INNOVATIVE STORAGE METHODS

May 01, 2004 9:30 PM  By

Even for a dyed-in-the-wool pragmatist, the phrase “innovative storage methods” might well conjure up a picture of slick, exciting modules gleaming with a high-tech luster. While some storage systems may well fit that description, innovation is just as often a matter of using existing components in a different or better way.

To be mundane, storage components in a distribution center usually involve racking, and the simplest and most immediately effective means of developing innovative storage systems in your DC is to look first at the racking systems already in place and consider how they might be modified. Racking is indispensable, and it serves three basic purposes:

Cube utilization

Rack 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.

Product protection

The rack structure eliminates the vertical crushing forces that would occur if goods were stacked on top of each other, and it protects the product from equipment impact.

Organization

Each pallet is stored in a unique location.

RACK TYPES

There are several storage methods to choose from. The first thing to do is to be aware of the most common rack types:

Floor stack

Although not technically “racking,” floor stack can be an important part of a storage system. It is very flexible (just about anything can be put there) because there is no structure to define limits for load height and width. This can be a benefit when used for temporary storage of fast-moving product (i.e., cross-docking). Product characteristics may limit stacking height, and 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 a location.

Selective

The most common type of rack, selective rack has each “bay” typically designed to store two pallets between the frames. The advantage of selective rack is — selectivity. Product is stored only one pallet deep off the aisle, and every level of storage is independent, so every pallet is accessible. Selective rack is typically the least expensive rack type. The downside of using selective rack is that the portion of the building’s footprint devoted to aisles can easily exceed 50%.

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 just one deep. This provides increased storage density while maintaining a good degree of selectivity. Deep reach rack does require special forklifts with extensions.

Drive-in/drive-through

Pallets are supported on their edges by rails or angle irons that run perpendicular to the aisle, rather than by beams that are parallel to the aisle. A drive-in/drive-through bay is one pallet wide. Because the pallet support rails are under the sides of the pallet, there is a gap between the support rails that allows the forklift to drive in or drive through the rack structure. Lateral bracing is either in the rear of the location or overhead. The biggest advantage of drive-in/drive-through is its high storage density. As product velocity in distribution facilities has increased over the past several years, drive-in/drive-through rack has become less popular, but in some situations it is still the most appropriate application.

Pushback

Pushback rack typically uses a series of nested carts that ride on an inclined rail. When the first pallet is stored in a 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. The process is reversed when the location is unloaded.

The advantages to pushback include high storage density and good selectivity. There is also a productivity advantage over some other high-density systems because a forklift is always working on the face of the rack, rather than reaching in or driving in to retrieve a pallet.

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 gravity to move the pallets from one end to the other. The biggest advantage to pallet flow is its ability to maintain first-in-first-out rotation of product in each lane. It is normally used in high-throughput operations to stage product for shipping, and to feed pallets to a case-pick line.

STOCK PROFILE

Determining what style of pallet rack is best for your operation will depend on several factors, the biggest of which is your inventory profile. This will require some analysis of your inventory now and in the recent past. If your business has seasonal fluctuations in order volume or buying patterns, 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 (e.g., number of SKUs, inventory turns) can then be applied to this base number.

Once you have created an inventory profile, you can determine the style of rack you need. A general indicator of proper rack style is the “Rule of Three,” which suggests that when pallets are stored in multi-deep locations, for proper efficiency each SKU should occupy at least three locations; for example, a SKU stored in two-deep rack locations should have a minimum of six pallets in inventory.

In addition to storing reserve pallets, you need to make plans for the picking operation. If the pick operation is in the same area as storage, the need for pick slots can be the primary determinant of the rack layout, especially if the pick slots must all be accessible from floor level. Pick slots must be sized so that pickers have few, if any, out-of-stock problems, and so that inventory is sufficient to keep replenishments at a manageable level.

Another factor to consider in determining storage layout is aisle width, which is usually determined by the type of material handling equipment in use. Generally speaking, the narrower the aisle, the higher the cube utilization in the building. However, too narrow an aisle will limit its use to one piece of equipment. If multiple pieces of equipment are in use, the slowest sets the pace. In either case, the speed with which pallets can be put away and retrieved in an aisle must match the demands of other areas. You do not want to achieve high storage efficiency at the expense of not getting product to where it needs to be in time to meet commitments to your customers.

HIGH RISE

Perhaps the highest storage efficiency is achieved by high-rise automated storage/retrieval systems (ASRS). These systems have a lot to offer, but there are some caveats about their use. First is capital cost. These systems are not inexpensive, and if land cost and labor cost are relatively low, it may be difficult to justify the investment. On the other hand, a lack of labor (rather than cost) may make automated systems the only option.

The second caveat relates to capacity. A high-rise system needs to be designed for full throughput and storage capacity from the beginning, even though it may be several years before volume reaches maximum. Alterations to the rack structure can also be a challenge. Some systems offer flexibility in their selection of rack type, or in their ability to add equipment as the volume increases. Few companies have the luxury of assuming that their business demands will not change during the life expectancy of an asset.

USE AND ABUSE

One important point is to consider is the amount of abuse to which the racking will be exposed. It is imperative to remember that in a warehouse, whoever argues with a forklift will lose. Material handling equipment that uses wire or rail guidance will inflict minimal, if any, damage to the storage rack. Standard forklifts and reach trucks are another matter. However, racks should also be designed to withstand the minor bumps that always occur in an operation. The busier the operation, the more potential for damage. Part of the solution is providing enough working room for both product storage and for operating equipment to reduce the potential for damage.

A mix of storage rack types will suit some operations best. Every operation is unique, and a careful examination of product flow and operational demands (hours/days of operation, volume fluctuations) will be necessary to make sure that the rack layout enhances efficiency, rather than creating bottlenecks.

Innovative storage methods do not always mean a significant capital expense. May I offer a personal example? I was looking at the silverware drawer in the kitchen and expressing concern that the drawer organizer did not provide enough room for all of our teaspoons. My daughter came and looked in the drawer also. She suggested I move the spoons to the knife compartment (which was much larger) and move the knives to the spoon compartment. Problem solved! Moral: Often it is worthwhile to bring in some outside help to help you understand how the entire system should function. Equipment vendors and material handling consultants can provide an extra set of eyes that can be a valuable resource to in-house personnel.

A properly designed rack storage system is a key component in the effort to achieve both customer satisfaction and continued growth.

Bruce H. Anderson has spent 28 years in operations, engineering, and consulting for the distribution industry, and can be contacted at BruceHAnderson@aol.com.

Rule of Three

2-Pallet Storage Locations
Pallets in Inventory Storage Locations Used (Positions) Percent Utilized
1 1 (2) 50%
2 1 (2) 100%
3 2 (4) 75%
4 2 (4) 100%
5 3 (6) 83%
6 3 (6) 100%
7 4 (8) 88%
8 4 (8) 100%
9 5 (10) 90%
10 5 (10) 100%
15-Pallet Storage Locations
Pallets in Inventory Storage Locations Used (Positions) Percent Utilized
1 1 (15) 7%
15 1 (15) 100%
16 2 (30) 53%
30 2 (30) 100%
31 3 (45) 69%
45 3 (45) 100%
46 4 (60) 77%
60 4 (60) 100%
61 5 (75) 81%
75 5 (75) 100%

Source: Bruce H. Anderson