2.4 Warehousing and Storage
2.4.3 Storage and Handling Systems
Storage and handling systems fall into two main categories: palletized and nonpal-letized. Some of the most common examples of the various types of storage and handling equipment available for these two systems are introduced in this section.
Palletized Storage and Handling Systems
As mentioned earlier, the most frequently used unit load in warehouses is the wooden pallet. Wooden pallets are popular mainly because they allow the use of standard storage and handling equipment, regardless of the size and characteristics of the goods on the pallet. Products in these types of systems either arrive on pal-lets or are palletized at the receiving areas, so they can benefit from the conve-nient-size load for their movement and storage.
Pallet Movement
There exists a wide variety of equipment for horizontal movement of pallets in warehouses. The most regular types include the following:
G Pallet trucks generally fall into two types: hand pallet trucks and powered pallet trucks.
Hand pallet trucks are manually operated trucks with two forks and a steering unit. The forks enter into the pallet slots and then can be raised slightly by a hand-operated hydrau-lic pump or by using a mechanical system of levers until the load is lifted off the floor, ready for transportation. Powered pallet trucks are similar to hand pallet trucks, except
they are electrically powered. They can be pedestrian- or rider-controlled, depending on their design. These types of trucks are faster than hand pallet trucks, and they are more suitable for moving greater loads over longer distances and more frequently[30,31].
G Tow and platform trucks are suited for horizontal movements of loads over long dis-tances, and they can be manually operated or battery powered. Using a tug towing several trailers can reduce the number of required journeys in warehouses. In situations where fumes or oil spillage may not be a hazard to operators or cause contamination of pro-ducts, diesel tow trucks can also be used[30].
G Conveyors are devices suited for continuously transporting material, especially where unit loads are uniform and the path and rate of their movements tend to remain unchanged.
Thus, conveyors are used mainly in cases where the frequent movement of material between specific points is required and the flow volume would justify the conveyors’
fixed costs. Conveyors may be driven using some source of power (traction conveyors) or without power—for example, as with gravity roller conveyors known as tractionless con-veyors. The latter is suitable for short distances, while the former is suited for longer and more controlled movement of material[30].
G Automated guided vehicles (AGVs) are driverless battery-powered vehicles controlled by computer. They are mainly used for material movement between determined points and in cooperation with other handling systems such as conveyors. They are programmable devices, and a variety of means may be used for their guidance such as underfloor wires or magnets, optical guidance through painted lines or strips, or, more recently, by laser-guided systems. AGVs benefit from obstacle detectors, so when there is an obstacle (e.g., a person or a truck) in their way, they stop[3,31].
These types of equipment are mainly used to move material horizontally.
However, as today’s warehousing usually involves with stacking, some lifting mechanism should be applied to place the pallets into their storage positions. We describe stacking equipment for palletized loads in the following section, but note that many of these lifting trucks are also commonly used for horizontal movement around the warehouse.
Pallet Stacking
To utilize the warehouse space more efficiently nowadays, pallet stacking methods are employed in most warehouses. Pallets are placed on top of each other or, more commonly, they are placed in storage racks by means of equipment capable of lift-ing pallets or loads. Again, a wide range of stacklift-ing equipment and liftlift-ing trucks exist that may also be used to horizontally move goods around the warehouse.
Some of the more common types are as follows:
G Counterbalanced forklift trucks (CB truks) carry their loads outside the chassis area and forward of the front wheels so that they can accommodate a great range of loads.
Because a lack of counterbalance would produce a turning moment that would tend to tip the truck forward, all the truck’s heavy components such as the engine and battery are located at the rear of the machine. A counterbalance weight also is built there to balance the overturning movement. Counterbalanced trucks are one of the most common devices used in warehousing because they are general purpose machines that can handle many different types of loads with the aid of various auxiliary attachments. CB trucks are robust and flexible, and they are available off the shelf from many different suppliers and in a
large range of capacities, ranging from 1 ton to about 45 tons. However, because they are large and heavy trucks, wide aisles of about 3.5 meters or more are required for their operation. Therefore, although counterbalanced trucks are great “yard trucks” for loading and unloading the vehicles, they are less suitable for indoor warehousing activities[3,30].
G Pallet stackers are quite similar to pallet trucks, except they have a greater range of lift-ing motion and can do more than just lift a pallet high enough to move it. A wide range of pallet stackers are available from simple pedestrian or so-called walkie stackers to stand-on and ride-on types. To store or retrieve pallets from racks, the truck legs are straddled around the bottom pallet or driven into the space under it. Unlike counterba-lanced trucks, pallet stacker trucks are fairly lightweight, and they can operate even in 90-degree turning aisles of 2 meters or less. The maximum capacity of these trucks is usually up to around 2 tons, and their lift height limitation is about 6 meters[3,24].
G Reach trucks are one of the most popular types of material-handling devices, lighter and smaller than the counterbalanced trucks. They carry their load within their wheelbase, so they can maneuver and operate in relatively smaller areas. In fact, typical models of reach trucks can handle standard pallets in aisles of between 2.7 and 3 meters. To place loads into or to retrieve them from storage racks, the truck turns 90 degrees facing the load location; a mast with a pantograph or scissors mechanism is then used to reach the storage rack to place or retrieve the load. Then the mast is retracted into the area enclosed by the wheels;
hence, when the truck travels, the load rests on the outrigger arms. Therefore, unlike CB trucks, no counterbalance weight is required and the truck length can be reduced. However, to support the load when the mast is extended beyond the outrigger arms, the counterbal-ance of the truck is used. Modern reach trucks are available with lift heights of up to 11.8 meters, and their capacities typically range from 1 ton up to 3.5 tons[30,32].
G Turret trucks provide greater stacking height (up to 12 meters or so), but they also require greater investment cost. Turret trucks are suitable for narrow aisles and confined spaces.
In fact, they can operate in aisles of between 1.5 and 2 meters. The rotating forks and mast allow the pallets to be picked up and retrieved from either side of the storage aisle.
The truck itself does not rotate during the storage or retrieval, so their bodies should be longer to increase their counterbalance capabilities.
G Specialist pallet stacking equipment exists in a wide range of equipment for specific types of storage and handling systems. These include narrow-aisle trucks, which have minimum aisle width requirement; double-reach trucks with telescopic forks that allow two pallets to be handled from the same side of an aisle; and stacker cranes, which are commonly used in automated storage and retrieval systems (AS/RSs).7
Nonpalletized Storage and Handling Systems
There are many types of products that cannot be palletized because of their special characteristics. Being too large, too long, or even too small or having some han-dling limitations such as being lifted from the top, many types of products are not suitable for palletization. Some electronic items, nut and bolts, steel bars, drums, paper reels, hanging garments, and carpets are examples. This section introduces some of the equipment and devices that are commonly applied in various storage and handling systems of these items.
7 Further details about these devices, along with the appropriate storage systems are described in the chapter 15 of reference [3].
Truck Attachments
A wide range of attachments are available for fitting to forklift trucks so that they can handle goods that cannot be touched by forks. These truck attachments enable additional degrees of movement for handling unit loads. However, the extra weight of these attachments should be considered in calculating the payload capacity of trucks and in determining the weight that can safely be carried by the truck. Some of the most common types follow:
G Clamp attachments can be used instead of forks to handle loads such as cardboard boxes, cartons and bales, drums, kegs, paper reels, and similar materials. They consist of shaped or flat side arms that are powered by a truck’s hydraulic system. The side-arm pressure is adjustable, but the clamps often exert severe pressure on loads, so they must have enough strength to resist such pressure[3,30].
G Rotating head attachments are suitable for situations in which the orientation of loads needs to be changed. A common example is newsprint reels, which are stored with their axes vertical, even though they are presented horizontally to printing machines[3].
G Load push pull devices are alternatives to forks, by which materials are positioned on slip sheets. Slip sheets take up little space in warehouses or shipping containers, and they avoid the cost of one-way pallets. Also, such devices enable nonpalletized loads to be pulled on to a platen, so they can be lifted, moved, and placed effectively as if they were a pallet load.
The major drawback of this type of material handling is its longer operation time[30].
G Drum-handling equipment are devices for handling drums or barrels. There are several types of forklift truck attachments, including cradle attachments, horizontal carry attach-ments, vertical carry attachattach-ments, and drum tines. These attachments are used to lift a number of horizontally or vertically oriented drums at one time.
G Booms are available in various types. They replace the truck forks for handling rolled items, such as carpet rolls, steel coils, and horizontal reels.
Cranes
The typical application of cranes in warehousing is for handling very heavy loads, such as metal bars, or for relatively lighter loads that are just too heavy to be han-dled manually. Cranes can provide movement of loads only in limited and predeter-mined areas. Some of the more commonly used types are as follows.
G Bridge cranes or overhead traveling cranes consist of a hoist mounted on a bridge made up of one or more horizontal girders that are supported at each end by carriages that travel along a pair of parallel runways that are installed at right angles to the bridge. The hoist moves along the bridge, while the bridge itself travels along the runway, so that the working area is fully covered[31].
G Gantry cranes consist of hoists fitted in a traverse trolley that can move horizontally along the bridge section and supporting columns or legs at each end of the bridge. These legs may run on ground rails, or they may be supported on wheels that allow the whole crane to traverse. Therefore, mobile gantry cranes are also suitable for outdoor applications.
G Jib cranes are lifting devices that travel along a horizontal boom—a pivoted arm called the jib—that is mounted on a pillar or on a wall. These types of crane are relatively inex-pensive and can handle loads within the arc defined by the radial jib[3,31].
G Stacker cranes have a forklift-type mechanism and are usually used in automated storage and retrieval systems (AS/RSs). The crane traverses on a track in the warehouse aisles, and the fork can then be lowered or raised to any levels of storage racks on either side of the aisle. The fork can then be extended into the rack to store or retrieve products.
Typical types of crane are electrically powered, and they may be used with a wide range of attachments for specific purposes. Different types of hooks, magnets, and mechanical clamps are common examples of these attachments.
Conveyors
As mentioned previously, conveyors are widely used in warehousing to move both palletized and nonpalletized loads over fixed routes. They may also be used to sort or accumulate products (short-term buffering) or as an integral part of packaging and order-picking operations. In general, two types of conveyer exist: nonpowered or gravity conveyers and powered conveyors.
In comparison to powered conveyors, gravity conveyors are more basic and less costly. They are normally used to move loads weighing up to several tons over short distances. The major types of gravity conveyors include the following[31,32].
G Spiral chute conveyors are normally used to convey goods between floors or to link two handling devices. They may have double or triple spiral runways for sorting and transfer-ring items to different levels.
G Wheel conveyors are constructed of a series of skate wheels mounted on a shaft or com-mon axle. The spacing of the wheels and the slope for gravity movement depend on the type and weight of the loads being transported. Wheel conveyors are commonly used for vehicle loading and unloading, and they are more economical than roller conveyors for light-duty applications.
G Gravity roller conveyors are an alternative to skate-wheel conveyors for heavy-duty applications. Roller conveyors can handle items with a rigid riding surface, and their slope depends on the load weight.
Powered conveyors are normally used for moving heavier loads over longer dis-tances. Some of the more frequently used types include the following:
G Live roller conveyors comprise a series of rollers, and they are suitable for moving heavy goods or loads with irregular shapes and sharp corners. To provide accumulation, rollers can be disengaged using force-sensitive transmission features. Powered roller conveyors can move loads horizontally and up 5- to 7-degree slopes[31,32].
G Belt conveyors comprise a continuous belt running on supporting rollers that provides complete support under the loads being transported. They are generally used for moving light and medium weight loads, and they provide considerable control over their orienta-tion and placement. Belt conveyors are suitable for paths with inclines or declines, and they can move loads with unusual shapes and configurations; however, no accumulation is provided[3,32].
G Slat conveyors consist of separately spaced slats, and they are generally used for heavy loads with abrasive surfaces that may damage the belt. They can provide control over the orientation and placement of the load[31,32].
G Chain conveyors carry loads directly on one or more endless chains. They are primarily used to move heavy loads or to transfer loads between sections of roller conveyors[3,31].
G Trolleys conveyors consist of chains or cable suspended from equally spaced trolleys run-ning in a closed loop path. Overhead trolley conveyors can handle loads up to several tons, and they are commonly used in systems with fixed path and paced flow[31].
Automated Guided Vehicles
Automated guided vehicles are introduced briefly under the section “Palletized storage and handling systems”. In addition to palletized loads, AGVs may be used for moving nonpalletized loads, especially large and heavy loads, including paper reels and automobile bodies. There exists a wide range of AGV types, and they can be guided using physical guide path (such as wire, tape, paint), or by nonphysical guide path (software). Changing vehicle path in latter method is easier, because the path is not physically fixed; however, absolute position estimates (e.g., from lasers) are required in those methods[3,32].
References
[1] Council of Supply Chain Management Professionals. Supply chain management defini-tions [online].http://cscmp.org/aboutcscmp/definitions.asp, 2000 (cited 14.11.10).
[2] J.C. Johnson, D.F. Wood, D.L. Wardlow, P.R. Murphy, Contemporary Logistics, sev-enth ed., Prentice Hall, Upper Saddle River, NJ, 1999, pp. 1 21.
[3] A. Rushton, P. Crouche, P. Baker, The Handbook of Logistics and Distribution Management, third ed., Kogan Page, London, 2006.
[4] S.C. Ailawadi, R. Singh, Logistics Management, Prentice Hall of India, New Delhi, 2005.
[5] R.H. Ballou, Business Logistics/Supply Chain Management: Planning, Organizing, and Controlling the Supply Chain, fifth ed., Pearson-Prentice Hall, Upper Saddle River, NJ, 2004.
[6] A. West, Managing Distribution and Change: The Total Distribution Concept, John Wiley & Sons, New York, 1989, pp. 181 195.
[7] J.R. Stock, D.M. Lambert, Strategic Logistics Management, fourth ed., Irwin McGraw-Hill, New York, 2001.
[8] G. Ghiani, G. Laporte, R. Musmanno, Introduction to Logistics Systems Planning and Control, John Wiley & Sons, NJ, 2004, pp. 6 20.
[9] M. Hugos, Essentials of Supply Chain Management, John Wiley & Sons, Hoboken, NJ, 2003, pp. 1 15.
[10] D.M. Lambert, J.R. Stock, Strategic Logistics Management, third ed., Irwin, Homewood, IL, 1993.
[11] U.S. Department of Transportation, Research and Innovative Technology Admini-stration, Bureau of Transportation Statistics, and U.S. Census Bureau. 2007 Commodity Flow Survey; Transportation Commodity Flow Survey, preliminary [online]. http://
www.census.gov/compendia/statab/2010/tables/10s1036.pdf, 2008 (cited 14.11.10).
[12] H.T. Lewis, J.W. Culliton, J.D. Steel, The Role of Air Freight in Physical Distribution, Division of Research, Graduate School of Business Administration, Harvard University, Boston, MA, 1956, p. 82.
[13] R.J. Sampson, M.T. Farris, D.L. Shrock, Domestic Transportation: Practice, Theory, and Policy, fifth ed., Houghton Mifflin, Boston, MA, 1985, pp. 68 85.
[14] KnowThis LLC. Principles of marketing tutorials [online]. http://www.knowthis.com/
principles-of-marketing-tutorials/managing-product-movement/modes-of-transportation-comparison, 1998 (cited 14.11.10).
[15] D.A. Deveci, A.G. Cerit, O. Tuna, Determinants of intermodal transport and Turkey’s transport infrastructure, in: METU International Conference in Economics VI. Ankara, Turkey, September 11 14, 2002.
[16] Institute of Logistics, Understanding European Intermodal Transport—A User’s Guide, Guideline No. 4, Institute of Logistics, Corby, UK, 1994.
[17] M. Yan, H. Yu, Logistics solutions for company A’s Latin American market. Research project, Simon Fraser University, Faculty of Business Administration, 2004, pp. 62 73.
[18] P. Kotler, G. Armstrong, J. Saunders, V. Wong, Principles of Marketing, second European ed., Prentice Hall Europe, London, 1999, pp. 894 907.
[19] D. Riopel, A. Langevin, J.F. Campbell, The network of logistics decisions, in:
A. Langevin, D. Riopel (Eds.), Logistics Systems: Design and Optimization, Springer, New York, 2005, pp. 12 17.
[20] M. Browne, J. Allen, Logistics of food transport, in: R. Heap, M. Kierstan, G. Ford (Eds.), Food Transportation, Blackie Academic & Professional, London, 1998, pp. 22 25.
[21] R.G. Kasilingam, Logistics and Transportation: Design and Planning, first ed., Springer, London, 1999, pp. 24 26.
[22] A.C. McKinnon, Physical Distribution Systems, first ed., Routledge, London and New York, 1989, pp. 26 30.
[23] L.E. Boone, D.E. Kurtz, H.F. MacKenzie, K. Snow, Contemporary Marketing, second Canadian ed., Nelson Education Ltd, Toronto, ON, Canada, 2010, pp. 390 395.
[24] J.A. Tompkins, J.A. White, Y.A. Bozer, J.M.A. Tanchoco, Facilities Planning, fourth ed., John Wiley & Sons, Hoboken, NJ, 2010.
[25] J. Drury, Towards More Efficient Order Picking, IMM Monograph No. 1, The Institute of Materials Management, Cranfield, 1988.
[26] J.K. Higginson, J.H. Bookbinder, Distribution centers in supply chain operations, in:
A. Langevin, D. Riopel (Eds.), Logistics Systems: Design and Optimization, Springer, New York, 2005, pp. 71 78.
[27] D. Waters, Logistics: An Introduction to Supply Chain Management, Palgrave Macmillan, New York, 2003, pp. 283 290.
[28] T. Gudehus, H. Kotzab, Comprehensive Logistics, first ed., Springer-Verlag, Berlin, 2009, pp. 342 344.
[29] J.M.A. Tanchoco, Material Flow Systems in Manufacturing, first ed., Chapman & Hall publication, London, 1994, pp. 107 109.
[30] K.C. Arora, V. Shinde, Aspects of Materials Handling, first ed., Laxmi Publications, New Delhi, 2007, pp. 146 170.
[31] K. Bagadia, Definitions and classifications, in: R.A. Kulwiec (Ed.), Materials Handling Handbook, second ed., John Wiley & Sons, New York, 1985, pp. 103 110.
[32] C. Hiregoudar, B.R. Reddy, Facility Planning and Layout Design, first ed., Technical Publications Pune, Pune, Maharashtra, 2007, pp. 31 56.
3 Logistics Strategic Decisions
Maryam SteadieSeifi
Department of Industrial Engineering, Amirkabir University of Technology, Tehran, Iran