Pallet rack
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A pallet rack, also referred to as “pallet racking,” is for stocking inventory that sits on pallets (or “skids”) that are stored in horizontal rows with multiple levels; pallet racks allow warehouse inventory to be stored more efficiently (called maximum storage density). The pallet racks link together to create a pallet rack system. Forklift trucks are usually required to place the loaded pallets onto the racks for storage because of their size and weight. Pallet racks are essential and ubiquitous to modern warehouses, retail centers, and other facilities.
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[edit] Structural Components
A pallet rack has several structural components:
- Load beams (also called step beams or box beams).
- Step beams have a 1⅝” step along their edge on which to rest the horizontal support component.
- Box beams have no inset and have four flat sides like a box. Load beams typically clip into or bolt onto an upright frame column.
- Upright frame columns (also called upright columns or uprights). Upright columns vary according to steel thickness, load requirements, and styles. The most common upright column is produced by rolling metal into the shape of a three-sided, cornered post, and is called an open-back roll-formed column. Holes or slots run up and down the column at standard intervals so that horizontal crossbeams can be mounted into the upright columns with pallet supports.
- Pallet supports are bolts or clips that fit into the holes or slots in the upright and come in various styles. Three common support styles are hat channel, tec screw, and clip-in style.
- Wire decks are used as support platforms to distribute loads more evenly. Wire mesh decking comes in various thicknesses and mesh dimensions. Wire mesh construction also allows for easy identification of shelf contents and prevents dirt and other debris from accumulating on the shelves because of the holes in the mesh.
Wire decking is made in three different styles. Most wire mesh decking has U-shaped channel supports, also known as struts, to support the load. With waterfall decking, the wire mesh extends across the top and down the front of the beam to provide more support, and is more desirable in the marketplace. Reverse waterfall decking can provide containment of a loose product to prevent the product from falling behind the rack system. Lay-in decking rests inside the step of the beam, and wire mesh does not waterfall over the beam. Some types of decking are manufactured with solid metal instead of wire mesh. Even though the solid decking provides a greater distributed weight capacity, it is discouraged by fire inspectors because sprinkler systems cannot spray through the shelves to levels below. - Footplates, also known as footpads, are at the base of columns and serve as anchors to give the rack more stability: bolts are inserted through the footpad’s anchor holes to attach the column to the concrete floor. Footplates are made of thicker steel and in some geographic locations, they must be of a certain size and seismic rating. Footpads increase the pallet rack’s overall stability and weight-bearing capacity.
- Shims are used when the uprights are resting on uneven floors; the shims, equal in size to the base of the uprights, are installed beneath the uprights to level the rack.
- Row spacers are sometimes used if uprights are arranged in back-to-back rows; the spacers are mounted between adjacent columns to ensure that the rows are kept straight and to give the pallet racks even more strength and steadiness.
- Diagonal braces and horizontal braces, also known as windows or supports, are often welded inside the upright to give the frame more rigidity and stability.
- Wall ties may be used for further support if the uprights are arranged in a row along a wall.
- Column protectors, also known as post protectors, are protective shields that can be installed around the base of an upright to minimize damage where forklifts might hit the upright. Damage to the base of a column can weaken the entire frame and could cause it to collapse. Column protectors are made of various materials such as polyethylene, ductile iron casting, and other durable materials.
- Guard rails are installed to increase protection for upright columns and for human safety when platforms or steps are attached to pallet racks.
[edit] Common Types
Many types of pallet racks are available with different designs to fulfill specific functions or create specific advantages. When deciding on the type of pallet rack to use, several basic considerations have to be taken into account:
- Desired storage density
- Floor space and building height
- Inventory accessibility
- Inventory rotation
- Item/load size and weight
- Optimal storage design
- Cost of materials and installation
Some of the most common types of pallet rack systems used include:
- Selective pallet rack systems are the most commonly used rack system because they are inexpensive and quick and easy to assemble. Selective pallet racks typically come in two configurations: a teardrop, or clip-in configuration, and a structural bolt-together configuration. The nickname “teardrop” comes from the fact that the holes on the column of the upright are shaped like a teardrop. Pallets then rest on the horizontal beams that are held in place by mounting clips.
Because the clips on teardrop configurations can be quickly moved, the shelves can be easily adjusted to different heights to accommodate various load sizes. This is convenient for a warehouse that needs to store a wide variety of product sizes. Another advantage of selective pallet rack systems is the easy accessibility they provide to all products at all times. Such accessibility is important if the inventory is rapidly depleted and restocked (called quick turnover). A selective pallet rack system is commonly used in a “big-box” distribution application, as well as in retail store inventory rooms, cold storage applications, wholesale stores, etc. - Structural pallet rack systems are very similar to selective pallet rack systems, but the horizontal supports are attached to the uprights with bolts and often have an increased weight-bearing capacity. Also, structural pallet racking can be designed into the structure of the building itself, so that the upright columns are simultaneously used to support the roof of the storage facility, in which case the structural pallet rack uprights replace the storage building’s vertical support I-beams.
- Very Narrow Aisle (VNA) is the use of selective pallet racking in a tighter configuration to provide maximum space utilization within a storage facility. These systems typically operate in conjunction with wire-guided or rail-guided reach-truck systems. A wire-guided system consists of a wire embedded in the concrete floor that provides tracking for the reach-truck. A rail-guided system consists of angle iron bolted to the floor down the length of each row. Typically, the angle iron is 4” by 3” and ¼” - ⅜” inches thick.
- Push-back pallet rack systems are designed around the principle of organizing space by depth rather than width. This depth arrangement greatly reduces aisle space and increases storage density. In this configuration, each row is multiple pallets deep, and often has wheeled carts that fit onto rails to take advantage of gravity, saving enormous amounts of energy for moving heavy pallets. When a forklift sets the pallet onto the cart, it drives forward and causes the pallet to bump the next pallet, causing the entire row of pallets to roll backwards. When removing a pallet from the front position the remaining pallets immediately stage themselves forward so that the next available pallet can be accessed.
- Pallet Flow and Carton Flow is a pallet racking system also designed for depth space; it uses a slightly inclined conveyor that makes pallets or totes move easily along a sloped plane. These systems are also called gravity flow or dynamic flow racking systems. The pallet flow system often has complex motion and braking systems to control the speed of the moving pallet.
- Drive-in and Drive-through (sometimes spelled Drive-thru) are storage rack configurations that allow the forklift to drive directly into the lane of stacked rows (called a bay). The difference between a drive-in and a drive-thru pallet rack system is simply whether the bays have an entry at only one end, or at both ends. Drive-in rack systems use a common entry and exit, while drive-thru systems have entry points at either end of the bay. Because a drive-in racking system has only one entrance, it uses what is called a “Last In, First Out” (LIFO) storage method. With only one entrance, the last pallet put into a row is necessarily the first one to be taken out. A drive-thru storage system, with two different entry points, can also use a “First-In, First-Out” (FIFO) storage method. With a FIFO system, pallets are loaded in one end and are pushed back to the other end, where they are then at the front of the row on the opposite side. The first pallet put into such a row is the first one taken out at the other end. This system is advantageous for material with an expiration date or wherever shelf life is a major concern.
Some of the disadvantages of the push-back, drive-in, and drive-thru systems are less access to all stock at any given moment (although if the stored product is all the same, it should not matter), and that such arrangements are more expensive than the simpler selective pallet rack storage systems. In large factory production facilities, however, push-back pallet rack systems are essential, since the efficiency of time and space is optimized.
[edit] Safety Considerations
Because of the size and weight of pallets, important safety factors have to be considered at all times:
- Pay attention to any loose components in the pallet rack system, and take the time to report any damage in the pallet rack frame; such frame damage could cause the pallets to fall.
- It is the owner’s legal responsibility to communicate this important warning to all who are around storage racks:
“Never climb on racks during or after assembly. Storage racks are not designed to be stepped on or climbed on. A slip or fall may result in serious injury.”
It is especially important to have highly visible warning signs if the pallet rack system is used in retail environments, such as wholesale centers, where the public is present. - Use only quality pallets that are not damaged. To save money, or perhaps from neglectful management, some warehouses use pallets until they become faulty and dangerous. Regular inspection of pallets for broken or fractured planks or stringers, protruding nails, and missing support blocks is essential. Damaged pallets can cause loading and unloading problems; for example, loose stringers can get hung up on the pallet racks, which can cause loads to fall from high positions. Also, faulty pallets can cause obstruction problems in flow systems by jamming certain pallet rack designs.
- Always ensure that the proper motorized equipment is being used for the application.
- Do not obstruct the end of aisles by staging pallets in these areas. Doing so can cause severe and potentially fatal injuries and accidents.
- Never overload or exceed the recommended load specifications for a racking system. Overloading may cause a catastrophic failure of your storage rack system.
[edit] References
This article incorporates text from Material Handling Exchange (http://www.m-h-e.com/pallet-rack-definition.html) [1]) (Retrieved with permission February 2007)
