Powered systems for small parts
Here's an overview of carousels, vertical lift storage modules, and AS/R systems for secure, high-density storage and more productivity.
By Tom Feare -- Modern Materials Handling, 8/1/1999
When your manufacturing or warehousing operation requires storing or staging of parts and packages, and these items are small in contrast to big, or unit load in size, there are a number of materials handling solutions that come to mind.Let's further narrow the definition of the requirements for your system to equipment that provides high-density staging or storing of these small items. Shop floor or warehousing space is limited in your existing facility, for example. So you want to keep the system footprint small. Or let's say you need to find the most cost-effective use of floor space and/or get the best cube utilization (maximize use of overhead space) that you can for the dollars which will be invested in new construction.
Finally, in today's tight job market for skilled labor, with relatively high wages paid to warehouse workers and plant operators, you want to get the most productivity out of the employees whose work will interface with or be dependent upon the small parts/loads system that you select.
Preferred is a handling solution that will reduce to a minimum any "walk and search" time spent by machine tool operators in the plant or by orderpickers in the distribution center or warehouse. In some cases, as much as two-thirds of an operator's or picker's time can be wasted on travel time back and forth from the storage equipment with some low-density, low technology handling solutions.
With the system parameters set thus far, you'll want to consider point-of-use or part-to-picker materials handling solutions. Generally, these solutions fall under the heading of powered or automated storage/ staging systems.
The powered component of these systems does the "walking," while computer hardware and software automating them provide the smarts and memory to carry out the swift, accurate searching and retrieving of items stored.
Horizontal carousels, vertical carousels, vertical lift storage modules (VLSMs), and the miniload and microload (as contrasted with unit load) versions of automated storage/retrieval systems (AS/RSs) all stage/store small items in high-density configurations fitting our parameters.
"Small," at this point might be better defined as less-than-caseload in size. Each of the four kinds of systems can be configured to hold varying shapes and weights of "small" items as well.
The four system technologies tend to maximize floor space and cube utilization, to varying degrees, and increase operator productivity as we'll see in more detail in the following.
Each type of system also provides a high degree of security and safety for items that, though they're small, may be costly. When staged or stored in these systems, parts or small packages are inherently protected from "inventory shrinkage." Active security measures may be added to enhance the protection already afforded by the passive security inherent in the systems should you need to safeguard very expensive items, such as computer chips, from potential employee theft.
Manufacturers have made ergonomics a design consideration in each of these systems. The latest models of either type of carousel, the VLSMs, and the miniloads and microloads generally feature a picking and replenishment station set at a comfortable working height for operators. Picking productivity and delivery efficiency of parts for assembly should reach high rates, thanks to the limits set on how much the employee needs to bend or stretch when handling individual parts or the containers or totes that the system uses as its storage media.
If you so choose, these systems generally interface readily with other materials handling technology. For example, inbound conveyor can deliver empty totes or containers filled with items for restocking; outbound conveyor can take away totes with picked orders or containers with subcomponents needed on an assembly line. Light-guided picking and bar code scanning are among options that can be added for greater efficiency and accuracy of transactions performed.
So let's get an overview of each of the four system types:
Horizontal carousels. You're familiar with the local dry cleaning store's use of this technology. Think of a carousel system for your facility as an industrialized grade of the equipment, albeit one with far more sophisticated software. Carriers rotate horizontally, and can be either bottom- or top-driven. Bottom-driven models generally are reserved for heavier-duty applications. Speeds usually range 60-90 ft/minute.
Floor space savings of 40% to 60% can be achieved with these carousels compared to storage units where the picker walks to the part. Cubic utilization runs as high as 80%. By starting with the single carousel's small footprint, then stacking carousels in tiers that are two- or three-units high, cube usage can be maximized. Check to see whether stacking these systems will require a full mezzanine or not, however, as that adds to costs.
Orderpicking productivity can be as high as 600 lines per hour by a single operator. One company's twin-bin design-which pairs up two carousels side by side and doubles the pick face area-is designed to enable a picker to do more than 800 lines per hour.
Recent applications of carousels in distribution include their use as "put" systems. By putting away all the ordered items for a single, large customer, for example, into the carousel until the full order is built, then pulling only these items and shipping them, the system provides accumulation capacity.
Manufacturing applications- besides many point-of-use and staging-for-assembly uses-include having the carousel function as a self-contained process plant--for drying or curing steps in production or for "burn-in" of chips in computer assembly.
Vertical carousels. In contrast to the "open" design of many horizontal carousels, the rotating shelves of a vertical carousel are hidden beneath an outer shell of cabinetry. Inside, however, the vertical carousel has much of the appearance of a horizontal carousel turned 90 degrees onto its side. Generally, a single motor and chain drive located along one side of the unit provide rotational power. Powered shelves move up or down at the operator's command or at the direction of software controlling the system.
The work counter or benchtop area where the operator replenishes or pulls items generally extends across the width of the unit. Its position tends to be set in the "golden zone" for ergonomically correct motions by the operator working at a waist level height. On a single move of the carousel's shelving, an operator can access (or put away) several different items or SKUs in bins or pans to the left side, center, and right side of the counter, maximizing productivity and minimizing the number of rotations necessary-given the right software to do so.
With their small footprints, vertical carousels can achieve high cube utilizations. Two back-to-back units can recover up to 75% of the floor space used previously by less dense storage systems.
Multiple vertical carousels can be arranged in various configurations: back-to-back, as just noted, or side-by-side, or side-by-side on both sides of a central aisle, for example. Units come in varying widths and heights. Customization includes equipping the carousels to function as refrigeration units and to provide storage of items in controlled atmospheres or under clean room conditions.
Vertical lift modules (VLSMs). Like vertical carousels, VLSMs offer footprint savings, increased overhead space utilization.
Looking much like a vertical carousel at first glance, the VLSM, however, differs significantly in its operational features. A chain-driven, vertically travelling extractor platform moves up and down and accesses trays stored on fixed shelving within the unit. Indeed, the VLSM operating mechanism more closely resembles that of an AS/RS unit (see following).
An access window and countertop in the VLSM unit enable the operator to get to parts and items within the trays delivered to the window by the extractor's most recent move. The operator then can slide trays into and out of the VLSM extractor platform for replenishment and picking.
Modularity in the design concept of the system is one major plus. The technology generally is installed in multiple unit configurations. By moving among several access windows, an operator can increase his or her productivity. Multiple windows plus the "smarts" of computer software will minimize operator wait time. Potentially, moreover, a three-unit VLSM system operating within the same footprint of a vertical carousel can triple the throughput of the latter compared to the former, given the right software to manage the VLSM.
Miniload/microload (AS/RS). Downsized from unit load, heavier-duty AS/RS technology, miniloads and microloads are designed to store and stage totes or containers of parts within a high-density system environment. These systems have one or more aisles within which a S/R device operates, accessing the totes (trays or pans also can serve as the storage medium) stored on racking or shelving. Unlike a VLSM, where the extractor travels only in the vertical axis direction, the design of mini- and microloads features an S/R device that can travel within its aisle both vertically and horizontally, as well as simultaneously. High-speed picking access thus is one of the technology's strengths.
Miniloads usually are built to vertical dimensions of 40 ft or less with container weights of 120 lb or less, one manufacturer advises. Multiple SKUs can be kept in a single tote or container; partitioning the containers will help separate the different items.
Supplier list on the web
Major manufacturers and suppliers on the kinds of powered storage systems described in this report are listed in the web version of this article along with their telephone numbers. Go to www.mmh.com for this list. Or you can find comparable information in our Casebook Directory Issue for 1999, or in the 2000 edition, out next month.
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