Drum- Buffer- Rope: Constraints Management

by James P. Tate on April 19, 2013

In the Theory of Constraints (TOC), or as it often called, Constraints Management, the technique DRUM-BUFFER-ROPE (DBR) is a key element. This term refers to the process of managing a constraining resource to maximize the throughput. Use of DBR is an excellent tool in manufacturing management because it allows you to maximize throughput even when there is no realistic way to expand the capacity of a production work center.

In TOC or Constraints Management, you have identified a work center or machine that is the limiting factor in plant throughput. This limitation could be due to the capacity of the machine, it could involve set up time factors, or the specialization of a work crew that cannot be easily expanded. Typically the constraining work center is so restricted that expansion of capacity is financially, or physically, impossible in the short or intermediate time frame. What to do?

The DBR technique allows you to work with the constraining work center by maximizing the existing capacity to the fullest. Let’s look at the components of DBR:

The Drum is the rate of production set by the constraint. This is a given attribute, there is no changing it. In the theory of constraints the master schedule will focus on the drum. Because this work center sets the pace for the entire plant, you schedule the drum work center to its maximum.

The drum schedule is designed to coordinate the constraint’s capacity and the customer requirements. The trick to scheduling the drum is to prioritize the work orders before releasing them to production. Once they are released, don’t change them. If a new order comes into the schedule with a higher priority, make it first in line (assuming all the materials are available) to enter the production stream at the first available time. Don’t stop the production flow to insert an order and don’t stop the drum work center to advance an order. This will only cost you production capacity and time, and make the following orders late to their customers. The last time you can insert a production order in the production stream is at the first work center. The Drum schedule drives the plant.

The Buffer of DBR is the inventory of ready work in front of the drum. Always have a buffer in front of the drum work center. The size of the buffer is determined by the time to set up the constraint for a new production run, and the size of the production order the constraint work center can accept. This buffer inventory is used to cover surprises that will occur, and insures that the drum is never down for a lack of work.

The rope of DBR is the communication line from the drum to the gating operation that monitors and limits the material introduced to the production flow stream. This communication line can be electronic or visual, but it must be timely. Any delay in communicating the status of the constraint and the flow of the production stream will cause you to keep more inventory on hand.

Once you have established the elements of the Drum-Buffer-Rope, you can begin to improve the flow. The application of Lean techniques to work centers upstream and downstream from the constraint will allow you to speed the flow of material to the constraint; and from the constraint to the shipping dock. This will reduce your cash flow and improve customer delivery time.

To maximize the constraint’s throughput, you must look at reducing set up times. Any delay that keeps the constraint from processing material (adding value) is a waste and must be eliminated or at best reduced. Product quality from the constraining work center is important. To have to replace a defect product or run the constraining work center a second time to achieve the proper quality is a waste. The maintenance of the constraint becomes important because unexpected downtime is a loss of capacity. This lost capacity can never be regained. All of these factors will reduce the output of the constraint and cost you profits.

Drum-Buffer-Rope is a powerful tool in any production stream with an severe limitation. Using this tool will enhance the profitability and throughput of the production operation.

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