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Make storage part of the flow process.

The storage function should really be looked upon as a staging activity in which materials are prepared for subsequent steps in the material flow process.

Gone are the days when storage was just concerned with "getting things out of the way." Also gone are the days when storage was concerned with holding a substantial amount of inventory to ensure that "we didn't run out of stock." Modern strategies such as just-in-time manufacturing, continuous flow, and cross-docking have greatly reduced the amount of goods stored, as well as the amount of time they are stored.

Through the use of buffers, accumulation zones, and related arrangements, the storage activity has taken on more of a "staging" function, whereby materials are held for a short term before being released to the next step in a process, be it assembly, orderpicking, or the like. Following this concept, the acronym AS/RS, which traditionally stood for "automated storage and retrieval systems," might now be replaced with "automated staging and routing systems."

The point is, storage today should be planned to be part of the materials flow process, rather than being a stationary, off-path break in an operation. Modern concepts in manufacturing, warehousing, and distribution require new ways of thinking about the storage function. And in today's facilities, that includes the role information systems play in the storage/staging function.

Planning for storage equipment needs involves first considering the form in which the product will be stored and retrieved. The major classes include small parts (or broken-case items), full cases, and unit or pallet loads.

Unit-load staging

The first series of illustrations shows different choices-essentially variations of storage rack-available for handling unit or pallet loads and some case loads. Most common is the selective pallet rack. One bay of this structure consists of two upright frames and a desired number of horizontal load beams that form storage levels. Each pallet or load faces an aisle in the front and in the rear, for easy retrieval.

As shown in the drawings, installing one rack behind another for two-deep storage of like items saves on aisle space. A load can be accessed from one direction, unless a lift truck is equipped with a double-reach mechanism.

The drive-in rack also reduces the number of aisles, by promoting dense storage of like, non-spoiling goods. For retrieval, the lift truck drives into narrow lanes within the rack for last-in, first-out storage.

The high-density concept is promoted even further with gravity flow racks or high-density flow systems. In a common version of this concept, loads index forward by gravity along downward-pitched wheel or roller lanes. One new variation uses air-cylinder power to move loads in a level lane, without any pitch, thus saving space. Other types keep the loads stationary, and use a shuttle to move in and out of lanes for deposit and retrieval.

A type of rack that is not new, but is now catching on rapidly is the push-back rack. This unit uses structural carts riding on rails to move loads forward in a high-density lane for last-in, first-out storage. Benefits include very high rack utilization, reduced handling time, and greater storage capacity.

Still another new variation involves an attachment that permits lift-truck forks to disengage from the mast and move into deep lanes, using an umbilical cord to supply power. No pitch is required for this high-density system, either.

Finally, automated storage/ retrieval systems (AS/RS) provide real-time material control with random storage under computer control. A floor-running S/R machine carries a shuttle horizontally and vertically to deposit and extract loads automatically within a very-narrow-aisle rack structure.

Parts storage solutions

Part and package storage options begin with the staple of most storage areas, shelving sections, which provide an economical choice for storing light-to-medium weight items. Units can be fitted with bins, drawers, shelf boxes, and doors to provide various levels of storage efficiency, activity, and security. And, double-deck shelving mezzanines can help make efficient use of overhead air spaces.

For high-density storage and efficient organization, modular drawers provide an excellent solution for small items such as electronic parts, hardware, and tools. And, the drawers can be locked for security. They can be fitted in cabinets or within shelving sections, depending on the application.

Horizontal carousels (heavy-duty, industrial versions of dry cleaner carousels) provide high throughput, achieving pick rates of 100 lines per hour. In a manufacturing setting, units can quickly feed parts and materials for kitting to the assembly floor.

Vertical carousels can handle the same type of small parts storage work, but with up to 70% less floor space. On the other hand, basic equipment costs can run up to 40% higher. Ranging in height to 35 ft, these units make good use of overhead space.

Vertical-lift storage modules resemble vertical carousels. However, the bins or trays do not rotate, they are stationary within a vertical housing. An insert/extract mechanism travels vertically on powered chains to provide storage and retrieval action.

Flow rack and flow shelving provide the high-density storage similar to their unit-load counterparts. Totes or packages index forward by gravity, generally along pitched flow lanes. Replenishment is from the rear.

Miniload or microload AS/RS perform the same function as unit-load versions, but within a smaller envelope. An S/R machine equipped with an extractor travels horizontally and vertically in a storage aisle. It moves totes or containers between storage locations and an orderpicking or kitting station that is located at one end of the aisle.

Layout and lift trucks

Once the basic storage devices and systems have been selected, the matter of storage area layout and configuration still remains to be decided. A particular range of options exists with regard to pallet storage racks, aisle size, and type of lift truck used to service the storage area.

At least part of the decision has to be based on throughput and level of activity. So, should you consider a conventional storage layout that typically uses 12 ft wide aisles, and a stacking height of four or five slots? Or, are you better off with a narrow-aisle (NA, 8 ft wide aisles) configuration, that increases the number of pallet positions within the same floor space and stacking height by 50%? Or how about a very narrow aisle (VNA, 5-1/2 ft wide aisles) system that stacks loads nine units high and rises to over 40 ft up?

Obviously the decision also affects the type and cost of lift trucks used to service the storage area. With a conventional system, standard sit-down counterbalanced trucks can be used, both at the receiving dock and at the storage area. And, a more compact version, the standup counterbalanced rider truck, has been dubbed the "dock-to-stock" truck because of its utility in both areas.

Certainly using one type of truck from receiving to storage to shipping provides obvious economies. But if the level of activity warrants it, using narrow-aisle equipment such as reach trucks exclusively for stacking and retrieving duties, while the all-purpose truck is used in other areas of the facility, can also provide economies through greater specialization and efficiency. And the added cost of VNA equipment--such as turret trucks-can be offset under certain high levels of activity by space savings and increased operating efficiencies. So it is a tradeoff that requires careful analysis.

Between the two extremes of conventional and VNA, the layout in the middle-narrow aisle using reach trucks--is one that can fit many requirements and pocketbooks, and should be looked at carefully. And the review should include an evaluation of which system best fits into today's format of continuous flow of materials and information.

RELATED ARTICLE: Storing and staging of unit loads

Single-deep selective pallet rack

Permits load to be stored or retrieved from front or rear.

Double-deep rack

Saves on aisle space by installing two sections back to back. Two-deep retrieval can be done with double-reach lift-truck attachment.

Drive-in rack

Promotes dense storage. Truck drives into lanes within rack for last-in first-out storage.

Push-back rack

Provides high-density storage using structural carts riding on rails to move loads forward within a rack lane.

High-density flow system

indexes loads forward by gravity along wheel or roller lanes. Some versions user air power to move loads level, without gravity or pitch.

Automated storage/retrieval system

Uses floor-running S/R machine to insert and extract loads automatically within narrow-aisle rack system.


RELATED ARTICLE: Ten key points for information flow storage

The following common storage activities offer an opportunity to improve information flow within this function. Review the list and check the column that best describes how data is collected and managed now. If five or more activities are done manually, it's time to consider some type of automatic data collection (ADC)-bar codes, RFDC, RFID, for instance, as well as a warehouse management system (WMS).


Manually Automatically

Identify pallets/cases/items -- -- Assign putaway locations -- -- Direct workers to storage locations: Record putaway location -- -- Record inventory status by SKU and location -- -- Confirm accuracy of putaway -- -- Link putaway to an individual worker -- -- Record each inventory move -- -- Update central database for every change in inventory status/location -- -- Manage replenishment of picking area -- --

RELATED ARTICLE: Storing and staging of small parts

Shelving section

Uses individual shelves that can be adjusted for different-sized parts and packages. Usable height can be extended using mezzanine layouts.

Flow rack or shelving

indexes bins or totes forward along wheel or roller lanes that are pitched to promote gravity flow.

Modular drawers

Provide high-density, organized, and secure storage for parts, tools, and valuable items. Drawers can be stacked in cabinets or used in shelving sections.

Horizontal carousel

Provides closed-loop system for rotating basket or bins that bring parts to a picking or replenishment station.

Vertical carousel

Stores bins of parts on shelves that rotate vertically in a continuous loop, returning bins to front window for picking/replenishment.

Vertical-lift storage module

Contains stationary bins from which parts are picked automatically by a vertical-traveling extractor.

Miniload or microloads AS/RS

Stores parts in totes or containers on both sides of one or more aisles. Horizontal and vertical traveling S/R machine picks totes, delivers them to end of aisle.

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Title Annotation:includes related article on information flow in storage process
Publication:Modern Materials Handling
Date:Apr 15, 1997
Previous Article:Starting off right in receiving; streamlining the inbound flow of materials and data will help improve efficiency of downstream operations.
Next Article:Taking time and cost out of work-in-process handling.

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