The Dynamics of Storage Media

Aug. 1, 2009
Selecting storage media is an essential part of inventory management. Here is a basic guide to the most common storage configurations and their unique applications.

Selecting storage media may not at first appear to be a strategic decision, but racking and shelving in cutting-edge manufacturing and distribution centers do much more than idly store parts, work in process or finished goods.

Storage media — rack systems, pick modules, shelving and more — are fundamental components of inventory management. Depending on how it's used, a storage strategy can help or hinder the flow of material throughout a facility and beyond the four walls.

No single storage configuration works for every application. The right approach varies based on desired storage density, available floor space and building height, physical properties of the material to be stored, desired level of access to inventory and inventory rotation needs.

Selective vs. High Density

Stationary selective pallet rack is the most common and economical type of racking system. Selective rack's major claim to fame is 100% selectivity — every pallet is immediately accessible. In addition, it's easily assembled and reconfigurable.

However, a gain in selectivity can result in a loss of storage density, essentially, space utilization. Though access is ideal, selective storage requires a large amount of aisle space, which restricts the amount of material that can be stored in a facility.

However, there is a way around this problem. Selective rack can be modified through a double-deep configuration, a simple adaptation of 100% selective rack in which one row of rack is placed behind another. While selectivity is reduced, twice as many pallets can be stored, which can increase productive floor space by 60% to 65%.

Drawbacks of double-deep storage, however, include the need for specialized lift trucks to retrieve pallets in the rear position as well as the potential for honeycombing — when vacant space occurs after pallet loads are retrieved.

High-density storage systems take storage beyond selective rack, allowing an operation to store much more material in less space with fewer aisles. High-density systems can increase usable facility space by up to 75%. Although some inventory positions are not immediately available, these systems maximize space utilization, a significant advantage for applications in which facility space is costly to maintain, as with freezer and cooler operations. High-density systems also allow for fast-moving, high-volume order fulfillment.

There are many types of high-density storage systems, each having strengths and weaknesses. Drive-in and drive-through systems are generally considered the most economical high-density configurations. In a drive-in pallet rack, product is stored on continuous rails, and lift trucks enter one side of the rack structure to place or retrieve pallets. Drive-in racks normally store two to eight pallets per lane and are designed for frequent pallet movement.

Drive-in and double-deep storage both support last-in, first-out (LIFO) inventory rotation, meaning the last pallet placed is the first pallet retrieved. LIFO rotation is most often used for material that is not time-sensitive or perishable.

Operations handling food or other perishable material requiring the opposite material flow — first-in, first-out (FIFO) — may consider a drive-through rack system, in which lift trucks load from one side and unload from another, allowing FIFO rotation.

Push-Back Rack

When inventory management calls for both selectivity and high storage density, push-back pallet rack is a viable option. In this LIFO storage configuration, a pallet is placed by a lift truck onto a cart and then pushed back into the rack by the next pallet being loaded. Carts are mounted on rails installed in the rack structure to make lanes, which are installed at a slight angle, allowing rear pallets to roll forward by gravity as lift trucks remove the front pallets.

Many experts recommend push-back rack for handling multiple SKUs. These systems can be up to six pallets deep, yet they still allow access to a range of SKUs. In general, push-back rack systems can accommodate 90% more product than selective racks yet increase selectivity by as much as 400% over drive-in racks.

Flow Rack

Flow rack is another step up in storage density. This high-density storage system is typically used for perishables and other time-sensitive material since it offers automatic FIFO rotation. Flow rack can accommodate pallets or cases, depending on order fulfillment requirements.

Some rack suppliers offer case flow rack designed for order picking of high-volume, fast-moving SKUs packaged in boxes or stored in bins. Case flow rack has shelves of rollers, wheel tracks or skate wheels set at a slight pitch, allowing product to flow via gravity from one side of the rack to the other. The idea is to bring product to the picker, therefore reducing unproductive travel time. Case flow rack can be installed in existing rack systems, and shelves can be reconfigured as business requirements change.

Alternatively, pallet flow rack, featuring lane depths of up to 180 feet, is considered extremely high-density storage. As with their case flow counterparts, pallet flow racks allow FIFO rotation, using gravity to slide pallets from an input end, down an incline to be picked at the opposite end. Pallet flow rack is ideal for storing perishable goods, but it can be used for just about any application.

Pick Modules

When it comes to complexity, the pick module likely tops the list. Pick modules combine rack systems with conveyors or other components and provide superior space utilization. They allow dense storage of products and the ability to fill orders containing multiple SKUs.

Designed for high-volume order fulfillment, pick modules have a lift truck aisle on one side and an order picking aisle on the opposite side. Material is placed into the system from the truck aisle and removed by order pickers on the pick side. These systems can be very complex, using multiple storage configurations, including pallet flow, carton flow, push-back and static storage, all at the same time. The goal is optimum efficiency in material flow.

Specialized Storage

One option known for its versatility is mobile storage, also called portable racks. Portable racks can be set up, moved, used and rotated where and when they're needed. These rack systems are best used for seasonal or fast-moving product. In addition to versatility, they offer the ability for an operation to receive product already placed in rack positions, thereby avoiding the need to unload pallets from a truck and then place them in a separate rack.

Some operations take a hybrid approach, combining stationary with mobile rack systems. For example, stationary selective rack can be used to store out-of-season goods or slow movers, while portable racks hold fast-moving inventory.

Suppliers of mobile, high-density compact pallet racks claim these systems can cut unproductive facility space in half, while offering maximum storage density and selectivity. These specialized systems have mobile carriages that can be divided into bays with an access aisle for each bay.

And, last but not least, operations handling long or awkward-shaped materials, such as piping and lumber, can benefit from cantilever racks. In this storage configuration, loads are supported on multiple arms for even weight distribution and unobstructed access.

Storage systems for inventory management are numerous, and that's no accident. Each is designed for a specific purpose and a precise need. Far from being just a static way to store materials, today's rack configurations play a vital role in a company's overall material handling objectives.

Expert Resources

This article was developed in consultation with the following suppliers of storage media. Contact any of them for more details.

Elite Storage Solutions

Intellia Liftparts Inc.

Interlake Mecalux


Steel King

Southwest Solutions Group

Unarco Material Handling