Lines and Cells

Q: Can I calculate OEE for a Line or Cell?

Daniel Högfeldt •    A line or a cell with many operations has to be looked at as one unit. Ideally all machines should run at the same rate to get a balanced one-piece flow through the line. Usually that is not the case though. Usually one piece of equipment is slower than the other equipments making it the bottleneck of the line/cell. Let us say the line consist of two machines. The first machine has an ideal cycle time of 60 seconds and the second machine has an ideal cycle time of 30 seconds. If that is the case ideally the bottleneck would run with an OEE of 100% but the faster machine could then only run at 50% OEE, because it had to wait for pieces to be completed by the previous machine.

If they both were measured and an average was made for the line the OEE would be 75%. If the line is looked at as one piece of equipment and is only measured on the bottleneck then the line OEE would be 100%. If on the other hand the ideal cycle time of the fastest machine was used and the measurement still was made on the bottleneck the line OEE would be 50%.

To really expose the bottleneck in the line the best way would then be to measure the OEE on the bottleneck using the ideal cycle time of the fastest machine. When improvements have been made there could be a different machine that is the bottleneck and then the measurement should be made on that machine instead. That way the bottleneck always will be in focus for improvement.

The other way would be to use the ideal cycle time of the bottleneck when measuring the line OEE. That would show more accurately how the line is performing based on what it is capable of performing. If data is collected correctly at the bottleneck it should show if the machine is spending much time waiting for pieces or waiting to send pieces. If it spends a lot of time waiting for pieces some machines upstream is the current bottleneck. Then focus has to be to fixing that problem. If the machine spends a lot of time waiting to send pieces the current bottleneck would be somewhere downstream. Then focus has to be there for improvements.

Master’s Thesis: PLANT EFFICIENCY A value stream mapping and overall equipment effectiveness study

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