Theory of Constraints Handbook - James Cox Iii [138]
Whether a production operation has a true bottleneck or not, as long as there are disruptions the need for a time buffer exists. The only way to ensure that the flow at the end of the system meets promised due dates is to provide protection from disruptions using time buffers. When there is a bottleneck in the system, and this is the case when we are dealing with the full DBR system, there is a need for an additional level of protection. Any time lost at a bottleneck, by the very definition of a bottleneck, will be Throughput lost for the entire system because this lost time cannot be recovered. Hence, if one hour is lost at the bottleneck, then effectively the total system will be down for one hour and we lose the Throughput that would have been generated during this time. The downtime at a bottleneck can arise from problems at the bottleneck itself (downtime, setups, etc.) or from the same problems upstream from the bottleneck. The bottleneck can be decoupled from the disruptions upstream if we can ensure that there is always material ahead of the bottleneck. The amount of material that is sufficient to provide adequate protection depends on the nature and distribution of upstream disruptions. Note that the constraint buffer at the bottleneck is not created by adding more time into the previously established time buffer. Since the bottleneck is the true constraint to flow, material naturally accumulates at this operation/resource. All other resources have protective capacity and should be able to keep the products flowing. However, when disruptions upstream are of such a nature as to prevent the accumulation of material at the bottleneck, they threaten to create downtime at the bottleneck. This must be avoided and can be done during execution control by monitoring the amount of work at the bottleneck and taking corrective action whenever the work queue at the bottleneck is dangerously low.
It is instructive to point out here that there are other types of buffers used in the overall management of flow in a supply chain. In addition to time buffers, three other types of buffers exist: capacity buffers, stock buffers, and space buffers, with respect to production planning and control systems. The capacity buffer is defined as the protective capacity at both constraint and non-constraint resources that allow these resources to catch up when Murphy strikes. Stock buffers are defined as a “quantity of physical inventory held in the system to protect the system’s throughput. Perspective: Stock buffers should not be confused with time buffers such as the constraint or shipping buffers” (Sullivan et al., 2007, 43).9 Stock buffers may be used for raw materials, WIP items (for example, at major divergent points in a V- or T-plant), and finished goods items to reduce lead time or protect against product variety. The TOCICO Dictionary defines space buffer as “Physical space immediately after the constraint that can accommodate output from the constraint when there is a stoppage downstream that would otherwise force the constraint to stop working” (Sullivan et al., 2007, 41).9 The idea is to keep the space buffer empty in the same manner as one tries to keep the constraint and shipping buffers full. BM should be used on each of these types of buffers to ensure effective operation of the constraint and high due date performance. They should also be monitored to ensure that they are not too large. Time buffers impact lead time, while stock buffers impact inventory investment.
The Rope
The final component of the DBR system is the rope—a mechanism that is used to control the flow through the system by controlling the flow at a small number of control points. The drum has created a master schedule that is consistent with the constraints of the system and is best able to satisfy customer demand. The time buffers provide the safety