Lesson 5: The Roadrunner and the drum

On the night shift, the finishing line downstream of the rolling mill sits half idle. The foreman shrugs: “They don’t give us enough work, so we slow down. Why would we race?”

Upstream, the shearing line has stacked plate to the ceiling. “Inventory? Not my problem. I hit my numbers.”

Ruth: “Both of them are doing exactly what the work ethic tells them to do.”

The current work ethic: look busy

Under the current ethic, every resource should work as much as possible. Upstream resources produce high levels of inventory nobody needs. Downstream resources, starved of work, slow down so they don’t look idle, and are then unjustly accused of not working enough. The “look busy” ethic is right for exactly one resource: the bottleneck.

A new work ethic: the Roadrunner rule

Goldratt borrowed an image from the cartoons: the Roadrunner is either standing still or running at full speed. Nothing in between.

When you have work, work as fast as you can. Otherwise, wait for work.

For the downstream resources, this removes the conflict: they race when work arrives, and nobody blames them for waiting. But it raises a question for upstream: what stops them from flooding the floor with inventory? The answer is the procedure Goldratt called Drum-Buffer-Rope.

What decides what we work on?

Two things: what the market wants, and what we are able to deliver. We can’t process everything at once, because capacity isn’t infinite. Which resource has the least? The bottleneck. So we schedule it first, considering its limited capacity.

The drum

Suppose orders for three products are waiting, and the bottleneck takes half an hour per unit:

Product	Units ordered	Bottleneck time
Plate type 1	30	15 hours
Plate type 2	6	3 hours
Plate type 3	10	5 hours

The bottleneck’s schedule is: type 1 from hour 0 to 15, type 2 from 15 to 18, type 3 from 18 to 23. This list of work for the bottleneck is the drum. All resources should be managed so the bottleneck is utilised as much as possible, so its schedule sets the beat for the entire operation. The drum is the key to synchronising everyone else.

Don’t be over-sophisticated building the drum. Which job comes first matters far less than having a drum.

And if no resource is short of capacity, the drum is simply the list of orders and their due dates.

The buffer

Murphy will strike upstream. To protect the drum, the right work must be waiting at the bottleneck even when upstream resources run into problems. So we release material a set time before the bottleneck needs it: the buffer. Because every other resource has protective capacity, this creates the bank of work in front of the bottleneck that we proved was needed.

The bigger the buffer, the more protection against Murphy, but also the more inventory and the longer the lead time. Choosing its size comes in the next lesson.

The rope

Tying the rope means choking the release of material in line with the drum and the buffer. With an 8-hour buffer, the release schedule is simply the drum schedule moved 8 hours earlier:

The drum starts type 2 at hour 15 and type 3 at hour 18. With an 8-hour buffer, when should their materials be released?

Both at hour 0, so nobody is idle
Hour 15 and hour 18
Hour 7 and hour 10

Drum time minus the buffer: 15 − 8 = 7, and 18 − 8 = 10. Type 1, needed at hour 0, should already have been released 8 hours ago. And the rule of the rope: don’t release work ahead of schedule, even if some work centres have nothing to do.

Expect the question from the floor: “Why won’t they let us work as much as we can?” That question is the culture change. DBR in three lines:

Drum: set the schedule of work at the constraint.
Buffer: set the time work is released before it is due at the constraint.
Rope: set the release schedule accordingly, and don’t release work early.

Now the Roadrunner ethic works for everyone: upstream resources can’t flood the floor because release is choked to the drum’s pace, and downstream resources race when work arrives.

Wandering bottlenecks

“But our bottleneck moves every few days!” Goldratt’s answer: if the bottleneck shifts that often, it isn’t because of the product mix; it’s because of local optima policies: efficiencies everywhere and big batches to save set-ups. Replace them with DBR and the real picture appears: most resources have ample capacity, and if a real capacity constraint exists, it sticks out like a sore thumb.

If you suffer from wandering bottlenecks, proceed as if no bottleneck exists, and let the market orders be the drum.

Check your understanding

Answer all five. Get four right to complete the lesson.

1. The Roadrunner rule says…

Always work at a steady pace, so that the flow stays even through the day
When you have work, work as fast as you can; otherwise wait for work
Look busy
Work overtime when behind

Full speed or standing still.

2. The drum is…

The buffer of material
The release schedule for raw material into the plant
The fastest machine
The schedule of work at the constraint, which sets the pace for all

Having a drum matters more than perfecting it.

3. A bigger buffer gives…

More protection against Murphy, but more inventory and longer lead time
Less inventory, because work waits less often for missing parts
Shorter lead time
No change

That is why its size must be chosen.

4. The rope means…

Releasing work as soon as anyone is idle
Tying workers to their machines
Not releasing work before the drum time minus the buffer
Expediting late orders

Choking the release is where the culture change happens.

5. If the bottleneck seems to wander, you should…

Buy capacity at each one
Act as if there is no bottleneck and use market orders as the drum
Reschedule daily
Measure each resource’s efficiency more closely to find the real one

Wandering is usually caused by local optima policies.