Pallet Design Modifications that Lower Unit Load Costs
Presented to Transpack 99
by John W. Clarke |
|
March 1999 |
Introduction
Approximately 90% of the U.S. domestic product is shipped on pallets. For most products, the pallet and packaging used to unitize, protect, and transport the product represent a significant percentage of costs. Most companies spend considerable resources in packaging development. Many do not give as much importance to the pallet design. Examples include:
- Pallet specifications that are 20-30 years old
- Pallet specifications that lack manufacturing details
- Specifications designed to minimize pallet price
The Center for Unit Load Design at Virginia Tech researches interactions between packaging, pallets, and material handling equipment. These 3 components determine until load material handling efficiency (and costs). It has been documented that pallets and packaging work as a system to determine overall unit load performance. There are many ways to modify pallet design that will save on overall unit load costs. For the vast majority of unit loads, one point is clear:
The Lowest-priced pallet is usually not the lowest cost pallet.
Reducing pallet price often drives up other factors, such as product damage, additional protective packaging, and labor for increased handling.
The Center designs and tests pallets manufactured of all materials, including solid wood, plywood, corrugated, molded paper, plastics, and metal. Although alternative pallet materials (non-wood) have gained significant market share in the last decade, wood pallets still comprise about 90% of the pallet market. Wood pallets are also the most easily modified pallet material, and the impact of design changes can be quickly determined using the computer model Pallet Design System (PDS).
The intent of this paper is to give packaging designers an introduction in the basics of wood pallet design and performance that can be used to facilitate communication with pallet suppliers to optimize overall unit load design. This paper focuses on solid wood pallets, but many of the design changes discussed in this paper will also apply to plastic, paper, and/or metal pallets.
Wood Pallet Terminology
A schematic drawing of a stringer class and block class pallet, along with the industry terminology for pallet components, is given below. Stringer pallets are more common in the United States. A skid is a stringer pallet with no bottom deckboards. Block pallets cost more to manufacture than the equivalent strength stringer pallets. The principal advantage of block pallets is full access on all 4 sides for both forklifts and pallet jacks (full 4-way). Stringer pallets allow only forklift entry on the sides if stringers are notched (partial 4-way entry). If stringers are not notched, pallet jacks and forklifts can only enter on the 2 ends and they are called "2-way."
The size of wood pallets is specified as the stringer length, followed by the deckboard length. Therefore, GMA pallets (Grocery Manufacturers' of America) are 48x40-inch pallets, not 40x48-inch pallets. The 48x40-inch pallet is the most common pallet size in the U.S., or approximately 1/3 of the pallets produced each year. Note that the GMA pallet, or grocery pallet, is not a specific design. The true grocery pallet specification was written in the 70's and used heavy-duty deckboards and stringers and high quality nails. Today's grocery pallets are a highly variable mix of trimmed-down designs that are frequently damaged after 1 or 2 trips.
The easiest, fastest, safest, and most economical method to design wood pallets is with a computer aided design procedure called the Pallet Design System (PDS). PDS users describe the pallet specifications, load analog, support conditions. PDS then estimates pallet strength, stiffness, durability, and the cost to use pallets. Users can then fine tune the design by changing input variables. The end result is the most economical pallet that will safely support the load under the required conditions. The PDS program was developed by Virginia Tech and is leased by the National Wooden Pallet & Container Association (NWPCA). Ask your pallet supplier if they can provide the PDS design service.
Wood pallets are usually manufactured of the most economically-attractive lumber species that provides adequate performance and that is readily-available in a given marketplace. While good pallets can be manufactured of almost any species, pallet specifications need to be adjusted to emphasize strengths and minimize weaknesses of each species. Nationwide companies will likely need different pallet specifications for different areas of the country. There are several species groupings for pallet parts. These are:
Class A, Class B, and Class C
Group I, Group II, Group III, and Group IV
Virginia Tech's PDS Species Classes 1-29
The Class A-C and Group I-IV are older classifications that group similar species according to density and shock resistance. Prior to development of PDS, these classifications were adequate for the majority of pallet customers. The PDS Species Classes are based on tests of actual pallet parts sampled from the pallet industry. The PDS Classes differentiate species to a greater extent than the older classifications, and allow greater accuracy when evaluating pallet performance using the PDS program. Pallet users may still encounter the other classifications, especially in older pallet specifications.
Pallet Design Modifications that Impact Packaging Performance
Improve Nail Quality - nails are the most neglected component of pallet manufacture. Nails are "out of sight, out of mind," until the nail head snags packaging. Lower quality nails will cost 3-5% of the pallet price. Upgrading to higher quality nails will only raise the pallet price by another 2-3% while more than doubling the life of the pallet. The use of properly inserted, high quality nails will also eliminate exposed nail heads, protruding nail points, and loose deckboards. This creates opportunities to eliminate or reduce pallet pads and other protective packaging.
Use Winged Pallets - Most pallets are manufactured flush, with deckboard ends flush with the outer stringer sides. A pallet wing refers to the length of deckboard overhang past the outer pallet stringers. Pallet stringers act as deckboard supports. The bending strength and stiffness of deckboards is determined in part by the free span between stringers. If a 48x40-inch pallet has 1½" wide stringers, the deckboard span between stringers is 17¾ inches ((40 - 3(1.5)) / 2). If the outer stringers are inset 2 inches (2" wing), the pallet still measures 48x40 inches, but the deckboard span is now only 15¾ inches. This modification should not add any cost to the pallet. Wings are not compatible with some products and handling systems.
Relocate Deckboards -deckboards are typically spaced evenly across the top and bottom pallet decks. If a consistent product size is used, deckboards should be placed to maximize packaging strength. For example, corrugated box corners should be placed over a pallet deckboard to utilize box corner stacking strength. If boards are not needed to support key parts of the packaging, use them to prevent product damage from forklifts. Most forklift tines are 40-46 inches in length. Place deckboards at likely tine tip locations to reduce tine tip damages. Relocating pallet deckboards should not increase the pallet price.
Increase Deckboard Thickness - Thicker deckboards are stronger, stiffer, and more durable than thinner deckboards. They also will reduce stresses imposed on packaged products. Most product and packaging is designed to withstand certain predicted compression stresses. Thinner deckboards will bend when stacked and will flex up and down due to shipping vibrations. The stringers directly under the deckboards, however, do not bend or flex. Therefore, thinner deckboards allow very high stress concentrations to develop directly above the stringers. Thicker deckboards better resist bending and flexing forces, providing a more flat and uniform surface for the product. They also reduce shipping vibrations transferred to the product. Thicker deckboards cost more, but will reduce stresses imposed on the product and packaging. The additional price to increase deck thickness should be compared with potential savings in protective packaging and opportunities for product redesign.
Increase Deckboard Coverage - Adding deckboards is not usually as cost effective as increasing deckboard thickness, but is the best solution for some product applications. More deckboard coverage will better protect products from forklifts, increase strength, stiffness, and durability, and will better support a variety of product sizes. Adding additional deckboards will increase pallet price, and should be compared with potential savings in protective packaging and opportunities for product redesign.
Specify Flatter Decks - No pallet is perfectly flat, although most molded plastic and plywood pallets are close. New wood pallet deckboards are manufactured to some target thickness with a tolerance around that thickness. Industry Standard manufacturing practices state that new pallet deckboards should be ± 1/16". Repaired pallets are often much more variable in thickness. Non-flat pallet decks create stress concentrations at the transition between thickness'. A flatter, uniform deck provides the opportunity to save on protective packaging, such as protective pads, and creates opportunities for product redesign. Pallet specifications should reflect the acceptable thickness variation for your product. You should not have to pay more for ±1/16" in new pallets. You can expect ±3/16 in repaired pallets. The price of plywood pallets or plastic pallets is significantly more than lumber pallets, but this difference is cost effective for many products.
Use 4-stringer Pallets - Pallets are typically manufactured using 3 stringers. Using 4 stringers makes pallets stronger when racked across the stringers. It also reduces the deckboard span between stringers, increasing deckboard strength, stiffness, and durability. In many cases, users may convert from 3 stringers at 1½" wide to 4 stringers at 1-1/8" wide, for no net increase in lumber volume. Four stringers, in combination with wings, makes an excellent platform for rigid products such as drums. Each drum is supported by 2 stringers, and deckboards are not stressed in bending. Therefore, deckboard dimensions may be reduced and pallet price may actually be lower. The interior stringers of four stringer pallets need to be spaced to ensure compatibility with existing handling systems.
Eliminate Stringer Notches - Notches allow forklift tine entry on the pallet sides. In many cases, notches are specified by default. Notches add to the pallet price while reducing strength and durability. Pallets with damaged stringers are less likely to protect products. If you do not need entry on the pallet sides, eliminate the notches and convert to a 2-way pallet.
Reorient the Pallet Stringers and Deckboards - A 48x40" pallet has certain advantages in the 48" direction and certain advantages in the 40" direction. Some products would benefit from a 40x48" pallet. Many wide pallets (deckboards longer than stringers) are more economical to reorient as long pallets (stringers longer than deckboards). This could increase or decrease pallet price, depending on the design. This is very likely to interfere with downstream handling systems, and should be closely evaluated before implementation.
Use Dry Pallets - Most wood lumber pallets are manufactured at a "green" moisture content, or 40-60% moisture content. Moisture sensitive products must use such protection as pallet pads, desiccants, plastic wrapping, etc. Kiln drying or air drying pallets to 20% MC or below will prevent most moisture problems. Many softwoods are readily available as kiln dried pallet lumber. Most repaired pallets are already at a lower moisture level. Plywood, plastic, and corrugated pallets are already dry at manufacture. Dry pallets increase the pallet price, but should be compared with savings in moisture barriers and lower product damages.
Consider Returnable pallets - Returnable pallet programs allow you to invest more in the pallet. The result is a higher quality, better performance pallet at a lower cost per trip. For example, a shipping pallet that costs $7.00 adds $7.00 to the price of the unitized product. A returnable pallet that costs $15.00, but that is used for 15 trips, adds only $1.00 to the cost of the product (assuming free return). Of course, returnable programs require return logistics. The return is not economical for many companies. If such programs were not feasible in the past, take another look with today's nationwide consolidated pallet companies. Another option is the pallet leasing through companies such as Chep USA and National Pallet Leasing Systems (NPLS). Several plastic pallet manufacturers are also offering leasing options. In general, returnable pallets must be stronger and stiffer than required to obtain the durability required for multiple trips. This extra strength and stiffness give you many of the advantages of the modifications listed above at a lower price than one-way pallets.
Finalizing the Pallet Design
Once you have developed a promising design, you should build a few prototypes and make sure they work through all phases of your packaging and material handling system.
Following successful field-trials, write a detailed pallet specification. At a minimum, this specification should include the following details:
- Pallet size and description
- Target component dimensions
- Tolerances about the target dimensions
- Allowable Species
- Lumber grade
- Lumber moisture content
- Nail Quality
- A Schematic drawing
Also require that wood pallets be manufactured to standard industry practices as outlined in ASME MH1.8M "Wood Pallets." There has been a sharp upturn in lawsuits related to low quality, improperly designed pallets that result in injury. Pallets properly designed with a PDS analysis and manufactured according to standard industry practices will minimize the risk of product damage and injury that leads to litigation.
Summary
Dust off the old pallet specs. Look for opportunities to modify the pallet
and packaging as a system. The optimum solution will use overall unit load
cost and performance, and not just pallet and packaging purchase prices, as
the measure of success.
