the table for the shorter spread. Picking from the full-spread table because most of the lift happens on the good side is one of the classic ways cranes go over.

The chart also assumes the crane is level within the manufacturer’s tolerance and set on ground that can carry it, which ties to the ground conditions rule in 1926.1402. On Black Belt clay after a week of rain, or on coastal sand and dredge fill near Mobile Bay, that assumption takes mats, cribbing and sometimes an engineer’s input to make true.

Deductions: from gross capacity to net capacity

Net capacity is what remains for the load after everything else below the boom tip is subtracted. Typical deductions include:

• The hook block or overhaul ball in use (the weight is usually stamped on it or listed in the manual)
• Slings, shackles, spreader bars, lifting beams and any below-the-hook device
• Hoist rope beyond a set length, where the manufacturer requires it
• An auxiliary boom head or rooster sheave, if installed
• A jib, either stowed on the boom or erected, using the chart’s deduction table; an erected jib usually costs far more than a stowed one
• A second hook block hanging from the auxiliary line, even when you are not using it

Parts of line and the range diagram

The capacity table assumes the hoist is reeved to carry that load. Hoist capacity equals the allowable line pull per part, from the chart or manual for your rope and drum, multiplied by the number of parts of line. If a block is reeved with four parts and the chart allows a certain pull per part, the hoist can handle four times that, minus any reeving efficiency the manufacturer applies. When that number is lower than the gross chart capacity, the reeving is the limit. Re-reeve or choose a different setup; do not lift past it.

The range diagram is the side-view drawing of boom lengths, angles and radii with tip heights marked. Use it to confirm the boom will clear a roof parapet, a pipe rack or a tree line, and to estimate the radius at a given angle when you plan a pick on paper. It also shows jib offsets and how much height you gain with each section, which matters on tight plant work where the load has to go over something to get set.

Worked example (illustration only)

The numbers below are invented, round figures chosen to show the arithmetic. They do not come from any real crane’s chart. Never use them to plan an actual lift.

Suppose an imaginary hydraulic crane is set on fully extended outriggers with full counterweight, lifting over the rear. The boom is out to 70 feet, and the load will sit at a 40-foot radius. In this made-up chart, the cell for 70 feet of boom at 40 feet of radius reads 20,000 lb. The pick is close to the edge of the cell, so we also confirm the set point will not drift past 40 feet once the boom deflects.

Deductions: the hook block weighs 600 lb, the slings and shackles 150 lb, and a stowed jib carries a listed deduction of 1,000 lb. Total deductions are 1,750 lb, so net capacity is 20,000 − 1,750 = 18,250 lb.

Next, the reeving. In the same made-up world the allowable line pull is 6,000 lb per part and the block is reeved with three parts, giving 18,000 lb of hoist capacity, which is less than 20,000 lb gross. Here, the reeving limits the lift to 18,000 lb gross, or 16,250 lb net after the same deductions. If the load weighs 15,000 lb, it fits, but with only about 8% margin to spare. If the job could swing over the side where the imaginary chart drops lower, the whole calculation has to be repeated for that area.

Mistakes that turn up again and again

Under 1926.1417(o), the crane may not be run over its rated capacity, and the operator must verify that the load is within it, either from a weight recognized by industry or a recognized calculation, or by starting the hoist and using a load weighing device, load moment indicator or rated capacity limiter to check whether the load exceeds 75% of the maximum rated capacity at the longest radius of the lift. If it does, the lift waits until the weight is verified. Most chart problems come down to one of these slips:

• Using the radius at pick-up and forgetting the radius at set-down
• Reading the full-spread table when an outrigger is short-jacked or not fully extended
• Leaving the stowed jib or second block out of the deductions
• Rounding radius or boom length down instead of up
• Trusting the load weight on a shipping ticket without checking what was added on site
• Ignoring the reeving limit or the minimum boom angle
• Assuming the chart covers wind; it is built for the manufacturer’s stated conditions, and gusts off the Gulf can push a load radius out in seconds