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.

07/06/2001


Plant Engineering - January 2001 MATERIAL HANDLING

Feature article
: Comparing unit load storage racks

S idebar:
Comparing rack operation

Table:
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.

          Types
          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

          <table ID = 'id4522105-112-table' CELLSPACING = '0' CELLPADDING = '3' BORDER = '1' STYLE = 'background-color: #FFFFFF'><tbody ID = 'id4522115-112-tbody'><tr ID = 'id4522117-112-tr'><td ID = 'id4522119-112-td' VALIGN = 'top' STYLE = 'background-color: #F8F2E6'></td></tr><tr ID = 'id4522127-114-tr'>Standard pallet (selective)Double (two) deepBridge acrossDrive inDrive throughGravity flowPush backMoveableCantilever</tr><tr ID = 'id4522195-135-tr'><td ID = 'id4522197-135-td'>
          Relative installed cost/unit load</td><td ID = 'id4522204-137-td'>
          $$</td><td ID = 'id4522210-139-td'>
          $$</td><td ID = 'id4522216-141-td'>
          $</td><td ID = 'id4522223-143-td'>
          $$$</td><td ID = 'id4522230-145-td'>
          $$$$</td><td ID = 'id4522236-147-td'>
          $$$$</td><td ID = 'id4522243-149-td'>
          $$$$</td><td ID = 'id4522250-151-td'>
          $$$$$</td><td ID = 'id4522256-153-td'>
          $$$</td></tr><tr ID = 'id4522264-156-tr'><td ID = 'id4522266-156-td'>
          Footprint</td><td ID = 'id4522272-158-td'>
          Large</td><td ID = 'id4522279-160-td'>
          Medium</td><td ID = 'id4522286-162-td'>
          None</td><td ID = 'id4522292-164-td'>
          Small</td><td ID = 'id4522299-166-td'>
          Small</td><td ID = 'id4522306-168-td'>
          Small</td><td ID = 'id4522312-170-td'>
          Medium</td><td ID = 'id4522319-172-td'>
          Small</td><td ID = 'id4522326-174-td'>
          Large</td></tr><tr ID = 'id4522334-177-tr'><td ID = 'id4522336-177-td'>
          Storage density</td><td ID = 'id4522342-179-td'>
          Low</td><td ID = 'id4522349-181-td'>
          Medium</td><td ID = 'id4522355-183-td'>
          Varies</td><td ID = 'id4522362-185-td'>
          High</td><td ID = 'id4522369-187-td'>
          High</td><td ID = 'id4522375-189-td'>
          High</td><td ID = 'id4522382-191-td'>
          Medium</td><td ID = 'id4522389-193-td'>
          High</td><td ID = 'id4522395-195-td'>
          Low</td></tr><tr ID = 'id4522403-198-tr'><td ID = 'id4522405-198-td'>
          Throughput</td><td ID = 'id4522412-200-td'>
          High</td><td ID = 'id4522418-202-td'>
          Medium</td><td ID = 'id4522425-204-td'>
          Medium</td><td ID = 'id4522432-206-td'>
          Medium</td><td ID = 'id4522438-208-td'>
          Medium</td><td ID = 'id4522445-210-td'>
          High</td><td ID = 'id4522452-212-td'>
          Medium</td><td ID = 'id4522458-214-td'>
          Low</td><td ID = 'id4522465-216-td'>
          Medium</td></tr><tr ID = 'id4522472-219-tr'><td ID = 'id4522475-219-td'>
          Effectiveness of space use</td><td ID = 'id4522481-221-td'>
          Fair</td><td ID = 'id4522488-223-td'>
          Good</td><td ID = 'id4522494-225-td'>
          Good</td><td ID = 'id4522501-227-td'>
          Very good</td><td ID = 'id4522508-229-td'>
          Very good</td><td ID = 'id4522514-231-td'>
          Excellent</td><td ID = 'id4522521-233-td'>
          Good</td><td ID = 'id4522528-235-td'>
          Excellent</td><td ID = 'id4522534-237-td'>
          Fair</td></tr><tr ID = 'id4522542-240-tr'><td ID = 'id4522544-240-td'>
          Load accessibility</td><td ID = 'id4522551-242-td'>
          Excellent</td><td ID = 'id4522558-244-td'>
          Fair</td><td ID = 'id4522564-246-td'>
          Excellent</td><td ID = 'id4522571-248-td'>
          Poor</td><td ID = 'id4522578-250-td'>
          Poor</td><td ID = 'id4522584-252-td'>
          Fair</td><td ID = 'id4522591-254-td'>
          Fair</td><td ID = 'id4522598-256-td'>
          Good</td><td ID = 'id4522604-258-td'>
          Excellent</td></tr><tr ID = 'id4522612-261-tr'><td ID = 'id4522614-261-td'>
          Load rotation</td><td ID = 'id4522621-263-td'>
          FIFO</td><td ID = 'id4522627-265-td'>
          LIFO</td><td ID = 'id4522634-267-td'>
          FIFO</td><td ID = 'id4522641-269-td'>
          LIFO</td><td ID = 'id4522647-271-td'>
          Either</td><td ID = 'id4522654-273-td'>
          FIFO</td><td ID = 'id4522661-275-td'>
          LIFO</td><td ID = 'id4522667-277-td'>
          FIFO</td><td ID = 'id4522674-279-td'>
          FIFO</td></tr><tr ID = 'id4522682-282-tr'><td ID = 'id4522684-282-td'>
          Aisles</td><td ID = 'id4522691-284-td'>
          Many</td><td ID = 'id4522697-286-td'>
          Medium</td><td ID = 'id4522704-288-td'>
          None</td><td ID = 'id4522711-290-td'>
          Few</td><td ID = 'id4522717-292-td'>
          Few</td><td ID = 'id4522723-294-td'>
          Few</td><td ID = 'id4522730-296-td'>
          Few</td><td ID = 'id4522736-298-td'>
          Few</td><td ID = 'id4522743-300-td'>
          Many</td></tr><tr ID = 'id4522750-303-tr'><td ID = 'id4522752-303-td'>
          Maximum unit loads deep/opening</td><td ID = 'id4522759-305-td'>
          1</td><td ID = 'id4522765-307-td'>
          2</td><td ID = 'id4523535-309-td'>
          1</td><td ID = 'id4523541-311-td'>
          15</td><td ID = 'id4523548-313-td'>
          10</td><td ID = 'id4523554-315-td'>
          15</td><td ID = 'id4523561-317-td'>
          5</td><td ID = 'id4523567-319-td'>
          1</td><td ID = 'id4523573-321-td'>
          1</td></tr><tr ID = 'id4523581-324-tr'><td ID = 'id4523583-324-td'>
          Maximum stacking height, ft</td><td ID = 'id4523589-326-td'>
          40</td><td ID = 'id4523596-328-td'>
          40</td><td ID = 'id4523602-330-td'>
          25</td><td ID = 'id4523609-332-td'>
          30</td><td ID = 'id4523615-334-td'>
          30</td><td ID = 'id4523621-336-td'>
          30</td><td ID = 'id4523628-338-td'>
          30</td><td ID = 'id4523634-340-td'>
          30</td><td ID = 'id4523641-342-td'>
          20</td></tr><tr ID = 'id4523648-345-tr'><td ID = 'id4523650-345-td'>
          Pallet selectivity, %</td><td ID = 'id4523657-347-td'>
          100</td><td ID = 'id4523663-349-td'>
          50</td><td ID = 'id4523669-351-td'>
          50 or 100</td><td ID = 'id4523676-353-td'>
          20</td><td ID = 'id4523682-355-td'>
          30</td><td ID = 'id4523689-357-td'>
          20</td><td ID = 'id4523695-359-td'>
          40</td><td ID = 'id4523701-361-td'>
          100</td><td ID = 'id4523708-363-td'>
          100</td></tr><tr ID = 'id4523715-366-tr'><td ID = 'id4523717-366-td'>
          Applicable material handling equipment*</td><td ID = 'id4523724-368-td'>
          W, N, VNA</td><td ID = 'id4523730-370-td'>
          N</td><td ID = 'id4523737-372-td'>
          W, N</td><td ID = 'id4523743-374-td'>
          W, N</td><td ID = 'id4523749-376-td'>
          W, N</td><td ID = 'id4523756-378-td'>
          W, N</td><td ID = 'id4523762-380-td'>
          W, N</td><td ID = 'id4523769-382-td'>
          W, N</td><td ID = 'id4523775-384-td'>
          VNA</td></tr></tbody></table>


          *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
          www.advancestorage.com

          Brute Fabricators
          www.bruterack.com

          W.C. Cardinal
          www.wccardinal.com

          EGA Products
          www.egaproducts.com

          Engineered Products
          www.engprod.com

          Frazier Industrial
          www.frazier.com

          Frick-Gallagher
          www.frickgallagher.com

          Hi-Line Storage Systems
          www.hi-linerack.com

          Hohl Machine & Conveyor
          www.hohl.com

          Interroll
          www.interroll.com

          Jarke
          www.jarke.com

          Kardex Systems
          www.kardex.com

          Keneco
          www.kenecoinc.com

          Kornylak
          www.kornylak.com

          LB Intl.
          www.loadbank.com

          Lyon Metal Products
          www.lyonmetal.com

          Modern Equipment

          Penco Products
          www.pencoproducts.com

          Prestrack
          www.prestrack.com

          Republic Storage Systems
          www.republicstorage.com

          Ridg-U-Rak
          www.ridgurak.com

          Schaefer Systems
          www.ssi.schaefer-us.com

          SJF Material Handling
          www.sjf.com

          Spacerak Div., Eugene Welding
          www.spacerack.net

          Stanley Vidmar
          www.stanleyvidmar.com

          Steel King Industries
          info@steelking.com

          Titan Rack
          www.titanrack.com

          Unarco Material Handling
          www.unarcorack.com

          United Steel Products
          www.usprack.com

          Vestil Manufacturing
          www.vestil.com

          Western Pacific Storage Systems
          www.wpss.com

          Wireway Husky
          www.wirewayhusky.com

          Worksmart Systems
          www.worksmartsystems.com

          [ Back to top ]





No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
2016 Product of the Year; Diagnose bearing failures; Asset performance management; Testing dust collector performance measures
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me