My family and I went camping with my brother-in-law and his family. We went to a place in Utah called Uinta National Forest — it was beautiful. We prepared well, got the tent and camping stuff ready, then headed to the camp site on Friday. When we arrived, we set-up camp, then we went on a hike.
That’s us above on a hike — that’s me with the blue shirt holding a baby; yeah, the Asian guy, that’s me. It was just absolutely beautiful up there. It was nice to be in nature.
During our hike, I couldn’t help but think of the Boy Scout chapter in Goldratt’s “The Goal”. You know, the chapter where Goldratt introduces the Drum-Buffer-Rope system.
Before explaining the Drum-Buffer-Rope (DBR) system, let me first explain some basic characteristics of systems, in general:
- Every system has a bottleneck.
- A bottleneck is a state of affairs where demand for service exceeds the capacity to serve.
- The Throughput of a system is dependent on the Throughput of the Bottleneck.
- Given (1), (2), & (3), for maximum output, a system ought to keep the bottleneck working at 100% capacity with little or no defects (scrap, waste (muda), time-traps).
- Given (4), Non-bottleneck processes should be working at less than 100% capacity, so as to not over-burden the bottleneck with large batches of work-in-process (WIP).
The characteristics above are basic to any system — manufacturing, clinical, software, or otherwise. Put on your Systems-wide Hat and think about your experience thus far, regardless of industry: bottlenecks are everywhere. That’s not the problem, but managing it is. Hence, the Drum-Buffer-Rope System.
Managing the Constraint is mostly about managing the non-bottleneck systems and making them “aware” how fast they should work — when they should slow down, when they should stop, or when they should increase pace and by how much. The Drum-Buffer-Rope system allows for a systems-wide awareness.
The Bottleneck or Constraint, acts as a Drum — it sets the rythm that the whole system should follow. In Lean Manufacturing, this is also called “Takt Time.”
There are situations when when upstream processes can’t produce as much as is needed by the Bottleneck; the result: the Constraint is starved and overall system output is compromised. So, we must have a buffer of inventory that is the size of the accounted-for variation is demand. This will help to level-out variation. A Buffer will assure that the Constraint never has to wait and, waiting is a form of waste.
Similarly, if upstream processes are producing more than the Constraint has the capacity to handle, then there’s going to be excess inventory sitting in front of the Constraint and, hence, a feast.
Put another way, the Buffer is the inventory and inventory is directly related to Lead Time (I explain this in 2 previous posts on Little’s Law here and here.
This phenomena is sometimes called the Feast-of-Famine Syndrome.
DBR is used to avoid either of these scenarios — the Feast or the Famine — by dictating the batch size and frequency of the inputs into the Buffer.
The Rope is a method by which the Constraint can signal to the upstream processes (non-bottleneck processes) when to slow down, when to stop, or when to produce faster and the quantity. This is called “Pull Scheduling” in Lean Manufacturing terms. In software, this can be implemented as a data structure called a Stack, with “Push” and “Pop” as the methods for pulling from the Stack.
Applications of Drum Buffer Rope
When I was with Amazon.com, I led a project where I investigated a production line that was experiencing a Feast/Famine scenario. There was a lot of waste on this line and it impacted daily production in a serious way. As the team lead, I set out to observe, interview the operators, and collect data on this line. I quantified the cost to Amazon that was a result of the Feast/Famine scenario — costs in terms of actual dollars resulting from missed orders, upgraded orders, overtime of operators, product damage, and safety issues.
The distribution you see above is best approximated by the Poisson Distribution, which means that the Mean and the Standard Deviation are approximately the same. What does this mean? The picture above graphically shows the
Famine Feast scenario — product in totes arrive at the constraint ALL at the same time.
Solution? I led a team of software and industrial engineers that re-engineered this line and we implemented a DBR solution, where the pack-rate at the Constraint would dictate what the upstream pick-rate should be. In other words, we made sure that the pick-rate would never be higher than the pack-rate. This solution worked and Amazon saved a lot of money and customers benefit.
Drum-Buffer-Rope is an intuitive solution to a seemingly complex problem. DBR is seen in many areas including Agile Development, Manufacturing, and Medicine (Emergency Room Visits). It’s an effective business tool to manage the constraints that every business faces.
Oh, by the way, camping was great!
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