Although pallet design is not rocket science, there is more to it than most pallet users generally imagine. There are ways you can make small changes that can have a big impact on pallet performance.
First, there’s a difference between strength and the safe working load capacity of a pallet. Quite frankly, if someone had historically been safely placing 2,000 lbs. on a pallet, and you collected 100 of those pallets to be tested to failure in a lab, the average load causing failure would probably be over 4,000 pounds. The safe working load is less than the strength. The difference accounts for variation in manufacture, variation in materials, and variation in how it’s used. Most pallet design software predicts the safe working load. Laboratory tests measure the strength or average load causing failure. Remember, the strength is always greater than the safe working load.
Stiffness, a critical component of the overall pallet design, is the measure of how much a pallet or pallet parts bend under load. When a pallet is racked, over time it will sag under the strain of the load. The same thing occurs to deck boards during stack storage. While the pallet may not actually break, the deflection of the pallet itself creates several problems affecting load carrying capacity, potential product damage and the smooth operation in materials handling systems.
Stiffer pallets are better. Stiffer pallets are more stable in shipping. Stiffer pallets amplify less the input accelerations from shocks and vibrations. They better protect packaged product. A good rule of thumb when designing pallets is to first be sure the pallet is strong enough to support the load, then look at ways to improve stiffness without adding cost. Durability is critical. Certainly, if the pallet is going to be reused, you want it to last quite a while before repair. Even if the pallet is used only one time, it needs to pass through the supply chain undamaged, because a damaged pallet will not protect the packaged product.
Pallet Design 101
The following is a basic tutorial in pallet design. First, you must remember that the function of a pallet is to facilitate product distribution and storage and to protect packaged products. Second, the limit states of pallet design and performance are strength and functionality. The two most important functionality characteristics of a pallet are stiffness and durability.
Third, the pallet is one of three parts of the unit load supply chain – the packaged product; the unit load handling, shipping and storage equipment; and the pallet. The pallet is the only part touching both other parts and is therefore literally and figuratively the interface. It is the critical component of the unit load supply chain.
Think of the pallet as a beam and the deck boards and stringers as beams. Beams hate spans. To increase the capacity of a pallet without adding cost, find ways to reduce spans in racks and conveyors and other equipment, such as retrofitting your load beam racks with supports. In chain or roller conveyors, one can add a strand or reduce roller spacing. Tighten drive in rack spans by one inch, still allowing room for the fork lift mast. Every inch counts.
To increase the strength of a pallet spanning its length you could increase the width of stringers or thickness of stringer boards in block pallets. However, it is best to focus on the center stringer or stringer board because it supports most of the weight in storage racks. Using wider center stringers is an efficient way to improve strength spanning the pallet length.
In a notch stringer, you can reduce notch depth. That’s a free change, typically. The notch depth can be reduced from 1-1/2" to 1-3/8" or 1.25" depending on the thickness of the bottom deck boards. Remember we have to be able to get a 1.5 inch thick fork tine in the notch.
Changing notch depth changes pallet strength. You can even change the fill radius from 1/2" to 1-1/2". That’s a free change that increases the strength of the pallet by 6-7%.
Consider switching to a two-way pallet. This gets rid of the notch. The question you have to ask yourself and your customer is how often are the notches really used by the owner of the pallet and their customer. A lot of times you find they’re never used.
If you get rid of them, number one, you save money, your pallet costs you less, but it becomes much more durable and much stronger. That standard 1-1/2" deep by 9" notch, six of those in a three-stringer pallet, reduces the racking strength across the length in half. You basically pay someone to put notches in a pallet and you reduce the strength, and the durability has been reduced.
Not all wood species are created equal in terms of performance. And not all hardwoods perform better than softwoods. A pallet designer can certainly change the species of wood in a stringer. You can switch to oak or a stronger wood in the stringer, or add a fourth stringer. The interior stringers should be grouped fairly closely together near the center to optimize the efficiency of adding a fourth stringer when spanning pallet length in storage racks with no interior supports.
Drive-in or drive-through racks have to have room for the mast of a forklift to pass between the supports. The pallet is typically spanning its width. The span doesn’t have to be 38" for a 40" wide pallet. Maybe it can be 36". Maybe it could be 35" or 34" and the mast can still move in and out. For every inch, you increase the capacity of the pallet.
Drive-in racks are probably one of the highest stress conditions to which pallets are exposed because you cannot retrofit these racks with interior supports. You could install push-back racks or flow racks. That would solve the problem and reduce the span. But until then, it’s all about the span and the drive-in rack.
The alternative is to increase the thickness of the bottom deck. In non-reversable pallets it is more efficient to focus on the bottom deck. Or a stringer can be added, but this is not efficient unless the stringer is needed for other reasons. Remember when adding a fourth stringer to space them no more than 6 to 8 inches apart if pallet jacks or trucks are being used.
The Advantages of Wing Pallets
Another design strategy is to add a wing on the pallet. Believe it or not, that increases the strength of a pallet spanning its length in a load beam storage rack. The reason is you’re shifting load away from the center stringer to the outer stringers to share it better. Once those stringers share the load better, the strength increases.
If you stack store product by placing one unit load one on top of another, consider adding a wing. Why does it work? Notice that you’ve reduced the span between stringers. This is a free change. This doesn’t cost anything. If there’s a disadvantage to wings, when you’re shipping sacked product, they can catch on things and perhaps damage sacks. Another option is to overhang the sacks so the wings will not tear them. For most other packaging, they’re fine. Do they break off sometimes? Yes, that’s another disadvantage of a wing. But there are significant advantages to performance. Racked across length is improved. Certainly, stacking performance is improved.
In the case of a block-style pallet, there are similar design concepts along the length called a cantilever or wings across the width. You can actually reduce spans by cantilevering the stringer board over the block. The stringer board now extends beyond the block. Just like in a winged pallet, the deck board would extend beyond a stringer board.
Other Stringer and Deck Board Changes
Another option is to change from three stringers to four narrower stringers. That will allow you to extract some wood from the top and bottom deck boards. This produces a pallet of equivalent or better performance without adding cost.
When you stack and store block-style pallets, the real member under the most stress is the stringer board. So instead of focusing on the deck boards in block-style pallets, you should focus on the thickness of the stringer board. Also, you can replace material with denser woods or higher quality lumber.
Fasteners & Nailing Patterns Matter
Resistance to damage, impacts durability and reusability of a pallet. Fastener selection greatly affects the resistance of the pallet to damage. You want to specify and use better, high-quality fasteners with quality threads. And if possible, you want to use more fasteners to provide stronger connections between boards. Especially in the end boards, most of the damage occurs to the top deck end boards. Instead of using three in a nominal six, what if you used four? Instead of using two in a nominal 4" board on the end of a pallet, what if you used three?
It is possible to put too many nails in a board, which could split stringers. But adding four or three in some situations significantly boosts performance and strength without affecting cost.
Final Thoughts
Put particular focus on the end boards because they receive most of the abuse by forklifts. Consider using denser woods in the end boards. You can put dense hardwoods (such as oak) in the end boards and use lower density woods in the interior like poplar or some of the softwoods.
After you’ve gone through the process of developing an optimum design, draft a detailed, well written specification. Always conduct field trials before finalizing the design changes. Always specify that the quality of parts and assembly meet or exceed that specified in the “Uniform Standard for Wood Pallets” (www.palletcentral.com) or ANSI/MHI MH1, Pallets Slipsheets and Other Bases for Unit Loads (www.mhi.org). These documents are available for free. The ANSI standard is comprehensive and includes standards for paper, plastic, metal and pressed wood pallet designs.
Finally, every pallet design should be periodically, reevaluated, as SKUs, customers, and supply chains change. Look for ways to optimize designs. Periodic supply chain audits are recommended.
Editor’s Note: Marshall (Mark) White is the former director of the Center for Packaging and Unit Load Design at Virginia Tech. He is a renowned expert on unit load and packaging design. He provides consulting for both pallet companies and pallet users. You can find out more about his company and modeling design software at http://www.whiteandcompany.net/index.html or call (540) 552-1158 or email customerservice@whiteandcompany.net
