andDonald D. EISENSTEIN
“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.)
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.
(If you do not see a simulation immediately following this paragraph, your browser does not support Java. You need to install the "Java Runtime Environment" (JRE), which is freely available here.
Here is a video of order-pickers in a warehouse operating under the bucket brigade protocol. (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..
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.)
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