Racking Right

Logistics Insight Asia, 1/1/2008

Double-deep, drive-in, drive-through, push-back? Corinne Kator sorts through the different types of pallet rack systems.

Pallet rack systems are among the most basic and yet useful materials handling tools. Variations of these simple steel structures create order and maximize storage density in a warehouse, making good use of floorspace and ceiling height.

A basic pallet rack consists of upright steel frames connected by horizontal steel beams. Pallets rest on the beams between the upright frames. Two frames and the corresponding beams form a bay. The number of pallet positions in a bay depends on the height of the frames and the spacing of the beams.

High-rise storage systems reach heights of 30 meters or more, but a typical rack height is six to eight meters, says Gary Slater of Unarco Material Handling, president of the Rack Manufacturers Institute (www.mhia.org/rmi).

The components of the upright frames – their steel posts and cross braces – can either be bolted or welded together. Welded frames are more common in the US, while bolted construction is more common in Europe.

There are also two choices for connecting frames and beams. Beams can be bolted to frames, or the two components can snap together using a slotted connection system.

Rack components can be made of structural steel or of roll-formed steel. Which type of steel is most suitable for which applications is often a point of discussion within the rack industry. The majority of racking sold in the US, says Slater, is made of roll-formed steel. Structural steel costs about five to 10 percent more than roll-formed, according to consulting firm Gross & Associates.

Several types of rack are available: low density systems that allow easy access to products but limited storage capacity; high density systems that make more efficient use of space but limit access to what they store; and dynamic systems that facilitate rotation of inventory.

Single-deep rack
Also known as selective rack, single-deep pallet rack, is by far the most common type of rack used in warehouses today. Because loads are stored only one pallet deep, single-deep rack provides the lowest density storage of any pallet racking system.

Single-deep remains popular, however, because it permits unimpeded access to every pallet it stores. Additionally, single-deep rack can be used in a variety of aisle configurations, can be paired with any type of lift truck and is the least expensive style of rack. According to “Rules of Thumb,” a pricing tool created by Gross & Associates, standard selective pallet rack costs approximately US$50 to $70 per pallet position.

Double-deep rack
As its name implies, double-deep rack stores one pallet load behind another in a rack structure that’s twice as deep as single-deep rack. While doubling storage density, this design limits access and flexibility.

To access the rear pallet in a doubledeep rack system, the front pallet position must be empty. Double handling is necessary unless pallets are stored on a last-in/first-out basis. And in most cases, two pallets of the same product are stored in a slot of double-deep rack, which limits the flexibility of a warehouse’s storage space.

Gross’ Rules of Thumb lists doubledeep pallet rack at $55 to $80 per pallet position. Putting away and retrieving loads in the rear position of a double-deep system requires a deep-reach lift truck.

Drive-in rack
Drive-in rack can be configured to store loads three, four or more deep, creating extremely dense storage. A drive-in system that stores pallets four high and five deep, for example, can hold 20 pallets in each bay.

In a drive-in system, lift trucks drive into the front of a storage bay, place a load in the desired position and then back out. To allow lift trucks into the bays, drive-in systems are designed without traditional beams placed across the bays. Instead, pallets rest on rails that run along the insides of the bays, perpendicular to the aisles.

Pallets in a drive-in racking system are stored on a last-in/first-out basis. Each bay is typically dedicated to a single product, so drive-in rack is best used in warehouses storing large quantities of the same product. But drive-in systems severely limit access to products stored in rear positions. As a result, say experts, this type of storage is best suited for situations in which an entire bay of products would be moved at once, such as in staging products for shipping.

Standard drive-in rack costs $60 to $90 per pallet position for systems three loads deep and $50 to $80 per position for systems four or more loads deep, according to Rules of Thumb.

Drive-through rack
Another means of providing high density storage is the drive-through rack, which has a similar design as the drive-in rack but allows lift trucks to enter and leave the bays from either end. This considerably increases access to pallet loads, but requires an aisle on both ends of the racking system. Its configuration allows loading from one end and emptying from the other, for first-in/ first-out product rotation.

According to Rules of Thumb, drivethrough rack costs approximately $65 to $105 per pallet position for systems three loads deep and $55 to $95 per position for systems four or more loads deep.

Gravity flow rack
The gravity flow rack combines a stationary rack structure with skatewheel or roller conveyor to create a dynamic storage system. Pallets are loaded into the back end of the rack and then travel down the slightly inclined lane of conveyor until they are retrieved from the front of the system.

Flow rack systems provide high density storage by storing products many pallets deep. Because each layer of flow rack is typically dedicated to a single product, these systems offer less storage flexibility than selective rack but more flexibility than drive-in or drivethrough rack where an entire bay isdedicated to one product.

A flow rack allows easy rotation of inventory on a first-in/first-out basis, making it a good choice for storing dated products. It can be used for picking by piece, carton, or pallet. Many rack manufacturers also offer carton flow rack that stores individual cartons rather than entire pallets.

Gross & Associates estimates pallet flow rack costs about five times as much as traditional selective rack – between $250 and $350 per pallet position for a standard system three or more pallets deep.

Push-back rack
A push-back rack combines a stationary rack structure with nested carts that move along inclined rails. Pallets are loaded from the front. The first pallet is placed on the top cart; when the second pallet is delivered, it pushes back the first pallet, exposing the second cart, and so on. Nested carts moving along inclined rails provide last-in/first-out storage in a push-back rack system.

Push-back racking systems are usually configured two to five pallets deep. Like flow rack, they offer dense storage without requiring a lift truck to enter the racking system or requiring an entire bay be dedicated to one product. However, unlike flow rack, push-back rack manages inventory on a last-in/first-out basis. Push-back rack requires less space than flow rack because rear access is not necessary.

Push-back rack costs roughly $95 to $135 per pallet position for a double-deep system and becomes more expensive as more carts are added, according to Gross’ Rules of Thumb. A push-back system storing pallets five deep costs about $175 to $200 per pallet position.

Because drive-in systems limit access to rear storage positions, they are best suited fortransfer of an entire bay of products.

drive-throughrack

gravity flow rack

Nested carts