Different Forms of Drum Buffer Rope Part 6

In my last post we began discussing the different types of constraints that can exist within a system.  Let's finish that discussion and then we will look at two different types of DBR systems.

Types of Constraints (Continued)

As discussed in my last posting, constraints can exist in one of two types.  The first type is the internal constraint— which means that the market demand for your product is higher than the capacity of the system to produce it.  Customers want much more of what you offer then what you can produce.  It’s a good situation to be in, but only up to a point.  If you can’t figure out a way to meet market demand, then your competitors will usually figure out a way to do it for you.  This situation is ideal for implementing traditional DBR to meet the demand and capture more market.

The second type of constraint is an external constraint.  In this case the market demand is less than your ability to produce.  The market is buying less, in some case much less than you can produce.  This is a less desirable situation, but one that nonetheless can exist.  This situation usually means that there is not an internal constraint to contend with. If this is the case, then it is somewhat improbable that traditional DBR will provide an acceptable answer.  Instead, in this situation, a modified or simplified form of DBR might be more practical.  Consider S-DBR.

Simplified Drum-Buffer-Rope (S-DBR)

The concept of S-DBR was developed by H.W. Dettmer and E. Schragenheim is defined in their book Manufactruing at Warp Speed1.  The S-DBR concept assumes that the constraint is external to the system and resides in the market segment.  Customers aren’t buying as much product as you can make, or there is significant variation in market demand, which can cause the constraint to float back and forth between internal to external locations.  In this situation, the constraint becomes interactive by moving between the market constraint (external) and the production constraint (internal).  This oscillating cycle between internal and external constraints can cause its own brand of chaos in deciding which market segments should be pursued and which ones might be better left alone.  Either way it is a decision that must be dealt with.

In the scenario of an external constraint, the drum is determined and activated only when the system has firm orders in place.  The rope is now determined by the orders that actually exist, which are released based on due dates.  If the orders exceed the capacity of the system, then the constraint has become internal and different actions must be taken.  This also assumes that the internal constraint will exist only for short periods of time and can be overcome by actions like implementing additional shifts or short-term overtime.  Dettmer and Schagenheim have argued, quite successfully, that the market is the true constraint of any system.  There is much more reading available about this concept at Dettmer’s Goal Systems web site.

Multiple Drum-Buffer-Rope (M-DBR)

There is another unique situation that can require the implementation of a third type of DBR, known as Multiple-Drum-Buffer-Rope (M-DBR). The situation for M-DBR is created when a single buffer location is required to supply products to more than one assembly line, and each assembly has its own drum that is keeping pace at a different rhythm. Figure 1 shows an example of an M-DBR configuration.  This configuration was used at an MRO facility to perform maintenance on a fleet of helicopters.

Figure 1

One other use for Multiple Drum Buffer Rope (M-DBR) could be in a hospital Emergency Department as depicted in Figure 2.

Figure 2

The Total View

Even with all the respectable improvements that can be achieved with a synchronized flow using traditional DBR, S-DBR or even M-DBR, there can also be some problems associated with achievement, especially with traditional DBR.  It’s not a bad problem, just one you need to be aware of.  When you follow Goldratt’s Five  Focusing Steps, it is possible during Step 2 (the exploitation step) that a constraint can be improved to the point that it is no longer the constraint, and at times this can happen very quickly.  When it does happen, you have effectively “rolled” the constraint to a new location, which means you only finished Step 2 before it is now time to go back to Step 1 again.  

The original system process that was considered to be the constraint today is no longer the constraint tomorrow.  These types of rapid system improvements can obviously cause some problems.  When a new constraint is identified in the system, then the system effectively has a new drumbeat.  When that happens, you also have to move the buffer location to reside in front of the new constraint, and you have to move the rope signal from this new location back to the release point at the front of the line.  In some systems it might be possible to roll the constraint several times to several different locations before an acceptable level of system stability is achieved.

This fast action of fixing and rolling the constraint can and does cause a certain amount of chaos in a system.  Workers will quickly become confused about “Who is the constraint today?”  Improvements can happen so fast that the negative effects of change will outweigh the positive effects of improvement.  This was a problem recognized early on by some implementers of TOC and DBR concepts, and there are some simple and robust solutions to overcome this phenomenon.

This completes our discussion on the various types of Drum Buffer Rope.  In my next post, we will discuss a completely different subject.

Bob Sproull