A stacker crane extracts a load by engaging it – usually under a lip, a rim or the base – and pulling it horizontally out of the slot. The load is applied at one point or along one edge, it is horizontal, and it is applied quickly. A container rated for stacking has been rated for a completely different load case, and the gap between the two is where most container damage in high-bay systems originates.
What the crane actually grabs
Most extraction mechanisms engage a feature: the underside of a rim, a lip at the base, or the base itself where it overhangs a rail. Whatever the feature is, it has to take the whole weight of the load plus the friction against the shelf, applied over a small area, in a direction the box was not designed for. Where the feature is a moulded lip this is a straight strength question. Where it is the base panel, it is a question of local stiffness and of whether the base is tied into the walls.
Friction against the shelf
The force needed to start a load moving is the friction between the box base and the shelf, which is the load weight multiplied by a friction coefficient. A box with a high-friction base on a steel shelf takes more force to start than a smooth-based one, and that force goes straight into the engagement feature. Where extraction forces are high, either the base finish has to change or the engagement feature has to be strengthened, and the first is usually easier.
The wall takes it next
Once the load starts moving, the force transfers from the engagement point into the box structure, and it travels through the base and up the walls. A wall that is stiff in compression vertically is not necessarily stiff in this direction, and the classic failure is a wall that tears away from the base at one corner. Where a fleet shows corner damage after a period in an automated system, extraction force is the first suspect and the base-to-wall joint is the first thing to look at.
Speed matters more than weight
A slow extraction applies roughly the static force. A fast one applies the static force plus an inertial component, and on a high-performance crane that component can exceed the static force. Where a system is running at its rated speed, the peak force is several times what a slow pull would produce. The container specification needs the peak, not the average, and it needs the acceleration profile rather than the cycle time.
Repeated loading is a fatigue case
A container in a high-bay system is extracted hundreds of times a year, each time with a force spike at the same feature. Materials that survive the first hundred cycles comfortably can fail at the thousandth, and the failure looks sudden. Where a fleet is being specified for this duty, the engagement feature should be designed for the cycle count, not for a single proof load, and it should be inspectable so the degradation can be seen before it becomes a failure.
What to do about an existing fleet
Three options: reduce the extraction force by changing the base finish or the shelf surface, slow the extraction to reduce the dynamic component, or add a reinforcement at the engagement point. The first is cheapest and usually effective. The second costs throughput. The third is a modification programme and is the most expensive of the three. Most sites end up doing the first, and most wish they had specified for it in the first place.
What to send the container supplier
Extraction method, engagement geometry, peak force, cycle count per year, and the shelf material. Five facts, all of which the crane supplier already has. Given them, a container can be built whose engagement feature will last. Without them, the container will be built to a stacking rating and then meet a load it was never specified for. The related dimensional question is in ASRS slot dimensions and box tolerance.