Systems Improvement Part 2

In my last post, I explained that in this series of posts, I wanted to write about what I refer to as system's improvement.  I presented a basic definition of what a system is as defined by [1] Arnold and Wade as presented in their paper entitled, A Definition of Systems Thinking: A Systems Approach.  In their white paper they presented what they referred to as, “The System Test.”  While Arnold’s and Wade’s intention was to use this test to verify the requirements for a system’s thinking definition, I explained that my use will be to outline the basic structure of a manufacturing system and the thinking that goes along with it.  I explained in my last post that I had changed the wording originally presented by Arnold and Wade to describe the three characteristics of a manufacturing system, namely its purpose, the elements within a system, and the interconnectedness of the system’s elements.  I finished my last post by stating that we will consider at a simple example to better understand system's thinking in manufacturing. 

Systems always exist to realize a specific purpose and in reality, the purpose should be viewed as the goal of the system or the objective toward which all effort should be directed.  If we are attempting to improve our current system, then we must do so with our goal in mind.  Improvement implies that change will be required from the system’s current status, but because changes to our system can be either good or bad, we must do so with the ultimate system goal clearly in the forefront. Let’s now consider at a simple example. 

In Figure 1 below, we see the cross section of a simple piping system used to transport water (i.e. its purpose) starting from Section A.  Each of the different pipe diameters represents the basic elements of this interconnected piping system.  This system is gravity fed with water entering Section A, then flows into Section B and continues until water reaches a receptacle directly beneath Section I.  Suppose there was an increasing demand for more water and you have been assigned the responsibility to satisfy this increased demand.  What would you do and why would you do it?

Figure 1

It should be apparent that if more water is required, then you must first identify that part of the system that is limiting or constraining the output of water through this piping system.  In Figure 1 we see that the constraining factor is Section E’s diameter.  It should be evident that in order to increase the output of water through this system, Section E’s diameter must be enlarged.  What the new diameter must be is completely dependent upon the demand requirement being placed on this system.  In other words, how much more water is required?

Figure 2

Figure 2 is this same piping system with Section E’s diameter enlarged to allow more water to flow through the system.  The new output of water has clearly increased, but is now limited by a new constraining factor, Section B.  If there was another surge in demand for water, then our focal point will now be Section B.  So how does this simple piping system relate to a typical manufacturing system?

Figure 3 is a simple, linear manufacturing system with individual cycle times listed for each interconnected step.  Parts, or raw materials, enter Step 1, are processed for 30 minutes and are then passed on to Step 2.  In Step 2, the semi-finished product is processed for 45 minutes and passed on to Step 3.  Step 3 requires 90 minutes to process the semi-finished product and then passes it on to Step 4 which requires 30 minutes to process.  When Step 4 is completed, the finished product is sent directly to either shipping or to direct sales or is stored in racks to satisfy future orders.

Figure 3

If we wanted to increase the output rate of product through this manufacturing system, the first thing we must do is to locate that part of the system that is the limiting or constraining factor.  Just like we identified Section E in our piping system as the constraint, we must do the same thing for this manufacturing system.  Whereas in our piping system, in order to identify the constraint, we simply looked at the volume of water passing through each pipe which was proportional to its diameter, as well as looking for a “back-up” of water waiting to pass to the next section. In our manufacturing system we must identify which step has the longest cycle time.  Here we see that Step 3, at 90 minutes, is clearly the longest cycle time, so it is labeled as the system constraint.  If we wanted to increase the output of this manufacturing system, we would undoubtedly need to reduce the time required at Step 3.

This system, in its current state, can produce one part every 90 minutes because that is the rate of the system constraint.  Even though Steps 1, 2 and 4 can produce parts at much higher rates than Step 3, the total system is limited or constrained by Step 3’s output rate. Table 1 is a step-by-step summary of cycle times and output rates for this system for a typical 8-hour day.

Step #	Cycle Time	Output Rate for 8 Hours
1	30 minutes	16.0
2	45 minutes	10.7
3	90 minutes	5.3
4	30 minutes	5.3

Table 1

Clearly, Step 1’s output capacity is dominant at 16 parts every 8 hours, while Step 2’s rate is approximately 11 parts every 8 hours.  The system constraint (Step 3) can only produce product at about 5 parts in 8 hours which then limits what Step 4 can produce.  That is, Step 4 can only produce what Step 3 delivers to it.

The question now becomes, in its current state, how fast should Steps 1 and 2 be running?  In my next post, we will answer this question and continue our discussion on systems improvement.

References:

[1] Ross D. Arnold and Jon P. Wade, A Definition of Systems Thinking: A Systems Approach, 2015 Conference on Systems Engineering Research