Mhlnews 474 Storage01 00

Storage Strategies: Carousels Keep Things Moving

July 1, 2005
Dynamic storage seems like a contradiction in terms. It is, however, the way many companies choose to handle inventory.

Inventory will always be part of manufacturing and distribution. Keeping inventories moving, making them part of the flow, reduces the cost of doing business. Current transport technologies and philosophies, for example, are designed to extend the movement of inventories outside the building. One strategy for keeping inventories on the move inside the building, particularly in manufacturing and kitting operations, is with carousels. Companies with limited space allotments, looking for high throughput of material, find that carousels offer this advantage, and more.

In the late 1980s and early 1990s, carousels got a bad rap, say manufacturers, because too many people installed them when it was not the proper technology. Carousels were not used properly because neither the users nor the installers were taking time to analyze the data and understand the user's true needs.

Today, a thorough analysis of stock keeping unit (SKU) profiles and inventory management technologies are leading customers to integrated systems that feature conveyors, pick-to-light systems and other pieces of equipment surrounding carousels.

Inventory management
When it comes to inventory management, two important measures of achievement are visibility—knowing what is on hand and where it is—and productivity—doing more with less. These are also the primary benefits of carousels.

Along with inventory visibility and productivity, another financial driver for carousels is ergonomics. Horizontal and vertical carousels present material to the operator at the correct height that minimizes walking, stretching and bending. It is, however, difficult to measure the cost saving from reduced potential for injury, so most practitioners cite reduced floor space and reduced manpower when talking about the benefits of carousel applications.

Horizontal carousels consist of a series of storage bins or locations, motor driven and linked to form a closed loop. The length of the loop is often limited only by the space available.

Vertical carousels are similar in design and function to horizontal carousels, however, they are more often enclosed in cabinet-like structures. They are limited in height by the ceiling of the building, typically 25 to 35 feet.

Controls for carousels are usually computerized, receiving movement instructions from the warehouse management system. They can be integrated with other manufacturing and distribution orderpicking schemes, using lights to direct the operator to a particular location.

There are numerous variations on these two basic carousel themes. The essential advantage of a carousel is that it brings the work to the operator. Included within that essential benefit are:

  • Space utilization. Material moves within the footprint of the storage area, reducing the need for aisles.
  • Productivity. An operator can be assigned to select orders from several machines; making his selection from one unit while another machine automatically positions itself.
  • Accuracy. Picking accuracy is improved through computer directed item selection.

Making the right choice
Under optimal conditions, a company could surround itself with its materialvendor network, eliminating the need to carry any inventory. With global supply chains that is almost impossible today. The first step in carousel selection is to determine how much inventory should be kept as buffer stock. How long could the company stay in business if material could not be brought in the back door? A realistic look at inventory requirements helps determine how much is actually needed, or what amount makes economic sense.

All of these elements were considered by DePuy, Inc. (Bridgewater, Mass.), a medical device manufacturer and distributor. Dave Johnson, director of distribution, says, "A surgeon might schedule tomorrow's surgery for noon, placing the order with us for the needed medical device today." Same-day turnaround is becoming common practice in the medical community. Needless to say, order accuracy is paramount.

Johnson's solution was a combination of horizontal and vertical carousels, as well as vertical lift modules supplied by Remstar International (Westbrook, Me.). Depuy's 92,000-square-foot warehouse annually ships medical devices to support more than a halfmillion surgeries. The products range from spine implants to kits for new elbows.

"The vertical and horizontal units, along with the order-selection software," says Johnson, "give us better than 99.9 percent order-selection accuracy." He adds that the units save floor space because of the high-density storage capability.

Working three shifts per day, six days per week, distribution center workers select orders from more than 14,000 active stock keeping units. Each order averages five or six lines. DePuy ships an average 2,200 orders each night, says Johnson.

DePuy's product line is typical of many carousel users: small items of high value. The price of many products range into the tens of thousands of dollars so tight inventory control is mandatory. "Our philosophy," says Johnson, "is to create an efficient and effective distribution system with as close to flawless execution as possible."

To help achieve those goals, bar code scanning is done several times during the order selection process. Order selection and verification from the horizontal carousels and vertical lift modules are automated, documenting the removal and placement of anything in the units.

Three horizontal carousels are grouped together, holding 3,800 SKUs. On the evening shift, a single operator, using the carousels' light trees and software that provides exact pick instructions, pulls outbound orders from this system. Faster moving items are staged within the carousels' ergonomic golden zone to limit stretching and bending. Typically, the operator will pick 500 orders in three or four hours using this system. On the day shift, emergency orders are picked from the carousels as well. Johnson says that before the installation of this technology, it took five operators picking batch orders to match this one person's productivity.

Two vertical carousels are located in the packing and redress area where product segregation and labeling are done. A vertical unit stages material prior to inspection, says Johnson, and keeps material separated and organized. In terms of getting the right product to the right place, this is the last link in the supply chain before a surgeon's hands touch the products.

Do Your Homework
While the interest in horizontal carousel applications is strong, Larry Strayhorn, president, Diamond Phoenix (Lewiston, Me.) says the interest in integrated systems is even stronger. "Given the adverse publicity carousels got in the late 80s and early 90s," says Strayhorn, "our business model has changed to designing technologies to work around the carousels."

As an example he sites Boeing (Chicago) and its C-17 Globemaster III factory. Any airplane with a 168-foot wingspan (the Wright brothers' first flight was 105 feet) is huge. Actually, the C-17's airframe is made of more than 200,000 different small pieces. Without the storage and picking capabilities of carousels, employees would have been required to push carts of parts across nearly 250,000 square feet of assembly space.

"[Diamond Phoenix] asked us a million and half questions," says Jim Brown, senior manager of C-17 inventory and receiving operations, Boeing, "and spent the time to know our business."

The integrated carousel storage and retrieval system they installed consists of 12 units storing about 65 percent of the warehouse inventory. Parts are transported in 400 totes on two levels of conveyor.

The picking efficiency improved five times over the manual system, and storage space was reduced by 56,000 sq. ft.

Know The Inventory
"You have to know your inventory and how it moves," says Chad Sullivan, director of distribution, Market America (Greensboro, NC). His company is a direct marketing firm selling a wide variety of neutraceutical products. "We have about 2,000 SKUs," he says, "of which 500 are slow movers." And, even though the products were slow movers, the number was increasing as business grew. Sullivan was limited in space and so opted to go vertical.

Sullivan says it's a balancing act to determine how much inventory to carry. "As a direct marketing company," he says, "we can't do just-in-time, but we can come close."

He worked with his inventory control people and the team from H‰nel U.S.A. (Pittsburgh) and determined, by moving about 75 percent (500 SKUs) of his cosmetic products into the vertical lift module, along with his slow moving items, he could free space in the pick-to-light areas without adding a mezzanine, conveyors and other expensive pieces of material handling equipment.

The units Sullivan is using, Lean-Lifts, are not strictly vertical carousels. This machine has a computerized positioning elevator that runs down the center, called an extractor, with storage shelves in front of and behind it for storing inventory. It does not use bins that are linked and move together. The extractor retrieves and puts back the requested container of products.

"The vertical lift gives us a small footprint for high-density storage," says Sullivan. "We know the products in the unit will only be requested once every week or two." He adds that with the vertical unit there is an added measure of security for products because the unit requires entry through a keypad and can be locked at night.

He uses the tower to store about 700 SKUs. "We can now handle those 700 SKUs with one person, whereas previously it required four people to handle 500 SKUs," he says.

Not Just Small Stuff
Carousels can be configured to handle anything from light bulbs to full pallet loads weighing thousands of pounds. It's all a matter of applying the right technology to the right application, says Berny McCabe of PCC Systems (Germantown, Wis.), a manufacturer of control automation system designs. "We're seeing integrated applications where the carousels are being installed upstream of the put-to-light stations, which are ahead of the quality control stations."

Consolidation and expanding product lines were challenges for JLG Corporation (McConnellsburg, Penn.), manufacturer of overhead cranes and other lifting machinery. Spools of cable and wires had to be manually retrieved from racks as high as 72 feet.

After the correct spool was selected and retrieved, most often it was dragged across the floor of the warehouse to a spot where the cable was measured and cut. This was a two-person job and involved moving other things on the floor to complete the task.

Working with J&D Associates (Middletown, Pa.) JLG installed a vertical carousel that can be operated by a single employee who retrieves and cuts the cable to length. The system has become so smooth, operations for on-line ordering have been added to provide just-in-time service to customers.

For more information on carousels and other automated storage and retrieval technologies, start with the product section of equipment manufacturers of the Material Handling Institute of America (

Vertical storage units and horizontal carousels, together with order selection software, ensure that DePuy's picking accuracy exceeds 99.9 percent.

Boeing can deliver parts faster and more accurately using this integrated carousel storage and retrieval system that features radio frequency scanning updates for project tracking.