Why do pallet loads shift during transportation? Pallet loads shift because braking, acceleration, cornering, vibration and road impacts put forces on the load that aren’t present when it is sitting on the warehouse floor. Poor pallet patterns, gaps between packages, inadequate load containment, pallet flex, and insufficient trailer restraint can all make the problem worse.
I used to see this all the time. Open the trailer door and there, with the load documents, would be a Polaroid photo showing a pristine, uniform-looking load when it was shipped. Unfortunately, the reality at the receiving dock was often very little like the picture, with leaning loads and cases falling off the back of the trailer and onto the tarmac.
The temptation is to blame the stretch wrap or the person who built the load. Sometimes that’s where the problem lies, but often there is more going on.
The Load Has to Survive the Trip, Not Just the Warehouse
A pallet load sitting on the floor is relatively easy to keep stable. Once it is in a trailer, it has to deal with acceleration, braking, cornering, vibration and the occasional pothole or hard stop. Those forces can expose weaknesses that weren’t obvious when the load was stationary.
Some interesting work by the International Safe Transit Association helps illustrate the point. In ISTA field research involving palletized bottled water, researchers tracked real pallets on real trucks and found loads that swayed, leaned and in some cases lost containment entirely, with bottles ending up on the pavement during ordinary turns. The more surprising finding was what happened back in the lab. The standard single-axis vertical vibration test widely used to evaluate packaging didn’t reproduce the failure at all. Only when researchers replicated the actual multi-directional motion recorded from the road, capturing the side-to-side sway and rotation that a purely vertical test misses, did the same instability show up on the test table. That’s a useful caution against assuming a load is transport-ready just because it passed a standard test.
That movement matters because a palletized load isn’t really one object. It is a system consisting of the pallet, packages, product and whatever is being used to hold everything together. If cases can move slightly against one another, or the entire load can move relative to the pallet, a small amount of movement early in the trip can become a much larger problem several hundred miles later.
Stretch Wrap Is Only Part of the System
More stretch wrap may improve some loads, but adding film isn’t a cure-all. Film selection, pre-stretch, containment force and the wrap pattern all matter, as does how effectively the load is secured to the pallet. Containment force can also be measured rather than judged simply by how tight the film looks, giving an operation a more repeatable way to compare wrap patterns or identify a wrapping process that is drifting out of specification.
Just as important is what the film is trying to contain. A pallet pattern with gaps between packages gives the load room to move. A tall load with considerable weight near the top will behave differently from a lower, more compact one. Product overhanging the pallet, weak lower cartons or a damaged or overly flexible pallet can add another layer of instability.
Packaging can also change during the trip. Corrugated exposed to high humidity or refrigerated conditions can lose strength, while vibration can gradually work packages out of position. A load that was square and reasonably stable when it left the plant may therefore become less stable as the trip progresses.
This is why a recurring shifting problem is better approached as a unit-load problem rather than simply a stretch-wrapping problem. ISTA’s broader transit-testing resources and technical papers are useful in this regard because they examine the packaged product within the distribution environment rather than treating a single component in isolation.
Don’t Forget What Happens Inside the Trailer
Trailer loading can make a good unit load better or worse. There is also an important distinction between product shifting on the pallet and the entire palletized load moving within the trailer. In practice, one can lead to the other.
Leave enough open space around a pallet, and the complete unit load may move when the truck brakes, accelerates or turns. Once it has room to gain momentum, it can strike an adjacent load or the trailer structure. A load that was perfectly stable when it left the shipping dock may not remain that way after repeated impacts.
That’s where load restraint and void management come into the picture. Depending on the freight and trailer configuration, cargo bars, load locks, straps or other appropriate restraint systems can help limit movement. Voids between loads can sometimes be managed with dunnage, inflatable dunnage bags or other suitable void-filling materials. The objective is to prevent the load from having enough room to move and build momentum in the first place.
Where and how pallet loads are positioned in the trailer can also affect the forces they experience and how readily they can move. The appropriate solution depends on the cargo, vehicle, loading pattern, and transportation environment. A cargo bar isn’t the answer to every shifting problem, any more than another few turns of stretch film are. But trailer restraint shouldn’t be overlooked when a load that leaves the plant in good condition repeatedly arrives out of position.
The Federal Motor Carrier Safety Administration’s cargo securement rules provide the broader U.S. requirement. Cargo must be secured so that it doesn’t shift on or within the vehicle to an extent that adversely affects the vehicle’s stability or maneuverability.
There can also be a cumulative effect. A single corner or rough patch of road may not be enough to noticeably shift a load, but repeated movement over several hundred miles can gradually work packages or entire pallet loads out of position.
Find Out Where the Movement Begins
If the problem is recurring, one of the simplest troubleshooting tools is still a camera. In my receiving days it was a Polaroid. Today, of course, everyone already has a camera in their pocket.
Photograph the load after palletizing, after stretch wrapping, and after it has been positioned and restrained in the trailer. Compare those images with what arrives at the customer’s dock.
That sequence can tell you quite a bit. If cases are already moving before shipment, start with the pallet, packaging, stacking pattern and load-containment system. If the unit load leaves square but consistently arrives shifted, take a closer look at trailer loading, open spaces between pallet loads, cargo restraint and what is happening during transportation.
The pattern of damage can also provide clues. Cartons working loose within an otherwise stationary pallet load point in a different direction from crushed corners where adjacent pallets have collided or a complete pallet that has migrated from its original position in the trailer. If the most obvious symptom is a load that has developed a pronounced lean, see Why Is My Pallet Load Leaning?
Carton position on the pallet can provide another clue. Unsupported edges and excessive overhang can affect package performance, while excessive setback can create other unit-load issues. We look at that question separately in Should Cartons Overhang a Pallet or Be Set Back From the Edge?
The answer isn’t necessarily a heavier pallet, stronger corrugated, another few revolutions of stretch film or another cargo bar. Those changes all cost money and may accomplish very little if they aren’t addressing the real weakness.
Find out what is moving, where the movement begins, and what is allowing it to happen. Then fix that part of the unit load or transportation process.
Find answers to more questions about pallet load stability, overhang, pallet capacity and transportation at our Pallet Problems hub.
Rick LeBlanc, EMBA, is the founder and editor of Reusable Packaging News (subscribe to the free newsletter) and editor of Western Pallet Magazine. A supply chain journalist with more than 30 years of experience, he specializes in pallets, reusable packaging, material handling and related operations. Rick is co-author of Pallets: A North American Perspective and Pallets & Progress: A Collected History of Pallets and Palletized Handling 1922–1945. He is also an advisory board member of the Virginia Tech Center for Packaging and Unit Load Design. Read Rick’s full bio.