A vehicle is rated to carry a mass. A container is rated to hold a mass. The two numbers look comparable and usually are not: the vehicle rating assumes the load is within a certain envelope and distributed in a certain way, and the container rating assumes a support condition that a lift deck may not provide. Where the two are simply compared as numbers, the system is specified on an assumption nobody has checked, and the failure shows up as deformed boxes or dropped loads rather than as an obvious overload.
What the vehicle rating really covers
A payload rating normally assumes the load sits within the deck footprint, with its centre of gravity near the middle, and that it is rigid enough to be lifted without bending. A container that overhangs the deck, or whose contents are concentrated at one end, or that flexes when lifted, is outside the assumptions even when the mass is inside the rating. Most vehicle suppliers will state the envelope if asked; most are not asked, and the mismatch is discovered when a load is dropped at the far end of a route.
What the container rating assumes
A box load limit is usually quoted for a box sitting on a flat, continuous surface – a floor or a pallet – with the load evenly distributed. On a lift deck supported at points or rails, the base is spanning rather than resting, and the supported load capacity is lower. How much lower depends on the base stiffness and the deck geometry, and it is not a number that appears on either datasheet. The conservative approach is to rate the box against the deck it will actually sit on, and that means testing rather than calculating.
Dynamic load is not static load
A vehicle accelerates, brakes, turns and lifts. Each of those adds to the effective load: braking at a modest deceleration can add twenty per cent to the force the container has to resist, and a turn adds a lateral component the box may not be designed for at all. Where a route includes tight turns or a fast cycle, the dynamic case governs, and it governs the container more than the vehicle, because the vehicle was designed for it and the container was not.
The stack case
Where a vehicle carries a stack, the bottom box carries everything above it plus the dynamic component. That is a straightforward stacking load and most containers are rated for it, but the rating is almost always for a static stack on a flat floor. Adding vehicle dynamics to the bottom box of a stack is the load case nobody quotes, and it is the one that produces crushed bottom boxes on automated routes that look well within rating. See the stack geometry discussion in height tolerance in AMR handling.
Margin, and how much is enough
Where both ratings are known and both assumptions checked, a margin of twenty per cent on the container covers normal dynamic effects and normal variation in contents. Below that, boxes fail early and unpredictably. Above about fifty per cent, the container is being over-specified and the extra cost buys nothing. Where the contents vary – and on most real lines they do – the margin has to be measured against the heaviest realistic load, not the average one.
How to check it in an afternoon
Load a box to the working weight, put it on the actual deck or a representative one, and run the actual route for a shift. Then measure the box: base flatness, wall squareness, and whether the stacking features still seat. Any permanent change means the combination is outside what the container will take, and the answer is either a heavier box construction, a lower stack, or a gentler motion profile. The first is usually cheapest.
What to specify
Working load, maximum load, the deck support geometry, and the route’s most demanding motion. Given those, a container can be specified for the real case. Given only a mass, the container is specified for a floor it will never sit on. Send us the deck drawing and the route profile and we will rate our containers against it rather than against a datasheet assumption.