How the drill works
The chart above the questions is made up for teaching. Its numbers match no crane and no exam. Each round picks a radius and a boom length, some of them printed on the chart and some of them between printed values. You choose the capacity the chart gives, and the answer shows the row and column used, the cell it came from and what the wrong choices were.
One round goes on to net capacity and one asks whether a payload fits, so you can see how a reading error carries into the answer. For more chart problems with deductions and percentages, use load chart practice. For the basics of finding the right chart and row, read how to read a crane load chart.
A three-step routine for in-between values
- Find the configuration first. Pick the chart, boom length, support setup and lift area, because a number only holds for the setup it was printed for.
- Bracket the radius and the boom. Find the printed values just below and just above each one. If either one is printed exactly, use it as it stands.
- Use the longer entries. Read the cell at the next longer radius and the next longer boom. If that cell is a dash, the answer is no rating. Then take off the deductions for net capacity.
Worked example 1: radius between two rows
Same invented chart. The radius is 15 ft and the boom is 70 ft. The boom is a printed column. The radius sits between the 10 ft row and the 20 ft row, so use the 20 ft row. The 70 ft column at 20 ft reads 26,000 lb. Blending the 36,000 lb and 26,000 lb cells would give 31,000 lb, and reading the 10 ft row would give 36,000 lb. Both of those are higher than the chart gives at 20 ft, so both overstate it.
Worked example 2: where a blend flips the answer
The radius is 35 ft and the boom is 60 ft, so both fall between printed values. Use the 40 ft row and the 70 ft column: 11,000 lb gross. The deductions are the 800 lb hook block and 500 lb of rigging, 1,300 lb in all, so net capacity is 11,000 − 1,300 = 9,700 lb. A 10,000 lb payload exceeds that by 300 lb. A blend of the four surrounding cells gives 14,750 lb gross, or 13,450 lb net, and would wrongly say the payload fits. That is the cost of blending: the answer changes sides.
These are original arithmetic examples on an invented chart, not exam questions and not any crane's chart.
Common misreads
- Blending two rows. It produces a value no one rated, and it reads high.
- Rounding to the nearest row. A radius of 31 ft is not a 30 ft lift. Nearest is not safe; longer is.
- Fixing only one side. Radius and boom length can both fall between values, so bracket both.
- Borrowing past a dash. A dash is no rating, not a prompt to read the next column over.
- Forgetting deductions. The chart number is gross; the net capacity is what remains after the hook block and rigging come off. The net capacity calculator walks through that step.
OSHA 29 CFR 1926.1417 covers operating within rated capacity and following the manufacturer's procedures. Which rule a chart applies to in-between values is set by that chart's notes, so confirm against your own.
Interpolation questions
Can I interpolate on a crane load chart?
Follow the notes printed on the chart you are using, because they say how to treat a radius or boom length that falls between entries. The conservative reading, and the one this drill uses, is to take the next longer radius and the next longer boom, which gives the lower capacity. That reading never gives you more than the chart rated. If a chart's notes say something different, the notes on that chart win.
Why use the longer radius instead of averaging the two rows?
Averaging gives a number nobody rated, and it usually reads high. Where a crane is limited by tipping, capacity falls roughly like one over the radius, a curve that sags below the straight line between two rows, so a straight-line blend overstates it. The next longer radius is a printed, rated value that sits on the safe side. Confirm the exact rule against your own chart.
What if my radius or boom length is past the last entry, or the cell is a dash?
Then the chart gives no rating for it. Do not extend the numbers past the last row and do not borrow the value from the cell beside it. A dash or a missing row means the answer is no rating for that configuration, so the lift needs a different setup or a chart that covers it.
Reading the right cell and carrying it through to a net capacity is the kind of chart reasoning the written exam tests. This drill shows the method and does not approve a lift.