A roller conveyor supports a box on a series of lines rather than a surface. That difference is small when the box is large and stiff, and it is everything when the box is small, light, or has a base that was designed for a pallet. The number that governs it is the pitch – the distance between roller centres – and it is fixed by the equipment long before anyone chooses a container.
Three rollers is the working minimum
The rule of thumb on a gravity or powered roller line is that the load should always rest on at least three rollers. Two is unstable, because any shift in the centre of gravity turns the box into a see-saw between them. Three means the load is always bridged, and small dimensional variation stops mattering. Translating that into a specification is straightforward: the shortest box that will run on the line should be comfortably longer than twice the pitch. Where a container family includes a small size, that is the one that will cause trouble, not the large one.
Where the base flexes between rollers
Between two rollers the base is unsupported, and it will sag by an amount that depends on the load, the span and the stiffness of the board or moulding. A small sag is harmless. A large one changes the effective length of the box as it travels, because the leading edge drops into the gap and the trailing edge lifts. On a long line this produces a characteristic shuffle, where boxes advance in small steps rather than smoothly, and it is one of the more common causes of a line that never quite reaches its rated throughput.
Rib direction relative to the rollers
Underside ribs running across the direction of travel – perpendicular to the roller axes – are generally the better arrangement, because each rib lands on a roller rather than falling between two. Ribs running along the direction of travel can drop into the gaps and give the box a tendency to sit on one rib only, which is the same instability as resting on two rollers. Where the base has a moulded perimeter, the same logic applies: a continuous perimeter running across the rollers bridges reliably, and one interrupted by drain holes or lightening cut-outs does not.
Light boxes are harder than heavy ones
Counter-intuitively, the lightest box is often the one that will not run. Powered roller lines depend on the load pressing the box against the rollers to generate traction. A very light empty container can skid, sit still while the rollers turn underneath it, or be pushed sideways by the smallest irregularity. Where a line has to move empties as well as fulls, the empty case is the design case, and it is worth testing the line with empties before signing off the container. The point is covered from the other side in why boxes jam on a conveyor line, where the empty container is again the usual culprit.
Curves, merges and the short box
On a straight section a short box is fine. On a curve it has to span rollers that are set at an angle to each other, and the effective support pattern gets worse the shorter the box is. Merges are worse again, because two streams have to be interleaved and the gap control depends on predictable acceleration. Where a line includes a curve or a merge, the minimum box length normally has to increase, and it is worth asking the equipment supplier for that figure rather than assuming the straight-section figure applies throughout.
What to change when the box will not run
Three options, in increasing order of cost. Increase the base stiffness, usually by moving to a heavier board or adding a perimeter feature. Lengthen the box so it always spans three rollers, which may be as simple as choosing the next size up in the family. Or reduce the roller pitch on the affected section, which is an equipment change and the most expensive of the three. Most lines end up doing the first, because it is a specification change rather than a capital one, and because it usually improves the box everywhere else as well. The same question from the equipment side is in whether PP crates run on roller conveyors.
Specifying the base for a roller line
Write down the roller pitch, the minimum box length, and the lightest and heaviest loads the line will see, and send all three with the enquiry. A container specified against those numbers will run. One specified against a pallet drawing and then put on rollers is a gamble, and the failure mode – intermittent, load-dependent, worst on the lightest boxes – is exactly the kind that is hardest to diagnose once the line is live.