 Levinson Productivity
Systems, P.C.William A. Levinson, P.E. Principal 570-824-1986  |
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Other pages on this subject:
The Theory of Constraints and Synchronous Flow Manufacturing
Online JavaScript simulation
The experiment shows that a "balanced" production line is an illusion.The simulation shows how random variation in a production line with balanced capacity reduces its throughput below the theoretical rate, and causes inventory to collect in the factory.
For each workstation, a single die is rolled (1-6). The workstation can process that many pieces (if they are available in queue). The station will ship the lesser of the die roll and the number of pieces that are waiting to be processed. For example, if six pieces are waiting and the die roll is 2, the station will ship 2 pieces. If only three pieces are waiting and the die roll is five, the three pieces will be processed and shipped.
Each workstation starts with 4 pieces in stock. Therefore (in theory) the manufacturing line "should" be able to ship an average of 3.5 pieces per turn. The following screenshot shows the first turn of the simulation. Although the average production die roll is (6+1+6+2)/4 = 3.75, a little bit above the expected average of 3.5, two die spots are wasted at Workstations 1 and 3 because they have only four pieces wating. Since every workstation in a balanced production line is a constraint, this is known as starving the constraint. Since time lost at the constraint is lost forever, these four wasted die spots can never be recovered and the factory's overall productivity will average less than 3.5 units a turn.
The question arises as to whether the system ever achieves a steady state in which each workstation has so much work-in-process that high die rolls are never wasted and the eventual production rate settles down at the expected 3.5 units per turn. The following simulation (with six workstations) suggests that this state is never achieved. Even though the system now contains more than 600 units of inventory, only three are available at Workstation 2 so there is a 50:50 chance that part of the die roll will be wasted! (And remember that this wasted capacity can never be recovered.) Also note that, although it may fluctuate up and down, the total WIP inventory seems to increase indefinitely.
This does not mean, however, that it is impossible to run a balanced factory at 100 percent capacity; all you have to do is reduce variation in processing and material transfer time to zero! Henry Ford wrote, in fact, "The idea is that a man must not be hurried in his work— he must have every second necessary but not a single unnecessary second" (My Life and Work, 1922). He succeeded in running a production line in this manner without generating the piles of inventory that appeared in The Goal (and in many real factories).
Download the program for free (UPDATED September 18 2005 using Visual Basic Service Pack 6)
Licensing and Terms of use: Permission is given to download this program, provided that no changes are made in them and that they are not shown for a fee (other than a usual dinner meeting fee for, as an example, a monthly ASQ, APICS, or SME meeting). Your sole remedy for dissatisfaction with this program is a full refund of the purchase price. :-)
Download the following files (right-click and select "save link target as" in Netscape Communicator), install them in the desired directory, and run setup.exe