Going forward by moving backward is how one author described a pull system. Others use the analogies of drums, buffers and rope to explain how to “pull” production through a manufacturing shop. There are numerous images one can use to visualize pulling goods through a plant.
Since pull systems are frequently contrasted to so-called “push” systems in which production is master scheduled to push production from one operation to the next whether needed or not, they are often thought of as merely scheduling or shop floor control tools. In fact, pull systems are much more. They are the heart of a synchronized factory. They permit synchronization by working backward through signals or triggers which cause production events to happen. The experience of backward motion starts at the finished goods warehouse or shipping area and signals the previous operation – final assembly – when more finished goods are needed. Final assembly in turn signals a previous operation – perhaps a component fabrication department – when more components are needed. Component fabrication shops may signal a preceding manufacturing department or a raw materials store-keeping area which would signal a vendor to make a delivery. The signals in a pull system are in fact inventory levels for fabrication and raw materials replenishment and may be either inventory levels or a forecast/schedule for finished goods replenishment.
With such a system in place, shipment of finished goods triggers withdrawal of components for assembly to replenish the shipped goods which triggers withdrawal of raw materials for fabrication to replenish the withdrawn components and so on through the triggering of a vendor shipment to replenish raw materials. This pull sequence system is known as a kanban system in Japan and in The Toyota Automobile Corporation where it was developed and refined. Internal Pull Up/Down Resistor More revealing of its simplicity is the fact that the kanban system is often still called the supermarket system because the concept originated in observation of American supermarkets by Toyota executives.
The Toyota executives observed that when customers withdraw goods from the small stocks on supermarket shelves, the stocks are replenished in small quantities by a stock clerk who checks the shelves and replaces only the quantity which was taken. The first pull signal came from the customer who withdrew the inventory and told the stock clerk how much to replenish. The Toyota executives reasoned that this supermarket concept could be adapted for management of a factory on a simple visual basis. Since it is impractical to have roving “stock clerks” in a factory, a card is used to communicate to the production foremen the fact that a shelf was empty. The Japanese word for card is kanban; hence the name kanban for a pull system.
THE KANBAN APPROACH
In a system which triggers production in backward motion, a system of signals is the means to communicate the replenishment of goods. The signal media in a “classic” kanban system are cards and containers. While there are many variations on the visual Kanban theme, the most instructive is the Toyota system. The Toyota system utilizes a specifically sized container for each part which cycles back and forth between the producing department and the using department (each may have specific store-keeping areas). Two cards (kanban) are used: a production kanban and a conveyance kanban. These kanban specify the part number, the container capacity and other data.
When a using department withdraws a container of parts, the conveyance kanban previously attached to it by the producing department is detached and placed in a collection box. When the most recently emptied container for the same parts is ready to be conveyed to its producing department, the conveyance kanban in the collection box is attached to it. At the time this empty container is received by the production department, the conveyance kanban is detached and attached to a recently manufactured full container of those parts which is then moved to the using department. The removal of the full container out of the producing department triggers production through removal of a production kanban attached to it which is placed in a collection box. The production kanban in the collection box are transferred hourly to a dispatch box and serve as the authorization for the foremen to produce those parts within a specific time frame and fill an empty container. When the container is filed the production kanban is attached to it and the container is placed in a store area awaiting transfer to the using department. This process repeats itself over and over again.