Theory of Constraints Handbook - James Cox Iii [117]
Louw and Page (2004) state that the determination of the time buffer lengths is a trial-and-error approach that consists of first determining the initial size of the time buffers through simple empirical rules (Srikanth and Umble, 1997; Tu and Li, 1998). The buffer lengths are then monitored and adjusted through a process known as Buffer Management (Goldratt, 1990; Schragenheim and Ronen, 1991). Goldratt (1990) suggests determining the initial buffer lengths by estimating the current average lead time of the tasks to the specific buffer origin and dividing it by five. Srikanth and Umble (1997) suggest the total time buffer for any product should be approximately one-half the firm’s current manufacturing lead time, whereas Schragenheim and Ronen (1990) suggest a constraint buffer size of three times the minimum cumulative processing time to the constraint.
Louw and Page (2004) use a procedure for estimating the sizes of the time buffers based on a queuing model in a multi-product open queuing network. Details of this network are beyond the scope of this chapter. Ye and Han (2008) use a mathematical approach to estimate both the time buffer and the assembly buffer sizes.
Weiss (1999) presents a queuing network using separated continuous linear programs, which he says is similar to DBR in that it tends to form buffers at the busiest stations.
Taylor (2002) points out that attempting to remove all system variability is not cost effective. It is better to buffer the constraint and to some extent buffer CCRs in order to protect them from starvation. Taylor simulated MRP, JIT, and DBR systems and compared their influence on a number of operations performance measures.
Some companies have been hesitant to start DBR because they do not know how to set the buffer sizes. Because adjustments to buffer size suggested by Buffer Management will quickly correct any initial buffer size estimate, companies should simply pick a conservative buffer size and get started.
Buffer Sizing and Lead Time
In a serial line with a single resource constraint, there should be two buffers—the time buffer at the constraint and the shipping buffer. The manufacturing lead time through the system should approximate the sum of the two buffer sizes. Even in arrangements that are more complex, this statement would be true unless there is a non-constraint assembly between constraint parts and non-constraint parts and one of the non-constraint parts had a longer lead time to the assembly point than the constraint part. In this case, the lead time should approximate the sum of the assembly buffer and the time allowed to flow from the assembly point to the shipping dock. This relationship, and its importance, is explained at length in Chapters 9 and 10 of this book and by Fry et al. (1991).
TOC and Distribution
Little has been written about the TOC solution to a distribution environment such as a supply chain. However, in the early 1990s, Goldratt utilized a distribution simulator to teach the TOC approach to distribution in various classes. Recently, Schragenheim et al. (2009) published a chapter on the distribution environment with a very thorough treatment. Imagine an environment in which a manufacturer produces a variety of products that are distributed via a set of warehouses to a larger set of retailers. Under traditional management, it is common for the retailer to order an entire season’s supply of an item to arrive before the season begins based on a forecast of what sales may be. However, forecasts are always wrong, so the retailer usually runs out of stock before the season ends or has excess stock left at the end of the season that must be sold at greatly reduced