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In the theory of constraints, the primary improvement effort is always to identify and streamline the bottleneck. Any problems on a bottleneck process affect the whole system. A problem on a non-bottleneck only affects that process.
In most operations, there is a single constraint, but there may, on occasion be a few that are relatively close in capacity.
The pace of the factory, therefore should be tied to the constraint. In order to prevent disruptions, a buffer is placed in front of the constraint so that it is never starved for work. Finally, all of the production processes are linked together so that there is no overproduction in non-bottleneck operations. He refers to this as “drum-buffer-rope”.
The theory of constraints advocates the use of small batches to help speed products through a system, which means ample application of SMED. Just as in Lean, the small batch sizes act to reduce inventory and streamline flow through the operation.
Continuous improvement efforts should be focused on a constraint. Any gain in that production process mean that the beat of the drum can be sped up, and production for the whole system will improve.
Eventually, the bottleneck process will improve so much that it will no longer be the constraint. At that point, there will be a new constraint, and it will be reassigned as the drum, with the improvement process starting all over.
While the primary effort should be on bottlenecks, do not neglect other operations. Not everyone in an organization will be involved in improving the constraint. They should still be making improvements in their own work area.