Dynamic storage
These flow systems provide a high-density environment for pallets and cartons. Here's where they work best.
By TOM FEARE -- Modern Materials Handling, 10/1/1999
Today's thinking in materials handling emphasizes staging rather than just storing materials in warehouses and distribution centers. Stressed too, is a continuous flow of raw materials and finished goods through the supply chain, streamlining movement to reduce inventory expenses and handling costs.Dynamic storage certainly addresses that profile. Strictly defined, dynamic storage handles loads (pallets, cartons, or cases) in lanes within staging/storage equipment and systems. Loads constantly move within a lane toward a pick face in a warehouse, for example. Or cartons of parts flow toward a point of use on an assembly line or at a work cell.
Dynamic storage thus differs from static storage where loads or items sit on shelving or standard selective pallet rack waiting to be picked or used.
Gravity flow rack is perhaps the most common type of dynamic storage. And at opposite ends of a spectrum are two other, lane-based, load-flow systems: push-back pallet rack and super high-density dynamic storage systems (see graphic).
With push-back racking there's a reversing flow of loads in lanes that are up to six pallets deep. Loads are pushed in, then flow out toward the pick face as each individual load is pulled. Storage density is greater than that for standard racking (see graphic) but less than that for flow rack or for certain, "super" systems.
At the other end of this spectrum of increasing staging/storage density and dynamics of load movement-compared to push-back racking-is very deep lane equipment. Some in the materials handling industry refer to this equipment as "super" HDDS (high-density, dynamic storage) equipment.
A super HDDS system in a DC might be configured so it stores pallet loads in racking that is stacked in lanes as many as seven-high to the ceiling. Meanwhile, individual lanes hold 10, 20, and on up to 60 pallets. Minimal space exists between the pallet loads in a lane.
Movement of loads within a HDDS system's storage/ staging lanes can be by conveyorized transport or on a subsystem of tracks and wheels. Some sort of braking or load control method will be a subsystem component as well. Gravity may power load movement through a system having inclined lanes, for example. Or load transport may be pneumatically or mechanically driven along level lanes.
Super HDDS systems move loads automatically or semi-automatically. These systems require less labor than a push-back rack system where a forklift operator's time to load and unload pallets is necessary.
Push-back rack systems rely upon sliding rack carts or other mechanisms to provide the internal dynamics once the load is within this environment.
High-density a plus
One benefit that each of these three types of equipment delivers is high-density. Old yardsticks of smart handling still apply today such as maximizing use of warehousing floor space with equipment with small footprints and increasing cube utilization by going up as high as possible to the ceiling with systems in a DC. High-density can be a cost saver.
Through the high-density nature of HDDS equipment, however, ready access to specific items is not easily achieved. Any one-of-a-kind pallet load lodged deep within a super HDDS storage/staging lane is hard to get at on short notice, unless prior efforts are made to separate it and store/stage it elsewhere in the DC.
Push-back racking offers more load selectivity than the denser HDDS systems, however, through storage of fewer pallet loads in a lane. There's a reverse flow to push-back rack, and product rotates on a last-in, first-out (LIFO) basis.
Determining needs
Potential end users of dynamic storage technology should start any determination of whether these systems are suitable for their application(s) by examining and listing their storage/staging requirements. Consideration should be given to both peak and average conditions. Included are: (1) Mix of SKUs (stockkeeping units) and quantities of each SKU. (2) Product rotation needs. (3) Throughput volume or the velocity of products to be moved through a system per day or per shift. (4) Floor space available, ceiling clear height dimension. (5) Inventory turns desired. (6) Compatibility of pallets to be used within the potential system's flow capability-cheap wooden pallets may create problems in some automated systems!
Rigid rules are hard to make on which system type best fits a certain set of requirements. Generally, however, the fewer the unit loads per SKU to be staged/stored, the more the end user will lean toward using a shallow depth HDDS system, a push-back rack, for example. Accessibility to specific lot numbers, date coded items, and the like is achieved more readily with shallow systems.
In contrast, when there are many unit loads to stage per SKU and control over lot number/date code of specific loads is less vital-or, put another way, when products can be managed in batches and serialized-then a deeper lane system provides this kind of distribution environment.
FIFO product rotation requirements tend to dictate a choice of the deeper systems, all other factors being equal. Some super HDDSs will give you both FIFO and LIFO.
High throughput needs and HDDS go hand in hand, with the deeper lane systems delivering the higher velocities of product flow with less need for labor or forklifts to move loads.
Where there are needs to maintain throughput at a high rate, yet to keep inventory levels low, flow rack systems-for pallets or cases-perform well. They provide productivity in order picking modules (graphic) and, in the case of carton flow systems, increasingly are seen as an efficient, space-saving solution to supplying materials and parts on a just-in-time basis to assembly lines and work cells in manufacturing. These flow rack systems also can be used to queue sorted or sequenced finished subassemblies ready for JIT delivery to the next plant in a stepwise process, as we saw recently at one factory.
Retrofitting pallet rack to turn it into carton flow rack has been one trend recently, particularly where needs change and more loads need to move through the supply chain in cartons or cases instead of unit load quantities. Check with suppliers of flow rack to see how this will work for your system.
Finally, there are the issues of matching up pallet type with a specific system and avoiding "hangups" or jams of an occasional pallet, particularly within the more automated systems. Finding your best solution to this problem is a complex matter-load weight, quality of pallet construction, type of pallet material used (soft or hard woods or plastic), and other factors all enter into the equation of what works, what doesn't. We encourage you to explore this issue in depth with potential system vendors along with other points made in this article.
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