Bucket brigade: Stocking shelves for fast order fulfillment

Featured in the Spring 2014 W. P. Carey magazine In colonial America, firefighting was carried out by bucket brigades. Starting at a water source, a line of people passed buckets filled with water to one another until the line reached the fire. Another line passed the empty buckets back to the start. The result was an almost continuous flow of water. In the 19th century, fire engines were invented that could pump water from ponds and rivers, and bucket brigades vanished as a firefighting technique. But in the last few decades, bucket brigades, or at least the concepts behind them, have made a comeback in industry. Both academics and practitioners have discovered that an orchestrated hand-off of tasks among workers in teams can generate efficiencies in 21st century warehouses and factories. The coordination of workers using a bucket brigade model can speed up manufacturing and also move finished products out of warehouses and into the hands of customers faster than many older methods. Supply chain management professor Scott Webster has been studying bucket brigades for much of the past decade. In 2012, he and two colleagues published findings that represented a major advance in the understanding and use of bucket brigades in warehousing. Webster and researchers Robert A. Ruben of Iowa State University and Kum-Khiong Yang of Singapore Management University found that the placement of products on a warehouse rack can have a big impact on the efficiency of a bucket brigade-style operation. "When people studied bucket brigades in the past, they assumed that the way you stock product in a warehouse didn't matter," Webster said. "But we found that if you can take advantage of specialization of labor, placement of product on a warehouse rack can matter a great deal. You can see increases in the output rate by as much as 20 percent." The research is detailed in the article, "Impact of Storage Assignment Decisions on a Bucket Brigade Order Picking Line," published in the journal Production and Operations Management. Improving an effective method In the 1970s, Toyota launched what is believed to be the first large scale industrial bucket brigade-style experiments, deploying teams of workers to sew the upholstery for car seats. Under the Toyota system, operators moved from one machine to the next and manually passed fabric to the next operator. The process saved time and improved efficiency, and it caught the attention of other manufacturers, some of whom copied the system. Bucket brigades soon spread to warehouses. In warehousing, a bucket brigade system starts with a group of workers assigned to pick orders from a storage rack. A list of items to be picked is posted at one end of the rack. A worker takes the list and picks items from top to bottom while moving forward, then hands off the partially completed pick list to another worker, who repeats the activities, then hands it to another worker. The cycle repeats until the order is filled. Early studies of bucket brigades in warehousing found significant improvements in pick rates for companies that used the technique. Revco Drugs, McGraw-Hill, Harcourt-Brace and Blockbuster Music all were found to have recorded notable productivity gains. Studies also identified a potential problem with warehouse bucket brigades — the phenomenon of “blocking." When a faster upstream worker must wait for a slower downstream worker to finish a task, then blocking occurs and the whole process slows down. However, researchers soon found a way to minimize blocking. By setting up the order of workers on a picking line from slowest to fastest, the process tended to move more smoothly, making blocking less of a problem Webster and his colleagues suspected that the pace of picking in a warehouse also could be affected by the placement of products on the rack. Whether a product was a slow or fast mover would be an important consideration, they speculated. A slow-moving product would sell only a handful of units a year, while fast-moving products sold many units daily. Webster, Ruben and Yang set out to determine the most effective way to distribute these products in the rack. Should fast-moving products be at the beginning or the end? Where should medium sellers go? What would happen if products were organized randomly, with slow, medium and fast sellers dispersed evenly throughout the rack? Simulating a modern bucket brigade To answer these questions, Webster and his colleagues used mathematical techniques to develop economic models of a warehouse bucket brigade. "The creation of a model begins with an understanding of the real system," Webster said. "You try to develop a simple representation of that system for the purpose of gaining insights into that system. If you have fifty different variables, it's hard to understand how those things interact without using models." After experimenting with different models, the researchers settled on two that seemed to have the most potential — one a purely mathematical representation of the system and another that used mathematical principles but was a closer simulation of actual conditions in a warehouse. Using computer programs, the researchers tested their hypotheses about how a warehouse would work under different arrangements of products on a rack. "The main thing that we found from the simulations is that in some settings, the way you store product in a rack can have a significant impact on output," Webster said. "These increases in productivity, as much as 20 percent, are almost free. They require minimal investment on the part of managers." In the most common warehouse setting, products that are "fast movers" should be at the end of the rack, or closest to the point where a conveyer will move the product to the area where products are boxed and shipped, according to Webster. "Slow movers," on the other hand, should be at front of the rack, closest to the point where workers start working from the pick list. Using skills wisely The reason that the organization of a warehouse rack matters is that workers in warehouses perform differently, according to Webster. "There are basically two tasks that happen in a warehouse — walking and picking," he said. "Some people are faster at walking and some are faster at picking." The greatest variation, though, is in picking skills, he said. "If you take two people, the rate at which they walk is not going to be that different, but you take a person that's been picking for three years and a person who has been picking for a week, and you are going to find real big differences. The person who's been picking for three years knows where everything is." In a typical warehouse, in which workers vary considerably in picking skills, putting slow moving products at the beginning of the line and fast moving products at the end will result in the biggest gains in output, Webster and his colleagues found. A person who is fast at picking will quickly shift down to the end of the line to handle the fast moving products coming off of the rack. "The reason this works is that you can take advantage of the specialization of labor," Webster said. "The person who is faster at picking is going to do more of the picking. The person who is slower at picking is going to be more of the walking." Lessons from nature Colonial firefighting and modern warehouses and factories are not the only places that bucket brigades have been found to be effective. Researchers also have found bucket brigades in nature. A species of harvester ant found in Spain, lives in colonies that survive on seeds. Smaller ants, which are adept at exploration, range far from the nest to gather seeds, which they pick up and start to carry back. But before they get far, larger ants take the seeds away from the smaller ants, bring the seeds back to the nest and drop them. Then the larger ants go back and the cycle repeats. "What is happening is that nature has devised a way to exploit specialization," Webster said. "The ant that is built for exploring is doing the exploring, while the ants built for carrying are doing the carrying." The same process happens in a well-designed bucket brigade, according to Webster. "The person who is able to do the really fast picking does the picking, while the worker who is slower at picking does more of the walking."  

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