Interface - The Journal of Wheel/Rail Interaction

WHEEL/RAIL INTERFACE

Applying Quality Concepts to the Wheel/Rail Interface
(Part 3 of 3)
By Stephen S. Woody • April, 2008

Part I of this article examined the results of a six-sigma project that Norfolk Southern initiated to improve the efficiency of its grinding program. Part 2 illustrated some the data analysis-related problems that NS experienced and learned from during other six-sigma projects. Part 3 describes two ideas for prioritizing and resolving wheel/rail interface issues.

If you follow the trends in the quality industry, you consistently see two underlying issues that can affect wheel/rail practitioners. The first issue is that the most important problem affecting a process is rarely properly identified and acted upon. The second issue is that when quality professionals do identify the proper action to take, they frequently cannot frame their recommendations in terms that cause their management to act upon those recommendations.

A branch of the quality field known as the theory of constraints can help to identify the most important problem in any process. The premise behind the theory of constraints is that a system is like a chain: it will never perform better than its weakest link. (The theory of constraints is really an extension of the Pareto Principal). Figure 1 shows a five-step process as an example of the mindset behind the theory of constraints. The number of units that can be produced per day is shown below each step. As the Figure indicates, the maximum number of units that the process can produce per day is 6 units because step C is the limiting factor, or the constraint.

Assume that a quality improvement project increases the production of step D of the process to 12 units per day as shown in Figure 2. However, the output of the process is still 6 units per day because the weakest link is still step C. Note that a great deal of time, money, and effort may have been spent on improving step D for no real benefit.

Now assume that a quality improvement project improves the output of step C of the process to 11 units per day as shown in Figure 3. Now the output of the process is 8 units per day because step D is now the new weakest link. If we want to improve our process output even more, we will now have to focus our efforts at step D. When the output of step D improves, step E will become the constraint.

Refer to Figure 1 and note that there are two possible courses of action to take depending on what the output of the process must be. If 6 units a day is an acceptable process output, then steps A, B, D, and E must be consolidated or slowed down since they would otherwise be creating excessive in-process inventory, which wastes money and manpower. If 6 units per day is not an acceptable process output, then step C must be worked on to increase the process output to the desired level.

The above example consists of independent steps. Real-life processes rarely have independent steps and interactions between factors can be the system constraint.

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