Every load chart is laid out differently, so this is a method rather than a map. Work it in the same order every time and the layout stops mattering.
Before a rating means anything
A rated capacity is only true under the conditions the manufacturer assumed. Before you trust a number, confirm:
- The chart is the one for that machine. Charts are written per model and serial number, and the crane's own chart must be in the cab.
- The crane is level — within one percent unless the manufacturer says otherwise.
- The crane is set up the way that table assumes — outriggers or tires, how far the beams are out, the counterweight fitted, the boom mode.
- You are reading the right working area — over the front, over the rear, or 360 degrees.
Chart ratings are static figures for ideal conditions: a freely suspended load, no wind, no side loading and no dynamic effect from snatching or swinging. Anything you add to that comes off your margin, not out of the chart.
The order to work in
- Write down how the crane is set up. Whether it is on outriggers or on tires, how far the outrigger beams are extended, what counterweight is fitted, and which boom mode it is in. A chart carries a separate table for each of these, so this is what decides which table you are allowed to read at all. Get it wrong here and every number after it is wrong.
- Find the radius — the horizontal distance from the axis of rotation to the centre of the hook, measured with the load suspended, not with the boom empty. Some charts are organised by boom angle instead.
- Find the boom length.
- Read the gross capacity where your boom length meets your radius, or your boom angle, in the column for your working area.
- Subtract the deductions to get net capacity. See Deductions: Gross Capacity to Net Capacity.
- Compare it with the load. If what you are picking is not below your net capacity, the lift does not go.
When your number is not printed on the chart
Radii, angles and boom lengths come in steps, and the lift you are planning will often fall between two of them. You do not average the two — you take the more conservative line:
- Between two radii: use the next longer radius.
- Between two boom angles: use the next lower boom angle.
- Between two boom lengths: compare both boom lengths and take the smaller rated capacity. This one is not a single direction — read both columns before you choose, and it is a common place to lose a mark.
Two things that can override the rating you picked
Having found a gross capacity in the table, there are two cases where that number is not the one you carry forward.
- The rope. Multiply your parts of line by the permissible line pull and compare it against the gross capacity you just read. If the rope figure is less, it becomes your gross capacity. Do this on every lift — on one part or on ten.
- A prohibited configuration. If the chart does not permit the set-up you are in, the gross capacity is zero. There is no rating to work from and no lift to plan.
Count your parts of line carefully. Four parts on a block is obvious. What catches people out is the configurations that do not announce themselves: a ball is one part of line, and lifting from the jib is one part of line, and neither the chart nor the set-up in front of you necessarily says so. One part is where the rope figure is at its lowest, so it is also where it is most likely to be what limits the lift.
Permissible line pull is not available line pull
A chart can carry both, and they are not interchangeable:
- Permissible line pull is the safe maximum for one part of line — the wire rope's breaking strength divided by the design factor. It is a rope figure. This is the one used for capacity.
- Available line pull is the actual pulling force the winch or hoist motor can deliver, and it falls as rope layers build on the drum. It is a machine figure: it tells you whether the winch can pull the load, not what the crane may lift. It is never used in a capacity calculation.
Shaded, bold or asterisked figures
Charts mark some ratings to show what is limiting them. A marked value is usually limited by the structural strength of the crane; an unmarked one is limited by stability, which is to say by tipping. The practical difference matters: a crane at a structural limit can fail without ever giving you the warning that a crane approaching a tip does. Always read the chart's own legend — the marking is not standardised between manufacturers.
The tables are not the whole chart
The pages around the capacity tables carry rules the tables do not: operating notes, the range diagram, the working area diagram, the deduction tables for load handling equipment, and the erection charts for a boom or jib. A rating read without its notes is a rating read wrong.