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Principle (Conservation of Material): In a stable system, over the long run, the rate out of a system will equal the rate in, less any yield loss, plus any parts production in the system.

Motivation

This is a very general principle and perhaps follows common sense. It simply states that whatever enters a line must eventually come out of the line. That is, material is neither created nor destroyed while in passage through the line. However, this simple principle may have interesting consequences as shown in the following example.

Examples

1. Push System: In push system the throughput is decided by the release rate into the system. Now suppose we increase the mean time to repair for a station in this line. The throughput of the line is unaffected by this change, no matter how much we improve the repair time! Since the principle of the material holds, what goes into the line must come out and over the long run the average throughput of the line is thus unaffected.

Throughput in Push system is unaffected by MTTR improvement

 

2. Definition of bottleneck as utilization: In a line the bottleneck station is defined not as the slowest line but as the one with the highest utilization. This is because even though the processing time in a station may be high, the amount of material through it may be smaller, so as to decrease the utilization of the machine. A situation is shown in the three-station line below. Suppose the processing time for Station 3 is higher than that of Station 2. However, some material leaves the line after Station 2 (due to yield loss etc.). Thus by the conservation of material principle we have less material and flowing through Station 3, and if the material loss is large enough it is possible that the utilization of the Station 2 becomes higher than that of Station 3 and hence become the bottleneck in the line.

Bottleneck defined by utilization as opposed to speed

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