Pull System
This time, let's exchange some ideas about the Pull System: its importance in lean philosophy, its function, and also the various types of Pull System.
I believe almost everyone has been to a supermarket. That's right, a supermarket, which nowadays is a common place where people go to buy all sorts of products.
So let's see: when does a person go to the supermarket?
As a rule, a person normally goes to the supermarket when they need to buy some product to satisfy a need. And how does this person make the purchase? Think about it for a moment, because here lies the model that is the subject of this article: the Pull System.
It's simple: when people need something, they go to the supermarket where what they want is stocked and buy (pull) the quantity they need. There you have the Pull System and, along with it, Just in Time. Let's follow this model and transfer it to the day-to-day activities of a factory in operation.
In production, the most important premise for implementing the Pull System is to have a stable production system. Stability ensures and enables continuous flow, that is, the sequence of adding value to the product one at a time, uninterruptedly through the subsequent processes. By acting this way, that is, with the next process pulling what is needed, when it is needed, in the quantity needed, we avoid building up unnecessary inventory of products between processes and also overproduction. On the other hand, by producing in continuous flow, one product at a time, any quality discrepancy is immediately noticed by the next process (when not detected in the process itself), thus avoiding scrapping the entire production lot, which is very common in conventional batch production. The lead time (the time to process the parts) becomes shorter, because it eliminates the time subsequent processes spend waiting for the whole lot to be processed.
Well, here are the great merits of the Pull System: a) avoiding inventory between processes and overproduction, b) achieving quality in the process, c) shorter lead time.
In lean thinking we have already seen that excess inventory, rework/scrap (non-quality) and waiting are some of the wastes. By applying the Pull System concept, as you can see, we are eliminating these wastes and emphasizing the first principle of lean thinking, which is Value, value from the customer's point of view. And believe me, the customer is the reason the business exists.
Therefore, when designing the process, whatever the activity, let's think about how to implement the Pull System, that is, how to serve the customer more economically, with quality and faster. These are the factors that distinguish success from failure.
So far we have seen the importance and function of the Pull System for lean thinking.
Now let's look at the ways of implementing the Pull System. To make it easier to understand, and for learning purposes, let's consider three forms of Pull System:
a. Supermarket Pull System – This is the most common and widespread form of pull production. In this system, each product has a store, a supermarket, where a certain known quantity of each item produced is kept. The important thing is that each process produces only what is needed to replace what was withdrawn from the supermarket.
To control the withdrawal and replenishment of the product, kanban is used, the silent manager of the information flow, which, if used properly, prevents unnecessary inventory from building up and promotes continuous flow without the need for a human manager.
For all intents and purposes, the supermarket is still inventory and, as such, still a waste, a necessary waste, to avoid another waste, which is waiting.
And the key point in determining the size of the supermarket and of the inventory under control is to fully know and master takt time.
b. Sequential Pull System.- A system used when there is a very large variety of products. The goal is to minimize total inventory by approaching make-to-order production. PPC (Production Planning and Control) must schedule heijunka production, establishing a correct mix in a given sequential order that must be strictly followed. If the sequence is broken, the whole Pull System is compromised. As you can see, for the system to work properly, the production lead time must be fully under control, stable and very short. This system requires very strict management to succeed.
c. Mixed Pull System – This is the most complex form, which in certain situations can bring the benefits of each system to a single production line. A practical example is producing spare parts on a regular production line. This means that the production process must be changed to produce the spare part, which follows a special Sequential Pull System schedule, while the line normally runs a Supermarket Pull System. Due to its nature, this system requires very rigorous management.
In the next article we will discuss JIDOKA.
Stay tuned.
