Thursday, March 26, 2015

Visual of Theory of Constraints

One of the subjects I've written about many time here on my blog is the Theory of Constraints (TOC).  I've explained to you that the throughput of any process or system is completely dependent upon the cycle time of the slowest operation.  I've also explained that unless and until you subordinate everything else to the constraint, your process will be "jammed up" with excessive work-in-process inventory (WIP).  And when this happens, there will be a negative impact on the overall processing time and your on-time delivery will be severly impacted in a very negative way.

For those of you who haven't followed my blog, I want to demonstrate visually the concept of the constraint (a.k.a. bottleneck).  The way I explain this is, first by demonstrating what happens within a simple piping system used to transport water, just like the following graphic.
In the above graphic we see a cross section of a piping system all with different diameter pipes.  If you wanted to increase the amount of water exiting this system, it should be obvious that only by increasing the diameter of Section E would you be able to accomplish this because Section E controls the rate at which water passes through each of these pipes.  The new size of Section's E's diameter would be dependent upon how much more water was needed.  So let's say you increased the diameter of Section E, then ask yourself what would happen?  The following graphic is what this system would look like after you increased Section E's diameter.
 What you see is more water flowing, but only to the extent of the next smallest pipe's diameter which in this case is Section B.  If you still needed more water, then it should be clear that you would have to open up Section B's diameter.  So how does this apply to a normal manufacturing process or a process of any kind for that matter?  The next graphic explains this very well.
Here we see a simple four-step process with raw materials entering into Step 1 where they are processed for 30 minutes and then passed on to Step 2 which requires 45 minutes to process.  The semi-finished material is then passed on to Steps 3 and 4 before exiting Step 4 as Finished Product.  So I ask you the same question I did for the piping system.  If you wanted to increase the output of this process, what would you do and why would you do it?  If you said you would need to identify the constraint and then reduce its time, you would be correct.  Even if someone had a bright idea to reduce Step 1's processing time from 30 minutes to 15 minutes, would finished product exit this system any faster?  The answer is no, because Step 3, with its extended processing time, controls the rate at which product is produced in this process.
Here's a new question for you.  At what speed should Steps 1 and 2 be running?  If you said 90 minutes, or the speed of the slowest step, you would be correct.  Why is this true?  Quite simply, because if you ran every step at its maximum capacity, the process would be clogged with WIP and what happens when a process is clogged with WIP?  The figure below demonstrates this effect.
Because Step 1 is faster then Step 2 and because Step 2 is faster than Step 3, the accumulation of WIP results.  And when this happens, the overall cycle time of the individual parts increases and on-time delivery deteriorates.
This past weekend I visited my three grandsons in Ponte Vedra, Florida.  We then traveled to Brandon, Florida to visit my mother-in-law.  We were traveling on Interstate 4 West when we noticed lots of police cars ahead of us in the East bound lanes.  Apparently a trailer had become disengaged from the truck pulling it and had skidded sideways blocking 3 or the 4 lanes of East bound traffic.  I began looking at my odometer and the back-up of cars and trucks was 6 miles long.  When I saw this my wife said, "Looks like there is a constraint!"  I got to thinking about what she said and the analogy to a process with a constraint was clear.
So think for a minute, what happens to the cycle time and on-time arrival to each of those vehicles when they run into an accident such as this one.  The total time required to travel to their destinations has increased dramatically and their arrival times increased proportionally.  So just like the process described above, where WIP increased, each of these vehicles was late getting to their destination.  So does it make any sense at all to run every step in your process as fast as you can?  Not if it results in extended cycle times and late deliveries.  These vehicles would not have "speeded-up" until the constraint is removed.  If the vehicles were limited to a single lane of traffic, they would only increase their speed when each new lane of traffic was opened up.  That's what continuous improvement is all about.
Bob Sproull 

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