Ten Ways to Improve Pallet Performance in your Warehouse
| by John W. Clarke |
|
May 15, 2000 |
Introduction
Why should a warehouse manager or equipment designer learn about pallets? Because pallets are the primary interface between the packaged product and today's highly automated material handling equipment. Today, approximately 90% of the U.S. domestic product is shipped on pallets. Although pallets are not the most glamorous part of the warehouse, they are important because they cost a lot of money! Some pallets cost much more than others. Some warehouses may want to use a high quality "6-Sigma" pallet, while others are better off shipping on Grade B repaired pallets. The optimum equipment to handle these 2 pallet quality levels is different. Many of the pallet specifications that are used to purchase pallets for automated warehouses are 10-20 years old, or lack critical manufacturing details that determine performance. The end user becomes frustrated when the new, multimillion-dollar material handling system does not perform to expectation.
Warehouses that use pallets, and equipment designers that recommend pallet designs for their systems, need to accurately specify the pallet quality level required for these systems. This paper will list the important components of an accurate pallet specification, and give you 10 points to consider that can improve pallet performance in your systems.
Pallet Terminology
The following pallet terminology applies to any pallet material. In general, any material can be used to manufacture a high quality or low quality pallet. Pallet designers can design to any performance level, at some cost.
The figure below shows schematic drawings of pallet styles and components. There are 2 general styles of pallets: Stringer-style and Block-style. 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 forklift, but not pallet jack, 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 a pallet 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, loads, and support conditions. PDS then estimates pallet strength, stiffness, durability, and the cost to use the pallets. Users can then optimize 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). Most quality wood pallet suppliers lease or have access to the PDS program.
Pallets of any material should be tested to determine the performance level. Common performance measurements are strength, stiffness, and durability. Test methods include ASTM D1185, ASME MH1, and ISO 8611.
Important Parts of any Pallet Specification
A proper pallet specification levels the playing field between pallet vendors. If pallet users search for the lowest price with a vague pallet spec, the end result is a poor quality pallet that often "costs" much more than it saves. How many pallet users complain of inferior pallets? How many low quality pallets reduce the functionality of highly automated material handling systems? This can be corrected through a pallet specification that includes the following details:
- Pallet size and description
- Target component dimensions (actual, not nominal sizes)
- Acceptable tolerances about the target dimensions (i.e. &#plusmn; ¼")
- Allowable materials (wood species, plastic formulation, ...)
- Material grade (100% recycled HDPE, Grade 2 lumber,...)
- Moisture content
- Nail Quality (not just length!)
- Coefficient of Friction
- Minimum Performance (Strength, stiffness, durability)
- A Schematic drawing
Also, you should require that wood pallets be manufactured to standard industry practices as outlined in Part 3 of ASME MH1 "Pallets, Slip Sheets, and other Bases for Unit Loads."
10 Ways to Improve Pallet Performance in Your Warehouse
As you develop your pallet specifications, consider these 10 ways to modify pallets and reduce problem interactions between pallets and material handling equipment.
Improve Nail Quality - In solid wood and plastic lumber pallets, nails are the most neglected component of pallet manufacture. Nails are "out of sight, out of mind," until a loose deckboard snags on a rack support. Most pallets are made of wood and nails. The wood costs about 55-60% of the pallet price, while nails are only about 3-5% of the pallet price. Upgrading to a high quality nail will only raise the pallet price by another 1-2% while more than doubling the durability of the pallet. How much do loose deckboards and exposed nail heads cost your warehouse each year? Paying 1-2% more for pallets with properly inserted, high quality nails will significantly reduce problems with exposed nail heads, protruding nail points, and loose deckboards.
Use Perimeter Base Pallets - The bottom boards on a pallet are the primary interface with most material handling equipment. All stringer style pallets, and many block style pallets, have bottom boards oriented in one direction. This is called a unidirectional base. Unidirectional bases can and do work well with equipment designed to handle this type base, but pallet movement is often delayed as pallets are reoriented before conveyance on rollers, chain conveyors, etc. Perimeter base block style pallets (available in any pallet material) have base components oriented in both directions. They may cost more, but handle faster and are less sensitive to equipment orientation.
Specify a Minimum Pallet Coefficient of Friction - Most material handling equipment has been developed in unison with wood pallets. On steel surfaces, wood pallets have a coefficient of friction (COF) of 0.40 to 0.60. That is, a 100-pound wood pallet would require 40 to 60 pounds of horizontal force to move that pallet on a steel surface. HDPE plastic pallets, however, have a COF of only 0.15 to 0.25. Low friction pallets can more easily slip in storage racks, or slide off forklift tines. When empty, low COF pallets often fail to convey on rollers. Most low friction pallets can be modified with rubber grommets or spray-on tackifiers to increase friction. Make sure that the pallets you specify meet a certain COF to ensure adequate performance in your system.
Eliminate Stringer Notches - Notches allow forklift tine entry on the pallet sides. In many cases, notches are specified by default. Cutting stringer notches actually adds to the pallet price, while reducing strength and durability. Pallets with damaged stringers are less likely to function on conveyors and are dangerous in racks. For a typical GMA-type pallet, eliminating stringer notches can double the pallet racking strength and durability and save 5% on the pallet price. If you do not need 4-way entry, eliminate stringer notches and convert to a 2-way pallet.
Increase Pallet Deck Stiffness -Stiffer pallets cause fewer equipment hang-ups than more flexible pallets. They also cost more, and the trend today in wood pallets is to use pallets with very thin deckboards. Unfortunately, this savings on pallet price often leads to stuck pallets in automated warehouses. Thin deck pallets can easily deflect 1-inch or more in racks. Thin boards are easier to break and often jam conveyors. Warehouse equipment can be designed to handle thin deck pallets, but at greater cost. In many cases, it is more economical to design the warehouse to handle low stiffness pallets. Other times, however, users are better off with a stiffer pallet deck and faster warehouse system. Specify a maximum pallet deflection (stiffness) for use in your system.
Specify Flatter Decks - No pallet is perfectly flat, although most molded plastic, paper, and plywood panel 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". Used and repaired pallets are much more variable in thickness and flatness. Your pallet specifications should reflect the acceptable flatness for your system. You should not have to pay more for ±1/16" flatness in new pallets. You can expect ±3/16 in used and repaired pallets. The price of plywood pallets or plastic pallets is significantly more than lumber pallets, but flatter pallets are cost effective for some systems.
Reorient the Pallet Stringers and Deckboards - A 48x40" pallet has certain advantages in the 48" direction and certain advantages in the 40" direction. Some systems would benefit from a 40x48" pallet. Many wide pallets (deckboards longer than stringers) are more economical to convert to 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. 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 may increase the pallet price, but offer stiffer decks, the potential for tighter dimensional tolerances, and fewer moisture problems in enclosed vans and intermodal containers.
Use "SPEQ" marked pallets - How can you be sure you get what you paid for? For wood pallets, there is a third party inspection program called SPEQ, administered by the National Wooden Pallet & Container Association (703 527-7667). Once you have gone to the effort of developing good pallet specifications, "SPEQ" marked pallets are guaranteed to meet your specifications. I would highly recommend the SPEQ program for competitive bidding between pallet vendors. You will get the most competitive bid from a level playing field. Using properly designed, SPEQ-stamped pallets will also help protect you in the event of litigation against your company.
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 at outsourcing return management with today's nationwide consolidated pallet companies. Another option is pallet leasing through companies such as Chep USA and National Pallet Leasing Systems (NPLS). Several plastic pallet manufacturers also offer 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 pallet design, you should test a few prototypes to ensure that they work through all phases of your material handling system. Seriously consider the SPEQ program. There has been a sharp upturn in lawsuits related to low quality, improperly designed pallets that result in injury. Pallets properly designed 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! Learn more about pallets. The Center for Unit Load Design publishes a list of educational resources on the pallet and container industry. Visit our website to download these materials. Check out the Center's Short Courses for Material Handling Equipment Suppliers and Pallet Users. As you update your pallet specifications, look for opportunities to design the pallet and material handling equipment as a system. The optimum solution will balance pallet and equipment efficiency and economy, and not just pallet and equipment purchase prices, as the measure of success.
