Pallet rack is not generally regarded as very glamorous or exciting; however, if your operation is facing a minor crisis in storage capacity or if you have some rack that has had a structural failure, things can get exciting in a hurry.
Racking serves three basic purposes. The first purpose is cube utilization. Racking provides the supporting structure necessary to stack products at greater heights than would be possible if pallets of goods were stacked on top of each other on the floor. The second purpose of racks is to protect product, not only by reducing crushing forces but also by guarding the product from equipment. The third is organization. Each pallet is stored in a unique, identifiable location.
Common Rack Types
There are several rack types from which to choose. The first thing to do is a general review of the most common rack types.
Floor Stack � Although not technically �racking,� floor stack can be an important part of a storage system. One advantage is its flexibility. Just about anything can be put there, since there is no structure to define limits for load height and width. This can be a benefit when the location is used for temporary storage of fast-moving product (i.e., cross-dock). Another advantage is storage density because locations can be of any depth; however, if the product characteristics limit stacking height, the space between the top of product and the ceiling structure represents unusable storage capacity. A big advantage of floor stack is the low capital cost since all that is required is paint or tape to mark the location.
Selective Rack � Selective rack is the most common type used in distribution centers. Each �bay� of rack is typically designed to store two pallets between the frames, but bays can also be designed to hold one or three pallets, depending on need. The major advantage of selective rack is its selectivity. Products are stored only one pallet deep off the aisle, and every level is independent, so every pallet is accessible. The downside is that the percentage of building footprint devoted to aisles easily exceeds 50%. Selective rack is typically the least expensive rack type.
Deep Reach � Deep reach is similar to selective rack, but the bays are constructed so that the pallets can be stored two deep off the aisle rather than one deep. This provides increased storage density while maintaining a good degree of selectivity. Special forklifts with an extended reach must be used with a deep reach rack.
Drive-In/Drive-Thru � This type of rack is designed to hold one pallet between the frames. Pallets are supported by �rails� or angle irons that run perpendicular to the aisle rather than by beams that are parallel to the aisle. Because the pallet support rails are under the sides of the pallet rather than across the front and back, there is a gap between the support rails that allows the forklift to �drive in� or �drive through� the rack structure. The difference between drive-in and drive-thru is drive-in has lateral bracing at the rear of the storage location with some overhead bracing at the front. Drive-thru has no lateral bracing at the levels where product is stored so the forklift can enter the rack structure from one aisle, drive through the rack and exit into a different aisle. All lateral bracing on a drive-thru rack is located at the top of the structure. The biggest advantage of drive-in/drive-thru is the high level of storage density. As product velocity in distribution centers has increased over the past several years, drive-in/drive-thru rack has become less popular, but there are situations where it is the appropriate application.
Pushback � Pushback typically uses a series of nested carts that ride on an inclined rail. When the first pallet is stored in the location, it is placed on the top cart. When the second pallet is ready to be stored, it is used to push back the first pallet. As this is done, the pallet and cart travel up the inclined rail, exposing the second cart, onto which the second pallet is placed. This process is repeated until each cart is loaded and pushed back, with the last pallet resting on the rails. When a pallet is unloaded from the front of the location, the pallets (and carts) to the rear roll forward one position.
The advantages to pushback include high storage density and good selectivity. There is also a productivity advantage over some other high-density systems because the forklift is always working on the face of the rack, rather than needing to reach in or drive in to retrieve a pallet. Cart-type pushback is limited to six-deep storage. Other pushback systems that do not use carts can store product deeper, but five or six deep is about as deep as most operations ever need.
Pallet Flow � In this type of rack, the pallets flow through the structure on rollers or wheels. The rails that support the rollers/wheels decline from the infeed side to the discharge side, using the force of gravity to move the pallets from one end to the other. Some systems require a braking mechanism in the rollers/wheels that keep the pallets from gathering too much speed as they roll forward.
The biggest advantage to pallet flow is the ability to maintain first-in/first-out (FIFO) rotation of product in each lane. It is typically used in high-throughput production operations to stage product for shipping and to feed pallets to a case-pick line.
Choosing the Right Pallet Rack
The biggest factor in determining the proper rack type is your inventory profile. This will require some analysis of your inventory. It is a good idea to evaluate several inventory �snapshots� for different times of the year. This inventory information should get down to the SKU level to identify the number of pallets for each SKU in your facility. Projections for future growth (i.e., number of SKUs, inventory turns) can then be applied to this base number.
Once an inventory profile has been created, the style of rack can be determined. A general indicator of proper rack style is the �Rule of 3,� which suggests that when pallets are stored in multi-pallet-position locations, each SKU should occupy at least three locations for proper efficiency. For example, an SKU stored in two-deep rack locations should have a minimum of six pallets in inventory. Any SKU with less than six pallets in inventory belongs in selective rack.
There are other important things to consider when developing a rack system layout. These include:
� How many SKUs need a pick slot that is accessible from floor level?
� What type of material handling equipment do I expect to use?
� How do I plan for future expansion?
One final point to consider is the amount of abuse to which the racking will be exposed. It is important to remember that in a warehouse, whoever argues with a forklift loses. Management oversight and periodic review are essential to keep racks in good condition; however, racks should also be designed to withstand the minor bumps that always occur in an operation. And the busier the operation, the more potential damage. Part of the solution is also providing enough working room for both product and for operating equipment so that the potential for damage is reduced.
Some operators find that a mix of rack types best suits their operations. In addition, a careful examination of product flow and operations demands will be necessary to make sure the rack layout enhances efficiency rather than creates bottlenecks. Often, it is worthwhile to bring in some outside help.
As companies have looked at ways to increase efficiency and profitability, the warehouse/DC is usually one of the last areas to be considered. However, in the last several years, it has been recognized that distribution and logistics play an important role in achieving both customer satisfaction and continued growth. A properly designed rack system is a key component in that effort and can provide a big boost to your company�s long-term success.
Bruce H. Anderson is the director of Industrial Engineering at The Stellar Group. E-mail him at banderson@thestellargroup.com.
