Why I Don’t Like Two-Bin Systems

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:

  1. 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.
  2. 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.

Why?

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.

So?

  • 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.

12 thoughts on “Why I Don’t Like Two-Bin Systems

  1. One very important consideration in setting up a pull system, two bin or otherwise, is that there needs to be a level pull. The more eratic the pull patterns the more problems there will be.

  2. Take a look at our two bin system which is used by major companies all over the world. Visit our website and have a look for yourself http://www.twinbin.com

    Thanks

    Mark’s Comment:
    After thinking about it, I went ahead and approved this comment since it is technically on-topic. It is however a commercial solicitation for a proprietary two-bin dispensing system. I invite everyone to take a look, and comment back here if it is worth it, or simply a solution in search of a problem.

  3. What about if there are too much parts like 20 kind of bolts and different washers and so on for assembly process , does bin system work?

  4. Fahad, your opportunity may be to reduce the types of bolts and washers. This will reduce total cost of ownership and then work better in the Kanban

  5. Have you seen the Indicator Bin from Akro-Mils? This is a single-bin Kanban system. The Indicator bin is a double ended bin; one side is blue and the other side is bright orange. A divider is placed near the back of the bin and reserve stock is placed in the bin behind the divider. The bins are placed on the shelf blue side out. When all stock is depleted from the blue side, the Indicator Bin is then flipped around to reveal the orange side. This bright orange color indicates that the bin needs restocked. The reserve stock behind the divider is now available for picking with enough inventory to last until the bin can be restocked. Check out the bins here http://www.akro-mils.com/Products/Industrial/Specialty-Storage-Bins/Indicator-Bins.aspx.

  6. There is a company called Pyxis in the medical industry that has a system with a set of buttons that can be programed and attached to the front of the bin. When the button is pushed central supply is notified exactly what needs replenishment. Saves a ton of foot traffic

  7. Hi Mark. Can you just clarify what system are you proposing over the two-bin system? I may have missed the point but I did not read into your suggested, better approach. Are you suggesting a bin system greater than 2 bins with lower quantities? If so, what would be a good quantity to start with? 1/2 * (average daily usage x lead time)? Thank you.

    1. Hi Jeremy –
      I apologize for the delayed response. I was having problems on the back-end of the site, and just got time to dig in and fix it so I could actually see your comment.

      What I don’t like about two-bin systems is not so much “two bins.” Rather, once people are locked in on “two bins” they tend to try to keep two bins even if rates and lead times change.

      When rates increase, the reflex is to make the bins bigger – put more parts in them. This, however, introduces a longer delay between the time the first part is taken from the bin and the last part is taken from the bin.

      In mathematical terms, the time to empty the bin is #parts x takt time.
      On the back end, the more parts must be put into the bin to replenish it, the longer it takes to turn around the empty bin.

      It is surprisingly easy to fall into the trap where the replenishment time for an empty bin exceeds the consumption time for a full one.

      A two-bin system is mathematically equivalent to a single kanban card following the rule of sending the card back when the first part is pulled from the container. If the math doesn’t work for one card, then two bins won’t work either.

      All kanban formulas work out to the same basic requirement.
      You need enough material to cover the circulation time of your replenishment signal from the time you send it until it returns.

  8. Hello
    We currently use a two bin system at my site and they want me to get rid of it and go to one bin. They do not like the look of the second bin sitting on bread racks on the prod floor. We have traceability requirements so we need to keep our lots segregated so going to one bin makes that a challenge without going back to bagging everything and that creates waste. I am looking for suggestions to other options that others might have.

    1. Not liking the appearance is hardly a rationale for changing a replenishment system, but… OK.
      The behavior of a replenishment system is math. At the end of the day, no matter what you are using to trigger replenishment, either a second bin has to be there just before the first runs out, or you have to wait for more.

      The only other alternative would be a secondary supermarket somewhere else, but that’s not much better.

      Rather than look for alternative solutions, I think the first step is to clearly define the problem.

      How many parts are in the bin?
      How long does it take to consume that many parts?
      How long does it take to refill a bin from the time you know you need more until that bin shows up on the shelf?

  9. Hello Mark,
    Very good insites. I still had one question. you stated `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`. how is it so?. If the production line stops in a 2 Bin System, then it should not be affecting the bin replineshment as the materials from the bin is already empty and the new material brought into will be kept in the place of the empty bin.
    This is how I felt. Pls clearify.

    1. Rohit – In a kanban card system set up by Toyota’s rules, the card is pulled (starting the replenishment process) when the *first* part is removed from the container. As you use the parts from that container, the card is going to trigger another one to be delivered. If your timing is good, that 2nd bin (with the single card) will arrive just as you are emptying the first one.

      When you empty the first bin, you pull the card from the second bin as you use the first part from it.

      Thus, a one-card system circulates two bins.

      In a Toyota-style kanban card system, bins are generic (they are not part specific) until the cards are attached to them for return-circulation to the supermarket.

      Thus, you need cards+1 bins for any given loop. Therefore, a two-bin system, and a single-card system are mathematically equivalent.

      The problem with “two bin systems” is people tend to stick with “only two bins.” As rates increase, they compensate by adding more parts to the bins (bigger bins) rather than either (1) working to circulate things faster or (2) increasing the number of bins for smaller, more frequent, replenishment.

      There are mathematical instances where increasing the number of items in the bins will extend the lead-time for replenishment beyond the time it takes to use all of the parts in the bin being consumed, which results in a shortage at the point of use. This is counter-intuitive, but it can happen.

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