Pallet rack basics: Behind the backbone
February 01, 2011 - MMH Editorial
If you’ve ever taken a placement exam, you might recall those analogy questions: “A is to B as C is to D”? Here’s one: Pallet rack is to a warehouse as a skeleton is to a body. Both provide structure and support to an overall entity. And, if part of either is broken, it’s very painful.
As end users expand their existing facilities or build new warehouses, new pallet rack is likely to be part of the conversation. What are most companies using today? “The shift in rapid order fulfillment has impacted the types of systems that are in demand,” says Dave Olson, national sales and marketing manager for Ridg-U-Rak (http://www.ridgurak.com). “End users are still looking for selective rack, but there’s been more activity in recent years in flow systems and multi-level pick systems for rapid fulfillment.”
There are great reasons to have either traditional rack or sophisticated rack for automated storage and retrieval systems in your facility. Both can have a significant impact on an operation’s ability to maximize inventory turns, minimize the amount of product on hand and turn it rapidly.
Efficiency continues to be a driving force in the materials handling industry, and the right racking solution in a warehouse can represent a company’s ability to remain competitive in the global marketplace. Proper planning can ensure complete and total efficiency in any warehouse,” says Linda Demke, CEO of Interlake Mecalux (http://www.interlakemecalux.com). “This permits the customer to establish a system that allows them to accurately understand all aspects of their inventory. Consequently, they are able to make quick and smart decisions and ultimately stay ahead of the competition.”
To determine which rack is right for your operation, let’s go back to basics.
Pallet rack systems are a key component of any warehouse or distribution center, and one of the most important tools in the materials handling industry. Variations of these steel structures keep product organized, provide storage, and maximize cube space from floor to ceiling. More importantly, they are central to getting product out the door. “A good rack system literally and figuratively supports order picking,” says Kevin Curry, national account manager for Steel King (http://www.steelking.com).
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 create 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 can reach 100 feet, but 20 to 25 feet is a typical rack height.
The components of the upright frames—the steel posts and cross braces—can be bolted or welded together. Bolted construction is more prevalent in Europe, while welded frames are more common in the United States.
There are also two ways to connect beams and frames: The components can be bolted to frames or interlock using a slotted connection system.
Rack components can be made of structural steel or roll-formed steel, with roll-formed being the most common in the United States. Structural steel typically costs about 5% to 10% more than roll-formed. According to Olson, raw materials make up a significant part of the cost of rack. For that reason, any increase in the price of steel will drive a percentage increase in the cost of the finished rack.
Rack systems can generally be divided into two broad categories: low-density systems that allow easy access to product, or high-density systems that make better use of space but limit access to stored product. Here’s look at what the different systems offer.
Single-deep rack, or selective rack, is the most common type used in warehouses and distribution centers. Since loads are stored one pallet deep, this is the lowest density storage of any system, but it allows unimpeded access to every pallet. It can be used in a number of aisle configurations and can be paired with any type of lift truck.
Single-deep pallet rack is also the least expensive rack option. According to “Rules of Thumb,” a pricing guide published by TranSystems, standard selective pallet rack costs about $50 to $75 per pallet position.
Aptly named, double-deep rack stores one pallet load behind another in a structure that’s twice as deep as single-deep. This design doubles storage, but it limits access and flexibility.
To access the rear pallet load, the front pallet position must be empty. Double handling is necessary unless pallets are stored on a last-in/first-out basis. In most cases, two pallets with the same product are stored in a slot of a double-deep rack, which limits flexibility and requires a deep-reach lift truck to access loads in the rear position.
“Rules of Thumb” lists double-deep rack at $65 to $90 per pallet position.
Drive-in rack can be configured to store loads four or more deep, creating very dense storage. For example, a drive-in system that stores pallets four high and five deep can hold 20 pallet loads in each bay.
In a drive-in system, lift trucks drive into the front of a storage bay, place a load in the designated position then back out. The rack is designed without traditional beams across the bays so lift trucks can maneuver in and out. Instead, pallets rest on rails that run along the inside of the bay, perpendicular to the aisles.
Pallets 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 for storing large quantities of the same product. They work well where an entire bay of product is moved at once, such as in staging product for shipping.
According to “Rules of Thumb,” standard drive-in rack costs $80 to $105 per pallet position for systems two loads deep, $75 to $105 for three loads, and $75 to $100 for four or more loads deep.
Drive-through rack is almost the same as drive-in, except that drive-through rack allows lift trucks to enter from either end. This means an aisle is required at both ends of the structure, but it increases access to pallet loads and can be loaded from one end and emptied from the other, enabling first-in/first-out product rotation.
According to “Rules of Thumb,” drive-through rack costs $80 to $120 per pallet position for systems two loads deep, $80 to $115 for three loads, and $80 to $110 for four of more loads deep.
Gravity flow rack
Gravity flow rack combines a stationary rack structure with skate wheel or roller conveyor to create a dynamic storage system. Pallets are loaded into the back end of the rack then travel down the slightly inclined lane of conveyor so they can be retrieved from the front of the system.
Flow rack systems provide high-density storage by storing product 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 than drive-in or drive-through.
This is a good option for storing dated products because it allows easy rotation of inventory on a first-in/first-out basis. It can be used for picking by the piece, carton or pallet. Carton flow rack that stores individual cartons are also available from many manufacturers.
Because there is more engineering involved in creating flow racks, they are more expensive than selective rack. According to “Rules of Thumb,” flow rack costs $135 to $260 per pallet position for systems two loads deep and $250 to $400 for three or more loads deep.
Push-back rack combines a stationary rack structure with nested carts that move along inclined rails. The first pallet, which is loaded from the front, is placed on top of the cart. When the second pallet is loaded, it pushes back the first pallet, exposing the second cart, and so on.
Usually configured two to five pallets deep, these systems offer dense storage. Like flow rack, push-back rack does not require a lift truck to enter the racking structure or require an entire bay to be dedicated to one product. Unlike flow-rack, however, push-back rack manages inventory on a last-in/first-out basis and requires less space than flow-rack because rear access is not necessary.
According to “Rules of Thumb,” push-back rack costs $110 to $140 per pallet position for a double-deep system and becomes more expensive as carts are added—$130 to $170 for three deep, $155 to $200 for four deep, and $175 to $220 per pallet position for a system five loads deep.
Protect your investment
With so much riding on your rack, it’s wise to protect your investment. Rack damage is the biggest risk to users, so replacing damaged components is critical, says Steel King president Jay Anderson, who is also the current president of the Rack Manufacturers Institute (RMI, http://www.mhia.org/industrygroups/rmi). All rack manufacturers offer a variety of options and accessories that reduce potential damage to rack structure, he says, and advises making the investment in protection.
Don’t skimp on protection or prevention. “Nothing lasts forever, and wear and tear can take its toll,” adds Curry. “Actually walk the aisles and inspect your rack frequently. This preventive step could save a major headache later on.”
Protect the code, track the load
Streamlined movement in and around pallet racking is crucial to an operation’s overall success. “About 80% of all industrial storage is on pallet rack,” says Frank Ceriello, vice president of sales for Aigner Index (http://www.aignerindex.com). “The ultimate goal is to know where everything is—all of the time.”
Bar codes provide a wealth of product data, but in a harsh warehouse environment, the quality of a bar code label can be compromised if it isn’t protected properly. An unprotected bar code label can be damaged by dust, dirt, grease, moisture or smudged label ink. Then it might not scan or it might scan inaccurately. Either scenario can cause serious and expensive shipping and inventory problems, not to mention the impact it could have on customer relations.
One simple solution for bar code protection is clear insertable plastic label holders. Available in many styles, these holders typically cost less than 5% of the total warehouse expenses and can generate returns in improved productivity, inventory accuracy, knowledge of product location, and flexible designation or location changes.
Once product is located, tracking its movement and operator performance can also pay off big. “Complete visibility to vehicle movement can bring gains in labor productivity, fleet optimization and safety,” explains Sarah Brisbin, vice president of marketing for Sky-Trax (http://www.sky-trax.com).
To determine the location of a forklift, a sensor mounted on top of the vehicle reads the optical position markers mounted in the racked area. The system can pinpoint the forklift to within a square inch the facility.
Special system reporting packages can measure distances traveled and time taken to complete a task. Traffic reports can identify bottlenecks and collisions. With this tracking system, users can go back and replay the data to see which truck contacted the rack and why. It can also determine who was driving and what was he carrying. Finding the answers to these questions quickly could identify the need for additional operator safety training, uncover a process flaw, or identify a damaged rack that requires immediate attention to prevent disaster down the road.
Look for the R-mark
If good rack goes bad, the results can be catastrophic, even deadly. So pallet rack suppliers have established strict technical guidelines to ensure product integrity and overall safety. To be sure the rack you’re considering meets these guidelines, look for the R-Mark.
The R-Mark Certification program is administered by the Rack Manufacturers Institute (RMI, http://www.mhia.org/industrygroups/rmi), an industry group within the Material Handling Industry of America (MHIA). To earn the R-Mark, rack manufacturers must submit sets of product data which is studied by RMI engineers to ensure the suppliers’ testing, calculations and resulting rack capacities meet the most current standards.
Only after product data is shown to meet the guidelines does RMI issue the R-Mark seal. Manufacturers are then authorized to use the R-Mark seal on published capacity charts and designs and display it on products built according to those designs.
You can visit the RMI Web site for the latest versions of rack standards (including MH16.1-2008-Specification for Industrial Steel Storage Racks and MH26.2-200-Specification for Welded Wire Rack Decking).