What happens and why
Why a queue grows in front of Herbie
The source releases a part a second, and Herbie processes a part in 2 seconds. Every 2 seconds two parts come to him, and one leaves. The difference piles up in front of him — in machine 3's buffer, on the belt, then in the buffers of machines 2 and 1, until the source simply has nowhere to put new parts. The queue grows until it takes up all the room in front of Herbie.
Why the machines after Herbie stand
Machines 4 and 5 can process a part a second, but Herbie gives them a part once every 2 seconds. Half the time they have nothing to do. That is why the idle meter shows about half the shift for them. Machines 1 and 2 stand too — not because they have nothing to do, but because they have nowhere to pass the part: Herbie's buffer is full.
Herbie has no idle time: there is always a queue in front of him. That is why the bottleneck is the machine with zero idle time.
Why Herbie's rhythm did not cut the output
The factory's output is Herbie's output: a part every 2 seconds, however much we feed. When the release went once a second, half the parts just stood in the queue. By feeding parts at Herbie's rhythm we removed the queue but did not take a single part away from Herbie: he still never waits a second.
| rhythm 1 s | Herbie's rhythm (2 s) | |
|---|---|---|
| made by 200 s | 94 | 94 |
| work in progress | about 20 | about 6–7 |
Parts standing in the queue are money invested in material and work that has not yet come back. Goldratt calls them inventory. The constraint's rhythm reduces inventory without touching the output.
Why only Herbie is worth speeding up
We sped up machine 1 — it began putting parts into the queue sooner, and Herbie takes them at the same speed. We sped up Herbie — the output of the whole factory grew. This is the main conclusion of the kit: a system's output is set by its constraint, and it is the constraint that is worth improving.
What's next
We set Herbie's rhythm by hand — with a switch, knowing that he works 2 seconds. But machines break down, dice upset the rhythm, and then Herbie either waits or is swamped. How to feed parts on a signal from the bottleneck itself — «drum — buffer — rope» — is the next kit in the series.