Theory of Constraints Handbook - James Cox Iii [694]
The unbalanced line approach focuses on the constraint and ensures that non-constraints have enough protective capacity to catch up to the constraint when “Murphy” strikes. Eliminating variation is still a priority in an unbalanced line. The difference is that the focus of the improvements are directed to what will rapidly improve and protect Throughput while reducing Inventories (or other investments) or Operating Expenses.
In summary, both designs are set up to meet customer demand. The balanced line works well when there is little or no variation in product mix, process times, or demand. The unbalanced line works well in the presence of variation in product mix, process times, and demand. While variation reduction is a priority in both designs, the difference is where and how many places one must focus and what the impact will be on the Throughput of the total organization. The constraint in the unbalanced line is managed very tightly. Efficiency and predictability at the constraint are important metrics. The non-constraints are measured on their effectiveness in keeping the constraint supplied—this is called time buffer management. The output of the total system is the overall top metric.
Work Behaviors
The balanced–unbalanced design decision dictates how resources will be measured and ultimately how they will behave. Lines with balanced capacity expect workers to work to Takt; unbalanced lines would have workers working to the “relay runner”3 work ethic. Figure 36-5 depicts the discord between working to Takt and working to the Relay Runner ethic.
FIGURE 36-5 TAKT or Relay Runner work ethic.
Once Takt is determined and the line is balanced, the operator is to work to Takt. This works well when there is little or no variation in product mix, process times, or demand. However, if you have negative variation in the actual versus planned processing time of an operation, the work is blocked from moving to the next operation at Takt time. This results in a negative impact on Throughput and typically calls for inserting coping mechanisms on the shop floor. When there is positive variation, the worker has no incentive to pass the work on quickly so there is little opportunity to do other value added work.
Behaviors common in a work to Takt time environment are the student syndrome and Parkinson’s Law. With the student syndrome, you think you have ample time to finish the task and therefore hold off starting the work until the last minute. If variation occurs after the last minute start, the work is finished late. Parkinson’s Law states that, “Each task will expand to fill the allotted time available.”
In this environment, improvements are masked due to these policies and behaviors. Early finishes of each operation are not passed on and late finishes by any operation can disrupt meeting the Takt time of the total system. This is the result of having protection that by policy is isolated within each operation and therefore cannot be aggregated to protect the total flow time. When Takt time is violated in one operation, the entire line suffers the consequences.
The relay runner ethic emulates a finely tuned relay race team. When work is present, the operator works head down at a fast pace that is consistent with quality and safety until the work is completed or he is blocked. Should the operator become blocked, he works on the next sequenced job until the previous work becomes unblocked. This eliminates the student syndrome and Parkinson’s Law effects while exposing improvement opportunities. In the relay runner environment, early finishes are passed on immediately and are aggregated to form time buffers that protect the constraint and the delivery to the customer from variation in process time or demand. Thus, the on-time delivery and Throughput of the system are protected even in the presence of significant variation.