Page 211 - How To Implement Lean Manufacturing

CHAPTER 12

Constraint Management

he management of bottlenecks is often overlooked in Lean applications because
it is not a large part of the Toyota Production System. It, however, is often a
Tstrong tool in the Lean Tool Box since it can be used very powerfully for process
improvements. In this book, I show numerous examples of bottleneck reduction which
then led to huge process gains. Often, bottleneck reduction provided the “low-hanging
fruit” for the early process gains. For this reason, it is included herein.

Bottleneck Theory

What Is a Bottleneck?
Bottlenecks are the limiting aspects of a process, much like how the neck of a wine
bottle limits the flow of its contents as you pour. Often, instead of “bottleneck,” we use
the more sophisticated term “constraint.” Every process has a constraint. There are con-
straints to our manufacturing processes, our engineering processes, and constraints to
our business process. Every process that has an objective has a constraint, unless the
objective is fully met. In a typical manufacturing process, the constraint is usually the
step in the process that has the longest cycle time. If so, it can normally be identified
with a standard time study, as described in Chap. 7.

Moving Constraints
In Lean Manufacturing, we are always striving to eliminate wastes. Often, this entails
using the tool of line balancing. When we balance a line, we try to design the process
steps so they all have the same cycle time. This brings about another phenomenon.
When the cycle times are very similar, a small variation in one step can cause a cycle-
time increase, making it the constraint at that moment. The variation disappears and
yet another process step incurs a minor variation that causes it to now be the long cycle-
time step, and thus the new process bottleneck. In this manner, the constraint moves
from one process step to another. We call this a moving constraint. (Refer to Chap. 19 for
an example of this.) If the variation of the individual process steps is not too large, we
are often not concerned with this moving constraint and do little about it. If, however,
the variation of the individual process steps is significant, this variation will measur-
ably affect the process performance and thus the process will not produce to the design
cycle time. In this case, there is a process loss. There is a cycle time loss. This loss can
be quantified by the Lean metric of Overall Equipment Effectiveness (OEE) discussed
in Chap. 4, and once it is quantified we can decide if we wish to work on this constraint
to improve the process.

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