A published load rating almost always describes the box at the bottom of a stack, because that is the box carrying the most. Turning that into a permitted number of layers is simple arithmetic with three inputs, provided each of them is defined rather than assumed.
The inputs
The load rating of the box, in the units the supplier quotes. The weight of a loaded box – part plus dunnage plus the box itself, not just the part. And the duration and temperature the rating was measured at. Three numbers, and the second is the one that is consistently underestimated.
The arithmetic
Subtract one loaded box from the rating, because the bottom box does not carry itself. Divide what remains by the weight of one loaded box. Round down, always. That is the number of layers above the bottom one, so the total stack is one more. Where the result is not a whole number, the fraction is not a margin – it is the difference between a stack that stands and one that leans.
The dynamic factor
For a stack that will travel, divide the rating by the dynamic factor for the route before doing the arithmetic above. The factor is large on rough or multi-leg journeys and small on smooth, direct ones. A supplier who ships regularly into a region will have a view; it is worth asking, because guessing it wrong in either direction costs money.
Where the estimate goes wrong
Three recurring errors. Using the part weight instead of the loaded box weight. Applying a static rating to a transit stack. And assuming the rating includes a safety margin, when in many cases it is a measured failure point with the margin already removed. Ask whether the figure is a working load or a failure load.
Checking it against reality
Stack to the calculated height, leave it for the full storage period, then measure the height before and after and look at the bottom box for permanent set. If the height has dropped and the box has not recovered, the rating was optimistic or the duration was longer than the test. Either way, one fewer layer.
What to write down
The permitted number of layers, the loaded box weight it was calculated from, and the storage duration and temperature it applies to. On the box, if possible, or on the packing instruction at minimum. A stack height limit that exists only in someone’s head is not a limit.
Where the calculation goes wrong
Usually in the loaded weight. The part weight is recorded and the box and dunnage are not, which understates the load on the bottom box by whatever the packaging weighs. On a light part, the packaging can be a substantial fraction of the total.
The second error is applying a static rating to a stack that travels. Route factor first, then the arithmetic. Where the two errors combine, the calculated layer count can be two layers too many, which is enough to fail in service.
What the number depends on
Loaded box weight, which has to include dunnage and the box itself. Duration, because the rating falls over time. Temperature, because creep accelerates with heat. And the route factor, which depends on how many times the load is handled. Four inputs, and the second is the one most often left out.
What we can do
We build PP corrugated turnover boxes and state the working load, the failure load and the duration separately, so the layer count can be calculated rather than guessed. Send us the part weight and the stack you need and we will do the arithmetic. Which of the two applies is explained in static versus transit load ratings.