Supermarkets turn up in Lean descriptions from time to time. Taiichi Ohno is often said to have come up with the idea for kanbans after seeing an American supermarket. Other authors suggest that Ohno had not been to the US at that time, and had the idea after reading about supermarkets, rather than seeing one. Either way, the superrmarket system where goods wait on shelves in a pre-defined number and customers remove them and thereby trigger stock replenishment, does have a lot of the ideas that later became important in Lean thinking.
The ideal in a Lean system is to have continuous flow. If you can’t get to that, at least straight away, a common compromise is to establish a ‘supermarket’ between two parts of the system where one by one flow is not currently possible.
The supermarket acts in the same way as the shop example above. The downstream part of the process, the customer, pulls supplies from the supermarket. This then triggers the upstream process to make more of the required product to replace the removed stock. The beauty of this is that the people working in the upstream process do not need to know the precise overall schedule – they just need to know that the item they produce has been removed, and therefore to make more, in the same quantity.
In a kanban system, the downstream process sends a supply kanban to the supermarket for the items it needs. These items are withdrawn, and a production kanban goes from the supermarket to the upstream process, to tell it to produce more of the required product or part.
I saw a version of the supermarket system recently in an industrial plant. The plant made a health care product, on a production line. The production line had a two part production process, and then a packaging process. In the production process, there were a set of carts beside the second process, which I will call Process Two. When one of the carts was emptied, this was the signal for Process One to manufacture enough product to fill the cart, which was then placed back in the supermarket.
The number of carts, the Production Manager told me, was calculated to ensure that Process Two could keep functioning to feed the downstream Packaging Process, even if there was unscheduled downtime in Process One. The supermarket had a role as a buffer, as well. The carts fulfilled the function of the production kanban. This was straightforward, as Process One and Process Two were within three metres of one another, so it was easy to see whether a cart had been emptied.
Much the same process can be used in healthcare, where one process feeds another. Operating theatre packs are an example of this, where removal of packs can, in a Lean system, trigger production of the same pack.
Photo courtesy of Hyena Reality at freedigitalphotos.net