Comparing unit load storage racks

You should consider these factors to determine the unit load storage rack system best suited to an application from both an economic and efficiency standpoint: characteristics of loads stored; accessibility, or selectivity, requirements; volume, throughput and rotation factors; material handling equipment and plant configuration. This article discusses these and other considerations.
By Plant Engineering Staff July 6, 2001

Plant Engineering – January 2001 MATERIAL HANDLING

Feature article
: Comparing unit load storage racks

S idebar:
Comparing rack operation

Comparative features of various storage racks

Manufacturers list:
Unit load storage rack manufacturers

Ron Holzhauer, Managing Editor, Plant Engineering Magazine

  • Load characteristics, accessibility, volume, throughput, and rotation are major factors in rack selection..

  • Pallet, bridge across, double deep, drive in and through, gravity flow, push back, moveable, and cantilever racks are available.

    • Several factors combine to determine the unit load storage rack system ( Fig. 1 ) best suited to an application from both economic and efficiency points of view.

    • Characteristics of loads stored. What products are being handled, and what are their dimensions and weights? How many pallets are required, and what is their condition? Pallets are available in several configurations, and wood or plastic construction.

    • Accessibility (selectivity) requirements. How many stock picking locations must be immediately available? Storage depths of one to more than ten pallets are available from various types of rack systems.

    • Volume, throughput, and rotation factors. How much material is maintained at any given time, and what is the time frame for moving it through the storage area? How is inventory control maintained: first in first out (FIFO) or last in first out (LIFO)?

    • Material handling equipment. Is the lift equipment counterbalanced, straddle, or outrigger; and wide, narrow, or very narrow aisle? What is the vehicle’s lifting height, capacity, and turning radius; and does it include a pantograph device for double-deep reaching? Will new equipment be purchased to satisfy a rack system? Higher racks and deeper storage require more expensive vehicles. Make sure vehicle operators are trained according to OSHA requirements and plant needs.

    • Plant configuration. Are there overhead obstructions or building columns to work around, and what is the ceiling height and floor condition? Also consider aisle requirements, fire protection, and seismic factors.

      • The relative merits of this information determine rack capacity and height, and beam and upright requirements. Most rack manufacturers offer computer programs to assist in the sizing and selecting processes.

        Other considerations, such as selectivity and handling equipment, determine the type of rack, number of aisles, picking speed, and storage density. For example, the greater the density, the less the selectivity and the slower the picking speed. Or, the greater the selectivity, the more floor space (aisles) required, compared to equal-capacity alternative storage approaches.

        Racks are as high as 40 ft, but 12%%MDASSML%%18 ft is more common. At the lower height, four or five storage positions (floor plus three or four) are commonly available.

        Typically, pallets hold 2000%%MDASSML%%3000 lb. Front and back cross beams are usually 96-in. or 108-in. wide, which accommodates two standard 40-in. 3 48-in. pallets placed in either orientation. Larger beams with higher capacities are available, but never exceed rated capacity. Rack openings are easily adjusted to handle the size of stored loads.

        It is not uncommon to find several different types of racks in the same facility. For example, one model might be used to temporarily store incoming raw materials, while another design is used for outgoing finished products.

        Racks are available in a variety of colors, constructed from structural or roll-formed steel, and anchored to the floor. The racks, particularly vertical uprights, should be visually checked on a regular basis for damage from vehicles. Various types of guards are available for rack protection. Consult the manufacturer before repairing a damaged rack.

        There are several types of unit load storage rack configurations available: pallet (selective), bridge across, double (two) deep, drive in and through, gravity flow, push back, moveable, and cantilever. The same basic rack is used for most of the options.

        Pallet racks are the most common type. They are relatively inexpensive and offer 100% selectivity and excellent picking efficiency, because each pallet sits in a front position. However, pallet racks provide low storage density because of the many aisles required, are more labor intensive, and require additional floor space.

        Bridge across is another version of the pallet rack. Lower beams are removed to create an aisle and material is stored over the passageway.

        Double-deep racks ( Fig. 2 ) feature the same basic configuration and components as pallet racks. However, a second row of storage is added, which creates four unit loads per bay opening. Storage density increases substantially, but selectivity and throughput decline. Reach trucks must be used for back row pallet placing.

        Drive-in racks allow vehicles to enter the storage area for pallet placement in a front-to-back approach. Two sections may be placed back-to-back, or one could butt against a wall. In either case, vehicles enter and exit from the same aisle. Density is excellent, but selectivity is quite limited. Vehicle drivers must exercise great care since they are operating within the rack system. No other approach provides more storage capacity per square foot of room.

        Drive-through racks are similar in components and features, except there are two access aisles. This type is appropriate where there is expiration-dated material or when shelf life is a major concern.

        Gravity flow racks use one aisle for pallet or carton deposit, and a second for retrieval. The load is placed on skate wheels or a roller conveyor, and the rack is inclined slightly downward toward the withdrawal end. Storage density is good, but there are a few moving parts to contend with and the higher cost could be a factor.

        Push back racks are another type of gravity system. In this approach, a single aisle is used, and pallets are placed on a carrier and pushed up the slight incline by the lift truck. Push back racks are often placed against a wall.

        Moveable racks ( Fig. 3 ) sit on a carriage that allows the creation of an aisle wherever needed to reach the desired stored products. Controls are placed on the supports or vehicle to move the carriage, access the desired load, and continually update inventory levels. This approach provides high density and selectivity, but low throughput. Safety beams, warning lights, and horns alert employees of carriage movement.

        Cantilever racks ( Fig. 4 ) are designed for storing long, bulky, awkward items, such as carpeting, tubing, pipe, or lumber. Single or double-faced support arms handle about 3000 lb, and have solid decking for holding palletized or nonpalletized loads. Selectivity is high, but density is low.

        The table below compares several characteristics of the various types of unit load storage racks.

        Comparative features of various storage racks

        Standard pallet (selective)Double (two) deepBridge acrossDrive inDrive throughGravity flowPush backMoveableCantilever
        Relative installed cost/unit load $$ $$ $ $$$ $$$$ $$$$ $$$$ $$$$$ $$$
        Footprint Large Medium None Small Small Small Medium Small Large
        Storage density Low Medium Varies High High High Medium High Low
        Throughput High Medium Medium Medium Medium High Medium Low Medium
        Effectiveness of space use Fair Good Good Very good Very good Excellent Good Excellent Fair
        Load accessibility Excellent Fair Excellent Poor Poor Fair Fair Good Excellent
        Load rotation FIFO LIFO FIFO LIFO Either FIFO LIFO FIFO FIFO
        Aisles Many Medium None Few Few Few Few Few Many
        Maximum unit loads deep/opening 1 2 1 15 10 15 5 1 1
        Maximum stacking height, ft 40 40 25 30 30 30 30 30 20
        Pallet selectivity, % 100 50 50 or 100 20 30 20 40 100 100
        Applicable material handling equipment* W, N, VNA N W, N W, N W, N W, N W, N W, N VNA

        *W = wide aisle; N = narrow aisle; VNA = very narrow aisle

        Manufacturers list:
        Unit load storage rack manufacturers

        The following companies provided input to this article by responding to a written request from Plant Engineering magazine. For more information about their storage rack product lines, check out their web sites.

        Advance Storage Products

        Brute Fabricators

        W.C. Cardinal

        EGA Products

        Engineered Products

        Frazier Industrial


        Hi-Line Storage Systems

        Hohl Machine & Conveyor



        Kardex Systems



        LB Intl.

        Lyon Metal Products

        Modern Equipment

        Penco Products


        Republic Storage Systems


        Schaefer Systems

        SJF Material Handling

        Spacerak Div., Eugene Welding

        Stanley Vidmar

        Steel King Industries

        Titan Rack

        Unarco Material Handling

        United Steel Products

        Vestil Manufacturing

        Western Pacific Storage Systems

        Wireway Husky

        Worksmart Systems

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