On the surface, a “two bin system” seems a great, simple solution to a part resupply process that could otherwise get complex.
And, on the surface, I don’t argue with that.
But two-bin has some limitations. And because it is so simple to set up, those limitations are frequently not understood or taken into account.
What is “Two Bin?”
Although it may be obvious to all, I want to define “two bin” to reduce the chance that what is “obvious” is actually less so.
“Two bin” is a simple pull system. The parts are supplied by two rotating containers. When a bin is empty, it is returned to the supplying process to refill. The second bin supplies parts while the first one is being filled.
The same system can be applied to things much bigger than “bins.” I have seen carts carrying fabricated steel parts weighing many hundreds of pounds set up the same way.
When does it work?
In general, two-bin is OK when two criteria are met:
- The parts are relatively cheap. That is you don’t worry too much about having more than you actually need. This comes with a warning, however. It is easy to have a ton of money tied up in relatively small excesses of hundreds of parts. It does all add up.
- This is critical: The time to replenish and return a full bin is short compared to the time to use the parts contained in a full bin.
In this scenario, the first bin is empty, and long before the Team Member has emptied the second bin, the first one is safely returned and is behind it on the shelf.
So What’s The Problem?
There are problems at two levels. I am going to emphasize the practical one, then talk about the philosophical one.
At The Practical Level
First, let me explain how this system would work if it were being done with kanban cards as signals.
One of the rules of kanban cards is that the card is removed from the container when the first part is consumed. That is, the container is NOT empty.
In this type of system, the number of containers circulating is +1 over the number of cards circulating. If you think about it, this makes sense. Let’s say there are five cards circulating, and the container size is 10. If all of the full containers were on the rack, and one part is used from the first container, then the rules state that the card is removed and signals bringing another bin.
If production stops at that point, the worst-case scenario of kanban is realized: The card is returned with another bin of 10 parts. There are now 5 full bins on the shelf, each with a card attached, plus the one bin of 9 parts.
What this has to do with two-bin: Two bin is mathematically identical to a one-card kanban loop.
At a practical level: Do the math for kanban. If your system, with your replenishment quantity and times won’t work with one kanban card, a two-bin won’t work either.
At a Philosophical Level
The problem comes in in practice. Two-bin is easy to set up, and frequently the people doing so don’t do the math. And it usually works.
What results is a pull system that has locked down the number of circulating containers (two), and if there are perceived problems the reflex is to alter the quantity of parts in each bin.
If the system is running a little close to the edge, the materials people will up the parts quantity per-bin from, say, seven parts to ten parts.
Then the system fails.
With kanban, you signal for replenishment when the first part is removed from the bin. Having more parts in the bin means it takes longer to empty the bin. Your supplier has more time to return with a full bin.
With two-bin, you signal for replenishment when the last part is removed from the bin. Having more parts in the bin means it takes longer to empty the bin. Adding more parts to the bin delays the notification to your supplier that you have started using parts.
While it doesn’t always happen, there are cases when adding even a few parts to the bins will cause two-bin to fail because of this.
No matter what system you use, you must thoroughly understand how it works, why it works. You must think through (and try) every step of the process, not just talk about it.
You must ask questions and understand every detail:
- If there are hundreds of bins, and they all have part-specific labels, how will they be sorted and routed to the appropriate supplying process?
- How will the bins be used to visually manage the replenishment process?
These are questions with obvious answers in card systems (that use generic bins), but are frequently not well thought through in the rush to implement the “simpler” circulating bin systems.
Also keep in mind: If you are not constantly monitoring actual use, execution and results against your assumptions and expectations of what should be happening, you might be using pull, but you are not applying the Toyota Production System.
- Go ahead and use two-bin if you want to. Just do the math and make sure it will work for you.
- However, if you use cards, or other signals, anywhere consider the complexity you are introducing with more than one system. The rules are different depending on the parts. Remember, you are asking your customers to keep track of all of this.
- Generally speaking, if your system is operating close to the edge, it is better to use more containers that are smaller. Things will circulate more quickly, and your supplying process will have a much better picture of your consumption patterns.
- Remember – your objective is to move closer to single-piece-flow. If you move away from that (with bigger containers) you are doing the opposite of kaizen.