The Moving Constraint

By Ron Pereira Updated on April 1st, 2008

bottleneck2.jpgLet me ask you a question. Is a moving constraint/bottleneck a good thing or bad thing?

I contend it’s a good thing. No, I contend it’s a great thing. Allow me to explain.

Let’s say there are three processes in your widget making factory – machining, painting, and assembly.

After your current state value stream mapping exercise you learn that machining is the constraint to the system due to a variety of reasons (e.g. cycle time, defects, availability losses, etc.).

Once this is understood you and your band of merry men (and women) attack these issues with reckless abandon. After a few weeks of focused kaizen, machining is no longer the constraint – assembly is.

In fact, if you are able to increase the output of assembly you will immediately sell more since your sales growth has gone through the roof due to your already best in class lead-times.

So, as you probably guessed, your team heads over to assembly with stop watches and video camera in hand. You study things and identify lots of excess motion, poor material presentation, and ineffective planning of the cells.

A few weeks later, assembly is rolling along like a well oiled machine… and wouldn’t you know it… that darn painting process is now the bottleneck!

I could go on and on with this post, but I think you get the point.

You see, I contend that moving constraints mean your business is in constant growth mode. And this constant growth is placing constant pressure on your manufacturing capabilities.

Conversely, if you work to balance your cycle times to takt time and that suffices for more than six months, I’d contend you are not growing your business.  And that, as they say here in Texas, ain’t good.

Do you agree?

  1. Stan Wilson

    April 1, 2008 - 9:39 pm

    I tend to agree. However, if you really have things balanced then the new constraint (once your sales increase) will be all your processes and not just one.

  2. Pete Abilla

    April 2, 2008 - 9:17 am

    Hmmm, I’m not sure placing a value-judgment of “good” or “bad” on moving bottlenecks is correct. Dynamic Systems, by definition, will always have contraints — it’s almost like a law of physics. The relevant question for us is really about exposing the contraints and managing and/or controlling them.

    Also, consider the scenario below:

    Step A: flow
    Step B: constraint
    Step C: flow
    – – –
    Step A: flow
    Step B: flow
    Step C: constraint

    When the constraint moved from Step B to Step C, it could mean that we are optimizing sub-process at the expense of the another process (or at the expense of the whole system). That situation doesn’t really say anything about growth or improvement — the folks in Step B feel good, but the folks in Step C are probably not happy. At the end of the day, the whole system still suffers.

    Here’s a case in point:

    Imagine a distribution center that has:

    receive, stow, pick, pack, ship.

    A few things are true in this situation:

    1) pick rate > pack rate = constraint at pack
    2) pack rate > ship rate = constraint at ship
    3) cash is realized only when things are scanned “ship”

    The phenomena of the moving bottleneck in this case is interesting because the challenge comes when we try to regulate the downstream-to-the-upstream (the rope). The countermeasure here is to really stay true to the notion of “pull”, otherwise push leads to either “feast” or “famine”.

    I’m rambling now — good post; got me thinking.

  3. Chris

    April 2, 2008 - 10:45 am

    I’m not sure I completely agree with you. In my situation for instance we are building a system that is a cog in the machine that is the Virginia class submarine. Obviously, the main contractors can only build so many submarines per year, so our production rate is VERY fixed, but we are still challenged to decrease cost and improve yields – part of this leads to doing more with less – space, people, movement, and inventory, any one of which can expose new constraints to work. Instead of selling more widgets, we look to grow our business in sales of systems that are related to this one, but used in different applications, with the same reliability, cost, and performance benefits of our current system.

  4. Ron Pereira

    April 2, 2008 - 6:23 pm

    Thanks for the excellent comments gents. Your insights have helped me think about this is a new way. That’s the coolest part of blogging… I aim to make folks think… and then they reply with comments that make me think. I love it!

  5. Scott Edwards

    April 3, 2008 - 4:50 pm

    I think Pete’s got the right idea. Constraints are a fact of life. The important thing is to know where it is and manage too it. In any system, especially production systems, there are going to be one, maybe two constraints and it will define the system’s capacity. therefore, assuming the market is not the constraint, any time lost at the constraint will be lost forever so it’s key to understand where it is so that you can ensure it never goes down. then if you improve it, you increase your throughput and then yes, it could definitely shift elsewhere where you start again. One downside I think of shifting the constraint constantly is the potential confusion to those working in the operation. If you manage to one constraint then improve such that it moves, make sure everyone is aware of the change and understands the implications.

  6. Ron Pereira

    April 3, 2008 - 8:52 pm

    Great comment Scott. Thank you.

    I think we are all really saying the same thing… any system will of course have a constraint.

    The point I was trying to make (from personal experience) is that most companies hope to grow their business. When this happens something in manufacturing normally needs to improve.

    And since you can only produce as fast as your slowest link (bottleneck, constraint, herbie, whatever you want to call it) this is the area that gets our focus.

    Then, once that constraint is broken, for lack of a better term, the constraint moves.

    Now then, the breaking of this initial constraint may allow us to meet the new customer demand… but what happens when Sal the sales guru goes out and grows the business even more?

    I’ll tell you (again from personal experience). A new constraint will be identified forcing us into action once again.

    I like to think of it as job security!

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