When choosing a pallet, it is imperative that the specified performance requirements of the pallet matches the actual accessed pallet capability to ensure the safety and functionality of the unit load. When it comes to pallets made from alternative materials, manufacturers often don’t know how to define performance since these manufacturers do not produce as many customized pallets as wooden pallet manufacturers. In contrast, PDS (Pallet Design System) users are fairly well trained in the understanding of load support conditions, load capacity, and even durability of wood pallets.
Pallet performance is typically measured by its strength and stiffness (in dynamic and static conditions) and durability. In dynamic conditions a pallet is subjected to bending, such as during warehouse racking, where the pallet has to carry its load with only side support. Alternatively, static conditions are where the pallet does not bend, such as during warehouse stacking, where load capacity is greatly increased by its weight fully supported by the floor. For example, a pallet that only has a load capacity of 2,000 pounds individually racked may also support 6,000 pounds on the floor while stacked three pallets high.
Strength is a measure of a pallet’s breaking capacity. To avoid the dangerous situation of loading a pallet beyond its breaking point, a designer utilizes a typical industry safety factor of two and a half times the average pallet breaking strength. For example, if a certain type of pallet on average breaks at 5,000 pounds of load, its strength capacity (load that can safely be put on the pallet) using the two and a half times safety factor, would be 2,000 pounds.
Stiffness, a critical component of the overall pallet design, is the measure of how much a pallet bends under load. When a pallet is racked, over time it will sag under the strain of the load. While the pallet may not actually break, the deflection of the pallet itself creates several problems. Since the pallet is no longer flat, it may no longer properly support its load resulting in potential damage to the product. Additionally, substantial deflection could prevent fork truck tines from properly inserting into the pallet. Finally, in sophisticated storage systems, such as automatic storage and retrieval systems (ASRS) typically seen in large automated warehouses, deflection may prevent shuttle carts (that have low tolerance for variations) from getting under a load to pick it up and move it.
Stiffness becomes even more important with alternative materials – particularly plastic pallets. Plastic pallets rarely break and so are often referred to as deflection limited on their load capacity. Stiffness is also a problem with paper-based pallets that can hold heavy loads, such as cement blocks, while sitting on the floor; however, when picked up by a fork lift, the load fails since the deck is not stiff enough to truly support the load.
Many pallets have been tested without obtaining deflection measurements, which should not be ignored. When comparing different pallet manufacturers or pallets of different materials, it is very important that both the strength and stiffness numbers be investigated and compared since even a small deflection can affect the product being transported or stored on a pallet. Under the ASTM test protocol the deflection limit is 1.9% of the greatest span, which is often the rack span. In a rack storage system where the supports are 44 inches apart, the deflection limit would be just over ¾ of an inch.
In the United States, the ASTM D1185 test protocol is the primary test to determine a pallet’s performance capacities, while in Europe the ISO 8611 method is used, with each standard having associated performance guidelines. A general purpose pallet is tested using the worst case load – a uniform, flexible load, which is simulated at the Center for Unit Load Design using an airbag.
Sometimes, when a pallet is only going to be used for a very specific load, the properties of the individual load itself can significantly influence the carrying capacity of that pallet. For example, several years ago during a pallet test at the center using car batteries, it was discovered that the particular way the batteries were stacked, enhanced by the addition of honeycomb pads between each layer, increased the operational pallet performance through load bridging. Load bridging is a phenomenon where the character of the load itself serves to distribute the load stressors to the outside edges of the pallet and then to the stronger rack supports. The airbag test, however, applies a uniform load across the entire pallet that eliminates any load bridging effect. In this case, the resulting difference between the airbag test performance and the actual battery load performance on the pallet was significant with the unit load safely performing nearly twice what was indicated by the airbag test. Unfortunately, in pooled and commodity pallets that aren’t designed to a particular load (and don’t even know what the potential load might be) the pallet must be tested using the worst case scenario.
The last performance measure is durability which measures how long a pallet is going to last. Tests for durability measure normal wear and tear on a pallet, such as how impact resistant a pallet is to fork tine impact on the top deck. Regardless of the material used in the pallet, the durability tests are the same. It should be noted, however, that pallets can purposely vary regarding their durability specifications. For example, a pallet that’s going to be used in a pooled system needs to last much longer than a pallet that’s designed for onetime use.
Understanding performance tests are important in terms of safety. If a pallet is loaded beyond its load capacity, the likelihood of breaking and bending increases. Besides ultimate pallet failure that can possibly lead to injury and death, loads that are too heavy for a pallet can also cause subsequent problems such as damaged product or logistics issues within the distribution environment. It really comes down to matching the performance needs of a pallet with the performance capability of that pallet to avoid future costly problems.
Ralph Rupert is the director of the Center for Unit Load Design at Virginia Tech, the premier pallet laboratory in the United States. For testing or performance consulting, contact the center at 540/231-7106 or rrupert@vt.edu or visit http://unitload.vt.edu/
