Applying Pull Systems


April 1, 2006
By Darren Dolcemascolo

As a follow up to our discussion on the universal application of lean, let's consider the concept of pull. How is it used? Most associate pull with a replenishment pull system, which is the most common application of the concept of pull. It works as follows. Let's suppose we have a factory with two major processes: injection molding and assembly. The factory has a supermarket consisting of injection molded components. As injection molded components are moved into the assembly cells such that a trigger point is met, a signal kanban is sent to the injection molding department to replenish the supermarket with molded components. In other words, as molded components are pulled from the supermarket, they are replenished by the molding machines. Similarly, the company may also have a finished goods supermarket such that the assembly cells replenish the finished goods market as customers pull product (i.e., as product is shipped). But this is not the only way that the concept of pull can be applied. This idea behind pull is to prevent the wastes of overproduction and inventory by making only what the customer has ordered.

Some situations, however, are not conducive to the supermarket pull system. A job shop, for example, would almost never use a supermarket pull system since the shop would make such a wide variety of product. Does this mean that the concept of pull does not apply to a job shop? Certainly not. In fact, the job shop by definition is only making what the customer has ordered. so, how might a job shop apply the concept throughout its value stream. First, as a customer places an order, a signal (order) is sent to the first step in the manufacturing process. The product then flows through various steps until it is ready for shipment. Let's suppose that there are areas within the process where inventory builds (i.e., areas that are not and cannot be one-piece flow). For example, let's suppose there is a heat treat process. How can we use the concept of pull to control inventory waiting for the process (and prevent excess inventory from accumulating). We can use a FIFO (First In First Out) Lane to control the flow of material into the heat treat area. This would do two things:

  • Limit the amount of material allowed to accumulate behind the process.
  • Ensure that product is processed in sequence (first in first out).

  • If the FIFO lane ever becomes full, this will serve as a visual signal. For example, a full FIFO lane might indicate that there is a problem in the process or that overtime must be worked (or some jobs might be off-loaded for heat treat). In a typical job shop, inventory would just pile up in front of the process until it is too late to meet the customer's delivery date. The pull concept applied in the form of a FIFO lane would help prevent a late delivery and prevent overproduction and excess inventory.

    FIFO lanes have other application; for example, the concept might be used in hospitals with patients for a process. When a certain number of patients accumulates, additional labor or equipment might be sent in to prevent too long of a wait. How might pull be applied to your organization?

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