Theory of Constraints Handbook - James Cox Iii [171]
Take the case of a CCR operation feeding another CCR operation mentioned before as a re-entrant line. One has to assume a minimal time difference between the two CCR operations of the same order. This time difference is required to make sure the parts that finished processing by the previous CCR operation will reach the next CCR operation. In this way, several back-to-back time buffers have to be included in the planning, forcing long total lead time. When S-DBR is implemented in such an environment, there is no predetermined schedule for CCR. The practical consequence is that whenever the parts for the next operation reach the CCR they become available for immediate processing based on the priorities at that time. This allows the total length of the production buffer to be shorter than the total of the back-to-back buffers used in DBR.
Most cases that seem to require sophisticated scheduling of the CCR should use S-DBR as a practical approach that leaves most of the complexity to the last minute decision by the people who know the rules well enough and who are exposed to the real-time priorities as set by BM. However, S-DBR also has some limitations.
The Cases Where S-DBR Does Not Fit
* * *
S-DBR has two necessary conditions:
1. Arbitrary sequence of processing the orders does not significantly impact the capacity of the resources. In other words, the sequence as such does not cause any resource to become a bottleneck.
2. The ratio of the touch time to the production lead time is very small (less than 10 percent before S-DBR is implemented, less than 20 percent with S-DBR on). Touch time means the net processing time along the longest chain of operations. This definition is intended to exclude cases where assembly of thousands of parts, done on different sets of resources, might have a long processing time, but as the majority of the parts are assembled in parallel, the actual production lead time is not so long.
* * *
The environment where the first condition does not apply is where the length of the setup depends not only on what is going to be produced, but also on what has just been produced. Such a situation is usually called a sequence-dependent-setup.20
When the difference between the setup times is very large, then S-DBR implementation might be problematic because an arbitrary sequence of processing orders, as dictated by BM, could easily turn a non-constraint into a bottleneck. The situation forces the producer to follow a certain sequence through the various products. Assuming that going through the whole cycle of products is a very significant portion of the standard lead time, the unavoidable result is that lead times might be quite long. What is even worse is that there is not much practical possibility to expedite any order because the sequence should not be changed. In other words, BM is able to show priorities but it is very difficult to follow them.
From this, one can deduce that even in a sequence-dependent-setup environment S-DBR can be applied if the total cycle time, the time between producing a product until it is possible to produce that product again, is short relative to the standard lead time. Another case where S-DBR is still fully applicable even in a sequence-dependent-setup environment is when there are several production lines, which provide good enough flexibility to expedite an order without wasting too much capacity.
The second condition mentioned above makes us aware that manufacturing environments with relatively long touch times (more than 10 percent of the lead time before S-DBR is implemented) might pose certain difficulties in applying either DBR or S-DBR. In some of the cases, what is described as a manufacturing environment is actually a multi-project environment, where each single order is actually a project. In such an environment, the planning has to include sequencing of all resources within each project in order to define clearly the longest chain; otherwise, the lead time might be significantly inflated. Schragenheim and