In my last post we discussed the various types of Drum Buffer Rope systems with both single drums and multiple drums. In this final post in this series I will discuss one additional usage of multiple Drum Buffer Rope and then present the total view of Drum Buffer Rope.
An
additional variant of this same concept can also exist in a
production/manufacturing situation where the buffer supports different lines,
but the buffered product is the same for each “I” line. An example of this situation might be a
maintenance and repair organization (MRO) for aircraft. The lines in this case would be equivalent to
hangar space. The drum would be the
length of time required to repair each aircraft, and the buffer is equal to the
number of aircraft waiting to enter the repair system. Each hangar can and does repair the same type
of aircraft. The difference becomes the
duration of the repair cycle time. Some
will be faster and some will be slower.
It’s the total repair time duration that determines the drumbeat in each
“I” line. Aircraft waiting in the buffer
would be required to have the repair problem isolated (to some reasonable
level), and fully ready to enter with the necessary
repair parts, the ability to move aircraft in and out of the hangar and a crew
waiting and ready to perform the repair.
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 Drum Buffer Rope. 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(s) to reside in front of the new constraint(s),
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 the Theory Of Constraints and Drum Buffer Rope concepts,
and there are some simple and robust solutions to overcome this phenomenon.
This completes my blog post series on Drum Buffer
Rope. In my next post, I will begin a new series of posts on a new subject.
Bob Sproull
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