What is “Pull”?

The fourth principle of lean is to allow the customer to “pull” from the producer. So what exactly does this mean?

Essentially the concept of pull means that we produce only when the customer asks for what we make. In the picture above, the end customer signals, “I need one now,” and the finished goods are shipped out immediately to the customer. The “pull” continues upstream as each “consuming” process requires what the “producing” process makes upstream, all the way to raw materials. In a lean organization, the pull signal will continue up into the supply chain.

There are are a couple major forms of pull. In the first form, orders are received from a customer, and are managed and released to production so that everything flows through in FIFO - First In, First Out order. If we were to represent this graphically, it would look like the figure below.

The second major form is the supermarket pull. In this situation, customer orders come in and are released to shipping to meet demand. Shipping withdraws the finished goods from a supermarket, and ships according to customer orders. When the items are removed from the supermarket, this triggers an order to replenish what has been removed in the form of a Production Kanban.

While most people think of kanban cards when they hear the word “kanban”, in fact “kanban” is essentially a signal that tells an upstream process what to do. It can come in the form of a card, an empty bin, an electronic signal, an empty space for materials, or many other forms.

When the upstream process receives the signal to produce more of what is needed downstream, they will in turn will use a Withdrawal Kanban to signal the need to withdraw materials from a supermarket between themselves and their upstream supplier. The removal of materials from that supermarket will then trigger another Production Kanban signal.

It is important to note that the supermarkets differ significantly from regular warehouse storage. Supermarkets contain a managed inventory level for each part, with maximum levels, minimum levels and reordering levels. The supermarket is always controlled by the upstream process. While these two forms are the most prevalent forms of pull, it is important to note that there are many variations which combine elements of both forms.