One way to demystify the concept of kanban is by thinking of simple examples of pull-replenishment systems in everyday life. Like when you decide to buy gas for your car. Does your gas tank get filled according to a schedule? Would you consider simply filling the tank once per week on Monday morning? I doubt it. If you did, you would find you sometimes really do not need gas Monday morning and other times you will run out before Monday. Most likely, when you notice the gas gauge approaching empty, you stop at a gas station. The same pull system behavior goes for most routine things we purchase in our households. The simple trigger is that we notice our inventory is running low on an item and at some point say, “Yep, I better go out and get some more of that.”
Not everything can be replenished based on a pull system; some things must be scheduled. Take the example of high-end products, like a Rolex, a sports car, or those killer high-tech golf clubs advertised by Tiger Woods. Whenever you are buying a special or single-use item, you have to think about what you want, consider the costs and benefits, and plan when to get it. In a sense, you create a schedule to purchase, since there is no immediate need for it.
Services are another type of purchase that isn’t immediate but has to be scheduled. For example, we recently had our septic tank cleaned. We had no way of telling if it was getting full and needed to be emptied. So we followed the generally recommended (and probably inaccurate) schedule for cleaning the septic tank—a push system. But now there is a device on the market that you can install that detects how full the septic tank is; when it reaches a trigger point, it indicates through a radio signal when to get it cleaned. If we invest in that, we can eliminate the need for a “scheduling system” and replace it with a pull-replenishment system—a signal to replenish (actually, to empty) based on actual usage rather than a vague guess about usage.
Because the pull system corresponds with actual usage or consumption, Toyota is constantly working to achieve the ideal of just-in-time replenishment. Using kanban, they are carefully monitoring and coordinating the use and replenishment of thousands of parts and tools, orchestrating specific schedules for replenishment, developing rules for when to pull the trigger to send a replenishment signal, calculating the maximum amount of inventory that will be allowed, and the like. The kanban/pull system works better than a schedule system for most business situations. But it still depends on small inventory buffers or “stores of parts”—and inventory is always a compromise. So the goal is to eliminate the “stores of parts” and move to true one-piece flow wherever possible.