The number printed in a capacity table is not what you may hang on the hook. It is what the crane can hold in total, including the hardware already hanging there. Getting from one to the other is a subtraction, and the marks are lost by forgetting an item rather than by getting the arithmetic wrong.
What the gross capacity figure has to cover
The figure in the table is not the weight of the object you are picking. It is the total the crane may support in that configuration, and everything hanging below the boom tip has to come out of it — the block, the rigging, a jib, the line. What is left once all of that is accounted for is what the object itself may weigh.
What gets deducted
The chart's own deduction table is the authority, not a list you memorised. Work down the table on the chart in front of you and take off everything on it that is fitted to that machine today.
| Jib | Deduct its effective weight — the figure the chart gives, whether the jib is erected or stowed. |
|---|---|
| Block | Deduct it if it is equipped. |
| Ball | Deduct it if it is equipped. |
| Rigging | Deduct the slings, shackles, spreader bars, lifting beams and anything else below the hook. |
| Attachments and other | Deduct whatever else is hanging off the crane, whichever items that chart lists. |
| Line (wire rope) | Deduct the line hanging off the crane where that chart says to. See below — this one varies. |
Use the actual weight of the block and the rigging, from the tag or the manufacturer's data, rather than the generic figure in your head. And stowing an attachment does not stop it counting — it usually just changes the figure.
The line deduction, which is the one that varies
There is no universal rule for deducting wire rope. You have to read the chart:
- Some cranes deduct all the line.
- Some deduct only the excess line, or the line not in use.
- Some do not deduct line at all.
A worked case for the middle one. You are making a lift on the jib, and the block is still hanging off the boom tip on four parts of line, twenty feet below it. Those four parts are not in use — so on a chart that deducts line not in use, the block and that twenty feet of rope, four parts of it, both come off.
The subtraction
Start from your gross capacity and take each item off in turn. The numbers below are invented to show the shape of the working, not read from any chart:
| Step | Example (lb) |
|---|---|
| Gross capacity from the table | 32,000 |
| Subtract the jib, at its effective weight | − 1,850 |
| Subtract the hook block | − 1,100 |
| Subtract the slings and shackles | − 340 |
| What is left is your net capacity | 28,710 |
Finally, compare it with the load
Net capacity is what the crane will give you. Against it goes everything you are about to pick. Weigh what you can: a certified weight beats a shipping document, which beats a calculation from dimensions, which beats an estimate. If you cannot establish the weight, you do not lift it.
If the load is not comfortably below the net capacity, the answer is not to try it and see. Shorten the radius, rig it differently, or bring a bigger crane.