Theory of Constraints: Drum-Buffer-Rope Methodology


Theory of Constraints: Drum-Buffer-Rope

The 5 Focusing Steps of the Theory of Constraints are summarized as follows (see earlier post for details):

Step Explanation
1. Identify system constraint Find the part of the process that limits throughput of system or rate at which goal is achieved.
2. Exploit system constraint Use incremental improvements or kaizen to the throughput by getting the most out of the system with existing resources.
3. Subordinate everything else Adjust rate of other activities in the process so that they are aligned with constraint; align organization to support decision.
4. Elevate system constraint Consider further actions to eliminate constraint, including additional investment in capital equipment and/or technology if necessary
5. Repeat process Once constraint has been eliminated, avoid inertia and search for next constraint to be removed.

One manufacturing execution methodology that utilizes the first three focusing steps is called “Drum-Buffer-Rope” after its three components:

Component Question Purpose
1. Drum What is the physical constraint or “drum” of plant? Throughput is maximized when constraint operates or “drum” beats at maximum capacity.
2. Buffer What work flows into the “drum”? Throughput is maximized when inventory feeds into the drum on a steady basis, and this system is called the “buffer”.
3. Rope When are new resources required for the “buffer”? Throughput is maximized when the signal requests new inventory to be fed into the drum on a timely basis, and this signal is called the “rope”.

Here’s another schematic comparison between the three components.

This system maximizes the throughput while minimizing both the inventory and the work flow that feeds the inventory into the constraint or “drum”.

There are actually two buffers required, one before the constraint called the “constraint buffer” (no surprise there), and then one right before the product ships out to the customer, called the “customer buffer.” Even if your factory is very efficient with regards to throughput with very little inventory feeding into the system, if your finished products stack up to the ceiling before they get shipped out to the customer, then that is another waste in the system that must be reduced.

This concludes the material on the Theory of Constraints that is expected to be known as part of Lean Tools & Techniques (chapter 2 of the Six Sigma Green Belt Body of Knowledge). The next posts will discuss the D of DMAIC, or “Define” phase of the Six Sigma process.

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