Bucket-brigade assembly lines

What are "bucket brigades"?

"Bucket-brigades" are a way of organizing workers on an assembly line so that the line balances itself.

Here is how it works. In an assembly line, products are progressively assembled as they move down the line, from worker to worker, toward completion. This is a familiar organization in manufacturing, but assembly lines are found in all types of industries, wherever work is passed from person to person in sequence.

A classic challenge in the management of assembly lines is to balance the assignment of work so that there are no bottlenecks in the flow. This is hard to do because it requires, first, knowing how much work is inherent in assembly; and then dividing that work appropriately among the workers. Balancing an assembly line is typically done by engineers and represents a significant project. But because bucket brigades are self-organizing, the need for such centralized engineering is reduced or even eliminated. Furthermore, bucket brigades are able to achieve better balance than any engineering team because bucket brigades redistribute the work based, not on estimates (time-motion studies), but on the time it actually took a particular worker to perform a particular task.

This idea may be found in the social insects, such as ants or bees, which are highly effective at organizing themselves even though without blueprint, plan, or management. Instead, global coordination emerges spontaneously, through the multiple interactions of many participants, each following simple rules. Similarly, when workers on an assembly line are organized into bucket brigades, they can function as a self-organizing system that spontaneously achieves its own optimum configuration, without special equipment, time-motion studies, work-content models, management, or software control systems.

The operation of bucket brigades is simple: Each worker carries a product towards completion; when the last worker finishes his product he walks back upstream to take over the work of his predecessor, who walks back and takes over the work of his predecessor and so on, until, after relinquishing his product, the first worker walks back to the start to begin a new product. If, in addition, workers are sequenced from slowest to fastest, then we call the system a "bucket brigade" and the workers will spontaneously gravitate to the optimal division of work so that throughput is maximized.

In this, the simplest version of bucket brigades, workers must maintain their sequence: No passing is allowed and so it can happen that one worker might be blocked by his successor. In such case we require that the blocked worker simply wait until his successor has moved out of the way, so he can resume work. (This waiting is not necessarily bad because it is the means by which the workers migrate to their optimum locations.)

Benefits

There is a reduced need for planning and management because bucket brigades make the flow line self-balancing.
Production becomes more flexible and agile because bucket brigades "tune" themselves, without time-motion studies or the other cumbrous endeavors of assembly-line balancing.
Throughput is increased because bucket brigades spontaneously generate the optimal division of work.
Secondary labor is reduced and quality improved because bucket brigades operate with the absolute minimal work-in-process.
Training and coordination are simplified because it is easy for workers to know what to do next.

See for yourself

In the simulation below, three workers assemble product as they move from left to right. Whenever the fastest (rightmost, red one) finishes a product (reaches the right end of the line), he leaves it and walks back to take over the work of his predecessor, who walks back and so on, until the first worker walks back to start a new product.

Note that the last, fastest worker sets the pace by "pulling" work into the system; and the slowest worker is always the one who starts new work.

The workers begin at arbitrary positions far from balance; but you will observe that they quickly gravitate to the optimum partition of work (indicated by the tick marks). Click anywhere on the simulation to restart the workers at random positions and observe how balance is quickly reestablished.

Here is a video of order-pickers in a warehouse operating under the bucket brigade protocol: Stream RealPlayer or download an mpg file (45MB). (You will see order-pickers pressing buttons on the flow rack; this is to communicate with the pick-to-light system and has nothing to do with bucket brigades.)

Here is a simple explanation of the self-balancing mechanism of bucket brigades.

Who is using bucket brigades?

Currently, bucket brigades are used mostly in distribution warehouses to organize order-pickers, in the apparel industry to organize garment-sewers, and in simple assembly processes.

We believe bucket brigades to be more widely applicable but feel that the greatest economic significance is in order-picking, which is very labor-intensive. A typical high-volume distribution warehouse employs hundreds of workers to pick orders and the work must be rebalanced daily, and sometimes more often.

Here is a list of some current users of bucket brigades. (Note, however, that implementations of bucket brigade sometimes differ in local ways depending on the context, so not all of these operate strictly "by-the-book" as we have described.)

McGraw-Hill:Order-pickers in distribution centers
The MusicLand Group: Order-pickers
Time Warner Trade Publishing/Little, Brown: Order-pickers
Bantam-Doubleday-Dell Distribution: Order-pickers
Harcourt-Brace: Order-pickers
Blockbuster Music: Order-pickers
Coach Leatherware: Much sewing is done by bucket brigade
Champion Products: All sewing is done by bucket brigade. Read about it here.
Subway: Corporate headquarters now recommends to their 13,000 franchises that they assemble sandwiches by bucket brigade. Read about it here.
Tug Manufacturing: Manufacturing airport tractors. Read about it here.
Mitsubishi Consumer Electronics America: Assembling large-screen televisions, packaging cellular phones. Read about it here.
United Technologies Automotive: Assembling automotive harnesses. Read about it here.
Revco Drug Stores, Inc. (now CVS): Order-picker in their national distribution center increased pick rates by 34%. Read about it here.
Anderson Merchandisers: In a two-week trial order-pickers increased production rate by 20% and reduced variance in pick rates by 90%. Furthermore, each order-picker set a career best. Read about it here.
Readers Digest: Order-pickers realized an 8% increase in pick rates and a 35% reduction in errors over their first picking season. Read about it here.
Wawa Convenience Stores, Inc.: Their (unionized) distribution center began using bucket brigades in an operation that picks full-cases to pallet jack. Order-pickers averaged a 25% increase in pick rates the first week. Read about it here.
Ford Customer Service Division: The most popular products were moved out of carousels and into flow rack and picked by bucket brigade. The pick rate increased over 50%. Read about it here.
The Gap: The Gap Distribution Centers supply three chains of retail stores: Old Navy, The Gap, and Banana Republic. They report a 25% improvement in throughput by using bucket brigades in their pick modules. Read about it here.
Walgreen's: Walgreen's uses bucket brigades in their distribution centers, supplying almost 4,000 retail stores. Details to follow.
Radio Shack: Radio Shack uses bucket brigades to coordinate order pickers in its distribution centers. Look for a mention in an upcoming issue of National Geographic in an article on "swarm intelligence".
Mother Nature: Amazingly, some species of ant have been reported to use bucket brigades to coordinate the transfer of food to the nest. Read about it here

Primary references

Most of the following are technical publications and will be of interest primarily to academics. Material of more general interest may be found here.

A production line that balances itself by J. Bartholdi and D. Eisenstein, in Operations Research 44(1):21-34 (1996), special issue on new directions in operations management. This is the original paper introducing bucket brigades and is the key academic reference. It includes a precise mathematical model of a bucket brigade as a dynamical system, analyzes its asymptotic behavior, and proves that production rate is maximized. Also includes a detailed justification of the appropriateness of the mathematical model.
Dynamics of 2- and 3-worker `bucket brigade' production lines by J. Bartholdi, L. Bunimovich, and D. Eisenstein, in Operations Research 47(3):488-491 (1999). Primarily for academics: A catalogue of all possible dynamic behavior of small bucket brigade lines plus mention of some open mathematical questions. Among the results: Evidence for the existence of mathematical chaos for some pathologically mis-configured lines.
Performance of bucket brigades when work is stochastic by J. Bartholdi, D. Eisenstein, and R. Foley (2001), Operations Research 49(5):710-719. Mathematical proof that bucket brigades are expected to perform well even when the work has an element of randomness to it.
Using Bucket Brigades to Migrate from Craft Manufacturing to Assembly Lines by J. Bartholdi and D. Eisenstein (2005), Manufacturing and Service Operations Management 7(2):121-129. A case study describing how a manufacturer of tractors used bucket brigades to migrate from craft assembly (one person assembles one tractor) to a semi-automated assembly line.
Bucket brigades on in-tree assembly networks by J. Bartholdi, D. Eisenstein, and Y. F. Lim (2006), The European Journal of Operational Research 168(3):870-879, special issue on balancing assembly and transfer lines. This shows how to use bucket brigades on a "tree" of merging sub-assembly lines so that all the sub-assembly lines are balanced and, moreover, they are all synchronized so that the assembly network produces product at regular, predictable intervals.
Deterministic Chaos in a Model of Discrete Manufacturing by J. Bartholdi, D. Eisenstein, and Y. F. Lim, submitted. A mathematical analysis showing that if a special type of bucket brigade is configured pathologically then fully chaotic behavior is possible.

Related publications

Task partitioning in insect societies: bucket brigades, Insectes Sociaux 49 (2002). In 1999 two Spanish biologists reported a species of ant that carries seeds back to the colony by passing each seed from slower to faster ants--bucket brigades! This paper analyzes how bucket brigades can form spontaneously if each ant simply grabs the first seed it can and carries it back to the nest.
D. Armbruster and E. Gel of Arizona State University have explored the dynamics of bucket brigades in which worker skills are changing or are multi-dimensional. With J. Murakami, they show that bucket brigades are robustly optimal in the presence of worker learning. In another paper they study a model in which a worker may be faster at one portion of the work but slower at another portion.
R. Villalobos of Arizona State University and his colleagues have written several papers about how bucket brigades can be especially effective in the presence of high labor turnover. They have also done some nice work implementing a bucket brigade production line at a United Technologies Automotive site.
Recovering cyclic schedules using dynamic produce up-to policies by D. Eisenstein, Operations Research 53(4):675-688 (2005). This uses the ideas behind bucket brigades to schedule a manufacturing resource amongst competing products. A simple rule tells what to manufacture next and for how long; the result is that the system gravitates to a sustainable and efficient manufacturing schedule.
A survey of the self-balancing production lines (“bucket brigades”) by A. Bratcu and A. Dolgui, Journal of Intelligent Manufacturing 16 (2005).

Other references, citations, discussions

A nice summary article in Supply Chain Digest by D. Gilmore (July 2007). (Subsequent to this, a reader wrote in to say that he had set up bucket brigades in his shipping area and “this has helped us considerably”.)
“The bucket brigade: a new approach to order-picking”, in Warehousing Tips by K. Ackerman, Ackerman Publications (2002).
Bucket brigades are discussed in the article “Swarm intelligence: A whole new way to think about business” by E. Bonabeau and C. Meyer, which appeard in the Harvard Business Review, May 2001, pp 107-114.
“Give productivity a boost without any investment in equipment” by S. Estersohn, in Distribution Channels magazine, May 1998, pp 51-54.
“Self-organization will free employees to act like bosses”, in the weekly column "The Front Line" by Thomas Petzinger, Jr., The Wall Street Journal, January 3, 1997. Petzinger writes "The new model for organizations is the biological world, where uncontrolled actions produce stunningly efficient and robust results, all through adaptation and self-organization."
The Center of Business Innovations at Ernst and Young LLP has sponsored colloquia on complexity science and business and have published books about each. You can read about bucket brigades in the book describing the second conference.
“Call out the bucket brigade!”, in The WERCsheet (a publication of the Warehousing Educational and Reseach Council), July/August 1996. A journalist's summary of our presentation on order-picking by bucket brigade, given at the national WERC conference in San Francisco, 1996. It does not quite get the fine points right but is a reasonable summary, with some second opinions.
Two students at the University of Canterbury in New Zealand have done an excellent honors project predicting how bucket brigades could improve manufacturing at a local company.
Two graduate students at the University of Lund in Sweden won a prize for their masters thesis, Evaluation of bucket brigades - a next generation order-picking strategy. The work was done with collaboration of Consafe Logistics.
Here is a lecture by John Bartholdi on self-organizing logistics systems.

Logistics Log

Wednesday, October 08, 2008