Theory of Constraints Handbook - James Cox Iii [695]
In summary, note that how you designed your line—balanced to Takt or unbalanced—will dictate if the system works to Takt or applies the relay runner work ethic. In recent years, there have been many “workarounds” offered to try to make a balanced line work to Takt time in the presence of variation. These “workarounds” often redesign the line to an unbalanced state.
Material Release
Another subtle difference in applying TOC or LSS to a system is how material is released into a system. Both systems are pull systems based on responding to customer demand. The main difference is that the TOC signaling method is based on time, while the LSS method is based on inventory.
As shown in Fig. 36-6, when there is demand on the time-based system (known as Drum-Buffer-Rope[DBR]) there is a signal sent to the constraint for scheduling purposes to meet a shipping request, and a signal is sent from the constraint to the beginning of the line (production control) for timing the release of material. As discussed earlier, this is an unbalanced line. The non-constraint resources have “catch-up” capacity to assure orders get to the constraint on time and to the customer on time even in the presence of variation. Buffer times are calculated from the constraint to the shipping point, called the shipping time buffer, and from material release to the constraint, called the constraint time buffer. These buffers will absorb variation in getting to the constraint and to the customer, thus protecting Throughput. Material is released based on the time buffers and the actual run time of the constraint. Material is only released into the system when there is a pull from the customer; therefore, the WIP in the system is based on customer need and what the constraint can produce. There is no standard number of units of WIP, but the WIP is based on the amount of processing time that it will take on the constraint resource.
In the time-based system, high variation in demand, product mix, and process times are accommodated through adjustments to the two time buffers. These time buffers act as shock absorbers to all of the operations preceding them. Instead of providing large buffers to accommodate variation at each individual operation, the Relay Runner work ethic allows the buffering to be aggregated just in front of the constraint and in front of the customer. The protective capacity of non-constraint resources coupled with the Relay Runner work ethic allows them to catch up when there are disruptions any place in the system. Some protective capacity is usually available at the constraint resource as well. This allows it to catch up when it is the cause of disruptions.
As shown in Figure 36-6, an inventory-based release system (Kanban Manufacturing System) is activated when there is a customer demand. A signal to produce, called “kanban” is sent upstream link by link, as material is pulled to satisfy and protect customer requirements. This process is continued until all supermarkets needing replenishment are filled. The Kanban system is a system of visual signals that triggers or controls material flow. The Kanban in each supermarket is set to restock each part to its “Standard Level” once the signal is sent to reorder. Kanbans synchronize work processes across a system. In this system, nothing is produced unless there is a signal to produce.
FIGURE 36-6 Release of material—time or inventory.
FIGURE 36-7 Replenishment system—time or inventory.
In systems with high variation in demand, product mix, or process times, the inventory-based system will not work effectively. In the inventory-based system, high variation in demand, product mix, or process times can lead to high variation in the Takt time, which can require frequent rebalancing of a balanced line. Variation can create wandering bottlenecks, which can disrupt the flow through the line and have a negative impact on the Throughput of the system.
Replenishment System
Another subtle difference between TOC and LSS is determining the size of raw material and finished parts