We confirmed that we have met our load containment standard; however, we continue to have load failures, especially after we began shipping by rail. How do we adjust our containment standard to prevent these failures?
Hi MaryAnn, thanks for sending in an excellent question. Let’s see if we can come up with a solution for you.
To understand why your load failures increased after switching to rail, we first need to know what is different about the forces applied to a pallet in a railcar.
There are three primary types of forces a load can experience during shipment. The most common are what I refer to as normal forces. These occur during standard highway conditions: starting, stopping, turning, and traveling uphill and downhill. These forces act as though a steady hand is pushing against the load’s side. They are relatively low in intensity (.5g to .7g), but last for extended periods of time. For effective load containment under these conditions, we must remove as much elasticity from the stretch film as possible. This is achieved in two stages: First in the wrapper’s carriage (pre-stretch), and then again as the load is wrapped (applied tension). If too much elasticity remains, the film will continue stretching during transit as normal forces are applied, which reduces load containment. While adding more film can offset this, it increases material costs and slows down the wrapping process. In your case, it sounds like your current standard was established by adding more wraps until load failures were minimized.
Now that we understand how the load containment standard was established and why it has worked in the past, let’s focus on how rail shipments differ. Rail shipments share many of the same normal forces as highway shipments, but the major difference comes from switching and coupling railcars. During switching, railcars are often sent along a sloped track, using gravity to move the car some distance, until it reaches the train and is coupled. The collision that happens during railcar switching, or as the train starts and stops, generates what we refer to as an Impact Force. This force is applied to each pallet in each railcar.
Impact Force is a very high force over a very short duration. During switching, the railcar can reach a velocity of 1.5-2.5 feet per second by the time it reaches the train. This coupling generates powerful impact forces, up to 10g (4-6 times greater than normal forces). Impact force typically occurs over a period of under 30 milliseconds, and as each railcar in the train is coupled, it will generate an impact force. If you have been near a railyard when a train is being assembled, then you would have heard a loud bang as each car is coupled. Even the routine starting and stopping of a train creates an impact force of around 3.5g applied to the railcar’s contents. While sitting in your car waiting at a railroad crossing, when a train stops and then restarts, you have likely heard the banging as the slack is taken out of the coupling between the railcars.
Now let’s get into a little physics. Impact loads have a much higher acceleration because the speed at which the load is traveling changes over an extremely short distance. The shorter the distance over which a load stops, the higher the acceleration and resulting force. For instance, if you are traveling in a car at 30mph and hit a brick wall, you stop immediately (50g or more). However, if you are traveling at 30mph and slam on your brakes, skidding for some distance before coming to a stop, you might only generate 2g. Applying this to rail transportation, the acceleration is high because the change in speed or velocity occurs over a very short distance, maybe 6 inches or less. In contrast, the traction between a vehicle’s tires and the road surface, along with speed, determines how far the vehicle will take to reach a stop, which means the acceleration and force generated are much lower. This is the fundamental difference between over-the-road and rail shipments.
If the load is fully unitized (all boxes, bags, trays, etc., are pulled tightly together by the stretch wrap), the impact force will be distributed evenly throughout the load, and the stretch film has a much better chance of providing containment. This is likely not the case for your loads. If components of the load are allowed to move independently, then the force that each box, bag, or tray applies to the stretch film will be significantly higher due to its higher inertia. The stretch film cannot counter that concentrated force. With each impact, the shift between layers increases until eventual load failure.
Similarly, if the load is not adequately tied to the pallet, the entire load can shift or slide off the pallet.
The two major takeaways to improve load containment when shipping by rail are: 1) Fully unitize the load, and 2) Tie the load to the pallet. There are several other considerations, such as the construction of the pallet, the placement and bracing of the pallet within the railcar, and following the loading guidelines published by the rail companies. Each of these factors plays a role in minimizing damage.
We have studied the science behind load failures and have developed a stretch wrap solution that provides exceptional load containment, especially if you are shipping by rail. In our lab, we can simulate the high-impact forces of railcar switching on a pallet. We incorporate reinforcement filaments into the film and stretch it to its peak level of performance. This maximizes load containment by unitizing the load, prevents web breaks as the load is being wrapped, and we do this while applying significantly less film. We would be happy to review your containment standard and recommend a cost-effective solution tailored to your shipments.
Thanks for asking!
