A side guide exists to keep a box tracking straight. It does this by leaving a gap small enough that the box cannot rotate far, and large enough that it does not bind. Those two requirements pull in opposite directions, and the gap that satisfies both depends entirely on how consistent the boxes are. Where the container tolerance is loose, no guide setting works well, and the line runs either with jams or with boxes arriving at stations visibly out of position.
The three things the gap has to absorb
Manufacturing variation, which is the spread between the widest and narrowest box in the fleet. Positional error, which is the skew the box arrives with, and which converts a length into an effective width. And deflection, which is how far the wall moves when boxes press against each other in a queue. Most guide settings are calculated from the first of these alone, and then the line is surprised by the other two. A box that is 3 mm over nominal and arrives at 2 degrees of skew presents an effective width noticeably greater than either figure suggests.
Why tight guides make things worse
The instinct when boxes arrive out of position is to close the guides up. This usually works for a day. What it does is convert a guidance problem into a friction problem: the box now contacts the rail continuously, the drive has to overcome that friction, and the contact accelerates wear on both the box and the rail. Worse, the boxes most likely to bind are the widest ones, so the fleet self-selects – the narrow boxes run fine and the wide ones stop, which is the opposite of what the line needs.
Where the tolerance actually comes from
On a corrugated PP container the width is set by the blank and the fold, and it drifts with board thickness, with humidity at the time of folding, and with how long the box has been stacked. Moulded containers drift less in service but have more tool-dependent variation between production runs. Neither is automatically better for automation. What matters is whether the supplier quotes a tolerance they will hold, and whether the fleet as delivered actually sits inside it. Measuring twenty boxes from the delivery is a ten-minute check that prevents a month of guide adjustment.
Height matters as much as width
A tall box against a low guide is a lever: the force from a queue is applied near the base, and the top of the box is free to lean. The effective width at the top of the box can therefore exceed the guide gap even when the base is comfortably inside it, which is why some lines show scuffing on the upper walls and nowhere else. Where boxes are tall relative to their width, either the guide height has to increase or the queue pressure has to be limited, and the second is usually easier.
What to do when the spread is too wide
Where an existing fleet has a wider spread than the line can absorb, the options are to sort the fleet, to relax the guide, or to accept a lower rate. Sorting is unglamorous and effective: measure, keep the boxes inside a band, and retire the outliers from automated lanes while leaving them on manual ones. Relaxing the guide works only up to the point where boxes start arriving at stations too far off centre to process, and that point is reached sooner than expected on lines with grippers or scanners.
Writing it into the specification
Give the container supplier the station tolerance – how far off centre the box can arrive and still be processed – rather than the guide gap. The guide gap is a consequence of the line; the station tolerance is a property of what the line has to do, and it is the number that determines whether the container is good enough. Where a supplier is given a station tolerance they can usually meet it, and where they are given a guide gap they will quote a box that fits the gap and nothing else. The base and wall behaviour behind this is covered in what a conveyor belt needs from a plastic box.
The check before sign-off
Run the widest and narrowest boxes in the delivery through the line, at speed, back to back. Both should track. If only one does, the guide is set for a nominal box that not enough of the fleet matches, and the line will run intermittently for as long as it is in service. Tell us the station tolerance your equipment needs and we will quote containers held to it.