Non-Constraints
Our improvement focus thus far has been on the constraint, but
now it’s time to turn our attention to non-constraints. You will recall that
the third of Goldratt’s five focusing steps is to subordinate everything else to the constraint. Just exactly what is
a non-constraint? In TOC jargon, a constraint
is any resource whose capacity is less than the demand placed on it, and a non-constraint is any operation whose
capacity is greater than the demand that is placed on it. So, theoretically,
constraints limit throughput, while non-constraints do not. But, as you will
see, the reality is that this is not always true. So, why did Goldratt believe
that it was so important to subordinate everything else to the constraint? To
quote [2] Debra Smith, “The ability to subordinate will define a company’s
ability to succeed with the Theory of Constraints. Exploitation of the constraint
is dependent upon effective subordination.”
The key role of non-constraints is to guarantee that the
constraint always has work exactly when it is needed, so as never to allow
starvation of the constraint. Constraint starvation translates directly into
lost throughput, which negatively impacts profitability. The most effective
method I have found to assure that constraint starvation does not occur, is by
using a TOC based scheduling system called Drum-Buffer-Rope (DBR) coupled with
Buffer Management.
DBR is designed to regulate the flow of product through a
production line, based upon the processing rate of the most constrained
resource, otherwise known as the Capacity
Constrained Resource (CCR). In a DBR system, the production rate of the CCR,
is equated to the rhythm of a drum. In
order to protect the drum (CCR) from starvation, a time buffer is placed in
front of it, which is the average amount of time required for raw materials to
be released into the process and processed by the up-stream non-constraints, in
time to reach the CCR. In order to
guarantee that product reaches the drum on time, a signaling mechanism,
referred to as a rope, connects the
drum (CCR) to the raw material release for the first operation. Therefore, the
first purpose of the rope is to ensure that the CCR is never starved.
By the same token, we want to guard against excess WIP entering
the system, and the rope prevents this as well. Incidentally, the derivation of
the term DBR is found in Goldratt’s book, [1] The Goal. So if you haven’t
ever read it, I strongly encourage you to do so. Because of the importance of
DBR, we will now spend some time focusing on the implementation of DBR.
Drum Buffer Rope
(DBR)
The
first step in any kind of TOC based implementation, is to correctly recognize
the constraint, or more specifically, the Capacity Constrained Resource (CCR).
The slowest resource in any production operation is the CCR, which in fact,
sets the pace for every other part of the process. The danger in out-pacing the
constraint will be an increase in both Operating Expense (OE) and Inventory ( I
). In fact, maximizing production at non-constraints, will always result in
excessive amounts of WIP, extended cycle times, more labor than is actually
required, more need for storage, and the need to spend more than is required to
secure the needed raw materials. It’s important to remember that what we are
trying to accomplish, is to deliver excellent due-date performance at minimum
inventory levels.
The
important learning here is that different resources, have different
capacities. But because of statistical
fluctuations and unexpected interruptions, which can never be totally
eliminated, your solution must consider these two phenomena. Murphy’s
expression was, “Anything that can go wrong, will go wrong.” We also believe that Thompson’s Law might
also apply. You’ve probably not heard
about Thompson, but his law states that anything that can go wrong, already
has, you just don’t know it.
The
cold, hard reality is that the constraint must be protected from “Murphy” (and
Thompson) at all times and DBR does this effectively. DBR utilizes three strategically placed
buffers to guard against Murphy (and Thompson) as follows:
- A buffer in front of the constraint to avoid starvation in the event that Murphy (and Thompson) strike any resource in front of the constraint.
- A buffer in front of assembly, if a constraint part is required to complete the assembly.
- A buffer in front of shipping, to assure on-time delivery, in the event that Murphy (and Thompson) strike upstream of shipping.
It
is important to understand that these three buffers are generally in the form
of time, rather than physical products.
The management of these buffers is critical to your success using DBR,
so the question becomes, if time is the buffer, how do you know how much time
is required?
In my next post, we will continue our discussion on Drum Buffer Rope by answering the question just posed.
Bob Sproull
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