Pallets are exposed to an exceptional variety of external forces. Researchers at Virginia Tech’s Center for Unit Load Design and elsewhere have studied to some extent how these forces are distributed over a pallet and within the individual components. However, there is a lot we still don’t know about specific localized stress levels.
Greater knowledge about stresses on each part of a pallet would provide the ability to enhance pallet design. For example, researchers might be able to find the best place to locate radio frequency identification (RFID) tags to prevent damage during transit.
Virginia Tech is developing a technique to map the static stress distributions and impact forces on pallets. Pallet companies could be interested in this developing research because it takes design enhancement beyond anything else available on the market today, including the research used for the Pallet Design System (PDS) analysis or any other current methods for studying pallets and unit loads.
It can be difficult to develop good data because pallets are subjected to a multitude of various stresses and impacts that leave no visible record to follow. The locations of applied forces on a pallet or container are only discovered after damage or failure has occurred.
Understanding the location, frequency of occurrence, and stress levels associated with shocks and impacts during pallet loading and unit load movement would provide valuable insight and could lead to improvements in both pallet and packaging design efficiency. Analyzing the distribution of static stresses imposed by loads on the pallet during warehouse rack or stack storage could lead to improved unit load designs that reduce product damage and packaging costs, and improve workplace safety.
For every action there is an equal an opposite reaction; therefore, under static conditions, pallets, as load bearing structures, transfer stresses to the packaged products that rest on top of them. Package designers must know where and to what degree stresses are being imposed on the pallet in order to efficiently design a package that protects the product when it is shipped on a pallet.
Static stress levels and how they are distributed on pallet surfaces under load are directly related to pallet deck stiffness. Design considerations that effect pallet deck stiffness and load distribution include: wood species, relative moisture content, span between stringers, deck board thickness, and deck board width.
Perhaps the most significant result of understanding the distribution of stresses applied to pallets would be the opportunity to improve the design of unit load material handling equipment in such a way to reduce stress levels and frequency of stresses applied to pallets during use.
One possible technology for mapping the distribution of static and dynamic forces on pallets during use is pressure sensitive film. These films indicate points or areas where contact by other objects has occurred by revealing a range of color intensity relative to the pressure incurred. Dark regions indicate the effective bearing area. The total range represented by these commercially available films appears to be appropriate for testing both static and dynamic forces imposed on pallets during use.
Research Methodology/Process
The Center for Unit Load Design has begun preliminary testing of pressure sensitive films to map stress levels on pallets. This research covers both static load stress distribution across pallet deckboards and localized impacts by fork lifts and pallet trucks. Two experiments were performed using the pressure sensitive film. One was to map the static stress distribution between the pallet deck and the packaging on the pallet. The other was to map the location and frequency of impacts by fork tines during unit load handling.
Static stress distributions were tested using Pressurex ® film, pallet parts, and corrugated that was compressed using a rigid steel load applicator.
The occurrence (frequency and location) of dynamic (impact) forces on pallets during handling was evaluated using one 48×40 GMA style pallet and pressure sensitive film. This pallet was then tested in the Center’s Fastrack lab, which simulates unit load warehouse handling and shipping environment. Fastrack handling cycle includes idle pallet storage, palletizing, shipping, transport, receiving, and three types of storage: static rack, flow rack, and block stacking.
For complete details on the methodology used in the above tests, read the complete article on the process at www.palletenterprise.com.
Results of Static Stress Distributions on Wood Pallet Decks
Packaging must be designed to resist the compressive forces of the pallet top and bottom deck boards during warehouse block stack storage. These compressive forces will depend on the stiffness of the pallet deck boards, the weight on each pallet and the number of unit loads stacked on top of each other. The compressive stresses (i.e., force per unit area) will also depend on the area of contact (bearing area) between the pallet deck and the packaged product while the load is being supported.
The Center’s working hypothesis is that the bearing area is less than the surface area of the deck boards because pallet deck boards deform or deflect under load. Depending on the stiffness of pallet deck boards, the ‘effective’ bearing area is less than the actual surface area of the pallet deck board by an unknown amount. The smaller the effective bearing area, the greater the compressive forces on the packaging and the stronger the package will have to be to withstand these compressive forces.
During the test, the pallet sections deflected. The pressure sensitive film shows various levels of pressure dark regions on the pressure sensitive film indicate the effective bearing area. Upon examination after testing, the dark area of the film of specimen 1B extends over the entire surface area of the deck board resulting in approximately 100% bearing area as the load reached 20 psi. Subsequently, the dark area of the film for the sample in 1A extends approximately 3.5-inches from each edge of the deck board before abruptly disappearing. This is equivalent to 44% bearing area for the whole deck board.
The pallet section in Figure 1B is 5.6 times stiffer than the sample section in 1A. Therefore, a 5.6 fold increase in pallet stiffness increases effective bearing area by 227%. If the performance of the packaging is compression limited, which is often the case when warehouse block stacking is used, then the required compression strength of the containers can be significantly reduced by using a deck board stiff enough to provide maximum bearing area. For example, on the stiffer deck pallet section in Figure 1B, container compression strength could be reduced by 56%.
A relatively small investment in pallet costs to ensure efficient pallet deck stiffness can result in a significant reduction in unit load costs. It is clear from this preliminary study that the use of pressure sensitive films has potential for determining the effective bearing areas and the compressive stresses between pallet decking and packaging placed on pallets.
Results of Pallet Handling in Fastrack
The results of the rough handling test conducted in the Center’s Fastrack can be seen in Figures 2A & 2B. After 15 cycles, contact patterns become clear. An important observation is the low frequency of fork tine contacts on the two side surfaces of the inside stringer. The left view (Figure 2A) shows no marks, while the right view (Figure 2B) indicates some contact was made. Another important observation as the result of this test scenario is the lack of impact indications between notches on the outside surface of the two outer stringers.
Significant impact frequency can be observed at the ends of the outer stringer surfaces. This is the result of fork tine contact from sluing the pallet and other inadvertent contacts made as the forklift operator attempts to handle the pallet. However, it appears the middle section of the outer stringers is outside this margin of forklift operator error. The few contacts that do occur to this area are the result of the forklift operator bumping these edges of the pallet into other things as he handles the pallet; for example, the pallet side scraped against the unit load sitting next to it as the pallet is lifted, or it might rub against a wall as the forklift rounds a corner. Contacts can also occur during trailer loading and unloading. However, the potential for these contacts is much smaller compared to fork tines impacting the ends of stringers.
From this study, it appears that one of the best areas on a pallet to locate bar codes, labels, and RFID tags is the area on the outside surface of outer pallet stringers between the notches. These preliminary results show this area to be the least susceptible to being damaged during handling. Tags should not protrude from the stringer surface since they would then become susceptible to other non-fork contact damage.
These preliminary results show that mapping technology can be useful in improving overall pallet and unit load design. More research is needed and is in the process of being done.
For more information or to request copies of Virginia Tech reports, contact Peter Hamner at the Virginia Tech – Center for Unit Load Design (phone: 540/231-3043; email: phamner@vt.edu).
